Fe5.0 Cr1.5 Mo1.0 V

0.35 C

Composition limits of H-13 based on the AISI/UNS (T20813) standards are (mass %): 0.32-0.45C, 0.20-0.50 Mn, 0.80-1.20 Si, 4.75-5.50 Cr, 0.30max Ni, 1.10-1.75 Mo, 0.80-1.20 V, 0.250 max Cu,0.03 max P, and 0.03 max S. Where specified, asresulfurized H-13, sulfur may be increased to0.06-0.15% to improve machinability.Besides the standard H-13 grade, various modified,premium, and superior grades of H-13 areavailable from hot work steel producers, usuallywith limiting phosphorus and/or sulfur levelsthat are below the standard composition limitsto improve toughness and thermal fatigueresistance and containing principle alloyingelements in particular ranges that may be outsidethe T20813 standard. Also, the premium gradesof H-13 within T20813 composition limits aregenerally produced by special refining andmetallurgical practices to control microstructureand especially carbide size and distribution.

1.0 GeneralThis medium alloy, martensitic, air hardening,ultrahigh-strength steel is similar to H-11 and H-11 Mod in composition, heat treatment, and manyproperties. The steels H-11, H-11 Mod, and H-13exhibit several properties that are important inairframe and landing gear applications, includingthe ability to be heat treated to an ultimate tensilestrength of 300 ksi while having excellent thermalshock resistance. These grades are typicallyhardened by austenitizing and cooling in air,flowing inert gas, oil, or hot salt bath. Upontempering, they show secondary hardnessmaxima in their tempering curves and, uponbeing double or triple tempered at 1050-1100F,typically develop the combination of high hardness(44-48 Rc) and high room temperature ultimatetensile strength (220-250 ksi) combined withgood fracture toughness and maximum fatiguestrength at room and elevated temperatures.H-13 steel is not as commonly used as H-11 Modas a constructional steel in ultrahigh-strengthapplications, although it can be substituted forH-11 Mod in cases where availability or the slightlybetter wear resistance and other characteristics ofH-13 offer advantages.All these steels are susceptible to hydrogenembrittlement and, in structural applications,must be protected with a corrosion-resistantcoating, even in mild atmospheric environments.Similarly, they must be protected against oxidationif exposed to air for prolonged periods of time attemperatures above 750F.The main compositional difference between H-13and H-11 Mod is higher vanadium content in

H-13, which leads to agreater dispersion ofvanadium carbidesand higher wearresistance. The H-13steel also has a slightlywider range of theother principalalloying elements,allowing producersflexibility in tailoringmechanical propertiesfor given heattreatments and applications. Premium andsuperior grades of H-13 have carefully controlledcompositions with low levels of sulfur andphosphorus and are produced by special melting,refining, and hot forging/rolling schedulesprimarily to achieve a fine microstructure andimprove toughness and thermal fatigue resistanceover conventionally produced H-13 grades. In afew cases, some H-13 producers employ longterm, high temperature, homogenizationtechniques with controlled cooling to refine thecarbide distribution and produce a more isotropicmicrostructure. Powder/particle metallurgygrades of H-13 are available with significantlyrefined distributions of carbides and sulfides (forthe high sulfur, free machining grade) toimprove toughness and thermal fatigue andwear resistance relative to conventional H-13 steelthat is normally produced by ingot metallurgy.Careful consideration of H-13 supply will assurea cost effective selection of steel grade for a givenapplication.Due to its good temper resistance and ability tomaintain high hardness and strength at elevatedtemperatures, H-13 is one of the most widelyused hot work die and mold materials fornonferrous and ferrous casting and hot formingoperations or for molding of plastics. Dies andtools made of H-13 can withstand workingtemperatures of 1000F or more. The steel hasdeep hardening characteristics, thereby allowinglarge sections to be cooled in still or fan stirred airor by pressurized inert gas from austenitizingtemperatures. It is the preferred material foraluminum and magnesium die casting dies aswell as for many other hot work die and toolingapplications, e.g., extrusion tools and containers,hot forging and stamping dies, hot shear blades,and plastic molds, where good thermal fatigue,hot erosion and wear resistance, and ability tomaintain hot hardness during operations attemperatures up to 1000F or higher are important.Due to the good thermal shock resistance of H-13in hardened tempers, dies and tools may be

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internally water cooled in service to preventundue softening. In many cases, H-13 hot workdies are nitrided or hard coated by other meansto improve wear resistance. (Refs. 12)

1.1 Commercial DesignationsH-13H13 (Alternatively)AISI H-13Premium AISI H-13ASTM H-13SAE H-13Material No. 1.234440CrMoV5Cast Grade CH-13GX40CrMoV5-1X40CrMoV5

1.2 Other DesignationsESR H-13 (ESR Electroslag Remelted)8407 SUPREME (Assab)8407 2M (Assab)Orvar Superior (Bohler-Uddeholm)Orvar 2M (Bohler-Uddeholm)W302 SUPERIOR (Bohler Uddeholm)ISOBLOC (Bohler Uddeholm)No. 883 (Carpenter)Extendo-Die (modified H-13 composition byCarpenter)Pyrotough 78 (modified H-13 composition byCarpenter)Nu-Die V (Crucible Specialty Metals)Nu-Die XL (Crucible Specialty Metals)Nu-Die ESR (Crucible Specialty Metals)DH2F (International Mold Steel)MTEK T90813 (MetalTek)THYROTHERM 2344 EFS (ThyssenKruppSpecialty Steels)THYROTHERM 2344 EFS SUPRA(ThyssenKrupp Specialty Steels)THYROTHERM 2367 SUPRA ESR(ThyssenKrupp Specialty Steels)THYROTHERM 2344 ESR MAGNUM(ThyssenKrupp Specialty Steels)VDC H-13 (Timken Latrobe Steel. Timken soldLatrobe in December 2006, and it is now LatrobeSpecialty Steels)TLS H-13 PQ (TLS Timken Latrobe Steel.Timken sold Latrobe in December 2006, and it isnow Latrobe Specialty Steels)LSS H-13 (Latrobe Specialty Steels)LSS H-13 PQ (Latrobe Specialty Steels)

1.3 SpecificationsASTM E45 Method AASTM E114ASTM A388ASTM A597ASTM A681SAE J437aSAE J438bSAE J467UNS T20813CAST UNS T90813FED-QQ-T570UNE 36072/2BS 4659 BH-13 (UK)BS 4659 H-13 (UK)EN ISO 4957: 2000 XCrMoV5-1AFNOR NFA35-590 Z 40 CVD 5 (France)DIN 1.2344 (Germany)DIN 17350 (Germany)JIS SKD61 (Japan)UNI X 35 CrMoV 05 KU (Italy)SS 2242 (Sweden)DC-9999-1 (GM Powertrain Group)AMTD-DC2010 (FORD Advanced ManufacturingTechnology Development)

1.4 CompositionThe composition of H-13 can vary depending onthe grade selected and particular offerings ofspecialty steel suppliers, who typically producestandard H-13 but may also produce premiumand superior grades of H-13 that have lower thanminimum levels of phosphorus and sulfur, arespecially refined, usually by vacuum arc remelting(VAR), to reduce segregation and inclusions,lower gas (N, O, H) levels, and spheroidize non-metallics. Special forging practices are used on H-13 steel to break up coarse alloy carbides andachieve a fine microstructure. Modified versionsof H-13 are also available from different H-13steel producers with some alloying elementsfalling outside of the standard T20813 H-13composition range in order to impart specialproperties for particular applications (Refs. 421).As H-13 steel is so widely used in die castingdies, which require high die life to reach break-even costs, the North American Die CastingAssociation (NADCA) has specified compositionsfor both premium and superior grades of H-13 intheir steel acceptance criteria and permissiblelimits of microcleanliness (Table 1.4.2). (Ref. 4)However, requirements for other applications arenot as critical, and the standards for H-13 steel

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acceptance for hot work dies and tooling differdepending on the industry.

1.4.1 [Table] Standard AISI/UNS chemicalcomposition of H-13 steel and typical compositionsof specially produced commercial grades of H-13steel available from specialty steel companies(Refs. 121)

1.4.2 [Table] Chemical composition (weight %)of critical alloying elements and impurities inpremium and superior quality grades of H-13steel (Ref. 4)

1.4.3 [Table] Maximum allowable limits ofnonmetallic inclusions according to ASTM E45Method A for H-13 steel for premium andsuperior quality grades of H-13 steel (Ref. 4)

1.5 Heat Treatment and Microstructure

1.5.1 NormalizingNormalizing is unnecessary and not recommendedfor H-13. (Ref. 3)

1.5.2 AnnealingAnnealing must be done after hot working andbefore hardening or rehardening. Most H-13material that is to be used for dies and tools thatrequire machining is purchased in the annealedor soft condition at a typical annealed hardnessof 192-235 HB. Annealing of H-13 is done byheating slowly and uniformly to 1575-1650F ina vacuum furnace, a controlled atmospherefurnace, or in a protective annealing pack toprevent decarburization. Holding time variesfrom one hour for thin sections and small furnacesto ten hours for heavy sections and large charges.For pack annealing, hold time is one hour perinch of cross section thickness. Upon removalfrom the furnace, H-13 should be cooled slowly(at 50F/h maximum) to about 1000F, then cooledat a faster rate to room temperature withoutaffecting hardness. (Ref. 3)This annealing treatment should result in afully spheroidized microstructure with a grainsize from ASTM 1 to ASTM 3. However, themicrostructure of annealed H-13 steel varies,depending on previous metallurgical processing,and several microstructural standards have beenestablished, including the often followedNADCA (North American Die CastingAssociation) acceptance microstructures forannealed H-13 steel used for die casting dies.Figure 1.5.2.1 shows acceptable (AS1-AS9) andunacceptable annealed microstructures (AS10-AS18) for H-13 steel according to NADCAspecifications for H-13 tool steel used in diecasting dies. Acceptable annealed microstructuresconsist of a ferritic matrix with a homogeneous

distribution of spheroidized carbides whenviewed at 500x after etching with 5% Nital. Thenumbering of the acceptable annealedmicrostructures from AS1 to AS9 does not denotea quality ranking, but shows typical variationswithin thick die blocks of H-13 steel. (Ref. 4)Banding microsegregation, indicative ofalternating layers of different compositionaligned in the direction of primary hot working,is also typically monitored in annealed H-13 steelblocks, and excessive banding microsegregationis cause for rejection for sizes above four inches inthickness. Figure 1.5.2.2 shows acceptable andunacceptable banding microstructures accordingto NADCA specifications for H-13 tool steel usedin die casting dies. (Ref. 4)It should be noted that outside of NADCA anddie casters, other major users of H-13 tool steel inhot work die/tooling applications or high-temperature structural components do not specifyannealed microstructure of H-13 steel; ratherchemical composition and hardness of theannealed H-13 steel are most commonly specified.

1.5.2.1 [Figure] Annealed quality ASmicrostructure chart for annealed H-13 steelshowing acceptable (AS1-AS9) and unacceptable(AS10-AS18) microstructures according toNADCAall microstructures at 500x and etchedwith 5% Nital (Ref. 4)

1.5.2.2 [Figure] Banding microsegregation chartfor annealed H-13 steel blocks showingacceptable and unacceptable microstructuresaccording to NADCAall microstructures at 50xand etched with Villellas reagent (Ref. 4)

1.5.3 Stress RelievingStress relieving of annealed H-13 steel after roughmachining and prior to finish machining andhardening is optional but is sometimes done tominimize distortion during hardening, especiallyfor dies or tools with complex shapes. This isdone by heating to 12001250F, holding for onehour per inch of cross section (minimum of onehour), then cooling slowly in air or in the furnaceto about 400F then in air to room temperature.(Ref. 3)Electro-discharge machining (EDM), commonlyused in finish machining of die insert cavitiesmade from hardened H-13 steel, can leave surfaceeffects such as unwanted residual stresses thatseriously affect tool life. Before stress relievingEDM-processed H-13 materials, the white layershould be removed by stone or sand blastingbefore stress relieving. Stress relieving after finishmachining is done by heating to 1000-1050F (or50F below the highest tempering temperature),holding for one hour per inch of cross section or

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for two hours minimum, then cooling in still airto room temperature. (Refs. 2021)

1.5.4 HardeningHardening heat treatment for H-13 steel byaustenitizing and quenching, usually followed bytempering, is a critical factor in the performanceof this material and must be carefully planned foroptimal results. H-13 steel experiences very littledimensional changes and shape distortion uponquenching and tempering.

1.5.4.1 [Figure] Schematic of NADCArecommended austenitizing, quenching, andtempering heat treatment cycle for H-13 steel(Ref. 4)

1.5.5 Austenitization/QuenchingFor austenitizing, typically done at 18501900Fand sometimes higher, surface protection againstoxidation and decarburization is necessary byutilizing vacuum or controlled atmospherefurnaces, salt baths, or pack protection. Preheatingof H-13 die blocks or tools for austenitizing canbe done in open furnaces at temperatures below500F. Pack containers with H-13 parts can besafely heated in a furnace to 1000F. Onceworkpieces or containers have reached temperature,they should be heated slowly in a protectiveatmosphere or vacuum to 1500F at 200F/hmaximum, held for one hour per inch of thicknessor per inch of pack container, then austenitized at18501900F for 1540 min, with longer times forthicker sections. Quenching is normally done instill or fan-blown air or in special pressurizedinert gas quench chambers that are mountedwithin vacuum furnaces. Sections greater thanfive inches in thickness require acceleratedcooling in forced air, pressurized gas, or interruptedoil quenching. In oil quenching H-13, the partshould be removed when black at about 900F,then cooled in still air to 150125F. Quenchingcan also be done in hot salt baths at 9501050Ffollowed by air cooling. Too rapid quenchinginduces distortions, which may offset criticaldimensions in dies and tools. However, to obtainoptimum properties, H-13 steel must be quenchedfrom austenitizing temperature at a minimumrate of 50F/min to below 1000F. (Refs. 321)Austenitizing of H-13 has been commonly doneat 1850F for many years since this steel wasdeveloped; however, as early as 1969, researchdone at Case Western Reserve University (CWRU)on thermal fatigue of H-13 steel (see 3.7.1) hadshown that austenitizing temperatures of 1950Fand even 2050F greatly improved thermal fatigueresistance. This was determined to be due to theincrease in resistance to softening under thermalstress cycling conditions simulative of aluminum

die casting, in spite of the larger grain size thatresults from high temperature austenitizing.(Refs. 2225)Prior to the CWRU work, which caused aparadigm shift in metallurgical thinking abouthardening hot work tool steels, it was generallyassumed that austenitizing of H-13 steel attemperatures of 19002050F would result in adecrease in fracture toughness relative toaustenitizing at 1850F due to an increase ingrain size (Figure 1.5.5.4). Subsequent researchhas established that austenitizing H-13 at 19502050F does not affect fracture toughness andmay actually increase it, while significantlydecreasing softening at high temperatures due tobetter carbide dissolution (Figure 1.5.5.5), thusexplaining the increase in H-13 die life in manyhot working applications after high temperatureaustenitization. The greater degree of dissolutionof carbides at higher austenitizing temperaturesgenerally results in higher as-quenched hardnessof H-13 steel, depending on component crosssection influencing the quench rate (Figure1.5.5.6). (Refs. 2228)Due to its high hardenability, quenching of smallsection sizes of H-13 components in still air fromaustenitization temperatures is often adequate toachieve a martensitic structure throughout and toavoid carbide precipitation at grain boundaries.Most accepted standards for achieving proper H-13 microstructures require cooling rates of >30F/min to produce proper microstructures andmechanical properties. H-13 parts must be cooledbelow 13001350F in less than 1520 min in orderto avoid grain boundary carbide precipitation,which can reduce toughness if excessive (Figure1.5.5.7). Once past this range, cooling can beinterrupted at 1050700F for a period of time toequalize temperature gradients in large andintricate sections without degrading mechanicalproperties.For large section sizes of H-13, portions mayenter into the bainitic region upon quenching(Figures 1.5.5.2, 1.5.5.3, and 1.5.5.8), resulting inreduced toughness upon tempering; in H-13, thetoughness of tempered bainite has been shown tobe inferior to that of tempered martensite due tothe presence of interlath carbides formed duringthe slow quench following austenitizing. At anaustenitizing temperature of 1870F (1020C), H-13steel has a transformation into the bainitic rangestarting at higher cooling rates than H-11 steel,probably as a result of the higher molybdenumcontent of H-13; however, at the higheraustenitizing temperature of 1975F (1080C) for H-13, the bainitic transformation starts at slowercooling rates that are similar to those for H-11(Figure 1.5.5.3). (Refs. 4, 2829)

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1.5.5.1 [Figure] Isothermal phase transformationdiagram for H-13 steel (0.40 C, 1.05 Si, 5.00 Cr,1.35 Mo, 1.10V) austenitized at 1850F (Ref. 3)

1.5.5.2 [Figure] Time-temperature transformationdiagram for premium grade H-13 steel (Thyrotherm2344 ESR Magnum, ThyssenKrupp SpecialtySteels, Inc.) austenitized at 18701920F (Refs. 17, 18)

1.5.5.3 [Figure] Kinetics of bainite transformationfor H-13 and H-11 steels compared at givenaustenitizing temperatures (Ref. 29)

1.5.5.4 [Figure] ASTM grain size versusaustenitizing temperature for H-13 steel andair cooled for 0.5-in. cube samples soaked attemperature for 2580 minutes (Ref. 28)

1.5.5.5 [Figure] Volume % carbide (95% confidenceinterval) in the as-quenched condition of aircooled (AC) small sections and larger 6- and 12-inrounds of H-13 steel as a function of austenitizingtemperature for given soak times (Ref. 28)

1.5.5.6 [Figure] As-quenched hardness as afunction of austenitizing temperature of aircooled (AC) small sections and larger 6- and 12-inrounds of H-13 steel for given soak times (Ref. 28)

1.5.5.7 [Figure] As-Quenched microstructurefor H-13 steel showing moderate carbideprecipitation at grain boundaries and finelydispersed carbides in matrix (Ref. 30)

1.5.5.8 [Figure] Continuous cooling transformationdiagram for H-13 steel austenitized at 1970Fshowing cooling curves A through E (Ref. 30)

1.5.6 Critical Temperatures (Ref. 31)Heating @ 100F/h:Ac1 1544F (temperature at which austenitebegins to form)Ac3 1634F (temperature at which ferrite toaustenite transformation is complete)Cooling @ 50F/h:Ar3 1475F (temperature at which austenitebegins to transform to ferrite)Ar1 1418F (temperature at which transformationof austenite to ferrite or to ferrite + cementite iscomplete)

1.5.7 StabilizingStabilizing is optionally done, sometimes oncomplex shapes, to remove residual stresses andto convert some of the residual austenite that mayhave formed in quenching H-13 from austenitizingtemperatures. In stabilizing H-13 steel, a briefstress relief/stabilizing temper is conducted at300 to 320F, followed by refrigeration at -150 to-320F. The steel may be used in this condition if ahigh hardness is required. Normally, to reducehardness, tempering is done immediately after

stabilizing after the part reaches room temperature.(Ref. 3)

1.5.8 TemperingTempering of H-13 steel components can beginas soon as they reach 120 to 200F upon quenchingfrom austenitizing temperature, i.e., assumingno stabilizing treatment is used. There is littlechange in as-quenched hardness with temperingtreatments up to 1000F, although a secondaryhardness peak occurs at ~950F caused bytransformation of retained austenite and formationof complex carbides. At temperatures above1000F, hardness drops and toughness increases.Thus, tempering of H-13 is usually done attemperatures of 1000 to 1200F, and preferablydouble tempering or triple tempering (cooling toroom temperature between each temper) toconvert residual austenite that may have formedin quenching from austenitizing temperature andalso to control final temper hardness. Convectionair tempering furnaces are used to heat large H-13 dies and tools at a moderate rate. Salt baths areacceptable for small parts but not for large orintricate shaped dies and tools, as they mightcrack due to thermal shock. Tempered hardnessdepends on austenitizing temperature, increasingwith increasing austenitizing temperature as seenin Figures 1.5.8.21.5.8.4. (Ref. 3)Tempering is usually done in cycles at a selectedtemperature for one hour per inch of thickness,cooling to room temperature, and retemperingusing the same time at temperature. The secondtemper is essential, and a third temper is beneficial(Figure 1.5.4.1). Final temper hardness is 53 to 40HRC, depending on initial austenitizingtemperature and total tempering temperatureand time.The relationship of hardness of quenched andtempered H-13 depends on temperingtemperature and time, often expressed by theHollomon-Jaffe parameter or master temperingparameter (MTP):MTP = (Temperature, F + 460) x (20 + Log Time,Hours) (Ref. 4)The relationship of hardness of H-13 steel on theMTP is shown in Figure 1.5.8.5. For example, toachieve 46 HRC in a small H-13 steel tool afteraustenitizing at 1850F, tempering may beconducted at 1080F for four hours, while a largetool soaked for 24 hours at 1025F to attainuniform temperature should also achieve 46HRC, as the MTP in both cases is 31,727. (Ref. 4)Another method based on the MTP for predictinghardness of tempered H-13 steel, assumingaverage composition and adequate quenchingfrom an austenitizing temperature of 1875F, is

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diagramed in Figure 1.5.8.6. The upper part ofthis diagram is applicable for a one hourtempering period, while the effect of longertempering time on hardness is determined byfollowing the slanted lines to a particular time inquestion and then vertically into the upper partwhere hardness can be read. For example, thediagram shows a tempered hardness of 47 HRCafter tempering at 1130F for 3 h, then temperingfor 3 h more for a total of 6 h gives a hardness of43.5 HRC, while still another 3 h temper totaling9 h further reduces the hardness to 42 HRC, etc.The higher the tempering temperature, the greaterthe sensitivity of H-13 steel to an increase intempering time, as indicated by the MTP above.(Ref. 32)After austenitization, air quenching, and doubleor triple tempering, H-13 steel should exhibit atempered martensitic microstructure withvarying amounts of tempered bainite along withvery fine undissolved carbides distributed in thematrix (Figures 1.5.8.7 and 1.5.8.8). In Figure 1.5.8.7,the variations are typical of those that occurwithin different sections of a heat treated blockof H-13 steel that had the different acceptableannealed and banded microstructures shown inFigures 1.5.2.1 and 1.5.2.2. (Refs. 4, 30)The microstructural changes that occur ontempering cause some slight dimensionalchanges in blocks of H-13 steel, and these mayneed to be compensated in the finishing of diesand tools that have tight dimensional tolerances.(Figure 1.5.8.9 and Table 1.5.8.10). (Ref. 3)

1.5.8.1 [Figure] Hardness variation with temperingtemperature for H-13 steel air cooled from 1875Fand tempered 2 h at temperature (Ref. 1)

1.5.8.2 [Figure] Hardness as a function oftempering temperature for H-13 steel austenitizedat 1850F and 1800F, air cooled, and doubletempered (Ref. 3)

1.5.8.3 [Figure] Hardness as a function oftempering temperature for H-13 steel austenitizedat 1950F and 1800F, air cooled, and tempered for2 h (Ref. 3)

1.5.8.4 [Figure] Tempering curve (double tempered2 h + 2 h) for H-13 steel after air quenching (AC)0.5-in. square samples from 1875, 1950, and 2050F(Ref. 28)

1.5.8.5 [Figure] Master tempering parametercurve for H-13 steel hardened by austenitizing at17501950F and quenching (Ref. 4)

1.5.8.6 [Figure] Time-temperature temperingdiagram for H-13 steel hardened by austenitizingat 1875F (Ref. 32)

1.5.8.7 [Figure] Acceptable microstructures (500x,5% Nital etch) of H-13 steel properly heat treatedby austenitizing, quenching, and tempering asper NADCA acceptance criteria (numbering HS1-HS9 does not denote a quality ranking as all areacceptable) (Ref. 4)

1.5.8.8 [Figure] Typical microstructure from thecenter of a six-inch-thick H-13 steel die hardenedcommercially in a vacuum furnace at 1850F anddouble tempered (2 h at 1050F + 2 h at 1100F)(Ref. 30)

1.5.8.9 [Figure] Dimensional change of H13 steelin a 1 x 2 x 6 in. block versus temperingtemperature (Ref. 3)

1.5.8.10 [Table] Typical dimensional changes inhardening and tempering H-11 and H-13 steels(Ref. 2)

1.5.9 Stress Accelerated TemperingH-13 steel, like other H-series steels in hot worktooling and dies, is often exposed to temperaturesjust below the standard tempering temperaturesof heat treatment and relatively high stressesimposed by either the pressures imposed by hotworking, thermal stresses imposed by sharptemperature gradients during casting or hotworking, or both. Thus, it is likely that thesestresses acting at high temperatures affecttempering mechanisms, hardness changes, andeven die and tool failure mechanisms in H-13steel components. These effects were notedinitially in the work of Benedyk, et al., first inrelation to the mechanism of thermal fatiguefailure and stress accelerated hardness change ofH-13 steel (see 3.7.1) and later in relation to themechanism of creep rupture. (Refs. 22-24)The work done at the Illinois Institute ofTechnology Research Institute (IITRI) on theinvestigation of stress accelerated tempering increep of H-13 steel utilized a variable section testspecimen (Figure 1.5.9.1) tested to failure attemperatures of 10001100F with hardnessdetermined over the range of cross sectionalareas, representing a 10:1 difference in appliedstress. In this work, the H-13 steel was suppliedas annealed 1-in. round bar, reduced by forgingand rolling from a cast 6-in. square billet. Thevariable section test specimens were heat treatedby austenitizing at 1850F, oil quenching,tempering at 1100F for 2 h, air quenching to roomtemperature, and retempering as before. Theoriginal hardness of the specimens after heattreatment was 46 Rockwell C, and the standardtempering curves showing further hardnesschange as a function of time at 1000, 1050, and1100F with no imposed stress are shown inFigure 1.5.9.2. Hardness changes due to stress

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accelerated tempering at stress levels of 25-80 ksiwere measured in the sectioned variable crosssection of specimens as a function of time duringcreep testing at 1000, 1050, and 1100F, and theresults are presented in Figures 1.5.9.31.5.9.5.(Ref. 25)

1.5.9.1 [Figure] Variable section creep specimenused to study stress accelerated tempering of H-13steel as reported in Figures 1.5.9.21.5.9.5 (Ref. 25)

1.5.9.2 [Figure] Tempering curves for H-13 steelconstructed from unstressed section of testspecimen shown in Figure 1.5.9.1 (Ref. 25)

1.5.9.3 [Figure] Hardness change (microhardnesssurveys converted to Rockwell C) in H-13 steelmeasured along variable section creep specimenshown in Figure 1.5.9.1 due to stress acceleratedtempering at 1000F (Ref. 25)

1.5.9.4 [Figure] Hardness change (microhardnesssurveys converted to Rockwell C) in H-13 steelmeasured along variable section creep specimenshown in Figure 1.5.9.1 due to stress acceleratedtempering at 1050F (Ref. 25)

1.5.9.5 [Figure] Hardness change (microhardnesssurveys converted to Rockwell C) in H-13 steelmeasured along variable section creep specimenshown in Figure 1.5.9.1 due to stress acceleratedtempering at 1100F (Ref. 25)

1.5.10 Retained AusteniteRetained austenite in H-13 steel that has beenquenched from austenitizing temperatures canresult in the formation of untempered martensitein operation, which will produce concentratedlocal stresses that promote brittle failure. Due toits low carbon content (~0.40%) and consequentlyhigh MS (martensitic start) temperature, H-13steel has small quantities (~1%) of retainedaustenite after austenitizing and quenching. Withdouble or triple tempering treatments, any minoramount of residual austenite is basically convertedto tempered martensite. (Ref. 33)

1.5.11 NitridingNitriding of H-13 is done after finish machiningand final heat treatment to produce an especiallyhard and wear resistant surface that is resistant tothe softening effect of heat at temperatures up tothe nitriding temperature. Nitriding also inducessurface compressive stresses, thereby improvingfatigue life. Since nitriding is normally carried outat 50F below the normal tempering temperature,the nitriding step can serve as the last temper in adouble or triple tempering heat treatment. Beforenitriding, the surfaces should be clean and free ofdecarburization.Gas nitriding is most common, although liquidbath nitriding, pack, and ion or plasma nitriding

are also done. Nitrided case depth (outer whitebrittle layer and more ductile diffusion layer) isa function of time and temperature, e.g., gasnitriding at 950F for 1012 h results in a casedepth of 0.0040.005 in., while for 4050 h, a casedepth of 0.0120.016 in. is obtainable. Thediffusion layer is 1030 times thicker than thewhite layer, depending on the type of nitridingprocess (Figure 1.5.11.1 and Table 1.5.11.2). Theouter white nitrided layer is brittle and itsthickness must be optimized to achieve best results.The demarcation of the white layer may notalways be clear, as grain boundary networks ofnitride may be present throughout the temperedmartensitic case of H-13 steel (Figure 1.5.11.3).

1.5.11.1 [Figure] Cross-sectional optical micrographsof gas nitrided, ion nitrided, and salt bathnitrided H-13 steel after austenitization at 1890Fand triple tempering (70X magnification) (Ref. 34)

1.5.11.2 [Table] Thickness of surface layers ofnitrided H-13 steel after austenitization at 1890Fand triple tempering depending on nitridingtreatment (Ref. 34)

1.5.11.3 [Figure] Gas nitrided case (24 h at 975F)produced on H-13 steel that was austenitized at1890F, triple tempered at 950F, and surfaceactivated in manganese phosphate (300Xmagnification) (Ref. 35)

1.6 Hardness

1.6.1 Room Temperature HardnessAnnealed hardness characteristics of H-13 steelare similar to those of H-10, H-11, and H-12steels and typically are measured by the Brinellhardness (HB) test, which for these annealedsteels ranges from 192 to 229 HB. Rockwell C(HRC) hardness measurements, typically usedto gauge the quality of austenitization andtempering heat treatments, range from 42 to 56HRC, depending on composition and heattreatment conditions. Vickers hardness (HV),Knoop hardness (KHN or KH), and 15N Rockwelltests are used to evaluate the superficial hardnessof H-13 steel in order to establish a comparison ofsurface and subsurface hardening (nitriding,carburization, nitrocarburizing, or other treatment)or softening (decarburization). Conversions ofHV, Knoop, and 15N Rockwell readings to HRCare readily available from standard tables.The hardness of austenitized H-13 steel sectionsdepends on several factors that includeaustenitizing temperature, section size, and rateof quenching (Figure 1.5.5.6 and Table 1.6.1.1).The measurement of room temperature HRC,determined as a function of austenitizing andtempering temperatures for specific times, gives

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a measure of the resistance to tempering of thehardened steel (Figures 1.5.8.11.5.8.6, Tables1.6.1.21.6.1.4).Hardened and tempered H-13 steel decreases inhardness upon heating in service and exposureto high temperature (at or near temperingtemperatures) for various times. The ability ofH-13 steel hardened under different conditionsto maintain its tempered hardness in service,sometimes called red hardness, can bemeasured by exposing hardened and temperedsamples to high temperatures for differentperiods of time (Figure 1.6.1.5). However, itshould be noted, that this does not always givea good indication of the combined effects ofexposure to stress and temperature in service(see 1.5.7).

1.6.1.1 [Table] Effect of oil and air quenching onroom temperature hardness of small H-13 steeltest pieces (1 in. round x 2 in. long) cut fromannealed bars austenitized for six minutes atindicated temperatures and quenched in oil orstill air (Ref. 31)

1.6.1.2 [Table] Effect of tempering on roomtemperature hardness of small H-13 test pieces(1 in. round x 1 in. long) austenitized at 1800,1850, 1900, and 1950F, quenched in still air, andtempered for two hours at indicated temperatures(Ref. 31)

1.6.1.3 [Table] Effect of tempering on roomtemperature hardness of large H-13 test pieces(4 x 4 x 4 in.) ground on two saw-cut ends,preheated at 1500F for one hour, austenitized at1850F for one hour in a controlled atmosphere,quenched in oil or still air, and tempered forthree hours at indicated temperatures (Ref. 31)

1.6.1.4 [Table] Resistance to tempering for H-13steel austenitized at either 1800 or 1850F anddouble tempered (3 + 3 h) at indicatedtemperatures (Ref. 36)

1.6.1.5 [Figure] Room temperature hardness orred hardness of H-13 steel hardened andtempered as indicated after exposure to elevatedtemperatures for 4-100 h (Ref. 36)

1.6.1.6 [Figure] Tempering curves of H-13 steel airquenched after austenitizing at 1750, 1850, and2050F showing hardness (Rc or HRC) as afunction of time at a tempering temperature of1100F (Ref. 22)

1.6.1.7 [Figure] Tempering curves for H-13 steelair quenched after austenitizing at 1750, 1850,and 2050F, tempering to 45 HRC, and showinghardness (Rc or HRC) as a function of time at atempering temperature of 1000F (Ref. 22)

1.6.2 Hot HardnessHot hardness of H-13 is the actual hardness ofthe steel measured by a penetrator at a giventemperature for controlled amounts of time ofapplication of the indentation load. Samples arenormally held for 30 min. at the testing temperaturebefore testing. The longer the indentor is in contactwith the steel under load, the softer the steelappears due to creep deformation. Thus, hothardness determinations are typically made inthe minimum time to apply the indentor load.Hot hardness gives an indication of strength atthe high temperature at which H-13 steel is used,and again it depends on heat treatmentconditions.

1.6.2.1 [Figure] Typical hot hardness of H-13 steelfor specimens oil quenched from 1850F anddouble tempered 2 + 2 h at indicated temperingtemperature (Ref. 37)

1.6.2.2 [Table] Brinell hot hardness of H-13 steelsamples (1 in. round x 7/8 in. long) hardened byoil quenching from 1850F and tempering for twohours at 1050F or test temperature if higher (Ref. 31)

1.6.2.3 [Table] Rockwell C hot hardness of H-13steel heat treated to indicated room temperaturehardness levels (Ref. 36)

1.6.2.4 [Figure] Effect of temperature on hot Brinellhardness (HB) of hardened (austenitized andquenched, untempered) H-13 steel (Ref. 44)

1.6.3 Nitrided Case HardnessMicrohardness testing, usually Vickers hardness(HV) or Knoop hardness (KHN or HK), or 15NRockwell superficial hardness are used to measurenitrided case hardness. A typical nitrided H-13surface has a hardened surface or case of about0.004 in. thickness and exhibits a case hardness ofabout 1, 0001, 250 HV or 6972 HRC (convertedfrom HV using standard hardness conversiontables).

1.6.3.1 [Table] Case and core hardness on -in.square test pieces of nitrided H-13 steel hardenedat 1850F and tempered at 1050F, ground, andnitrided as indicated (Ref. 31)

1.7 Forms and Conditions AvailableH-13 steel in conventional, premium, andsuperior grades (Table 1.4.1) is most commonlyavailable in wrought form as bar, rod, billet, andforgings in the annealed or pre-hardenedcondition. However, it is also available as wire,sheet, and plate. Round bar stock is supplied insizes of 0.50 to 25 in. diameter; square bar stock issupplied in sizes of 8 to 24 in. or more. Surfacefinish and dimensional tolerances on bar stock

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vary and are specified. Die blocks for die cast andhot work tooling are supplied in annealed or pre-hardened (austenitized and tempered) conditionsas rounds of as much as 36 in. in diameter andflats in a range of sizes up to 20 in. x 40 in. in areaor more with a varying thickness to accommodatedie and tooling sizes and with special orders ofshape and size available. Typically, the cost of thematerial used for die casting and hot work diesrepresents 1015% of the total die cost. (Refs. 519)Cast H-13 steel (designated as CH-13) billets andingots and even shapes are available in theannealed condition. (Ref. 40)H-13 tool steel powders (predominantly sphericalshape) of varying compositions and particle sizedistributions (1250 microns), used for powdermetallurgy, metal injection molding, and claddingapplications, are also available. (Ref. 41)

1.8 Melting and Casting PracticeH-13 steel is most commonly air arc melted(AAM) in an electric furnace in heat lots of 2550tons. The AAM H-13 melt is then degassed byone of several techniques and often followed byvacuum arc remelting (VAR) or electroslagremelting (ESR). Alternatively, vacuum inductionmelting (VIM) followed by VAR or ESR is usedfor heat lots of up 515 tons. After melting andsolidification, the next operation before hot workingby forging or rolling is soaking pit practice orhomogenization, conducted at temperatures at ornear 2000F, in order to level the microsegregationand relieve the thermal residual stresses thataccompany solidification. This is followed bylong holding and slow cooling to hot workingtemperature for ingots that undergo immediatehot working or to room temperature for ingots tobe stored. Hot working by rolling or forging isimportant in refining the cast metallurgicalstructure; however, the larger the ingot size, opendie forging is commonly done because standardhot rolling pressures do not refine the internalportions of large ingots in applications slated forproduction of large H-13 block sizes. (Ref. 42)For production of premium and superior gradesof H-13 steel, ESR or VAR processes are necessary.In fact, some specifications for die materials onlymake exceptions in cases where availability ofESR or VAR material is wanting for small stocksuch as rounds of less than three inches diameter.(Ref. 20)

2.0 Physical and Environmental Properties

2.1 Thermal Properties

2.1.1 Melting RangeThe melting temperature of H-13 steel rangesfrom 2500F to 2700F and depends on composition,mainly carbon content, i.e., the melting pointdecreases with increasing carbon content withinthe specified range.

2.1.2 Phase ChangesCritical temperatures of ferrite to austenite andaustenite to ferrite or ferrite and cementitetransformations on heating and cooling aregiven in 1.5.6. Generally, the amount of retainedaustenite in H-13 after austenitizing and quenchingis minimal (~1% or less) and is transformed upondouble and triple tempering (1.5.10).The two major types of carbides formed in H-13steel upon heat treatment are the V-rich MC typeand the Cr-rich M7C3/M23C6 types. The compositionof these carbides in H-13 steel varies widely, andthe size and distribution of these carbides inannealed microstructures depends greatly onprior metallurgical processing and austenitizing/quenching/tempering conditions. (Refs. 2628)

2.1.3 Thermal Conductivity

2.1.3.1 [Table] Thermal conductivity of H-13 steelas a function of temperature (Ref. 2)

2.1.3.2 [Table] Thermal conductivity of Thyrotherm2344 ESR Magnum premium grade AISI H-13steel at indicated temperatures (Ref. 18)

2.1.4 Thermal Expansion

2.1.4.1 [Table] Average linear thermal expansioncoefficient of H-13 steel over indicated temperaturerange (Ref. 44)

2.1.4.2. [Table] Average thermal expansioncoefficient of Nu-Die V (Crucible Steel) and VDC(Timken Latrobe Steel) H-13 steel from roomtemperature (RT) to indicated temperature(Refs. 12, 16)

2.1.4.3 [Table] Average thermal expansioncoefficient of Thyrotherm 2344 ESR Magnumpremium grade AISI H-13 steel over indicatedtemperature range (Ref. 18)

2.1.4.4 [Table] Average thermal expansioncoefficient of Crucible CPM Nu-Die EZ patentedH-13 particle metallurgy sulfurized steel fromroom temperature (RT) to indicated temperature(Ref. 43)

2.1.5 Specific HeatThe mean specific heat of H-13 steel over thetemperature range 32212F is 0.11 Btu/lb-F.

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Values of specific heat (heat capacity) of H-13steel increase linearly with temperature, from 0.125at 212F to 0.148 Btu/lb-F at 1300F. (Refs. 17, 18, 44)

2.1.6 Thermal DiffusivityThermal diffusivity () of H-14 steel can bederived from physical properties (k = thermalconductivity, = density, and c = specific heat)utilizing the general equation = k/c.

2.2 Other Physical Properties

2.2.1 Density/Specific GravityRoom temperature density of H-13 steel is 0.28lb/in3. Room temperature specific gravity of H-13steel is 7.761.

2.2.1.1 [Table] Density of Thyrotherm 2344 ESRMagnum premium grade AISI H-13 steel over thetemperature range 701200F (Ref. 18)

2.2.2 Electrical PropertiesElectrical Properties are similar to H-11; refer tothe H-11 chapter (Code 1218) for this information.

2.2.3 Magnetic PropertiesThis steel is highly magnetic, but becomesnonmagnetic at temperatures of 14001500F(Curie temperature) and above. Inductionheating is disturbed by heating above the Curietemperature, thereby drastically reducing theinduction heating rate above that temperature.

2.2.4 EmittanceEmittance is similar to H-11; refer to the H-11chapter (Code 1218) for this information.

2.2.5 Damping Capacity

2.3 Chemical Environments

2.3.1 General CorrosionThe general corrosion resistance of H-13 steel islow and surface protection is required. For H-13dies and tooling, temporary protection duringstorage is recommended by first cleaning thesurfaces and then coating with oil or corrosioninhibiting solvent or water-base solutions. Duringhot working service, H-13 dies and tooling areoften coated repeatedly with emulsions andlubricants that protect the surface from generalcorrosion. Nitriding or nitrogen implantationinto the surface of H-13 steel has been shown toincrease its corrosion resistance in salt water(Figure 2.3.1.1). As shown in 2.3.1.1, H-13 specimenstreated by nitrogen PIII (plasma immersion ionimplantation) exhibited passive regions underpolarization conditions in salt water, with acorrosion potential as low as -0.42 V for a PIIIH-13 nitrided specimen against -0.52 V for theuntreated specimen. Also, the passive regioncurrent density of PIII nitrided H-13 was as

much as two orders of magnitude lower than foruntreated H-13, indicating a higher corrosionresistance for the PIII treated H-13.

2.3.1.1 [Figure] Potentiodynamic polarizationcurves of AISI H-13 steel measured in a 3.5%NaCl solution at pH = 6 on an () untreatedsample, (----) nitrogen plasma immersion ionimplantation (PIII) processed sample (T=300C,t=12 h), and () nitrogen PIII processed sample(T=450C, t=9 h) (Ref. 45)

2.3.2 Stress CorrosionH-13 steel, although it has good stress corrosionresistance in comparison to other high strengthsteels due to its high chromium and molybdenumcontent, will still fail prematurely when exposedunder stress to corrosive media. Use of premiumand superior grades of H-13 steel and appropriateheat treatment (higher tempering temperature tolower yield strength) improve stress corrosionresistance.

2.3.3 Oxidation ResistanceThe chromium content of H-13 steel improves itsoxidation resistance over steels with lower alloycontent. In service, at temperatures at or belowthe tempering temperature, H-13 steel oxidizesin still air at low rates. However, the oxidationrate of uncoated H-13 steel climbs as a functionof temperature above 1000F and is severe ataustenitizing temperatures, necessitating avacuum or endothermic atmosphere to preventexcessive oxidation and the decarburization andloss in surface hardness that accompaniesoxidation of carbon steels. Nitrided H-13 steelsurfaces also oxidize when heated for longperiods in air furnaces and ovens, commonlydone on dies and tools to facilitate production,thereby losing their hardness and protectivesurface properties.Oxidation has been implicated as one of themechanisms of crack propagation in thermalfatigue, with the lower density oxide acting towedge open the crack; EDS (electron dispersionspectral) analysis of the oxide within the thermalfatigue cracks of an H-13 steel specimen hasindicated that it is predominantly iron oxide witha chromium oxide layer at the metal-oxideinterface. (Ref. 22)Not all oxidation of H-13 steel components is bad,since properly prepared oxide films on H-13 steeldies and tooling are beneficial with respect topreventing soldering of molten metal to the diesand tooling during casting and to preventsticking of solid workpiece metal during hotmetalworking. Dense oxide films on H-13 diesand tooling, prepared by careful heating in airfurnaces (often with proprietary atmospheres) at

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temperatures below the lowest temperingtemperature, are beneficial in preventingsoldering and sticking of metal during casting ormetalworking. Oxide films prepared for suchpurposes have blue, blue-black, or blacksurface coloration and not only prevent solderingand sticking but also act to reduce the surfaceheat transfer coefficient. (Ref. 22)

2.3.3.1 [Figure] Comparative oxidation resistanceof H-13 and maraging steels at 1000F (Ref. 46)

2.3.3.2 [Figure] Cross sectional view of oxidebuildup in thermal fatigue crack in hardenedH-13 steel specimen: (a) 50x, nital etch and (b)500x, nital etch (Ref. 22)

2.3.4 Hydrogen ResistanceA few parts per million of hydrogen dissolvedin steel can cause hairline cracking and loss oftensile ductility. In casting dies for aluminum inparticular, soldering and washout of H-13 steeloften results in die failure due to the high velocityof impinging molten aluminum. In the hardenedcondition, H-13 steel is most susceptible tohydrogen embrittlement. Resistance to hydrogenembrittlement decreases as strength increases,particularly when the strength level exceeds150180 ksi, and the effects of hydrogenembrittlement increase as strain rate decreases.Prevention of failure due to hydrogen damage,especially in structural applications in mildatmospheric environments, involves protectionwith a corrosion-resistant coating and/orimparting compressive residual stresses on thesurface by shot peening or other means. If H-13steel parts are welded, the welding rods shouldbe kept dry in storage and during welding, asadsorbed water is a major source of hydrogen.(Ref. 55)

2.3.5 Corrosion/Erosion by Molten MetalCorrosion/erosion (washout) by molten aluminum,zinc, or other metals that react with H-13 steel isaccelerated by turbulence and increase in melttemperature. In aluminum die casting dies inparticular, soldering and washout of H-13 steeloften result in die failure due to the high velocityof impinging molten aluminum. Immersion testsconducted in molten aluminum have shownthat the dissolution of H-13 steel decreasessignificantly with nitrided coating thicknessFigure 2.3.5.1). (Ref. 47)In an accelerated die washout test conducted onH-13 steel test pins (Figure 2.3.5.2 and 2.3.5.3)impinged by molten A356 aluminum alloy at1350F, various types of PVD (physical vapordeposition) coatings on H-13 steel pins havesignificantly increased washout resistance

(Figure 2.3.5.3). Among the various PVD coatingstested, a CrC PVD coating was especiallyeffective. (Ref. 48)

2.3.5.1 [Figure] Variations in corroded/erodeddepth of immersion test specimens of hardenedand tempered H-13 steel as heat treated andmachined and separately nitrided by gas or ionnitriding maintained in a molten Al-Si-Cu (KSALDC) aluminum alloy at 1300F for 43 h (Ref. 47)

2.3.5.2 [Figure] Schematic diagram of acceleratedwashout testing arrangement at Case WesternReserve University (CWRU) with molten aluminumalloy injected into die cavity at ~70 in/s (Ref. 48)

2.3.5.3 [Figure] Test pin design and positionwithin die cavity of CWRU accelerated washouttest arrangement shown in Figure 2.3.5.2 (Ref. 48)

2.3.5.4 [Figure] Microstructural cross sections(500x) PVD coated pin specimens tested in CWRUaccelerated washout arrangement shown inFigure 2.3.5.2 (Ref. 48)

2.3.5.5 [Figure] Effect of various types of PVDcoatings shown in Figure 2.3.5.4 on washoutresistance of H-13 steel die casting core pins inCWRU accelerated washout test shown inFigures 2.3.5.2 and 2.3.5.3 (Ref. 48)

2.3.6 Liquid Metal Induced Embrittlement ResistanceLiquid metal induced embrittlement (LMIE) is thecatastrophic brittle failure of a normally ductilemetal when coated with a thin film of a liquidmetal while stressed in tension. In most cases, thestress needed to initiate and propagate cracksoccurs instantly, with fracture propagating at440 in/sec through the entire test specimen. Aswith other types of high strength steels, H-13 steelin the heat treated condition can be embrittled byvarious liquid metals, notably lead, antimony,cadmium, zinc, and others. (Ref. 49)U-bend liquid metal embrittlement tests (Figure2.3.6.1) of heat treated H-13 steel recently conductedin pure zinc melts at the TPTC (ThermalProcessing Technology Center) of Illinois Instituteof Technology (IIT) indicate a low resistance toLMIE in this medium. Results are shown inFigure 2.3.6.2 for embrittlement tests conductedin molten zinc baths maintained at either 970 or1050F for hardened H-13 steel plates in thelongitudinal and transverse rolling direction afteraustenitizing and air quenching at 1750, 1850, or1950F, with some specimens triple tempered (2 +2 +2 h at 1100F #9 and #11), and some specimenssingle tempered for the same period (6 h at 1100F);as a control, a single tempered specimen wastested in a neutral molten salt bath (#12) andexhibited no failure. (Ref. 50)

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2.3.6.1 [Figure] Schematic drawing of TPTC U-bend liquid zinc embrittlement test procedureconducted on H-13 steel plate specimens: originalannealed specimen bent into U-shape, heattreated, bent to an incipient plastic flow conditionat a displacement of d (left), displacement heldby fastener, and immersed in a molten zinc bath(right) (Ref. 50)

2.3.6.2 [Figure] Liquid zinc embrittlement TPTCU-bend test data (Y-axis: time to fracture in hours;X-axis: test conditions as shown in Figure 2.3.6.1)conducted on H-13 steel plate specimens (Ref. 50)

2.3.7 Solid Metal Induced Embrittlement ResistanceEmbrittlement can occur in heat treated H-13steel below the melting temperature of the solidin certain liquid-metal embrittlement couples;this type of failure is known as solid metalinduced embrittlement (SMIE). The severity ofSMIE generally increases with temperature, witha sharp and significant increase in severity at themelting point of the embrittling metal. Above themelting point, the embrittlement has all thecharacteristics of liquid metal embrittlement.Many high strength steels such as heat treatedH-13 steel have experienced failures due to SMIE.(Ref. 51)SMIE has been identified as a failure mechanismin extrusion dies made of hot work steels such asSKD61 (Japanese steel similar to H-12 steel) andH-13 steel for the extrusion of Al-4.75Zn-1.25Mgalloy hollows, thus implicating the zinc in thealloy as the embrittling agent. (Refs. 52-53)In analyzing SMIE and LMIE transport mechanisms,Gordon determined that bulk liquid metal flowpenetrating the tips of sharp cracks is the likelytransport mechanism in LMIE, but surface selfdiffusion is the likely transport mechanism inSMIE. (Ref. 54) As the melting point of zinc is787F and it has a high vapor pressure and shortvapor transport time at its melting point, the hotmetalworking of an aluminum alloy with highzinc content may under certain conditionscontribute to brittle failure. (Ref. 54)

2.4 Nuclear EnvironmentNeutron irradiation of H-13 steel at low andmoderate strength levels leads to an increase inthe ductile to brittle transition temperature(DBTT) and a decrease in energy above theDBTT as determined by Charpy V-notch impacttests. The degree of damage caused by a nuclearenvironment depends on neutron dose, neutronspectrum, irradiation temperature, and steelcomposition and properties. Vacuum degassingand control of phosphorus content helps toreduce susceptibility to neutron embrittlement.(Ref. 56)

3.0 Mechanical Properties

3.1 Specified Mechanical Properties

3.1.1 NADCA H-13 207-97 H-13 Steel AcceptanceCriteria (Ref. 4)Acceptance criteria fo H-13 steel used in castingdies for aluminum have been established byNADCA and involve impact capability testingof all mill product forms with a thickness greaterthan 2.5 in. The specimen blanks should be removedfrom the short transverse orientation (base ofCharpy V notch parallel to the longitudinaldirection of the parent block, slab, or bar) in thecenter of the parent block of steel as per ASTMA370. The blanks should be heat treated byaustenitizing at 1885F for 30 min, oil quenching,double tempering at 1000F for 2 h minimum eachtemper (air cooled between each temper) toachieve a final hardness of 4446 HRC, andmachining to final Charpy V impact specimensize per ASTM A370. Acceptance criteria, basedon five specimens tested at room temperaturewith values of highest and lowest specimensdiscarded are as follows for the remaining threeresults:Premium Quality H-13 8 ft-lb average with 6 ft-lbsingle minimum valueSuperior Quality H-13 10 ft-lb with 8 ft-lb singleminimum value

3.1.2 Other H-13 Steel Acceptance Criteria (Refs. 521,40, 41)H-13 steel in its various wrought product formsis typically purchased on the basis of H-13 steelproducer or user specifications that not onlyinclude mechanical properties but also forgingand heat treatment procedures.

3.2 Mechanical Properties at RoomTemperature

3.2.1 Tension Stress-Strain Diagrams and TensileProperties

3.2.1.1 [Table] Typical longitudinal roomtemperature mechanical properties of H-13steel bar oil quenched from an 1850Faustenitizing temperature and tempered atdifferent temperatures (Ref. 1)

3.2.1.2 [Table] Transverse mechanical propertiesat room temperature of air melted and electroslagremelted (ESR) H-13 steel large section bars oilquenched from 1850F and double tempered (2 +2 h) at 1090F to a final hardness of 48 HRC (Ref. 1)

3.2.1.3 [Figure] Room temperature tensileproperties of H-13 steel in relation to hardnessand Charpy V-notch impact energy (Ref. 44)

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3.2.1.4 [Table] Room temperature tensileproperties of annealed and heat treated H-13steel (Ref. 16)

3.2.2 Compression Stress-Strain Diagrams andCompression PropertiesCompression yield strength of H-13 steel isequivalent to tensile yield strength at room andhigh temperatures. Tensile yield strength dataare commonly used to estimate compressive yieldstrength of H-13 steel components in service.

3.2.3 ImpactTo achieve maximum impact toughness in service,H-13 steel specimens should be quenched fromaustenitizing temperatures at a minimum rate of50F/min and preferably at 25F/min to below1000F (see 1.5.5). The preferred method fordetermining impact toughness of H-13 steel isCharpy V-notch testing, and an average CharpyV-notch toughness in the short transverse directionof 8 ft-lb or more at room temperature is consideredthe standard for high quality H-13 steel (see3.1.1). However, it is difficult to characterize theimpact resistance of H-13 steel from roomtemperature tests alone, especially consideringthat this steel is most often used at temperaturesup to 1000F and higher (see 3.3.3 and 3.3.9).

3.2.3.1 [Table] Longitudinal Charpy V-notchimpact properties at room temperature of H-13bar air cooled from an 1850F austenitizingtemperature and tempered at differenttemperatures (Ref. 1)

3.2.3.2 [Table] Transverse Charpy V-notch impacttoughness at room temperature of various gradesof H-13 steel at 4546 HRC (Ref. 43)

3.2.3.3 [Table] Izod impact properties at roomtemperature of H-13 steel specimens machinedfrom -in. square bar stock to 0.394-in. square,preheated to 1500F, austenitized at 1850F, oilquenched, and tempered at indicated temperaturesfor 2 h (Ref. 31)

3.2.4 Bending

3.2.5 Torsion and Shear

3.2.6 Bearing

3.2.7 Stress Concentration

3.2.7.1 Notch Properties

Sharp notches can significantly reduce theability of H-13 steel to withstand impactand shock loads.

3.2.7.1.1 [Figure] Effect of notch radius onthe impact strength of Izod type specimensof H-13 steel heat treated to 48 Rockwell Cand tested at room temperature (Ref. 61)

3.2.7.2 Fracture Toughness

Plane strain fracture toughness (KIc) testresults for heat treated (austenitized, aircooled, and tempered) H13 steel at roomtemperature are given in Table 3.2.7.2.1and Figure 3.2.7.2.2. (Refs. 58, 28) Asmentioned in 1.5.5, decreased quenchingrates of H13 steel sections fromaustenitizing temperature can result inthe formation of upper bainite along withpreferential precipitation of carbidesalong prior austenitic grain boundaries,which decreases toughness in the heattreated component (Figure 3.2.7.2.3 and3.2.7.2.4). (Ref. 76)

Correlations have been made between KIc,Charpy-V notch (CVN), and HRC testdata for heat treated H13 and H11 steels.(Ref. 77) For these steels, having roomtemperature yield strength (ys) in therange 150230 ksi and hardness in therange 4056 HRC, the correlation between(KIc/ys)

2 and CVN/ys is given in Figure3.2.7.2.5 Linear regression analysis of KIc,HRC, and CVN at room temperature forH13 and H11 steels shows the followingrelationship:

KIc (estimated) = 4.53(CVN)1.11(HRC)-0.135

for which experimental and calculatedvalues of KIc are plotted in Figure 3.2.7.2.6.(Ref. 77)

3.2.7.2.1 [Table] Longitudinal plane strainfracture toughness of H-13 steel air cooledfrom 1920F and tempered two hours attemperature (Ref. 58)

3.2.7.2.2 [Figure] Room temperature planestrain fracture toughness (KIc) of smalland large size specimens of H-13 steelversus austenitizing temperature (a) andtempered hardness (b) after all specimenswere double tempered at 1100F (2 + 2 h)for austenitizing soak times and coolingconditions given (25 min soak and airquench for small specimens and 50 and 60min soak and simulated laboratory quenchfor 6- and 12-in. rounds) (Ref. 28)

3.2.7.2.3 [Figure] Variation of roomtemperature plane strain fracturetoughness KIc of heat treated H13 steel,austenitized at 1870F for 30 min, quenchedat various rates, and tempered to 44 HRC,as a function of quench rate (Ref. 76)

3.2.7.2.4 [Figure] Charpy V-notch (CVN)impact values of H13 steel quenched at

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various rates after austenitizing at 1870Ffor 30 min and tempered to 44 HRC (Ref. 76)

3.2.7.2.5 [Figure] Correlation between(KIc/ys)

2 and CVN/ys at roomtemperature for H13/H11 steels (Ref. 77)

3.2.7.2.6 [Figure] Experimental values ofKIc at room temperature compared withvalues of KIc calculated from CVN andHRC data at room temperature (see linearregression relationship in 3.2.7.2) for H13and H11 steels (Ref. 77)

3.3 Mechanical Properties at VariousTemperatures

3.3.1 Tensile Properties

3.3.1.1 [Figure] Effect of elevated temperature ontensile strength of H-13 steel heat treated to roomtemperature Rockwell C (HRC) hardness valuesgiven (Ref. 44)

3.3.1.2 [Figure] Effect of elevated temperature(C x 1.8 + 32 = F) on tensile and 0.2 yield (creep limit)strength (N/mm2 x 0.145 = ksi) and reduction inarea of H-13 steel heat treated to roomtemperature strength level given (Ref. 17)

3.3.1.3 [Figure] Flow stress (determined frommodified Johnson-Cook model of flow stressand tuned by OXCUT computer program andexperimental data from lathe machiningexperiments) of H-13 steel at temperatures of8001200C (1472-2192F) and strain rates of 6 x 103

9 x 105 1/s for H-13 steel originally at 46Rockwell C (HRC) hardness (MPa x 0.145 = ksi)(Ref. 57)

3.3.1.4 [Table] Elevated temperature tensileproperties of heat treated H-13 steel (Ref. 16)

3.3.2 Compression Stress-Strain Diagrams andCompression PropertiesCompression yield strength of H-13 steel isapproximately equivalent to tensile yield strengthat room and high temperatures. Tensile yieldstrength data are commonly used to estimatecompressive properties of H-13 steel componentsin service.

3.3.3 Impact

3.3.3.1 [Table] Charpy V-notch (CVN) propertiesof Nu-Die V (AISI H13) steel air cooled to roomtemperature from 1825/1875F, and doubletempered (two hours minimum) to original roomtemperature hardness (Rockwell C or HRC)indicated and tested at elevated temperatures(Ref. 58)

3.3.3.2 [Figure] Longitudinal Charpy V-notchimpact resistance versus austenitizing temperaturefor H-13 steel specimens austenitized for

treatment times given, air quenched (small sizespecimens) or cooled to simulated quenching of 6-and 12-in. rounds, double tempered at 1000F (2 +2 h), and subsequently tested at roomtemperature and 800F (Ref. 28)

3.3.3.3 [Figure] Longitudinal Charpy V-notchimpact resistance versus austenitizing temperaturefor H-13 steel specimens austenitized for treatmenttimes given, air quenched (small size specimens)or cooled to simulated quenching of 6- and 12-in.rounds, double tempered at 1100F (2 + 2 h), andsubsequently tested at room temperature and800F (Ref. 28)

3.3.3.4 [Figure] Longitudinal Charpy V-notchimpact resistance versus austenitizing temperaturefor H-13 steel specimens austenitized for treatmenttimes given, air quenched (small size specimens)or cooled to simulated quenching of 6- and 12-in.rounds, double tempered at 1150F (2 + 2 h), andsubsequently tested at room temperature and800F (Ref. 28)

3.3.3.5 [Figure] Longitudinal Charpy V-notchimpact resistance versus tempered hardness forH-13 steel specimens austenitized at 1875, 1950,and 2025F for soak times given, air quenched(small size specimens) or cooled to simulatedquenching of 6- and 12-in. rounds, tempered, andsubsequently tested at 800F (Ref. 28)

3.3.4 Bending

3.3.5 Torsion and Shear

3.3.6 Bearing

3.3.7 Stress Concentration

3.3.7.1 Notch Properties

3.3.7.2 Fracture Toughness

Plane strain fracture toughness (KIc)measurements were made at temperaturesup to 800F on four types of high strengthsteels, including H13 steel, for the purposeof developing a drill bit used in triconeroller drill bits capable of sustainedoperation at the temperatures encounteredin drilling geothermal wells. (Ref. 78)Short rod (chevron-notch) fracturetoughness (KIv) tests (Figure 3.3.7.2.1)were conducted on H13 steel specimenscut from blanks of 0.5-in. diameter by0.75-in. length, originally cut from a 4.13-in. diameter x 3.54-in. forging using stockclose to the forging surface. (Ref. 79) Thespecimen axes were parallel to the forgingaxis. The H13 specimens were heat treatedby austenitizing at 1825F, air quenching,and double tempering at 1000F (2+2 h),resulting in an average room temperature

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yield strength of 238 ksi and elongationof 9%. After heat treatment, the chevron-notch slots were ground as shown inFigure 3.3.7.2.1.

It should be noted that, for high strengthsteels, the minimum specimen diameter ofa short rod fracture toughness specimento give size independent test results thatmeet the requirements of a valid planestrain fracture toughness (KIc) measurement,i.e., KIv ~ KIc, was estimated to be

3.5 Fatigue Properties

3.5.1 High-Cycle FatigueThe better the microstructural homogeneity of H-13 steel, the higher the fatigue strength. This isshown in the comparison of fatigue strength ofelectric arc air melted (AAM), electroslag remelted(ESR), and ESR-Isodisc (a patented H-13microstructural control process of BhlerEdelstahl GmbH) in Figure 3.5.1.1. (Ref. 38) Axialor tension-tension fatigue tests conducted on heattreated longitudinal H-13 steel specimens alsohave shown the benefits of ESR H-13 over electricarc air melted H-13 steel (Figure 3.5.1.2). However,in fully reversed or tension-compression fatiguetests, while no significant differences were notedbetween longitudinal specimens of ESR and airmelted H-13 steel, a significantly better life wasnoted in the transverse ESR H-13 steel specimens(Figure 3.5.1.3). (Ref. 62)A general formula for estimating fatigueproperties of most metallic materials includingH-13 steel is based on the universal slopesmethod developed by Manson and coworkers atNASA Lewis:

t = (3.5u/E)Nf-0.12 + D0.6Nf

-0.6

where t is the total strain range and the mechanicalproperties u, E, and D are the ultimate tensilestrength, modulus of elasticity, and true ductility(determined from the % reduction in area (RA) byln(100/(100-RA), and Nf is fatigue life in numberof cycles. The universal slopes equation is relativelyaccurate for estimating room temperature fatiguebehavior in small laboratory specimens. Itbasically indicates that strength predominatesover ductility for good high-cycle fatigue life atlow values of total strain range. When applied toestimating high-cycle fatigue life at hightemperatures, values for u, E, and D are selectedat the high temperature of interest at moderatestrain rates used in tensile testing. (Ref. 63)

3.5.1.1 [Figure] Fatigue strength (N/mm2 x 0.145 =ksi) of smooth and notched specimens tested atroom temperature in fully reversed tension-compression (R = -1) of hardened H-13 steel inrelation to microstructural homogeneity achievedby electric arc air melted, ESR, and specialtreatment (patented Isodisc process) (Ref. 38)

3.5.1.2 [Figure] Tension-tension (R = +0.2) fatiguecurves determined at 60 Hz for longitudinalspecimens of air melted () and ESR () heatsof H-13 steel in the hardened condition(austenitized at 1850F, oil quenched, and doubletempered 2 + 2 h at 1090F to 48 HRC hardness)(Ref. 62)

3.5.1.3 [Figure] Fully reversed tension-compression(R = -1) fatigue curves determined at 60Hz forlongitudinal ( ESR, air melted) andtransverse ( ESR, air melted) H-13 steel inthe hardened condition (austenitized at 1850F, oilquenched, and double tempered 2 + 2 h at 1090Fto 48 HRC hardness) (Ref. 62)

3.5.2 Low-Cycle FatigueLow-cycle fatigue is an important failure mode,especially in dies and tooling subjected tocomplex stress states over a range of temperatures.The universal slopes equation developed byManson (3.5.1) for estimating low-cycle fatiguelife indicates that true ductility is more importantthan ultimate strength in low-cycle fatigue athigh levels of total strain range, although for low-cycle fatigue it has been found to only work wellfor estimating room temperature low-cyclefatigue behavior in small laboratory specimens.At relatively high temperatures and levels of totalstrain, it overestimates Nf significantly due tocreep effects that induce time dependent failures.In cases of combined fatigue and creep-rupturedamage, when taking values for u, E, and D atthe high temperature of interest at moderatestrain rates used in tensile testing, a cycle-reductionfactor called the 10% rule (fatigue life Nf istaken as 10% less of that predicted by the universalslopes equation) offers a good approximation oflow-cycle fatigue life for H-13 steel. Manson andHalford have developed a more accurate methodof estimating low-cycle fatigue life at hightemperatures and low frequencies of loading forcases when the cycle-reduction factor is less than10% and Nf < 10

5 that takes greater account of thecreep rupture damage that may occur in suchcases and have applied it successfully to hot worktool steels such as H-13 and other hightemperature alloys. (Ref. 64)

3.6 Elastic properties

3.6.1 Poissons RatioThe room temperature Poissons ratio of H-13steel is 0.28, but it increases with temperature in anonlinear fashion, approaching 0.30 at 1000F and0.33 at 1400F.

3.6.2 Modulus of Elasticity

3.6.2.1 [Table] Modulus of elasticity of Thyrotherm2344 ESR Magnum premium grade H-13 steel asa function of temperature (Ref. 18)

3.6.2.2 [Table] Modulus of elasticity of VDC H-13steel as a function of temperature (Ref. 16)

3.6.3 Modulus of Rigidity

3.6.4 Tangent Modulus

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3.6.5 Secant Modulus

3.7 Thermal Fatigue PropertiesThermal fatigue differs from conventional fatigueat high temperatures in that the stresses thatproduce thermal fatigue failure are induced byrepeated temperature gradients at the surface(thermal stresses). Thermal fatigue failurestypically initiate at the surface in a multiple crackpattern that gradually deepens, a phenomenoncalled heat checking in dies and tooling, whicheventually causes failure. Thermal fatigue is oneof the main causes of heat checking in H-13 steeldies and tools used in die casting and hotworking, with cracks deepening every cycle tothe point where the dies and tools are useless andmust be replaced. Heat checking due to thermalfatigue is the main limiting factor, for example, indie life in aluminum die casting. Various thermalfatigue tests have been developed to assessthermal fatigue or heat checking resistance ofH-13 steel using thermal cycles that attempt tosimulate those that occur in service; however, theresults of these tests do not necessarily correlatewell with each other. Only relative comparisonsof H-13 variables (H-13 steel grade, heat treatment,coatings, etc.) in a particular thermal fatigue orheat checking test seem to be relevant.

3.7.1 CWRU Thermal Fatigue TestThe thermal fatigue test developed at Case WesternReserve University (CWRU) in the 1960s hasbecome the standard of comparison of H-13 steeland other die materials for the aluminum diecasting industry (Figure 3.7.1.1). The CWRUsquare specimen, water cooled from the inside,results in thermal fatigue cracking along each ofthe four corners, and all the cracks are measuredperiodically as a function of number of cycles,then averaged. This test was developed to simulatespecifically the thermal fatigue behavior thatcauses heat checking in aluminum die castingdies. In the initial work done in testing H-13steel in this test, the discovery was made of theunexpected advantages of high temperatureaustenitizing of H-13 steel in significantly raisingits thermal fatigue and heat checking resistance(see 1.5.5 and Figure 3.7.1.2) and the role of stressaccelerated softening as an important mechanismin thermal fatigue failure (see 1.5.9). (Ref. 4, 22-25)The CWRU test has been very useful in isolatingquality of H-13 steel in terms of thermal fatigueand heat checking resistance. For example, thetest has clearly distinguished various qualities ofH-13 steel (Table 3.7.1.5) against the standardheat of base H-13 steel, obtained from an argondegassed melt that was forged into a 5 x 20 in.block from a 32-in. square ingot, and established

an order of merit (Figure 3.7.1.3); the poor thermalfatigue resistance of a particular heat of vacuumremelted H-13 (vac. remelt #1) was found to bedue to an inadequate forging reduction of theingot. Thermal fatigue testing at CWRU of H-13steel with various coatings and of competitivehigh temperature materials continues today.(Ref. 48)

3.7.1.1 [Figure] Schematic drawing of CWRUthermal fatigue test used in studying H-13thermal fatigue and heat checking resistanceshowing apparatus used to dip specimen intomolten A380 aluminum at 1300F (upper) andwater cooled H-13 steel test specimen (lower)(Refs. 2224, 48)

3.7.1.2 [Figure] Thermal cycle used indetermining thermal fatigue resistance of H-13steel in the CWRU test for different austenitizingtemperatures (Figure 3.7.1.3) and various typesand grades of H-13 steel (Figure 3.7.1.4) (Refs. 2224)

3.7.1.3 [Figure] Thermal fatigue behavior in theCWRU test (Figure 3.7.1.1) of H-13 steelaustenitized at 1750, 1850, 1950, and 2050F, airquenched, and all tempered to 45 Rc (HRC)showing average maximum crack length inmicrons () (upper) and summation of total crackarea (lower) as a function of number of cycles(Refs. 2224)

3.7.1.4 [Figure] Thermal fatigue behavior in theCWRU test (Figure 3.7.1.1) of various types andgrades of H-13 steel austenitized at 1850F, airquenched, and all tempered at 1100F to 45 Rc(HRC) showing average maximum crack lengthin microns () (upper) and summation of totalcrack area (lower) as a function of number ofcycles (Refs. 2224)

3.7.1.5 [Table] Chemical composition and inclusionratings of H-13 steel materials tested for thermalfatigue resistance in the CWRU test (Figure 3.7.1.1)(Refs. 2224)

3.7.2 IITRI Thermal Fatigue TestThe thermal fatigue test developed at IITRI (theformer Illinois Institute of Technology ResearchInstitute) utilized a stack of disc shaped specimenswith a peripheral fin that were subjected toalternating heating and cooling cycles in fluidizedbeds (Figure 3.7.2.1). The material tested wasmachined from 2.5-in. diameter bar stock forgedfrom induction air melted and cast 6-in. ingots ofH-13 steel. The IITRI results confirmed some ofthe observations made in the CWRU testregarding effects of composition, inclusion level,and hardness of hardened H-13 steel on thermalfatigue resistance. Also, as in the CWRU thermalfatigue tests, stress accelerated tempering

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occurred in the IITRI thermal fatigue testsconducted on hardened H-13 steel. (Refs. 25, 66)Stress accelerated tempering was also noted inthe thermal fatigue tests conducted at IITRI. Theeffect of the length of the thermal cycle between1100 and 400F was also determined. Besides theshort cycle shown in Figure 3.7.2.1, a longthermal cycle was used (Figure 3.7.2.4), and theresults were compared (Figure 3.7.2.5). Thehardness noted in the highly stressed fin sectionwas always less than that to be expected fromstandard temperature-time tempering data inunstressed pieces of H-13 steel hardened andtempered to the same initial condition. (Ref. 25)

3.7.2.1 [Figure] Heating and cooling curves forH-13 steel during a short cycle of thermal cyclingbetween 1100 and 400F in the IITRI thermalfatigue test (Refs. 25, 66)

3.7.2.2 [Table] Influence of initial temperedhardness of H-13 steel on thermal fatigueresistance measure in the IITRI thermal fatiguetest according to the cycle shown in Figure 3.7.2.1(Ref. 66)

3.7.2.3 [Figure] Number of cycles to crackinitiation of H-13 steel in IITRI thermal fatiguetest according to the cycle shown in Figure 3.7.2.1as a function of cleanliness of the steel as rated bythe number of +4 mm oxide inclusions at 320xmagnification in 40 random fields (Ref. 66)

3.7.2.4 [Figure] Heating and cooling curves forH-13 steel during a long cycle of thermal cyclingbetween 1100 and 400F in the IITRI thermalfatigue test (Ref. 25)

3.7.2.5 [Figure] Tempering of IITRI thermal fatiguefins (HRC hardness noted on fins) during shortand long thermal cycling for 25016,000 cycles(Ref. 25)

3.7.3. Thermal Fatigue Tests of H-13 Steel SuppliersVarious thermal fatigue tests have been devisedby H-13 steel suppliers and even a few users inorder to be able to distinguish quality of the steelin relation to thermal fatigue resistance and tooptimize metallurgical processing accordingly.These tests differ significantly in method of heatingand cooling; however, the comparative data areuseful in assessing effects due to microstructureand processing, especially if care is taken toduplicate the thermal fatigue conditions in service.For example, the comparative thermal fatiguedata in Figure 3.7.3.1, generated by Crucible SteelCo. in their thermal fatigue test show thefollowing order of merit for similarly hardenedH-13 steel: (1-best) CPM Nu-Die EZ powdermetallurgy produced resulfurized H-13 steel, (2)premium H-13 steel/Nu-Die XL, and (3-worst)

conventional resulfurized H-13 steel. This testalso indicates the deleterious effect of the EDMd(electrodischarged machined) surface on thermalfatigue of H-13 steel. (Ref. 12, 43).EWK-GmbH and ThyssenKrupp Specialty SteelsNA have developed a thermal fatigue test basedon high frequency induction heating and waterquenching (Figure 3.7.3.3) to evaluate heat checkingof H-13 steel and modified versions of H-13 steel.A comparison was made of the thermal fatigueresistance of a superior grade of H-13 steel ofstandard composition (Thyrotherm 2344 EFS Supra)with a modified version of H-13 steel (Thyrotherm2367 EFS Supra) having a variation in the contentof Si (lower), Mo (higher), and V (lower) than thestandard AISI H-13 composition. The modifiedH-13 composition (see Table 1.4.1) had asignificantly better thermal fatigue resistancerelative to the standard superior grade H-13composition (Figures 3.7.3.4 and 3.7.3.5) uponevaluation of crack patterns over 2, 0008, 000heating and cooling cycles. (Ref. 68)

3.7.3.1 [Figure] Thermal fatigue data from CrucibleSteel Co. test (alternate immersion of squarespecimens in molten A380 aluminum at 1250Fand quenching in a water bath at 200F withexamination for cracks every 10, 000 cycles)(Ref. 43)

3.7.3.2 [Table] Thermal fatigue/heat checkingcomparison of AISI H-13 and premium H-13 steelspecimens (1.75 x 2.38 in.) heat treated to 47 HRCtested at 1,500 cycles of alternating heating at1300F and quenching into hot water (Ref. 67)

3.7.3.3 [Figure] Thermal fatigue testing apparatusof EWK/Thyssen Krupp Specialty Steel: Aratmosphere to avoid corrosion, induction heatingto 1200F, water bath maintained at or near roomtemperature and at a pH = 10.5 to improvewetting, 2 x 2 x 0.40 in. H-13 steel samples cutfrom 30 x 8 in. forged slabs in transverse directionand rough machined, samples were hardenedand tempered to 4446 HRC and ground to a finefinish (Ref. 68)

3.7.3.4 [Figure] Total crack length measured onsamples of a superior grade of AISI H-13(Thyrotherm 2344 EFS Supra) and a modifiedgrade of H-13 (Thyrotherm 2367 EFS Supra) heattreated to 4446 HRC and tested in the EWK/Thyssen Krupp Specialty Steel thermal fatigueapparatus shown in Figure 3.7.3.3 (Ref. 68)

3.7.3.5 [Figure] Microscopic evaluation at 200x ofsome thermal fatigue cracks developed in samples(Figure 3.7.3.4) from the EWK/Thyssen KruppSpecialty Steel thermal fatigue test (Figure 3.7.3.3)(Ref. 68)

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4.0 Fabrication

4.1 FormingIn the fully annealed condition, H-13 steel can beformed to a limited extent by most commonmethods.

4.1.1 Hot Working

ForgingH-13 steel is normally forged in open or closeddies at 19002200F and should not be forgedbelow 15001600F. A typical forging practiceconsists of heating slowly to about 1500F andthen more rapidly to the forging range of 19002200F. After forging, the H-13 workpiece shouldbe cooled slowly and annealed as soon aspossible. (Refs. 17, 31)Conventional grades of H-13 do not have aspecification on prior hot forging reductions orpractice, although inadequate hot forgingreduction and practice may compromise somephysical and mechanical properties, e.g., thermalfatigue (see 3.7.1) and elevated temperatureimpact resistance. Premium and superior gradesof H-13 steel typically have been forged fromthree directions (triaxial forging) in order tominimize longitudinal, transverse, and thicknessproperty differences. Forging reductions thatassure a minimum ratio of 5:1 in each directionare necessary to obtain optimum physical andmechanical properties. Should there be a need toreforge the as-received material, the originalphysical and mechanical properties may nolonger apply.

4.2 Machining and GrindingRough machining (usually, turning, milling,and/or drilling) is generally performed on H-13steel in the fully annealed condition, in which itsmachinability is considered fair, somewhat lessthan the A steels and better than the D steels.Fully annealed H-13 steel has an averagemachinability rating of about 70% of the ratingfor annealed 1% C tool steel or about 45% of thatfor B1112, which is used as a standard referencesteel for machinability ratings. Nominal speedsfor milling of annealed H-13 steel (200-250 HB)with HSS and carbide cutters at standard cutterfeeds of 0.001-0.003 in./tooth are 70-75 sfm forrough milling and 95-100 sfm for finish milling;for hardened H-13 steel (48-50 HRC) under thesame conditions the nominal speeds for roughand finish milling are 30 and 35 sfm respectively.Grindability of H-13 steel decreases as hardnessincreases and is a function of vanadium andchromium content, both of which adversely affectgrindability; however, use of cubic boron nitride

superabrasive wheels with a resinoid or vitrifiedmatrix and in metal-plated monolayer CBN(cubic boron nitride) wheels can significantlyincrease H-13 steel removal rates as well as wheellife. (Ref. 69)Electric discharge machining (EDM) is widelyused for production of hardened H-13 steel diesand tools. However, EDM generates a harmfulwhite layer of martensite on the surface, whichmust be removed by careful grinding, stoning, orpolishing. After removal of the white layer, adouble temper at 50F below the highest temperingtemperature used in hardening is sometimesrecommended in order to stress relieve thesurface. (Ref. 70) Although a white layer doesnot usually form during conventional machiningoperations at normal machining speeds, a whitelayer can form on the surface during the highspeed turning of H-13 steel bars in the hardenedcondition (5456 HRC) using CBN cutting tools.(Ref. 71)On H-13 extrusion dies with complex configurationsand passages, abrasive flow machining (AFM) isoften used to finish surfaces and edges byextruding viscous abrasive media through oracross the die. AFM is used to deburr, polish,radius, and remove the white layer producedby EDM on H-13 steel surfaces and edges. (Ref. 72)Sulfurized H-13 steel with sulfur additions ofabout 0.20% has a much better machinabilityrating than standard H-13 compositions (~75% ofthat for B1112). For the same turning and millingconditions, machining of sulfurized H-13 requiresabout 30% less time. In grinding operations,sulfurized H-13 steel in annealed or hardenedconditions has much better grindability thanH-13 with standard limits on sulfur (

however, welding of H-13 steel dies involvesrisks such as cracking. Thus, several guidelineshave been established for proper welding of H-13steel. (Ref. 4-20)For welding annealed H-13 steel, the part shouldbe preheated to 6501000F, preferably in a furnaceto assure a uniform temperature. Uncoated H-13steel filler rod (0.100.17 in. diameter) withtungsten inert gas (TIG) welding at 100150 AD.C. using Ar protective gas should be used. Thetemperature of the part should be kept above600F, with reheating applied as required untilwelding is complete. Slow cooling after weldingcan be achieved in an insulating medium. As-welded hardness is typically 4852 HRC. Afterslow cooling, the part then is given a full anneal.For welding hardened H-13 parts, such as dies ortooling, the same procedure that is used forwelding annealed H-13 may be used, preferablyusing a preheating temperature close to 1000F. Itis recommended that the welded part be placedin a furnace kept at the preheat temperature andfurnace cooled to room temperature. To furtherrelieve welding stresses and make the hardnessin the weld zone come closer to the base metal,the welded part should be tempered at 3050Fbelow the original tempering temperature.

4.4 Surface TreatingGas, salt bath, fluidized bed, or ion nitridingand ferritic nitrocarburizing are all effective

procedures for nitriding H-13 steel and producinga hard, abrasive resistant surface having a casehardness of 6070 HRC (see 1.5.11 and 1.6.3). Forhot working dies and tools, deep nitriding to acase depth of >0.010 in. is not recommended.H-13 parts that have been deep nitrided need tobe lapped or surface ground to remove the thin,outer brittle layer. Also, dies with sharp cornersor critical projections may not benefit fromnitriding and may actually have shortened livesdue to breakage. Selective nitriding can also bedone to produce a hard nitrided case only whereneeded. Copper plating is preferred for stoppingoff areas that do not need nitriding. Stop-offs thatcontain lead should not be used, as lead embrittlesH-13 steel. (Refs. 13) The Aluminum ExtrudersCouncil has made several recommendations fornitriding H-13 steel aluminum extrusion diesand promoted the benefits of re-nitriding them toincrease life substantially. (Ref. 73)Physical vapor deposited (PVD) and chemicalvapor deposited (CVD) coatings have been usedon H-13 dies and tools to protect H-13 steelagainst corrosion/erosion of molten aluminum(2.3.5) and to improve its wear resistance in hotmetalworking dies and tools. Duplex coatingsinvolving plasma nitrided pretreatment andeither a PVD or plasma-assisted CVD or PVDcoatings have been successful in prolonging H-13steel die life in aluminum extrusion dies comparedto conventional nitriding. (Refs. 7475)

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References

1. Metals Handbook Ninth Edition. Volume 1. Properties andSelection: Irons and Steels, American Society for Metals(now ASM International), 1978, pp. 43739.

2. Metals Handbook Ninth Edition. Volume 3. Properties andSelection: Stainless Steels, Tool Materials and Special-PurposeMetals, American Society for Metals (now ASMInternational), 1980, pp. 42169.

3. HEAT TREATERS GUIDE: Practices and Procedures forIrons and Steels, ASM International, 2nd Ed., 1998, pp.600-605.

4. Premium and Superior Quality H-13 Steel and HeatTreatment Acceptance Criteria for Pressure Die Casting Dies:NADCA #207-2003, North American Die CastingAssociation (NADCA), River Grove, Illinois, 2003.

5. Allegheny Ludlum H-13 Tool Steel, UNS T20813,DATA SHEET.

6. Assab Steels 8407 SUPREME Hot Work Steel, DATASHEET.

7. Bohler-Uddeholm ORVAR H-13 Hot Work Tool Steel,DATA SHEET.

8. Bohler-Uddeholm SUPERIOR H-13 Hot Work ToolSteel, DATA SHEET.

9. Bohler-Uddeholm ISOBLOC H-13 Hot Work Tool Steel,DATA SHEET.

10. Carpenter Pyrotough 78 Hot Work Die Steel. DATASHEET.

11. Carpenter No. 883 Hot Work Die Steel (Red-Tough)(AISI H-13), DATA SHEET.

12. Crucible Nu-Die XL (Premium Quality AISI H-13)Tool Steel, DATA SHEET.

13. International Mold Steel DH2F Pre-Hardened H-13-Type Mold Steel, DATA SHEET.

14. International Mold Steel Premium H-13 Hot Work Dieand Mold Steel, DATA SHEET.

15. Latrobe LSS H-13 Tool Steel, DATA SHEET.

16. Latrobe VDC H-13 Tool Steel, DATA SHEET.

17. ThyssenKrupp Specialty Steels THYROTHERM 2344,DATA SHEET.

18. ThyssenKrupp Specialty Steels THYROTHERM 2344ESR MAGNUM, DATA SHEET.

19. ThyssenKrupp Specialty Steels THYROTHERM 2367SUPRA ESR, DATA SHEET.

20. Premium Grade H-13/Hot Work Tool MaterialStandard Specification: Die Insert and Heat TreatingSpecification, Spec. No. DC-9999-1, GM PowertrainGroup, 6-29-2001.

21. Die Insert Material and Heat Treatment PerformanceRequirements: General Applications, Spec. No. AMTD-DC2010, FORD Advanced Manufacturing TechnologyDevelopment, 17 February 2003.

22. Benedyk, J. C., Thermal Fatigue Behavior of H-13 DieSteels, Ph.D. Thesis, Case Western Reserve University,January, 1969.

23. Benedyk, J. C., Moracz, D. J., and Wallace, J. F.,Thermal Fatigue Behavior of Die Materials forAluminum Die Casting, Paper No. 111, 6th SDCE(Society of Die Casting Engineers) International DieCasting Congress, November 1619, 1970.

24. Dunn, R. P., Thermal Fatigue Behavior of H-13 Steel &Other Die Materials for Aluminum Die Casting, Report of the Task Group on Materials & SurfaceTreatments for Dies, Die Casting Research Foundation1969 Annual Meeting, October 8, 1969.

25. Nolen, R. K., Benedyk, J. C., and Howes, M. A. H.,Investigation of Strain Softening Occurring duringThermal Fatigue, Internal Research and DevelopmentProject Final Report on Project B1051, IIT ResearchInstitute, October 23, 1969.

26. Purohit, S. and Tszeng, C., Carbide Dissolution in H-13Tool Steel during Austenitization, TPTC (ThermalProcessing Technology Center, Illinois Institute ofTechnology) Project Review, 2003.

27. Pacyna, J. and Mazur, A., The Influence of Grain Sizeupon the Fracture Toughness of the Hot-Work ToolSteel, Scandinavian J. of Metallurgy, No. 12, 1983, pp.2228.

28. Schmidt, M. L., Effect of Austenitizing Temperature onthe Structure and Mechanical Property Behavior ofLaboratory Treated Specimens and Large Section Sizesof H-13 Tool Steel, Paper No. G-T87-006, 14th SDCE(Society of Die Casting Engineers) International DieCasting