Application and Characteristics of Low-Carbon MartensiticStainless Steels on Turbine Blades

Hwa-Teng Lee1, Feng-Ming Liu2 and Wun-Hsin Hou3,+

1Department of Mechanical Engineering, National Cheng Kung University,No. 1, University Road, Tainan 701, Taiwan, R. O. China2Department of Business Administration, Hsing-Kuo University, Taiwan,No. 600, Sec. 3, Taijiang Blvd., Annan District, Tainan, Taiwan, R. O. China3R&D Center, Gloria Material Technology Corp.,No. 35, Hsin Chung RD, Hsin Ting District, Tainan City, Taiwan 730, R. O. China

410M1 (0.17%C-11.6%Cr-0.18%Nb) and 410M2 (0.17%C-10.2%Cr-0.38%Nb-0.84%Mo-0.2%V-0.05%N) martensitic stainless steels aremodified from the basic martensitic stainless steel 410 (0.12%C-12%Cr). They contain Nb and are utilized in the blades of turbines forgenerating power. This study investigates the heat treatment characteristics, microstructure and secondary hardenability of 410M1 and 410M2.

The precipitation hardening of 410 occurs at 400°C but that of 410M1 or 410M2 occurs earlier at 300°C. The peak hardening of 410occurs at 450°C but that of 410M1 or 410M2 occurs at 500°C. Clearly, addition of Nb improves the mechanical properties of steel at hightemperature.

Under quenching conditions, 410M1 and 410M2 are lath martensites. 410M2 contains not only Nb but also Mo, V, and N, which improveits secondary hardenability over that of 410M1. From the characteristic chart of quenching and tempering, the tempering softening and theincrease in impact toughness of 410M2 are delayed as a high tempering temperature range of 650°C to 670°C is reached. This phenomenon isobserved by FE SEM and proves that NbC-carbide with 20³40 nm are precipitated in the matrix. This investigation studies the effect of alloydesign on its toughness, secondary hardenability, microstructure and applications. [doi:10.2320/matertrans.M2014307]

(Received August 25, 2014; Accepted January 14, 2015; Published February 20, 2015)

Keywords: martensitic stainless steels, turbine blade, precipitation hardening

1. Introduction

Stainless steels combine superior resistance against high-temperature corrosion with favorable mechanical properties;they are often used at high temperature, along with super-alloys. Stainless steels for elevated-temperature applicationshave various grades, including ferritic grade, martensiticgrade, austenitic grade, precipitation-hardening grade, valvesteels, and cast heat-resistant alloys.

Based on their strength and heat resistance, martensiticstainless steels can be grouped as follows.1)

Group 1: Type 410, 403 and 416 (least strength and heatresistance)

Group 2: Greek Ascoloy and type 431Group 3: Moly Ascoloy and M152Group 4: H-46 and type 422 (greatest strength and heat

resistance)The addition of Nb to form H-46 improves its high-

temperature creep. Nb is a strong carbide-forming element,and small amounts of Nb are typically added to low alloysteels to improve their mechanical properties. In ferritic,austenitic and martensitic steels, the addition of Nb alsoimproves for various purposes. For example, in 347, Nb isadded to form Nb carbide and to prevent the precipitation ofchromium carbide at grain boundaries, thereby improvingcorrosion resistance.2) 18Cr-10Ni austenite has high nitrogencontent, and some studies have found that Nb interacts withcarbon and nitrogen more easily than does Ti, Zr and V, andthus forms Nb nitrides. Nb nitrides are finer than the nitridesthat are formed with Ti, Zr and V, and they fix manydislocations, effectively increasing yield and strength.

The addition of 0.5mass% Nb to ferritic stainless steelsthat contain 17­18mass% Cr was studied. The steels thusformed were found to have a higher wear resistance thanother steels that contain 1 to 3mass% Nb, because form thesmallest NbC particles and have both higher hardness andwear resistance.3) In another study of the effect of ageing onthe creep properties of 20Cr-25Ni-Nb steel, constant loadcreep tests at 225MPa performed on a material with twomicrostructures. Comparison of the creep behavior ofmaterial that was aged for 500 h with that aged for 10,000 hrevealed an increase in creep rate and ductility and areduction in rupture life. The properties change with agingbecause the coarsening of a fine Nb(CN) matrix precipitatesincreases the interparticle spacing and reduces the resistanceto recovery, increasing the creep rate.4) Only fine Nb(CN) caneffectively improve creep properties.

However, few studies of the applications of martensiticstainless steels that contain Nb have been performed. Thesesteels are widely used in the manufacture of power generationturbine blades.

Low-carbon martensitic stainless steels that contain Nb areused in turbine blades because Nb improves strength andcreep characteristics at elevated temperatures. Internationalstandards describe steel grades but the makers of turbineblades modify their chemical compositions to stabilize theirperformance and improve service life. In this work, 410M1and 410M2, which are modified martensitic stainless steelsthat contain Nb, are used in power generation turbine bladesand their microstructures and mechanical properties arestudied.

Low-carbon martensitic stainless steels are used in variousindustries because of their low cost, high strength andtoughness. Type 410 is basic, general-purpose stainless steel+Corresponding author, E-mail: sin@gmtc.com.tw

Materials Transactions, Vol. 56, No. 4 (2015) pp. 563 to 569©2015 The Japan Institute of Metals and Materials

that is used for bolts, pump shafts, and steam valves, atroom temperature. At high temperatures, this steel exhibitsfavorable corrosion resistance and strength5,6) and so isutilized to make turbine blades that will not be operated atexcessive temperatures.

With respect to the mechanical requirements of turbineblades, when the blades will be exposed to temperaturesabove about 480°C for only a short period, their mechanicaldesign need only take into account their short-term tensileproperties. At the temperatures that exceed about 0.4 timesthe melting point of the material, the full range of effectsbecome evident. Blades on spinning rotors in turbine enginesslowly grow during operation and must be replaced beforethey touch the housing. When the operating temperature is toexceed 480°C, the design process take into account suchproperties as creep rate, creep-rupture strength, creep-ruptureductility, and creep-fatigue interaction.

Among metallic heat-resistant materials, 410 do notperform so well:1) its tensile strength and stress rupturestrength at elevated temperature are inadequate for its use inturbine blades at high temperatures.

The basic steel 410 should be used at temperatures of lessthan 400°C, but when some alloying elements such as V, Nband N are added to it, it can be used at temperatures up toapproximately 650°C. In martensitic stainless steels thatcontain 13% chromium, adding just about 0.05mass% Nbonly slightly increases tempered hardness at a temperingtemperature of below 600°C, but the increase is much greaterat 650°C, because fine NbC carbides precipitate in the matrix,and these carbides retard the recovery of dislocations.7) Nb-containing alloys such as H-46 typically have a favorablestress-rupture characteristic (creep resistance) over shorttesting times (100 to 1000 h) but they lose this strengthadvantage over periods of about 10,000 h or more. Thefavorable effects of Nb additions on short-term stress-ruptureproperties are attributed to the precipitation of finelydispersed NbC. The favorable effects tend to decline as thetempering temperature is increased, and a more coarselydispersed precipitate is therefore formed.1)

Since Mo can reduce the rate of formation of NbC,high-strength low-alloy steels are generally formed by thesimultaneous addition of Nb and Mo to reduce the growthrate of carbide and to increase its strength.8)

V and Nb very easily react with carbon to form fine alloycarbides. At elevated temperatures, these carbides can inhibitthe migration of grain boundaries and thereby improve thecreep properties. The simultaneous addition of Nb and V ismore effective than adding them separately.9,10)

In martensitic stainless steel, nitrogen can increase the high-temperature strength by solid-solution strengthening andsecondary hardening. Nitrogen atoms, nitrides or carboni-trides may be responsible for retarding the motion of austenitegrain boundaries at elevated temperature and reducing the sizeof grains. In steel that contains 13% Cr, the amount ofchromium in the carbides and their lattice parameters decreaseas the nitrogen content of the steel or alloy increases, makingthe finer and more uniformly distributed.11)

At different tempering temperatures, 410 precipitatesdifferent carbide structures, such as M3C, M7C3 and M23C6;the latter forms at 480°C and M3C disappears when the

tempering temperature is increased to 650°C. The amountof M7C3 carbide decreases as the tempering temperatureincreases. At higher tempering temperatures, the finer M7C3

carbide aggregates to become coarse Cr23C6 carbide.Tempering softening then reduces the hardness.12)

M2C carbide is usually absent from a Cr-C system butmaterial that contains 12³17mass% Cr forms M2C carbideduring tempering. This carbide is the intermediate phaseof (Fe, Cr)3C before it is transformed to (Fe, Cr)7C3. Theaddition of Mo, N and V stabilizes the carbide and increasessecondary hardening.9) Tempering in the appropriate temper-ature range causes secondary hardening by the precipitatestrengthening of M7C3 carbide. The elements Mo, W, Ti andNb delay the coarsening of M7C3 carbide with increasingtemperature and bring forward the formation of M2C carbide,increasing secondary hardening.13)

410M1 and 410M2 are formed by modifying 410 byadding the alloying elements of Nb, Mo, V and N to improvetheir high-temperature mechanical properties. Most turbinemakers use 410M1 in the lower-temperature stage and410M2 in the higher-temperature stage. Table 1 presents theirchemical compositions: 410M1 is similar to 410Cb but its Nbcontent exceeds that of 410Cb, so its strength at elevatedtemperature is greater.

410M2 has a higher Nb content than 410M1 and otheralloying elements V, Mo and N, which improve its high-temperature strength and creep strength. 410M2 is similar toH46 but contains more carbon and alloying elements and istypically used in turbine blades that are used at very hightemperatures. The high alloying element content facilitatesthe macro-segregation of carbides, and the production ofcoarse grain and duplex grains, which defects are notacceptable in turbines. Preventing the macro-segregation ofcarbide is critical to the manufacture of 410M2.

410M1 is tensile-tested at 800°F (427°C) to determinewhether it meets the mechanical requirements for use inturbine blades. 410M2 requires a higher tensile strength andmust pass a 1200°F (649°C) stress rupture test.14,15)

The increase need for nuclear power plants has led toincreases in the amounts of 410M1 and 410M2 used in turbineblades in such plants. To increase the turbine generationefficiency, 410M1 is gradually being replaced by 410M2.

This thesis study compares the thermal characteristics,mechanical properties and micro-structures of 410M1 and410M2.

2. Experimental

In this investigation, rolled round bars of 410M1 and410M2 with a diameter of 65mm are used. Table 1 presents

Table 1 Chemical composition of tested specimens (unit: mass%).

SteelGrade

C Mn Si P S Ni Cr Mo Nb V N

410Max.0.15

Max.1.0

Max.1.0

Max.0.04

Max.0.03

®11.5³13.5

® ® ® ®

410M1 0.17 0.52 0.48 0.016 0.002 ® 11.6 ® 0.18 ® ®

410M2 0.17 0.39 0.39 0.020 0.008 0.46 10.2 0.84 0.38 0.20 0.05

H.-T. Lee, F.-M. Liu and W.-H. Hou564

the chemical composition. The bars are melted in an electricalair melting furnace and then remelted in the Electro-SlagRemelting Process (ESR). Reducing the oxide inclusionand sulfide contents not only improves the high-temperaturebrittleness but also reduces the transversal segregation andshrinkage, while refining the micro-structure, to extent thelifetime of the steel at severely elevated temperatures.

The experimental procedure involves taken from the rolledbars, the pre-machine specimen size are 55mm © 11mm ©11mm. The quenching lasts for 40min and is performed atvarious austenitic temperatures, before forced air is used tocool the specimens to room temperature. Holding time intempering is 80min. Following heat treatment, to ensure thatthe test data are accurate, specimens are ground using a facegrinding machine to remove the decarburized layer to athickness of approximately 0.5mm from each face, and thena 45° notch is machined to form a Charpy impact specimen.Before the impact test is performed, hardness is tested at bothends of each impact specimen.

Micro-structural specimens are an impact specimen andground using #180, #600 and #1000 sandpapers. Aftergrinding, the specimens are polished using 3 µm and 1µmdiamond paste and etched using Vilella to observe theirmicrostructures.

3. Results and Discussion

3.1 Quenching and tempering hardnessThe tempering curve of 410 low-carbon martensitic

stainless steels reveals that secondary hardening occurs at400 to 450°C, appearing to reach a maximum around 450°C.9)

Figures 1 and 2 plot the tempering curves of 410M1 and410M2. Secondary hardening occurred at approximately300°C and the maximum hardness was not reached untilaround 500°C.

When the tempering temperature of 410M1 exceeds therange for secondary hardening, the hardness becomesinversely proportional to the tempering temperature. How-ever, the tempering curve of 410M2 from 650°C to 670°Creveals a delay in the drop in hardness owing to the fineprecipitation of the NbC carbide. The nano-sized alloycarbide inhibits the slippage and climbing of dislocations andexhibits a favorable anti-tempering softening characteristic.As the tempering temperature is increased above 700°C, theobvious drop in hardness is caused by the growth of NbCcarbide and the formation of coarser M23C6 carbide.1)

The amount of alloying elements that are dissolved in thematrix increases with the quenching temperature. Since, thematrix dissolves more alloying elements, the amount ofprecipitation is increased, and so the hardness is higher. Theincrease in the amount of tempering carbide precipitatesincreases the Ms point and the retained austenite more easilydecomposes. Therefore, the tempering hardness increaseswith the quenching temperature.

The specifications of 410M2 require that it undergoes astress rupture test at 650°C. At that tempering temperature, itis strengthened by NbC precipitation and exhibits better high-temperature creep.

410M1 did not exhibit a delay in tempering hardnesssoftening. The two alloys have the same carbon content but

when heat-treated under the same conditions, the hardnessof 410M2 exceeded that of 410M1 by about 5 HRC, aspresented in Fig. 3. The greatest difference of about 10 HRCappeared following tempering at 650°C³750°C. Since410M2 contained added alloying elements Mo, N, V andmore Nb than 410M1, 410M2 is better for use at elevatedtemperatures because the alloying elements reduce the size ofthe carbide, increasing strengthening by precipitation.

3.2 Micro-structural observationThe tempering hardness curves in Fig. 3 clearly demon-

strate the difference between 410M1 and 410M2, whosecause is revealed by observation of the microstructures.

Figures 4(a), (b) and (c) present the microstructure of410M1 following heat treatment under various conditions,observed under an optical microscope. Quenching at temper-atures from 1100°C to 1180°C, it yielded lath martensiteof medium size. However, quenching 410M2 at 980°Cproduced extra-fine lath martensite, as presented in Fig. 4(d).The fine martensite represented a fine austenite grain

Fig. 1 Hardness of 410M1 after quenching and tempering.

Fig. 2 Hardness of 410M2 after quenching and tempering.

Application and Characteristics of Low-Carbon Martensitic Stainless Steels on Turbine Blades 565

structure. At elevated temperatures, fine grains improvedtoughness at room temperature but worsened creepingstrength. 410M2 should not be quenched at 980°C, and hasbetter creep properties than 410M1. The sizes of lathmartensite following quenching from 1150°C to 1180°C areall similar.

The carbide of Nb has a high melting temperature and doesnot completely dissolve at 1100°C or 1180°C. Therefore,some undissolved carbide was found on the grain boundariesand in the matrix following quenching at 1180°C. The

microstructure that is presented in Fig. 4(f ) and Fig. 5reveals no grain growth and the quenching temperature at1180°C is therefore acceptable.

A comparison of the SEM microstructures of 410M1 and410M2 in Fig. 6 indicates that 410M2 has rounder andcoarser residual primary carbides. A comparison withquenching at 1150°C and tempering at various temperaturesreveals that tempering at 550°C precipitates slim and rod-likecarbides. When the tempering temperature is increased to650°C, the carbides become coarse and round, and remainhighly concentrated and distributed along the martensitic lath.When the tempering temperature is increased to 750°C, thecarbides grow into coarser M23C6 carbides. Their concen-tration falls and the carbides become more scattered.

Figures 7(d), (e) and (f ) present the microstructures of410M2. The shapes of the tempering precipitated carbides aresimilar to those in 410M1 but the carbides are smaller than in410M1, particularly following tempering at 750°C. Presum-ably, the addition of Mo reduces the rate of diffusion and therate of carbide formation. 410M2 contains nitrogen, whichcan reduce the size of the carbide structures and refine theprecipitated carbide.

FE SEM was used to observe the microstructure of 410M2that was tempered at 550°C. Figure 7(d) shows no fine NbCcarbide precipitate in the matrix. When tempered at 650°C,the matrix contains fine NbC carbide precipitates sizes of 20to 40 nm, as described in the literature.1) Figures 7(e) and (g)present the microstructures. The nano-carbides are easilyobserved following tempering at 650°C and became fewerand more scattered as the tempering temperature wasincreased to 750°C. These microstructures reflect the delayin tempering softening to temperatures of 650°C to 670°C.410M2 is strengthened by nano-carbide precipitation.

The morphology of a martensitic structure is dominatedby its carbon content. When the carbon content is below0.5mass%, lath martensite is formed. 410M1 and 410M2 arelow-carbon martensitic stainless steels, with the same carboncontent and similar amounts of the main alloying element.Therefore, these steels have the same lath martensitequenched microstructures. The martensitic lath did notbecome too coarse due to over-quenching or too fine due tounder-quenching and 410M1 can be quenched at a temper-ature from 1100°C to 1180°C. However, the martensitic lathsin the microstructure of 410M2 following quenching at980°C are too fine, indicating the quenching temperature istoo low. Comparing the tempered microstructure reveals that

Fig. 3 410M2 has higher hardness than 410M1 and exhibits delayedtempering softening at 650°C.

Fig. 4 Quenched micro-structures of 410M1 and 410M2. Micro-structureof 410M2 following quenching at 980°C is extra fine, (a) 410M1following forced air cooling at 1100°C; (b) 410M1 following forced aircooling at 1150°C; (c) 410M1 following forced air cooling at 1180°C;(d) 410M2 following forced air cooling at 980°C; (e) 410M2 followingforced air cooling at 1150°C; (f ) 410M2 following forced air cooling at1180°C.

Fig. 5 Microstructure of 410M1 that was quenched at 1180°C.

H.-T. Lee, F.-M. Liu and W.-H. Hou566

the precipitated carbide in 410M2 is finer than that of 410M1because 410M2 contains more of the added alloying elementsMo, V and N. At a high tempering temperature, thesealloying elements exhibit delayed diffusion and they delaythe growth of the precipitating carbide. Hence, 410M2outperforms 410M1 at elevated temperatures. The micro-structure that is observed by FE SEM demonstrates that thelarge amount of NbC carbide precipitated is responsible forthe delay in tempering softening.

3.3 Impact TestFigures 8 and 9 presents the results of the impact tests on

410M1 and 410M2. When the steel is tempered at 300°C to500°C, impact toughness decreases as the tempering temper-ature increases. The impact toughness was lowest followingtempering at 500°C to 550°C, and is affected by secondaryhardening that is caused by fine alloy carbide precipitation.When the tempering temperature exceeds 600°C, the impacttoughness of 410M1 exceeds that of 410M2.

The amount of alloy carbide that is dissolved into matrixincreases with the quenching temperature. As the matrixcontains more alloying elements when the tempering temper-ature is higher, the precipitation of alloy carbide increases thehigh-temperature strength but negatively affects toughness.

The Nb content of 410M2 exceeds that of 410M1 and 410M2also contains Mo, V and N. These elements easily combinewith carbon to form fine carbides, so 410M2 exhibits greaterprecipitation but have lower impact toughness at roomtemperature. 410M2 is used at elevated temperatures, so itscreep property is more important than its impact property.

The tempering of 410M1 above 700°C rapidly increases itsimpact toughness. Toughness is greatly increased becauseM23C6 carbide growth and tempering hardness are clearlyreduced as the tempering temperature is increased.

The toughness of 410M2 increases considerably onlywhen it is tempered at the lowest quenching temperatureof 980°C. This fact is explained by the microstructure inFig. 4(d). The grains are extra fine because the quenchingtemperature is minimal and a large amount of undissolvedresidual carbide at the grain boundary retards grain growthduring quenching. When 410M2 is quenched at 980°C andtempered at 700°C, the toughness is improved not only bythe undissolved residual carbide, which easily becomesnuclei of M23C6, but also by the reduction of the dissolutionof alloying elements in the matrix and of precipitationstrengthening.

Fine grains improve the toughness of 410M2 whichcan therefore be used under severe conditions. The creep

Fig. 6 410M2 contains many residual primary carbides after quenching but the tempered carbide is finer than that in 410M1.

Application and Characteristics of Low-Carbon Martensitic Stainless Steels on Turbine Blades 567

Fig. 7 Increasing the tempering temperature transforms the M7C3 carbide rods into larger M23C6. No NbC carbide is precipitated in410M1 and 410M2 upon tempering at 550°C. During tempering at 650°C, NbC carbides precipitate in the matrix of 410M2. Thesecarbides form coarser carbide upon tempering at 750°C.

Fig. 8 Impact chart of quenched and tempered 410M1. Fig. 9 Impact chart of quenched and tempered 410M2.

H.-T. Lee, F.-M. Liu and W.-H. Hou568

resistance is more important than toughness and explainswhy 410M2 cannot be quenched at 980°C. Increasing thequenching temperature grows the grains and dissolves morealloying elements into the matrix, increasing precipitationstrengthening, and thereby reducing toughness at roomtemperature. This effect is particularly evident in thetempering temperature range of 650°C to 670°C and iscaused by nano-carbide NbC precipitation. This phenomenondisappears when the tempering temperature rises above700°C, consistent with the tempering hardness curves that arepresented in Fig. 9 and Fig. 2.

As the tempering temperature raises above 700°C, the410M2 toughness are increases, but remains below that of410M1, demonstrating that 410M2 is stronger at a hightemperature. The toughness of 410M2 is less than that of410M1 at room temperature, but owing to its better stressrupture properties, 410M2 is more suitable than 410M1 foruse in turbines at high temperature.

4. Conclusions

(1) The secondary hardening of 410 steel occurs upontempering at 400°C. The maximum hardness is reachedfollowing tempering at 450°C. 410M1 and 410M2exhibit not only secondary hardening at 300°C but alsoa delayed maximum hardness at 500°C, because theaddition of Nb increases precipitation hardening andimproves high-temperature tensile strength.

(2) High-temperature tempering causes 410M2 to precip-itate refine carbide, improving precipitation strengthen-ing, causing the hardness of 410M2 to exceed thatof 410M1 by approximately 5 to 10 HRC. The finercarbides cause 410M2 to exhibit greater precipitationstrengthening and resistance to tempering softening.

(3) When 410M2 is tempered at temperatures of 650°C to670°C, tempering hardness softening is reduced and therate at which the toughness increases is reduced by fine

NbC carbide precipitation; the sizes of the carbidesare 20 nm to 40 nm. The nano-carbides increase theresistance to stress rupture and 410M2 is more suitablefor use in turbines at high temperatures than is 410M1.

(4) NbC carbide precipitation clearly affects the temperingcurve from 650°C to 670°C. The temperature of 650°Cat which the stress rupture test is performed falls withinthis temperature range.

(5) 410M2 acquires an extra fine structure when quenchedat 980°C. This steel has excellent toughness, but whenused at elevated temperatures; its stress rupture propertyis its most important property. Therefore, 410M2 shouldbe quenched at temperatures above 980°C.

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