20th ASM Heat Treating Society Conference Proceedings, 9-12 October 2000, St. Louis, MO, ASM International, 2000

Microstructure and Fatigue Resistance ofCarburized Steels

J.P. Wise, G. Krauss, D.K. MatlockColorado School of Mines, Golden, Colorado

Abstract

A summary is presented of recent research on themicrostructure and bending fatigue performance of severalcarburizing steels. Two fatigue optimization approaches wereexplored: alloy chemistry and processing. Compositionswere varied with respect to sulfur, phosphorous, manganese,and silicon. In each case, maximum composition limits of theelements were identified that resulted in decreases in thepresence of fatigue crack initiators. The suppression ofmanganese sulfides, intergranular cementite, and intergranularoxides substantially increased bending fatigue endurancelimits. A carburizing process study showed that reductions incase carbon content, through low carburizing potentials andreheat treatments, resulted in less retained austenite, largersurface residual stresses, and consequently, higher endurancelimits. Shot peening created the most dramatic increases infatigue performance, due to the generation of very largeresidual stresses. The entire set of data summarized in thisreport was statistically analyzed to reveal broad trends inbending fatigue research. Prior austenite grain size in the caseand surface residual stresses had the largest effects on bendingfatigue endurance limit. Case grain refinement was mosteffectively achieved through reheat treatments, while shotpeening was the primary means to significantly increasecompressive residual stresses in the specimen surface.

carbon steel case and a low-carbon steel core. When this steelcomposite is quenched to martensite and tempered, the highhardness and strength of the case microstructure, combined withthe favorable case compressive residual stresses developed duringquenching, produce a high resistance to fatigue.

This work identifies the important chemistry, processing, andmicrostructural variables that govern the bending fatigueperformance of carburized steels. The discussion concentrates onrecent bending fatigue research and is divided into two sections:effects of alloy chemistry and effects of processing techniques.All fatigue research was performed with a constant specimengeometry, offering the unique opportunity to perform a statisticalanalysis to extract broad trends in bending fatigue behavior.

Fatigue Research

Introduction

Steel gears fail in bending fatigue at nominal stresseswell below the bulk yield strength of the material due to thepresence of microstructural features that locally raise theapplied stress. Maximum gear performance is achieved whenthe metallurgist understands how microstructures respond toapplied stress and how to control, or compensate for, thesemicrostructures through alloy and process design.Carburizing improves gear performance by enhancing surfaceproperties. The addition of carbon into the surface of acomponent creates a composite material consisting of a high-

Testing. The studies cited throughout this work were part ofa large effort to assess the role of alloy chemistry .heat treabnent,and shot peening on the microstructure and bending fatigueperformance of several carburizing steels. All testing was donewith the cantilever beam bending fatigue specimen shown inFigure I. The specimen incorporates a 3.2-mm radius designed tosimulate a single gear tooth. Bending fatigue testing wasperformed on commercial test frames. Specimens were cyclicallyloaded by a combination of a rotating weight and a static loadimposed by a constant displacement. Tests were run at afrequency of 30 Hz with a ratio of minimum to maximum stress of0.1. Keeping specimens in tension at all times preserved fracturesurfaces for later examination. S-N curves for each experimentalcondition were created using approximately 40 specimens. Threetests were performed at each stress level, and endurance limitswere defined as the maximum stress that produced threeconsecutive run-outs at ten million cycles.

Several specimen characteristics were kept constant to isolatethe effects of alloy chemistry and processing approaches on fatigueperformance. Following machining, the specimens werechemically polished to create a uniform surface finish prior tocarburizing. All specimens were carburized to a nominal casedepth of I mm to 50 HRC, which resulted in constant residualstresses near 100 MPa in compression at the specimen surface.

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The relatively low residual stresses were a result of the type ofspecimen used. The small specimens were easily through-hardened upon quenching. Consequently, much lowerresidual stresses were generated in the test specimens ascompared to those achieved in actual gears.

Figure 2. Overview of fatigue fracture in a carburized SAE 4320steel, showing elliptical b"ansgranular crack centered on anintergranular initiation site marked by arrow. SEM micrograph.

Design of Alloy ChemistryFigure I. Cantilever beam bending fatigue specimen.Measurements are in millimeters.

Specimen Failure Mechanisms. The fatigueperfonnance of the steels reviewed in this work was closelyrelated to the mechanism by which the initial fatigue crackfonned. In direct-quenched gear steels with moderate to goodendurance limits of 1000-1300 MPa, the initial fatigue crackpredominantly fonned as a result of the separation of prioraustenite grain boundaries at the component surfacel. Thisintergranular fracture is related to phosphorous segregation to,and cementite fonnation on, austenite grain boundaries2.Intergranular initiation sites were composed of a single grainboundary or often extended several grains into the interior ofthe component. High endurance limits, above 1400 MPa,were usually associated with transgranular crack nucleationwithin the prior austenite grains3. Prior austenite grain sizescontrolled the mode of crack initiation, as initiation modeshave been successfully altered from intergranular totrans granular by refining the austenite grain size throughreheat treatments after carburizing4. Figure 2 shows a typicalfracture path of a bending fatigue specimen failure. The crackin the figure begins with a small region of grain boundaryseparation. This intergranular crack forms as soon as aninitial stress is placed on the specimens. The sharp tip of theintergranular crack then becomes the starting point for stabletransgranular fatigue crack growth. When the transgranularcrack reaches a critical size, it becomes unstable and finaloverload fracture of the specimen begins.

While guidelines for alloy chemistry for optimum fatigueresistance are continually being refined, some desirablecharacteristics are generally agreed upon. The case should consistof a high-carbon tempered martensite with some retained austenite.Ni, Cr, Mn, and Mo increase a steel's hardenability and preventthe formation of non-martensitic transformation products uponquenching. In addition, Ni is often added to improve martensitetoughness. Large carbides, oxides, and other second-phaseparticles are to be avoided. These structures may either combinewith other features that initiate fatigue cracks, or in the absence ofsuch features, serve as the sole source of fatigue crack initiation.Consequently, S, P, and strong oxide-formers (Cr, Si, Mn) must becarefully controlled. Finally, small prior austenite grains sizes arebeneficial to fatigue resistance, as discussed previously. Finecarbides, nitrides, and carbonitrides of AI, V, Nb, and Ti pin grainboundaries at carburizing temperatures.

The following is a review of recent research on alloychemistry effects on the microstructure and fatigue performance ofcarburized steels. In each case, an element, or combination ofelements, formed an undesirable second-phase that contributed tofatigue crack initiation by weakening of prior austenite grainboundaries. The goal of these works was to identify critical levelsof the elements to avoid significant loss in fatigue resistance.

Sulfur: The Effect of Non-Metallic Inclusions. Sulfurexists in steel primarily in the form of manganese sulfideinclusions. While MnS particles greatly improve machinability.they act as stress concentrators to facilitate fatigue cracknucleation. The effects of MnS on fatigue performance weredemonstrated by a study that systematically varied sulfur content

in an SAE 8219-type steell. The amount of manganese sulfideinclusions present in the microstructure was found to beproportional to the bulk sulfur content. As the number of sulfides

1153

increased, they became more likely to be associated withfatigue crack initiation sites in bending fatigue tests. In allalloys, the initial fatigue crack consisted of a cluster ofseparated grain boundaries at the specimen surface. The MnSinclusions, however, apparently provided all extra source ofstress concentration to facilitate fatigue crack nucleation, asshown in Figure 3. Figure 4 shows how the increase in bulksulfur content from 0.006 to 0.029 weight percent resulted in adrop in the endurance limit from 1260 to 1070 MPa.Consequently, a gear steel designer must weigh this loweredfatigue performance against gains in machinability associatedwith higher sulfur levels.

Phosphorous: Grain Boundary Cementite Formation. Inmedium carbon steels, tempering is required to cause intergranularfracture. For example, tempered marten site embrittlement (TME)and temper embrittlement (TE) develop after heating between 260-370°C and 375-575°C respectively6.7. However, in steelscontaining more than 0.5 weight percent carbon, susceptibility tointergranular embrittlement occurs immediately after quenchingand persists after tempering at temperatures below those at whichTME develops. This phenomenon, termed quench embrittlement,is characterized by cementite formation on austenite grain

boundaries during austenitizing and/or quenching2.Phosphorous strongly promotes quench embrittlement by

its segregation to austenite grain boundaries and its stabilizingeffect on cementite. A study on the effects of phosphorous contentin a carburized SAE 4320 steel showed that high phosphorouslevels enhance phosphorous segregation to austenite grainboundaries and considerably lower bending fatigue endurancelimits2. As shown in Figure 5, when bulk phosphorous contentexceeded 0.017 weight percent, endurance limit droppedsignificantly. An increase in phosphorous from 0.017 to 0.031weight percent reduced endurance limits from 1075 to 875 MPa.Despite the beneficial effects of low phosphorous levels onendurance limits, fatigue crack initiation was still characterized byintergranular cracking in all direct-quenched alloys.

Figure 3. Manganese sulfide inclusions at the fatigueinitiation site in a carburized SAE 8219-type steel. SEM

micrograph.

1400

0.006 S

0.015 S1200(I) Figure 5. Effect of phosphorous content on the S-N curve of

carburized 4320 steel. Concentrations are expressed in weight

percent.1100

0.029 S

Manganese and Silicon: Intergranular Oxidation. Theatmosphere within a gas carburizing furnace is composedprimarily of CO, CO2, H2, H2O, and N2. Carbon potential iscontrolled by the addition of a hydrocarbon gas such as methane orpropane. In addition to these gases, a small amount of oxygen isinevitably present, causing surface oxidation during carburizing.Silicon, manganese, and chromium commonly form oxides whenexposed to a gas carburizing atmosphere. Oxides typically formon austenite grain boundaries to a depth of 10 to 20 ~, due to the

10001.0E+04 1.0E+OS 1.0E+06

Cycles to Failure

1.0E+O7 1.0E+O8

Figure 4. Effect of sulfur content on the S-N curve of acarburized SAE 8219-type steel. Concentrations areexpressed in weight percent.

1154

1600

.

...\

\,

~

.,

'\,6

\

'\'\

ease of oxygen diffusion along these paths. Similar to theMnS inclusions discussed previously, oxides can facilitatenucleation of fatigue cracks. In addition, surface oxidationremoves elements such as chromium and manganese thatprovide hardenability to the matrix. This may result in theformation of non-martensitic microstructures such as ferrite,bainite, or pearlite upon quenching8. The formation of thenon-martensitic structures compromises both strength andcompressive residual stresses.

A study of the effects of high oxide-potential elements ina gas carburized SAE 4320-type steel has shown that controlof alloy chemistry can virtually eliminate intergranularoxidation (IGO) and subsequently increase fatigueperformance9. In a low-Cr modified 4320 steel, silicon levelswere systematically reduced from 0.34 to 0.07 weight percent,resulting in the corresponding reduction in the IGO depth asseen in Figure 6. With the decrease in stress concentrationassociated with the oxides, bending fatigue endurance limitsimproved from approximately 1100 to 1500 MPa, as shown inFigure 7. Using the low-Si alloy (0.07 Si) as a base,manganese contents were subsequently reduced from 0.65 to0.31 weight percent in an attempt to further improve oxidationand fatigue properties. The depth of oxidation did notappreciably decrease with lower Mn levels; however,endurance limits improved from 1250 to 1350 MPa with theMn reduction. The low-Mn alloy had very limited oxidecoverage on the near surface grain boundaries. Thus, bothoxide depth as well as the extent of grain boundary coverageseem to impact the ease of fatigue crack initiation at prioraustenite grain boundaries. Figure 8 depicts the intergranularfracture origin of a high-Mn high-Si alloy, showing patternson the fatigue initiation site indicative of oxide formation.

. .."'.

" -

'"

'"

'\

. ..

1000

0 2 4 6 8 10 12

Surface Oxidation Depth (~m)

14 16

Figure 7. Influence of intergranular oxidation depth on the

bending fatigue endurance limit of a carburized modified 4320steel.

Figure 8. Oxide coverage on the intergranular fracture origin of a0.6Mn-0.14Si modified 4320 Steel. SEM micrograph.

Intergranular oxidation has also been successfully controlledthrough processing techniques; however, a reduction in surfaceoxidation does not always translate into improved fatigueperformance. Unlike conventional gas carburizing, plasma andvacuum carburizing are performed in atmospheres that do notcontain oxygen, and the degree of IGO is very small. In addition,these processes can achieve higher carburizing temperatures andcarbon potentials, significantly reducing carburizing time.However, excessive austenite grain growth and difficult carbonpotential control is often associated with these techniques, leadingto reduced fatigue performance.

16

-E 14~.c 12-a.Q)C 10c:o~ 8tU

"0"x 6O

~ 4tU't:=' 2

C/)

0 0.1 0.2 0.3

Silicon Content (wt. %)

0.4

Figure 6. Effect of silicon content on oxidation depth in a

carburized modified 4320 steel.

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Process Design: Carburizing and Shot Peening potential that maximizes case strength without compromisingresidual stress and resistance to embrittlement.

An investigation was performed to study the response of acarburized SAE 4320 steel to carbon potentials of 0.6. 0.85, 1.05.and 1.25 weight percent 11. Effects on retained austenite content,residual stress state. and bending fatigue endurance limits wereassessed. A reheat step after carburizing was added to theprocessing of some specimens to refine austenite grain size and tolower the solute carbon content in the case through the formationof carbides. Figures 9-a and 9-b show how increased carbonresulted in higher retained austenite contents and lower residualstresses at the specimen surface. Consequently. bending fatigueendurance limits were found to decrease with increasing carbon asshown in Figure 9-c. The plots also show that the reheattreatments successfully reduced the solute carbon in the case.contributing to the observed improvements in retained austenite.residual stress. and endurance limit over conventionally treatedsteels.

Shot Peening. Shot peening was evaluated with the use ofsoft steel shot, hard steel shot, and a two-step (hard shot + lowintensity-small shot) peening treatment on a carburized SAE 4320steel12. All peening processes significantly reduced retainedaustenite at the specimen surface, from 28 volume percent in theas-carburized condition to less than 9 volume percent after allpeening treatments. Surface hardness increased from 57 to 61ARC, and compressive surface residual stresses dramaticallyincreased from 100 to almost 1000 MPa. As a result, bendingfatigue endurance limits increased from 1070 to over 1400 MPafor all peening treatments, as shown in Figure 10.

Additional shot peening studies were performed oncarburized SAE 4023, 4320, and 9310 steelsl3. Similar to theresults discussed above, compressive residual stresses at thespecimen surface increased from 100 to as high as 1450 MPa afterpeening. Shot peened alloys consistently achieved endurancelimits near 1500 MPa, approximately 250-350 MPa higher than in

typical unpeened specimens.

Statistical Analysis of Specimen Characteristics andTheir Effect on Bending Fatigue Performance

Carburizing introduces a gradient in the surfacemicrostructure and mechanical properties. The hardness andstrength of martensite increase with increasing carbon content.Therefore, the highest hardness of a hardened carburized steelis at or close to the surface, typically in the range of 58-62HRC. Hardness may peak at a distance from the surface iflarge amounts of retained austenite, which is much softer thanmartensite, offset the high hardness of the martensiteimmediately adjacent to the surface where the carbon contentis at its highest. Increasing carbon content significantlylowers Ms temperatures and depresses the entire temperaturerange for martensitic transformation to below roomtemperature. Consequently, there are almost alwayssignificant amounts of retained austenite in the cases ofcarburized steels quenched to room temperature. Retainedaustenite plays a large role in the fatigue of carburized steels,and amounts on the order of 20 to 30 volume percent arecommon in the near-surface cases of direct-quenched gears.At the high strain amplitudes present in low-cycle fatigue,retained austenite undergoes deformation-assistedtransformation to martensite, extending fatigue lifelo.Conversely, retained austenite may be detrimental in high-cycle fatigue, for the retention of austenite reduces hardnessand compressive residual stresses, facilitating fatigue crackinitiation.

Two heat treatment approaches were considered in theresearch reviewed below: direct quenching and reheating.Direct quenching is the most common carburizing treatment.The component is typically quenched in oil from thecarburizing temperature and is then tempered to achieve thefinal properties. Reheat treatments add an additional step tothe direct-quenched method. After quenching from thecarburizing temperature, the component is intercriticallyaustenitized before a final quench. Carbide particlesprecipitate during the intercritical treatment, reducing thematrix carbon content.

Shot peening mechanically strengthens the surface of acarburized steel component through work hardening and thetransformation of retained austenite to martensite.Furthermore, the cold work and martensitic transformationinduce large compressive residual stresses. Shot peening mayalso affect surface roughness. The impact of the steel shotoften smooths coarse machining marks, reducing stressconcentrations that may playa role in fatigue crack initiation.

The myriad of variables that can affect bending fatiguebehavior make it very difficult to determine exactly what shouldbe the focus of research programs devoted to increasingperformance. The studies presented above offered a uniqueopportunity to apply statistical methods to a large set of data in aneffort to extract significant trends to aid in chemistry and processdesign. In each research program summarized above, a broadspectrum of specimen characteristics was recorded: retainedaustenite content, prior austenite grain size of the case and core,extent of intergranular oxidation, surface roughness in thespecimen root, and the case profiles of residual stress, hardness,and carbon content. The analysis below begins with correlationcoefficient calculations to determine those variables that show thestrongest association to endurance limit. Using multiple regressiontechniques, these variables are then considered in the constructionof a quantitative model to predict fatigue performance.

Carburizing: Carbon Potential and ReheatTreatments. As discussed earlier, strength gains realizedthrough carburizing increase with carbon potential, yetexcessive carbon results in incomplete martensitictransformation upon quenching. Large amounts of retainedaustenite diminish compressive residual stresses and loweryield strengths, resulting in a decrease in high-cycle fatigueperformance. Furthermore, high-carbon steels are especiallyprone to quench embrittlement. Consequently, optimumbending fatigue performance depends on selecting the carbon

1156

30

/A Single Heat Treatment

O Single + Reheat Treatment

D

//

< 20-0Q)c: 18"tU

~ 16

14

0.4 0.6 0.8 1.2

Case Carbon Content (wt. pct.)

(a)

1.4

Figure 10. Effect of shot peening on the S-N curve of a carburizedSAE 4320 steel.220

""ia:J

"0.00a>

CI:

200 .Single Heat Treatment

D Single + Reheat Treatment

180

,"

Q)-> CIS

.-0.~ ~ 160

Q)-"- (/)

~ ~ 140

O~(.)cnQ) 120U

~~ 100cn

800.4 0.6 0.8 1.2

Case Carbon Content (wt. pct.)

(b)

1.4

1600

A Single Heat Treatment

D Single + Reheat Treatment

Correlations Between Endurance Limit and SpecimenCharacteristics. The correlation coefficient, r, provides a guideas to which variables have the most pronounced effect on thevariable of interest -in this case endurance limit. Specifically, thecorrelation coefficient is a measure of the strength of the linearrelationship between two variables. When a linear relationship isnonexistent, the r-value is close to zero. A perfectly lineardependence results in an r-value of lor -I, depending on whethera positive or negative linear relationship exists between the twovariables.

Several variables were considered to determine those with thestrongest correlations with bending fatigue endurance limit:surface carbon content, surface hardness, case depth to 50 HRC,residual stress, retained austenite, case and core grain size, depthof intergranular oxidation, and surface roughness in the specimenroot along the longitudinal axis of the specimen. Residual stressand retained austenite profiles into the case exist for many datasets; thus values from the surface as well as from deeper into thecase were considered. Figure II displays the absolute value of thecorrelation coefficient of each variable with endurance limit.Absolute values were used so the strengths of the relationshipscould be easily compared. The plot shows that surface residualstress and prior austenite grain size in the case exhibit the strongestrelationships with endurance limit.

In the statistical design of an experiment to measure theimpact of a variable, one would select values for that variable thatwould span an appreciable range to insure the variable waseffectively tested. In the present study, however, the analysis wasperformed within the confines of test conditions having beenalready established. This non-ideal experiment setup wasmonitored to insure false conclusions were not drawn. Forexample, intergranular oxidation data do not vary significantly.Almost all of the surface oxides were found to be about 10 ~mdeep, and thus the oxidation correlation results should not beapplied to a wide range of depths. Conversely, case grain sizevaried to a large degree, from 4 ~m to 18 ~m, across the total dataset. Specimen characteristics that varied little include oxidation

~ 1500Q.

~.~ 1400E

:::i0)Uc: 1300CIS...::I

"0c:

W 1200 ,

1100

0.4 0.6 0.8 1.2

Case Carbon Content (wt. pct.)

(c)

1.4

Figure 9. The effect of carbon potential on (a) retainedaustenite, (b) surface residual stress, and (c) the bendingfatigue endurance limit of a carburized SAE 4320 steel.

1157

depth, retained austenite, the surface roughness parameters,and to a certain extent residual stress.

type dependence on grain size was chosen because of itssignificance in materials strengthening. The negative sign on theresidual stress coefficient is due to the convention used in thiswork that defines compressive stresses as negative.

Figure 12 is a graphical representation of the regressionmodel detailed in Table 1. The figure is a contour plot showinglines of constant endurance limit as a function of case graindiameter and surface residual stress. Moving from the top-right tothe bottom-left of the plot, case grains become finer and residualstresses more compressive, yielding higher endurance limits. Themultiple regression equation is only valid over the range in whichthe variables were tested. Consequently, the plot should only bereferenced for case grain diameters between 4 and 18 I.1In andresidual stresses between approximately O and 1500 MPa incompression. Within this region, the standard error of the model is121 MPa. Figure 13 compares the experimental data to valuespredicted by the model.

IEnduranceLimit(MPa)= 750+ 1352x (Case Grain Diarn.()Jm»)-2

-0.19 x (Surface Residual Stress (MPa»

(2)0 0.2 0.8 10.4 0.6

Correlation Coefficient, I r I

Figure 11. Correlations of measured specimen characteristicsto bending fatigue endurance limit. Table I: Multiple Regression Model Predicting Bending Fatigue

Endurance Limit

Multiple Regression Model for Endurance Limit. Amultiple regression analysis was used to describe the bendingfatigue endurance limit by an equation of the general form:

Endurance Limit = Po + PlV. + P2V2 + P3V3 +... (I)

Determiners of Grain Size and Residual Stress. In aneffort to guide the design of alloys to achieve fine grains andhighly compressive residual stresses, correlation calculations wereperfonned to determine which specimen characteristics moststrongly affect the two variables of interest. It was necessary toomit shot peened alloys from the calculations to obtain meaningfulresidual stress correlations. Shot peening creates high compressiveresidual stresses that exaggerate the effects of other variables thatmay have just happened to have extreme values in shot peenedstudies. Case grain size correlations encompass all data, includingshot peened alloys. Figure 14 shows the results of the correlationcalculations, limited to the variables that had the strongestcorrelations. Significant correlations to case grain size are sharedwith core grain size and several alloying additions. Residual stresscorrelations show the effects of several elements as well as surfacehardness, core grain size, and case depth.

where the coefficients are constants and Vi, i=I,2,3.., representthe independent variables. Three statistical measurementswere used to choose the combination of variables for themodel: the multiple coefficient of error (R1, the F-statistic,and the P-value. R2 is a measure of how well a model fits a setof data. The F-statistic is a test of the hypothesis that at leastone of the regression coefficients in Eq. I is not zero, or is adetermination of whether the model is more useful than nomodel at all. The P-value is derived from the two-tailed t-distribution. It is the probability that a regression coefficientcan be considered statistically equivalent to zero. Insummary, the goal of the regression analysis is to create amodel that maximizes the R2 and F-statistic while minimizingthe P-values for the independent variables.

A regression model to describe bending fatigueendurance limit was created by starting with a very complexequation containing all the specimen characteristics andsubsequently simplifying the model by eliminating variableswith large P-values. With the reduction in number ofvariables, the fit to the experimental data was less accurate,yet overall confidence in the regression coefficients increased.The most appropriate model seems to be one that containsonly the effects of case grain size and surface residual stress.The details of the model are shown in Eq. 2 and Table I.While all functional relationships between endurance limitand grain size produced equally good results, a Hall-Petch

1158

correlations. The lack of any strong relationships to case grainsize reflects the dependence of grain size on variables other thanmicrostructure and chemistry .Many reheat treatments thatproduce fine grain sizes were part of this study. and they can easilyovershadow the effects of other variables. The carbon potentialstudy reviewed previously found that case grain sizes werereduced nearly in half from a diameter of9.4 ~ to 4.8 ~m when areheat step was added.

Core grain size

-Aluminum

.Manganese

Nickel

Silicon

Case GrainSize

0~(/)

..:Q)u(\1'-(\1

~uc:Q)E

.(3Q)0.

cn

I Sulfur

Surface Hardness

j Phosphorous

~ Core grain size

I Case Depth

Surface

Residual

Stress

Figure 12. Graphical representation of regression model inTable I depicting constant endurance limit contours as afunction of case grain size and surface residual stress.

0.80 0.2 0.4 0.6

Correlation Coefficient, I r

Figure 14. Correlation of specimen characteristics to case grainsize and surface residual stress.

The residual stress calculations revealed a few relationshipsthat may be significant. The levels of sulfur and phosphorous aremost likely not determiners of residual stress despite the relativelyhigh correlation coefficients for these elements. The surfacehardness relationship may be indicative of a more completemartensitic transformation upon quenching, which would beexpected to influence the residual stress state. The depth of thecarburized case, also, would be expected to impact the stress stateat the surface. Like grain size. residual stress was found to dependprimarily on processing, not chemistry or microstructuralcharacteristics. In all cases, shot peening dramatically increasedcompressive residual stresses from values of 100 MPa in unpeenedspecimens to 1100-1500 MPa after peening.

Figure 13. Comparison between experimental endurancelimits and model predictions.

Perspectives on the Statistical Analysis. The statisticalanalysis consolidated data taken over a decade of work by severalresearchers on many different alloy compositions. Despite themany sources of variation inherent with this experimental setup, afew very significant determiners of bending fatigue endurancelimit emerged: case grain size and surface residual stress. A fewfactors that are known to affect fatigue performance were notexposed in the broad study of the fatigue data. For example, thedetrimental effects of the impurity elements, sulfur andphosphorous, discussed earlier were not revealed in this analysis.Both bulk S and p varied little in the total data set. Similarly, thestatistical analysis did not reveal retained austenite as a significant

The case grain size correlations show some interestingrelationships. The high correlation with core grain size isexpected, because those factors that promote a fine case grainsize usually affect the core similarly. A cause and effectrelationship between case and core grain size is most likelynot present. The second strongest correlation to case grainsize is aluminum content. In light of the grain refining natureof aluminum nitride, the correlation is expected. Nitrogendoes not show a strong correlation to case grain size; however,nitrogen levels were fairly consistent across the researchprojects and thus would not be expected to show significant

1159

means to significantly increase compressive surface residual

stresses.

factor in fatigue, although an austenite effect is well known.The relatively constant carburizing conditions used throughoutthe studies confined retained austenite contents to a narrowrange, between approximately 20 and 25 volume percent.This lack of variation prevented an accurate assessment of theimpact of retained austenite.

Acknowledgements

This project, and the research reviewed in this work, wassupported by the Advanced Steel Processing and ProductsResearch Center, an industry/university cooperative researchcenter at the Colorado School of Mines in Golden, CO. Theauthors would like to acknowledge all the researchers thatcontributed to the work presented here: I.L. Pacheco, K.A. Erven,R.E. Cohen, I.A. Sanders, D.I. Medlin, R.S. Hyde, I. Kristan, andB.E. Cornelissen.

Conclusions

The following conclusions are based upon data takenover a decade of work by several researchers on manydifferent alloy compositions. It has been shown thatsignificant gains in the fatigue performance of gas-carburizedgear steels can be realized through proper control of alloychemistry and processing. Significant results are as follows:

References1. Elevated sulfur levels produced a high density of MnSinclusions. The MnS inclusions act as stress concentratorsthat facilitate fatigue crack initiation. As bulk sulfur contentin a carburized SAE 8219-type steel increased from 0.006 to0.029 weight percent. the endurance limit dropped from 1260to 1070 MPa.

J K.A. Erven, D.K. Matlock, and G. Krauss, "Effect of Sulfur on

Bending Fatigue of Carburized Steel", Journal of Heat Treating,vol. 9, no.1, 1991, pp. 27-35.

2 R.S. Hyde, D.K. Matlock, and G. Krauss, "Quench

Embrittlement: Intergranular Fracture Due to Cementite andPhosphorous in Quenched Carbon and Alloy Steels", Proceedingsof the 40th Mechanical Working and Steel Processing Conference,

pp.921-928.

2. Phosphorous strongly promotes intergranular fatiguecrack initiation through its stabilizing effect on cementite.When phosphorous levels exceeded 0.017 weight percent in acarburized SAE 4320 steel, the endurance limit droppedsharply. An increase in P from 0.017 to 0.031 weight percentreduced endurance limits from 1075 to 875 MPa.

3 J.L. Pacheco and G. Krauss, "Microstructure and High Bending

Fatigue Strength in Carburized Steel," Journal of Heat Treating,vol. 7, no.2, 1989, pp. 77-86.

3. Intergranular oxidation, associated with gas carburizing,was effectively controlled by reducing the levels of highoxide-potential elements. Reductions in silicon in a low-CrSAE 4320 steel resulted in smaller oxide depths and higherendurance limits. Reductions in manganese improved fatigueperformance of the low-Si alloy to a lesser degree, primarilyby reducing the extent of oxide coverage on austenite grainboundaries.

4 R.S. Hyde, G. Krauss, and D.K. Matlock, "The Effect of Reheat

Treatments on Fatigue and Fracture of Carburized Steels,'. SAEReport No.940788, Society of Automotive Engineers,Warrendale, PA, 1994.

5 R.S. Hyde, R.E. Cohen, D.K. Matlock, and G. Krauss, "Bending

Fatigue Crack Characterization and Fracture Toughness of GasCarburized SAE 4320 Steel," SAE Report No.920534, Society ofAutomotive Engineers, Warrendale, PA, 1992.4. Reductions in case carbon content, through lower gas

carburizing potentials and the use of reheat treatments,resulted in less retained austenite and higher compressiveresidual stresses, leading to improvements in the endurancelimit of a carburized SAE 4320 steel.

6 N. Bandyopadhyay and C.J. McMahon, Jr., "The Mico-

Mechanisms of Tempered Martensite Embrittlement in 4340- TypeSteels", Metallurgical Transactions A, Vol. 14A, 1983, pp. 1313-1325.

5. Shot peening significantly enhanced compressive surfaceresidual stresses from 100 MPa to over 1000 MPa in severalcarburizing steel grades. As a result, bending fatigueendurance limits of 1400 to 1500 MPa were achieved, 250-380 MPa higher than those of unpeened specimens.

7 Mo Gutmann, Po Dumoulin, and M. Wayman, "The

Thermodynamics of Interactive Co-segregation of Phosphorousand Alloying Elements in Iron and Temper-Brittle Steels",Metallurgical Transactions A, Vol. l3A, 1982, pp. 1693-1711.

6. A statistical analysis of the entire set of bending fatiguedata identified prior austenite grain size in the case andsurface residual stress as the most important factors thatdetermine endurance limit. A multiple regression model topredict endurance limit achieved an R2 value of 0.56. Casegrain refinement was most effectively achieved through reheattreatments, while shot peening was found to be the primary

8 W.E. Dowling Jr., W.T. Donlon, W.B. Coppel, R.A.

Chernenkoff, and C. v. Darragh, "Bending Fatigue Behavior ofCarburized Gear Steels: Four-Point Bend Test Development andEvaluation", SAE Report No.960977 , Society of AutomotiveEngineers Inc., Warrendale, Pa., 1996.

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9 B.E. Cornelissen, "Alloying and Processing Approaches to

Improved Bending Fatigue Performance of Carburized GearSteels", Ph.D. Thesis, Colorado School of Mines, Golden,Colorado, June, 1999.

10 M.A. Zaccone. J.B. Kelly. and G. Krauss. "Stain Hardening

and Fatigue of Simulated Case Microstructures in CarburizedSteel... Carburizing: Processing and Performance. ASMInternational. Lakewood. Colorado. July 12-14. 1989, pp.249-265.

11 D.J. Medlin, B.E. Comelissen, D.K. Matlock, G. Krauss,

and R.J. Filar, "Effect of Thermal Treatments and CarbonPotential on Bending Fatigue Performance of SAE 4320 GearSteel", SAE Report No.1999-01-0603, Society of AutomotiveEngineers Inc., Warrendale, Pa., 1999.

12 J .A. Sanders, "The Effects of Shot Peening on the Bending

Fatigue Behavior of a Carburized SAE 4320 Steel", M.S.Thesis, Colorado School of Mines, Golden, Colorado, August,1993.

13 D.J. Medlin, "Bending Fatigue of Carburized and Shot

Peened Alloy Steels", Confidential Advanced SteelProcessing and Products Research Center Report, ColoradoSchool of Mines, December, 1996.

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