AISI 52   100, 440C, REX20, and Crucible CRU80 steel samples were exposed to 16   different heat treatments to vary the levels of retained austenite. Rockwell   C hardness measurements, optical microscopy, and compression testing were   used to compare the properties of the different steels

Clemons, K.

Es-Said, Omar S.

Lorraine, C.

Ogren, J.

Salgado, G.

Taylor, A.

Umin, P.

English

Loyola Marymount University

Digital Commons@Loyola Marymount Universityand Loyola Law SchoolMechanical Engineering Faculty Works Mechanical Engineering10-1-2007Effects of Heat Treatments on Steels for BearingApplicationsK. ClemonsC. LorraineG. SalgadoA. TaylorJ. OgrenSee next page for additional authorsThis Article is brought to you for free and open access by the Mechanical Engineering at Digital Commons @ Loyola Marymount University andLoyola Law School. It has been accepted for inclusion in Mechanical Engineering Faculty Works by an authorized administrator of DigitalCommons@Loyola Marymount University and Loyola Law School. For more information, please contact digitalcommons@lmu.edu.Repository CitationClemons, K.; Lorraine, C.; Salgado, G.; Taylor, A.; Ogren, J.; Umin, P.; and Es-Said, Omar S., "Effects of Heat Treatments on Steels forBearing Applications" (2007). Mechanical Engineering Faculty Works. 2.http://digitalcommons.lmu.edu/mech_fac/2Recommended CitationOrtiz, D., Abdelshehid, M., Dalton, R., Soltero, J., Clark, R., Hahn, M., Lee, E., Lightell, W., Pregger, B., Ogren, J., Stoyanov, P., and Es-Said, O. S., 2007, "Effect of Cold Work on the Tensile Properties of 6061, 2024, and 7075 Al Alloys," Journal of Materials Engineeringand Performance, 16(5), pp. 515-520.AuthorsK. Clemons, C. Lorraine, G. Salgado, A. Taylor, J. Ogren, P. Umin, and Omar S. Es-SaidThis article is available at Digital Commons@Loyola Marymount Universityand Loyola Law School: http://digitalcommons.lmu.edu/mech_fac/2Effects of Heat Treatments on Steels for BearingApplicationsK. Clemons, C. Lorraine, G. Salgado, A. Taylor, J. Ogren, P. Umin, and O.S. Es-Said(Submitted July 31, 2006; in revised form August 29, 2006)AISI 52 100, 440C, REX20, and Crucible CRU80 steel samples were exposed to 16 different heat treatmentsto vary the levels of retained austenite. Rockwell C hardness measurements, optical microscopy, andcompression testing were used to compare the properties of the different steels.Keywords advanced steels, bearing applications, compressionstrength, heat treatments1. IntroductionHistorically, 52100 and 440C steels have been widely usedfor bearings in space systems for the last four decades. TheAISI 52100 is a high-carbon chromium alloy steel developedspecifically for the use of ball bearings. Its high resistance towear and plastic deformation, without fracture, are what makethis steel a high-quality choice for ball bearing applications(Ref 1). The 440C is a high-carbon martensitic stainless alloythat displays toughness and excellent corrosion resistance (Ref2). 52100 is widely used in many space systems because of itshigher strength and hardness compared to 440C, the latter isused where corrosion can be a concern.Recently, hybrid bearings consisting of silicon nitride ballsand steel raceways have shown improved fatigue properties.New class of bearing steels harder than 52100 is needed inhybrid bearing systems in order to take advantage of increasedfatigue properties without reduction in the load bearingcapacity. The load bearing capacity of the hybrid bearingsystems is reduced compared to the all-steel bearing systems ofthe same size due to the higher modulus of the silicon nitrideballs (Ref 3). These ball bearings will be providing a rollingsurface for the shaft of the turbo pumps that move the liquidoxygen and liquid hydrogen to the shuttles main engine(Ref 4, 5).The REX20 or AISI M62 is a cobalt free, super high-speedtool steel that can only be made through a powder metallurgyprocess (Ref 3). REX20 and its vacuum processed version calledVIM CRU20 have emerged as replacements for 52100 withsubstantially higher hardness, and hybrid bearing systems withsilicon nitride balls and CRU20 raceway are under developmentfor space applications (Ref 3). More recently, PM processedCRU80 steel was developed as a potential replacement of 440C,with increased hardness over 440C while maintaining similarcorrosion resistance. The CRU80 is a new steel consisting ofonly four elements, and is still undergoing experimentation, butdisplays many favorable mechanical properties. REX20 andCRU80 gain their strength mainly from the martensite transfor-mation of the matrix, but fine distribution of carbides alsocontribute to their strength. Carbides also give good wearresistance. All of these steels contain a large percentage ofcarbon, to have enough carbon in the steel matrix for strongmartensite formation even after forming primary carbides ataustentization temperatures. These high-carbon steels tend topossess more of the desired properties pertaining to applicationsof ball bearings (Ref 3).In this study, the processing-structure-property relationshipsare studied in the new bearing steels (REX20 and CRU80) andare compared to the baseline 52100 and 440C steels. Eventhough the hardness values are widely used as a qualitativemeasure of the load capacity of bearings, it is shown that thecompressive yield strength is a better indication of the loadcapacity, and that the amount of retained austenite affects theload capacity (Ref 3). For each steel, four different heattreatment schedules were developed to produce differentmicrostructures with varying retained austenite contents tostudy the interrelationships of hardness, compressive yieldstrength, and the retained austenite.2. Experimental ProcedureThe as-received 52100, 440C, REX 20, and CRU 80 havethe nominal compositions shown in Table 1.The bars were cut in 0.635 cms (0.25-inch) thick discs. Atotal of 48 samples were received, 12 for each steel. Smallbaskets of 316 stainless steel wire mesh were used as a sampleholder during heat treatments.A schedule of 16 different heat-treatments were performed,4 heat treatments per steel (Table 2). Heat-treatments consistedof preheating samples at 843 C (1550 F) using a SybronThermolyne 30400 furnace, and then austenitizing samples in aKeith furnace at temperatures of 843-1215 C (1550-2220 F);immediately followed by warm high-speed oil quenching at48.9 C (120 F). Pre-heating was done to ramp the temper-ature of the samples prior to austenitizing and to preventthermal shock. One heat treatment per steel was carried outK. Clemons, C. Lorraine, G. Salgado, A. Taylor, J. Ogren, P. Umin,and O.S. Es-Said, Mechanical Engineering Department, LoyolaMarymount University, Los Angeles, CA 90045-8145. Contacte-mail: oessaid@lmu.edu.JMEPEG (2007) 16:592–596 ASM InternationalDOI: 10.1007/s11665-007-9075-6 1059-9495/$19.00592—Volume 16(5) October 2007 Journal of Materials Engineering and Performancewith cryogenic quenching at -48.9 C (-120 F) immediatelyafter oil quenching to increase the formation of martensite(Treatment 1). The cryogenic quench was prepared by sub-merging dry ice into ethanol. The final step was tempering, itwas conducted at temperatures ranging from 135 to 537 C(275-1000 F). A Thermolyne Oven Series 9000 was used fortempering samples with low temperatures and the Barnsted/Thermolyne 30400 furnace was used to temper samples thathad to reach high temperatures. 52100 and 440C steels weredouble tempered, which means that the samples were temperedand taken out of the oven, air cooled, and tempered again. REX20 and CRU 80 were triple tempered.After the heat-treatments, the samples were ground, pol-ished, and etched in accord with standard metallographicprocedures (Ref 8). An Olympus PME3 microscope was usedfor measuring the depth of decarburization, and for studying themicrostructure. Rockwell C hardness measurements were alsoperformed. Compression testing of 0.5 inches diameter spec-imens was performed in accordance with ASTM standard E 9-89a (Ref 9).3. Results and DiscussionOptical microscopy, hardness measurements, and compres-sion strength tests were performed on all of the test specimens.The results were as follows:3.1 DecarburizationAn effect of the heat treatments is the formation of adecarburized layer during austenitizing because the heattreatments were performed in an air furnace. The carbonallows the steel to be hardened by forming martensite oncooling. Strength of the martensite phase of the steel dependson the carbon content in the martensite. During heat treatments,the thermodynamic chemical potential of carbon in the ambientenvironment may not be the same as that of the carbon contentin the steel. The result is the diffusion of carbon from the steelto the furnace atmosphere. The surface of the steel is thendecarburized making it much softer because the differentcarbon carbides are not able to form because of the low carboncontent (Ref 3, 5).Measurements of the depth of the decarburized layers weretaken from samples which undergone heat treatment (H.T) #3.Figure 1 shows the decarburized layers depth for 52100 and440C, while Fig. 2 shows them for REX20 and CRU80. TheREX20 had the largest decarburized layer and 440C had thesmallest decarburized layer. The austentization temperature forREX20 was the highest and for 440C was the lowest. Thisheat treatment was performed in air and accounts for the depthof the decarburized layers. The numerical values of thedecarburized depths for the test specimens are presented inTable 3.The difference between the greatest depth, REX20, and theleast depth, 440C, is 175lm or 4.5 times greater.Table 1 Chemical compositions of AISI 52100 (Ref 2), 440C (Ref 2), REX20 (Ref 6), CRU80 (Ref 7)Type of steelChemical % composition, wt.%C Cr Mn Si P S Mo V WAISI 52100 0.98-1.10 1.30-1.60 0.25-.45 0.15-0.35 0.025 max 0.025 max ... ... ...440C 0.95-1.20 16-18 1.0 max 1.0 max 0.04 max 0.03 max 0.75 max ... ...CPM REX20 1.3 3.75 0.35 0.25 ... 0.06 10.5 2 6.25Crucible CRU80 2.35 14 ... ... ... ... 1 9 ...Table 2 Heat-treatment schedules for 52100, 440C, REX 20, and CRU 80Alloy Treatment Pre-heat/time AustenitizationWarm oilquenching(O.Q.)CryogenicquenchTemperingstep. (T.S.) # of T.S.52100 1 ... 1550 F (843 C)/30 min 120-130 F(49-55 C)-120 F(-49 C)/1 HR325 F (163 C)/2 HR 252100 2 ... 1550 F (843 C)/30 min O.Q. ... 325 F (163 C)/2 HR 252100 3 ... 1650 F (899 C)/30 min O.Q. ... 325 F (163 C)/2 HR 252100 4 ... 1650 F (899 C)/30 min O.Q. ... 275 F (135 C)/2 HR 2440C 1 1550 F (843 C)/30 min 1950 F (1065 C)/15 min 120-130 F(49-55 C)-120 F(-49 C)/1 HR350 F (177 C)/2 HR 2440C 2 1550 F (843 C)/30 min 1950 F (1065 C)/15 min O.Q. ... 350 F (177 C)/2 HR 2440C 3 1550 F (843 C)/30 min 2050 F (1121 C)/15 min O.Q. ... 350 F (177 C)/2 HR 2440C 4 1550 F (843 C)/30 min 2050 F (1121 C)/15 min O.Q. ... 275 F (135 C)/2 HR 2REX20 1 1550 F (843 C)/30 min 2175 F (1190 C)/15 min 120-130 F(49-55 C)-120 F(-49 C)/1 HR1000 F (537 C)/2 HR 3REX20 2 1550 F (843 C)/30 min 2175 F (1190 C)/15 min O.Q. ... 1000 F (537 C)/2 HR 3REX20 3 1550 F (843 C)/30 min 2220 F (1215 C)/15 min O.Q. ... 1000 F (537 C)/2 HR 3REX20 4 1550 F (843 C)/30 min 2220 F (1215 C)/15 min O.Q. ... 800 F (427 C)/2 HR 3CRU80 1 1550 F (843 C)/30 min 2100 F (1149 C)/15 min 120-130 F(49-55 C)-120 F(-49 C)/1 HR975 F (524 C)/2 HR 3CRU80 2 1550 F (843 C)/30 min 2100 F (1149 C)/15 min O.Q. ... 975 F (524 C)/2 HR 3CRU80 3 1550 F (843 C)/30 min 2175 F (1190 C)/15 min O.Q. ... 975 F (524 C)/2 HR 3CRU80 4 1550 F (843 C)/30 min 2175 F (1190 C)/15 min O.Q. ... 500 F (260 C)/2 HR 3Journal of Materials Engineering and Performance Volume 16(5) October 2007—593Steel 440C had a lower decarburized depth compared to thatof 52100. It was expected to find a larger decarburized layer inCRU80 because it had the largest amount of carbon content.However, the results showed that REX20 had the largestdecarburized depth. A possible reason for such a largedecarburizated layer in REX20 may be explained by thealloying composition of the metal. REX20 showed to havesignificant amounts of alloying elements such as molybdenum,vanadium, and tungsten which might enhance the activity ofcarbon. Alternately the high content of chromium in CRU80might explain the decrease of activity of carbon. The activity ofcarbon is decreased by the presence of chromium. In the case ofhigh carbon contents and low-alloy chromium steels, carbidesprecipitate which contain chromium like (Fe, Cr)3C) and (Cr,Fe)7C3). After tempering, the chromium content of the carbidesincrease and the amount of cementite decreases until at aminimum (Cr, Fe)7C3 remains as the only stable precipitate.Thus the activity and solubility of carbon are decreased in steelscontaining chromium (Ref 10).3.2 HardnessHardness measurements were conducted on the specimensto compare their values with the standard industrial baselinehardness values. Hardness values were also taken after heatFig. 1 Decarburized layer of 52100 (left) and 440C (right) at 200 · magnification under heat treatment 3. The decarburized layer is identifiedas the thin light region located 1/4 (right to left) of the photograph. 52100 had a depth of 75 lm (3 mil) and 440C had a depth of 50lm (2 mil)Fig. 2 Decarburized layer of REX20 (left) and CRU80 (right) at 200 · magnification under heat treatment 3. The decarburized layer is identi-fied as the light region located 1/4 (right to left) of the photograph. REX had a depth of 225lm (9 mil) and CRU80 had a depth of 125lm(5 mil)Table 3 Decarburization depths of test specimensSample Decarburization depth (lm)52100 75440C 50CRU 80 125REX20 225594—Volume 16(5) October 2007 Journal of Materials Engineering and Performancetreatments to study the effect of varying the heat treatments andamount of retained austenite on the mechanical properties.Table 4 shows both the standard hardness values and theexperimental hardness values.It was observed that some of the experimental valuesresulting from heat treatments 2-4 of the 440C, REX20 andCRU 80 had slightly lower values than that of the standardhardness values. Overall, the hardness values of all steels underheat treatment #1 were within the standard hardness. For heattreatment #2, 440C and REX20 exhibited lower hardnessvalues than the baseline hardness values. For heat treatment #3,only the 440C had hardness values that were less than thebaseline hardness values. Heat treatment #4 produced a lowerhardness values for REX20 and CRU80 when compared to theother heat treatments. In every heat treatment other than #4,REX20 attained the highest hardness of all the steels.3.3 Compression TestingThe results of the compression testing are shown in Table 5and are compared to the hardness values. From the result of thecompression testing, REX20 had the highest hardness andcompressive yield strength and flow stress than all other alloysfor the same heat treatment except for heat treatment #4.Although the results for the 52100 alloy were lower than theREX20 for most heat treatments, the hardness, compressive andflow stress values were still comparable. It is interesting to notefrom the 52100 alloy that different microstructures (differentpercent of retained austenite) can result in similar hardnessnumbers, yet have different yield strengths and load capacity.Thus, as pointed out in a previous study (Ref 3) traditionalpractice of using hardness values for estimating bearing loadcapacity is not valid.Table 4 Rockwell ‘‘C’’ hardness values comparison ofthe industrial baseline to the experimental averageobtained in the researchIndustrial baseline valuesSpecimen52100(Ref 2)440C(Ref 2)REX20(Ref 6)CRU80(Ref 7)HRC range 60-66 58-65 66-68 61-63Experimental averaged valuesH.T # 52100 440C REX20 CRU801 64 59 66 642 62 57 65 613 64 51 66 624 63 54 62 59Table 5 Compression data for the different steelsSteel Heat treatment Hardness (HRC) Compressive YS Mpa (Ksi) Maximum stress Mpa (ksi) (Flow stress at 5% strain)52100 1 64 2890 (419) 3620 (525)52100 2 62 ... ...52100 3 64 2553 (370) 3475 (504)52100 4 63 2574 (373) 3688 (535)440C 1 59 2265 (329) 3153 (457)440C 2 57 ... ...440C 3 51 731 (106) 1380 (200)440C 4 54 659 (96) 1471 (213)REX20 1 66 3445 (500) 4103 (595)REX20 2 65 ... ...REX20 3 66 3075 (446) 4337 (629)REX20 4 62 1360 (197) 2373 (344)CRU80 1 64 2319 (336) 3113 (452)CRU80 2 61 ... ...CRU80 3 62 2413 (350) 3310 (480)CRU80 4 59 1818 (264) 2650 (384)Fig. 3 Optical microstructures of Rex 20 and CRU 80Journal of Materials Engineering and Performance Volume 16(5) October 2007—5953.4 MicrostructureThe microstructure, Fig. 3, of CRU80 in heat treatments 1 to3 is compared to each other and to that of REX20 (heattreatments). It is noted that the microstructures at a magnifi-cation of 1000· are similar.Chiefly the austenitization temperature was raised to dissolvemore carbon into solution. The carbon stabilizes the retainedaustenite. The properties were supposed to be correlated tomicrostructure. Unfortunately optical microscopy did not revealclear differences between the different heat treatments in an alloysystem or between different alloys under the same heat treatment,Fig. 3. Accordingly a correlation between microstructure, hard-ness and compression testing values was not obtained.4. ConclusionsWithin the limitations of this experiment it was concluded:1. That the heat treatments performed had significant effectson the mechanical properties of the steels. The REX20had the highest decarburization layer under the sameenvironment as the other steels. 440C showed to have athinnest amount of decarburization layer.2. For hardness measurements, the REX20 had overall the high-est hardness values while 440C had the lowest hardness val-ues. Heat treatment #1 had generally the highest hardnessvalues while heat treatment #4 had the lowest hardness values.3. This study confirms previous studies that the traditionalpractice of using hardness values alone for estimatingbearing load capacity in steels is not valid. Compressiontests should be used.4. A correlation between microstructure hardness and com-pressive strength values was not obtained in this study.AcknowledgmentsThe authors thank the National Science Foundation (NSF),Grant No. EEC0353668. Ms. Esther Bolding is the programmanager. The authors thank Ms. Maria Guerrero and Mr. Salim Es-Said for their help in the preparation of the manuscript. Drs.Michael OBrien and Woonsup Park from Aerospace Corporationare gratefully acknowledged for supplying the materials, suggest-ing the topic, and performing the compression testing.References1. General Bearing Corporation. http://www.generalbearing.com/JGBR/specs.htm. West Nyack, New York2. New England Miniature Ball Corporation (NEMB). http://www.nemb.com3. W. Park, M.R. Hilton, and A.R. Leveille, Microstructure, Fatigue Lifeand Load Capacity of PM Tool Steel Rex20 for Bearing Applications,Lubric. Eng., 1999, 55(6), p 20–304. M.J. OBrien, M. Pressure, and E.Y. Robinson, Failure analysis of threeSi3N4 balls used in hybrid bearings, Eng. Failure Anal., 2003, 10,p 453–4735. Eastern Tool Steel Services Inc. http://www.easterntoolsteel.com/hea_treatment_of_toolsteels.htm. St. Marys, Pennsylvania6. Crucible Service Center: Tool Steel and Specialty Alloy Selector http://www.crucibleservice.com7. B. Hann, P. Kilonsky, D. Smith, and M. Sperber, Wear and CorrosionResistant PM Tool Steels for Advanced Bearing Applications.Proceedings of the 35th Aerospace Mechanisms Symposium, AmesResearch Center, 20018. Petzowig. Metallographic Etching, Berlin-Stuttgart, American Societyfor Metals, 1978, p 61–689. ASTM Designation: E 9-89a (Reapproved 1995). Standard TestMethods of Compression Testing of Metallic Materials at RoomTemperature, 1995, p 99–10610. H. Erhart and J. Grabke, Equilibrium Segregation of phosphorous atgrain boundaries of Fe-P, Fe-C-P, Fe-Cr-P, and Fe-Cr-C-P alloys, Met.Sci., 1981, 5, p 401–408596—Volume 16(5) October 2007 Journal of Materials Engineering and Performance

Effects of Heat Treatments on Steels for Bearing Applications

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Effects of Heat Treatments on Steels for Bearing Applications

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