118 004 1THS THE EFFECT OF MARTEMP ERING ON THE ENDURANCE LIMITS OF TWO ALLOY MACHINE STEELS Thesis for the Degree of M. S. MICHIGAN STATE COLLEGE Samuel Mercer, Jr. 1950 THE EFFECT OF MARTEMPERING . ON . . THE ENDURANCE LIMITS OF TWO ALLOY MACHINE STEELS By SAMUEL MERCER, JR. A THESIS Submitted to the School of Greduate Studies of Michigan State College of Agriculture and Applied Science in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Mechanical Engineering 1950 THFGIF ACKNOWLEDGMENT The author wishes to express his gratitude to Professor Howard L. WOmochel for his guidance and assist- ance and to the many faculty staff members, employees and students of Michigan State College whose suggestions and aid were instrumental in the completion of this work. Rooms 11 11131;}; OF CONTENTS Introduction Theory of Martempering Purpose and ScOpe of Investigation Preparation of Specimens Heat Treatment Testing Procedure Discussion of Results Conclusions Bibliography and References 111 Page «luu—v 10 12 14 15 36 38 gmnonucnou The process called martempering was deve10ped by B. F. Shepherd and was described in the metallurgical literature for the first time early in 1943.1 Fundament- ally the treatment is an interrupted quenching procedure applied to the hardening of steels. As described by Shepherd, the first stage of the treatment is identical with normal steel hardening practice. The material is heated to some suitable temperature above the upper criti- cal and held at this temperature long enough to permit a reasonably complete and uniform austenitic structure to form. Where common practice consiSts of quenching from the austenitizing temperature directly into a cooling medium at approximately room temperature, martempering re- quires that the steel be cooled in two distinct steps. The first step in the cooling procedure for martempering consists of quenching the steel into some medium (usually molten salt) which is maintained at a temperature slightly above that at which martensite begins to form (the so-called Ms temperature.) The steel is held in this quenching medium until the temperature is approximately uniform throughout the entire mass of the piece (usually five minutes per inch 1. Shepherd, 3. F., Martempering, Iron Age, Jan. 26, 1943, Pp 50-52, Feb. 4, 1943, pp 45-45. of section.)2 The formation of martensite is then per- mitted to take place by removing the piece and allowing it to cool in any convenient manner to room temperature. The usual procedure is to cool the piece slowly in air. I In the past, the term martempering was used in con- nection with interrupted quenching procedures for which the quenching medium is held at about 400°F with no re- gard for whether this was below the MS temperature. This was followed by holding and cooling as previously des- cribed. At this time the term martempering is generally accepted to imply cooling quickly to the Ms temperature, holding until a uniform temperature is attained, then cooling to room temperature to form martensite. Hewever, practical considerations in cooling the steel and the present state of our knowledge of the temperatures at which the austenite to martensite reaction is initiated are such that a great many of the practical heat treat- ment procedures described as martempering processes pro- bably involve quenching into a medium being held at a temperature somewhat below the true Ms temperature.3 2. Three Keys to Satisfaction, Climax Molybdenum Co. 3. An examination of isothermal transformation diagrams for an SAE 4140 steel appearing in Republic Steel Corpora- tion's "Atlas of Isothermal Transformation Diagrams" and in Climax Molybdenum Company's book "Molybdenum Steels Iron Alloys" revealed that the diagrams are not in agree- ment. That-indicates that our knowledge of M3 temperatures is probably not reliable. And in addition to that fact, it is known that the austenitizing temperature chemical composition, prior austenite grain size and homogeneity, undissolved carbides and nonmetallic inclusions all exert an influence on the transformation temperatures. It is a fortunate circumstance that the percentage of transformation that occurs after cooling below the Ms temperature is usually small for the first rather large increments of temperature drOp. 4’5 Indications are that comparable advantages are realized from the procedure regardless of whether the quenching medium is slightly above or below the MB temperature. The one attractive advantage claimed by Shepherd for martempering over the customary steel hardening tech- nique was an appreciable improvement in the dimensional stability of the part being hardened. So many investi- gators have verified Shepherd's claim that it is accepted as fact. THEORY OF MARTEMPERING The explanation of the reduction in warping and quench cracking that is realized by using the martempering treat- ment lies in the time relations between the formation of martensite in the center and outside portions of the piece. Figures 1 and 2 illustrate these relations for martemper- ing and normal quenching practice. By quenching steel directly into some medium at room temperature, a large tem- perature gradient exists across the section of the piece ‘ 4. Boyer, Howard E., gge Interrupted Quench and Its Practi- gal Aspects, ASM Transactions, 19 7, Vol. 38, p. 209. 5. Archer, R. S., Briggs, J. 2., and Loeb, C. M., Jr., I‘Lolvdbenum Steels Irons Alloxg, New York, Climax Molydbenum Company, 1948, pp 19-34. TEMPERATURE TEMPERATURE 33v :13 “9 aiN3 F VELTRAMSFORMATION .Mk “— TIME '- LOGI $CALE FIGURE 1. Schematic Representation of Transfor- mation for Normal Hardening Practice. TRANSFORMATION \_, TIME - LOG SCALE. FIGURE 2. Schematic Representation of Transfor- mation for Martempering. -4- during the entire cooling period. As a result of this temperature gradient, martensite does not form simultan- eously throughout the piece, but forms at different times depending on its location. Since the outer layers of material have the more rapid cooling rate, martensite forms there while the core is still austenitic. In most cases the difference in cooling rates between the inner and outer fibers is so great that virtually complete transformation of the outer fibers to martensite has occur- red before the inner fibers have cooled to the M3. Thus, an extremely hard shell of martensite locks in the untrans- formed austenite at the core. Accompanying the austenite to martensite reaction is a decrease in the density of the transformation pro- duct. Since austenite is somewhat plastic, it cushions the shock of transformation and helps to avoid the build- ing up of high internal stresses. However, after the first stages of transformation in the conventional quench- Iing procedure a condition is reached such that all of the remaining austenite is located in the core of the piece. 'When this core of austenite cools below the Ms temperature, it attempts to expand as transformation occurs. But the hard, confining shell of martensite does not readily yield Plastically and extremely high internal stresses are creat- ed within the material. Unfortunately, the resulting 6 stresses on the outer fibers are tensile in sense and are manifested by warpage and quench cracking. In the martempering process the cooling proceeds rapidly at first with great disparity between interior and surface temperatures. But the coolant is at some temperature above the Ms; consequently, the cooling pro- cess is arrested long enough to permit the temperature to become equallized throughout the mass before any marten- site forms. Subsequent cooling is such that only a very small temperature differential exists between inner and outer fibers. Therefore, the formation of martensite occurs throughout the entire piece at virtually the same rate. By avoiding the formation of a non-yielding marten- sitic shell on the outside of the piece, expansion of the steel upon transformation is less restricted. The uncon- fined matrix of austenite is relatively plastic and cush- ions the transformation resulting in a lower intensity of internal stress and avoidance of warping and cracking. There are certain disadvantages attendant upon the use of the martempering treatment. All other things be- ing equal, you cannot obtain through martensitic hard- ness in as large a bar by martempering as you can by oil or water quenching because of the relative slowness of 6. Compressive stresses in the outer fibers could prove beneficial to fatigue strength. Shot peening, rolling, pressing an d tumbling are used to obtain superior fati- gue prOperties by inducing such beneficial surface com- pressive stresses. cooling rate in a hot bath. This disadvantage may be- come even more pronounced in the future in view of the fact that new data on MS temperatures places them at higher temperatures than previous data. In addition, a more closely controlled heat treating procedure requiring more expensive equipment and a more precise knowledge of the prOperties of the steel being treated are required than for the conventional hardening practice, However, in many applications, these disadvantages are outweighed by the prOSpect of lower spoilage and less finish machin- ing. And the literature gives hints of other advantages not fully exploited at this time. gunmen AND scorppr INVESTIGATION The manifold uses of metals for applications where high operating Speeds and low weight are requisites have focused attention on their fatigue properties. Many in- vestigations have been made and volumes have been written about the nature and causes of fatigue failures so that designers can now utilize this hard-won knowledge to pre- dict and avoid fatigue failures in service. It has been conclusively shown that the underlying cause of all fatigue failures is a highly unfavorable local stress condition which produces a minute rupture.7 7. Battelle Staff, Prevention of The Paglure of Metalg Under Repeated Stress, New York, John Wiley & Sons, Inc., 1941. - 7 - This first tiny fracture acts as stress raiser to further prOpogate the rupture with each application of the load. Ultimately, the rupture will have Spread, in the form of a crack, to such an extent that the portion of the piece still intact will be unable to support the load and gross failure occurs. Notches, scratches, abrupt changes in section tend to reduce the nominal stress at which this type of failure occurs, because high local stresses ac- company them. Where favorable stresses are induced and stress raisers avoided an improved fatigue resistance results. These facts logically lead to the conclusion that if two materials are alike in every respect ex- cept that one is martempered and the other quenched and tempered, the martempered Specimen will exhibit superior fatigue properties by virtue of having a less unfavorable distribution of residual stresses. The first work substantiating the premise that mar- tempering gave improved fatigue strength over the normal quenching procedure appeared in 1948.8 Forsythe and Carreker presented data on an SAE 1095 steel showing an improvement in endurance limit of over twenty-five per- cent resulting from martempering. These data demon- strated conclusively that martempering does enhance fati- gue prOperties. But in view of the fact that SAE 1095 is extremely sensitive to quench cracking and that 8. Forsythe, A. C. and Carreker, R. A., Fati ue Limit of S.A.E.,lg95 After Various Heat Treatmentg, Metal Progress, November, 1948, pp. 683-685. - 8 _ Forsythe and Carreker used a low draw temperature in their experiments, much doubt remained as to whether the improvement achieved would be appreciable for steels drawn to lower hardness values. It was felt that data on some alloy machine steels drawn at higher tempera- tures would be more Significant. For this study SAE 4140 and SAE 6140 were selected. The basis for choosing these was that both were on hand, and that they enjoyed reasonably wide pOpularity in applications where medium high strength and ductility were required. Information on these steels revealed them to have been used in the oil quenched and tempered condition for a great variety of applications where the tensile strengths were in the range 100,000 to 200,000 PSI.9 It was decided that the steels used would be drawn to a hardness of about Rockwell 350. This would result in a tensile strength well within the range of strengths commonly used. It was anticipated that the improvement in fatigue strength resulting from martempering in this study would be smaller than in Forsythe's and Carreker's study for two reasons: 1. The higher draw temperature would effect a greater relief of the residual stresses, and 2. The comparison would be made with an oil quenched steel instead of a water quenched steel. 9. U.S.S. Carrilol Steels Allol Steels for the Special Jobs of Industry, Carnegie-Illinois Steel 00., Pittsburgh and Chicago, 1948. - 9 - To determine whether the magnitude of the improvement at a low hardness level gave promise of making the more costly martempering treatment economically attractive was the objective of this work. PREPARATION OF SPECIMENS The SAE 4140 and SAE 6140 steels were obtained in the form of one-half inch round hot rolled bar stock. All specimens of like material were machined from the same bar. The specimens were machined all over before the heat treatment, but the tapered ends were left over- size to permit a post tempering finish grind. After heat treatment the test sections of the Specimens were polished in successive stages using graded abrasives through a 00 paper. The final polish was longitudinal using a soft cloth dipped in oil and a 600 grade alumi- num oxide abrasive powder. This series of polishing operations removed about three-thousandths of an inch from the diameter of the test section. It is held that not only the scratches due to machining, but that any cracks and sore Spots in the surface layers of mater- ial that might have resulted from the action of the quenching medium were removed by the polishing. The taper ends were then ground between centers using a tool post grinder so that the final form of the specimens was that illustrated in Figure 3. - 10 - zmguwan 2.9.2.”. Foam “Nam mumB. mace—2 .N.N_ / 028-... “Ewe .330 m NNDQWA HT I II1 n I 11!. W ”I A! ‘II T m N' I I «>33 I e 5.. 0Q? -11.. HEAT TREATMENT A quenching fixture like that shown in Figure 4 'was used for mounting all Specimens. Its use made it possible to quench all specimens simultaneously and to minimize distortion by holding them vertical during their entry into the quenching bath. A Leeds and Northrup, 15 KW, Vapo-Carb furnace was used for austenitizing all Specimens. The furnace was permitted to come up to llOOOF at which time the speci- mens were inserted. Upon placing the Specimens in the furnace the flow of cracked hydro-carbon fluid through the furnace was adjusted to produce an atmosphere pre- viously established as being inert with respect to the steel being used. One hour was required for the furnace to come to the austenitizing temperature of 15750F. Following a one hour soaking period, half of the speci- mens were quenched in oil. At the same time the re- maining half of the Specimens were cooled in a Hoskins electric salt pot being maintained at a temperature of 500°F. During cooling, the specimens were vigorously agitated in the cooling medium. The oil quenched speci- (mens were permitted to cool to room temperature in the oil. The other specimens were held in the 500°F salt for one minute and then removed and cooled in air. All specimens were drawn for one hour at approxi- mately 1100°F in an electric Lindberg draw furnace to a hardness of about Rockwell 35C. - 12 - h‘__~r//'-\-l_____—_ FIGURE 4. Quenching Fixture for Holding 18 Fatigue Specimens -13- After finishing the specimens as previously described, S-N curves were determined for each of the four groups of specimens. TESTING PROCEDURE A Baldwin-Southwark, R. R. Moore high speed rotat- ing beam fatigue testing machine was used to test each specimen. This machine produced a uniform bending moment on the Specimen while it rotated the Specimen at'a nor- mal Speed of 10,000 RPM. Because of variations in the supply line voltage, the speed of the machine was ob- served to vary between 9,000 RPM and 11,000 RPM during the progress of the tests. It is held that these speed variations were not sufficiently great to have material- ly affected the significance of the results obtained. Investigations by Oberg and Johnson,10 Krouse,1l and Roos, Lemmon, and Ransom12 on the effect of Speed in fatigue testing substantiate this point of view. After each test the hardness and microstructure of each specimen were observed to insure uniformity. The 10. Oberg, T. T. and Johnson, J, B., Fatigue Propertigg of Metals Used in Aircraft Construction at 3450 andAIOQQQ C cles, Proc. Am.Soc.Test.Mat., 1937, Vol. 37, part II, pp 195-205. 11. Krouse, G. H., A High Speed Fatigue Testing Machine and Some Tests of Spged Effect on Endurance Limit, Proc. Am.Soc.Test.Mat., 1934, Vol. 34, part II, pp 156-164. 12. Roos, P. K., Lemmon, D. C., and Ransom, J. T., Influ gnce of Type of Machine, Range of Speed,gand Specimen Shape on Fatigue Test Dgpg, Am. Soc. Test. Mat., Bulletin No. 158, May, 1949, pp 63-65. -14- average hardness values for each specimen are given in Table I. These represent the average of five hardness checks at selected points across the section of the piece. It was found that all pieces were hardened uni- formly throughout and that all specimens within any one group exhibited an average hardness of within 1.5 Rockwell C of the average of the entire group. The maxi- mum hardness variation observed for any one Specimen was 2 Rockwell C. Ekamination of the microstructures revealed a uniform, tempered martensitic structure probably con- taining some tempered bainite. There was no evidence of carburization or decarburization during heat treatment. Figures 5 to 8 are typical of the final structures that were obtained. DISCUSSION OF RESULTS As was anticipated, the endurance limits of the SAE 4140 and SAE 6140 steels in the martempered and drawn condition were greater than the endurance limits of the same steels in the oil quenched condition when drawn to the same hardness. The increase in endurance limit, shown in Figures 9 and 10, was approximately 3% and 13% for the SAE 4140 and SAE 6140 steels respectively. It may be argued that the difference in endurance limits found could be attributed to variations in hard- ness of the Specimens, since all specimens in the groups - 15 - SAE 4140 Oil Quenched & Drawn Martempered & Drawn Specimen Rockwell C Specimen Rockwell C Ngmpg; Hardness Eggpgg Hardness 4-1 35.0 4-11 35.0 4-2 35.0 4-12 34.5 4-3 34.5 4-13 36.0 4-4 34.5 4-14 36.0 4-5 34.5 4-15 36.0 4-6 35.5 4-16 35.5 4-7 35.0 4-17 35.0 4-8 34.5 4-18 35.5 Average 35.0 . Average 35.5 SAE 6149 Oil Quenched & Drawn Martempered & Drawn Specimen Rockwell C Specimen Rockwell C Ngmpgg Hardness Npgpgg Hardness 6-1 34.0 6-11 35.0 6-2 34.0 6-12 35.0 6-3 34.0 6-13 35.0 6-4 35.0 6-14 35.0 6-5 34.0 6-15 34.5 6-6 34.0 6-16 35.0 6-7 35.0 5-17 35.5 6-8 36.0 6-18 35.5 Average Average 35.0 34.5 TABLE I: Final Hardness Values for Fatigue Specimens FIGURE 5. Microstructure of SAE 4140 Treatment: Oil quench and tempered Hardness: Rockwell 35 C Magpification: 500 X - 17 - FIGURE 6. Microstructure of SAE 4140 Treatment: Quenched in salt at 500°F, held for one minute, air cooled, and tempered. Hardness: Rockwell 35.5C Magnification: 500 X -18- FIGURE 7. Microstructure of SAE 6140 Treatment: Oil quenched and tempered Hardness: Rockwell 35 C Magpification: 500 X - 19 - FIGURE 8. Microstructure of SAE 6140 Treatment: Quenched in salt at 500°F, held for one minute, air cooled, and tempered. Hardness: Rockwell 35 C Magpification: 500 X - 20 - 0mm HHoaMon mo mmoapnam om semen a poaapudm OQHQ mdm mom mo>hdo 20m 0 madam“ oesaesa oe nosoau eo .oz . z oH boa hoe mom I? o , spawn e noeoeoso ado I? ? Sacha w peaceavpumz emu awe. ems. / o: no 0 seapamomsoo om no OCH «CH OHH sseils - g Isd cost _ 211 - 0mm Haoaxoom Mo mmoappsm ow Semen d,po:ophmm cede mew How mo>hdo zsm 0H mmbon onfiawmm op moaoho mo Hones: I z boa eoa moa «as I/ AW JY/Iflr xiv l/nmml.eheam s noeoeoeo Hno U o / i o semen a pmnomsopnmamuwvy. ///// em am. we. 6> no 0 Soapamomeoo om mm om no OOH mOH OHH was ONH ssealg — s Isd ooor - 252 - that were compared did not exhibit exactly the same hard- nesses. However, in plotting the results the individual hardnesses were considered and the curves that seemed most representative in the light of this hardness data were drawn. Data found in Hoyt13 and in a recent issue of the SAE JournaIlattest to the probable correctness of these curves. In the former, the tensile strengths of SAE 4140 and SAE 6140, which have been oil quenched and drawn to a hardness of 35 Rockwell C are given as 176,000 PSI and 160,000 PSI respectively. Based on this data the endurance ratios of the oil quenched Specimens were both approximately 0.52, which is consistent with gener- ally accepted valuescf endurance ratio. In the latter reference, an S-N curve is Shown for an SAE 4140 steel which had been oil quenched and drawn to a hardness of 35 Rockwell C. The endurance limit of 90,500 PSI com- pares very favorably with the endurance limit reported in this paper. In view of these facts and the fact that the results were consistent and reasonable, the improve- ments in fatigue strengths found can undoubtedly be attri- buted to the lower thermal and transformational stresses that resulted from the martempering treatment. However, it would seem that the improvements in fatigue strengths l3. Hoyt, Samuel L., Metals and Alloys Data Book, Reinhold Publishing Corp., New York, 1943, p 77 and 87. 14. Boegehold, A. L., Sglection of Steel for Agtomobile Parts, Part IV, SAE Journal, Nov. 1949, pp39-48. _ 23 - would be more nearly alike percentagewise, since both types of steel were similarly treated to approximately the same final hardness. Some insight as to a possible explanation can be gained from the results of prelim- inary tests made on the steels. A series of preliminary experiments was performed in an effort to determine the salt temperature and hold- 4 ing time that was to be used in martempering the materials employed. Cylindrical Specimens one-half inch in diameter by one-half inch in height were used. Some were quenched in water, some in oil and some in molten salt. The salt -¥ quenched specimens were held for various times, ranging from a few seconds to five minutes, in molten salt at various temperatures, ranging from 700°F to 400°F, fol- lowed by cooling in air. Typical as quenched hardness values are shown in Table II. It was evident from a scrutiny of these data of the microStructures, and of the isothermal transformation diagrams available for these materials that the selection of a salt temperature to be used for martempering was a compromise. The use of higher temperatures yielded low hardness values, indicating that a partial isothermal transformation of austenite to bainite had occurred in the salt bath. The use of lower salt temperatures to avoid the formation of bainite made it very likely that considerable transformation of austenite to martensite had occurred before a uniform temperature in the sample - 24 - SALTAQUENCHED & AIR COOLED 600°F SALT 500°F SALT 400°? SApg 51:6 SAE SAE SAE SAE SAE SAE gag; glgg gggg 4140 6140 &;59 glgg 2 Sec. 310 320 320 .340 .400 400 5 Sec. 370 360 460 47c 480 50c. 15 Sec. 440 450 520 54c 53c 560 30 Sec. 510 510 53c 57c 53c 57C 1 Min. 460 48C 52c 57c 53c 57c 2 Min. 47C 48C 52C 57C 53C 570 3 Min. 47C 48C 520 56C 530 57c 5 Min. 480 480 510 54c 530 570 WATER QUENCHEQ SAE 4140 - 590 SAE 6140 - 610 OIL QUENCHED SAE 4140 - 550 SAE 6140 - 590 TABLE II: As-quenched Rockwell Hardness of Preliminary Test Specimens For Various Treatments. - 25 - had been obtained. This was undesirable Since it was the purpose of martempering to avoid such a condition. The other alternative was to use only a very short hold- ing time in the salt bath. Unfortunately, it was found that even with the small Specimens used, the cooling rate was not sufficiently great to avoid the knee of the S-curve unless a holding time of more than fifteen seconds was used. It was decided that the best compro- mise was a salt temperature of 500°F and a holding time of one minute. Typical as-hardened microstructures for the group of preliminary specimens quenched into 500°F salt are shown in Figures 11 through 18. Evidence presented by Holloman, Jaffe, McCarthy 15 indicated and Morton15 and by Forsyth and Carreker that tempered bainitic structures and mixed structures of tempered martensite and bainite exhibit lower endur- ance limits than does a tempered martensitic structure. (It is evident from the microstructures and hardness values in Figures 11 through 18 that neither the oil quenching or martempering treatment employed produced a structure that was free of bainite. This suggests that a smaller prOportion of bainite in the SAE 6140 than in the SAE 4140 might account for the disparity between relative 15. Hollomen, J. H., Jaffe, L. D., icCarthy, D. E., and Morton, M. R., The Effects of Microstructure on The Mechanical Properties of Steel, ASM Preprint 25, 1947. 16. Forsyth, A. C. and Carreker, R. P., Fatigue Limit of SAE 1095 After Various Heat Treatmentg, Metal Progress, Nov., 1948, pp 683-685. -26- FIGURE 11. Microstructure of SAE 4140 Treatmgpt: water quenched from 1575°F Hardness: Rockwell 59 C Magnification: 500 X Microstructure of SAE 4140 FIGURE 12. Oil quenched from 1575°F Treatment: Rockwell 55 C Hardness: 500 X ificat on: -28- FIGURE 13. Microstructure of SAE 4140 Treatment: Quenched from 1575°F into salt at 500°F, held for two seconds, and cooled in still air. Hardness: Rockwell 32 C Magpification: 500 X - 29 - FIGURE 14. Microstructure of SAE 4140 0 Treatment: Quenched from 1575 F into Salt at 500°F, held for five seconds and cooled in still air. Hardness: Rockwell 46 C Magnification: 500 X - 3o - FIGURE 15. Microstructure of SAE 4140 Treatment: Quenched from 1575°F into salt at 500°F, held for thirty seconds, and cooled in still air. Hardness: Rockwell 53 G Magpification: 500 X - 31 - FIGURE 16: Microstructure of SAE 4140 Treatment: Quenched from 1575°F into salt at 500°F, held for one minute and cooled in still air. Hardness: Rockwell 52 C Magnification: 500 X - 32 - FIGURE 17. Microstructure of SAE 4140 Treatment: Quenched from 1575°F into salt at 500°F, held for three minutes, and cooled in still air. Hardness: Rockwell 52 C Magnification: 500 X -33.. FIGURE 18. Microstructure of SAE 4140 Treatment: QueBched from 1575°F into salt at 500 F, held for five minutes, and cooled in still air. Hardness: Rockwell 51 C Magnification: 500 X -34.. improvements of fatigue strength effected by the mar- temper and draw treatment. Further credence is lent to this explanation by an examination of the aS-quenched hardness values for the two series of steels shown in Table II. Observe that for those Specimens quenched in salt at 500°F there is a wider variation in hardness between the water quenched and oil quenched, and between the oil quenched and salt quenched Specimens for the SAE 4140 group than for the SAE 6140 group. This implies that a greater proportion of tempered bainite was present ' an r' 5.; P'mfiM-n. . «amply n...- I v" in SAE 4140 specimens than was present in SAE 6140 fati- gue specimens. Unfortunately, it is difficult to dis- tinguish tempered lower bainite from tempered martensite. Because of that, no Supporting evidence could be gleaned from a microstructural examination of the fatigue Speci- mens. The presence of bainite, as evidenced by the darker appearance of the etched structures, in the as-hardened microstructures of all except the water quenched speci- mens was very eanperatihg. It was desired that the for- mation of bainite be avoided in view of the inferior mechanical prOperties attending its presence. To avoid bainite, the use of a steel of higher hardenability suggests itself as a logical solution. It happened that the series of preliminary tests included an SAE 3340 group of steel. But even with this steel of relatively high hardenability the oil quenched and salt quenched Specimens exhibited a lower hardness and a darker etch- ing microstructure than did the water quenched Specimens. The implication of these data is that only in pieces of gggz small cross section is it possible to avoid the formation of bainite by martempering or by oil quenching. CONCLUSIONS The primary objective of this work was to determine whether the improvement in fatigue strength realized by martempering a medium alloy machine steel over that re- , S‘s). . . If‘t. _ I. '. sulting from the customary oil quenching treatment was sufficiently great to give promise of proving economi- cally advantageous when the steel was tempered to a medium hardness. These experiments Show that for the steels used and the conditions stated there was an im- provement realized, and for the SAE 6140 the improvement was appreciable. Since these results were obtained with Specimens that were specially prepared to avoid stress concentrations and since stress concentrations are in- variably present in practical designs, it can be inferred that the use of martempering to avoid high residual stresses arising during transformation would prove even more beneficial in practical heat treatments. From this it would Seem that further research in this vein will demonstrate that for certain machine steels drawn to a medium hardness the martempering process will prove -35- economically advantageous. In addition, this work sug- gests the probable nature of future researches bearing on this problem. AS the presence of bainitic structures seems to be detrimental to the fatigue prOperties of a steel, more promise of improvement is held for steels of high harden- ability than for those of lower hardenability. However, I with steels of high hardenability a considerable amount of austenite might be retained. Upon tempering, this retained austenite would transform to bainite and it's possible that the beneficial effects of martempering 7* could be completely offset. Since stress concentrations exist in most practi- cal designs, more significance would be attached to tests on notched Specimens than would be attached to tests on unnotched Specimens. Because the improvement in fatigue resistance has been shown quite conclusively to be the result of lower residual stresses arising during heat treatment, the determination of residual stresses in the Specimens used probably would yield some interesting and Significant data. Possibly the X-ray diffraction tecnnique could be used, since at this time it is the only non-destructive method of making such determinations. Unfortunately, it is a very time consuming and expensive test yielding results of doubtful accuracy. - 37 - BIBLIOGRAPHY and REFERENCES BOOKS Archer, R. S., Briggs, J. 2., and Loeb, C. M., Jr., Molybdenum - SteelSLIronsi Alloys, New York, Climax Molybdenum Company, 1948. Bain, E. 0., Functions of the Alloying Elements in Steel, Cleveland, AOSOMO ’ 1939. 'AA'JO‘. Battelle Staff, Prevention of the Failure of} {etals I Under Repeated Stress, New York, John Wiley & Sons, Inc., 1941. MAGAZINE ARTICLES Boegehold, A. L., Selection of Steel for Automobile Parpg, Part IV, SAE Journal, Nov.,1949. Forsythe, A. C., and Carreker, R. A., Fatigue Limit of SAE 1095_After Various Heat Treatments, Metal Progress, Nov., 1948. Hollomen, J. H., Jaffe, L. D., McCarthy, D. E., and Morton, M. R., The Effects of Microstructure on Tpp Mechanical Properties of Steel, ASM Preprint 25, 1947. Krouse, G. H., A High Speed Fatigue TestinggMachine and Some Tests of Speed Effect on Endurance Limit, Proc. An]. SOC. T8813. Ilia-to, 1934, V01. 34. Oberg, T. T., and Johnson, J. B., Fatigue Properties of Metals Used in Aircraft Construction ati34.500 and 10.600 Cycles, Proc. Am. Soc. Test. Mat., 1937, Vol. 37. Roos, P. K., Lemmon, D. C., and Ransom, J. T., Influence of Type of Machine, Range of Speed and Specimen snap; on Fatigue Test Data, Am. Soc. Test. Mat., Bulletin No. 158, May 1949. Shepherd, B. F., Martempering, Iron Age, Jan.28,Feb.4,l943. PAMPHLETS Atlas of Isothermal Transformation Diagrams, Carnegie-Illinois Steel Co., Pittsburgh and Chicago. Three Keyggto Satisfaction, Climax Molybdenum Co.,New York. U.S.S. Carilloy_Steels Alloy Steels for the Special Jobs of Indust , Carnegie-Illinois Steel Co., Pittsburgh and ‘0. 'l r" 3 F‘} IBRA UNIVERSITY E T I .H v . CC N A ) . k H .