ENGINEERING RESEARCH INSTITUTE UNIVERSITY OF MICHIGAN ANN ARBOR. MICH. SECOND PROGRESS REPORT TO MATERIALS LABORATORY WRIGHT AIR DEVELOPMENT CENTER ON AN INVESTIGATION OF THREE FERRITIC STEELS FOR HIGH-TEMPERATURE APPLICATION by Task No. 73512 March 15, 1956 March 15-, 1956

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SUMMARY This report presents the progress made in an investigation of the relationship between types of heat treatment and the elevated-temperature properties of low-alloy ferritic steels for high-temperature applications. The work period covered by the report was from December 15, 1955 to March 15, 1956. Complete data from a survey of the effect of hardness level on the hightemperature properties of several typical microstructures of a Cr-Ni-Mo(SAE 4340) steel and a 1. 25Cr-0. 75Si-0. 5Mo-0, 25V (" 1 7-22 -A'S) steel are presented. Survey creep and rupture tests were run on several structures which were tempered to a hardness of 350 BHN (t 20 BHN) so that comparisons could be made with the results of previous identical tests performed on the same structures tempered to a hardness of 300 BHN (+ 20 BHN). The results indicated that all structures of the "17-22-A"S steel increased in strength properties at the higher hardness level. The increase was substantial at 700~ and 900F, but only slight at 1100'F. The ductility was invariably lower at the higher hardness level. The results of the tests on the SAE 4340 steel indicated that the only significant changes in strength properties occurred in the oil-quenched, martensitic structure. Inthis structure, the increase in hardness produced moderate increases in strength at 700' and 900~F and slight decreases in strength at 1000'F. The other 4340 structures experienced no- significant increases or decreases in strength. The general heat-treating characteristics of the'new "17-22-A"V (1. 25Cr0. 75Si-0. 5Mo-0. 8V) steel were established in the form of an approximate isothermal transformation diagram. The diagram and photomicrographs of four isothermal structures -- as well as the oil-quenched and the normalized structures -- are are presented, Preliminary experiments designed to produce structures of mixed bainites in all three subject steels are described. The specific heat-treating conditions selected for 4340 and "17=22-A"V are presented, as well as photomicrographs of the resulting mixed bainitic structures,

1 INTRODUCTION This progress report, the second report issued under Air Force Contract No. AF 33(616)-3239, covers work done from December 15, 1955 to March 15, 1956. The present investigation is a continuation of previous work done at the University of Michigan for the Wright Air Development Center concerning the fundamentals of heat treatment of ferritic steels for service at elevated temperatures in such spplications as jet engines and airframes. The major items presented in the report include: 1. Complete data from the survey of the comparative high-temperature properties of 4340 and "117-2-A"S steels at 300 and 350 Brinell hardness levels. 2, The general relationships between heat treatment and structure have been established for the new steel added for the present contract, "17-22-A"V steel, in the form of an approximate isothermal transformation diagram. 3. Based on this diagram, a number of the treatments have been carried out to prepare specimens for creep and rupture tests. The previous work involved surveys of the high-temperature properties of several low-alloy, ferritic steels in the form of forged rotor wheels and in the form of bar stock given various continuous cooling and isothermal heat treatments. The results of these studies are to be found in WADC Technical Reports 53 —277 -Part I, 53-277-Part II,and 55-388o (References 1, 2, and 3)* The steels being studied in the current phase of the investigation include a Ni-Cr-Mo (SAE 4340) steel, a 1. 5Cr-0 75Si-0. 5Mo-0. 25V ("'17-22-A"S) steel, and a 1.Z5Cr-0.75Si-0 5Mo-0,8V (" 17-22-A"V) steel. The phases of research to be included in the investigation include: (1) completion of work initiated under the previous contract on the effect of higher hardness level on the high tempera* References are given at the end of the report

2 ture properties of several typical structures for the three steels; (2) a general s r:vey of the relationship between properties and structures of the new, higher stre " 17-22-A"V steel; (3) determination of the influence of transformation over a range of temperatures, using stepwise isothermal transformations in the bainitic range for all three steels; (4) an evaluation of the effect of an homogenization normalize prior to final heat treatment; and (5) a study of the effect of various double heat treatments to determine possible prior history effects on response to heat treatment as measured by high temperature properties and microstructures. TEST MATERIALS The SAE 4340 steel being used is the same heat that was used in the previous work. Additional "17-22AA"'S and "117-22-A"V stock for work under the present contract were both supplied gratis by the Timken Roller Bearing Company. The chemical compositions of the subject steels were reported by the manufacturers to be as follows: Steel Heat C Mn Si Cr Ni Mo V SAE 4340 19053 0.40 0o 70 0.30 0.78 1.75 0.26 "17-22-A"S 24797 Oo 30 0.63 0.60 1. 25 0.25 0.52 0.25 "117-22-A"IS 10420 0.29 0.61 0.67 1.30 0.18 0.47 0.26 "17-22-A"V 11833 0.29 0.70 0.71 1.43 0.31 0.51 0.81 PROCEDURES The procedures employed during the period covered by this report may be described under three topics: Effect of Hardness Level, General Survey of the Response of "17-22-A!'V Steel to Heat Treatment, and Development of Mixed Bainitic Structures. These correspond to items (1), (2), and (3),respectively, listed in the Introduction.

3 Effect of Hardness Level The procedure followed in studying the effect of hardness level on the elevated-temperature properties was described in detail in the First Progress Report, dated December 15, 1955. Briefly, several typical microstructures for each steel were selected for creep-rupture testing at the 350 BHN hardness level so that comparisons could be made with exactly similar tests which had been conducted at the 300 BHN hardness level under the previous contracts, The microstructures selected for testing at the 350 BHN level were necessarily limited to those having an "untempered" hardness considerably above the 350 BHN level. For tie SAE 4340 and "17-2Z-A"S steels the structures selected were: SAE 4340 "17 22 -A"S 1. Oil Quenched 1. Oil Quenched 4. Upper Bainite 2. Normalized 2. Normalized 5, Middle Bainite 3. Lower Bainite 3. Lower Pearlite 6. Lower Bainite These were suitably tempered to produce a hardness of 350 BHN. Survey type creep and rupture tests were then conducted in the temperature range of 700~ to 1100~F. The details of the heat treatments used to produce these structures at the 350 BHN (+ 20 BHN) hardness level are given in Table I. General Survey of the Response of "17-22-A"tV Steel to Heat Treatment Isothermal Transformation Diagram: As the initial step toward an understanding of the heat-treating characteristics of the new 1. 25Cr-0. 75Si-0. 5Mo-0, 8V ("17-22-A"V) steel, the (approximate) isothermal transformation diagram was determined. The work involved in the "S" curve determination constituted a major part of the experimental work accomp.lished during the period covered by this report. The details of the procedure fol lowed are given on the next page.

4 Approximately 100 samples were prepared by cutting 50 - 3/16-inch thick wafers from a 1-foot length of 1-inch round bar stock and then cutting each wafer in half. A small hole was drilled near the periphery of each sample so that it could be attached securely to the end of a 2-foot length of 14 gage chromel wire. The samples were divided into groups of 10, each group being used to follow the transformation of austenite at one given temperature. The course of the decomposition or transformation of austenite at any given temperature was determined as follows: A group of ten samples was put into an electrically heated furnace (air atmosphere) at 1850~F. After 30 minutes the samples were withdrawn from the austenitizing furnace and quenched into an agitated molten salt bath maintained at the given transformation temperature by external electric resistance coils. At various, predetermined times throughout the course of the transformation a sample was withdrawn from the salt bath and quenched in water. The transformation times were selected with the aid of the "'S" curve for "'17-22-A"S steel which is somewhat similar in composition to "17-22-A"V. Each sample was numbered after it was quenched from the salt bath. The semicircular wafers were again cut in half so that the freshly cut surface could be polished for metallographic examination. All ten samples were mounted in a single bakelite mount, hand polished, etched with 2% nital, and examined under the microscope with magnifications up to X1000 diameters. It was assumed that the sample in which the first trace of transformation product was visible at X1000D and the sample in which the last trace of martensite* was visible at X1000D corresponded to the initiation and completion of the transformation. The transformation times of these two samples were plotted on a temperature vs. log transformation time diagram, This procedure was repeated for each group of 10 specimens, covering transformation temperatures from 600' to 1320~F in 100~F or less intervals. * Representing the last trace of untransformed austenite,

5 The curves drawn through all points representing the start and the finish of the transformation at each temperature constitute the isothermal transformation diagram. Design of Heat Treatments: The types of heat treatments used to produce the various microstructures of the low-alloy, ferritic steels previously studied for this investigation have included: 1. Oil Quenching and Tempering 2. Normalizing and Tempering 3.. Isothermal Transformations to a. Upper Pearlite b. Middle Pearlite c. Lower Pearlite d. Upper Bainite e. Middle Bainite f. Lower Bainite Structural differences within these basic treatments which have been studied include variations in:(l) final tempered hardness for all structures; (2) austenitizing temperature for the oil-quenched and the normalized (0. 8-inch rounds) structures; and (3) effective section size (0. 8-inch and simulated 3- and 6-inch diameter rounds) for the normalizing or air-cooling treatment. It is planned to include all of these in the "17-22-At'V survey. The procedures followed in selecting the specific conditions for the heat treatments were as follows: 1. The same section sizes as were used in the previous studies were selected, i. e., 0. 8-inch rounds for oil quenching and normalizing, and 0.4-inch rounds for the isothermal transformations. Insulating firebrick cylinders will be used around 0. 8-inch rounds to simulate the air cooling of 3- and 6-inch dia

6 meter rounds. 2. Previous work on "17-22.A"V on a different project indicated that for normalizing, an austenitizing temperature of 1850'F gave better properties than 1750~ or 1800'F. On this basis, 1 hour at 1850'F was selected as the standard austenitizing treatment for normalizing. To eliminate austenite composition, homogeneity, and grain size as a variable in the basic microstructures, the same austenitizing conditions were selected for the oil-quenched and the isothermally transformed structures. 3. The transformation temperatures and times for the isothermal heat treatments were selected by means of the isothermal transformation diagram determined for this heat of "17-22-A"V. 4. The tempering conditions used to bring each structure to the final desired hardness level were selected with the aid of tempering curves (hardness vs. time at constant temperature). Each tempering curve was determined by cutting a representative, heat-treated bar into a number of slugs suitable for hardness testing and tempering the slugs for various times at a constant tempering temperature. The tempering temperatures were selected on the basis.of past experience with a similar steel but containing less vanadium. Development of Mixed Bainitic Structures Basically, the interest in the development of microstructures containing a mixture of the upper- and lower-temperature forms of bainite resulted from the knowledge that bainitic structures formed during continuous cooling (as in normalizing) are made up of an intimate mixture of bainites formed over a range of temperature. It was believed that one step toward a better understanding of these structures could be gained through a study of mixed bainitic structures of known composition -- that is -- composition with respect to kind (as determined by the temperature at which they were formed) and amount (volume percent) of the bainites

7 present. It was also thought that certain controlled mixtures of bainite might be found to possess high-temperature properties which are superior to those of any of the bainitic structures tested thus far. The suggested heat treatment for producing mixed bainitic structures was simply to use two or more transformation temperatures in a stepwise, isothermal treatment. Specifically, the sample would be austenitized in the usual way, quenched into a salt bath at a relatively high bainitic temperature, held until the transformation has progressed the desired amount, quenched into a second salt bath at a lower bainitic temperature, held until the transformation has progressed the desired additional amount, and so on until the sample contained the desired mixture of bainites. The isothermal transformation diagrams may be used in selecting the transformation temperatures (which determine the types of bainite in the structure, but the transformation times which will produce the desired amounts of the various bainites must be determined by quenching out a series of samples at each of the lower, subsequent transformation temperatures and examining them metallographically. For the sake of simplicity and to avoid excessive losses in time in securing additional equipment, structures containing only two kinds of bainite were selected for study under this contract, All three subject steels will be studied in this respect.

9 on which deformation the strength was based on. Lower Pearlite (Tempered): 1. At 700~ and 900"F, the creep and total deformation properties were both superior for the harder condition. 2. At 1100F the 350 BHN structure was stronger than the 300 BHN structure with respect to rupture, creep, and total deformation; but it was inferior with respect to ductility. The difference was much greater at the lower stresses suggesting the possibility that the low stress data for 300 BHN material is abnormally low and should be checked. Upper Bainite (Tempered): 1. The harder structure had substantially higher strength at 700~ and 900'F. At 1100'F there was little difference between the two hardness levels. Middle Bainite (Tempered): 1. At 700~ and 900'F the material at the higher hardness level showed marked superiority with respect to creep and total deformation strengths. 2. The rupture data at 1100'F showed little or no difference in strengths at 41,000 psi, but at 19,000 psi the 350 BHN structure was somewhat superior. The 350 BHN material exhibited inferior ductility as usual. 3. The creep and total deformation at 11O00F pointed in every instance toward the harder structure as being slightly superior. Lower Bainite- (Tempered): 1. The 350 BHN structure tested at 700 and 900F was greatly superior to the 300 BHN structure with respect to creep and total deformation strengths. 2. At 1100F the 350 BHN structure had only slightly higher rupture, creer and total deformation strengths. The ductility of the harder material was inferioi as usual.

10 SAE 4340 Steel Oil Quenched and Tempered: 1. At 700' and 900'F the 350 BHN structure had the better creep and total deformation strengths. 2. At 1000,F the softer structure proved to be somewhat superior with respect to rupture, creep, and total deformation strengths. 3. The ductility at 1000~F was unexpectedly high for the 350 BHN materialrunning from 10 to 80 percent higher than that for the 300 BHN material. Normalized and Tempered: 1. At 700'F the 350 BHN material had inferior creep strength and superior total deformation strengths. 2. At 900*F-the 350 BHN structure had superior creep strength and inferior total deformation strengths. 3. In all cases at 1000'F the material at the higher hardness level exhibited inferior rupture, creep, and total deformation properties and superior ductility. Lower Bainite (Tempered): 1. The structure at the higher hardness level showed lower creep and total deformation strengths at 700'F. 2. At 900 F the 350 BHN material exhibited superior rupture and creep strengths but somewhat inferior total deformation strengths. 3. The data at 1000F indicated that the harder structure had somewhat superior short-time rupture and creep strengths, but slightly inferior total deformation strengths.

11 General Survey of the Response of "t17-22-A"V Steel to Heat Treatment Isothermal Transformation Diagram: The approximate isothermal transformation diagram is presented in Figure 1. The general shape of the diagram is characteristic for this type of steel. That is, it shows the ferrite and pearlite region (upper curves) separated from the bainite region (lower curves) by a considerable range of temperature in which the austenite will not begin to decompose even after relatively long time periods. It is of interest to note that the very short times to the start of the bainite reaction below 900'F would limit the depth to which this steel can be fully trans - formed to martensite. Another interesting result of the isothermal study was the fact that the isothermal pearlite observed in this heat of t17-Z2-A"V did not have a well-defined lamellar structure. While a few instances were found in which the eutectoid mixture was somewhat lamellar, most of the time the eutectoid was simply a local area darkened by a concentration of minute spheroids of carbides. Microstructural studies revealed that austenitizing treatments of 30 to 60 minutes at 1850F left a trace of the original carbides undissolved. The diagram of Figure 1 therefore is based on material austenitized at a temperature slightly below that required for complete solution of carbide. Design of Heat Treatments. The specific heat-treating conditions for "17-22-A"V which have been selected to date include four isothermal treatments based on the isothermal transformation diagram, and the standard oil quenching and the normalizing treatments. The heat treatments are summarized on the following page:

12 Structure Oil Quenched Normalized Lower Pearlite Upper Bainite Middle Bainite Lower Bainite Section 0. 8-inch 0. 8-inch 0.4-inch Size round round round Heat Treatment 1 hr. at 1850'F, Oil Quenched 1 hr. at 1850F, Air Cooled 1 hr. at 1850'F, Isothermally Transformed at 1200'F for 5 hrs., Water Quenched 1 hr. at 1850F, Isothermally Transformed at 850'F for 2 hrs., Water Quenched 1 hr. at 1850'F, Isothermally Transformed at 750'F for 0.3 hrs., Water Quenched 1 hr. at 1850F, Isothermally Transformed at 650'F for 0.2 hrs., Water Quenched 0.4-inch round 0. 4-inch round 0.4-inch round Specific tempering conditions have been determined only for the oil-quenched and the normalized structures at the 350 BHN hardness level. The tempering curves for these two structures are presented in Figures 2 and 3. On the basis of the curves, the tempering treatment selected for both the oil quenched and the normalized bars was 2 hours at 1250'F. Photomicrographs of the oil-quenched and the normalized structures are shown in Figure 4. The oil-quenched bar was essentially 100% martensite. There were, however, a few spears of bainite at about the mid-radius of the 0. 8-inch round. At the center of the round the martensite at 100 diameters appeared as varying light and dark areas. This is believed to be due to chemical inhomogeneity. The normalized bar was 100% bainitic. At 1000 diameters magnification the structure appeared quite similar to middle bainite. At 100 diameters the effect of chemical inhomogeneity again appeared as alternate light- and dark-etching areas. Photomicrographs of lower pearlite and upper, middle, and lower bainite are presented in Figures 5 and 6. The pearlite formed isothermally at 1200'F did not exhibit a definite lamellar structure. Rather, the eutectoid mixture appeared simply as small areas of closely grouped spheroids of carbide. The eutectoid areas comprised approximately 5 percent of the microstructure —the remainder

13 being ferrite. The upper, middle, and lower bainite structures were similar to those observed in the "17-22-A"S steel. At the higher temperature (850~F) the transformation to bainite seemed to stop after the first hour or two, making it necessary to include about 40% martensite in the structure. At the lower temperatures (750' and 650'F) the reaction went to completion in a relatively short time, giving finer and more acicular bainite the lower the transformation temperature. Development of Mixed Bainitic Structures The preliminary experiments used in selecting the specific heat-treating conditions for the mixed bainitic structures yielded the following results: Steel Heat Treatment Microstructure SAE 4340 1 hr..at 1750~F, Isothermally Transformed Step- 60% Upper Bainite wise at 850F for 1 hr. and 650'F for 1 hr., + 40% Lower BainWater Quenched ite "17-22-A"V 1 hr. at 1850'F, Isothermally Transformed Step- 60% Upper Bainite wise at 850'F for 5 min. and 650'F for 45 min. + 40% Lower BainWater Quenched ite "'17-22-A"S 1 hr. at 1750~F, Isothermally TransformedStepwise at 900 and 700'F. The transformation times have not yet been determined. Photomicrographs of the mixed bainitic structures for SAE 4340 and "17-22-A"V are presented in Figure 7. The two kinds of bainite are readily distinguishable at X1000D in the 4340 steel but not in the "17-22-A"V.

14 DISCUSSION Effect of Hardness Level Strength properties of the various structures in the "17-22-Al'S steel at the 350 BHN hardness level were generally higher than those observed at the 300 BHN hardness level. The differences were substantial at 700' and 900F; however, at 1100~F the differences, even though fairly consistent, were so small as to be hardly significant. In no case was there arny.idication of a substantial reduction in strength values as the hardness was raised from the 300 BHN to the 350 BHN level. At 11000F, the harder material generally compared more favorably in the longer time, lower stress tests. The reverse might have been the case if greater instability from higher hardness had been a factor. It is important to note, however, that the specimens wfiich ruptured at 110OF all showed lower elongation and reduction of area for the harder material. One factor which ought to be considered in judging the final significance of the results for "17-22-A"S is that two heats of steel were involved in the comparison, All except three of the tests at 300 BHN were on specimens from Heat No. 24797. The other three tests at 300 BHN and all of the tests at 350 BHN were on specimens fromr Heat No. 10420. However, because the data of the three "common heat" comparisons invariably followed the same trends shown by the "different heat" comparisons, it is quite certain that the differences between heats were not large enough to affect the trend of the influence of hardness. The results for the SAE 4340 steel are not so easily generalized. It is clear, however, that in none of the structures tested was the 350 BHN material either greatly superior or inferior to the 300 BHN material. The only comparisons in which the 350 BHN material exhibited (moderately) higher strength values consistently were for the tempered martensite structure at 700' and 900'F. For the most part, however, the harder material was observed to have lower strength values as

15 frequently as it was observed to have higher strength values. The lack of a general superiority of the 4340 steel at 350 BHN over the same steel at 300 BHN is attributed to structural instability. It can be seen from Table I that the tempering conditions used to obtain a hardness of 350 BHN were much less severe than those used to obtain a hardness of 300 BHN. The result was that in 9 of the 13 tests conducted at the 350 BHN hardness level, the testing temperature was equal to or greater than the tempering temperature. It is quite likely that the continued tempering of these specimens during testing resulted in a lowering of their strength values. General Survey of the Response of "17-22-A"V Steel to Heat Treatment There are several items concerning the determination and the applicability of the isothermal transformation diagram which need some discussion. The term "approximate", as applied to the isothermal transformation diagram, was intended to mean simply that certain portions of the diagram are not well defined because of insufficient data. The accuracy of the individual data with respect to the measured temperatures and times and the metallographic examinations was considered to be good. Specifically, (1) the austenitizing temperature was controlled to within + 5'F of 1850~F, (2) each transformation temperature was controlled to within ~ 3~F of the nominal value, (3) the transformation times were measured to within t 1 second for the short times and to within t 1 minute for the long times, and (4) two observers agreed upon the results of the metallographic examinations of the "borderline" samples. In using any isothermal transformation diagram it is important to note what austenitizing conditions were used in determining the diagram. The reason for this is that the positions of the lines in the diagram may be shifted considerably by large variations in the composition, homogeneity, and grain size of the austenite. Since the temperature and time of the austenitizing treatment are the major factors gov~

16 erning these properties of the austenite, it is important to know the austenitizing treatment used in the determination of the diagram. Heat to heat variations in chemical composition and the other production variables can also affect the transformation diagram. In view of these considerations, it is advised that the isothermal transformation diagram herein presented was determined for Timken Heat Number 11833 of the "17-22-A"V steel austenitized at 1850'F for 30 to 60 minutes, starting from the asfurnished (hot-rolled) condition, and that under any conditions varying greatly from these the diagram must be considered as being only a "first approximation". Several check tests were run using an austenitizing time of 60 minutes rather than 30 minutes. No difference in the transformation times could be detected between the 30 and 60 minute austenitizing treatments. No checks were made on the effect of varying the austenitizing temperature, but it is estimated that the given transformation diagram would be relatively accurate for austenitizing temperatures as low as 1800' or 1750~F and as high as the coarsening temperature. The microstructures of "17-22-A"V which have been developed thus far are somewhat similar to the corresponding structures in the "17-22-A"S steel. The most salient difference in appearance is the much finer grain size of "17-22-A"V. In this connection it is interesting to note that the austenitizing temperature for "17-22-A"V was 100'F higher than that used for "17-22-A"S. The persistence of the fine grain structure of the "17-22-A"V at high temperatures is attributed to the effectiveness of undissolved carbide particles as grain growth inhibiters. Another notable difference between the two "17-22-A" steels was that on normalizing the 0. 8-inch rounds, the "17-22-A"S steel was about 85% bainite plus 15%o martensite, while the "17-22-A"V was 100% bainite. The shapes of the tempering curves (see Figures 2 and 3) for the oil-quenched and the normalized structures of '17-22-A"V indicated the presence of secondary hardening effects. This is what would normally be expected in a steel containing appreciable amounts of carbon and vanadium.

17 Development of Mixed Bainitic Structures The microstructures observed in the specimens which were isothermally transformed stepwise in the bainitic range were surprisingly different for 4340 and "17-22-A"V. In the 4340 steel, the high- and low-temperature forms of bainite are readily distinguishable; whereas, in the "17-22-A"V steel the two forms of bainite can hardly be distinguished. In Figure 7, the darkest as well as the lightest areas in the 4340 micro are the lower bainite; the medium-dark areas are the upper bainite. This illustrates the difficulty in identifying the various types of bainite in a single specimen with the aid of the microscope alone. The identification of the bainites in Figure 7(a) was accomplished by observing a series of four samples which had been transformed 0, 15, 30, and 60 minutes, respectively, at 650~F after the initial transformation at 850'F for 1 hour. CONCLUSIONS Effect of Hardness Level 1. Strength properties of the various structures in the "17-22-A"S steel at the 350 BHN hardness level were generally higher than those observed at the 300 BHN hardness level. The differences at 7000 and 900'F were substantial; however, at 1100'F the differences were so small as to be hardly significant. 2. The 350 BHN structures in "17-22-A"S steel at 1100F tended to compare more favorably in the longer times lower stress tests. 3. Without exception the ductility of the "17-22-A"S steel at fracture was lower at the higher hardness level, 4. Strength properties of the oil-quenched and tempered structure in the SAE 4340 steel at 700 and 900eF were increased moderately as the hardness level was raised from 300 BHN to 350 BHN. At 1000'F, however, the same structure was somewhat inferior at the higher hardness level.

18 5. The strength properties of the other structures in the SAE 4340 steel were not consistently different at the two hardness levels. FUTURE WORK Future work will consist primarily of completing the heat treating and ma-n chining of specimens, and conducting the planned program of creep and rupture testing. The other phases of the investigation involving the study of the effect of (1) a prior normalize and (2) double heat treatments will be started in the near future. REFERENCES: (1) A. Zonder, A, I. Rush, and J. W. Freeman, "High Temperature Properties of Four Low-Alloy Steels for Jet-Engine Turbine Wheels", Wright Air Development Center Technical Report 53-277, Part I (November, 1953). (2) A. I. Rush and J. W. Freeman, "High-Temperature Properties of Four LowAlloy Steels for Jet-Engine Turbine Wheels", Wright Air Development Center Technical Report 53-277, Part II (February, 1955). (3) K. P. MacKay, A. P. Coldren, A. I. Rush, and J. W. Freeman, "A Survey of the Effect of Austenitizing Temperature and Rate of Continuous Cooling on the Structure and 700' to 1200F Properties of Three Low-Alloyed Steels", Proposed Wright Air Development Center Technical Report 55-388 (September, 1955).

I TABLE I Structures and Heat Treatments Used in the Study of the Effect of Hardness on Properties of SAE 4340 and "17-22-A"S Steels Average Hardness Tempering Conditions Structure Initial Heat Treatment Before Tempering (BHN) 300 BHN 350 BHN - Oil Quenched (100% Martensite) Normalized (65% Bainite + 35% Martensite) Lower Bainite (100% Fine Bainite) SAE 4340 Steele 1 Hour at 1750~F, Oil Quenched (0.8 in. round) 1 Hour at 1750~F, Air Cooled (0.8 in. round) 1 Hour at 1750~F, Isothermally Transformed 1.5 hrs. at 650F, Water Quenched (0.4 in. round) 585 385 430 10 hrs. at 1100~F 1.5 hrs. at 1000~F 1 hr. at 1100'F 0.5 hr at 900~F 1.25 hrs. at 1100F 0. 5 hr at 900~F "17-22-A"S Steel* Oil Quenched 1 Hour at 1750~F, Oil Quenched (0.8 in. round) (100% Martensite) Normalized 1 Hour at 1750~F, Air Cooled (0.8 in. round) (85% Bainite + 15% Martensite) Lower Pearlite 1 Hour at 1750FF, Isothermally Transformed 10 hours at 1150FF, Water (40% Pearlite + Quenched (0.4 in. round) 60% Ferrite) Upper Bainite 1 Hour at 1750~F, Isothermally Transformed 2 hrs. at 900'F, Water (60% Bainite + Quenched (0.4 in. round) 40% Martensite) Middle Bainite 1 Hour at 1750'F, Isothermally Transformed 0.5 hr. at 800'F, Water (97% Bainite + Quenched (0.4 in. round) 3% Martensite) Lower Bainite 1 Hour at 1750FF, Isothermally Transformed 0.2 hr. at 700~F, Water (100% Fine Bainite) Quenched (0.4 in. round) * All values given are for Heat No. 10420. 525 355 375 465 360 365 1 hr. at 1300'F 10 hrs. at 1200~F 12 hrs. at 1200~F 16 hrs. at 1200~F 4 hrs. at 1200~F 12 hrs. at 1200~F 3 hrs. at 1200~F 15 hrs. at 1100~F 3 hrs. at 1100~F 12 hrs. at 1100~F 8 hrs. at 1100~F 12 hrs. at 1100~F I

TABLE I! Comparison of Rupture, Creep, and Total Deformation Data at the 300 and 350 BHN Hardness Levels for "17-22-A"S Steel Tested in the Range of 700' to 1100~F Test Rupture Elongation Reduction of Area Def. on Loading Min. Creep Rate Time to Reach Specified Total Deformation (Hours) Temp. Stress Time (hrs) (% in 4D) (%) (%) (%/Hr) U 0.2 Percent 0.5 Percent 1.0 Percent Structure (IF) (psi) 300 B HN 350 BHNN 3 00 IBHN 350 BHN 30 0 BHN 350 BHN 300 BHN 350 BHN 300 BHN 350 BHN 350 B 300 BHN 350 BHN Oil Quenched 700 115,000 289.0 a >1125.7 (Tempered) 900 70, 000 756.0 a > 888. 0* 1100 41,000 23.4 a 52.7 1100 19,000 850.0 a e 712.7 Normalized 700 115,000 132,0 a >1205.0 (Tempered) 900 70,000 >1482.0 a >1205.0 1100 41,000 112.0 a 92.8 1100 19,000 900.0 a e 1211.4 Lower Pearlite 700 115,000 ad >1296.0. (Tempered) 900 70,000 >1205.0 a >1176.0 1100 41,000 42.0 49.1 I 1100 19,000 f 677.2 1100 15,000 652.0 a >1148.7 Upper Bainite (Tempered) 700 115,000 147.0 a >1176.8 900 70,000 686.0 a >2376.7 1100 41,000 51.5 58.9 r1 1100 19,000 796.0 a 851.2 Middle Bainite 700 115,000 >1827.0 a >2544.2 (Tempered) 900 70,000 >1648.0 a >2448.6 1100 41,000 88.2 a 83.6 1100 19,000 815.0 a 938.1 Lower Bainite (Tempered) 700 115,000 59.4 a >1298.0, 900 70,000 1456.0 a >1200.0 1100 41,000 92.8 119.1 1100 21,0.00 889.0 a f 1100 19,000 f 1278.9 19.8 63.3 0.6700 0.538 0.00950 0.00016 b b b b 1.0 ^'1500 30.3 - 64.0 - 0.3780 0.365 0.00384 0.00028 b b 3.0 30.0 50.0 ^1500 28.0 5.5 27.5 i0.0 0.1730 0.224 0.00650 c c b b 40 c 24. 0 4.0 2.0 - 5.5 0.1050 0.102 0.00152 0.00150 17.0 30.0 170.0 220.0 420.0 492.0 21.0 - 61.9 - 0.6600 0.623 0.0220 0.000047 b b b b 1.0 >1205.0 - - - - 0.3350 0.352 0.0003 0.000079 b b 24.0 175.0 1400.0 >1205.0 2.5 2,0 3.1 2.4 0.2120 0.1980 0.00614 0.00420 b 0.1 26.0 -10.0 -60.0 2.0 1.0 - 0.8 0.0850 0.096 0.00063 0.00050 80.0 55.0 580.0 500.0 800.0 1050. 0 19.0 - 61.0 - - 0.843 - 0.000540 b b b b b <0.5 - - - - 0.4060 0.279 0.00223 0.000385 b b 2.0 60.0 53.0 1180.0 8.5 5.5 8.7 5.1 0.2260 0.231 c c b b 4.0 c 12.0 4.5 - 3.5 - 0.0780 - 0.00178 - 22.0 - 1850 - 385.0 15.5 - 17.1 - 0.0650 0.0630 0.00340 0.00068 54.0 72.0 107.0 500.0 218.0 1025.0 20.2 - 62.0 - 0.7100 0.424 0.0180 0.00011 b b b ' 10.0 <1.0 2000 30.0 - 59.5 - 0. 3550 0.3900 0.00504 0.000064 b b 1.0 70.0 50.0 >2376.0 7.2 5.0 8.6 7.1 0.2690 0.2700 c 0.0224 b b 4.0 5.0 13.0 24.0 5.8 3.0 6.6 4.1 0.1100 0.087 0.00140 0.00155 8.0 65.0 177.0 230.0 447.0 490.0..- - 0.6100 0.513 0.00029 <0.00001 b b b b 45.0 >>2544.0.... - 0.3230 0.309 0.00014 0.000052 b b 65.0 510.0 2500 e >>2448.0 5.1 2.5 4.9 3.3 0.2170 0.232 c c b b 6.0 9.0 19.0 40.0 4.0 -,1.0 3.0 '1.0 0.0960 0.105 0.0015 0.00081 30.0 35.0 222.0 360.0 575.0 735.0 18.8 24.0 5.0 2.0 2.0 4.0 66.7 56.2 4.0 5.6 - 0.8150 0.5110 0.0452 0.000047 b - 0. 3500 0. 3480 0.001.15 0.000072 b 2.5 0.2520 0.2400 0.0165 0.0090 b 0.1740 - 0.00113 - 6.0 4.7 - 0.0940 - 0.00085 b b b b <1.0 >>1298.0 b 12.0 550.0 362.0 >>1200.0 b 9.0 22.0 32.0 64.0 198.0 - 604.0 - 60.0 - 375.0 - 855.0 a Heat No. 24797: all others from Heat No. 10420 b Value exceeded on loading c Unavailable because of insufficient data d Ruptured on loading e Extrapolated or interpolated value f No test run under these conditions > Greater than (Test was discontinued at this time) ~> Much greater than < Less than Approximately Test discontinued because of accidental overheat

TABLE IMl Comparison of Rupture, Creep, and Total Deformation Data at the 300 and 350 BHN Hardness Levels for SAE 4340 Steel Tested in the Range of 700- to 1000F Test Rupture Elongation Reduction of Asea Def. on Loading Min. Creep Rate Time to Reach Specified Total Deformation (hours) Temp. Stress Time (hours) (% in 4D) __ (%) (% /hr) 0. Z Percent 0.5 Percent- 1. Percent Structure (F) (psi) 300 BHN 350 BHN 300 BHN 35BHN 30 BHN BHN 300 BHN 350 BHN 30 BHN 33050 BHN 300 BHN 350 BHN 300 BHN350 BHN 300 BHN 350 B OilQuenched 700 90,000 >1350.0 >1222.9 - - - - 0.430 0.342 0.00027 0.000138 b b 2.0 350.0 675.0 '3000e (Tempered) 900 55,000 381.0 887.4 19.5 11.0 39.5 12.3 0.269 0.292 0.01480 0.00546 b b 2.0 2.0 13.0 20.0 1000 31,000 160.0 e 143.1 11.0 12.5 15.0 17.0 0.149 0.376 0.02500 c 1.0 - 3 5 - 16 0 1000 20,000 780.0 e 693.9 12.0 21.5 15.0 23.0 0.099 0.113 0.00380 0.0060 7.0 '2.0 47.0 15.0 190.0 72.0 Normalized 700 90,000 >1294.0 >1224.9 - - - - 0.467 0.408 0.00016 ' b b 1.0 25.0 1000.0 v1700 e (Tempered) 'i 900 55,000 842.0 >1177.7 12.0 - 22.3 - 0.260 0.381 0.00414 0.0016 b b 8.0 <1.0 64.0 5.0 1000 31,000 371.0 268,1 5.5 12.5 7.4 13.5 0.126 0.164 0.00505 0.01260 N5.0.<10. 10.00 145.0 40.0 1000 20,000 1392.0 1090.1 5.0 7,0 4.0 7.0 0.090 0.106 0.00114 0.00213 20.0 N50 2.0 2 55.0 650.0 275.0 Lower Bainite 700 90,000 >1485.0 >1223.6 - - - - 0. 502 0.482 0.00016 0.00022 b b V1.0 b 3000 e 575.0 (Tempered) 900 55,000 897.0 1410.3 18.5 5.0 15.4 6.3 0.250 0.298 0.00530 0.0020 b b 8.0 * 5.0 51.0 35.0 1000 55,000 f 13.5 - 19.0 - 34.8 - 0.350 - c 0 b 1000 31,000 210.0 263.7 9.4 10.0 17.0 10.1 0.161 0.220 0.01790 A 0Q&. 1.0, 1.0 v 6.0 ' 6.0 26.0 20.0 1000 20,000 f 1074.6 - 8.0 - 7.4 - 0.161 - 0.00244 - 2.0 - 18.0 - 123.0 1000 13,000 >1035.0 - - - 0.055 - 0.00053 - 32.0 - 300.0 - 1100e - b Value exceeded on loading c Unavailable because of insufficient data d Ruptured on loading e Extrapolated or interpolated value f No test run under these conditions > Greater than (test was discontinued at this time) >> Much greater than < Less than 4 Approximately

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E A4 -/,I v/....... I....:~ -," I (I l X1OOD X1OOOD (a) Oil quenched friom 1850'F - 476 Avg. BHN (b) Normalized from 1850'F - 374 Avg. BHN Figure 4.- "17-22-A"V Bar Stock (a) As Oil Quenched from 1850'F and (b) As Normalized from 1850'F.

XI00D X1000D (a) Isothermally-transformed at 1200~F for 5 hrs. 5% Lower Pearlite + 95%o Ferrite. (b) Isothermally transformed at 850'F for 2 hrs. 60% Upper Bainite + 40% Martensite. Figure 5.- " 17-22-A"V Bar Stock as Transformed Isothermally to (a) Lower Pearlite and (b) Upper Bainite.

X100D X1OOOD (a) Isothermally trafnsformed at 750~F for 18 min. 100% Middle Bainite. (b) Isothermally transformed at 650~F for 12 min. 100% Lower Bainite. Figure 6.- "17-22-A"V Bar Stock as Transformed Isothermally to (a) Middle Pi777I1 4 -Bainite and (b) Lower Bainite.

XIOOD X1000D (a) SAE 4340, isothermally transformed stepwise at 850'F for 1 hr. and 650'F for 1 hr. 60% Upper Bainite + 40% Lower Bainite. (b) "17-22-A"V, isothermally transformed stepwise at 850'F for 5 min. and 650'F for 45 min. 60% Upper Bainite + 40010 Lower Bainite. Figure 7. - Mixed Baintic Structures as Produced in SAE 4340 and "17-22-A"V by Stepwise Isothermal Transformations in the Bainitic Range.

UNIVERSITY OF MICHIGAN 3 9015 02827 3913