DEPARTMENT OF ENGINEERING RESEARCH UNIVERSITY OF MICHIGAN ANN ARBOR Report on CONSTANT STRESS CREEP TEST CHARACTERISTICS OF 0.50 Mo STEEL K-22 AT 850 AND 10000~F by A. E. WHITE J. W. FREEMAN Project Number 309 Report Number 1 for Project 25 of the ASME-ASTM JOINT RESEARCH COMM1IITTEE ON THE EFFECT OF TEMPERATURE ON METALS November 23, 1943

CONSTANT STRESS CREEP TEST CHARACTERISTICS OF 0.50 Mo STEEL K-22 AT 850 AND 10000F. The ASME-ASTM Joint Research Committee on the Effect of Temperature on Metals sponsored a cooperative program on short-time testing of steels to predict long-time load-carrying ability. The program was designated as Project 25. The investigation was limited to a 0.35 carbon steel designated K-20 and a 0.50 Mo steel designated K-22. The present report presents the constant stress creep test characteristics of steel K-22 at 850 and 10000F. This work was sponsored at the University of Michigan by the Joint Committee. In addition to the standard creep test results sufficient data were taken to permit the evaluation of the Hatfield Time-Yield Stresses and the German DIN (DVM) Creep Strength values. Single tensile tests were also conducted at each temperature. Typical microstructures of the original material and the creep test specimens are included. Test Material The test material for this investigation was hotrolled 0.50 PMo steel supplied by Project 25 of the ASME-ASTM Joint Research Committee on the Effect of Temperature on MIetals. This committee designated this particular heat of steel as K-22.

2. The reported chemical composition was as follows: Chemical Composition, Per Cent C In N S Si Ni Cr Mo,16.66.015.027.24.13.09.53 The steel was produced in a 100 ton basic open hearth furnace. The ladle deoxidation consisted of the addition of 50 per cent ferrosilicon and 1.4 pounds of aluminum per ton. The test material was rolled to one-inch round bar stock. The McQuaid-Ehn grain size was.reported to be 6-8. The hot-rolled bar stock was heat treated by heating at 1650~F. for 1-2/3 hours and air cooling. It was then drawn by reheating to 1200~F. for 2 hours and air cooling. The bars were then machine straightened and stress-relieved by reheating to 12000F. for two hours and air cooling. The resulting Brinell hardness was 131-137. The microstructure, as shown in Chart 1, was ferrite and partially spheroidized fine pearlite. The actual grain size was 7-8. The specimens for this investigation were taken from bar 23A. Exoerimental Procedure Four constant stress creep tests were conducted at both 850 and 10000F. Temperature distribution and control was in accordance with ASTM recommended procedure. The elongation of

3. the specimen was measured by means of an extensometer system attached to the gage length of the specimens. The extensometer rods projected from the bottom of the furnace and actuated mirrors mounted on rollers. The optical beam to the telescope gave a sensitivity of 2.8 millionths of an inch per inch for the two-inch gage section of the 0.505-inch diameter specimens. During the first five days of the tests the extension of the specimens was read at frequent intervals in order to obtain data for Hatfield Time-Yield Stress and DIN (DVM) values. The creep tests were'all run for time periods in excess of 1000 hours and in some cases as long as 2000 hours. The original material and the highest stress creep specimen at each temperature was subjected to metallographic examination. Sections were taken lengthwise to the bar and in the case of the creep specimen at the center of the gage section. Short-Time Tensile Tests A single short-time tensile test was conducted at both 850 and 1000~F. The tests were conducted on a 60,000 pound hydraulic tensile machine equipped with a furnace for automatically controlling the specimen temperature within the prescribed limits. An optical extensometer system measured the elongation of the specimens until 0.2 per cent offset was obtained. The ultimate strengths were then determined at a constant head speed

4. of 0.3 inches per minute, Stress was applied in increments of 2500 pounds per square inch during the stress-strain data measurements and the strain measured between each increment. The sensitivity of the extensometer system was three millionths of an inch per inch in a two-inch gage section. The stress-strain curves obtained are shown in Figure 1 and the data obtained are tabulated below: Tensile Offset Yield Strength Proportion- Elong ReducTemp. Strength Lb./Sq.In. al Limit ation,% tion of ~F. Lb,/Sq.In. 0,2 Lb,/Sq.In. in 2 In. Ar 850 58,500 26,250 29,250 19,500 34.0 78.7 1000 49,250 25,800 28,000 15,500 33.0 83.3 Only single tests were conducted for the purpose of comparing results from our equipment with those of other co? operators. Hatfield Time-]Yield Stredss The Hatfield Time-Yield Stress is defined as the stress causing a deformation of 0.000048 inches per inch between the 24th and 72nd hours of a test, The necessary data were obtained from Figures 2 and 3 and are tabulated below:

5. Temperature Deg. Fahr. 850 1000 Stress Lb./S. In. 20,000 22,500 25,000 27,500 5,500 7,500 10,000 12,500 Deformation Between 24th and 72nd Hours Time-Yield Stress Inches per Inch Lb./Sq.In. 0.000085 0.000140 18,000 0. 000190 0,000390 0.000014 0.000022 10,000 0.000045 0.000094 The Time-Yield Stress was obtained by plotting stress against the deformation in Figure 7. DIN (DvWM) Values The creep resistance will be within conventional limits if the creep rate between the 25th and 35th hours does not exceed one per cent per 1000 hours and the permanent deformation does not exceed 0.2 per cent after 45 hours. The necessary data, obtained from Figures 2 and 3, are summarized in Table I. The creep rates and permanent deformations are presented graphically in Figure 8. This figure shows that the creep rates and permissible deformations are very close together at 850~F. and the limiting creep stress is defined by the allowable deformation at 26,000 pounds per square inch.

Table I DIN (DVM) Test Data for 0.50 M5o Steel K-22 at 850 and l000F. Temperature Deg. Fahr. 850 Stress Lb./S. In. 25-35th Hr. Creep Rate /1000 Hour 45th Hr. Estimated Elastic Deformation Deformation In./In. _InLIn. Permanent Deformation Per Cent 20,000 22,500 25,000 27,500 0.24 0.38 0.50 1.20 0.035 0.065.1O40 0.265. 0.00132 0.00170 0.00227 0.00362 0.00026 0.00039 0.00060 0.00076 0.00070 0.00070 0.00070 0.00070 0.00020 0.00029 0.00039 0.00051 0.062 0.100 0.157 0.292 0.006 0.010 0.021 0.025 1000 5,500 7,500 10,000 12,500 0x

7. At 1000~F. the deformations are far less thin the allowable values. Likewise the creep rates are much lower than the permissible 1,0 per cent per 1000 hours. A rather dubious extrapolation of the creep rates indicates a permissible stress of 15,000 pounds per square inch. Constant Stress Cree Tests Four tests were run at both 850 and 1000F. The stresses were selected to cover the creep rate range from 0.01 to 0.10 per cent per 1000 hours. The time-elongation curves are shown as Figures 4 and and the creep rates for 500 hour time intervals are summarized in Table II. The 800 to 1300 hour time period was selected for determination of creep strength since the data are most complete for this time interval and the rates for most of the tests are reasonably constant. The stresses and corresponding creep rates are plotted to logarithmic coordinates in Figure 6 and the following creep strengths were obtained from the curves. Temperature Stress, Lb./Sq.In. for Indicated Creep Rate De.Fahr. 0.01/1000 Hours 0s.o0/1000 Hours 850 20,000 28,500 1000 5,200 15,000 The higher stresses at both temperatures show decreasing rates over the time periods considered for these tests. It

Table II Creep Rates for 0.50 Mo Steel K-22 at 850 and 1000~F. Temperature __Deg. Fahr-. $50 1000 Stress Lb. /Sa. In. 20, 000 22,-500 25,000 27, 500 5,500 7,5.00 10,000 12,500 Creep Rates, 500 to 1000 Hr. Q. 010....0l..o 0.040 0.058 0.013 0.019 0. 45 0.070 % per 1000 Hours, for Indicated Time Intervals 800 to l000 to 1500 to 1300 Hr. 1500 Hr. 2000 Hr.. _ _ _ _ 0.026 0.046 0.08 0. 012 0.020 0.043 0.065 0.010 0.-045 0.066 0.043 0.062 0.044 0.062

9* is probable that longer time tests would revise the strengths upward. The only slight tendency of the lower stresses to decrease with time probably indicates that the time required for minimum rates would be extended to several thousand hours. An experimental reason for irregularities in some of the curves was not observed. MetallograDhic 1Examination The microstructure of the original material and the highest stress creep test specimens at each temperature are shown in Charts 1, 2 and 3. Metallographic specimens were taken lengthwise to the specimen at the center of the gage length. The grain size was 7-8 and the carbide areas were fine slightly spheroidized pearlite. Very little difference in structure could be found between the original material and the creep test specimens. Discussipon of Re sultsA tabular comparison of the results of the short- and long-time tests follows: Test Strength, Lb./Sq.In. 850 1000~F. Hatfield Time Yield 18,000 10,000 DIN (DVM) 26,000 15,000 Constant Stress Creep Test 0.01%/1000 Hr. Strength 20,000 5,200 0.10%/1000 Hr. Strength 28,500 15,000

10. At 850F, the Time-Yield test predicted the Q.01% per 1000 hour creep strength rather closely while the DIN (DVM). test predicted the 0*10% per 1000 hour strength. At 1000PF,, however, the Time-Yield stress was intermediate while the DIN (DVM) test again predicted the 0Q10% per 1000 hour strength. It should be recognized, however, that the DIN (DVM) stress at 1000~F. was based on a very dubious extrapolation of test data.

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Chart 2 Microstructure of Steel W22 (0o50Mo) Completed Creep Test Specimen 1453 Hours at 850~Fo Under a Stress of 27,500 Pounds Xi00D~j I K 4~~~~.^- A d: I X100D

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