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 RM E57K22a 
 
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 RESEARCH MEMORANDUM 
 
 ELEVATED-TEMPERATURE COMBINED STRESS-RUPTURE PLUS 
 
 FATIGUE STRENGTH OF WASPALOY HAVING DIFFERENT 
 
 AGING TREATMENTS AND/OR MOLYBDENUM CONTENTS 
 
 By C. A. Hoffman and M. B. Hornak 
 
 Lewis Flight Propulsion Laboratoiy 
 Cleveland, Ohio 
 
 UNIVERSITY OF FLORIDA 
 DOCUMENTS DEPARTMENT 
 120 MARSTON SCIENCE LIBRARY 
 P.O. BOX 117011 
 GAINESVILLE, FL 32611-7011 USA 
 
 NATIONAL ADVISORY COMMITTEE 
 FOR AERONAUTICS 
 
 WASHINGTON 
 
 February 25, 1958 
 
f^/l 03 3- l'^ 3?^ ^^''^ 
 
 NACA RM E57K22a 
 
 NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS 
 
 RESEARCH MEMORANDUM 
 
 ELEVATED-TEMPERATURE CCMBINED STRESS -RUPTURE PLUS FATIGUE 
 
 STRENGTH OF WASPALOY HAVING DIFFERENT AGING 
 
 TREATMENTS AOT)/OR MOLYBDENUM CONTENTS 
 
 By C. A. Hoffman and M. B. Hornak 
 
 SUMMARY 
 
 An investigation was conducted to determine if the combined stress- 
 rupture plus fatigue strengths of three groups of Waspaloy with different 
 aging treatments and/or molybdenum contents could be correlated with 
 their resultant stress -rupture ductilities and notch-rupture strengths. 
 
 Fatigue tests were run at 1500° F with direct tensile cyclic stresses 
 superimposed upon direct tensile mean stresses. Mean" tensile stresses 
 were selected from 1500° F stress-rupture data to produce stress-rupture 
 failure times of about 10, 100, and 500 hours. Cyclic tensile stress 
 levels were chosen to equal 12.5 to 90 percent of the mean tensile 
 stresses. 
 
 A possible direct relation between combined stress -rupture plus 
 fatigue strength and stress -rupture ductility of Waspaloy was indicated. 
 However, no clear relation between combined stress-rupture plus fatigue 
 strength and notch-rupture strength was found. If heat treatment and/or 
 composition alone (i.e., without regard to their effects upon basic prop- 
 erties such as ductility, etc.) are considered, the results indicate that 
 the double-aging heat treatment (compared to a single age) increases the 
 combined stress-rupture plus fatigue strength of Waspaloy and that the 
 double-aging treatment plus increased molybdenum content causes further 
 improvement in the combined stress -rupture plus fatigue strength of 
 Waspaloy. 
 
 INTRODUCTION 
 
 Waspaloy is of interest as a gas turbine-bucket alloy because of its 
 low strategic material content and its relatively good stress-rupture 
 properties. The developer of this alloy had investigated the stress- 
 rupture strength, stress -rupture ductility, and notch-rupture strength 
 
NACA R14 E57K22a 
 
 of three lots of Waspaloy; these lots had different aging treatments 
 and/or molybdenum contents. For 1500° F, the data revealed that the 
 three lots possessed fairly similar stress-rupture strengths but dif- 
 fered in both notch-rupture strength and stress -rupture ductility. Two 
 of these lots, group A (single aged) and group B (double aged), which 
 were from the same heat and contained 3 percent molybdenum, were notch 
 strengthened; the third lot (group C) which was double aged and con- 
 tained 7 percent molybdenum, either had the same strength or was slightly 
 weaker when notched. The first lot of Waspaloy possessed the lowest 
 stress-rupture ductility whereas the third lot had the highest stress- 
 rupture ductility. Since the third lot had the best stress-rupture duc- 
 tility but not the best notch-rupture strength, stress-rupture ductility 
 and notch-rupture strength might not be directly related. 
 
 One might hypothesize that among groups of material having comparable 
 stress -rupture properties, the group exhibiting either or both the great- 
 est stress-rupture ductility or notch-rupture strength would have the 
 greatest time to failure in combined stress-rupture and fatigue (partic- 
 ularly where the fatigue component is quite large); hence, a preliminary 
 study of the foregoing three groups of Waspaloy was carried out to deter- 
 mine if a correlation might exist between combined stress-rupture plus 
 fatigue life and stress-rupture ductility and/or notch-rupture strength. 
 
 In this investigation, endurance tests were run at 1500° F with 
 direct tensile vibratory stresses superimposed upon direct tensile mean 
 stresses for three lots of Waspaloy. The amplitude of the cyclic stresses 
 was as great as 90 percent of the mean stresses. 
 
 MATERIALS, APPARATUS, AND PROCEDURE 
 
 Pratt & Whitney (who developed Waspaloy) furnished heat-treated 
 stress -rupture specimens. These specimens were from the same lots and 
 had the same heat treatments developed by Pratt <y Whitney for the speci- 
 mens referred to in the INTRODUCTION. The chemical analyses and the de- 
 scription of the heat treatments furnished with the specimens are given 
 in tables I and II, respectively. Grain size and initial hardnesses ob- 
 tained at the NACA Lewis laboratory are also presented in table II. For 
 convenience, the designations A, B, and C will continue to be used for 
 the single aged - 3 percent molybdenum, double aged - 3 percent molyb- 
 denum, and double aged - 7 percent molybdenum groups, respectively (ta- 
 bles I and II) . 
 
 The specimens used in this investigation are illustrated in figure 
 1. The surface finish of the specimen test section was 5 to 10 micro- 
 inches root mean square. 
 
KACA RM E57K22a 
 
 Commercial direct tensile stress fatigue machines operating at ap- 
 proximately 2000 rpm (fig. 2) were used. A description of these machines 
 together with the operating procedure is presented in references 1 and 2. 
 
 Combined stress tests were run at the Lewis laboratory on specimens 
 at 1500° F with direct tensile cyclic stresses superimposed upon direct 
 tensile mean stresses. The mean tensile stresses were selected to yield 
 1500° F stress-rupture lives of approximately 10, 100, and 500 hours at 
 zero vibratory stress. The mean stresses selected to give these above 
 approximate lives differed somewhat for each of the three groups of spe- 
 cimens. The amplitudes of the cyclic tensile stresses were chosen to 
 equal 12.5, 25, 50, 87, and 90 percent of the mean tensile stresses. The 
 stress-rupture ductility, the notch-rupture, and a portion of the stress- 
 rupture data used herein are those already referred to in the INTRODUCTION. 
 
 A statistical survey of all tests showed that during any one test, 
 the average values of the maximum and minimum loads (i.e., mean loads) 
 were generally within 1 percent of the desired loads and the extreme 
 values of the maximum and minimum loads were generally within 5 percent 
 of the desired loads. (in general, the extreme values of the maximum or 
 minimum unit stresses were within 1500 psi of the desired values with 
 the greater number of the tests controlled within 1000 psi.) 
 
 Samples of as -received Waspaloy were examined to determine the micro- 
 structure of groups A, B, and C after heat treatment and prior to testing. 
 
 Fractured specimens were examined macroscopically and classified 
 according to the type of failure (stress-rupture, stress-rupture plus 
 fatigue, and fatigue) as described in reference 2. Microspecimens were 
 mounted to show typical intergranular or transgranular cracking and to 
 show fracture surface appearance for each of the three groups of Waspaloy. 
 
 Diameters of the failed specimens were determined at the failure 
 zone. These data were used to compute the percent reduction in area at 
 failure. Hardness measurements, obtained in Rockwell-A units and con- 
 verted to Rockwell-C units, were taken on a transverse section 1/16 to 
 l/s inch below the fracture surface. 
 
 RESULTS AND DISCUSSION 
 
 Combined Stress-Rupture Plus Fatigue Tests 
 
 The metallurgical data presented in table II show that group C 
 Waspaloy (7 percent molybdenum and double aged) had higher stress -rupture 
 ductility as mentioned previously and somewhat higher hardness than groups 
 A and B (3 percent molybdenum alloy, single aged and double aged, 
 respectively) . 
 
NACA EM E57K22a 
 
 Both 1500° F stress-rupture and notch-rupture data are plotted in 
 figure 3. It can be noted from this figure that the NACA stress-rupture 
 data tend to lie somewhat above the Pratt & Whitney data for groups A 
 and C, whereas for group B the data lie on the same curve. The differ- 
 ence in the case of groups A and C are likely random effects. Hence, a 
 single rupture curve has been drawn for each group. A study of this 
 figure indicates that groups A and B are notch strengthened in stress 
 rupture, while group C is either unaffected or slightly notch weakened. 
 The stress -rupture properties of the three groups can be considered es- 
 sentially the same in view of the small observed differences. 
 
 The results of this investigation are presented in figure 4 as a 
 plot of the mean stress against time to failure at 1500 F with the 
 cyclic stress ratio held constant. (The cyclic stress ratio is defined 
 as the ratio of the amplitude of the cyclic stress to the mean stress.) 
 In order to conveniently evaluate the data presented in figure 4, the 
 data have been cross-plotted in figure 5 as cyclic stress ratio against 
 time to failure at 1500° F and constant mean stress. A study of figure 
 5 indicates that for the conditions studied, stress ratios of a given 
 magnitude generally caused greater reductions in the life of group A 
 (notch strengthened) than in group B (notch strengthened) and had least 
 effect upon group C (slightly notch weakened or unaffected). At the low 
 mean stress (20,000 psi) and high cyclic stress ratios (0.50 to 0.90) 
 groups B and C appeared to have considerably better resistance to com- 
 bined stress than group A. 
 
 Consideration of the foregoing results suggests that a capacity for 
 notch-rupture strengthening does not necessarily result in improved com- 
 bined stress-rupture plus fatigue strength. In fact, a tendency towards 
 notch weakening may not necessarily be expected to be harmful. 
 
 As can be noted from table II, the stress-rupture ductility of group 
 C was appreciably better than of groups A or B and B was slightly better 
 than A. Further, as indicated previously, group C had the best combined 
 stress-rupture plus fatigue strength, group B had intermediate strength, 
 and group A had the lowest strength. Thus, there is indication of a pos- 
 sible relation between stress-rupture ductility and combined stress- 
 rupture plus fatigue strength. 
 
 Reductions in area at failure (dynamic ductility) in combined stress- 
 rupture plus fatigue tests plotted against time to failure are presented 
 in figure 6. A study of this figure indicates that group C is generally 
 most ductile followed in ductility by groups B and A over the range of 
 cyclic stress ratios and mean stress levels studied. Further, it can be 
 noted that the ductilities of the three groups decreased as cyclic 
 stresses increased and tended to become quite small (l to 2 percent) and 
 about the same order of magnitude at the higher cyclic stress ratios. 
 This particular behavior is quite interesting since it demonstrates that 
 
NACA RM E57K22a 
 
 while differences in dynamic ductility (as measured herein) become very 
 small, combined stress-rupture plus fatigue strengths still may differ 
 considerably for the three groups. Hence, one might conclude that com- 
 bined stress-rupture plus fatigue strength is not directly related to 
 dynamic ductility. 
 
 If the heat treatment and/or composition of the alloys studied in 
 this investigation are considered alone (i.e., without regard to their 
 resultant effects upon the basic properties of ductility, etc.), the fol- 
 lowing observations may be made: Double aging has improved the combined 
 stress-rupture plus fatigue strength of the Waspaloy with the 3 percent 
 molybdenum content. Additional molybdenum in conjunction with the 
 double-age heat treatment further improved the combined stress-rupture 
 plus fatigue properties. 
 
 Metallurgical Evaluation of Failed Specimens 
 
 The structures of the three groups of Waspaloy after heat treatment 
 and prior to testing are presented in figure 7. Groups A and B have 
 about the same grain size, while group C average grain size is slightly 
 finer. Group C has a duplex grain structure and pronounced spheroidiza- 
 tion of the grain boundaries. Inasmuch as a nonuniform grain size struc- 
 ture has generally been considered to cause reduced life in turbine 
 buckets, the possibility of a duplex grain structure causing reduced 
 life under conditions of combined stress-rupture plus fatigue may be 
 raised. However, group C with a duplex grain structure is indicated as 
 having as good or better strength than the groups with a uniform grain 
 structure. 
 
 The macroscopic appearance of the specimens at failure is summarized 
 in table III. Photomicrographs illustrating the three types of failure 
 are presented in figure 8. There did not appear to be any significant 
 difference in the macroscopic failure behavior associated with the three 
 groups of Waspaloy. 
 
 Stress-rupture fracture appears to have been initiated intergranu- 
 larly and then progressed in a predominately transgranular fashion. This 
 was the case for all three groups of Waspaloy. Intergranular initiation 
 of fracture is illustrated in figure 9(a) for groups A and B; figures 
 9(b) and (c) illustrate intergranular initiation of cracking in group C, 
 where the surface grains were fine and coarse, respectively. The progres- 
 sion of fracture through groups A and B and through group C is illustrated 
 in figures 10(a) and (b), respectively. 
 
 The fatigue areas fractured transgranular ly in all three groups of 
 Waspaloy. Fatigue fracture of groups A and B is illustrated in figure 
 ll(a) and of group C in figure ll(b). Intergranular failure along the 
 sides of fatigue-failed specimens was quite prevalent. 
 
MCA RM E57K22a 
 
 Stress-rupture plus fatigue failures combined the metallographic 
 features of both stress-rupture failed specimens and fatigue failed 
 specimens. 
 
 The microstructures of the failed specimens do not offer any appar- 
 ent explanation for the differences in the combined stress strengths of 
 the three groups of Waspaloy. 
 
 The hardnesses at failure are presented in figure 12. These data 
 do not show any pronounced trend between the hardness at failure and 
 cyclic stress ratio; however, group C hardness values are higher than 
 groups A and B. 
 
 SUMMARY OF RESULTS 
 
 This investigation, conducted 'to determine if the combined stress - 
 rupture plus fatigue strengths of three groups of Waspaloy could be corre- 
 lated with either their stress -rupture ductilities or notch-rupture 
 strengths, yielded the following results: 
 
 1. No clear relation occurred between combined stress -rupture plus 
 fatigue strength and notch-rupture strength. Contrary to what might be 
 expected, a group of specimens with least notch-rupture strength proper- 
 ties (slightly notch weakened or unaffected) had the best combined 
 stress-rupture plus fatigue properties. 
 
 2. A possible relation between combined stress-rupture plus fatigue 
 strength and stress -rupture ductility was observed. However, no relation 
 between combined stress-rupture plus fatigue strength and dynamic ducti- 
 lity (ductility at failure in combined stress) was observed. 
 
 3. If the heat treatments and/or compositions of the alloys studied 
 in this investigation are considered alone (i.e., without regard to their 
 resultant effects upon the basic properties of ductility, etc.), the 
 following may be observed: Double aging has improved the combined stress- 
 rupture plus fatigue strength of the Waspaloy with the 3 percent molyb- 
 denum content. Additional molybdenum, in conjunction with the double- 
 aging treatment further improved the combined stress -rupture plus fatigue 
 properties. 
 
 4. The high molybdenum (7 percent) Waspaloy group had a duplex grain 
 structure; this type of structure might be questioned as having a dele- 
 terious effect upon resistance to combined stress -rupture plus fatigue. 
 However, this group generally exhibited best combined stress strength. 
 
 Lewis Flight Propulsion Laboratory 
 
 National Advisory Committee for Aeronautics 
 Cleveland, Ohio, December 10, 1957 
 
NACA RM E57K22a 
 
 REFERENCES 
 
 1. Ferguson, Robert F.: Effect of Magnitude of Vibratory Load Super- 
 
 imposed on Mean Tensile Load on Mechanism of and Time to Fracture 
 
 of Specimens and Correlation to Engine Blades. NACA RM E52I17, 1952. 
 
 2. Hoffman, Charles A.: Strengths and Failure Characteristics of AMS 
 
 5765A (S-816) Alloy in Direct Tensile Fatigue at Elevated Tempera- 
 tures. Proc. ASTM, vol. 56, 1956, pp. 1063-1080. 
 
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NACA RM E57K22a 
 
 11 
 
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 Figure 2. - Direct tensile stress fatigue machine. 
 
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 Time to failure, hr 
 
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 Figure 3. - Variation of time to failure at 1500° F with static stress. 
 
NACA EM E57K22a 
 
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 1000 
 
 Time to failure, hr 
 
 (c) Group C (7 percent molybdenum, double aged). 
 
 Figure 4. - Variation of time to failure with cyclic mean stress at constant stress ratio and 1500° F. 
 
14 
 
 NACA EM E5TK22a 
 
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NACA EM E57K22a 
 
 15 
 
 
 
 
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16 
 
 NACA EM E57K22a 
 
 
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 ("b) Group Bj transverse section; X750 
 
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 (c) Group Cj longitudinal section; XlOO 
 
 (d) Group C; longitudinal section; X250 
 
 Figure 7- - Structure after heat treatment and prior to testing; electrolytically 
 etched with HC]>HN0_+H20. Magnification reduced 42 percent in reproduction. 
 
NACA RM E57K22a 
 
 17 
 
 (a) Stress-rupture failure ; X3 (b) Stress-ruptxire plus fatigue failure; X3.5 
 
 C-46623 
 
 (c) Fatigue failure; X3.5 
 
 Figure 8. - Three types of failure of Waspalloy. Magnification 
 reduced I7 percent in reproduction. 
 
18 
 
 NACA BM E57K22a 
 
 
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NACA EM E57K22a 
 
 19 
 
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 cMmfm^' 
 
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 C-46625 
 
 (t)) Fractiu-e in specimens of group C. 
 
 Figure 10. - Stress-rupture fractiu-e surfaces. Electrolytically 
 etched with HCL+HHOj+HgO ; X250. Magnification reduced 6 percent 
 in reproduction. 
 
 Load 
 
 Load 
 
 \ 
 
20 
 
 MCA EM E57K22a 
 
 i 
 
 Load 
 
 (a) Specimens of groups A and B. 
 
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 Figure 11, - Fatigue fractvire surfaces. Electrolytlcally etched with 
 HCL+HNOg+H-O ; X250 . Magnification reduced 6 percent in reproduction. 
 
 Load 
 
NACA EM E57K22a 
 
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 NACA - Langley Field, V. 
 
UNIVERSITY OF FLORIDA 
 
 3 1262 08106 590 5 
 
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