Elevated-temperature combined stress-rupture plus fatigue strength of waspaloy having having different aging treatments ...

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Material Information

Title:
Elevated-temperature combined stress-rupture plus fatigue strength of waspaloy having having different aging treatments and/or molybedenum contents
Series Title:
NACA RM
Physical Description:
21, 2 p. : ill. ; 28 cm.
Language:
English
Creator:
Hoffman, Charles A
Hornak, M. B
Lewis Research Center
United States -- National Advisory Committee for Aeronautics
Publisher:
National Advisory Committee for Aeronautics
Place of Publication:
Washington, D.C
Publication Date:

Subjects

Subjects / Keywords:
Nickel alloys -- Fatigue   ( lcsh )
Metals -- Fatigue   ( lcsh )
Genre:
federal government publication   ( marcgt )
bibliography   ( marcgt )
technical report   ( marcgt )
non-fiction   ( marcgt )

Notes

Bibliography:
Includes bibliographical references (p. 7).
Statement of Responsibility:
by C.A. Hoffman and M.B. Hornak.
General Note:
Cover title.
General Note:
"Report date February 25, 1958."

Record Information

Source Institution:
University of Florida
Rights Management:
All applicable rights reserved by the source institution and holding location.
Resource Identifier:
aleph - 003852979
oclc - 54102874
sobekcm - AA00006134_00001
System ID:
AA00006134:00001

Full Text


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RM E57K22a


SEARCH MEMORANDUM
ING EATMENTS AND/OR MOLYBDENUM CONTENTS
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:By C. A. Hoffman and M.. B. Hornak
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.Lewis Flight Propulsion Laboratory
.:.:Cleveland, Ohio....
SEI~SgVATED-TEMPERATURE COMBINED STRESS-RUPTURE PLUS

j:?i .. *"* r.. :. .

. IG TREATMENTS AND/OR OLYBDENM CONTENTS

SByC. A. Hoffman and M. B. Ho rak

SLewis Flight Propulsion Laboratory
,. .. Cleveland, Ohio

t .:;: UNIVERSITY OF FLORIDA
.i. DOCUMENtS DEPARTMENT
ir'.'.. .. 120 MARSTON SCIENCE LIBRARY
+ +i .1:.- By: C. AP.O. BOX 117011
I a:' Q"JNESVILLE, FL 32611-7011 USA
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1I7 ADVISORY. COMMITTEE

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Sary9 25, 1958
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NACA RM E57K22a


NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS


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


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 15000 F with direct tensile cyclic stresses
superimposed upon direct tensile mean stresses. Mean'tensile stresses
were selected from 15000 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 RM E57K22a


of three lots of Waspaloy; these lots had different aging treatments
and/or molybdenum contents. For 15000 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 15000 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 & 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.








NACA 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 15000 F with direct tensile cyclic stresses superimposed upon direct
tensile mean stresses. The mean tensile stresses were selected to yield
15000 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, 67, 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
1/8 inch below the fracture surface.


RESULTS AND PISCUSSIOII

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 RM E57K22a


Both 15000 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 15000 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 (1 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 transgranularly in all three groups of
Waspaloy. Fatigue fracture of groups A and B is illustrated in figure
11(a) and of group C in figure 11(b). Intergranular failure along the
sides of fatigue-failed specimens was quite prevalent.







NACA 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-ruptLre plus fatigue
strength and stress-rupture ductility was observed. However, no relation
between combined stress-rupture plus fatigue strength and dynamic ducti-
lity ductilityy 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-
ienumr 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 E52117, 1952.


2. Hoffman, Charles A.:
5765A (S-816) Alloy
tures. Proc. ASTM,


Strengths and Failure Characteristics of AMS
in Direct Tensile Fatigue at Elevated Tempera-
vol. 56, 1956, pp. 1063-1080.














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NACA RM E57K22a


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NACA RM E57K22a


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NACA RM E57K22a


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NACA RM E57K22a


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NACA RM E57K22a


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