Elevated temperature crack growth in nickel-base alloys in carburizing environments
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in a H2-4 pct C H 4 environment. The growth rates in this environment were identical to those in high purity helium, indicating that the carburizing environment had no effect on the fatigue behavior. This result was surprising since gas mixtures of this kind will produce significant carburization of nickel-base alloys at higher temperatures, and carburization can markedly alter the mechanical properties of such alloys. 2 In view of the fact that carburizing atmospheres are widely prevalent in industrial operations it seemed desirable to further explore fatigue behavior in such environments. In this study a brief examination was made of the fatigue properties of NIMONIC* alloy~ *Trademark of the lnco family of companies.
115 at intermediate temperatures in various carburizing environments. EXPERIMENTAL PROCEDURE
blanks were heat treated at 1190 ~ for 2 h, air cooled, followed by 6 h at 1090 ~ and air cooling. Compact tension fracture toughness specimens of the dimensions shown in Fig. I were machined from the blanks. After machining the samples were fatigue precracked at room temperature. Fatigue testing was done in a facility described previously. ~ This unit consists of a mullite environmental retort installed in a closed-loop fatigue machine. Crack lengths were measured by a compliance technique using a displacement gage mounted outside the test chamber. This technique avoids a number of problems when testing in corrosive environments. Heating was provided by a split-tube furnace surrounding the retort. The starting gases used in this study were helium (99.995 pct pure), hydrogen (99.95 pct pure) and methane (98.0 pct pure). These gases were mixed and passed through a calibrated flow regulator to the test retort. Tests were run at a gas flow rate of approximately 1000 cc/min at pressures slightly above atmospheric. Test specimens were brought to temperature and held approximately one hour in the environment before loading. All tests were run under load control using a sinusoidal waveform and a R ratio of 0.10. Most tests were run at a frequency of 0.10 Hz, but some tests at higher and lower frequencies were also conducted as described below. Several creep tests under static load also were conducted. The fracture surfaces of many o f the specimens were examined by scanning electron microscopy. In addition several tests were stopped short of failure and the specimens cross-sectioned and examined optically and by SEM.
Material for this study was taken f r o m commercially produced stock of NIMONIC alloy 115. The chemical composition is given in Table I. Sample S. FLOREEN is Research Fellow, Inco Research & Development Center, Suffern, NY 10901. C. J. WHITE, formerly with Inco Research & Development Center, Suffern, NY, is now with General Electric Corporate Research Center, Schenectady, NY. Manuscript submitted April 20, 1981.
Table I. Chemical Composition of NIMONIC Alloy 115 Used in Present Study Cr
Co
Mo
AI
Ti
C
B
Zr
Ni
14.3
13.2
3.3
4.9
3.7
0.15
0.016
0.04
Bal
ISSN 0360-2133 / 81 / 1111-197
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