Elevated temperature fatigue crack growth in incoloy alloy 800 in sulfidizing environments
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I.
INTRODUCTION
C R A C K growth in creep and fatigue at elevated temperatures has become a problem of considerable concem in recent years. One aspect which has received increasing attention is the role of the environment on crack growth. A number of studies have shown that air or oxidizing environments may increase crack growth rates by an order of magnitude compared to behavior in inert environments. Thus far almost no attention has been paid to sulfidizing environments. Preliminary tests by Floreen and Kane I indicated that the fatigue crack growth rate of INCONEL* alloy
tip often blunted the crack and halted further growth. This necessitated the use of fairly high stress intensity ranges in the tests so that crack advance would take place more rapidly than crack stoppage, especially for a relatively low-strength material such as alloy 800. Many of these tests were necessarily conducted at stress-intensity factors above the limits of linear elastic fracture mechanics conditions. The data, therefore, should be used with caution for design considerations. However, the results should be reasonably consistent for comparison of the environmental effects, which was the primary aim of the study.
*Trademark of the Inco family of companies.
II.
718 at 650 ~ was severely affected by exposure to He-5 pct SO2 and He-0.5 pct H2S gas mixtures. Unstressed material exposed to these environments showed only interference colors on the surface and no signs of subsurface attack. Cracking generally occurred by a mixture of transgranular and intergranular modes. Scarlin 2 tested IN-738LC at 850 ~ and 100 Hertz frequency in a mixture of powdered oxides and sulfates through which was passed a gas mixture of air and .015 vol pct SO2 and .015 vol pct SO3. Only a slight increase in crack growth rate compared to tests in air was observed, perhaps because the high frequency minimized the time needed for any environmental effects to be manifested. Outside of this limited work, it appears that no studies of elevated temperature crack growth in sulfidizing environments have been conducted. Part of the reason for this lack of data in sulfidizing environments may be due to the problems associated with performing such tests. In the present study fatigue crack growth rates in INCOLOY* alloy 800 specimens were measured in some He-SO2 and He-H2S environments at temperatures ranging from 316 to 650 ~ As discussed below, and as found in the earlier study,~ the formation of sulfide particles at the crack
Test material was commercially made plate stock of INCOLOY alloy 800. The chemical composition is given in Table 1. Specimen blanks were solution annealed in air one hour at 1145 ~ and air cooled to produce an equiaxed grain structure with an average grain size of 0.07 mm. Compacttype (CT) specimens of the dimensions shown in Figure 1 were machined from the heat treated stock. Before testing, . the samples were precracked in fatigue at room temperature and at low stress intensity ranges. Testing was done in the facility described in Reference 1.
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