An investigation of the deformation and aging characteristics of Inconel X-750 at elevated temperatures

Abstract

Results of the aging studies in the temperature range of 1300-1600°F show that substantial hardness increase occurs for all temperatures within the first minute of aging. The initial increase in hardness at 1500°F is associated with clustering and this clustering leads to the generation macrostresses. Precipitation of coherent γ' was found to follow the clustering stage and this precipitation was found to follow a "C-curve" time-temperature relationship. Although regions denuded of γ' have been reported in the vicinity of carbides after long aging periods, no evidence of such denudation was found in samples aged up to 30 minutes in the range of 1300-1600°F.

All testing was done on the R.P.I. Gleeble, and the resultant microstructures were examined using both optical and transmission microscopy. Fracture morphology studies were also made using standard replication techniques supplemented by scanning electron microscopy.

The mechanical response to stress in the temperature range 1000-1750°F was obtained from short-time tensile tests at a series of loading rates ranging from 0.16 in./min. to 16.0 in./min. Aging studies were carried out in the temperature range of 1300-1600°F. In addition, the effect of stress and strain on the aging process was examined.

The problem of post-weld heat-treatment cracking in precipitation hardenable high temperature alloys has often limited the commercial use of these materials. this cracking usually occurs while welded assemblies are being heated through a narrow temperature range, 1200-1600° F, during a thermal stress relief treatment. This investigation was undertaken to determine the factors that influence this form of cracking in a relatively simple age-hardenable superalloy, Inconel X-750.

Although evidence was found that both fracturing of the carbides and separation at the carbide-matrix interface occurred as a result of plastic deformation, grain boundary carbides appeared to inhibit grain boundary sliding. No evidence was found of embrittlement due to either stress-induced or strain-induced precipitation of either γ' or carbides during the short-time elevated temperature tests.

A mechanism has been proposed which explains the phenomenon of post-weld heat-treatment cracking of Inconel X-750.

The results of the short-time elevated temperature tensile tests revealed that the tensile ductility of Inconel X-750 loaded at rates of 0.16-16.0 in./min. exhibited a minimum in the vicinity of 1600°F. The ductility at the minimum was found to be strain rate sensitive and failure in the temperature range where the ductility minimum occurred was found to be associated with grain boundary sliding.