Zig-Zag Grain Boundaries and Strength of Heat Resisting Alloys

The role of zig-zag grain boundaries with coarse carbides for the deformation and fracture behaviors in creep was studied in an austenitic heat resisting steel having wide range of hardness. With increase in hardness, the rupture strength increases until maximum strength is reached at some critical value of hardness and then decreases in spite of decrease in creep rate. From the analysis of fracture mode, it was clarified that the transition of fracture mode from ductile to brittle with increasing hardness is responsible for this variation in the rupture strength. Zig-zag grain boundaries with coarse carbides can remarkably increase the rupture strength by increasing the critical hardness as a result of preventing the formation and the growth of cracks and cavities along grain boundaries. It was found that the combination of zig-zag grain boundaries and high hardness is quite beneficial to improve the rupture strength and that for this purpose, two step cooling process, i.e. direct furnace cooling to about 9OO'C after solution heating followed by rapid cooling, is very effective for both Fe base and Ni base super-alloys. For Ni base superalloy containing low carbon, however , directly aging treatment to about 9OO'C is more useful than two step cooling process.