Evaluation of the Mechanical Properties for Reversion Heat Treated Thermally Embrittled Duplex Stainless Steel

Abstract

The thermal aging embrittlement of duplex stainless steels is one of the key material property degradation that would limit their industrial applicability. In this investigation, we study the effect of reversion heat treatment on the mechanical properties of the thermally embrittled steels. The samples were solutionized, aged, reversion heat treated and re-aged. The tensile strength of the aged sample had increased with respect to the solutionized condition because in aged condition, the ferrite phase was spinodally decomposed into iron rich alpha (α) and chromium rich alpha-prime (αˈ) precipitates and also the chromium nitride precipitates was found along with these precipitates. The 60 minutes reversion heat treated samples showed a maximum recovery in tensile strength of upto 92% with respect to the solutionized condition because the temperature of 550 ⁰ C is above the (α + αˈ) miscibility gap, the ferritic phase was homogenized again. In other words, Ferich α and Cr-rich αˈ prime precipitates which were formed during ageing become thermodynamically unstable and dissolve inside the ferritic phase. As the reversion heat treatment time was increased the recovery induced was decreasing because of the formation of the secondary austenite and R-phase in the ferrite phase. The 60 minutes reversion heat treated sample was then again re-aged at 475 ⁰ C for varying periods inorder to check the re-embrittlement rate and the applicability of the reversion heat treatment. The tensile and impact strength had again increased significantly, which was almost similar to that of the aged sample and there was no much difference in the re-embrittlement rate. The high resolution transmission microscopy revealed that the spinodal decomposition again remained as the primary mechanism for the embrittlement of the ferrite phase. From fatigue test it was clear that fatigue strength was sensitive to the heat treatment condition. In this study the fatigue strength in embrittled state was higher than solutionized and reversion heat treated condition. In all heat treated conditions, the major resistance to crack growth came from α/γ phase boundaries because the α/γ phase boundary offers more resistance to the slip transfer as compared to α/α and γ/γ grain boundaries.