Formation of w-shaped microstructure and large hardening depth in 304 austenitic stainless steel through cryogenic explosion shock treatment

Abstract

In the present work, cryogenic explosion shock treatment (CEST) was used to treat 304 austenitic stainless steel (304 ASS), a w-shaped microstructure and a large hardening depth formed, and the corresponding mechanical properties were enhanced. The grain size distribution, volume fraction of α′-martensite and dislocation density in CEST treated 304 ASS and untreated ones were characterized by various techniques. The observation results showed that a gradient microstructure with five distinct layers formed in CEST treated specimen. According to XRD analysis, the volume fraction of α′-martensite and dislocation density in CEST treated specimen were found to follow a w-shaped trend. A numerical simulation was carried out for clearly illustrating the shock wave propagation process in CEST. The reflection of the explosive shock wave is identified as the cause for the formation of the w-shaped microstructure. Moreover, mechanical properties of CEST treated specimen were measured by Vickers hardness testing. The experimental results indicated that maximum hardness increased by 111.6% at the top surface, 65% at the central tensile layer, and 68.1% at the bottom surface. Combining microstructure characterization and mechanical properties testing, various significant strengthening mechanisms in CEST treated specimen were proven: grain refinement, martensitic transformation, dislocation slip and twinning.

Keywords: 304 austenitic stainless steel, w-shape microstructure trend, cryogenic explosion shock treatment, large hardening depth