Modification of S phase on austenitic stainless steel using fine particle shot peening

Hardening of the S phase by fine particle shot peening was investigated. The S phase created by carburizing has an austenite phase expanded by supersaturated diffusion of interstitial carbon. The austenite phase has good strain hardenability. Therefore, the expanded austenite structure is also expected to have it. With plasma carburizing for 14.4 ks, S phase of ca. 15 μm was produced by carbon diffusion on the austenitic stainless steel surface substrate. Fine particle shot peening was conducted on the S-phase surface with two different grits and different shot speeds controlled using air pressure of 0.1–0.9 MPa. Then the layer hardness was measured and the microstructures of specimens were analyzed. The carbon concentration profiles were also measured using glow discharge optical emission spectrometry. Results showed that the S phase can be hardened without cracking by shot peening, and that the origin of the hardness increase is strain hardening of the S phase itself.

► Fine particle shot peening can increase S phase hardness with no cracking.
► No newly developed phase was observed in the S phase layer.
► S phase maintained an FCC structure despite the severest peening conditions.
► Higher peening power enables the hardening of substrate beneath the S phase layer.