Grain growth and precipitation in nanostructured 304SS after heat treatment

Specimens of 304 stainless steel (304SS) were ultrasonic shot peening (USP) treated for refined and gradient microstructure. The specimens were then heat-treated at 500 °C, 600 °C, and 700 °C for 2 hrs, respectively. The hardness and microstructure of the untreated, shot peened, and heat-treated specimens were investigated. Grain growth and nanoscale precipitation were apparent only in the 700 °C heat treatment specimen and the microstructural analysis was focused on that specimen. The gradient microstructure from the top were characterized as: (i) nanocrystalline layer, with very little grain size of ∼200 nm, (ii) ultrafine grain layer in ~μm size with nanosize M₂₃C₆ in grain interior, and (iii) deformed coarse grain layer, with grains in ~50 μm and the M₂₃C₆ were in the grain interior and boundaries. The nanoscale precipitates, distributed on the original lamellas and deformed twin boundaries, inhibited the grain growth, and strengthened the peening affected layers.