High-temperature phase transformations in AISI 316 stainless steel infused with concentrated interstitial carbon

Abstract

For the austenitic stainless steel AISI 316 treated by low-temperature carburization, we report a first study on high-temperature phase transformations (up to the melting point). This study was carried out on uniformly carburized foils of AISI 316. Upon heating, M23C6 was found to precipitate in grain boundaries between 900 and 1100 K. At around 1325 K, these precipitates dissolve. Upon heating to 1473 K and subsequent cooling, the carbide M7C3 forms in grain boundaries and then also within the austenite grains. The Cr-depletion caused by precipitation of Cr-rich carbides converts austenite to ferro-nickel, causing the material to become ferromagnetic. Unless oxygen is available at the alloy surface, carbon is not lost from the alloy. Cooling down already after reaching 1173 K retains a substantial fraction of the initial interstitial carbon in solid solution, meaning that AISI 316 foils surface-engineered by low-temperature carburization have significant “accident tolerance.” Even after exposure to temperatures close to the melting point (1473 K), the mechanical properties will be improved over those of non-carburized AISI 316, owing to precipitation hardening by an exceptionally high volume fraction of intra-granular carbide.