A Theoretical Study on the Formation of Carbide and Chromium-Depleted-Layer in Austenitic Stainless Steel

Abstract

It is well known that the occurrence of intergranular corrosion in austenitic stainless steel is caused by the chromium depletion at the grain boundary, where the chromium carbide formed. However, no clear physical interpretations for the formation of carbide and chromium depleted layer have been given yet.
The purpose of this paper is to summarize mathematically what is the rate determing process for the formation of carbide.
It is concluded that: 1) the diffusion of carbon to the grain boundary is too rapid to be the rate determing process; 2) as the chromium diffusion is relatively slow below 500°C, the supply of chromium to the grain boundary determines the rate of precipitation; and 3) above 550°C, the formation of carbide itself is the rate determing process.
From the above conclusions, the Time-Temperature-Sensitization (TTS) diagram can be calculated under certain assumptions.
The theoretical TTS diagram agrees well with the experimental results.