Further Observations on Sensitization of Chemically Stabilized Stainless Steels^⋆

Niobium additions to an 18Cr:8Ni type matrix reduce carbon solubility to such an extent that any “solution treatment” below at least 1150 C causes stabilization. Consequently, no Cr₂₃C₆ precipitates at lower temperatures to sensitize the structure. Further observations on Type 321 suggest that two types of TiC precipitate from solid solution in an 18:8 type matrix. The size of the TiC nucleus decreases with the precipitating temperature. Above 1050 to 1100 C the initial TiC is probably incoherent, large enough to be stable and resistant to ferric sulfate-sulfuric acid solution. Below 1050 to 1100 C the initial TiC, known as “dot TiC” or “TiC on dislocations”, is probably coherent, not large enough to be stable without further growth, and not resistant to ferric sulfate-sulfuric acid solution. During holding at temperatures below 1050 to 1100 C, stabilization occurs as the TiC on dislocations agglomerates to larger, incoherent particles. The time required increases as the temperature decreases down to the minimum TiC nucleation temperature near 610 C. Cold work makes it easier for the coherent particles to become incoherent, in effect facilitating approach to equilibrium carbon solubility at any temperature. Once chromium carbide forms, dissolved titanium eventually reacts with it, forming TiC and releasing chromium to desensitize the structure. This reaction can occur, given sufficient time for titanium diffusion, at any temperature at which chromium carbide nucleates. It is much more rapid than back diffusion of chromium from the matrix.