Sensitization of manganese-stabilized austenitic steels

Abstract

In an earlier study of the phase stability of manganese-stabilized austenitic steels based on Fe-12 Cr-20 Mn-0.25 C (wt %), significant precipitation of tau carbides (M23C6) was found to occur both in the matrix and at grain boundaries during aging near 600°C. Because of the similarity of the observed microstructures to those of sensitized steels and the evidence of preferential grain boundary attack in extraction replicas, the susceptibility of these alloys to sensitization relative to a type 304L nickel-stabilized stainless steel was investigated with the use of corrosion tests and analytical electron microscopy. Four Mn-stabilized steels (IT-2 - IT-5) were prepared with different minor additions (Table 1). These alloys and the type 304L steel were solution annealed at 1150°C (SA) and subsequently aged for 2 h at 650°C. The steels were checked for sensitization in both the SA and aged conditions by immersion in a boiling, acidified CuSO4 solution for 48 h (modified-Strauss test, ASTM A 262-E). The degree of sensitization was indicated by the extent of grain boundary attack and the tendency for grain boundary failure during immersion or post-immersion bending. The type 304L steel was not sensitized in the SA condition and only mildly sensitized as aged (only slight intergranular cracking during bending). On the other hand, though not sensitized in the SA conditions, the aged Mn-stabilized steels were severely attacked during the modified-Strauss test. They generally fractured intergranularly under their own weight or internal stresses after removal from the solution. This indicated a high degree of sensitization at 650°C.