Origin of low temperature toughness in a 12Cr-10Ni martensitic precipitation hardenable stainless steel

Though ductile to brittle transition (DBT) is a typical feature of body centered cubic materials, the present 12Cr-10Ni precipitation hardened martensitic stainless steel exhibited excellent low temperature impact toughness. This was, however, dependant on the aging temperature. 250 °C aging led to higher toughness both at room temperature and at −196 °C (77 K). Specimens aged at 400 and 500 °C, on the other hand, displayed significantly lower sub-zero impact properties. Though martensite packet size, and size distribution, were identical between the two ageing treatments; there was clear evidence of second phase coarsening: from very fine precipitates of less than ~ 10 nm to relatively coarser second phase of ~ 5-25 nm range. It is suggested that precipitate coarsening and associated loss of coherency are the limiting factors to the DBT performance of this important class of material.