Metallurgical stability and the fracture behavior of ferritic stainless steels

The influence of aluminum on the thermal stability of ferritic stainless steels has been investigated using two commercial alloys— Armco Type 18SR and AISI430. Two reaction stages have been detected in these alloys during aging at 475 °; each stage is accompanied by changes in the hardness, yield strength, strain-hardening exponent, and elongation to fracture. The initial stage is attributed to the precipitation of carbide and nitride particles and the second stage to the precipitation of the chromium- rich a’ phase. The 430 alloy exhibits more pronounced changes than 18SR during the first stage due to the higher concentration of interstitials retained in solution after quenching. The effects of the second- stage aging reaction are detected after shorter aging times in the 18SR alloy and are more pronounced than in the 430 alloy, consistent with the influence of aluminum on the coherency strains associated with a’ precipitation. The fracture mechanism in both alloys changes from ductile dimples in the solution- treated and quenched condition to a mix of ductile dimples, intergranular fracture, and transgranular cleavage with increased aging times. Longitudinal cracking at the grain boundaries precedes failure of the aged alloys in tension; it is attributed to the combined effects of void initiation at fine grain boundary precipitates, a’ embrittlement that limits localized plasticity, and the transverse stress components resulting from triaxiality after the onset of necking.