Fracture toughness and martensitic transformation in type 316LN austenitic stainless steel extra-thick plates at 4.2 K

Abstract

The plane strain fracture toughness of type 316LN nitrogen-strengthened austenitic stainless steels depends on their stability at 4.2 K, which is characterized by the Md30 index. Deformation microstructures and martensite that developed at the crack tip of type 316LN steel extra-thick (535 mm) plates at 4.2 K were evaluated. Two kinds of type 316LN steel, LF and HF, represented low and high fracture toughness, respectively. The Md30 of the plates correlated well with their respective fracture toughness values, where a higher Md30 indicated lower fracture toughness. The plastic strains developed in austenite grains at the crack tip for the HF were higher and more homogeneous and extensive than those of the LF. The volume of alpha '-martensite detected in the HF was lower than that of the LF. Planar slip bands with the primary slip system and alpha '-martensite were dominant in the LF. In the HF, however, microslip bands with multiple slip systems, deformation twins, and alpha '-martensite formed. For the LF, extensive martensite leaves with multiple variants were formed at the crack tip. The martensite leaves grew along the high stress field at crack tip developed by the crack extension. For the HF, alpha '-martensite platelets were dispersed along {111} slip bands. Shear bands containing martensite were favored by low plastic strains and constituted a smaller fraction of their total density. Therefore, low plastic strains were responsible for an increase of alpha '-martensite formation at the crack tip. An alloy design having a higher Ni, Mn and Mo, and lower N content in its chemical composition would be favorable for type 316LN steel to provide a higher fracture toughness owing to higher stacking fault energy.