Analysis of thermal embrittlement of a low alloy steel weldment using fracture toughness and microstructural investigations

Abstract

A thermally aged low alloy steel weld metal is investigated in terms of its fracture toughness and microstructural evolution and compared to a reference. The main purpose of the study is to investigate the effects of embrittlement due to thermal ageing on the brittle fracture toughness, and its effects on the influence of loss of crack tip constraint. The comparison of the investigated materials has been made at temperatures that give the same median fracture toughness of the high constraint specimens, ensuring comparability of the low constraint specimens. Ageing appears to enable brittle fracture initiation from grain boundaries besides initiation from second phase particles, making the fracture toughness distribution bimodal. Consequently, this appears to reduce the facture toughness of the low constraint specimens of the aged material as compared to the reference material. The microstructure is investigated at the nano scale using atom probe tomography where nanometer sized Ni-Mn-rich clusters, precipitated during ageing, are found primarily situated on dislocation lines.