Thermo-Mechanical failure of SS 630 alloy during heat treatment

ASTM A564 Grade 630 alloy, commonly referred to as 17–4 PH alloy is used in various engineering applications where high strength, hardness and corrosion resistance are essential. These superior mechanical properties are achieved through the Precipitation Hardening (PH) process. This study focuses on the impact of residual stress/strain on thermo-mechanical failure while undergoing heat treatment in accordance with ASTM A564 /A564M-19a of type H1025 and H1100 as part of the PH process to replicate a failure observed in the industrial samples. It was conducted through a novel static immersion method using pre-stressed Modified Turski (MT) specimens. The residual stresses were estimated using a Finite Element (FE) model informed by Profilometry-based Indentation Plastometry (PIP). The study successfully replicated the industry failure in 7 different instances which included variations in stress/strain induced in specimens and thermal aging time and temperature. Time stamped studies conducted during aging showed a rapid increase in hardness, yield stress and ultimate tensile stress in the first 10 min. Analysis was conducted through fractographic studies of the cracked MT specimens, changes in mechanical properties and published literature. The study speculates that the increase in brittleness combined with relaxation of residual stress/strain near the u-notch of the specimen resulted in the formation of cracks, which propagated during the aging period in the direction of maximum principal stress. Based on these findings, it is recommended to include an intermediate step of annealing at subcritical temperatures to relax residual stresses/strains before initiating the PH process to mitigate the occurrence of such failures.