Atomistic Simulation of Grain Boundary Segregation and Embrittlement in Low Alloy Steel Welds Under Temper Embrittlement Conditions

This study applies molecular dynamics and Monte Carlo atomistic simulation methods to investigate grain boundary segregation of phosphorus and antimony in low alloy Cr-Mo steels and the resulting embrittlement mechanisms, with direct relevance to temper embrittlement concerns in thick-section pressure vessel welds fabricated for Nigerian petrochemical installations. Temper embrittlement is a critical degradation mechanism in Cr-Mo steel pressure vessels subjected to post-weld heat treatment and in-service temperatures of 350 to 550 degrees Celsius, and atomistic simulation offers a route to understanding the thermodynamic driving forces and kinetics of segregant accumulation that is inaccessible to purely experimental approaches. The embedded atom method (EAM) potential is employed for the Fe-Cr-Mo-P quaternary system, validated against first-principles DFT binding energy calculations for P at symmetrical tilt grain boundaries of the Sigma 3 and Sigma 5 type. Monte Carlo simulations using the Metropolis algorithm are applied to determine equilibrium grain boundary compositions as a function of temperature and bulk P content. A grain boundary decohesion criterion based on the Rice-Wang thermodynamic model is used to translate P coverage into embrittlement severity. Molecular dynamics fracture simulations are performed on notched bicrystal specimens to quantify fracture energy reduction as a function of P coverage. Findings establish the critical grain boundary P coverage threshold for embrittlement onset and the activation energy for P grain boundary diffusion. Keywords: atomistic simulation, grain boundary segregation, temper embrittlement, Cr-Mo steel, pressure vessel.