An analysis of thermal autofrettage process with heat treatment

Abstract

In this work, a finite element method (FEM) analysis of a thermal autofrettage process coupled with heat treatment is carried out. The objective is to strengthen a cylindrical pressure vessel with compressive stresses on the outer as well as inner walls. In the proposed method, after thermal autofrettage, the outer wall of the cylinder is heated above the lower critical temperature to cause austenization of the cylinder in its vicinity. When the cylinder is quenched to room temperature, it retains beneficial compressive residual stresses at both the inner and outer walls. The analysis considers temperature and microstructure dependent material properties of AISI 1080 steel. Phase transformation kinetics is incorporated through user defined subroutine UMATHT in commercial FEM package ABAQUS®. The simulation results indicate that heat treatment after thermal autofrettage converts tensile residual stresses at the outer wall to compressive residual stresses. On the other hand, it does cause a slight relaxation of compressive residual stresses at the inner wall. Nevertheless, with the ability to retain significant amount of compressive residual stresses at both the walls, the process can mitigate the problems of stress-corrosion cracking and fatigue failure due to tensile residual stresses at the surface.