The effects of PWHT on the toughness of weld HAZ in Cu-Containing HSLA-100 steel

Abstract

A study was conducted to examine the effects of postweld heat treatment (PWHT) on the impact toughness and microstructures in the weld heat affected zone (HAZ) of Cu-containing HSLA-100 steel. The Gleeble, thermal/mechanical simulator, was used to simulate the weld HAZ. The details between toughness and PWHT of HAZ were studied by impact test, optical microscopy (O.M.), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). The decrease of HAZ toughness in the single thermal cycle simulation relative to the base metal's toughness is ascribed to the coarsened-grain formed by heating to 1350°C. While the increase of HAZ toughness in the double thermal cycle simulation relative to that in the single thermal cycle is due to the fine ferrite (α) grain transformed from austenite (γ) by heating to α/γ two phase region. Cu precipitated during aging, which increases the strength of the base metal, is dissolved during the single thermal cycle to 1350°C and remains in the matrix during subsequent thermal cycle simulations. It precipitates by introducing PWHT suggesting that the decrease of toughness in the triple thermal cycle simulation (TP1=1350°C, TP2=800°C and TP3=500°C) does not occur from the Cu precipitation. The behaviors of Cu-precipitates in the weld HAZ are similar to those in the base metal. The PWHT at 550°C shows the highest hardness and the lowest toughness in the HAZ, whereas PWHT at 650°C shows a reasonable toughness and strength combination suggesting the possibility of improving the toughness of HAZ.