Evaluation of cross-weld properties of quenched and tempered pressure vessel steel before and after PWHT

Abstract

Longitudinal and circumferential welds in transportable pressure vessels are produced by submerged-arc welding using a single vee preparation and multiple weld runs. Quenched and tempered (QT) steels, which are commonly used for transportable pressure vessels, require mandatory postweld heat treatment (PWHT), at temperatures between 540 and 590 °C, regardless of plate thickness. This paper reports on the weld procedure and cross-weld performance evaluation of weldments in 11 and 12 mm QT steel plate before and after PWHT. Root bend (180°) tests showed no evidence of cracking or tearing in any of the samples tested. After PWHT, tensile tests indicate a reduction in the ultimate tensile strength of all samples and a decrease in yield strength for the 12 mm sample only. The hardness results were consistent with the tensile test results because they revealed significant softening in the HAZ and WM as a result of PWHT. Additionally, residual stress measurements before and after PWHT showed that PWHT is effective in reducing surface residual stresses to levels low in the elastic range. It is concluded that although PWHT exerts the beneficial effect of reducing residual stresses, the ductility of the weld region is satisfactory without PWHT, and PWHT decreases the cross-weld tensile strength.