Studying the influence of the interpass temperature on the heat-affected zone of an API 5L X65 steel welded pipe joint through computational and physical simulations

Abstract

Welding costs can be reduced by increasing the interpass temperature (IT). However, this can adversely impact the mechanical properties of the heat-affected zone (HAZ) of the welded joint; therefore, it is necessary to evaluate the influence of higher ITs on the HAZ. The influence of the IT on coarse-grained HAZ (CGHAZ) and intercritically reheated coarse-grained HAZ (ICCGHAZ) of an API 5 L X65 steel pipe joint welded with gas metal arc was investigated. A microstructure-predicting method (MPM) was built from numerical and thermodynamic simulations to examine the CGHAZ and ICCGHAZ. The results were validated using optical and scanning electron microscopy as well as dilatometry. Vickers microhardness and Charpy V-notch (CVN) impact tests were performed on physically simulated specimens. For all ITs considered herein, granular and lath bainite were observed in the CGHAZ; and bainite, ferrite, and Martensite–Austenite–Carbides (M-A-C) were observed in the ICCGHAZ. Increasing the IT did not significantly change the hardness of the CGHAZ and ICCGHAZ, but it decreased the CVN impact energy of these zones owing to presence of granular bainite and massive M–A. Contrary to that of the ICCGHAZ, the impact energy of the CGHAZ was greater than the DNVGL-ST-F101 standard specified limit (45 J). Based on the MPM and experimental results, it can be concluded that increasing the IT leads to embrittlement of the HAZ.