Use of acoustic emission to detect reheat cracks during post-weld heat treatment

Abstract

The phenomenon of stress relief cracking or reheat cracking can occur in the heat affected zone (HAZ) of welded joints during post-weld heat treatment (PWHT). The coarse grained region, which results from heating at elevated temperatures in the austenitic region, is the most susceptible to this problem. The present research programme was aimed at determining, by means of an acoustic emission measurement system (AEMS), the instant of propagation of reheat cracks during PWHT. The AEMS was fixed to a modified implant test device that allowed the loading of a specimen subjected to thermal cycles similar to those typically experienced by welded components in stress relief treatments. Tests were carried out with this setup using different load levels for the same thermal cycle and welding condition. Welding was done by means of the metal active gas process. The study was developed using a high strength low alloy (HSLA) steel commercially produced in Brazil. Test results have shown that the AEMS is adequate for monitoring the formation and growth of reheat cracking during PWHT.