Correlating pipe dimensions and success of local heat treatment: Developing nomograms to deduce heat treatment parameters

Local post-weld heat treatment (PWHT) is the only option for heat treating field welded joints. Quite often, the success of local PWHT in alleviating the residual stress and tempering the material within the soak band (SB) is dependent on the ability to achieve the required heat treatment temperature and maintain through-thickness temperature gradient (TTG) within the specified limits at the end of heating cycle, whence soaking begins. Field observations reveal the inadequacy of AWS D 10.10 specified parameters viz. rate of heating (ROH), heat band (HB) and insulation band width in not achieving the required TTG for certain pipe dimensions. Although prior works have attempted to address this issue by widening the HB, the capacity of the heating equipment often pose a limitation. In such cases, reducing ROH is a plausible alternative. With, no such prior studies seen in literature, an exhaustive finite-element analysis simulating the local PWHT on 81, SA106GrC pipe samples (diameter and thickness varied in 9 levels each) was performed, thrice. First as per AWS recommendations, second by halving the code deduced ROH and third by doubling the code deduced HB. The trend of important outcomes (TTG, power source rating and energy consumption) with great significance to the heat treatment industry were also compared and analysed. Two nomograms were developed to serve as a ready reckoner for field heat treater in not only assessing the adequacy of heat treatment parameters but also with possible alternatives in achieving the desired TTG using field-available power source.