Continuous cooling transformation behaviour and toughness of heat-affected zones in an X80 line pipe steel

Abstract

A continuous cooling transformation (CCT) diagram was developed using thermal simulation techniques to replicate the coarse-grain heat-affected zone (CGHAZ) of an X80 line pipe steel. Specimens were heated to a peak temperature of 1350 °C with a 1 s hold time, followed by cooling at several rates between 7.6 and 356 °C/s. The thermo-mechanical specimen design was compatible with sub-size instrumented Charpy impact testing, which allowed the toughness to be evaluated. Following dilatometry measurements during continuous cooling, specimens were prepared and examined using optical and scanning-electron microscopy. In addition, the initial CGHAZ microstructure (upper bainite) was reheated to a peak temperature of 850 °C for 1 s (double cycled) and cooled at three different cooling rates (10, 5 and 2 °C/s). This allowed the influence of intercritical reheating, which occurs in multi-pass welds to be investigated in terms of microstructure, tensile strength and notch toughness. It was observed that the microstructure consisted of effective grain (matrix) and untempered MA when cooling at 10 or 5 °C/s. However, when reheating was followed by a cooling rate of 2 °C/s, the microstructure consisted of a combination of effective grain and a more benign distribution of tempered MA, which provided better low temperature impact toughness.