The Effect of Post Weld Heat Treatment on the Microstructure and Creep Damage Susceptibility in Grade 92 Steel

Ferritic steels including low alloy steel and creep strength enhanced ferritic (CSEF) steels have shown the potential for creep failure in the weld metal, heat affected zone (HAZ) or fusion line. Details for this behaviour have been frequently linked to metallurgical risk factors present in each of these locations which may drive the evolution of damage and subsequent failure.

This work is focused on three weld samples fabricated from a commercially sourced Grade 92 steel pipe section. These weld samples were extracted from the same welded section but were reported to exhibit failure in different time frames and failure locations (i.e. HAZ of parent, fusion line and weld metal). The only variables that contribute to this observed behaviour are the post weld heat treatment (PWHT) cycle and the applied stress (all tests performed at 650 °C).

In this work detailed microstructural analysis was undertaken to precisely define the locations of creep damage accumulation and relate them to microstructural features. As part of this an automated inclusion mapping process was developed to quantify the characteristics of the BN particles and other inclusions in the parent material of the samples. It was found that BN particles were only found in the sample that had been subjected to the subcritical PWHT, not those that had received a renormalizing heat treatment. Such micron sized inclusions are a known potential nucleation site for creep cavities and this is consistent with the observed failure location in the HAZ of the parent in the sample where these were present. In the absence of BN inclusions, the next most susceptible region to creep cavitation is the weld metal. This has an intrinsically high density of sub-micron sized spherical weld inclusions and this is where the majority of the creep damage was located in all the renormalized samples.