Precipitation sequences during carburisation of Cr-Mo steel

When Cr–Mo steel is joined to mild steel and the joint held at an elevated temperature the carbon tends to migrate from the mild steel to the Cr–Mo alloy. This migration is driven by the higher chemical potential of carbon in the mild steel, and leads to a steady accumulation of carbon in the Cr–Mo steel at the weld junction. The present work demonstrates that the carbon enrichment has a strong effect on the sequence of carbides that forms during aging, and the results are rationalised using thermodynamic phase stability calculations. Similar data are reported for other dissimilar metal joints.