Several phase transformations occur during post-weld heat-treatment in the vicinity of the fusion line between a low-alloy steel and an austenitic stainless steel, driven by large gradients of carbon chemical potential across adjacent regions (bainite / martensite / austenite). Cementite dissolution in the low-alloy side provides carbon which diffuses towards martensite and austenite where Cr-rich carbides precipitate. Crystal structures, volume fractions and size distributions of carbides were quantified as a function of the distance from the fusion line, as well as carbon content in the martensitic matrix. Results are interpreted on the basis of local compositions measured for carbon, chromium and nickel. They allow to identify the key points for phase transformation modeling in this complex three-phase configuration.