Black lines in welds of P91 and P92 martensitic heat resistant steels

By means of optical microscope, scanning electron microscope, energy spectrometer, focused ion beam and transmission electron microscope, the distribution characteristics, stuctural characteristics and formation reason of black lines in submerged arc welding welds of P91 and P92 martensitic heat resistant steels were studied. The results show that the black lines were different in length, mainly located between the columnar crystals of the weld, and were composed of M₂₃C₆ type carbides with a linear arrangement of face centered cubic. The reason for their formation was that Cr and Mo elements segregated between the columnar crystals during welding, M₂₃C₆ carbides precipitated on the columnar crystal bondary during post weld heat treatment, and appearred as black lines under the optical microscope after corrosion.