Fractographic Investigation on Solidification Crack in the Varestraint Test of Fully Austenitic Stainless Steel : Studies on Fractography of Welded Zone (III)

概要

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Fractographic study is made on hot cracks in fully austenitic stainless steel weld metal in order to elucidate the reasons why the weld metal is susceptible to hot cracks. Two types of hot cracks, that is, solidification cracks and ductility-dip cracks, are induced by the Trans-Varestraint test, and the crack surfaces are observed with a scanning electron microscope (SEM) and analyzed with an energy-dispersive type analyzer (EDX). SEM observation reveals that a feature of the solidification crack surface is gradually changing together with a drop in crack opening temperature. That is to say, the feature is dendritic Type D in higher temperature region and flat Type F in lower temperature region. The crack surface of Type D is supposed to have been covered with liquid phase at the moment of the crack-opening, judging from the morphology and the distribution of chromium on the crack surface. The crack surface of Type F is apparently flat, but it has many hollows where low melting inclusions are detected. The reason of occurrence of Type F is a decrease in liquid phase and an increase in grain boundary migration. The ductility of the weld metal is very low around Type D and gradually raised in Type F. Furthermore, the feature of ductility-dip crack surface and the behaviour of low melting inclusions enriched in sulphur or phosphorus on the crack surfaces are discussed.
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