Microstructural factors that determine the weldability of a high Cr-high SiHK 40 alloy

The modified HK 40 heat resistant steel with higher chromium and silicon co ntent has improved resistance to carburization and metal dusting but presen ts serious weldability problems as the material is fissured during the weld ing process. In order to know the microstructural factors that determine th e weldability of the alloy, several welding repair tests were carried out a nd the causes that origin cracking during welding process were determined. The purpose of this study was to determine the phases that participate in t he cracking and to analyze their effect on the weldability of the alloy. Microanalysis of the aged metal, cracked during welding, showed that the eu tectic carbides modified their composition regarding the as cast condition, diminishing the chromium content and increasing the iron, nickel, silicon, carbon and molybdenum contents. It was also detected inside the eutectic c arbides the presence of phases rich in carbon, silicon, molybdenum and vana dium. These transformations should have enhanced the alloy susceptibility t o cracking, particularly in the area of the eutectic carbides, and hence th ey increased the fragility and impaired the weldability properties of the a lloy. The cracking observed was intergranular-type and spreads through the eutectic carbides. M23C6 carbides, austenite, nickel silicide (Ni31Si12) an d sigma phase were found in the aged alloy by X-ray diffraction analysis. ( C) 2000 Elsevier Science S.A. All rights reserved.