The modified HK 40 heat resistant steel with higher chromium and silicon content has improved resistance to carburization and metal dusting but presents serious weldability problems as the material is fissured during the welding process. In order to know the microstructural factors that determine the weldability of the alloy, several welding repair tests were carried out and the causes that origin cracking during welding process were determined. The purpose of this study was to determine the phases that participate in the cracking and to analyze their effect on the weldability of the alloy. Microanalysis of the aged metal, cracked during welding, showed that the eutectic 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 carbides the presence of phases rich in carbon, silicon, molybdenum and vanadium. These transformations should have enhanced the alloy susceptibility to cracking, particularly in the area of the eutectic carbides, and hence they increased the fragility and impaired the weldability properties of the alloy. The cracking observed was intergranular-type and spreads through the eutectic carbides. M23C6 carbides, austenite, nickel silicide (Ni31Si12) and sigma phase were found in the aged alloy by X-ray diffraction analysis.