A cast heat-resistant alloy able to withstand carburization and metal dusting was developed to be used in the conducting pipes of reformed gas heaters for the direct reduction process. However, this alloy exhibits serious weldability problems in its as-cast condition and during the repair of aged pieces. Study of the microstructural and technological factors that affect weldability indicate that the use of low heat input (in the range of 630-950 kJ m(-1)) and a maximum interpass temperature of 150degreesC favors the welding repair of the high Cr-Si modified HK40 alloy. A solution annealing treatment prior to welding was necessary to obtain acceptable weldments. The commonly known buttering technique was not successful in this type of alloy. A series of microstructural phenomena, such as precipitation of secondary carbides, formation of intermetallic compounds (sigma phase and nickel silicide), as well as transformation of primary carbides, were observed to occur during aging, and they are suspected to be responsible for reducing the weldability of aged parts. (C) 2002 Elsevier Science B.V. All rights reserved.