Effect of cooling rate and powder characteristics on the soundness of heat affected zone in powder welding of ductile cast iron

In an application concerning large forming dies made of ductile cast iron, a number of failures were reported at the location of some of the weld repairs. Metallurgical investigations pointed out the cause to be mainly related to the presence of micro cracks in the heat affected zone (HAZ) of the oxyacetylene powder weld surface repairs. Using controlled cooling conditions in order to simulate repair of large forming dies, powder welding with nickel base self-fluxing alloys was carried out. It is shown that in certain conditions involving relatively high cooling rates which could be encountered when welding very large parts with standard preheats combined with the use of the harder nickel base powders, micro cracks can form in the HAZ of ductile cast iron. The cracking process is proposed to be dominated by the stress fields induced on the toe of weld deposit by the shrinkage effect of the hard nickel base alloy. The cracks initiate mostly at the interface of graphite nodules and then propagate through the martensitic matrix.