A lot of trial and error is involved when it comes to repair welding of service exposed cast material like HP40-Nb used in the radiant section of steam cracking furnaces and many, different weld repair strategies are proposed in literature and applied by plant operators and weld fabricators. After service at high temperature cast alloys show a lack of weldability. This is caused by oxidation, the low ductility of high carbon castings after exposure to high temperatures due to ageing and the formation of brittle carbides and or nitrides. This will result in cracks in the heat-affected zone due to the shrinkage stresses and (high) restraint encountered during repair welding. Four methods are generally used to enhance weldability: solution annealing before welding, buttering, shot peening and high preheating temperatures during repair welding. With this research project, the Belgian Welding Institute wanted to make a scientific bench mark to optimize weld repair. An assessment of the mechanical properties at room and service temperature is made. Recommendations are formulated to make safe and reliable repair welds in the future.