Problems in Repair-welding of Duplex-treated Tool Steels

Castings of aluminum and plastics are of high importance in the industrial world. They are usually made using permanent metallic moulds for pressure die-castings and injection molding. Die-tools are usually produced from chromium maraging hot work tool steel and are heat treated to hardness between 29 and 48 HRC, 1-3 . Die life is a mayor factor of a die-casting process and it strongly affects the productivity of mass production. Depending on a cast or mold application, the typical damage and failure mechanisms may differ. Thermal fatigue cracking is the most important life limiting failure mode in dies for die-casting, 4-6 . Thermal fatigue cracking is often observed on the tool surface as a network of fine cracks or as individual and clearly pronounced cracks 7 . Formation of the thermal fatigue cracks leads to loss of surface material in form of small fragments. Other common reasons for damage are tension cracks caused by constructional notches, local adherence of a casting alloy to the tool i.e. soldering, and steel erosion promoted by the cast molten metal or plastic flow, 8-10 . Molds for plastic injections are subjected to lower working temperatures while pressure cycles are more demanding and, therefore, mechanical fatigue damage and overload failures might occur. Life of dies can be improved using proper surface treatment, e.g. nitriding, PVD coating, chrome plating, oxidation processes, and duplex-treatments. Plasma nitriding of tools results in easier separation of a product, less frequent cleaning of the die-core system and an increase in service life by 20 % to 50 % 11 . In literature there are reports that plasma nitriding improves thermal fatigue behavior due to high residual stresses in the diffusion layer and improvement of its tempering resistance. It is also reported that thermal cracks remain localized in the compound layer 12 . Oxidation is a process that creates a lubricant oxide film to prevent soldering and adhesive wear in high pressure die casting tools. Hard coating based on nitrides and carbides of transition metals, e.g. CrN, CrC, TiAlN, TiB2, are also used to protect casting tools surfaces, 13 . The aim of PVD coatings is reduction of erosion, soldering, and corrosion but it was shown that they fail to improve the thermal fatigue resistance of hot working steels in die-casting conditions. None of the above mentioned surface treatments provide an optimum solution for all failure mechanisms,