P 942 – Increase of the wear resistance of hot forging tools by utilization of the cyclical edgezone hardening
The life span of hot bulk metal forming tools is limited by damage resulting from a complex combination of cyclic loads. Especially the surface layer of a tool is under a high amount of thermal, mechanical, tribological and chemical stress during the production process. The resulting load spectrum leads to tool failure with smaller life span quantities compared to other manufacturing methods. The main cause of failure is wear in the shaping areas of the tool. That is why different wear reduction techniques for hot forging dies have been discussed in a number of research studies so far. Despite the significant progress being made by applying hard coatings or nitriding the tool surface, no solution with a satisfying cost-benefit ratio has been found so far. Recent measures to reduce wear require either a high investment or high tool costs which outweigh the life span benefit by far.
This study analyses a simpler and more economic method of wear reduction. A new hot working tool steel has been developed for this purpose reducing the expenses to the costs of the additional alloying elements. The alloying composition is based on the hot working tool steel 32CrMoV12-28 (material no. 1.2365 / AISI H10) with varying ratios of the austenite stabilizing eleUmformments manganese, nickel and cobalt . The objective is to lower the material’s austenite start temperature and manipulate the microstructure in the tool’s surface layer. The material’s intelligent reaction to the thermomechanical loads should induce an increased rehardening effect maintained over a great deal of forging cycles. This hardened surface layer acts as continuous wear rotection.
The research has shown that lowering the austenite start temperature through alloying can increase the rehardening effect if the thermomechanical loads duringforging cause the surface layer to austenize. In the tool’s thermomechanically highly stressed regions exceeding the austenite start temperature the following quenching lead to the formation of a re-hardened layer. The tool regions where the thermomechanical stress is insufficient for microstructural transformation and re-hardening develop tempered layers with less hardness. These increase wear and decrease tool’s ife span. The new alloy’s full potential can only be benefitted from if the process parameters and thermomechanical loads are known and optimized. The study has also shown that further improvement is possible by combining the new alloy steel with a nitriding treatment. This caused higher wear resistance in the tool regions that were not re- hardened leading to an overall increase of the tool life quantity.
The research project IGF-No. 445 ZN from the Research Association for Steel Application was supported by the Ministry of Economic Affairs and Energy through the German Federation of Industrial Research Associations (AiF) as part of the programme for promoting industrial cooperative research (IGF) on the basis of a decision by German Bundestag. The research project has been carried out at Institut für Umformtechnik und Umformmaschinen, of Gottfried Wilhelm Leibniz Universität Hannover and at Institut für Werkstoffkunde, of Gottfried Wilhelm Leibniz Universität Hannover.
Only available in german language.
B.-A. Behrens, J. Puppa, H.-J. Maier, F. Nürnberger