P 803 – Comparative evaluation of properties of traditional and innovative materials and processing concepts for forged components




P 803 – Comparative evaluation of properties of traditional and innovative materials and processing concepts for forged components

The objective of the research project was to verify the efficiency of precipitation hardened case-hardened steels and of precipitation-hardened bainitic steel with new alloying concepts for forgings in direct comparison with steels used up to now.
For the case-hardened steels the materials 20MnCr5 (WS1), 15CrNi6 (WS3), 18CrNiMo7-6 (WS4) with controlled sulphur content and 18CrNiMo7-6 completely desulphurised (WS5) as well as the two modified variants 20CrMo5mod (WS2) and 26MnCrNiMo6-5-4mod (WS6) were used as conventional reference materials – both provided with the micro alloying elements V, Ti and Nb. The modified bainitic steel was compared to the conventional quenched and tempered steel 42CrMo4 and to the conventional AFP steel 38MnVS6.
For all materials the fundamental mechanical properties as well as the fatigue strengthand fracture toughness values were determined. The high-strength case-hardened steel 26MnCrNiMo6-5-4mod shows core strength of 1750 MPa which is significantly higher than that in the application formulated core strength of 1400 MPa but with only acceptable toughness properties. The fatigue strength in the smooth and blind-hardened condition is, as expected, greater than that of the reference case-hardened steels. In the notched, case-hardened condition the fatigue strengths of all examined case hardened steels are in a scatter band. This is caused to the properties of the surface layer which are the same for all case- hardened steels. In case of the case-hardened drop-forged chains and the spur gears, the numbers of stress cycles to fracture or the fatigue strengths of all examined steels were in a common scatter band – just as with the notched samples. However, in the event of the casehardened and additionally shot-blasted gear shafts the fatigue strength of the WS6 26MnCrNiMo6-5-4mod is significantly higher than of the reference steel 18CrNiMo7-6.
The alloying concept which is used here for the adjustability of the strength properties is economically less favourable than for the case hardened steels based on MnCr, CrMo or CrNi but comparable to the 18CrNiMo7-6.
The yield strength of > 800 MPa defined as the target was not achieved for the bainitic steel. The bainite shows acceptable ductility properties. The smooth specimens of the modified bainite tend to have better fatigue strength than the reference quenched and tempered steel 42CrMo4, and the notched specimens even significantly better. For the modified bainitic material the component tests using the example of the steering lever resulted in lower number of stress cycles to fracture compared to the reference material.
The bainitic steel 12CrMnMoV7-5mod is more economical than the conventional quenched and tempered steel when used for components. Essentially, this is caused by the absent of heat treatment step, thus from the controlled cooling without subsequent reheating. One objective of further research activities is to examine the microstructural cause of the good fatigue strength as well as to improve the toughness properties of the bainite.

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Dr.-Ing. B. Kaiser, Dr.-Ing. D. Schwerdt