Volume 73, Issue 3

Powder metallurgy ferrous precision parts are employed in numerous industrial sectors. In case of high mechanical loading, as e. g. in automotive applications, heat treatment is frequently required after the press-and-sinter manufacturing. Here the specific features of PM parts have to be considered, in particular the presence of open and/or graded porosity. In the present study it is shown that processes such as sinter hardening, low pressure carburizing and plasma nitriding are particularly suited for sintered parts since the problems usually caused by the porosity can largely be avoided. By appropriate selection of the alloying system combined with optimum sintering and heat treatment, components can be obtained that are competitive to parts machined from wrought steels but offer additional benefits such as attractive NVH (noise-vibration-harshness) behavior.

The wear properties of industrial pure iron treated by nitriding-quenching and carburizing-quenching were investigated. Both the nitrogen martensite and carbon martensite produced by each treatment exhibited the same high degree of wear resistance. The results of sliding tests showed that the nano-indentation hardness near the surfaces of both samples had increased. The surface hardening of the nitrogen martensite was more noticeable than that of the carbon martensite. EBSD analysis revealed that plastic strain had been induced on a relatively large scale (around 20 μm) after sliding in the case of the nitrogen martensite. It was found that the high wear resistance of the nitrogen martensite led to strain hardening through the effect of the load applied in the sliding test.

The effect on the bainite formation of carburizing (0.6 wt.% C), nitriding (0.12 wt.% C-0.25 wt.% N) and carbonitriding (0.7 wt.% C-0.25 wt.% N) of a 23MnCrMo5 low-alloyed steel in the austenitic field was examined by in situ high-energy synchrotron X-ray diffraction (HEXRD) and transmission electron microscopy. The enrichment in nitrogen induces strong acceleration of the bainite transformation kinetics in carbonitrided steel compared to carburized steel, despite the γ-stabilizing character of nitrogen. This is attributed to the nucleation of ferrite on CrN nitrides, which precipitated during the enrichment, either at γ grain boundaries or intragranularly. AlN, VN and MnSiN 2 nitrides were observed as well, with much smaller number density. They formed frequently aggregates with the CrN nitrides. The bainite microstructure is much finer than in initial or carburized steel. It shares some common features with intragranularly nucleated bainite, i. e. acicular ferrite. From HEXRD, the chronology of the phase formation (ferrite and precipitates) during bainite formation as well as cell parameter evolutions are analyzed.

Industrial furnaces, especially in steel industry but also in non-ferrous, ceramic or chemical industry, are often fired with fossil fuels like natural gas, oil or coke. Considering the transition to green energy, it is necessary to enable the use of renewable energies like wind and solar energy in fossil-fueled furnaces. Resulting from this motivation, a hybrid recuperator for combustion air preheating with off-gas heat and electric energy is being developed in a public research and development project. The hybrid recuperator consists of a conventional tube bundle module and an electric heating module. The design of the prototype is based on numerical investigations to determine pressure loss, increase in temperature and heat transfer. This article introduces the concept of the new hybrid recuperator and initial results of numerical investigations for a small-scale hybrid recuperator geometry.