Volume 73, Issue 2

Lower bainite can indeed grow below martensite start temperature ( M S ) in a commercial alloy. Ultrahigh-strength low-alloyed 42CrMo4 steels are austempered below M S . The preliminary martensite and lower bainite packets formed during austempering partition the prior austenite grains effectively. The obtained ultrafine-grained nanostructured microstructure enhances the mechanical properties of the alloys significantly. Morphology and overall microstructure are examined by reflection optical microscope and scanning electron microscopy. Substructures of lower bainite and virgin/tempered martensite are revealed by transmission electron microscopy. With increasing duration of austempering, a clear transition from cleavage to ductile rupture was observed by fractography. Effects of grain partitioning and the risk of embrittlement have been discussed concretely. The better understanding of the microstructure austempered below M S will feedback to the materials development activities both in conventional and new materials areas.

A number of studies on bainite transformation at low temperature have revealed that bainitic ferrite formed super-saturated in carbon. The most sensible explanation to this is the increased solubility of a tetragonal ferrite lattice, by virtue of synchrotron radiation and X-ray diffraction results, as well as ab-initio calculations. The question is if this increased tetragonality in bainitic ferrite is the result of a disordered distribution of carbon atoms in ferrite or the result of local carbon clustering (ordering) in association with a locally increased tetragonality. This development of carbon-enriched and carbon-depleted zones that leads gradually to the formation of a modulated structure was reported in the early stages of decomposition of martensite. In the present work, new experimental and theoretical results on the distribution of carbon in bainitic ferrite will be shown trying to shed new light on the nature of bainite transformation.

Carbonitriding serves to increase the strength and wear properties of steel components. The carbon and nitrogen concentration and the depth distribution decisively determine the resulting properties. Optimal profiles create an ideal microstructure of martensite, residual austenite, finely distributed nitrides, carbonitrides and inherent compressive stress in the surface zone. The reliability of carbonitriding heat treatment process is strongly dependent on the possibilities of process control. Previous investigations aimed at measuring the nitrogen potential of the atmosphere by means of an ammonia sensor in the exhaust gas as well as by a wire sensor. Carbon potential is conventionally controlled using an oxygen probe. In order to further increase the process reliability of carbonitriding, simulation of the carbon and nitrogen profiles and of any precipitation of carbides and nitrides are necessary. The first step is the determination of interdependent alloying influences on the carbon and nitrogen contents. The carbon and nitrogen activities in the atmosphere and the alloy surface are near equilibrium after long-time carbonitriding. Depending on the composition of the material, significantly different effects can be described. These influences must be considered. In addition to the reached equilibrium content, the diffusion, the position of phase boundaries in the phase diagrams and the formation of precipitation with influence of carbon and nitrogen as well as interactions with other alloying elements are to be worked out in order to further develop controlled carbonitriding for the reliable adjustment of the heat treatment results.

A study of a proposed plasma nitriding system, exploiting the hollow cathode effect, with treated part biased to anodic potential (Anodic Plasma Nitriding in Hollow Cathode – HCAPN) is presented. The aim of the study was to investigate the differences and similarities with conventional Active Screen Plasma Nitriding (ASPN). At the same time, the experimental results i. e. optical and scanning electron microscopy study of white and diffusion layer along with micro-hardness measurements are presented. Furthermore, the ammonia formation during the nitriding process was studied at different temperatures. We showed that the amount of ammonia reaches a maximum value at 700 K and at higher temperatures the amount of ammonia gradually decreases. This indicates that at the higher temperatures more and more of the formed ammonia dissociates (decomposes) on the hot surfaces of the sample and cathode, transferring the nitrogen to these surfaces, as in the case of classic gas nitriding.

Today Low Pressure Carburizing (LPC) followed by High Pressure Gas Quenching (HPGQ) is an established process to produce high performance components for conventional steels in order to combine high surface hardness and fatigue strength. On the other hand, thanks to its ability to combine good quality and cost saving, PM process is more and more popular in the automotive industry (especially in the transmission applications as synchronizer hubs/rings, sliding sleeves, …). In order to answer the increasing application performance requirements, chromium-alloyed grades are often used because they provide a cost-efficient way to reach high mechanical properties. However these materials need specific sintering conditions and cannot be surface hardened by conventional gas carburization and oil quenching. Thus, the alternative LPC & HPGQ applied to PM parts is a very promising, cost-efficient, and environmental solution to produce high performance parts. This paper presents the last trends and results of this solution.