Effect of surface treatments on mechanical properties of low alloy sintered steels

Powder Metallurgy (PM) is a net- shape and cost effective technology used for the production of steel parts having good mechanical properties and geometrical precision. In the conventional press and sinter process, the voids among the powder particles cannot be completely eliminated, and the as sintered microstructure contains a certain amount of residual porosity. Mechanical properties are consequently lower than those of the corresponding wrought steels [1]. In particular, the fatigue resistance is significantly affected by porosity; crack tends to nucleate in correspondence of clusters of pores, and to propagate along the network of interconnected pores [2, 3]. Fatigue resistance can be improved on increasing the density, reducing pore size and pore clustering and enlarging the sintered ligaments between pore, or, similarly to wrought steels, by thermochemical (carburizing and nitriding) or mechanical treatments (shot peening). Carburizing consists in a surface carbon enrichment, which gradually decreases towards the core. After quenching high carbon martensite is formed at the surface, characterized by high hardness and a compressive residual stresses suitable for wear and fatigue resistance. Low pressure carburizing is a variant of the conventional gas carburizing performed under sub-atmospheric pressure with pressurized gas quenching. It is quite attractive for carburized PM sintered steels, for two main reasons. 1. Porosity increases the surface exchange area, enhancing the risk of oxidation mainly in Cr and Cr-Mn steels. Low pressure carburizing uses propane or acetylene, as carburizing gas, which does not contain oxidizing agents. 2. Quenching oil remains entrapped in the open porosity, and has to be eliminated. The possibility to combine low pressure carburizing with gas quenching results in clean parts as well as lower distortion. However, the combination between the very high carburizing potential of LPC and the large surface area of porous steels results in overcarburizing, with the precipitation of grain boundary carbides in Cr steels, and the formation of retained austenite in the case in Cr free ones [4, 5]. This problem can be solved by either increasing density, to close the residual porosity, or rolling and shot peening, to eliminate the surface porosity. Nitriding is based on the nitrogen enrichment of the surface layers of steel. On the base of nitrogen content the surface microstructure can be divided in two zones: the compound and the diffusion layer. The former is in principle a ceramic layer, whilst the latter consists in the base matrix hardened by solid solution and by the precipitation of nitrides. The nitride precipitation induces a compressive residual stress field which offers a resistance to the nucleation and propagation of the fatigue crack, improving the fatigue resistance. In order to obtain a hardened and deep diffusion layer the steel has to contain alloying elements with a high affinity for nitrogen, as chromium and molybdenum. Nickel and manganese have a negligible interaction with nitrogen. Among the different nitriding processes, plasma nitriding is recommended for sintered steel. Plasma nitriding is less sensitive to porosity than gas nitriding due to the particular mechanism of nitrogen diffusion (volume diffusion) which allows a uniform diffusion front on the steel surface and a homogeneous nitrogen distribution [6, 7]. Therefore, a preliminary surface densification is not necessary. Shot peening is a flexible and cost effective solution to improve the fatigue performances of mechanical parts, as gears and springs, thanks to the compressive residual stress generated below the surface and the surface work hardening. The improvement in fatigue resistance is more effective if shot peening is applied on case hardened steels, because of the more stability of the compressive residual stresses. Since the fatigue strength of sintered steels strongly depends on the material density, shot peening is a useful technique to improve such property, owing to the densification of the surface layer [8, 9]. The fatigue cracks nucleates beneath this layer and since it cannot propagate in a compression field, it moves towards the core. This PhD thesis is part of the an international research project, “Höganäs Chair project- fourth round†, financed by Höganäs AB, world leader in the production of ferrous powders, involving four research institutions: Trento University, Technique University of Wien, Carlos III University of Madrid and Slovak Academy of Science, Institute for Materials Research, Kosice. The aim of the project is to carry out a cooperative study to design highly performing structural steels by the conventional Powd