Bulk Heat Treatment and Surface Modification
Research focus
The Peer review has evaluated this group as Average
The request of increasing mechanical performance can be satisfied by optimizing the properties of materials by specific heat treatments and surface modifications. According to the different applications, the requirement can be different, implying the improvement of the bulk properties or only of the surface of the part. A first specific research subject about bulk heat treatments is concerned with sinter hardening of steel fabricated by Powder Metallurgy (PM). The mechanical properties of PM materials are mainly related to microstructure and density. In the as-sintered condition, most of the hypo-eutectoidic ferrous PM materials show a ferritic-perlitic microstructure, with different constituent fraction depending on carbon content. For this reason a secondary heat treatment is required, to optimize mechanical properties. However, drawbacks associated with the heat treatment are increase of production cost and loss of part precision. Recently, sinter hardening revealed a fast-growing PM process in which the high cooling rate experienced after sintering is set to produce high strength materials with an “as sintered” martensitic microstructure. The parts are “quenched” during the cooling stage inside the sintering furnace and the cooling rate is high enough to prevent ferritic and perlitic transformations, allowing the transformation from austenite to martensite or, in the long run, to bainite. The main benefits of this innovative process are: (1) elimination of the traditional secondary heat treatments, with considerable cost advantages; (2) reduced distortion of the parts, owing to slower cooling rates compared with oil quenching; (3) oil-free parts, since cooling is carried out in the same gaseous atmosphere used for sintering. Powders formulated for sinter hardening must possess hardenability that is high enough to prevent perlitic and even bainitic transformations at the cooling rates commonly experienced in sintering furnaces: a few Celsius degrees per second between 850°C and 400°C. The ability of these materials to form martensite during the cooling stage is related to their chemical composition and to the local cooling rate. These specific powder grades contain hardenability-enhancing elements, such as Ni, Mo, Cu and, in many cases, Mn and Cr, but at low percentages. To increase hardenability, these alloying elements must be in solid solution. However, because of the unavoidable solid-solution hardening, these elements also reduce the compressibility of the freestanding powders. A second issue of interest and of fast growing activities involves surface conditioning, either by thermochemical innovative process and by surface coating. Indeed, the possibility to modify both the chemical composition and the microstructure of the surfaces in metals and alloy, allows satisfying the increasing performance requests of industrial manufactures. 122 In particular, surface fatigue failure has been studied with regard to surface hardening and thermochemical treatments of steels. Initiating of fatigue damage was investigated and it was related to the state of surface: roughness, toughness of material and of hardening layers, residual stress induced by fabrication process and/or hardening treatment. Damage of an heat treated surface, for example, originates when contact stresses rises over fatigue resistance of material, according to Hertzian theory. Pure rolling contact experimental tests have shown that the cracks started at the surface and can propagate only in the presence of lubricant . They were interesting interpretation and comparisons in damage phenomena involved in contact surface fatigue for different materials and surface treatments. Coatings also play an important role in both ferrous and non ferrous alloys, especially in the aerospace and automotive industries. In lightweight alloys, typical properties restricting the use in engineering application are the low mechanical behaviour, especially for use in high temperature service, and the inadequate corrosion resistance in several media. Therefore, the application of tailored coatings able to increase not only the wear and corrosion substrate performance, but also the strength. Attention is particularly given to Physical Vapour Deposition (PVD) and Thermal Spray (TS) coating. Although the ceramic coating exhibits a high hardness, such property is not sufficient to avoid operational defects connected with micro cracks propagating along the interface with the substrate or through the film thickness. Therefore, systematic investigation were carried out , also considering the evaluation of the residual stress distribution induced by the coating manufacturing process that affects both the adhesion between coating and substrate and the mechanical properties of the coated component such as the fatigue limit. By means of experimental tests and numerical simulation, correlations between microstructural parameters, residual stress distribution and mechanical properties of both ferrous and non ferrous alloy coated by PVD or TS layers were defined.
Dipartimento di afferenza
Docenti afferenti
Full Professors
Giuseppe Silva
Marco Boniardi
Assistant Professors
Fabrizio D’Errico
Nora Lecis
Barbara Rivolta