Low temperature carburising (LTC) allows a significant hardness increase, with a consequent increase in wear resistance, without deteriorating corrosion behaviour. However, wear resistance strongly depends on contact conditions, therefore this work focuses on the dry sliding behaviour of LTC-treated AISI316L austenitic stainless steel against several countermaterials (AISI316L, LTC-treated AISI316L, hard chromium or plasma-sprayed Al2O3–TiO2). LTC produced a hardened surface layer (C-supersaturated expanded austenite), which improved corrosion resistance in NaCl 3.5% and increased wear resistance, to an extent which depends on both normal load and countermaterial. The best results were obtained when at least one of the contacting bodies was LTC-treated, because this condition led to mild tribo-oxidative wear. However, LTC did not improve the behaviour in terms of friction.

Bench tests and laboratory dry sliding tests were carried out on components for radial piston hydraulic motors involved in a boundary-lubricated sliding contact, with the aim of investigating the tribological behavior and improve their durability. Failure analysis of real components after bench tests (carried out on a rotating shaft, consisting of a quenched and tempered 36CrNiMo4 steel, coated by a Ni7Al layer deposited by Air Flame Spray, sliding against a carburized E470 steel contacting element) identified the main wear mechanism as two-body abrasion, due to hard particles detached from the AFS coating which induce severe plastic deformation on both the carburized steel contacting element and on the soft Ni-based matrix of the coating on the rotating shaft. The same wear mechanism was reproduced also in laboratory dry sliding tests, carried out on a block-on-ring tribometer under dry sliding conditions. Laboratory dry sliding tests allowed the investigation of the influence of normal load and sliding distance on friction and wear behavior of the tribological system under investigation.

Low Temperature Carburizing (LTC) allows the surface hardness of austenitic stainless steels to increase without significantly compromising their corrosion resistance. This thermochemical treatment is successfully applied on an industrial scale, however, the long processing time, resulting from the low temperatures involved, leads to high costs and thus low competitiveness compared to more conventional surface modification treatments. In an attempt to overcome these limitations, low temperature carburizing plasma assisted treatment has been developed on a laboratory scale, in which the surface activation is carried out by means of processes based on the use of a high energy density H-2/CH4 mixture. The laboratory treatment, performed with a 2% mixture of CH4, has proved to be suitable to form a surface layer of expanded austenite, with thicknesses ranging between 18 and 35 micron and hardness values ranging from 450 to 850 HV. While, higher (approximate to 1100 HV) and more reproducible hardness values were detected in the samples subjected to industrial LTC processing. The treated samples were subjected to dry sliding wear tests, using a slider-on-cylinder tribometer (stationary sliders: low temperature carburized AISI 316L, rotating cylinder: untreated AISI 316L). The coefficients of friction were comparable for samples treated on an industrial and laboratory scale; the greatest problems have been detected with respect to the wear behavior of samples treated on a laboratory scale, due to non-uniformity in the hardness values. In any case, the treatment on a laboratory scale led to an appreciable increase of the wear resistance of the AISI 316L compared to the untreated material. Under the maximum tested load (10 N), the wear volumes of the samples with higher hardness were comparable with those of the industrially treated specimens. In conclusion, the plasma treatment has proved its effectiveness, even though the prototype apparatus used for the experiment does not allow a homogenous effect to be obtained on the whole surface of the treated specimens, although the size is reduced. It is believed, however, that the problem can be overcome by operating with a larger chamber, where the local effects related to variations in curvature of the samples, typical of the plasma treatment, may be less critical.

ABSTRACTThis paper deals with the influence of interference fit coupling on the fatigue strength of holed plates made of a medium‐carbon forging steel (35 KB2), heat treated by quenching followed by tempering, up to a hardness of about 350 BH, obtaining a sorbitic microstructure. Tensile and impact tests showed an ultimate tensile strength of about 1100 MPa, a yield strength of about 1000 MPa, an elongation to failure of 15% and an impact toughness KV of 43 J at room temperature. Axial fatigue tests were performed on holed specimens with or without a pin, made of the same material, press fitted and still left into their central hole. The tension–tension fatigue tests have been performed with a stress ratio R = 0.1. The effect on fatigue strength was investigated both experimentally and numerically. Three different conditions were investigated by using open hole specimens, specimens with 0.6% of nominal specific interference and specimens with 2% of nominal specific interference. The experimental stress‐life (S–N) curves pointed out an increased fatigue life of the interference fit specimens, compared with the open hole ones. The numerical investigation was performed in order to analyse the stress field by applying an elastic plastic 2D simulation, with commercial finite element software. The stress history and distribution around the interference‐fitted hole indicate a significant reduction of the stress amplitude produced by the external loading (remote stress) because a residual and compressive stress field is generated by the pin insertion.

Low-Temperature Carburizing (LTC) is a thermochemical treatment that improves the wear resistance of austenitic stainless steels without decreasing their corrosion resistance. However, LTC does not contribute to the decrease in the coefficient of friction in steel-to-steel contacts, which take place in many tribological applications involving LTC-treated parts. Therefore, in this work the dry sliding behavior of a duplex system consisting of PA-CVD hydrogenated amorphous carbon (a-C:H) topcoat on LTC-treated AISI 316L, was investigated by a flat-on-cylinder tribometer, at room temperature and in laboratory air, so as to evaluate the influence of (i) the polishing procedure before a-C:H deposition, (ii) the LTC interlayer and (iii) the type of countermaterial (untreated or LTC-treated AISI 316L). The results showed that (i) the polishing step prior to a-C:H deposition to obtain low roughness and therefore low friction (and also low wear of the countermaterial) must be carried out before LTC, otherwise the thickness of the layer is modified uncontrollably. (ii) LTC improves the load bearing capacity of the substrate, thus leading to increased adhesion and longer coating life. (iii) Finally, the a-C:H topcoat proved to be effective in reducing both friction and wear compared to the uncoated LTC-treated steel (up to the transition load).

The present work is aimed at evaluating the feasibility of the linear friction welding process to produce dissimilar joints between a AA2124/25%vol SiCp composite and a 2024 Al alloy, illustrating and correlating their microstructural and mechanical properties. Optical microscopy (OM) and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) were used to characterize the effects of the welding process on the microstructure of the LFW joints. Tensile tests were carried out on joints welded at three different welding parameters. Axial fatigue tests were carried out under stress control and the corresponding S–N probability curves were computed. The mechanisms of failure were investigated by SEM analyses of the fracture surfaces. In the LFW joints almost no blending between the MMC and the base Al alloy was detected, while good particle distribution and no clustering were found on the MMC side. Mechanical testing demonstrated that high quality dissimilar joints, characterized by good tensile and fatigue properties, with respect to the AA2024 base material, were obtained by means of LFW. Fracture was usually located in the weld center or in the thermo-mechanically affected zone (TMAZ), due to the plastic flow which the material underwent during the solid state welding process.

The effect of cooling rate after hot rolling on the final microstructure and mechanical properties of a microalloyed medium C steel was investigated. The microstructure was characterized by optical microscopy; the mechanical behavior was studied by hardness, tensile and instrumented Charpy V-notch impact tests carried out at room and sub-zero temperatures. The results of microstructural analysis indicate that a low cooling rate of 0.7 degrees C/s led to a mixed microstructure consisting of perlite, pro-eutectoid ferrite and bainite, while an increase of the cooling rate to 7.5 degrees C/s favored the formation of martensite and acicular ferrite. This latter microstructure, in turn, induced an increase in the tensile strength of the steel, with a reduction of its elongation to failure, and superior impact toughness. Analyses of the fracture surfaces with scanning electron microscopy confirmed the influence of the two microstructures on the failure mechanisms of the steel. (C) 2012 Elsevier Ltd. All rights reserved.