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AISI 316L steel, subjected to a low temperature carburizing treatment (kolstering), has been examined by Mechanical Spectroscopy (MS) and nanoindentation to determine the Youngs modulus of the surface hardened layer (S phase). MS results showed that the average value of elastic modulus of S phase is 202 GPa, a little higher than that of the untreated material.Nanoindentation tests, carried out with loads of 5, 15 and 30 mN, evidence a modulus profile vs depth: E is ~ 400 GPa at a distance from the surface of ~ 110 nm, then decreases to reach the value of the steel substrate (190 GPa) at 33 μm.These results, together with X-ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES) measurements of carbon concentration profile, can be explained by considering the presence of a very thin surface layer, different from S phase and consisting of a mixed structure of Diamond-like carbon (DLC) and tetrahedral carbon (taC).Furthermore, the same experiments have been carried out also after heat treatments at 450 °C to correlate the modulus change to the decomposition of the metastable S phase leading to the formation of (Cr,Mo)C and Cr23C6 carbides in a Cr-depleted austenitic matrix.

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a Austenitic stainless steels are characterized by good corrosion resistance in different environments, but their use is limited because of low hardness and poor wear resistance. Conventional thermochemical surface treatments for improving the mechanical strength of steels induce Cr carbide precipitation and thus are detrimental to corrosion resistance. A low-temperature (<470 C) plasma treatment has been developed to overcome this problem and to reduce simultaneously the costs and the time of process. This paper reports the results of a microstructural characterization performed on a series of AISI 316L steel samples treated by plasma-assisted low-temperature carburization in different conditions. Microhardness tests and X-ray diffraction indicated that the best results are achieved by employing a gas mixture with 2% of CH 4 in H 2 . XPS and AES were used to examine the chemical composition of the 20-mm-thick hardened layer. The results revealed that this layer is not homogeneous because a 2-mm-thick overlayer of graphitic nature forms on the surface. Furthermore, only the plasma treatment with 2% of CH 4 guarantees that the whole carbon remains in solid solution, whereas for higher CH 4 amounts in the gas mixture, carbide precipitation takes place.

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The conventional heat-treatment standard for the industrial post hot-forging cycle of AA7050 is regulated by the AMS4333 and AMS2770N standards. An innovative method that aimed to improve toughness behavior in Al alloys has been developed and reported. The unconventional method introduces an intermediate warm working step between the solution treating and the final ageing treatment for the high resistance aluminum alloy AA7050. The results showed several benefits starting from the grain refinement to a more stable fracture toughness KIC behavior (with an appreciable higher value) without tensile property loss. A microstructural and precipitation state characterization provided elements for the initial understanding of these improvements in the macro-properties.