Hardfacing Plastic strain High load tribological test Work-hardening Strain-induced phase transformation a b s t r a c t Aeronautic forging dies are subjected to very high loads and temperatures for a long contact time between the pre-heated parts and dies. Cobalt-based hardfacings are commonly deposited on dies and their main wear mechanism is large plastic deformation of the die radii.This paper deals with the wear damage mechanisms of three different cobalt-based hardfacings: Stellite 21 deposited by a MIG process, Stellite 21 and Stellite 6 deposited by a LASER process. The tribological tests are carried out on a high load Ring on Disc tribometer at room temperature. The postmortem investigations are undertaken by SEM observations, micro-hardness measurements as well as by X-ray diffraction analyses.Results show that the increase of the hardness, in order to improve the wear behaviour, can be achieved by a higher carbon content and by a lesser iron dilution that depends on the deposition process. A very important work-hardening, up to 90%, is also observed under sliding conditions and a relationship is established between the increase of the micro-hardness and the plastic strain level. Two different plastic strain mechanisms are observed. For high (MIG) or low (LASER) iron dilution levels, the plastic strain causes respectively a reorientation of grains or a FCC to HCP phase transformation; the latter being associated with a lower friction coefficient.

X-ray microtomography can be used to characterise objects undergoing fabrication by additive manufacturing. During the layer-by-layer building process, it can provide key information about geometry, roughness and it can even reveal typical defects such as lack-offusion porosity, gas pores or cracks. Usually, objects are built with varied processing parameters and then characterised post-mortem. In the present work, we describe our custom-designed additive manufacturing chamber allowing in situ 3D-non-destructive characterisation to be performed during layer-by-layer construction using synchrotron X-ray microtomography. Scans before (subsequently to powder deposition) and after local laser melting are acquired for every layer. A few examples of such a characterisation demonstrate the ability of the setup to reproduce conditions close to those used in conventional laser powder-bed fusion devices and to reveal key phenomena.

A technique for the microstructural study of steels, based on the use of matrix dissolution to collect the very low number density precipitates formed in martensitic steels, has been considerably improved. This technique was applied to two different grades of alloy, characterized by high nickel and cobalt contents and varying chromium, molybdenum and vanadium contents. The technique was implemented at temperatures ranging between 900°C and 1000°C, in order to accurately determine experimental data including the crystallographic structure and chemical composition of the carbides, the carbide solvus temperatures, and variations in the chemical composition of the matrix. These experimental investigations reveal that the solubility of molybdenum in FCC carbides can be very high. These results have been compared with the behavior predicted by computational thermodynamics, and used to evaluate and improve the thermodynamic Matcalc steel database. This upgraded database has been validated on three other steels with different chemical compositions, characterized by the same Fe-Cr-Mo-V-C system.

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