Fe – Co – Mo alloys show hardening behaviour comparable to that of precipitation hardened Al-alloys, which is characterised by a low hardness after solution annealing and a significant increase in the hardness during subsequent ageing at higher temperatures. In this study, the precipitation behaviour of Fe-25% Co-15% Mo (wt.%) has been studied applying non-isothermal ageing. High sensitivity differential scanning calorimetry and atom probe tomography have been used to characterise the precipitation sequence. Three overlapping exothermic reactions, which are related to precipitation reactions, are observed in the temperature range of 400 to 700°C. The investigations have shown that precipitation starts with diffusion of Mo, followed by the formation of the intermetallic μ-phase which reaches its equilibrium composition at a temperature of about 615°C.
An intensive research work was carried out in the frame of a RFCS (Research Found for Coal and Steel) project, to investigate the influence of different deep cryogenic treatments (DCT) on hardness, strength, toughness and wear resistance of AISI M2. Short and a long classical DCT, providing the soaking at temperature close to the boiling point of liquid nitrogen (−196 °C) for 6 h and 20 h, respectively, were carried out prior to and after tempering. Furthermore, a third short DC route, providing temperature cycling between RT and low temperature was also considered. Care was taken to avoid stabilization of retained austenite or self tempering due to wait at room temperature prior to DCT and/or tempering. All treatments were calibrated to get 840 HV 10, in order to compare the properties of steel with the same reference hardness. DCT does not allow the complete transformation of retained austenite in the investigated high speed steel, due to the stabilizing effect of alloying elements. Tempering is necessary to completely transform this phase and to allow proper secondary hardening. If carried out after quenching, DCT shifts the secondary hardness peak to lower temperature, evidencing the need to adjust the tempering parameters to avoid overtempering. The microstructure didn't show any significant influence of DCT in terms of carbides distribution, due to the conditioning of martensite at low temperature. The same can be also concluded for the other properties (toughness, tensile strength and wear resistance), which are practically the same for samples having the same hardness.
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