Bruno De Cooman

Martensite transformation of sub-micron retained austenite in ultra-fine grained manganese transformation-induced plasticity steel

Abstract The influence of the chemical composition and the grain size of retained austenite in an ultra-fine grained medium manganese steel on the martensite transformation kinetics was investigated by means of dilatometry. A modified Koistinen–Marburger type exponential function was developed for the martensite transformation kinetics, which includes the influence of the size and the composition of the austenite. Whereas the ultra-fine grained retained austenite had a remarkably low Ms temperature, the kinetics of its transformation to martensite was accelerated.

Thermal Stability of Retained Austenite in TRIP Steel After Different Treatments

Thermal decomposition of retained austenite in TRIP steel was investigated by means of differential scanning calorimetry (DSC), The decomposition curve was abnormal, and the decomposition temperature and the activation energy were measured by the Kissinger method, which were all higher than those in quenched steel. The thermal decomposition data of samples soaked in liquid nitrogen after TRIP treatment were all similar to those without additional low temperature treatment. It indicated that there is a high thermal stability in retained austenite of the TRIP steel at low temperature, which was also proved by XRD analysis.

Neutron diffraction study of the low-temperature ageing of martensite in dual-phase steel

Abstract Neutron diffraction measurements were used to obtain a better understanding of the ageing behaviour in dual-phase ferrite + martensite steels. Therefore, the influence of different ageing treatments on the tempering of low-alloyed 0.4 and 0.7% C FeMnCrMo-martensite and 0.1% C FeMnCrMo dual-phase steels was investigated. On tempering at 170 and 350°C, the sequential precipitation of η- and θ-carbides, as well as the loss of tetragonality was observed in the martensite. The decrease in austenite lattice parameter after tempering is due to stress relaxation in the material. These observations offer an acceptable explanation for the large yield stress increase after ageing at 170°C.

Neutron diffraction analysis of martensite ageing in high-carbon FeCMnSi steel

Abstract A high-carbon metastable austenite phase was quenched in liquid nitrogen to obtain fresh athermal martensite. Different ageing treatments were performed in order to study the transformation of the martensite phase and the formation of carbides. Neutron diffraction experiments give detailed information on the transformations during the annealing treatment itself. The tetragonal athermal martensite transformed immediately, i.e. during the heating to the ageing temperature, to cubic martensite. Ageing at 400°C resulted in the decomposition of the austenite in ferrite and carbides and the formation of bainite. Those carbides could be determined as -carbides transforming in -carbides after extended annealing times. As was expected, ageing at 170°C resulted in the formation of a small amount of carbides while the austenite phase remained stable.