A physically based model for the isothermal martensitic transformation in a maraging steel

Isothermal transformation from austenite to martensite in steel products during or after the production process often show residual stresses which can create unacceptable dimensional changes in the final product. Tn order to gain more insight in the effects infiuencing the isothermai transformation, the overall kinetics in a low Carbon-Nickel maraging steel is investigated. The influence of the austenitizing température, time and quenching rate on the transformation is measured magnetically and yields information about the transformation rate and final amount of transformation. A physically based model describing the nucleation and growth of martensite is used to explain the observed effects. The results show a very good fit of the experimental values and the model description of the transformation, within the limitations of the inhomogeneities (carbides and intermetallics, size and distribution in the material and stress state) and experimental conditions

In situ observations on the mechanical stability of austenite in TRIP-steel

In-situ tensile deformation tests have been performed on a high Al TRIP steel (composition 0.26 wt. % Si, 1.5 wt. % Mn, and 1.8 wt. % Al) displaying the transformation-induced plasticity (TRIP) effect, while monitoring the phase transformation by means of X-ray microdiffraction in transmission geometry. Due to the small beam size ($\rm 25 ~\mu m \times 25~\mu m$) every retained austenite grain appears as a discrete spot on the diffraction patterns. The diffraction patterns are treated like a powder pattern for different $\eta$-angles, with $\eta$ representing the angle between the tensile direction and the normal direction of the diffracting {200} planes. The disappearance of austenite {200} reflections is analyzed during as a function of the imposed stress and orientation. Grains with $\eta = 0$ or 90$^{\circ}$ tend to transform to martensite more easily. A unique feature of this microdiffraction experiment is the possibility of detecting the average carbon concentration of the retained austenite as a function of stress. Direct proof has been obtained that austenite with a lower carbon content $\rm x_c$ transforms at lower stress levels. The average $\rm x_c$ increases from 1.0 to 1.05 wt. %. This increase indicates a relatively narrow distribution of the carbon content

Retained austenite: transformation-induced plasticity

The deformation-induced phase transformation of metastable austenite to martensite is accompanied by macroscopic plastic strain and results in significant work hardening and the delayed onset of necking. Steels that exhibit such transformation-induced plasticity (TRIP) effect possess high strength-ductility ratios and improved toughness. Since the stability of the retained austenite (RA) phase is the rate controlling mechanism for the TRIP effect, the factors affecting the chemical and mechanical stability of RA in CMnSi TRIP steels are discussed. It was suggested that chemical stability plays a more important role at low strains, whereas other factors become responsible for RA behavior at higher strains. The importance of optimizing the processing parameters to achieve the desirable level of austenite stability is highlighted. Finally, the influence of mechanical testing conditions and the interaction between the phases during tensile testing are also detailed