A FEM based framework for simulation of thermal treatments: Application to steel quenching

The kinetics of bainite and martensite formation in steel 41Cr4 has been determined using dilatometry for a range of cooling rates. It is demonstrated that all fraction curves can be adequately described with a previously proposed model using a single set of input parameters. This validation shows that the model can give accurate predictions of the CCT diagram of bainitic steels provided the model can be tested against a single experimental fraction curve to determine the model parameters for the given composition.

The kinetics of bainite and martensite formation in steels during cooling

Deformation behavior of austenite/supercooled austenite in a medium-carbon Si–Al-rich alloy steel is investigated on a thermomechanical simulator and an optical microscope Effects of ausforming temperature, strain rate and strain on martensite start temperature ( M S ) and ausformed martensite microstructure are studied in detail Results show that power-law-type work hardening is caused during supercooled austenite deformation at 300 °C; and the work hardening followed by steady-state flow is caused during the deformation at 600 and 900 °C The ausforming can decrease M S , refined lath martensitic microstructure and slightly enhanced hardness With decreasing the ausforming temperature the M S and martensite lath thickness decrease markedly The strain and strain rate do not have strong effects on the M S and lath thickness In addition, the ausforming can decrease the tetragonality of martensite and facilitate the formation of dislocation substructured martensite

Austenite deformation behavior and the effect of ausforming process on martensite starting temperature and ausformed martensite microstructure in medium-carbon Si–Al-rich alloy steel

Effect of tensile stress on the formation of S-phase during low-temperature plasma carburizing of 316L foil

The effect of ausforming on the stability of retained austenite in a C-Mn-Si bainitic steel was investigated through metallography, X-ray diffraction and dilatometry. The geometrical relationships of the amount of bainite transformation and the volume fractions of retained austenite with deformation strains were studied. The results show that the degree of promotion of small strains on bainite transformation is nonlinear because of the dual effects of accelerated nucleation and retarded growth caused by ausforming. The transformed bainite fraction first increased and then decreased with increased small strains. It indicates that there is a maximum degree of the promotional function corresponding to a certain small strain at low temperature. Although small strains promote bainite transformation, a larger quantity of retained austenite exists at room temperature due to the suppressed martensite transformation during the cooling process after bainite transformation. The carbon content in retained austenite increases with the amount of baintie transformation, which contributes to the stability of austenite. Compared with the stabilizing effect due to carbon enrichment, mechanical stabilization caused by ausforming has a decisive effect on determining the volume fraction of retained austenite after isothermal bainite transformation.

Effect of ausforming on the stability of retained austenite in a C-Mn-Si bainitic steel

Theoretical analysis shows that the fraction of pearlite formed from nucleation is additive and that from growth is not. A modified additivity model is established with two continuous cooling experiments. Calculations of Ae 3 temperature for proeutectoid ferrite formation under stress and nucleation rate as well as incubation period of ferrite or pearlite transformation under stress are successfully made. Kinetics equations for ferrite and pearlite transformations under stress are expressed from modification of J–M–A equation with addition of a stress item. The acceleration effect of stress on bainite formation is mainly attributed to the increase of diffusivity of solute atoms and even iron. By consideration of the grain size effect, Patel and Cohen’s equation expressing the effect of stress on M s is modified. Calculations of M s for fcc → bcc(bct) and fcc → hcp under stress are introduced. An equation showing the relationship between strain and nucleation of martensite which can well explain the morphology of martensite formed under stress is mentioned. Appearance and mechanism of mechanical stabilization of austenite in martensitic transformation, i.e., the lowering of M s , resulted from the work hardening of austenite, are different from retardation of bainite formation under stress, i.e., after B s raising, occurring the retardation of bainite growth resulted from hindrance by defects.

T.Y. Hsu

Papers

Additivity hypothesis and effects of stress on phase transformations in steel