Gérard Beck

Phase transformations and generation of heat treatment residual stresses in metallic alloys

Today, a better understanding of the development of microstructures and internal stresses during heat treatment is achieved through modelling and numerical simulations.The key point is to take into account the phase transformations and the induced couplings with the thermomechanical behaviour of the material.These phenomena and their present modelling are reviewed for different metallic alloys, mainly steels, aluminium and titanium alloys.

PREDICTION OF RESIDUAL STRESSES

This chapter discusses methods for prediction of residual stresses. The residual stresses only constitute the particular value of the stresses developed by a process when the action of the thermal or mechanical effects having created them has finished. Using numerical calculation, it is now possible to know the evolution of the stresses during cooling and, thus, to arrive at the quantitative prediction of the residual stresses. This prediction demands that two distinct groups of data be known: (1) what the material undergoes: temperature, gradients, deformations, etc, and (2) its behavior. Following this, it is necessary to set up a model of realistic mechanical behavior, then to introduce the data in this model. Finally, a numerical method of solving the problem is required.

Deformation of Eutectoid Steel During Pearlitic Transformation Under Tensile Stress

ABSTRACT The effect of tensile stress (0 to 80 MPa) on the isothermal pearlitic transformation of a 0, 8% C steel has been studied in the case of a transformation between 663 and 683°C. The variations of strain versus time (dilatometry) and the transformation kinetics (variation of electric resistance of samples under tensile stress) were obtained using a dilatometer generating fast thermal and mechanical cycles. The main phenomena observed and measured are as follows : pearlitic transformation rate increases with applied stress. The total strain ΔLT during pearlitic transformation increases linearly with stress above a critical value. This critical value and the slope of the curve ΔLT=f (δ) depend on the transformation temperature. Comparison of deformation and transformation kinetics reveals that the plastic deformation under tensile stress is not a linear function of the progress of the transformation. The plastic deformation is considerable during the first 20–30% of transformation and increases slowly with further transformation.

Influence of quenching stresses on the hardenability of steel

Abstract The effect of quenching stresses on hardness was emphasized by comparing the hardness of (i) a massive sample, (ii) a Carney-type sample and (iii) a dilatometric sample, cooled in several different quenching media. When the steel was cooled without quenching stresses, the level of hardness was higher than inside a massive sample for the same cooling rate. The effect of known stresses on the hardness was studied with a dilatometer by which means a variable tensile stress was able to be applied during the thermal cycle. When quenching in oil at 25 °C was simulated, an applied stress of the order of the yield stress of the material caused the hardness to decrease by 123 HV. Nevertheless a plastic deformation applied to austenite in the range 900-575 °C had no observed effect on the hardness value. When a plastic deformation above 575 °C and elastic stresses under 575 °C were combined, the observed hardness was near that of the massive sample.