S. Denis

Influence of stresses on the kinetics of pearlitic transformation during continuous cooling

Abstract The effect of stress/strain on the kinetics of pearlitic transformation is reviewed. Our results on the pearlitic transformation of an eutectoid carbon steel under applied uniaxial tensile stresses are analyzed. From these results, we have modelized the effect of the internal stresses, generated during cooling in a solid specimen, on the transformation kinetics. This model is based on a shifting in time of the IT curves of the steel as a function of the internal stress condition (deviatoric part of the stress tensor). A coupled thermal, phase transformation, stress calculation, which includes this model, is applied to the cooling of an eutectoid carbon steel cylinder. The calculated results show that the internal stresses have an important effect on the kinetics of transforamtion and on the temperature evolutions. The calculated cooling laws are compared with those obtained by experiment and the validity of the model is discussed.

Modeling of the effect of the β phase deformation on the α phase precipitation in near-β titanium alloys

Abstract A model has been developed in order to describe the influence of a prior plastic deformation of the β phase above the transus on the precipitation of the α phase occurring during subsequent cooling. The model relies on the calculation of the nucleation and growth rates of the α precipitates at the grain boundaries, based on a model presented formerly. Two major modifications have been made: first, the geometrical representation of the β microstructure accounts for subgrains resulting from the deformation process; second, the calculation of the nucleation rate is dependent on the conditions of the plastic deformation. A careful analysis of the main parameters of the model has led to a distinction between several assumptions: Widmanstatten colonies are likely to cross the subgrains during their growth; and the critical width of the transition from allotriomorphs to Widmanstatten plates is likely to decrease when the misorientation angle of the grain boundary decreases. Calculations performed to assess the influence of the strain and strain rate on the transformation kinetics are in good agreement with previous measurements.

Microstructural analysis of alumina chromium composites by X-ray tomography and 3-D finite element simulation of thermal stresses

Abstract X-ray microtomography is used to measure volume fraction and connectivity of the metallic phase in an alumina–chromium composite. Reconstructed images are used as input data for a finite element calculation of the residual thermal stresses. Results confirm the main trends shown by similar calculations previously performed on less-realistic finite element models.

Dendritic growth and crystalline quality of nickel-base single grains

Abstract It is a usual observation that subgrains exist in nickel-base single grain components solidified by the lost wax process. The associated misorientations are generally small, but they can eventually lead to casting defects in the case of highly complex mold shapes. This work presents an attempt to relate the formation of subgrain boundaries with the development of the dendritic solidification microstructure. Experimental investigations have been undertaken on cast components made of AM1 nickel-base superalloy designed for high temperature turbine blades. Single grains were obtained by means of a grain selector at the bottom of each part. Metallographic observations have been made to characterize the dendritic array, together with gamma diffraction to measure the crystalline quality of the material and X-ray topography for mapping of misorientations on a dendritic scale. Small misorientations between dendrite stems have been found at the upper end of the selector which lead to the formation of subgrains. Moreover, during the growth process, the total mosaicity of the material increases, firstly as a consequence of an increase in the misorientations between subgrains, and secondly because of a decrease of the internal quality of each subgrain. It is proposed that misorientations are due to thermomechanical stresses which build up during λ′ precipitation at temperatures slightly below the solidus temperature of the alloy.

On the contribution of internal mismatch stresses to the high-temperature broadening of gamma-ray diffraction peaks in a Ni-based single crystal

Abstract Recent results of a two-dimensional finite−element modelling of the elastic distortions resulting from the lattice mismatch between γ− and γ−phases have been used to estimate the relative contribution of internal stresses to the broadening of γ-ray diffraction peaks, which has been observed at high temperatures in the case of some nickel-based superalloy single crystals. In the case of CMSX-2 alloy, it is found that the magnitude of the broadening of {220} and {200} peaks calculated when taking the elastic distortions into account is in good agreement with the experimental observations.

FINITE ELEMENT CALCULATION OF THE MICROMECHANICS OF A DIFFUSIONAL TRANSFORMATION

A unit cell of the material with suitable boundary conditions is modelled by a 3D F.E. mesh, over which the transformation will develop in a successive way. We use two different descriptions of the transformation, namely a spherical growth of nuclei of the new phase and a random progression model. An external stress state is applied to the cell, resulting in a transformation plastic strain, the evolution of which we study versus the progress of the transformation. We have chosen the example of an isothermal pearlitic transformation of a steel

Non-coherent interfaces in diffuse interface models

We propose a description of incoherent and semi-coherent interfaces within the framework of diffuse interface modelling: accounting for the discontinuity of tangential displacements is an issue addressed by introducing a new field, called the incoherency field, located in the diffuse interface and able to release the shear stresses tangential to the interface. After having demonstrated the ability of the model to recover trends obtained by sharp interface models of sliding interfaces, the process of loss of coherency is investigated by assuming a local process which would result from the competition between the shear stress release at the interface and the interfacial energy increase.

Effect of tensile stresses on bainitic isothermal transformation

The effects of tensile stresses on isothermal bainitic transformation were studied in the case of a 35MV7 steel. The modification of transformation kinetics and the presence of transformation plasticity is shown in a first step. Furthermore, the effect of stress on the morphological modifications of the ferrite laths is illustrated. The role of the stress on these changes is analysed.

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.

Cooling of a Rotating Cylinder by a Subcooled Planar Jet - Influence of the Surface Velocity on Boiling Regime

Cooling from impinging jet is nearly compulsory in steel industry processing especially in Run Out Table processing and steel tube production because of the high heat transfer provided by the boiling of the subcooled water jet. As far as metallurgical phase transformations, residual stresses and deformations in the workpiece are concerned, the temperature drop during cooling must be controlled thanks to a full understanding of the heat transfer mechanisms. One of the main characteristic using jet impingement is that the transition boiling regime may exist for very high superheat and thus the Leidenfrost temperature is higher than in pool boiling; consequently, boiling curves generally have a particular shape in the transition boiling regime which is usually called “shoulder of flux”. In this study, an innovative experimental quenching device has been used for analyzing the effect of the wall velocity of the surface to be cooled on the boiling curves (i.e. heat transfer) and we especially point out that the “shoulder of flux” (i.e. transition boiling regime) is strongly dependent on the surface to jet velocity ratio (r*). We found that a very small increase of the wall velocity has a high influence on shoulder of flux collapse.