A new kinetic model for anisothermal metallurgical transformations in steels including effect of austenite grain size

A continuum mechanical description is presented for thermoplastic materials in the process of solid-solid phase transition. The material is assumed to be characterized by three different internal state variables: two internal variables which specify the crystallographic structural change during the plastic deformation, and a set of scalar internal variables which describes the extent of phase transition. Applying Edelen's decomposition theorem, the plastic quantities are determined from the dissipation potential, while the elastic quantities are specified by the internal energy. The explicit form of the flow rule and the evolutional equations for the internal variables are derived. The constitutive equations for the stress and the entropy are obtained in rate-type. It is shown that the continuous cooling transformation (C-C-T) diagram and the isothermal time-temperature-transformation (T-T-T) diagram could be derived from the theory developed here. The infinitesimal case is discussed in detail.

A thermomechanical description of materials with internal variables in the process of phase transitions

A study has been made of constitutive relations for a time–dependent inelastic material within the framework of continuum thermodynamics, and of the heat conduction equation and transformation kinetics for processes involving phase transformations. Calculations of the stress, temperature, and structures of a half–spaced body, heated and cooled from the surface have been made using the finite element method, and are presented as an illustrative example. The effects of coupling between stress, temperature, and metallic structures, and of time dependence on the constitutive model for melting during welding are evaluated.MST/14

Coupling between stress, temperature, and metallic structures during processes involving phase transformations

In this paper, the various interactions between the different parameters involved during quenching, i.e. temperature, phase transformation, and stress–strain, are investigated and a constitutive model of the material during the process is presented and discussed. The influence of the parameters has been studied, by computer calculations using a constitutive model, for three steel cylinders, and the results indicate that in some cases the constitutive model seems important. In none of the cases studied in the paper is the influence of the stress state on the transformation kinetics very great.MST/11

Interactions and constitutive models for calculating quench stresses in steel

This paper provides a state-of-the-art review on the investigations into the residual stresses in metallic structural materials generated by grinding. The materials covered include steels, titanium alloys, and nickel-based superalloys. The formation mechanisms of the residual stresses and their impacts are specifically discussed. Some major influential factors on the residual stresses formation in grinding, such as grinding wheel characteristics, dressing techniques, grinding parameters, cooling conditions, and properties of workpiece materials, are analyzed in detail. These include experimental measurement, modeling, simulation, knowledge-based monitoring, and fuzzy analysis. Finally, the paper highlights some important aspects of grinding-induced residual stresses for further investigation.

Review on grinding-induced residual stresses in metallic materials

T he problems of quenching-stress analysis are critically reviewed and an adequate simple theory is discussed. The theory accounts for both plasticity and volumetric changes due to phase transformation accompanying the thermal-hardening of a group of simple steels that are characterized by C-shaped time-temperature-transformation diagrams. The volume dilatation in the absence of stress is assumed to be a linear function of the separate specific volumes and weight fractions of the constituents (pearlite, austenite and martensite). With use of the classical relationships of a formal theory of transformation kinetics, the amounts of pearlite and martensite are expressed in terms of the temperature and the temperature-history. The specific forms of such functions are given. In order to account for the influence of phase transformation on plastic properties, the non-isothermal plastic flow-rule is generalized, and a thermal-hardening parameter is introduced which is identified with the amount of pearlite. Variational principles and bounding inequalities associated with the fundamental rate-problem are considered. As an example, the problem for a rapidly, uniformly-cooled half-space is solved. The variations of the residual stress and the final amount of martensite with distance from the outer surface are given, for several values of the rate-of-cooling. The results suggest that the residual stress vanishes on the plane containing approximately 30–35 per cent of martensite.

T. Inoue

Papers

Determination of thermal-hardening stress in steels by use of thermoplasticity theory