Finite element modeling of transient heat transfer and microstructural evolution in welds: Part II. Modeling of grain gr
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I.
INTRODUCTION
THE process of fusion welding subjects the parent plate in the vicinity of the weld, known as the heat-affected zone (HAZ), to rapid localized heating and cooling. This induces a variety of microstructural changes, as illustrated in Figure 1. As a result, the mechanical properties of HAZ metal are invariably different from and often worse than those of the parent plate. Not all of the microstructural changes illustrated in Figure 1 are observed in all materials. For example, phase transformation occurs in ferritic steels (referred to as the recrystallized zone in Figure 1) but not austenitic steels. In this research program, we have attempted to adopt a finite element heat-transfer model for welding tjj to predict microstructural changes in the HAZ. The current initial study considers austenitic stainless steel in which grain growth dominates microstructural evolution. The model which is developed, however, can be used to treat more complex materials in which phase transformations are important. Previously, Ashby and Easterling t2~ have computed the dissolution temperature of carbides and nitrides based on the composition of the material. They assume that below the dissolution temperature, particles pin the grain boundaries and no grain growth occurs. Above the dissolution temperature, particles are assumed to instantaneously dissolve and grain growth proceeds. Ion et alJ 31 have extended the kinetic model to include precipitate coarsening. In it, they allowed carbides or nitrides located in the HAZ to either coarsen or dissolve depending on the peak temperature and the duration of the heating cycle. However, they proceeded with the same methodology where the precipitation dissolves instantaneously and unimpeded grain growth occurs upon reaching S.E. CHIDIAC, Research Associate, is with the Structures Laboratory, Institute for Research in Construction, National Research Council Canada, Ottawa, ON K1A 0R6, Canada. D.S. WILKINSON, Professor, Department of Materials Science and Engineering, and F.A. MIRZA, Professor, Department of Civil Engineering and Engineering Mechanics, are with McMaster University, Hamilton, ON L8S 4L7, Canada. Manuscript submitted October 18, 1991. METALLURGICAL TRANSACTIONS B
the dissolution temperature. In this study, a revised kinetic model has been incorporated for particle dissolution where the grain growth is controlled by the grain pinning force that is determined as a function of the current particle volume fraction. Thus, effects due to the rate of heating on particle dissolution and grain growth can be included. The objective of this second part of a two-part article is to present a kinetic model where grain growth is controlled by the progressive changes in volume fraction. The physics of the proposed kinetic model, along with a method of implementing it using the finite element method, is presented. The potential of the kinetic model is then explored by comparing its results for austenitic stainless steel to both experimental data and results of other models avail
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