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I.

INTRODUCTION

COMPOSITE tubes can be made by joining stainless steels and low-alloy steels. These tubes are used in applications where it is difficult to meet the demands for both mechanical strength and corrosion resistance with a component made of a single material. Due to wide differences in the chemical composition between the two steels in a component, considerable transport of elements occurs over the interface. The interdiffusion of elements over the interface between the steels can result in composition changes and precipitation of phases near the interface. The change in chemical composition and formation of phases at the interface region can affect the mechanical properties of the composite tube. One interesting feature in joints between steels of different alloy composition is the uphill diffusion of carbon, i.e., carbon can diffuse from regions of low carbon concentration to regions of high carbon concentration. The carbon diffusion in this type of materials can result in precipitation of carbides at the joint, as can be seen in Figure 1. The microstructural changes in the steels at the joint can affect the properties of the joint, and, thus, it is of great practical importance to be able to predict structural changes that can occur during heat treatments in production and service. This problem has been tackled both experimentally[1,2,3] and by application of numerical calculations.[4,5] With the development of a general software for diffusion calculations, such as DICTRA[6] it has become possible to make calculations for this type of problem, provided that the thermodynamic and kinetic data are available. It is, thus, tempting to make calculations for a typical alloy combination of a stainless steel and a low-alloy steel in order to gain a better understanding of the carbon diffusion in these types of systems. II.

NUMERICAL CALCULATIONS

The computer simulations were performed using the software package DICTRA. In order to perform accurate simulations, one needs an accurate thermodynamic description ˚ GREN, THOMAS HELANDER, Graduate Student, and JOHN A Professor, are with the Department of Materials Science and Engineering, Division of Physical Metallurgy, The Royal Institute of Technology, S100 44 Stockholm, Sweden. Manuscript submitted April 16, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A

of the system, which is available within the THERMOCALC system.[7,8] It is also necessary to have access to accurate diffusivity data that have been assessed by Jo¨nsson[9,10] for the present system. Recently, Engstro¨m et al.[11] developed a model capable of handling diffusion in multicomponent, multiphase systems, provided that there is a continuous matrix phase in which the diffusion can be assumed to occur. This model has now been used for calculations on a combination of a typical stainless steel and a low-alloy steel. In that model, the effect of precipitates blocking the normal diffusion paths was handled by introducing a so-called labyrinth factor. The diffusivity is multiplied by the labyrinth fac