A process model for the heat-affected zone microstructure evolution in duplex stainless steel weldments: Part II. Applic
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I. INTRODUCTION
WELDING of duplex stainless steels is encumbered by a number of different service problems, including reduced heat-affected zone (HAZ) toughness and corrosion resistance.[1,2] Because these problems have a clear metallurgical origin, modeling of the HAZ transformation behavior in such weldments has been the topic of a number of previous investigations.[2–14] The increased understanding of the reactions taking place in the HAZ has, in turn, made it possible to develop a new generation of duplex stainless steels, where welds with acceptable properties can be achieved with conventional methods, provided that appropriate welding parameters are employed. From a productivity standpoint, the use of low heat input welding processes such as electron beam (EB) or laser welding offer additional cost advances, particularly in cases where deep penetration welds can be produced in one pass. On the other hand, the characteristic high cooling rates associated with these welding methods give rise to an adverse austenite-to-ferrite balance in the weldment, with consequent degradation of mechanical and corrosion properties.[15] Normally, the problem with a low austenite content in the fusion region can be handled by the use of overalloyed filler metals,[16] but this will not influence the HAZ phase balance. Hence, austenite depletion within the high peak temperature region of the weld HAZ is likely to be a problem both during electron beam and laser welding of duplex stainless steels. Process modeling is currently seen as a commercial strategic issue by the manufacturing industry to reach the goal of faster product development and optimization of process parameters and mechanical properties.[17] In the context of H. HEMMER, Section Head, and S. KLOKKEHAUG, Researcher, are with the Institute of Energy Technology, N-2027 Kjeller, Norway. Ø. GRONG, Professor, is with the Department of Materials Technology and Electrochemistry, Norwegian University of Science and Technology, N7491 Trondheim, Norway. Manuscript submitted August 12, 1999.
METALLURGICAL AND MATERIALS TRANSACTIONS A
welding, research is now being directed toward the implementation of physical-based microstructure models in a finite element (FE) environment to predict the response of a specific class of alloys to the thermal field imposed on the parent material by the electric arc or the electron beam.[18] A key issue here is to keep the different components of the model as simple as possible, without loss of essential ingredients. This makes the solutions quick and easy to implement at the same time as the computational effort is minimized. Both aspects are important from a user’s point of view and will eventually determine whether the FE process model can be applied as a predictive tool in ordinary production welding. The present article describes the development of a process model for electron beam welding of different grades of duplex stainless steels, e.g., SAF 2205 and 2507. A number of attractive features are built into the FE code, i.e., (1) a separa
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