Modeling Solid-State Phase Transformations of 13Cr-4Ni Steels in Welding Heat-Affected Zone
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TRODUCTION
SINCE its elaboration in the late fifties, 13Cr-4Ni soft martensitic stainless steel has been extensively preferred to low carbon steel for the manufacturing of hydraulic turbines as it offers better mechanical properties, toughness, and corrosion resistance.[1,2] Moreover, this steel is weldable, has a higher yield strength than common austenitic stainless steels, and it is less expensive, owing to a low alloying nickel content. These benefits made this material a good candidate for the manufacturing of Francis type hydraulic turbines, which are large and complex structures generally made of multiple cast components assembled with homogeneous Flux Cored Arc Welding (FCAW) deposits. Despite the advantages of 13Cr-4Ni steels, fatigue damage initiation and growth in the welded regions continue to be a recurrent problem that must be addressed by hydraulic powerplant operators.[3,4] Numerous factors promote
J. B. LE´VESQUE, J. LANTEIGNE, and D. PAQUET are with the Institut de Recherche da˜ Hydro-Que´bec, 1800 Boul. Lionel-Boulet, Varennes, QC J3X 1S1, Canada. Contact e-mail: [email protected] H. CHAMPLIAUD is with the E´cole de technologie supe´rieure, 1100 Rue Notre-Dame O, Montre´al, QC H3C 1K3, Canada. Manuscript submitted August 5, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
fatigue damage such as geometry, microstructures, casting defects, weld porosity, excessive cyclic loading, and residual stresses that are inherent to welding operations. Several studies have addressed the interaction between welding residual stresses and fatigue crack initiation and growth.[5–8] Nevertheless, a better understanding of the welding residual stresses build up and of their interaction with the hydraulic load applied on the turbine requires the development of appropriate numerical tools. Once developed, these tools can be used to simulate the welding operations and to model the associated solid-state phase transformations the material undergoes. This will allow modeling the microstructure distribution and use of appropriate constitutive law for residual stresses calculation. Moreover, it enables to assess the effect of the welding process on fatigue crack behavior by numerical methods. During welding, 13Cr-4Ni soft martensitic stainless steels experience different solid-state phase transformations.[9] Figure 1 shows an example of a dilatometric curve of subsequent heating and cooling. Upon heating from room temperature, the martensite to austenite transformation is characterized by a volumetric contraction. It starts at temperature (Ac1 ), which depends on the heating rate. This transformation is diffusive and its kinetics is driven by numerous factors such as temperature, soaking time, chemical composition, and microstructural morphology. In some circumstances
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Temperature, C Fig. 1—Dilatation as a function of temperature during heating and cooling of 13Cr-4Ni soft martensitic sta
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