No abstract

UNS S41500 is a grade of 13Cr-4Ni martensitic stainless steel utilized in hydroelectric turbine products. In this study, the microstructural characteristics, mechanical properties, and through-thickness residual stresses of electron-beam (EB) welded butt joints in 90-mm-thick UNS S41500, assembled using a single pass autogenous process, were evaluated after post-weld heat treatment (PWHT). The results of the longitudinal residual stresses, measured using the contour method, indicated that the applied PWHT was effective in stress relieving and reducing the hardness of the weldment through tempering of the “fresh” martensite present in the microstructure after EB welding. The static tensile properties that were evaluated in the directions transverse and longitudinal to the weld seam demonstrated high performance of the joints with conformance to the requirements of not only ASME Section IX, ASME Boiler & Pressure Vessel Code–Section IX: Qualification Standard for Welding and Brazing Procedures, Welders, Brazers, and Welding and Brazing Operators, but also ASTM A240, Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications, for unwelded UNS S41500. The Charpy impact energies indicated that the toughness of the welds greatly surpassed the minimum acceptance criteria specified in ASME Section VIII, ASME Boiler & Pressure Vessel Code–Section VIII: Rules for Construction of Pressure Vessels. Also, bend testing of transverse weld cross sections displayed no discontinuities on the tension side of the bent joints. These results provide the essential data for validating a manufacturing process for assembly of high-performance joints in an important hydroelectric turbine material.

Heavy-section assembly of hydroelectric turbine runner materials using single-pass, autogenous EBW was demonstrated to penetrate a 90-mm-thick butt joint. The welding-induced distortions and residual stresses were characterised to understand the impact of the materials and process conditions (e.g. preheating and/or PWHT). Using 3D optical measurements, the angular distortions of EB-welded UNS S41500 and CA6NM steels were determined to be 0.13°and 0.38°, respectively. The longitudinal residual stresses, measured through the contour method, had a M-shaped distribution throughout the thickness with minimum (∼−500 MPa) compressive stresses in the FZ and maximum (∼600 MPa) tensile stresses in the HAZ. After PWHT, the tensile and compressive stresses reduced to ∼100 MPa.

Fatigue damage is commonly encountered by operators of Francis type hydraulic turbine runners made of 13Cr-4Ni soft martensitic stainless steel. These large and complex welded casting assemblies are subjected to fatigue crack initiation and growth in the vicinity of their welded regions. It is well known that fatigue behavior is influenced by residual stresses and the microstructure. By including solid-state phase transformation models in welding simulations, phase distribution can be evaluated along with their respective volumetric change and their effect on residual stresses. Thus, it enables the assessment of welding process on fatigue crack behavior by numerical methods. This paper focuses on modeling solid-state phase transformations of 13Cr-4Ni soft martensitic stainless steel, used for manufacturing hydraulic turbine runners, occurring upon welding. It proposes to determine the material parameters of the models for both the austenitic and the martensitic transformation by nonisothermal dilatometry tests. The experiments are conducted in a quenching dilatometer with applied thermal conditions as experienced in the heat-affected zone of homogeneous welds. The activation energy and the kinetic parameters of the austenitic transformation from fully martensitic state are measured from the experimental results. The martensitic transformation modeling from a fully austenitic domain is also presented.