Vít Jan

Simulation of dissimilar weld joints of steel P91

Abstract Results of computer simulations of long term service exposure for weldments of the CSN 15 128/P91 and SK3STC/P91 steels are presented and compared with corresponding results of phase and composition experiments. The welded material P91 (EU designation: X10CrMoVNb 9–1) represents progressive chromium steel alloyed with molybdenum, vanadium, carbon, and nitrogen. The CSN 15 128 (13CrMoV 2–5) material is low alloy Cr–Mo–V steel. The SK3STC alloy (12CrMo 10–10) represents the consumable electrode material. The stability of the weldment microstructure is investigated at elevated temperatures (500–700° C). The simulation method is based on the Calphad approach complemented with the theory of multicomponent bulk diffusion, local conditions of phase equilibrium, and the assumption that diffusion is the process that controls the rate of phase transformation. Significant phase profiles, concentration profiles, and phase transformation processes in the diffusion affected zone are simulated, investigated, and compared with experimental results. The potentially deleterious carbon depleted region inside each weld joint is discussed. The method described can be used to predict microstructure instability in weld joints.

Local Changes of Microhardness in Dissimilar Weld Joints after High Temperature Exposure

The paper presents results obtained during evaluation of dissimilar weld joints of creep-resistant steels. During high temperature exposure of dissimilar weld joints, alloying elements were redistributed across the weld interface. These diffusion effects can cause local changes of microstructure and have a direct effect on local mechanical properties in weld interface area. Carbon and nitrogen have the strongest influence on changes of mechanical properties of steels. . These local changes of mechanical properties have a strong influence on the reliability and the service live of the whole welded structures. The dissimilar joints of the austenitic steel/martenzitic steel type was studied. Laboratory weld joints were prepared and annealed at different temperatures for different time periods. Microhardness profiles across the weld interface were measured and the influence of long-term, high temperature exposure on the changes of local microhardness was evaluated. Results were compared with pseudo-binary phase diagrams and with the literature.

Influence of multiple electron-beam remeltings on the characteristics of HVOF and CGDS sprayed conicraly coatings

Nanostructured CoNiCrAlY bond coatings were deposited onto a Ni-based alloy (Inconel 718) by both HVOF and CGDS spraying techniques. Subsequently, the deposits were remelted by an electron beam up to depth of about 100 μm, which resulted in the removal of defects on the substrate to the bond coat interface. This paper examines the influence of the parameters used for EB remelting, including multiple remelting on the microstructural changes, phase modification and the final state of the coatings. The amount of porosity in the coatings and the surface roughness has been evaluated. Scanning electron microscopy and X-ray diffraction were performed in order to characterize the phase modification before and after the applied treatment. The results indicated that multiple remelting improved the coating in terms of porosity, surface roughness decrease, mechanical strength and chemical homogeneity. This study also demonstrates that the CGDS deposition represents a promising alternative for CoNiCrAlY bond coat manufacturing.

Microstructure Evaluation of Heterogeneous Electron Beam Weld between Stabilised Austenitic and ODS Ferritic Steel

The weldability of advanced heat resistant ODS metallic materials in combination with conventional materials is a prior requirement for their wider use in energy production. The microstructure of ODS steels is composed of alpha iron based matrix with dispersed oxide particles. Due to heating during conventional welding, the microstructure and properties of the resulting weld joints are affected and the joints often become the weakest point of the structure. The electron beam welding with its reduced heat affected zone size may be an answer in this. The presented article is focused on thorough metallographic evaluation of the structure of heterogeneous electron beam welds which combine stabilized austenitic stainless steel with the MA956 ferritic ODS steel. EB welded joints were evaluated by light and analytical electron microscopy including EDS and phase EBSD analyses in the as-welded state and after post-weld heat treatment. Mechanical properties of the weld were evaluated from the results of micro hardness profiles. Achieving an appropriate structure of such welds and correct welding parameters are crucial aspects for future successful application of similar joints in energy industry