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The bimetallic welds between ferritic low alloy steels and austenitic stainless steels are widely used in the heat piping transport systems of nuclear power plants for connecting the heavy section low alloy steel components with those of high temperature stainless steel pipes. The operating experience of major nuclear power plant components has recently shown that bimetallic joints can jeopardize the plant availability and safety because of increased incidences of failure. In shielded metal arc welding process, the occurrence and severity of weld defects mainly depend upon the type of electrode filler wire and the electrode coating ingredients used. The use of nickel based filler metals is no longer considered as the final solution for unexpected failures of bimetallic welds due to incidences of hot cracking. In the present paper, an attempt has been made to design and develop an intermediate electrode based on CaO–CaF2–SiO2 ternary phase diagram system and nickel as an additional electrode coating ingredient using mild steel as a filler wire for the bimetallic weld joint. The extreme vertices methodology has been used to design 21 electrode coating formulations. The quadratic regression models for weld metal chemistry, ultimate tensile strength, impact toughness, macrohardness, diffusible hydrogen content, and corrosion rate in terms of electrode coating ingredients, have been developed and checked for adequacy using analysis of variance. The work aims at studying the individual as well as combined effect of electrode coating ingredients on the measured weld responses and microstructures of the weld. Also, the electrode coating formulations suggesting multiobjective optimized solutions have been proposed.

The present work is mainly focused on the investigation of Cr3C2-25NiCr coatings reinforced with 5 % and 10 % of yttria-stabilized zirconia (YSZ) nanoparticles deposited on the CA6NM turbine steel by using the high-velocity oxy-fuel technique. The coatings were analyzed by scanning electron microscope (SEM)/energy-dispersive X-ray spectroscopy (EDS). The phase identification of a crystalline material was done with the X-ray diffraction (XRD) technique. The SEM/EDS analysis showed that dense and homogeneous coatings were developed by the reinforcement of YSZ nanoparticles. The peaks of XRD graphs of Cr3C2-25NiCr coating reinforced with 5 % and 10 % of YSZ nanoparticles revealed that the chromium and carbon were present as a major phase, and the presence of nickel, yttrium, and zirconium was observed as a minor phase. The porosity level decreased up to 32 % and 45 % by the addition 5 % and 10 % of YSZ nanoparticles as compared with conventional Cr3C2-25NiCr coating. The surface roughness values for coated samples were found to be 5.03, 4.89, and 4.28. The nanocomposite coatings reinforced with 10 % YSZ nanoparticles exhibited the highest microhardness value (1,251 HV). The Cr3C2-25NiCr coatings reinforced with 10 % of YSZ nanoparticles resulted in low porosity, low surface roughness, and high microhardness. During the coating process, the nanoparticles of YSZ flow into the pores and gaps that exist in the coatings and provide a better shield to the substrate material. The Cr3C2-25NiCr with 10 % of YSZ nanoparticles showed better results in terms of mechanical and microstructural properties during the investigation.

Bimetallic welds (BMWs) have been a necessity within the steam generators of nuclear power plants, where the heavy section low alloy steel components are usually connected to stainless steel primary piping systems. These welds represent zones with metallurgical discontinuities, which tend to localize the strains and cause failures. The other critical issues which must be taken care of while welding of bimetallic joints include solidification cracking, thermal fatigue and residual stresses in welds. The occurrence and severity of all these problems associated with bimetallic welds depend upon the type of welding process used along with the other variables like welding consumables, heat input etc. In shielded metal arc welding (SMAW), welding electrode consumables/coatings play an important role in deciding the weld metal quality. The development of welding electrode coatings is quite difficult due to very rapid complex reactions taking place during welding and the involvement of many controlled process variables. The present paper is an effort to highlight the general issues and challenges for the systematic and scientific development of welding electrode coatings for bimetallic welds. The experimental procedure contains three sets of coated electrodes having varying ferro-alloys, silica and carbon content. In the first set of electrodes, non-ferrous elements are also introduced into the weld metal through flux addition. Further, the welded joints are subjected to various tests and then evaluated to ascertain their mechanical as well as metallurgical behaviour.

In this present work, an attempt has been made to optimize the wire electric discharge machining using muli-response optimization technique based on Taguchi's design approach. 16MnCr5 Alloy steel was selected as workpiece material. The response parameters viz. material removal rate and surface roughness were optimized by varying types of electrode, pulse on time (T on ), pulse off time (T off ) and peak current (I p ). The Taguchi's L 18 mixed orthogonal array has been used for planning and designing the experiments. ANOVA was used to find the contribution and significance of different process parameters on the response parameters. The results clearly indicated that pulse on time (T on ) was the most influential factor for the material removal rate and surface roughness. Optimal level of process parameters was used to perform the confirmatory experiments which verified the improvements in performance characteristics. Keywords List of symbols

The aim of this paper is to investigate the WC-10Co-4Cr coatings reinforced with 5 % and 10 % of yttria-stabilized zirconia (Y2O3/ZrO2; YSZ) nanoparticles deposited on the CA6NM turbine steel by using the high-velocity oxy-fuel (HVOF) thermal spraying technique. In the HVOF technique, the hot jet of the semi-solid particles strikes against the workpiece and creates a layer of coating of varying thickness on the substrate material. The coatings fabricated with HVOF were analyzed by scanning electron microscope (SEM) / energy-dispersive x-ray spectroscopy (EDS). The phase identification of a crystalline material was made with the x-ray diffraction (XRD) technique. The mecha­nical properties in terms of porosity, surface roughness and microhardness of the nanocomposite coatings were also evaluated. The SEM/EDS analysis showed that dense and homogeneous coatings were developed by the reinforcement of YSZ nanoparticles. The peaks of XRD graphs of WC-10Co-4Cr coating reinforced with 5 and 10 % of YSZ nanoparticles revealed that the WC was present as a major phase and W2C, Co3W3C, Co, Co6W6C, Co6W and Y2O3/ZrO2 nanoparticles were observed as a minor phase. The porosity level decreased up to 42 and 56 % by the addition 5 and 10 % of YSZ nanoparticles as compared with conventional WC-10Co-4Cr coating. The surface roughness values for WC-10Co-4Cr conventional coating, 95 % (WC-10Co-4Cr) + 5 % YSZ and 90 % (WC-10Co-4Cr) + 10 % YSZ nanocomposite coated samples were found to be 5.03, 4.89 and 4.28 respectively. The nanocomposite coatings reinforced with 10 % YSZ nanoparticles exhibited the highest microhardness value (1278 HV). The WC-10Co-4Cr coatings reinforced with 10 % of YSZ nanoparticles resulted in low porosity, low surface roughness and high microhardness. During the coating process, the nanoparticles of YSZ flow into the pores and are dispersed in the gaps between the micrometric WC particles and provide a better shield to the substrate material. The WC-10Co-4Cr with 10 % of YSZ nanoparticles showed better results in terms of mecha­nical and microstructural properties during the investigation.