welding-alloys.com 10 The combination of these alloying elements and their microstructural properties makes STELLOY cobalt based superalloys well suited for applications requiring wear resistance, corrosion resistance, and hightemperature stability. Even with low carbon cobalt alloys like STELLOY 6BC or 21, differences in thermal expansion between cobalt and steel can cause stress-related defects. In some cases, a buffer layer helps reduce residual stresses, lower the risk of cracking, and improve metallurgical transition and compatibility. Nickel based buffer layers offer better stress absorption and transition control than cobalt based alternatives. Key considerations in buffer layer selection: • Dilution effects: the first layer of STELLOY weld deposit is particularly prone to iron dilution, reducing its hardness, wear resistance, and corrosion resistance. A well-chosen buffer layer helps to minimise iron content in the final wear-resistant surface. • Nickel based buffer layer: applying a ductile nickel based buffer layer, such as 625, with low preheat and interpass temperature, ensures a smoother transition, reducing embrittlement and cracking risks. Nickel 276 is even more effective than 625, as its lower niobium content prevents the formation of low-melting eutectic phases, which can contribute to hot cracking in the first weld layer. • Austenitic stainless steel base metals: while stainless steels generally do not form brittle martensite, hardfacing with cobalt based cored wires can lead to carbon migration, causing sensitisation and reduced corrosion resistance. A nickel based buffer layer can help mitigate this effect. Hot shear blade hardfaced with STELLOY 21-LD Ball valve hardfaced with STELLOY 6-TIG Rubber mixer hardfaced with STELLOY 6-G
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