According to a 2025 technical guide from TWI, welding carbon steel to austenitic stainless steel introduces a critical challenge: carbon migration. At elevated temperatures, carbon diffuses from the high-carbon side (carbon steel) into the stainless steel, creating a decarburized zone in the carbon steel and a carburized layer in the stainless. This can lead to loss of mechanical strength and increased susceptibility to corrosion.
The standard mitigation is to use a nickel-based filler metal, such as ERNiCr-3 (Inconel 82), which acts as a diffusion barrier. The nickel-rich chemistry suppresses carbon migration and accommodates the difference in thermal expansion coefficients between the two base metals. Proper joint design also matters: a buttering layer on the carbon steel side before completing the weld reduces dilution and controls the heat-affected zone microstructure.
Key considerations for dissimilar metal joints include:
• Selecting filler metal with sufficient alloying to prevent martensite formation at the fusion boundary.
• Controlling interpass temperature below 150°C to minimize thermal stress.
• Post-weld heat treatment is generally avoided because it can accelerate carbon migration; if required, a low-temperature stress relief (around 620°C) with rapid cooling is used.
These principles are essential for piping systems in refineries and power plants where carbon steel and stainless steel meet.
What is your go-to filler metal for carbon steel to stainless steel welds?
Source: TWI, 2025