Carbon migration is a well-known metallurgical phenomenon in dissimilar metal welds between carbon steel and austenitic stainless steel. During elevated temperature service, carbon atoms diffuse from the carbon steel side into the weld metal, depleting the heat-affected zone of carbon and forming a soft decarburized layer. Simultaneously, a hard carbide-rich zone develops on the stainless steel side, which can embrittle the joint.
Per the Welding Research Council (WRC) bulletins on dissimilar metal welding, the driving force for carbon migration is the difference in carbon activity between the two materials. Austenitic weld metals, especially those with high chromium content, have a higher affinity for carbon, creating a chemical potential gradient. The migration rate accelerates above 400°C and becomes significant above 500°C, which is why post-weld heat treatment or service exposure in this range requires careful evaluation.
Key factors that control carbon migration include:
• The carbon content of the base metal: higher carbon in the steel increases the driving force.
• The nickel content of the weld metal: higher nickel reduces the carbon activity gradient.
• The use of an intermediate buffer layer, such as Inconel 82 or 625, which acts as a carbon diffusion barrier.
• The service temperature and time: longer exposure at elevated temperature increases the decarburized zone width.
In practice, for dissimilar welds in refinery piping or boiler tubes, a buttering layer of nickel-based alloy is applied to the carbon steel side before welding to the stainless steel. This practice, recommended by API RP 571 for damage mechanisms in the refining industry, effectively minimizes carbon migration and extends joint life.
How do you manage carbon migration in your dissimilar metal weld procedures?