Residual carbon ferrite
Residual carbon ferrite is a matrix phase in steels that holds residual carbon in forced solution. It occurs at highly undercooled transitions and can lead to hardness problems.
Physico-Chemical Mechanism of Action
Residual carbon ferrite forms when carbon in the weld is not fully dissolved and remains as a supersaturated solution in the ferrite structure. This happens especially at heavily undercooled transitions where the ferrite cools faster than it can absorb carbon atoms.
Metallurgical Relevance & Welding Risks
Residual carbon ferrite leads to increased hardness and toughness of the weld, which can cause cracks and hot cracks in the material. It also impairs the formability and machinability of the steel.
Shop Practice & Action Guidelines
To avoid residual carbon ferrite, it is important to ensure controlled cooling of the weld. This can be achieved by using cooling water or special cooling jackets. The cooling rate should be set so that the ferrite has enough time to absorb carbon atoms and allow a homogeneous structure to form.