Tertiary cementite
Tertiary cementite forms at low temperatures in unalloyed steels with low carbon content from ferrite. It is very fine and can lead to cracks.
Physicochemical mechanism of action
Tertiary cementite forms when carbon in the weld zone exchanges out of ferrite at low temperatures. This happens because the solubility of carbon in ferrite decreases sharply with falling temperature.
Metallurgical relevance & risks in welding
Tertiary cementite can lead to very fine cementite precipitates that can cause cracks. This separation occurs especially in unalloyed steels with extremely low carbon content and is a significant factor for the hardness and mechanical strength of the weld.
Workshop practice & instructions
To avoid tertiary cementite, careful control of the cooling rate should be carried out for unalloyed steels with low carbon content. Cooling should be slow to minimize the formation of cementite precipitates. Recommended cooling times vary by steel type, but generally should be between 30 and 60 seconds.