Secondary cementite
Secondary cementite forms during cooling of hypoeutectoid or hypereutectoid steels at the austenite grain boundaries from austenite. It marks the start of pearlite formation and can lead to increased hardness and grain size.
Physicochemical Mechanism of Action
Secondary cementite forms by the precipitation of iron carbide (Fe3C) from austenite during cooling. This process begins when the temperature drops below the eutectoid point and the metal transforms into a granular structure.
Metallurgical Relevance & Welding Risks
The formation of secondary cementite can lead to increased hardness and grain size, which can impair the toughness of the steel. This can cause cracking or distortion during further cooling. Additionally, cementite can affect weld quality and lead to increased stress concentration.
Workshop Practice & Instructions
To avoid secondary cementite, cooling should be controlled. For hypoeutectoid steels (with low carbon content), slow cooling is recommended. For hypereutectoid steels (with high carbon content), cooling should be faster to prevent cementite formation. Specific temperatures and times vary by steel type and welding process.