Proeutectoid cementite
Proeutectoid cementite forms during slow cooling of hypereutectoid steels above the eutectoid temperature at austenite grain boundaries. It forms continuous shells around the grains and can lead to increased hardness and toughness.
Physicochemical Mechanism of Action
Proeutectoid cementite forms during cooling of hypereutectoid steels (with more than 0.77 % carbon) above the eutectoid temperature. On slow cooling, cementite precipitates at the austenite grain boundaries and forms continuous shells around the grains.
Metallurgical Relevance & Welding Risks
Proeutectoid cementite increases the hardness and toughness of the weld, leading to improved strength. However, it can also cause increased stress buildup and cracking if cooling is too fast or if the structure is uneven.
Shop Practice & Action Guidelines
To promote proeutectoid cementite, cool hypereutectoid steels slowly (t8/5 > 25 s) and make sure cooling is even. Avoid rapid cooling methods like water quenching or strong cooling gas, as this can lead to uneven structure and increased cracking.