Martensite finish temperature
The martensite finish temperature is the point at which the martensite structure is fully transformed on cooling. Below this temperature, retained austenite remains in the material and causes a distorted crystal structure.
Physicochemical Mechanism of Action
The martensite finish temperature marks the end of the martensitic transformation on cooling. Austenite is converted into martensite, which leads to a finely distorted crystal structure.
Metallurgical Relevance & Welding Risks
Below the martensite finish temperature, retained austenite remains in the material. This can lead to a finely distorted crystal structure and higher hardness, which hurts the toughness of the steel and can cause cracks.
Workshop Practice & Instructions
To achieve a homogeneous martensite structure, cooling must happen within the critical temperature window. That means the weld must not cool too fast (t8/5 < 5 s), since this leads to glass-hard martensite and cracks. Also make sure cooling is not too slow (t8/5 > 25 s), since this can lead to a finely distorted crystal structure and loss of toughness.