Martensite start temperature
The martensite start point is the temperature at which austenite transforms to martensite without diffusion. This leads to a strong, glass-hard structure.
Physicochemical Mechanism of Action
When cooling steels with high carbon content (e.g., HSS), a solid solution of Fe and C forms in austenite. When the temperature drops below the martensite start point, carbon atoms can no longer be accommodated in the austenite crystal structure. The rapid, diffusionless transformation leads to a glass-hard microstructure with high stress buildup.
Metallurgical Relevance & Welding Risks
Cooling too fast can cause the martensite start point to be reached, resulting in a strong, crack-prone microstructure. This increases susceptibility to hot cracking and reduces steel toughness. Cooling too slowly, on the other hand, can prevent complete transformation of austenite to martensite, leading to a fine microstructure with higher toughness.
Shop Practice & Action Guidelines
To achieve a homogeneous microstructure and minimize hot cracking susceptibility, controlled cooling is essential. This can be done using cooling water or special cooling media. Exact cooling temperature and time depend on steel type and desired properties, but it's important that cooling is neither too fast nor too slow.