Critical Cooling Rate for Martensite Formation
The critical cooling rate is the minimum rate at which no martensite forms. If this rate is exceeded (t8/5 < 5 s), glass-hard, crack-prone martensite forms.
Physical-Chemical Mechanism of Action
The critical cooling rate is the minimum rate at which no martensite is formed. This happens because the cooling is too fast for the grain to grow large enough and for the diffusion process to complete fully.
Metallurgical Relevance & Welding Risks
If the t8/5 time exceeds this rate (t8/5 < 5 s), glass-hard, crack-prone martensite forms. This leads to increased embrittlement and a higher risk of hot cracks in the weld.
Workshop Practice & Action Instructions
To avoid a pure martensitic structure, the cooling rate must stay below 5 seconds. This means the grain grows large enough and the diffusion process completes fully. With excessively long t8/5 times (t8/5 > 25 s), the grain coarsens and the toughness of the steel drops.