Critical cooling rate
The critical cooling rate is the minimum rate at which a material is cooled to form a martensitic structure and avoid diffusion-controlled phases like pearlite or bainite. It affects the hardness and formability of the material.
Physical-Chemical Mechanism of Action
The critical cooling rate refers to the rate at which a material is cooled to form a martensitic structure. If cooling is too slow, diffusion-controlled phases like pearlite or bainite form. With faster cooling, the material stays martensitic and develops high hardness.
Metallurgical Relevance & Welding Risks
Too fast cooling can lead to glass-hard martensite, which reduces toughness and increases cracking. Too slow cooling can lead to diffusion-controlled phases like pearlite or bainite, which reduces hardness and formability.
Workshop Practice & Action Instructions
To achieve an optimal cooling rate, pay attention to the following: For high-carbon steels (e.g., HSS), cool quickly to get a martensitic structure. For low-carbon steels (e.g., austenitic steel), cool more slowly to avoid diffusion-controlled phases. The exact rate depends on the specific material properties and the desired weld strength.