Transformation Residual Stresses
Transformation residual stresses arise from time-shifted volume expansions during lattice transformations during cooling. They are especially pronounced with martensite formation and lead to hardness and cracking.
Physical-Chemical Mechanism of Action
Transformation residual stresses occur when the metal transforms into different lattice structures during cooling. These processes are not simultaneous and cause volume expansions that are offset in time. For example, during martensite formation, the metal suddenly hardens and decreases in volume, which causes stresses in the material.
Metallurgical Relevance & Risks in Welding
These stresses can lead to hardness problems and cracks, especially if the metal does not cool uniformly. During martensite formation, the weld can become very hard and vulnerable to mechanical loads. This increases the risk of hot cracks and fracture.
Workshop Practice & Action Guidelines
To minimize transformation residual stresses, it is important that the metal cools evenly. Use suitable cooling agents like a water bath or air flow. For martensitic steels (e.g., HSS), control the cooling rate to get a sufficiently sharp weld without making it too hard. Recommended cooling times are between 10 and 30 seconds depending on the steel type.