Subcritical annealing
A thermal treatment below the eutectoid temperature aimed at stress reduction without significant alteration of the microstructural state.
Subcritical annealing is a heat treatment below the transformation temperature. The structure doesn't change, but you relieve internal stresses, mainly from welding or cold forming. The material becomes softer and more stable without risking new hardening or a different structure. If you do it wrong, e.g., too hot or too fast, it can backfire. Then the stress stays in or you even get new cracks because the material doesn't relax evenly. Especially with thick or constrained parts, you have to be careful not to do more harm than good. In practice: you put the part in the furnace, heat it to the right temperature below the Ac1 line, and let it soak long enough. Then you cool slowly, preferably in the furnace or in air, so the stress can really relieve. The exact temperature and time depend on the steel; you have to follow the data sheet.