SR Stress relief
Stress relief · Tratamiento térmico · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “SR 550 °C 2 h after welding” states a stress relief requirement.
What controls
Heating to a moderate temperature, below the transformation temperature, held for a time and followed by slow cooling, with the aim of reducing residual stresses built up by welding, heavy machining, forming or casting without appreciably altering hardness or structure. It is a process requirement: what is certified is that the cycle was carried out (temperature, time, cooling rate), because residual stress level is not something routinely measured in a machine shop.
How to verify
What can be verified is the cycle record: temperature-time chart and heat treatment certificate, with load identification. As an indirect check, hardness is measured before and after to confirm it did not change, and the part's movement is verified geometrically by measuring reference dimensions before and after. Truly measuring residual stresses requires laboratory methods, not shop-floor ones.
Common mistakes when interpreting it
- Specifying it in the hope of correcting existing distortion: it relieves stresses, it does not straighten parts.
- Confusing it with annealing and accepting a drop in hardness as normal.
- Considering the part stress-relieved without the cycle record, which is the only real evidence it was done.
How to get it in the workshop / if it comes out of tolerance
Its practical effect is that the part stops moving between operations. On large weldments or heavily roughed parts, inserting it between roughing and finishing saves more time than it costs. Do not expect it to change hardness: if the part comes out noticeably softer, the temperature was too high and the treatment turned into something else. Leave stock for the subsequent finishing, because during stress relief the part settles and moves.
If the part still moves during finishing, the cycle was too short or the temperature too low for that material, or the stresses are being generated by the machining itself: reduce depth of cut and alternate faces. If hardness dropped, the temperature overshot. On weldments that distort heavily during stress relief, check how they were supported in the furnace: a poorly supported part sags under its own weight when hot.
Related symbols
Other symbols in Heat treatment
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