Machining induces stresses in parts. The bigger and more complex the part, the more the stresses. These stresses can cause distortions in the  long term. If the parts are clamped in service, then cracking could occur. Also hole locations can change causing them to go out of tolerance. For these reasons, stress relieving is often necessary. 

Typically, the parts that benefit from stress relieving are large and complex weldments, castings with a lot of machining, parts with tight dimensional tolerances and machined parts that have had a lot of stock removal performed. 

Stress relieving is done by subjecting the parts to a temperature of about 75 C (165 F) below the transformation temperature,line A1 on the diagram, which is about 727 C (1340 F) of steel—thus stress relieving is done at about 650 C (1202 F) for about one hour or till the whole part reaches the temperature. This removes more than 90% of the internal stresses. Alloy steels are stress relieved at higher temperatures. After removing from the furnace, the parts are cooled in still air. 
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