Official Answer
Answer
'Blueing' or stress-relieving is a thermal process where carbon steel springs are heated to approximately $600^{\circ}F$ to $700^{\circ}F$ ($315^{\circ}C$ to $370^{\circ}C$) for a specific duration. This temperature is below the transformation range but high enough to allow for the redistribution of residual stresses induced during the coiling and waving operations. This process stabilizes the spring's dimensions and improves fatigue life by reducing the peaks of internal tensile stress. A side effect is the formation of a blue-black oxide layer (magnetite, $Fe_3O_4$), which provides a very mild degree of corrosion resistance, though additional oiling or phosphate coating is usually required for industrial environments.
