Here are  8 reasons why you might want to consider stress relieving the steel before machining your parts.

1. High carbon grade of steel. Alloy grades over 0.40 carbon and carbon grades above 0.50 carbon can often benefit from stress relief.
2. Heavy draft to make size. Heavy draft can add cold working strain which can set up stresses in the part.
3. Small diameter parts. The percentage of cold work (strain) is higher for the same draft reduction as diameter decreases.
4. Long parts. Stresses tend to display  and their effects increase longitudinally.
5. Assymetric parts– and parts with large differences in section or mass.
6. To increase mechanical properties. At lower stress relieving temperatures, the hardness, tensile strength, and elastic properties of most cold drawn steels increase.
7. To decrease mechanical properties. At higher stress relieving temperatures, hardness, tensile strength and yield strength are reduced while % elongation and 5 reduction of area are increased.
8. To reduce distortion off the machine. Usually stress relieving is used as a last ditch effort to reduce the distortion  that presents after machining a part with some or many  of the characteristics given above.

Stress relieving is a lower than  the material’s critical point thermal treatment also known as strain drawing, strain tempering, strain annealling, strain relieving, or pre-aging. It is performed to modify the the magnitude and distribution of of residual forces within a cold drawn steel bar, as well as to modify the mechanical properties.

Thanks Seth at Sixthman Blog for the photo.