Stress-relieving
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Metal Stress Relieving

Stress relieving is a general term in heat treating, describing a wide range of processes. Generally, metal stress relieving involves heating a part to a temperature at which the yield strength is sufficiently low to the point which internal stresses can relieve themselves. Higher temperatures and longer times are beneficial and will yield a part with lower internal stress. Temperatures are always below the point at which there is a change of phase in steel.

Process of Stress Relieving Heat Treatment – Points to Consider
Stress relieving is a heat treatment process used for materials ranging from plastics and steel to non-ferrous materials, such as copper alloys and aluminum. For non-ferrous materials, temperatures are comparatively low, ranging from about 200F for plastics and 900F for copper alloys.

  • When stress relieving steel and iron, the typical temperatures range from 1000F to 1300F.
  • Stress relieving steel is always done at temperatures below the range at which the austenite phase begins to form.
  • When aiming for the lowest possible internal stresses, it is important that metal stress relieving is completed at temperatures at or near 1300F. This process is called sub-critical annealing because the temperatures used are near the point at which austenite begins to form.
  • Metal stress relieving in an open fire furnace does cause surface oxidation, which ranges from discoloration at lower temperatures to a fine scale at sub-critical temperatures.
  • If final surface finish is of concern, oxidation can be avoided by using a nitrogen atmosphere or stress relieving in a vacuum.
  • There is a substantial difference in cost for these processes, so it is wise to communicate these concerns prior to industrial heat-treating services.

 

Stress Relieving Recommendations from ThermTech
Typically, there is very little distortion during stress relieving. However, some considerations may be required for long slender components or plates that are heavily cold worked. If shot blasting is done after stress relieving, this operation does impart compressive stresses at the surface of the material and can cause distortion. Cast materials with low ductility or with geometries that include stress risers can increase the probability of cracking during the heating portion of the stress relief cycle. These materials may need a slow ramp rate to the stress relieving temperature.

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