Vacuum Stress Relieving
Stress relieving is a general term in heat treating describing a wide range of processes. Generally 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.
ThermTech has capability to stress relieve components up to 13’ long, 8’ wide and 4’ high. If surface oxidation is of concern stress relieving can be done in atmosphere or vacuum. Please call ThermTech for size capacities in this situation. Therm Tech’s staff of professionals is always available for advice if questions arise.
Stress relieving is a process used on materials ranging from plastics, 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 to 900F for copper alloys. When stress relieving steel and iron the typical stress relieving temperatures range from 1000F to 1300F.
Stress relieving steel is always done at temperatures below the temperature at which the austenite phase begins to form. When having the lowest possible internal stresses is important 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.
Stress relieving in an open fire furnace does cause surface oxidation which ranges from discoloration at lower temperatures to a fine scale at subcritical temperatures. If final surface finish is of concern oxidation can be avoided by using a nitrogen atmosphere or stress relieving in vacuum. There is a substantial difference in cost for these processes so it is wise to communicate these concerns prior to heat treatment.
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 relieve cycle. These materials may need a slow ramp rate to the stress relieving temperature.