In this column, we will discuss the process of stress relief and compare it to the effects of annealing.
During the manufacturing process, residual stresses are formed within the part. These could be beneficial (compressive at the surface) or detrimental (tensile at the surface). Just the presence of residual stress, either compressive or tensile, can cause problems with dimensional stability of the part, or cause problems with distortion.
In the typical process to manufacture a steel or aluminum part, incoming material is purchased; shaped by forming, forging, or casting; machined; heat treated; and finish processed (final machining, plating, or painting). This is illustrated in Figure 1.
In many circles, the use of stress relief and annealing are used interchangeably. However, these are distinctly different processes. Previously, this column has discussed annealing [1] but a short review is probably in order.
In full annealing, for steel, the part is heated to above the upper critical temperature and becomes fully austenitic. For aluminum, the part would be heated to the solution heat-treatment temperature that is appropriate for the alloy. The same would be true for titanium. The part is then allowed to slowly cool from the elevated temperature. A uniform, and soft microstructure consisting of ferrite and pearlite would be formed in steels.
With inter-critical annealing, the part is heated to above the lower critical temperature (in steel about 1,333°F or 722°C), but below the upper critical temperature where the part transforms completely to austenite.
For sub-critical annealing, the part is held at a temperature below the lower critical temperature. Holding at temperature for very long times will result in a spheroidized structure.
The primary reason for annealing is to change the microstructure to make it more uniform, softer, and to increase ductility.
Stress-relief
The purpose of stress relieving is to reduce the residual stresses present from forming or machining. It is not done to change the metallurgical properties or the microstructure. Cold working, grinding, machining or thermal cutting can produce significant residual stresses in the steel. During heat treatment, and specifically during the heating cycle, these stresses relieve themselves. This manifests as a change in geometry. Further, stress relief is also accomplished between manufacturing steps to provide a residual stress-free part, that will not bind or change shape as additional manufacturing steps are performed. It can also be done after heat treatment to improve fatigue strength. It will also improve dimensional stability of the part.
For many low-alloy steels that have been severely cold-worked, heating slowly to 200-425°C (400-800°F) for a relatively short time (2-4 hours) will reduce the residual stresses. For alloy steels, most stress relieving is performed 480-540°C (900-1,000°F) for 2-4 hours. This will reduce most of the residual stresses present. If the part is stress relived after heat treatment and tempering, a temperature is used that is approximately 30-50°C below the tempering temperature. In all cases, the parts are slow cooled after stress relief to prevent the reintroduction of thermal residual stresses.
Parts do not have to be heat treated (quenched and tempered) to be stress relieved. In these cases, temperatures as high as economically feasible are used. For heat-treated steels, parts are usually stress relieved at temperatures 50-100°F (28-56°C) below the tempering temperature, after finish machining or grinding.
Performing a stress-relieving operation is commonly done in the aerospace industry after each primary step in the manufacturing process. For instance, after forming or machining, an aluminum part may be stress relieved at 300°C (572°F) for a up to 2-3 hours. Again, the goal is to mitigate residual stresses and not change the microstructure.
In each step of manufacturing, a residual stress vector occurs with a different magnitude and direction. As manufacturing progresses, the manufacturing vectors sum to form a final residual stress state. This results in a warped or distorted part, or one that could potentially have detrimental residual stresses present. The use of stress relief processes throughout the manufacturing sequence can reduce the detrimental residual stresses.
The stress-relief cycle can also identify the main contributions to distortion and residual stresses. By applying a stress-relief process, and measuring the shape change before and after stress relief, the various residual stress contributions to distortion can be identified. As an example, a bearing manufacturer was having problems with distortion during heat treatment. Increasing the martempering temperature resolved many of the distortion issues, but there were several parts that stubbornly defied our efforts to reduce distortion.
A detailed process was implemented. A series of parts were stress relieved prior to heat treatment, and parts were heat treated and quenched. The distortion resulting from the heat-treatment process was identified. The resulting distortion was less than the distortion observed with normal production parts. By systematically working backwards in the process, by implementing a stress relief, and measuring parts before and after stress relief, they were able to identify that the stamping process was a major contributor to their overall distortion. More attention to worn dies and uniform application of stamping lubricant drastically reduced the overall distortion of parts produced.
Conclusions
The purpose of stress relieving is to relieve residual stress within the part from manufacturing processes, such as forming or machining. It is often performed prior to heat treatment to reduce distortion of heat-treated parts. Stress relief of parts after final grinding or machining increases the part’s thermal and dimensional stability. Finally, reduction of detrimental residual stresses improves part life and reliability during use.
Should you have any questions regarding this article, or have suggestions for other articles, please contact the author or the editor.
References
- MacKenzie, D.S., “Understanding different types of heat treatment: Annealing,” Gear Solutions, January, 2019, pp. 20-22.
