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Stress relief is a heat-treatment process that relies on slow cooling to achieve its desired effect, and it is influenced by a number of factors including the internal stress induced into the parts from the various manufacturing methods and prior processing.
Stress relief is a heat-treatment process that relies on slow cooling to achieve its desired effect, and it is influenced by a number of factors including the internal stress induced into the parts from the various manufacturing methods (e.g., bending, shearing, forging, sawing, machining, grinding, milling, turning, welding, etc.) and prior processing. The application end use ultimately defines the allowable stress state. So how does one perform a stress-relief operation? Let’s learn more.
Processes that depend on slow cooling (e.g., annealing, normalizing, stress relief) do so for a number of reasons – to relieve stresses, improve chemical homogeneity, soften a material for subsequent operations (e.g., machining), refine grain size and for such reasons as embrittlement relief or magnetic properties. As a general rule, the larger or more complex the part, the greater the amount of internal stress present.
Stress relief can be differentiated from other slow-cooling processes in that it is most often performed below the lower critical temperature (Ac1). Time at temperature depends on such factors as the complexity of the part, and enough time must be allowed in order to achieve the desired reduction in residual-stress level. Following stress relief, the steel is cooled at a sufficiently slow rate to avoid formation of or reintroduction of excessive thermal stresses. No microstructural phase changes occur during the stress-relief process.
How slow is slow?
Stress relief most often requires a “still-air cool” so as not to reintroduce stress into a material, but what does this really mean? A still-air cool (quench) can be defined as cooling at a rate of 40°F (22°C) per minute or faster to 1100°F (593°C) and then at a rate of 15-25°F (8-14°C) per minute from 1100-300°F (593-150°C). Below 300°F (150°C) any cooling rate may be used.
How do we perform a stress-relief operation?
For carbon steels, stress-relief operations are typically performed at 105-165°F (40-75°C) below the lower critical temperature – in the range of 930-1200°F (500-650°C). It is also important to understand that the elimination of stress is not instantaneous, being a function of both temperature and time for maximum benefit. Typically, one hour per inch (25 mm) of maximum cross-sectional area (once the part has reached temperature) is required. After removal from the furnace or oven, the parts are air cooled in still air. Rapid cooling will only serve to reintroduce stress, and this is the most common mistake made in stress-relief operations. This cycle is estimated to remove more than 90% of the internal stresses. Stress relief on alloy steels is often done at (slightly) higher temperatures.
For tool steels, the process is similar. It is common to perform a stress-relief operation in the temperature range of 500-550°C (925-1025°F), allowing the parts to slowly cool to room temperature before subsequent operations.
For stainless steels, the situation is more complex. Stress relief is done in the range of 290-425°C (550-800°F), which is below the sensitization range. Stainless stress relief depends on the form of the material, the operation being performed (e.g., machining) or whether a completed assembly will be stress relieved.
Poor Man's Stress Relief
In hardening, rapid cooling/quenching alone or in combination with pre-existing internal stresses can result in unwanted distortion, brittle fracture and, if near welds in certain grades of metal, stress corrosion cracking. For this reason, a number of heat treaters introduce a “stress-relief hold” during hardening or case-hardening treatments. This involves heating of a workload to an intermediate temperature – in the range of 1000-1300°F (538-705°C) – and soaking for a period of time equivalent to one hour per inch of maximum cross-sectional area. The idea is to allow for stress relaxation so that more predictable dimensional change occurs on quenching.
|Fig. 1. Effects of recovery and recrystallization on grain structure|
Stress Relief of Springs
Stress relief is one of the most common heat-treating processes used in spring manufacturing as well as the manufacture of other wire-formed products. Drawing, forming and machining induce stresses in all wire products. These stresses can cause loss of tolerance, cracking and distortion and contribute to in-service failures. For these reasons, stress relieving is often necessary and, in many cases, mandatory.
In addition to removing stresses, stress relief returns the material to a strength level approximately equivalent to where it was prior to forming. Studies have shown that the interstitial elements pin the lattice defects in the atomic structure of the metal, resulting in this increase in mechanical strength.
To completely eliminate residual stresses in helical springs through stress relief, the material must be heated high enough to fully recrystallize. This is not practical in spring manufacturing since the recrystallization process significantly reduces the material’s strength and, therefore, its usefulness in spring applications. On the other hand, an elevated-temperature recovery process (i.e. stress relief) can eliminate the majority of residual stresses without significantly deteriorating the material’s strength (Fig. 1). The temperature required to accomplish the recovery process depends on the material type and processing history (i.e. carbon steel vs. alloy steel, cold drawn vs. oil tempered, etc.) The SMI Encyclopedia of Spring Design provides recommendations for proper recovery.
Temperature, time and time at temperature are key process variables, and these have been documented elsewhere. In general, heating steel to a temperature of about 165°F (75°C) below the transformation temperature (Ac1) for about one hour (or until the entire part reaches temperature) will allow for the removal of most internal stresses. Typical temperature ranges are:
- 1025-1200°F (550-650°C) for unalloyed and low-alloy steels
- 1115-1300°F (600-700°C) for hot-work and high-speed tool steels
For many alloy steels, little or no stress relief occurs at temperatures less than approximately 500°F (260°C) while approximately 90% of the stress is relieved by 1000°F (540°C). The maximum temperature for stress relief is limited to 55°F (30°C) below the tempering temperature used after quenching from the hardening process. After removing from the furnace or oven, the wire must be cooled in still air. If cooled in any other manner, stresses are reintroduced into the part.
Many other severely cold-worked or bent shapes can be heated between 400-800°F (205-425°C) for a relatively short time to help reduce internal stresses. Alternative stress-relief processes (e.g., vibratory stress relief, rapid tempering/stress relief) are covered in the references.,,
The Final Word
The influence of internal stress can be positive or negative, which means that one must understand the design application in order to apply the proper stress-relief operation (at low or high temperature). In lightly stressed parts where dimensional tolerances are not critical, the presence of internal stress is not as great a concern as a highly stressed component that must hold dimensional stability over time or where the service application is such that excessive distortion or even fracture may occur. IH