Stress relieving is applied to both ferrous and non-ferrous alloys and is intended to remove internal residual stresses generated by prior manufacturing processes such as machining, cold rolling and welding. Without it, subsequent processing may give rise to unacceptable distortion and/or the material can suffer from service problems such as stress corrosion cracking. The treatment is not intended to produce significant changes in material structures or mechanical properties, and is therefore normally restricted to relatively low temperatures.
Carbon steels and alloy steels can be given two forms of stress relief:
- Treatment at typically 150-200°C relieves peak stresses after hardening without significantly reducing hardness (e.g. case-hardened components, bearings, etc.):
- Treatment at typically 550-680°C (e.g. after welding, machining etc.) provides virtually complete stress relief.
Non-ferrous alloys are stress relieved at a wide variety of temperatures related to alloy type and condition. Alloys that have been age-hardened are restricted to stress relieving temperatures below the ageing temperature. Austenitic stainless steels are stress relieved below 480°C or above 900°C, temperatures in between reducing corrosion resistance in grades that are not stabilised or low-carbon. Treatments above 900°C are often full solution anneals.
Stress Relief treatments can be performed to customer specifications or to industry standards such as AS4458-1997.
Stress relieving prepares metals for further processing or for the intended service conditions. They control the ability of materials to be machined with ease, perform without distortion in service, be subsequently hardened or carburised aiding in reducing distortion values, or to resist corrosive environments.
All commercial alloys or mild steel fabrications can stress relieved during the manufacturing process to provide the necessary qualities required for performance.
- Stress relieving of carbon or low-alloy steel fabrications is frequently the last heat treatment applied, so it must be ensured that the mechanical properties of the materials treated will not be adversely affected.
- Stress relieving between machining operations can be performed on pre-treated material. The stress relieving effectiveness may have to be reduced to prevent loss of mechanical properties.
- Many austenitic stainless steels require fast cooling after high-temperature stress relief or solution annealing. A degree of distortion or reintroduction of residual stresses is inevitable in such cases.
- The size and shape of items that can be stress relieved, annealed or normalised depends on the type of equipment operated by the heat treat facility. For large items, check the availability of suitably-sized facilities at an early stage.
- Most stress relieving operations are carried out in air, but protective media are also available. In air, alloys are subject to discolouration or scaling depending on the alloy and temperature used. Post-treatment cleaning may therefore be required.
- There is always a risk of distortion/sagging when high-temperature treatments are applied to vulnerable thin-wall vessels or large, heavy components. Your heat treater may have to use special supports or fixtures to combat this problem.
All of the following information should be included if possible. If uncertain, ask your heat treater before producing a specification:
- The process: this could be stress relieve, indicate if bright treatment is necessary, or if treatment in air is satisfactory.
- Material: type, grade, and the standard from which the material will be supplied , with a drawing, composition and mill certificate where available.
- Any general standards applicable (national, international or company) that contain relevant details which must be adhered to. For Example AS4458 -1997
- Existing condition; e.g. details of any prior heat treatment, such as hardening and tempering, solution treatment and ageing, intended to establish mechanical or other properties.
- Requirements for any special certificates or data to be provided by your heat treater.
- Other services required; e.g. straightening (with working limits), cleaning/blasting, laboratory or specialised NDT services etc.
- Heat Charts or Time Temperature Graph requirements or Certificates of Conformance
Guidance and information is always available from our experience heat treatment professionals.
Our procedures and work instructions are fully documented under our AS 9100 and ISO 9001 quality management systems.