Stress Relief Basics

Stress Relief Basics By S. Hornsey B.Sc VSR (Africa) cc

August 2004

Structural designers and engineers must be aware of residual stresses in fabrications and of the common methods used to relieve these unwanted stresses. Stress – the sources are numerous, but fortunately there are a variety of ways to relieve it. No I’m not talking about the hectic pace of our personal lives, although it certainly can apply, but instead I’m referring to the fabrication and machining of metals. Residual stress is an internal stress that is not a result of externally applied loads. If stress build up in the structure is excessive, the fatigue life of the metal is considerably reduced.

Importance of Stress Relief Cold working, hot rolling, grinding, machining operations, quenching treatments, welding and thermal cutting can all induce high levels of residual stresses into metal. The nature of residual stress, its distribution, and prediction of the level within a metal is a complex and not completely understood phenomenon, but you can be sure it is present. Welding in particular, because of the rapid thermal expansion and contraction created along a very localised area, is a prime source of residual stress. A very high heat source is applied to a small area relative to the cooler surrounding area. That point, where the arc is directed is rapidly heated from ambient temperatures to a temperature that can often be in excess of 4,500°C. The metal expands as it is brought to a molten state. As the molten weld pool solidifies along the joint, there is resistance to its shrinkage by the already solidified weld metal and the unmelted base metal adjacent to the weld. This resistance creates a tensile strain in the longitudinal and transverse directions of the weld. Distortion is often the result, and if the stress is excessive, buckling, stress corrosion cracking and a shortened fatigue life is often possible. All welds will have some residual stress, and it can never be totally reduced to zero strain. But the level of stress can be very high depending upon certain conditions. Heat input, base material thickness, rate of cooling, the restraint of the component and of course the welding process all play major roles in the level of residual stress induced into a component. Thermal or Non-thermal There are two major approaches to stress relieving; thermal and mechanical. A major difference between the two is thermal treatment, which in addition to relieving stress, will also effect a metallurgical change in the material, which is often unwanted. A postweld heat treatment entails uniform heating of the weldments, holding at temperature, and then a carefully controlled cooling. As the base metal becomes hotter, it becomes weaker. Once a certain temperature is reached, there is a reduction in the material yield strength, and it is thereby relieved. The effect often visibly manifests itself in the partial straightening of a distorted component. For carbon and low alloy steels, stress relieving is commonly performed in the range of 570°C-760°C although many specifications call for temperatures of up to 1050°C. The temperature at which stress relief occurs varies from 100°C up to 500°C according to the particular metal concerned. In general the higher the melting point, the higher is the temperature for stress relieving. The time at which the component is held at temperature is dependant upon the thickness of the material and its chemical composition. A commonly used method of stress relieving weldments is by postweld heat treatment its effectiveness is dependant on the control exercised in bringing the component to temperature and then its subsequent cooling. It therefore should only be performed by those knowledgeable in its application. Shot Peening Shot peening is a cold working method that reduces stress. Small round balls, or shot, are projected onto the surface of the component. The shot imparts small indentations into the surface which induces a compressive stress. The tensile residual stresses at the surface of the component must overcome the compressive stress for a fatigue crack to initiate. If properly applied the compression works to counteract

the tensile stresses. Fatigue cracks have a low probability of developing in the shot peened area. Caution must be taken to ensure the shot peening operation is performed with knowledge of its variables. There are three important variables to control its application; surface compressive stress, maximum compressive stress, and depth of compressive stress. The velocity of the shot is another controlling factor. If the impingement of the shot is too deep, detrimental stresses may be induced negating the desired results. Vibratory Stress Relief A commonly used method of stress relief is by vibration. A mechanical vibrating device is attached to the component. The vibrations resonant frequency can be accurately controlled and monitored by specially designed machines. The amount of time the component is subjected to the vibration is usually dependant upon its weight. The advantage of the process is that the vibration can be applied during or after welding and in the case of machined components very close to final machined sizes and in some cases following machining to achieve complete stability. Many theories abound as to how the process achieves the results the most common of which is that the applied force redistributes the stress distribution within the component by means of plastic deformation of the metals grains and thereby reducing sharp peaks of residual stress. As with Thermal treatment the effect often visibly manifests itself in the partial straightening of a distorted component. Many test reports indicate that Vibratory Stress Relief can obtain results compatible and in many cases exceeding those achieved using Thermal Treatment. An everincreasing list of users serves to confirm this. With comparison to the previous mentioned methods of stress relief VSR offers the user the following advantages: 1. A time reduction of up to 50:1. 2. No discoloration or scaling of materials. 3. No reduction in material yield strength. 4. Graphic printouts upon completion of VSR. 5. No surface damage e.g. shot peening 6. Complete portability of the equipment. 7. Enormous savings in cost as the work is carried out at the client’s premises. 8. The ability to treat up to final machined sizes. 9. The ability to treat large components (up to 200 ton) on site 10. Energy savings of up to 500:1 in comparison with TSR Should you require further information regarding any of the afore mentioned processes or additional technical reports please contact: Steve Hornsey B.Sc(Eng)MSAIW,CGLI (Dist weld),HND(MechEng)M.S.A.I.MechEngs VSR(Africa)cc PO Box 12272 Leraatsfontein 1038 tell (013) 6500702 / 6500287 fax (013) 6501308 Email [email protected] Website http://www.vsr-africa.com