FORMULA 62 Resonant Vibration Method For Reducing Residual Stresses in Welded or Machined Fabrications 1725 Monrovia Avenue, Building A1 • Costa Mesa, Californiawww.stressreliefengr.com 92627 VIBRATION SHAPING THE FUTURE What Are Residual Stresses and Where Do They Come From? For many people involved in the metal- working trades, the subject of stress relief is something they are not well versed in. As a result, stress relief is a subject they would just as soon like to avoid. With a little technical assistance, the average layman can get a basic understanding of residual stresses and how to deal with them. With this knowledge, he will be better prepared to evaluate shop problems and find a |solution that is effective. The following information is designed to answer some of the most frequently asked questions about stress relieving. Residual stresses, by definition, are introduced into the material in one or How Does It Work? those stresses in an elastic body that is free more of the following ways: thermal, from external force or restraint and metallurgical, mechanical and chemical. Low frequency vibrations are used as a carrier temperature gradients. An incompatibility Since these are the processes that make to deliver high amplitude energy to a metal of regions in the metal created by up our metalworking trades, it is only fabrication, or machined part. The non-homogenous plastic deformation is the right to assume that, at some point in heavy vibrations produce a load that is principal cause of these internal stress time, a stress relief treatment may be superimposed on the existing stress patterns systems, whether they are in an required. that result in a reduction of peak residual individual part or in an assembly of parts. stresses. This produces a more dimensionally This mismatch or misfit between adjacent stable product and reduces the random regions of the same part distorts the What is "Formula 62"? distortion that often occurs in unstable neighboring regions. This condition can be workpieces. either extremely damaging or very "Formula 62" is a resonance based beneficial to the part depending on the method of vibratory stress relief magnitude and direction. Compressive developed by Stress Relief Engineering Are Resonant Vibrations stresses created by shot peening can be Co. Workpieces are subjected to low Effective? good while tensile stresses created during frequency, high amplitude vibrations for welding can be bad. a short period of time based on the Resonant vibrations have been found to be the weight of the workpiece. This allows the most effective means for reducing residual Residual stresses are hard to visu- residual stresses to be reduced to a much stresses by vibration. The resonant frequency alise, difficult to measure and extremely lower level where static equilibrium is vibration method has a much more pro- difficult to calculate or predict, yet they are restored. The resonance method is used nounced stress redistribution just as important in the function of a part by researchers around the world in stress compared to the subresonant (subharmonie) as are externally applied forces that are relief studies using vibration and is frequency methods. High amplitude resonant more easily measured and calculated. currently considered an industry vibrations are very efficient in significantly Residual stresses are fundamentally standard. reducing peak residual stresses in weldments. www.stressreliefengr.com 2 Are There Any Limitations? There are some limitations as with any metalworking process. The process is not recommended for extrusions or severely cold worked items. On very large, very long or open space frame type structures, the vibration may need to be applied at several locations, which does require more time. Very small items in large quantities are more easily treated thermally in batches. In welding situations where vibration is used during the welding process, this method is most compatible with SMAW, GMAW and GTAW welding processes. Other welding practices may present formidable How Do You Know When Can I Stress logistical problems. When It's Done? Relieve A Part? As with any treatment process, one must Parts may be stress relieved virtually at any What Can I Use It On? follow a recipe to achieve success. In a point in the manufacturing process where thermal process, treatments are governed the part is accessible. The most typical The process can be used on a wide by a practice that dictates so many degrees applications allow for stress relief at key range of ferrous and non-ferrous metals. per hour per inch of thickness. In the junctures in the manufacturing process, Typical materials are: carbon steels, vibratory or non-thermal mode, the i.e. after rough machining, boring, stainless steel, aluminium, cast iron, treatment is predicated on a time vs. weight grinding, etc. For welded fabrications, manganese, incolnel, etc. these are a few basis where the aggregate weight of the stress relieving can be performed during of the metals that can be treated in a part, including any associated tooling or welding which is very helpful in preventing variety of conditions such as: wrought or fixturing attached to the part, is used to residual stress build-up that can cause cast, forged, formed, welded, ground, determine the length of time or treatment weld cracking or distortion of some polished or machined. after resonance has been determined. sections. Because the fusion process produces large temperature gradients in a short period of time, residual stresses are more dynamically active which can require stress relief during welding, immediately after welding or in an ongoing program of routine stress relief on a daily basis. As the time to completion increases for a fabrication, so does the risk of distortion related problems. Since large magnitude tensile residual stresses can reduce the fatigue life of welded assemblies, ample thought should be given to stress relieving all welded assemblies. www.stressreliefengr.com 3 How Long Does It Take? metal using the right analytical tools. Is It Difficult To Whether you are using heat, vibration or a The actual length of time for treatment is cryogenic technique for stress relief, you Set Up And Use? based on weight per each application. should only rely on known and approved Not at all! There are a few rules that must be Treatment time can range from a short analytical methods for determining when followed and they apply to all applications. The fifteen minute period to an hour or more stresses have been reduced. This is very first requirement is to isolate the depending on the size of the item an important because, not only do we need workpiece, as much as is practically possible, whether or not it needs to be treated at to know the magnitude of the stress so it is free to vibrate. In cases where this is not more than one location. concentration, but we also need to know possible, the vibration unit must be located as whether it is tensile or compressive. close as possible to the work area. The unit There is no simple tool or technique must be directly coupled to the workpiece in How Can I Tell available that can supply all of these data order to transfer the vibrational energy into the When Stresses requirements for a three dimensional workpiece. This can be accomplished by using object. Have Been clamps, fasteners or adaptive fixtures. Measurement techniques used by Once these conditions are met, the part Reduced? researchers are well known and we must be vibrated for a minimum period of time strongly support them. We highly recom- The most direct way, and the most based on weight. Parts can be vibrated for mend x-ray diffraction per ASTM E915-83, rewarding, is to resolve a basic longer periods without suffering any fatigue high speed hole drilling with strain gauges dimensional instability problem. This is damage or loss of tensile strength. per ASTM E837-85 or for ferromagnetic quite apparent when standard materials, the Barkhausen Noise Analysis The equipment is designed for ease of measurements techniques confirm that the method, for making any kind of determi- operation and is used both in shop environ- part is now holding dimensions that nation that stresses have been reduced. ments and field applications by your personnel. previously moved in random fashion. However, in most stress relief applications, there is no major dimensional problem to overcome and the part looks the same before and after treatment. What now? Is there any simple way to tell how well we've done? Unfortunately, no! There have been some very technically weak and scientifically unsupported methods promoted over the years that allegedly indicate when stress relief occurs. Some of these methods claim that motor current changes are a means to indicate when the part is relieved or that vibration frequencies change as the stiffness of the part changes due to less resistance from stress or the part undergoes a vibration fre- quency change from an unnatural mode to a natural mode. While we all would like to see some evidence of change, it just isn't that simple. Real quantitative proof of stress relief efficiency can be obtained for virtually any www.stressreliefengr.com 4 Can It Replace Thermal? In case where the thermal method is used solely to promote dimensional stability of the part for alignment purposes, surface integrity or service conditions, the vibration method can be used as a much more cost-effective replacement. This method also allows the machinist or fabricator to treat parts at various points throughout the manufacturing process that otherwise would be too costly or impossible to do, due to surface considerations. The thermal stress relief process, while adequate for many metals, does have some serious limitations that make vibratory stress relief a far better choice.