Residual Stresses: Friend or Foe?
Residual stresses are inherently introduced to forged products during their deformation. These stresses influence the integrity of load-bearing parts. Their effects depend on their nature (tensile or compressive), location and magnitude. Tensile residual stresses’ integrity can be very detrimental, particularly for products used in the as-forged condition. In contrast, compressive residual stresses can be a lifesaver, if tailored appropriately. Hence, numerical and experimental methods should be carried out to characterize the residual stresses, and they should be taken into account for forged products to prevent premature failure and to improve service life.
Forging is one of the most preferred production techniques based on significant and non-homogenous plastic deformation. As a result, residual stresses that influence the structural integrity of load-bearing parts are inherently introduced to forged products. Consequently, it is very important to characterize and handle the residual-stress fields in forged products.
According to continuum mechanics, when an external force is applied to a continuous body, internal forces are induced and distributed through this medium. The magnitudes of such forces are defined by their intensity (i.e., by the amount of force per unit area of the surface on which they act). In discussing internal forces, this intensity is called stress.
Having said that, continuous bodies or engineering materials may experience internal stresses, even if there are no externally applied forces. These so-called residual stresses are self-balanced, and locked-in stresses may be present in materials without the application of any external forces. Residual stresses may superimpose with applied loads so that they have direct impact on the performance and service life of parts. Therefore, residual stresses play a vital role, particularly in the structural integrity of engineering products, and they must be accounted for in all manufacturing methods, including forging.
Residual stresses can be induced into materials inherently (as in forging), or they can be deliberately introduced by means of mechanical surface treatments such as shot peening and laser peening. The main mechanism of residual-stress generation is any source of “misfit” in the material, one of which is the non-uniform plastic deformation induced by forging.
Assume a material in 2-D with square shape. If it is plastically deformed by upsetting and made rectangular, then there will be no macroscopic residual stresses as long as the deformation is introduced uniformly. Therefore, it must be kept in mind that in order to generate residual stresses, there should be a misfit in the material. In addition to plastic deformation after processes such as forging, misfits can also originate from the difference in the thermal-expansion coefficient, yield strength and stiffness inside a material, which is the case for almost all engineering materials that include more than one constituent.
Temperature gradients due to welding, phase transformations and grain boundaries are other sources of discontinuities for residual stresses to be generated. Schematic representation of most commonly observed residual-stress-generation mechanisms can be seen in Figure 1.
The most common classification of residual stresses is according to their size. Type I residual stresses are the self-balanced stresses across a large distance, and they can affect all or part of a component. Type II residual stresses can be described across grains, and the causes of these stresses are the anisotropy and heterogeneity of grains in polycrystalline materials. Type III residual stresses are atomic-level stresses formed due to crystalline defects like vacancies or interstitial atoms. Type I residual stresses are also called macrostresses, and Type II and Type III stresses are microstresses.
Residual stresses can be modeled by numerical and experimental techniques. For instance, there are Fe-based simulation programs specifically developed for forging operations. Use of such software requires substantial expertise, however, since it is very easy to obtain misleading results, especially if the assumptions and parameters are not realistic.
At NORM Group, we have employed numerical simulations of forging processes since the early 2000s. An example of the forging-induced residual stresses of a special nut is presented in Figure 2.
In contrast, experimental techniques are generally classified based on the nature of their damage to the test piece during measurement. They are either destructive or nondestructive techniques. All destructive techniques are based on the elastic relaxation that occurs when a layer of material from a stressed sample is removed. Hence, residual stresses are elastically relaxed. The resultant displacements are obtained and converted to strains from which residual stresses are back-calculated. Hole drilling, slitting and contour methods are the most widely used destructive measurement methods in academia and industry.
For nondestructive techniques, material characteristics such as atomic d-spacing (the atomic distance between crystallographic planes) or the material response to magnetism or sound are employed. Residual-stress determination with these techniques usually requires measurement of the characteristic features for unstressed and stressed material. X-ray diffraction, synchrotron X-ray diffraction, neutron diffraction and Barkhausen are the preferred nondestructive residual-stress measurement methods.
Destructive and nondestructive measurement techniques have different resolution and penetration depth to which residual stresses can be obtained (Figure 3). Furthermore, each technique has its own advantages and limitations. Therefore, choosing suitable methods for a particular application is very crucial. It is a very good practice to compare experimental residual stresses obtained from two different methods and numerical results to achieve reliable and accurate results. Detailed information regarding the residual-stress measurement methods can be found in G. Schajer’s Practical Residual Stress Measurement Methods.
Another key factor concerning macroscopic residual stresses is that material has to be deformed plastically in order to generate residual stresses. In this respect, a clarification has to be made between residual stresses and pre-stressing by shrink-fit, particularly applied for the dies of cold-forging operations.
Pre-stressing by shrink-fit is employed with a minimum of two dies without any plastic deformation. The main intention of the shrink-fit is to induce a compressive stress field into the dies, which will superimpose with applied tensile loads. When the final stress levels are less tensile, the service life of the dies will be improved. As soon as the die assembly for shrink-fit is separated (i.e., external forces are removed), however, there will be no stresses in the dies. On the other hand, residual stresses are permanent, even though there are no externally applied loads.
Residual Stresses: Friend or Foe?
Owing to the nature of forging, a significant amount of non-uniform plastic deformation is inevitably introduced into components. For hot forging, the operating temperature must be above the recrystallization temperature of the material to be forged. Therefore, the plastic deformation-induced misfit required for residual-stress generation is minimized owing to reformed grains.
For cold forging, on the other hand, macro and micro residual stresses have to be taken into account. The residual-stress distribution is crucial for products used in the as-forged stage, and characterization of the residual stresses is important to estimate the part’s expected service life, particularly under fatigue loads. Any significant tensile residual-stress field will accelerate crack formation, resulting in early failure under operating conditions. Therefore, application of stress-relieving heat treatment can be considered for this type of product. There are well-defined standards for stress-relieving procedures depending on material type and properties.
Most cold-forged products have to be heat treated to meet the mechanical requirements (e.g., ductility, hardness, yield strength and ultimate tensile strength) defined by the quality standards or user specifications. Considering steel as the raw material, the most common heat-treatment procedure will include quenching and tempering. The austenite phase transforms into the highly stressed metastable martensite phase during quenching, followed by tempering during which the microstructure of martensite is modified.
Owing to tempering, in which the products are held at elevated temperatures, the prerequisite misfits necessary for macroscopic residual stresses are minimized. Similar to steel products, aluminium-, titanium- and nickel-based superalloys are generally heat treated after forging not only to obtain isotropic and desired mechanical properties but also to minimize residual stresses.
In addition to inherently induced and mostly detrimental residual stresses, they can also be introduced intentionally. For instance, mechanical surface treatments, like shot peening and laser peening, are used to induce beneficial compressive residual stresses to increase the fatigue life of engineering products.
In the open literature, there are studies exploring the effects of shot peening on the service life of hot- and cold-forging dies. It was reported that the fatigue life of shot-peened dies used in hot and cold forging were increased by a factor of two and more than three times, respectively. Furthermore, such surface treatments can also be applied to forged products to increase service performance.
According to a study conducted for hot-forged connecting rods used in the automotive industry, the fatigue strength was increased about 50%, which was associated with the induced beneficial compressive residual stresses after shot peening. Application of shot peening and laser peening for prolonged service life and higher fatigue strength is a common practice, particularly for safety-critical applications in the aerospace, automotive and defense industries.
Residual stress is one of the most important concepts of structural integrity. The effect of residual stresses depends on their nature – tensile or compressive, location and magnitude. Therefore, tensile residual stresses threatening a component’s structural integrity can be very detrimental, particularly for products used in the as-forged condition. On the other hand, compressive residual stresses can be a lifesaver if tailored appropriately.
Hence, numerical and experimental methods should be carried out to characterize the residual stresses, and they should be taken into account for forged products to prevent premature failure and to improve service life.
Authors M. Burak Toparli, Sezgin Yurtdas¸, Emrah Kılınçdemir and Barıs¸ Tanrıkulu are all mechanical engineers at NORM Group, Izmir, Turkey. Questions may be addressed to Senior R&D Engineer M. Burak Toparli at email@example.com.
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