During the manufacturing process of coil springs, the imparted residual stresses can affect the performance and effective service life of the components. Because processes such as fabrication and forming tend to generate a significant quantity of tensile residual stress, which produces unfavorable conditions in the surface of the spring, peening processes are often applied to mitigate the negative effects of residual tension. 

Depending on the type of coil spring, the locations of failure can vary; therefore, it is extremely important to characterize the residual stress present in a given spring before and after shot peening in order to understand which areas to target. For example, in compression springs, the inner diameter (ID) is the area most likely to have tensile residual stresses that can contribute to in-service failure (see Figure 1). Torsion springs, on the other hand, most often fail at the outer diameter (OD). Finally, in tension springs, the inner radius of the hook is the most common source of failure.

Upon the formation of a spring wire into a coil, as well as the subsequent heat treatment process, a complex residual stress field is generated in the spring. In fact, even if a spring wire were shown to have no net residual stress prior to coil formation, significant tensile residual stresses would be expected to be observed on the ID of the set coil. For instance, the wire shown in Figure 2 begins in a stress-free state. When it is bent elastically (shown in orange), the wire exhibits compressive stresses on the ID and tensile stresses on the OD. When the load is removed from the wire, it becomes stress free again. 

However, when elasto-plastic bending that surpasses the yield strength is applied to the material (shown in gold), as would necessarily occur during formation of a permanent spring coil set, the wire does not spring back fully when the applied bending load is removed. As demonstrated by the blue line in Figure 2, the formed coil is tensile on the ID and compressive on the OD, with the greatest magnitude of tensile residual stress being present on the ID. When the spring is actually in service and applied stress is added to the existing residual stress, the total stress present in the part may contribute to premature failure under fatigue loading. 

A powerful yet cost-effective method that is often applied to impart compressive stresses into the surface of a spring is shot peening. The shot peening process serves to generate a compressive stress state and cold-worked layer near the surface of the circumference of the spring wire (shown in Figure 3). This cold-worked layer of compressive stress will improve fatigue resistance through the effective depth of the shot peened layer (shown in grey). Because the net sum of all the residual stresses across any cross section of a component is always zero, the introduction of compressive stress must be balanced with the inevitable generation of tensile residual stresses. With the application of shot peening, the imparted tensile residual stresses are located below the surface, where they are less likely to negatively affect the component. 

Moreover, when shot peening is performed, a number of factors can be optimized in order to best suit the material type and its mechanical properties: the applied peening pressure, the hardness of the shot relative to the coil wire, the coverage, the peening nozzle angle, and so on. This will impact the effective depth and the magnitude of the compressive layer imparted to the particular spring being peened.

Although there is a reliable method by which to quantify the intensity of a given peening process, it is not able to quantify the residual stress state of the peened component directly. This method involves the use of a thin strip of steel, known as an Almen strip, that is placed inside the shot peening chamber. The peening intensity is defined by the amount of deflection in the Almen strip after peening. However, because there could be multiple different residual stress fields produced from the same Almen strip intensity (due to material properties, shape of the component, residual stress state prior to peening, hardness of the shot relative to the hardness of the workpiece, peening equipment/setup, etc.), the residual stresses in the component must be measured directly in order to ensure the success of the shot peening process. 

X-ray diffraction (XRD) has long been the industry-approved method of quantifying residual stresses in shot peened components thanks to its high spatial resolution and ability to characterize stress gradients at and near the surface of the part, where tensile stresses are most likely to contribute to premature failure (i.e., where cracks tend to initiate). The x-ray diffraction method is based on measuring the Bragg angle of a diffraction peak in crystalline materials such as metals and ceramics. The angle of a diffracted x-ray beam, θ, is related to the atomic lattice spacing, d, via Bragg’s law: nλ = 2dsinθ, where λ is the wavelength of the incident x-ray beam and n is an integer multiple of the wavelength (Figure 4). By measuring the diffraction angle for a given wavelength, the d spacing, and thus the strain, can be calculated for the sampled volume. The stress can then easily be calculated using elasticity theory.

XRD residual stress characterization has proven successful at assessing numerous peening processes in many applications and industries, both in laboratory and field settings (using portable equipment). Because of this, XRD is a valuable tool for developing and optimizing processes and peening parameters in many different components. Components treated with conventional peening/blasting media such as cast or ceramic shot, cut wire, and glass beads, as well as those treated with more unconventional treatments, such as laser shock peening, have been successfully characterized using XRD. The effects of shot peening can be enhanced using XRD analysis to obtain greater value: better return on investment, product quality, and component performance, as well as minimized fatigue and stress corrosion, decreased production costs, and reduced component weight. Characterizing peened components can also serve to minimize conditions such as over-peening or unnecessary peening time, providing great economic benefit.

XRD residual stress measurement systems are more advanced and robust than ever, capable of fitting into almost any component geometry thanks to advanced design configurations. In fact, coil springs (and other components with small IDs) can often be measured directly in the inner diameter (see Figure 5, for example) without sectioning or destroying the component. In the case of a spring that is too small to find a suitable equipment configuration, careful sectioning can be performed prior to residual stress characterization to gain access to the inside of the spring.

When residual stress characterization is performed to verify the effectiveness of a peening process, XRD measurements are typically performed at the surface and through the effective depth of the peened layer (i.e., through the peened cold-worked layer and below). Many times, in order to confirm the true effect of the peening process, a preliminary residual-stress-versus-depth profile is collected on an un-peened part for comparison. As demonstrated in Figure 6, the ID of an as-formed coil spring (shown in red) has a remarkably different stress profile than the same spring’s OD (shown in orange): where the ID is characterized by tensile residual stresses, the OD is compressive. 

Since the ID’s stress profile was tensile, further testing was necessary to ensure that shot peening effectively reduced harmful stresses at that location. Spring sets formed with identical conditions and procedures were peened using a 230R shot single-peening process (shown in gold) and a 460H/230R dual-peening process (shown in blue). The depth of the peening effects can be observed in Figure 6 where the residual stress profiles of the peened springs (gold and blue) reach the baseline ID profile (red). 

Quantitative data sets can be used in conjunction with fracture mechanics software to predict a component’s fatigue life and damage tolerance. Alternatively, data sets can be used with fatigue testing to determine the level of residual stress that would be needed to achieve the desired fatigue life. In addition, once these analyses have been performed and verified via testing of a statistically significant number of samples, residual stress specifications can be created and added to design prints or stated as a delivery requirement. Results can inform suppliers or buyers of spring components that the specified residual stress levels have been met such that optimal fatigue life can be achieved in service. 

As demonstrated above, quantitative XRD residual stress measurements can equip manufacturers with the necessary data to enhance shot peening processes. Because the data provided is both quantitative and reliable, specifications can be accurately determined and peening processes can be fine-tuned to meet customer requirements. Obtaining a residual stress specification is a necessary step for those who design components that will be subjected to shot peening, as this data helps ensure that suitable product quality is achieved while saving valuable time and money and preventing critical failures caused by unknown fatigue resistance.

For Information: 

Proto Mfg. Inc.

12350 Universal Drive, Taylor, Michigan

USA, 48180-4070

Tel. +1.734.946-0974

E-mail: info@protoxrd.com

www.protoxrd.com