Residual stresses are a little counter-intuitive to the idea that a stress is defined as a load acting over an area. Residual stresses are stresses that remain after a force has been removed. They are contained within the component without any external force acting on it. Residual stresses form in two main ways: deformation of a material due to external forces, or a volumetric change in the material (usually due to a metallurgical phase transformation). The key component with both of these is that they occur more in one region compared to another. It is the variation between two regions in close proximity that causes a residual stress.
Residual stresses can be considered beneficial or detrimental to the operation of a component as stresses are additive. This means that if there is a tensile residual stress and the component is operating in a way that causes tensile stresses, the stresses need to be added to give the overall stress in that region. These are considered detrimental residual stresses as they act to increase the overall stress on the component. On the other hand if the residual stress is compressive, and the component is still experiencing tensile stresses the overall stress is reduced. This is because the stresses are acting in different directions. Compressive stress is usually designated as a negative stress and tensile positive; the addition of these gives a smaller stress than the tensile applied.
One of the most widely used processing techniques in the spring industry to form beneficial residual stresses is shot peening. Shot peening is the process of accelerating countless small, rounded particles (shot) at the surface of a component. Shot can be a variety of sizes and materials depending on: the material the target component is manufactured from, the size of the target component and the residual stresses required. Within the spring industry shot is typically ferrous (cast or conditioned steel) or non-ferrous (glass and ceramic) and the shot size is usually between 5 and 25% of the wire diameter.
Shot peening is performed on springs, and other components, as it is known to increase the fatigue life. One of the reasons for this increased life is due to compressive residual stresses imparted on the surface of the component where the operational stresses are at a maximum. This report will attempt to describe how these residual stresses are formed and why they are beneficial.
Terms and Definitions
Before getting into the technical details of residual stresses formed due to shot peening, it is worth looking at a number of terms used in this report and defining them.
Residual Stress: The stress that remains within a material when the external force (or stress) has been removed. They can be defined in terms of their direction (usually compressive or tensile) and can be detrimental or beneficial to the operation of the component.
Elastic deformation: The non-permanent deformation of a material or component when exposed to a load (stress). This means when the load is removed the material will return to its original shape. An example of this is a spring operating between two safe stress limits.
Plastic Deformation: The permanent deformation of a material or component when exposed to a load (stress). When the load is removed the material will not return to its original shape. E.g. a wire being coiled into a spring.
Yield Strength: The stress at which the deformation of a material changes from purely elastic to elastic/plastic. When the yield strength of a material is exceeded it will not return to its original shape.
Formation of stresses
Residual stresses are formed during shot peening due to the mechanical deformation of material causing both plastic and elastic deformation at, or close to, the surface. When shot is accelerated at the surface of a component, under the correct parameters, it will force material out from the impact point, creating a small indent in the surface, Figure 1(a) and (b). As this indent remains after the shot rebounds off, plastic deformation has occurred. The plastic deformation of the surface is shown as red in Figure 1. In most materials, and especially metals, plastic deformation does not exist in isolation, elastic deformation also occurs. Elastic deformation occurs in regions where the force was not large enough to exceed the local yield strength. The region of elastic deformation is shown as green in Figure 1. To understand the formation and interactions between the regions of deformation we need to look at the impact of a shot on the surface in stages.
As the shot impacts the surface of the component it applies a force, or an impulse, to the material, causing deformation. If the force is high enough then the local yield strength is exceeded and some of this material will plastically deform. As the distance from impact into the surface of the material increases, the effect of the impulse imparted by the shot diminishes. This leads to a transition, at some critical depth, from regions of plastic deformation to a region of purely elastic deformation. This interaction between plastically and elastically deformed regions give rise to the residual stresses.
Showing the formation of residual stresses due to shot peening (a) The shot is accelerated towards the work piece, (b) the shot creates regions of plastic (red) and elastic (green) deformation, (c) once the force is removed the elastic material tries to spring back to its original shape and (d) the plastically deformed material restricts the spring back crating a compressive residual stress at the surface
The example shown in Figure 1 only focuses on a single shot hitting the surface of a component but as more and more shot impacts the surface of the material a (near) continuous layer of plastic deformation will be present on the surface of the component.
The compressive stresses at the surface are beneficial as they oppose the stresses acting on the surface of the component during operation. As the stresses acting on a component are compounded to give the overall stress, the negative compressive stress needs to be overcome before the material starts to exhibit a positive stress which could lead to failure. For example if the compressive residual stress is 50 MPa and a spring is operating between corrected stresses of 100 MPa and 500 MPa the actual stresses seen by the spring are 50 MPa and 450 MPa. This gives an improved fatigue life, as though a lower stress was applied to the un-peened spring.
This isn’t the end of the story, however. As there is now a residual compressive stress at the surface of the peened component, there is also a subsurface tensile component to balance it out. Figure 2 shows a diagram representing the distribution of residual stresses on a shot peened work piece. Close to the surface of the material the stresses are compressive but as the depth increases the compressive stress reduce and eventually become tensile. The modulus of the maximum tensile stress is less than the modulus of the maximum compressive stress, but the tensile stress is present to a greater depth. The areas between the line and the y-axis are the same.
Showing the distribution of residual stress with respect to depth below the surface.
As residual stresses are additive, this can be overlaid with the stress acting on a spring during operation to give the overall stress in the system. Figure 3 shows the stress distribution of a shot peened spring under load. The stress created by the load (red line) on the spring reduces with distance from the surface but as the residual stress (blue line) becomes tensile, the overall stress (green line) in the system at this depth becomes higher than the stress applied. This means that the highest stress seen on a spring could be subsurface if the residual tensile are particularly high.
Showing the effect of the residual stresses formed by shot peening on the stress experienced by a spring under load.
This can be detrimental to the operation of the spring as any stress raisers in this region (e.g. inclusions) have more stress on them than if the spring had not been peened. In this scenario the spring will probably fail prematurely compared to the same spring which was not shot peened. Overall, however, shot peening will increase the fatigue life of a material because there are significantly more stress raisers on the surface than subsurface. This probable location of stress raisers, along with other benefits of shot peening (work hardening, surface cleaning and blunting of stress raisers) work together with the residual stresses to dramatically increase the life of a spring.
As well as the shot peening described above there are other methods of peening to create residual stresses at the surface of components, e.g. laser peening. Instead of using shot to create the force, laser peening creates a shockwave very close to the surface of the material. The shockwave travels into the surface of the component transferring its energy to the substrate which deforms the material structure. Material at the surface, closer to the initiation of the shockwave, absorbs more energy causing more deformation than subsurface. Similar to shot peening, this forms a strain field due to the plastically deformed region resisting the spring back of the elastically deformed region, giving the beneficial residual stresses.
Heat treating the spring after shot peening is sometimes known as a stress relieving heat treatment. This is not technically true, the low temperature heat treatment after shot peening (180°C-220°C) is not high enough to eliminate the stress profile, although some stresses will be relieved. The temperature is at the low end of the recovery temperature range, not within the high recovery (annealing) or recrystallisation range necessary for residual stress elimination. This is, obviously, a good thing as the residual stresses are beneficial. The heat treatment is done to restrict the movement of mobile dislocations created during shot peening. Previous experiments performed by IST have shown that springs not heat treated after shot peening take a larger set than those which were heat treated. This is because shot peening produces large densities of mobile dislocations which increase the degree of plastic deformation. The heat treatment stops this occurring by reducing the mobility of these dislocations or annihilating them. Reducing the movement is done due to precipitates formation during aging, blocking motion. The heat treatment also gives the mobile dislocations energy to move through the crystal structure until they are stopped or are annihilated. This increases the local yield strength reducing the set back to pre-shot peened levels.
From this initial shot peening methodology numerous variations have been developed to try and improve on the results seen. In the spring industry two variations are most prevalent; dual shot peening and warm shot peening.
Dual shot peening involves shot peening the component a second time with a smaller shot. This increases the coverage of the surface and imparts more compressive residual stresses to improve the fatigue life of the component. Warm shot peening on the other hand affects the mechanical properties of the component, to increase the residual stresses at the surface. While at the increased temperature the yield strength of the material is lowered, without affecting the microstructure of the material. This allows plastic deformation to occur at greater depths. Increasing the amount of residual compressive stresses on the surface of the wire.
In conclusion, residual stresses from shot peening are created due to regions of plastic and elastic deformation occurring at the surface of the wire. The plastically deformed region is compressed by the elastically deformed material below the surface, trying to return to its original shape after the impulse cause by the shot is removed. As the surface residual stress opposes the stresses acting on the spring during operation, they reduce the overall stress at the surface increasing the fatigue life of the spring. Although shot peening will increase the life of the spring on average, under certain circumstances it can reduce the life of a spring. A spring with a stress raiser just below the surface might failure prematurely as this region will have residual stresses act in the same direction as the applied stress, causing premature failure compared to a spring with a similar defect which has not been shot peened. Laser peening, dual peening and warm shot peening all impart residual stresses using the same method of interaction of plastic and elastic deformation at the surface of the material. Although method of creating these stresses or magnitude of these stresses may be different using these different methodologies, the creation and action of the stresses are the same giving the spring an increased fatigue life.