in Vol. 11 - September Issue - Year 2010
Residual Stresses by Design
Figure 1: Hardness (Rc) and XRD peak integral breadth as a function of heat treatment temperature for various alloys
Figure 2: Plot of the residual stress change/relaxation due to heat treatment of shot peened springs at various temperatures
Residual stress can be present in components as a by-product of manufacturing operations. The residual stress patterns and profiles are often dismissed as "product of the processes". The patterns and profiles can also be considered as characteristic of the details of the manufacturing process that created them, and knowledge of the residual stresses can then be used to compare products and processes. Attempts may be made to reduce residual stresses by stress relieving heat treatments, or by mechanical treatments that include uniform yielding.
In some manufacturing methods operations are included to introduce controlled levels of residual stresses at the surface of the part. It is well known that surface compressive stresses can delay or even prevent fatigue crack initiation in parts that carry cyclic loads. The phenomenon is more pronounced in high-cycle fatigue environments (where the fatigue life improvement may be orders of magnitude) than in low-cycle fatigue (where doubling the fatigue life may represent a significant improvement). Perhaps more important than the overall increase in the fatigue life, it is normal for the fatigue lives of treated families of parts to lie in tighter groups than their untreated equivalents, so the treated parts have longer, more predictable lives. Other properties are also modified by the introduction of compressive residual stress. For example, resistance to stress corrosion cracking is also markedly increased.
In this article we will look at traditional and new processes available to deliberately introduce controlled levels of residual stress to modify parts’ performance in service, and we will consider what may happen to those residual stresses during the life of the part.
The oldest and most basic of peening processes is the manual use of a hammer with a hemispherical end to repeatedly hit one surface of a sheet metal component supported against a mandrel, forming the sheet into a curve whose OD is the surface being hit. The first time I saw this being done I thought the curve should go the other way, after all, the hammer was depressing the metal, not lifting it! In fact, the round end of the hammer displaces the surface material sideways, in the plane of the sheet, expanding the surface. Of course, the material below the surface resists the expansion, and in the process of balancing the forces within the material the cold-worked, dimpled surface is left with a compressive stress, reacted by a net tensile force in the remainder of the thickness.
The post-Industrial Revolution equivalent of hammer peening is shot peening. In this process, spherical particles (originally of cast steel), are propelled at the surface being treated. There are two common propulsion systems; one uses compressed air passing through a nozzle (or nozzles) to accelerate the media, the other uses a spinning wheel (or wheels) to sling the media. The first offers the ability to aim well-defined streams of shot, by moving the nozzles, and the second fires large numbers of shot particles in a very short time. The range of media types available includes various sizes and hardnesses of cast steel shot, conditioned (i.e. worn in) cut wire, and ceramic and glass beads. The effect on the component receiving the peening treatment depends primarily on the mass and velocity (intensity) of the media. The intensity of the media stream is established and monitored in tests on standard steel test coupons (Almen strips). The arc heights generated in a series of controlled, timed peening exposures are used to plot a graph of curvature against time, a "saturation curve". The effect on the part also depends on the mechanical properties (particularly hardness and work hardening rate) of the component itself. For this reason, exposure of the part to the media stream is controlled by the time required to achieve close to 100% coverage, and is not related to the time taken to saturate the Almen strip.
Just like hammer peening, shot peening introduces plastic deformation to the surface by displacing material away from the point of impact, creating a dimple. Because the displacement of material is sideways, the non-uniform yielding during impact is tensile in the plane of the surface, so the residual stress left as each shot bounces away is compressive. The residual stresses introduced by shot peening are typically near the local compressive yield strength, accompanied by high levels of local plastic strain. (As complete coverage is approached, most of the surface has actually received multiple, overlapping impacts, which change the local mechanical properties of the material.) The degree of modification can sometimes be monitored by careful micro-hardness testing of prepared metallographic sections. More conveniently, this can be done by observing the breadth/width of the X-ray diffraction peaks used to measure residual stress profiles through the surface region. Heat treatment with slow cooling has the effect of relieving or reducing microstresses as well as decreasing the dislocation density, thus decreasing the hardness (see Figure 1).
It is difficult to assess the condition or quality of a peened surface non-destructively. For this reason, tight control of the peening process and documentation offers the best method to demonstrate that required peening has been applied.
During the service life of a peened component environmental effects can modify the properties of the protective peened surface. Designers of parts that rely on peening to achieve a fatigue life need to be aware of the processes that can interfere with their intentions.
Moderate temperatures can cause the residual stresses to relax gradually. The process of stress relaxation is faster in materials that have lower high temperature strength and is accelerated in material with greater levels of cold work. Both these concepts are consistent with metallurgical principles: Stress relaxation is accelerated at higher temperatures, as the mechanical properties of alloys decline with increasing temperature, and the presence of high levels of plastic strain eases the process further because the distorted crystal structure promotes diffusion, the thermally activated process by which atoms move within solid metallic materials, eliminating the beneficial effects of peening (see Figure 2). This feature should lead the designers of parts and processes to avoid over-peening and promote processes which introduce the required stresses without unnecessary plastic deformation.
It is understood that peened surfaces include material that is stressed close to the compressive yield point of the material, and that local plastic deformation will modify the residual stress distribution throughout the component. In fact, additional yielding in the peened surface due to a single application of a compressive load can modify or even destroy the beneficial residual stress field. In the absence of the compressive layer a peened surface may contain topographic features that promote rather than inhibit fatigue crack initiation.
Compressive residual stresses in one region of a component are balanced by tensile forces elsewhere in the part. These balancing forces need to be considered in the design of the component. For example, if both surfaces of a sheet 0.030" thick are peened to a depth of 0.005", only 0.020" is available to react to the compressive forces in the surfaces. That core material may be close to 50% of its tensile yield strength, and operational loads have not yet been applied! When the core does yield, and the applied load is subsequently removed, the component’s residual stress distribution will be severely modified, even to the extent that the residual stresses in the surfaces become tensile.
Mathematical models may be used to predict the effects of service environments on the residual stresses in components, but these models are very complex and still require practical verification for individual applications, designs, materials and peening processes via experimental methods.
Non-Conventional Peening Processes
Peening may have been used to improve surfaces since before the Industrial Revolution, but in the past decade modern technologies have emerged to compete with the established processes. One such process involves transforming the energy of a laser beam into a shock wave to install a compressive stress in the surface. The process is well controlled and precise in its application, and can leave the topography of the original surface essentially unchanged. Residual stresses are installed without much of the plastic strain that accompanies multiple overlapping shot impacts, and it is claimed that this feature slows relaxation of residual stresses by diffusion-controlled processes. Similar claims have been made for processes which have relatively large contact areas between the media and the work-piece, where plastic strains are minimized by other means, thought to be related to friction between the tool (a ball or roller) and the surface. While the effect these processes have on fatigue life can be measured, the effects of specific process variations on the residual stress profiles, and comparisons of the new and existing methods can be accomplished more directly by measurement of the residual stress profiles. It is tempting to rely on comparisons of Almen strip arc heights, but similar arc heights can be generated by residual stress distributions that are significantly different, and have more or less impact on component life.
Carburising, typically employed to generate a hard, wear resistant layer on part of the surface of a steel component, e.g. teeth of gears, or an integral raceway, is accomplished by placing the component in an atmosphere that releases carbon atoms onto the surface. The carbon is absorbed into the steel, and diffuses inwards to form an enriched layer. In subsequent heat treatments, the enriched layer forms a hard case over the tougher, more ductile core material. The case is not only harder, its increased carbon content causes it to expand relative to the core material, leading to the case being in compression. This combination of properties makes the contact faces wear resistant, and the teeth resistant to bending fatigue. Induction hardening, in which the surface to be hardened is locally heated and then quenched, generates similar hardness, but without the same level of residual stress. Shot peening can compensate by adding compressive stresses at the roots of the teeth.
Repair and Rework of Peened Parts
Components of complex and valuable machines can be expected to undergo overhaul or repair several times in their working lives. Repairs may include re-application of peening over the entire surface, or application of peening to areas of the surface that have been modified for other, local repairs. Repair peening processes include some techniques such as rotary (or flapper) peening that are not commonly used in original manufacture.
In some cases it is possible to use X-ray diffraction methods with electropolishing to assess the residual stress profile following service, then to blend out the shallow polished spot, and reapply the original surface treatment locally or to the whole component if necessary. In families of components operating in similar environments the type and extent of modification to the original surface of sacrificial examples can provide information about the actual operating environment of the parts and the condition of the rest of the family. This information can then be used in the evaluation of related components to assess their fitness for continued service.
The deliberate inclusion of controlled residual stresses in the design of components offers a method of managing and reducing some anticipated fracture mechanisms. Overall, the use of compressive residual stress in the design of components can extend operating life, reduce the weight and size of components for the same performance, and allow the use of materials not normally suited to specific chemical environments. However, the installation of protective residual stresses may bring risks if all the effects of peening and interactions with the service conditions and applied stresses are not considered. It is particularly important to understand the full nature of the treated component and its material, and to use appropriate methods of process control to ensure correct processing.