VOL. 15 May ISSUE YEAR 2014
in Vol. 15 - May Issue - Year 2014
Strategies for Choosing Residual Stress Treatments
LXRD Laboratory Residual Stress Measurement System
RS distribution around a hole. Compressive stress colored blue.
Measuring RS in a gear
Peening processes introduce compressive residual stress in the treated surface. The residual stress may be credited with significant improvement in fatigue life, or enhanced resistance to stress-corrosion cracking. The credit can be used to increase the safety margin of a component in a given environment, reduce the weight of the component, extend its service life, or, in some circumstances, increase its load bearing capacity. Many methods are available to introduce residual stresses at the surface of a part. This article explores some of the less well-known techniques and some that can be applied to specific components and materials.
Ideally, surface treatments should be identified during the design cycle so that they can be placed in the manufacturing sequence. This is particularly important with respect to inspection operations: Adding compressive residual stress before inspections can prevent detection of surface cracks and porosity by dye penetrant, and can mask features that would be inspected visually, such as grain size or surface roughness.
Characteristics of a Peened Surface
The most obvious change to a surface will probably be to its topography. In shot peening, the distribution of dimples on the surface is a witness of the coverage provided by the process, which provides an essential Quality Control metric. In service, topography is most important when surfaces are loaded and in contact. Surfaces covered in dimples may not do well in loaded contact with similar surfaces because the contact points are at the raised edges of the dimples on each surface. The local contact loads are high, possibly leading to adhesive wear.
In general, the compressive residual stress introduced by peening is accompanied by a zone of mechanically deformed material. Plastic deformation modifies the mechanical properties of the material locally, typically elevating the yield strength. The disturbed crystal structure of the peened material has an increased dislocation density, and the multi-directional plastic strain of the shot peening process produces a particularly tangled dislocation structure. At elevated temperatures, a high dislocation density can accelerate diffusion of interstitial solute atoms and vacant lattice sites, leading to early loss of the increased surface strength, and accelerated relaxation of the compressive residual stress.
A typical shot peened surface has a moderate compressive residual stress at the surface, becoming more compressive below the surface before returning to the stress level of the bulk material. At its most compressive, the residual stress can be similar in magnitude to the compressive yield of the heavily deformed material.
Remembering that residual stress and applied stresses are additive; applying even a single compressive load cycle to a peened surface that is already internally loaded to the compressive yield will cause local plastic deformation and alteration or loss of the residual stress. Similarly, a compressive surface residual stress is reacted by a tensile stress in the core of the component. Depending on the cross sectional area of the part, the reacting stress may be significant, and will add to any applied tensile load. If the core should yield in tension, the internal stress system of the part will re-balance, with reduction or even inversion of the surface residual stress!
Evaluating Peened Surfaces
Residual stress can be measured non-destructively at the surface by an X-ray diffraction technique. Depth profiles require multiple X-ray measurements, alternating with successive removals of thin layers of material by electro-polishing. The altered surface region of peened material can be seen in carefully prepared cross-sections and in transmission electron microscope images. When the altered zone is deep, the degree of alteration may be measured by micro-hardness measurements on metallographic samples. Plastic strain can be estimated quite precisely from broadening of the X-ray diffraction peaks collected during the measurement of residual stress profiles.
The profile of the residual elastic stresses and the corresponding profile of local plastic deformation together determine the initial properties of the treated surface. They also influence the way in which the properties will develop or deteriorate with exposure to the service environment. The topography of the peened surface will contribute to the way the part interacts with mating components.
In some applications, it is necessary to consider the implications of surface modifications beyond the introduction of a beneficial residual stress. These considerations may weigh in favour of a particular peening process, or require a combination of processes to optimise the component¡¦s properties in the service environment. Processes that may be carried out with shot peening include chemical modifications of the surface, such as carburising or nitriding, thermal treatments, which can stabilize the deformed microstructure, and multiple treatments, which may be used to enhance specific aspects of the surface. The use of these hybrid processes requires a thorough understanding of the materials and processes involved, and a demonstration of their success in the application concerned.
Compressive Residual Stress and Holes
Holes in stressed components present a number of issues in design, manufacturing and Quality Control for critical components. Several techniques have been used to improve the properties of hole walls:
Ballizing (also known as Ball Burnishing) improves the surface finish of holes, and imposes tight dimensional control on diameter and roundness. A hard ball is forced through a slightly undersized hole. While this process does generate a circumferential compressive stress, axial smearing on the wall may be considered undesirable.
In split sleeve cold working a disposable sleeve is placed between the hole wall and the tool, eliminating sliding contact between the tool and workpiece, but leaving a witness of the sleeve's split-line in the bore.
Split mandrel cold expansion uses a hollow mandrel with multiple splits that allow the mandrel to partially collapse as it is inserted through the hole. A pin is passed through the mandrel to solidify it before the mandrel is pulled back through the hole, expanding it. The process differs from Ballizing in the optimised shape of the mandrel and the ability to be used when there is access to only one side of the part.
The cold expansion processes are claimed to generate compressive residual stress fields up to one diameter deep around a hole up to half-an-inch diameter, with minimal plastic deformation. All may require some finishing, removing witness marks of the tooling's split lines or smeared surfaces, and the ends of the holes may need to be deburred. The large diameter of the residual stress fields may also be a concern, particularly if adjacent, large holes are treated, because the compressive residual stresses will be reacted by residual tensile forces elsewhere in the part, and these tensile forces will add to tensile service loads.
Modern Peening Processes
Some modern surface treatments equip the modified surface to resist deterioration with time in service, particularly where there is exposure to elevated temperature. These processes also generate residual stress fields with relatively low levels of plastic strain.
Laser Shock peening develops residual stress profiles that are deep, but which may not approach the material's yield strength. The process uses pulses of laser light to generate a shock wave, which propagates into the component. The area impacted by each pulse can be a few square millimetres, and can be very precisely placed. Laser Shock can be used to target specific areas of a component to combat recognised peak tensile loads, and may be combined with conventional shot peening to give a uniform topography and overall compressive residual stress and plastic strain.
In general, peening with larger diameter shot generates deeper residual stress fields, smoother surfaces and less plastic strain near the surface. Conventional shot peening uses shot less than about 0.05 inches diameter. Some modern techniques use very large diameter polished balls to produce surfaces with these characteristics.
Gravity peening, in which the balls are allowed to fall from a height of a few feet onto the surface to be treated, has been used to treat the fan blades of large aircraft engines. Of course, since gravity provides the kinetic energy of the balls, only one surface of a part can be peened at a time.
In one variety of ultrasonic peening, ultrasonic transducers supply kinetic energy to large diameter balls in an enclosure. Collisions between balls transfer energy from one to another, leading to the enclosure being filled with balls flying in all directions. A part placed in the enclosure is impacted from random directions, over its entire surface.
In a second process using ultrasonic transducers as a source of kinetic energy, captive pins or blocks in an array are driven onto the surface. The array can be moved over the surface, manually or by a robot. A similar technique can be applied to peen forming.
Stabilising Peened Surfaces
The residual stress fields of some materials, notably hardened and tempered steels, may be stabilised by peening them at a slightly elevated temperature. The process is known as warm peening, and it takes advantage of dynamic strain aging to pin newly formed dislocations in the material's crystal lattice during peening.
Using steel as an example, carbon atoms usually occupy interstitial sites (i.e. between the metallic atoms), but they can move within the material at temperatures as low as 300¢XC. They tend to migrate to low energy sites, such as around newly formed dislocations, where they retard further movement of the dislocation. The dislocations created during warm peening are so securely pinned by their accompanying clouds of carbon atoms that subsequent strain requires the creation of new dislocations. The surfaces produced by warm peening are more stable and so retain their residual stress and mechanical properties better than conventionally peened surfaces.
Peening Hardened Steel Surfaces
Case hardening by carburising is used to generate a carbon-rich layer near the surface of selected areas on near-finished steel parts. The process is one of two hardening treatments commonly used in manufacturing gears. After heat treatment, the surface is significantly harder than the core, and so has improved wear resistance. The surface also has a compressive residual stress, which improves resistance to both contact fatigue and tooth-bending fatigue. These properties of gear teeth can be further enhanced by shot peening the loaded flanks and filet radii of the teeth prior to finishing the profiles of the teeth. It may be necessary to use a harder steel shot, to match the hardness of the treated surface.
Shot peening is also applicable to Induction Hardened gear teeth, and generates a consistent and reliable residual stress system in the surface of the teeth.
A wide variety of residual stress solutions are available to enhance the properties of the surfaces of materials and components. Selecting the optimum system for a particular application requires an in-depth understanding of the component and its operating environment, and the characteristics of the surface enhancement techniques that are available. X-ray diffraction techniques are available to measure the essential properties of treated surfaces, the elastic residual stress profile and the plastic strain profile in newly treated surfaces, and in parts that have been in service.