Mechanical Surface Treatments

Several techniques are used to improve the surface properties of manufactured components mechanically. The most common methods are listed and described under:

1. Shot Peening
2. Laser Shot Peening
3. Water-jet Peening
4. Ultrasonic Peening
5. Roller Burnishing
6. Explosive hardening

1. Shot Peening:

In shot peening, the workpiece surface is continuously impacted by many numbers of cast steel, glass, or ceramic shot (small balls), which make overlapping indentations on the surface. This action, using ceramic shot sizes that range between 0.125mm to 5 mm in diameter, causes plastic surface deformation at depths up to 1.25 mm. Plastic deformation is not uniform throughout the thickness of the part and shot peening causes compressive residual stresses on the surface, thus improving the fatigue life of the component by delaying the initiation of fatigue cracks. Unless the process parameters are controlled properly, the plastic deformation of the surface can be so severe that it can damage the surface. The extent of deformation can be reduced by gravity peening, which involves larger shot sizes, but fewer impacts on the workpiece surface.

Shot peening is used extensively on shafts, gears, springs, oil-well drilling equipment, and jet-engine parts, such as turbine and compressor blades. However, note that if these parts are subjected to high temperatures, the residual stress will begin to relax (thermal relaxation) and their beneficial effects will be diminished greatly.

An example is gas-turbine blades performing at their operating temperatures.

2. Laser Shot Peening:

The laser shot peening process also called laser shock peening and first developed in the mid-1960s, In this process, the workpiece surface is subjected to planar laser shocks (pulses) from high-power lasers. This surface-treatment process produces compressive-residual-stress layers that are typically 1 mm deep with less than 1% of cold working of the surface. Laser peening has been applied successfully and reliably to jet-engine fan blades and materials such as titanium, nickel alloys, and steels for improved fatigue resistance and some corrosion resistance. Laser intensities necessary for the process are on the order of 100 to 300 J/cm² and have a pulse duration of about 30 nanoseconds. Currently, the basic limitation of laser shot peening for industrial, cost-effective applications is the high cost of the high-power lasers (up to 1 kW) that must operate at energy levels of 100 J/pulse.

3. Water-jet Peening:

In water jet peening more recently developed process, a Water jet at pressures as high as 400 MPa impinges on the surface of the workpiece, inducing compressive residual stresses and surface and subsurface hardening at the same level as in shot peening. The water jet peening process has been used successfully on steel and aluminum alloys. The control of process variables (jet pressure, jet velocity, the design of the nozzle and its distance from the surface) is necessary to avoid excessive surface roughness and surface damage.

4. Ultrasonic Peening:

The ultrasonic peening process uses a hand tool based on a piezoelectric transducer. Operating at a frequency of 22 kHz, it can have a variety of heads for different applications.

5. Roller Burnishing:

The roller burnishing process, is also called surface rolling. In this process, the surface of the component is cold-worked by a hard and highly polished roller or set of rollers.
The process is used on various flat, cylindrical, or conical surfaces (Fig. 1).

Roller burnishing improves surface finish by removing scratches, tool marks and pits and induces beneficial compressive surface residual stresses. Consequently, corrosion resistance is improved, since corrosive products and residues cannot be entrapped. In a variation of this process called low plasticity burnishing, the roller travels only once over the surface, inducing residual stresses and minimal plastic deformation.

Internal cylindrical surfaces also are burnished by a similar process, called ballizing or ball burnishing. In this process, a smooth ball is pushed through the length of the hole.

Roller burnishing is used to improve the mechanical properties of surfaces as well as their surface finish. It can be used either by itself or in combination with other finishing processes, such as grinding, honing, and lapping. The equipment can be mounted on various Computer numerical control (CNC) machine tools for improved productivity and consistency of performance. All types of metals can be roller burnished.

Roller burnishing is typically used on hydraulic system components, seals, valves, spindles, and fillets on shafts.

6. Explosive hardening:

In explosive hardening, the surfaces are subjected to high transient pressures through the placement and detonation of a layer of an explosive sheet directly on the workpiece surface. The contact pressures that develop as a result can be as high as 35 GPa and can last about 2 to 3 μs. Significant increases in surface hardness can be achieved with the explosive hardening method, with very little change in the shape of the component. For example, railroad rail surfaces are explosively hardened.