If you are involved in the manufacture or refurbishment of metal components then it is likely their working life can be improved by the mechanical surface treatment of shot peening.

Historically this method has been used since man first worked metal and is named after the most useful of engineering tools, the ball peen hammer. Shot peening simply stated replicates the action of the striking blow of the hand tool by using thousands of small pieces of shot be they metallic, ceramic or other synthetic material and bombard the surface to create a plastic deformation of the component thus increasing it's resistance to wear and potentially increasing the load bearing ability of the peened item.

The surface of all produced metal items have small marks or imperfections such as imperceptible fissures, very small stress induced cracks or simply surface marks. All these surface imperfections can be increased in size by regular cyclic stressing of the affected area which can change the size of minor imperfections.

When an item is shot peened the medium used produces miniscule indentations in the treated area which causes the surface to be plastically reformed and the coverage of the process ensures that a layer forms which will resist the reaction of stressing caused by tensile forces.

The following are ways in which the process of surface finishing by shot peening can be an advantage to you.

Cyclic Loading. Any item which is subject to a repeat stress application as in turbines, rotary drilling, equipment used to steer a machine and springs, gearing, and the shafts which carry them, can have their working life extended by using the process of shot peening. The many thousands of repeat usage cycles can be increased by the use of this process and the product will have it's working life extended by many years if the item is shot peened.

Stress Corrosion Cracking. SCC can only occur when you have corrosion and application of tensile stressing. With the removal of one these criterior SCC cannot occur. By shot peening the product you cause an alteration to take place and the surface becomes a compressive stress area which means that it is not possible for SCC [Stress Corrosive Cracking] to take place as you have eliminated one of the requirements, tensile stress. By the application of the shot peening process the wear resistance of threaded items can be greatly improved. The reduction of high stresses to the root thread area and their relief areas causes greater SCC resistance and extends the working life of these items.

Galling. This adhesion between two metal surfaces causing miniscule pieces of metal to remove themselves from the parent surface and attach themselves to a new surface is most evident in threaded items. Surface finishing by using shot peening in an ambient situation prevents the deformation of the two parts and hinders this occurring and prevents the adhesion of surfaces caused by galling.

Heat Teated Componentss. Where you would use heat treatment to increase wear resistance but because of production limitations this is not possible the action of shot peening can increase the surface hardness of many materials by as much as 250 %. Materials successfully treated in this manner are steel, both stainless and high carbon, manganese, inconel, aluminum, stellite brass and hastelloy. With an increased surface hardness the component will have a greater working life.

Surface Densification. If you use a coating process on your items such as plasma coating or a spray finish the application of shot peening will serve to make the surface more dense thus closing the surface porosity and increasing the coating density will also increase the life span of sintered products.

Welding Stress. The residual stress induced by welding can be much reduced with the application of shot peening on the welded joint with the increased life of the weld and the alteration to compressive from tensile.

The above are only some of the benefits of shot peening and its application in surface finishing. The increase in part strength due to the shot peen effect on the surface also allows parts to be produced smaller and lighter.

 

Author: Chris Charlton
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