Shot Peening & WPC Treatment - Special Treatment, Part 1 - Tech

We know what you're thinking and no we're not talking about a VIP pass at a hot club. We're talking about processes that can help you make power, make stuff last longer, improve reliability, and reduce or eliminate parts breakage. We're talking about stuff like Isotropic polishing, shot peening, Cryogenic treatment, and WPC. What the hell is this stuff? Are these medical tricks to help you live longer? No, they won't make you live one bit longer. Will these treatments give you gobs of power? Well not usually but they can help you make power and give you a lot more consistently high power over the life of an engine. Are these treatments expensive? Usually not, plus they provide quite a bit of good for very little cost.

Shot Peening
John Almin, a production engineer at Buick, invented shot peening. He noticed an improvement in metals' mechanical performance properties after they had been abrasively cleaned by shot blasting. He studied the phenomena and developed the methodology for the shot peening process as we use it today. Shot peening is blasting a metal part with small steel balls of a controlled size and hardness at a controlled velocity with a controlled amount of coverage. No matter what the guy at your machine shop may tell you, shot peening is not sand blasting, shot cleaning, or bead blasting.

Shot peening does many things to improve the properties of metal parts. The main one is the drastic improvement in the fatigue strength of a metal part. Fatigue strength is not the parts ultimate strength, which shot peening doesn't change very much, but the number of times that a part can be stress cycled before it fails. A good visual example would be if you bent a paper clip in your hand until it broke. Let's say you could bend the paper clip 10 times before it broke, if you increased the fatigue strength by 100 percent, it would then take bending the clip 20 times before it broke. The amazing thing about shot peening is that it can usually improve fatigue strength in most parts and metals by 100 percent. In some types of parts, particularly welded assemblies or rough-machined parts, shot peening can improve fatigue strength by a staggering tenfold.

You might wonder how shot peening works. The reasons are a little complicated but it goes like this. First you need to understand a little bit about the mechanics of cracking. Metal consists of a geometrically arranged crystal lattice structure of metallic atoms. This structure gives metal a grain much like wood. Like cracks and other defects in wood, slip planes, a defect similar to cracks but at an atomic level can form in the crystalline lattice of metal atoms. The stress from cyclic bending can concentrate in the junctures of these slip planes allowing them to separate and become the micro fractures that a failure-causing crack can propagate from. Most of the stress and strain due to flexing occurs on a parts' surface away from its center or neutral axis. Because of this, slip planes, micro fracturing, and the cracking of a part that ultimately leads to its failure, nearly always starts at the surface. Since cracking is a surface-generated phenomenon, surface treatments are particularly effective in reducing and eliminating fatigue failures.

Many processes used in making car parts add places where stress can concentrate and cause cracks to form. Machining operations like turning, milling, drilling, and grinding leave tool marks and scratches. At a micro level these tiny surface defects allow the stress to concentrate, forming slip planes and micro cracks. These defects are called stress risers. Sharp edges in the geometry of a part also create stress risers. The high heat of welding causes distortion and internal stress that can promote cracking. An inclusion in welds formed by contaminates or oxidation also generates stress risers. For these reasons, a part that is welded and then machined is a recipe for a crack-induced disaster.