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

Mike Kojima
Feb 26, 2009
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Turp_0902_01_z+shot_peening_wpc_treatment+glowing_exhaust_manifold Photo 1/14   |   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.

Turp_0902_02_z+shot_peening_wpc_treatment+shot_peening_graph Photo 2/14   |   This chart shows that shot peening can greatly increase the amount of load cycles to failure or increase the safe load cycle limit for no failures. It's common for the limited to be increased two to threefold for most metals.

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.

Turp_0902_03_z+shot_peening_wpc_treatment+sp_treated Photo 3/14   |   As seen in this highly magnified cross section of a metal part, shot peening greatly refines the grain making it harder for a crack to start.

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.

Sometimes stress risers with their related slip planes and micro cracks start to have oxidation form in the defects. This layer of oxidation acts like an expanding wedge in these small areas, which helps get the metal separated, greatly speeding crack formation. This is called stress corrosion fracture. It's common in areas where two dissimilar metals are in close proximity, like a piece of steel bolted to aluminum. Some hard alloys with certain types of heat treatment are also particularly sensitive to this sort of cracking as well. Stress corrosion fractures are particularly insidious because they are hard to detect and usually result in sudden catastrophic failure.

Now that you know how cracks form, we can explain how shot peening works to prevent them. The force of hardened shot impacting the metals' surface at high velocities causes plastic deformation of the metal, which cold works the surface and refines the grain. Imagine a tiny forging hammer hitting the part all over millions of times. The shot hitting the part forges the metals surface on a micro scale. The peening action of the shot seals up stress risers as well as relieves welding stress. The compressive stress caused by cold working breaks up the metals' large crystal lattice structure and makes it more homogeneous with a finer lattice network to a depth of 0.003-0.004 of an inch. The uniform distribution of compressive stress around the outside of the part and the fine grain skin that is formed by the peening action of the shot strongly resists the formation of cracks. Slip planes have a hard time starting in a finely divided homogeneous structure and stress has a hard time finding an area to concentrate in an area under uniform compressive stress. Stress corrosion fracture has a hard time propagating in this fine-grained compressed layer as well.

It's important for the shot used to be uniform in size and round. Irregularly shaped media can produce sharp-edged holes instead of nice round dimples. Sharp-edged holes can produce stress risers in a part and hurt fatigue strength. That's why it's important not to confuse abrasive cleaning and sand blasting with shot peening

Shot peening is a near-miraculous process. It helps just about any metal last longer and work better. Shot peening can be used to help with breakage problems on many parts. Axles, rods, crank fillets, gears, and springs all benefit from shot peening. Shot peening is cheap-the cost to shot peen a bunch of parts is usually under $100. Many techniques can be used to get the most out of shot peening. One common example is two-stage shot peening. A part with complicated surface geometry like a splined axle or a gear is first hit with a large shot to develop a good amount of surface working, then hit with a finer shot to extend the surface working into the roots of the gear teeth or the splines. Parts that are heat treated, induction hardened, or nitrided require the use of special hardened shot and higher velocities. When looking for a shot peener, it is good to find one who has aerospace or motorsports experience and to stick with their recommendations. An experienced shot peener can help you with the right process for your situation.

Shot peening's only disadvantage is that it can leave a somewhat pebbly surface that can cause some distortion of finely machined parts. Surfaces that must remain smooth with tight tolerances, like crank journals, pistons, seal surfaces, bearing bores, and cylinder walls can't be shot peened. Delicate parts, like bearing and piston rings, can't either. Sometimes connecting rods and crankshafts must be straitened and resized after shot peening. Are you having a problem with parts breakage? You might want to try shot peening it.

Turp_0902_07_z+shot_peening_wpc_treatment+wpc_comaprison_graph Photo 11/14   |   In this test done by a major OEM manufacturer, WPC outperformed these various types of coating in friction reduction by a large amount.

WPC
WPC is a Japanese process closely related to shot peening except it's done on a much smaller scale. WPC is a well-kept motorsport secret used extensively at high levels of the sport, like JGTC. Recently WPC has been gaining popularity in North America in venues like Top Fuel and Pro Stock drag racing, IRL, and NASCAR. WPC is also gaining acceptance from the OEM auto manufacturers as a cheaper and better performing alternative to coatings for wear resistance and friction reduction. WPC, like many Japanese companies, is very secretive about their exact process, but we've been able to glean a few important details from indirect observation. As we mentioned before, WPC is very much like shot peening in that it involves hitting a part with spherical projectiles to produce surface compressive stress, plastic deformation, and grain refinement. However, WPC differs from shot peening because the peening media is much smaller and harder and the part is hit with much higher velocities. To the casual observer, the WPC blasting media resembles baby powder.

We conjecture that the material is some sort of ultra-hard ceramic, like silicon nitride in the low micron range, and the velocities are over sonic in speed. We also feel that sometimes additives like zinc and moly are added to the mix to add some lubricity or extreme pressure characteristics to the surface. At high velocities, the additives are probably embedded into the surface of the metal at a molecular level, making their effects permanent and long lasting. We asked the folks at WPC to confirm this but they sort of just smiled and said nothing.

WPC offers many advantages over shot peening. Due to the small, light projectiles and high velocities, WPC offers a higher amount of compressive stress and an increase in the degrees of grain refinement to the base metal. The velocities are so high that in addition to cold working there is also a melting phenomena going on at a very micro level as well. The result is an extremely fine-grained, slip plane-less nano crystalline structure with a high degree of surface hardness, unlike shot peening, which has only a slight affect on surface hardness and strength. Because of this ultrafine-grained surface, WPC has superior fatigue strength gains and stress corrosion fracture resistance to shot peening.

Turp_0902_12_z+shot_peening_wpc_treatment+friction_reduction_ratio Photo 12/14   |   Shot Peening & WPC Treatment - Special Treatment, Part 1 - Tech

Due to the small mass of the media, the affected zone of WPC treatment is less than 0.001 of an inch, much shallower than shot peening. The WPC media is so fine that the surface dimpling can't be seen with the naked eye and can only be observed with a microscope where the final finish is about the same surface roughness as micro polishing. Due to this smoothness, WPC can be done everywhere and can be applied straight to areas where dimensional control and surface finish is critical, like bearing bores, pistons, cylinder bores, camshafts and cam followers, and seal surfaces. Due to the smallness of the media, WPC, for the most part, doesn't affect the dimensions of a part. Due to the light mass of the media, WPC can also be used for fragile dimensionally critical parts like piston rings and even bearings. Nearly every engine part can benefit from WPC treatment.

Another big advantage that WPC has is a big reduction in friction. WPC's micro-dimpled surface helps reduce friction in three ways. First, the dimpling helps maintain an oil film, which reduces metal-to-metal contact. Second, the dimpled surface helps reduce contact area in general to reduce friction. Third, the hard surface with anti-friction and extreme pressure materials embedded into it is a slippery and longwearing surface in its own.

Turp_0902_13_z+shot_peening_wpc_treatment+rotational_fatigue Photo 13/14   |   he WPC process was applied to bearings with this increase in load bearing capacity. Greatly reduced bearing wear was also noted in the test.

These properties make WPC an excellent process for treating cylinder bores, pistons, rings, wristpins, gears, and anywhere reduced friction can help. Test data by a major OEM manufacturer has shown that WPC treatment of pistons can reduce friction and wear of pistons by twofold over the best skirt coatings. Recent testing by a major forged performance piston manufacturer has also shown WPC to be more effective to the reduction of skirt wear and scuffing over any coating presently on the market.

WPC treatment of bearings has also shown to be very beneficial. Even though bearings are made of soft metals like aluminum, lead, tin, and zinc, they can be WPC treated with no change in dimension. The treated bearing shows a reduction in friction and an improvement in fatigue strength to where their load bearing capacity has improved from 20 to 50 percent.

WPC treatment has proven to work well on solving problematic transmission issues as well. WPC can obviously be applied to the gears and shafts. It can also be applied to cases and housings. WPC has found that some cases of transmission gear failure in some cars can be traced to flex in the transmission case, which allows the gears lash to change under load. WPC treating the case reduces this and transmission life is improved. Take note of this for your transfer case Evo owners. DSM owners and builders of the WRX and Sentra SE-R should pay attention here as well.

The only drawbacks to WPC are that it's much more expensive than shot peening and there is currently only one place in the country doing it. Although it is still relatively inexpensive for the multiple benefits it provides with its improvement of fatigue strength, reduction of friction, and improvements in wear resistance, and it's cheaper than coatings.

We have used both shot peening and WPC extensively on our race and project cars and found these processes to save us a lot of money in parts failure and maintenance, as well as reducing our DNF rate in racing.

Superior Surface Finish
Surface imperfections exposed to high cyclic stress loading lead to pitting, fretting, and crack initiation. During the WPC treatment process surface imperfections such as microcracks and micropores are effectively removed. This leads to a finish unrivalled by conventional alternatives.

Although the WPC process is usually invisible to the naked eye, under a microscope, the low-friction, tough, microdimpled surface is apparent. This surface is formed with no change in dimensions or distortion in the part. The surface also removes stress risers caused by machining or other forming processes.

It is well known that nearly all fatigue failures propagate at the surface. The superior surface hardness of WPC treated parts is a result of a localized energy release. This causes the matrix of the metal component to form a more compact nano - crystalline structure. Crack iniation is considerably reduced once parts have been treated.

Turp_0902_14_z+shot_peening_wpc_treatment+bearing_load Photo 14/14   |   Shot Peening & WPC Treatment - Special Treatment, Part 1 - Tech
ENGINE MODEL/YEAR PISTON SKIRT FRICTION RATIO
1 {{{Insight}}} 2000 molybdenum 1.4%
2 Insight 2000 graphite 1.4%
3 Insight 2000 electrolysis nickel 2.0%
4 Insight 2000 WPC treatment 2.0%
5 {{{Civic}}} 2000 WPC treatment 2.2%
6 {{{Odyssey}}} 2000 WPC treatment 2.6%

WPC has a significant improvement in fatigue strength over shot peening.

TEST CONDITIONS
Rpm range 7,640
Oil grade 5W-30 (sj)
Oil temp {{{100}}} oc
Oil pressure 0.1 Mpa
Clearance 100-101 m
Load rate 100kn/50 min.
Shaft rating s45c (aisi 1045)
  Hrc 55
  0.2 Ra µm

Test Results
Measurements were recorded when there was a sharp increase in temperature or the shaft rotational torque exceeded 30 nm.

BEARING RUN 1 RUN 2 AVE TEMP
Stock Bearing 54 66 60 160 C
WPC Treated Bearing 68 76 72 {{{200}}} C

Metal Improvement Company
www.metalimprovement.com
201.843.7800

WPC Treatment
www.wpctreatment.com
310.782.8647

Shot Peening Advantages:
Greatly improves fatigue strength and stress corrosion fracture resistance.
Very cheap.
Many companies are very experienced with the process.
Readily available in most cities.
Great bang for the buck process.

Shot Peening Disadvantages:
Pebbly surface finish not suitable for bearing, seal, or close-tolerance areas.
Can distort close tolerance parts.
Can damage soft or fragile parts.

WPC Advantages:
Greatly improves fatigue strength and stress corrosion fracture resistance.
Improves surface hardness for reduced friction and improved wear resistance.
Improves surface smoothness for reduced friction and wear resistance.
Ultra-smooth surface is good for bearing, seal, or close-tolerance areas.
Does not distort close-tolerance parts.
Does not damage soft or fragile parts.
Not easily visible to the naked eye most of the time.
Can take the place of several different processes to save time and money; for instance, shot peening then coating.

WPC Disadvantages:
It costs more than shot peening.
The process is only available in one place in North America; parts to be treated must be shipped. WPC has good service and fast turnaround.
The process is secret so you just have to trust them!

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By Mike Kojima
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