Shot Peening? Why Do We Do It?
Shot peening has 4 major benefits in the crafting of titanium bicycle frames:
Shot peening is a cold working process in which the frame is bombarded with small spherical metal balls called shot at a precise angle. Shot act like tiny ball peen hammers and create a uniform dimpled texture on the surface of the frame. This compacts the outer layer of the material.
The processes of butting and welding titanium tubes to make a bike frame are known to create tensile stresses in the frame material. Tensile stresses make the area in question want to pull itself apart. This is a bad property to impart to a bicycle frame as any minor notch or micro crack in the frame will want to propagate and further compromise the material. The induced tensile stresses are most concentrated in the heat affected zone — the area of the welds. Thus, strength is compromised precisely where you would like it to be greatest.
Shot peening of a welded titanium joint substantially increases both fatigue strength and fatigue life as compared to the same joint which is not shot peened. Shot peening imparts what is called residual compressive stress which counteracts the residual tensile stress, which is created in the process of cutting, grinding and welding. Typically, fatigue strength of a welded titanium joint after shot peening is double that without shot peening. Fatigue life is enhanced by shot peening to an even greater degree.
By shot peening the frame after it is welded together, we are able to relieve the stresses in the material providing compressive qualities, which are known to reduce micro cracking and enhance fatigue life. Without stress relieving, each of the tubes will retain tensile stresses which tend to conflict with one another. Stress relieving allows the component tubes of the frame to work together as designed, acting as a unified structure rather than a collection of competing parts.
The shot peening process work hardens the surface of the tube, while giving it a finely textured surface. These two properties together create an attractive finish that is highly resistant to scratches. If scratched, the scratch is harder to see because the surface is textured. The textured surface glitters in the sun in a manner similar to that of a pearl paint job.
Some people may confuse shot peening with sand blasting or bead blasting. At IF, we use sand blasting on our steel frames to remove contaminants. We also use it impart a microscopic tooth to the surface of the metal to provide a mechanical bond with the paint. Bead blasting is used for cosmetic purposes to provide a uniform finish to the surface of the metal. Neither sand blasting nor bead blasting improve the mechanical properties of the metal.
Shot peening is used precisely because it improves the performance characteristics of the finished parts. It is used in the aerospace industry, in high performance cars and motorcycles, and in light weight bicycle stems and bars where light weight and high strength are performance imperatives.
What's the big deal? Don't a lot of frame builders shot peen their frames?
To the best of our knowledge we are the only titanium frame builder using the shot peening process. This process should not be confused with bead blasting, which is used to provide a cosmetic finish to ti bikes. Any company that claims to bead blast over a shot peened finish does not understand shot peening. Any type of finish polishing brushing or bead blasting applied after shot peening negates the benefits of shot peening.
See www.shotpeening.com for more info.
Who does the shot peening of IF frames?
We do not do the shot peening in house. We use Metal Improvement Company in Wakefield, MA, which specializes in shot peening high performance parts for the aerospace industry. At Metal Improvement our frames are peened along side of parts for the space shuttle and F14 jet engine rotors.
See www.metalimprovement.com for more info.
Why can't IF do its own shot peening?
To shot peen properly requires the use of very large, sophisticated, and expensive computer-controlled equipment, and trained personel. Shot Peening really is high-tech stuff, and it's tough to get right. We decided to leave it to the specialists. In addition, we are confident in the established leadership, reputation, and quality of Metal Improvement.
Someone told me that shot peening can tear the metal if the angle of attack is less than 85 degrees. Is that true?
The shot peening flow at less than 85 degrees is known as overspray. This neither helps nor hurts the frame surface. It does nothing to a previously unimpacted surface and it does not detract from a previously impacted surface.
Why did IF decide to use shot peening on their Titanimum frames?
The primary reasons are that we are pleased with the mechanical properties we achieve with shot peening and we are delighted with the lustrous finish. The finish beautifully matches the painted panels we apply to our frames which often have pearl or metallic qualities, and it makes the frame stronger in the process. Another important reason is that we were not happy with the established ways of finishing ti frames. While these are less expensive, brush and polish finishes are purely cosmetic. In addition, applying those finishes is a dirty and possibly unhealthy job which no one enjoys doing.
Why not give the customer a choice in finishes?
While we considered offering bead blasting, brushed and polished finishes as alternatives, we became so convinced of the mechanical and cosmetic merits of shot peening that we concluded anything else would be offering less than the best.
Unfortunately, it is not possible to both shot peen and have final finishes employing these other methods. Shot peening must be the final finish with the exception of paint, as bead blasting, brushing, or polishing would destroy the mechanical benefits of shot peening.