Shot peening
Is shot peening the same as sand blasting?
What is compressive residual stress? How is it beneficial to my part? How do you measure it?
Is shot peening the same as stress relieving?
What is x-ray diffraction testing and why is it used?
How often should I re-run a saturation curve?
Is the saturation point the same as the 100% coverage point?
Is my peening intensity the same as the almen arc height at my processing time for my part?
How do I run a saturation curve?
What effect does impingement angle have on intensity?
Is more peening coverage better for my parts?
What is 100% vs. 125% coverage?
Is there a difference between SAE and MIL spec shot peening media?
How do I test the hardness of shot peening media?
What is better, cast steel or cut wire shot?
How do I test the size of media? What is a Ro-Tap, and do I need one?
Does the shot have to be round for shot peening?
How can single robotic nozzle be as fast as a multi nozzle machine for processing parts?
What does TBC and MG mean, and how do they relate to US standard test sieves?
Can you smooth a surface with shot peening?
Why do you shot peen parts?
Shot peening enhances the surface of the part by replacing potential tensile stress with a surface compressive residual stress.
Is shot peening the same as sand blasting?
No. Shot peening's primary function is to enhance the fatigue life of a part, whereas, sand blasting or abrasive grit blasting is to clean or prepare a part for a coating.
What is compressive residual stress? How is it beneficial to my part? How do you measure it?
Compressive stress is the stress applied to materials resulting in their compaction (decrease of volume). When a material is subjected to compressive stress, then this material is under compression. Shot peening produces compressive stress on the surface peened. This compression helps the part resist cracking in turn extending the service life of the component.
Is shot peening the same as stress relieving?
No. Shot peening actually induces a thin layer of compressive stress on the surface of the part which replaces surface tensile stress. Stress relieving is a heat treatment process to remove both tensile and compressive stresses.
What is x-ray diffraction testing and why is it used?
X-ray diffraction testing is a way to measure the stress in a material. This allows one to quantify how much stress is in an area of a part so the actual amount of stress can be measured for testing purposes. In order to determine the depth of the compressive stress, multiple x-ray diffraction measurements are made as layers of the test coupons are electro-polished away.
How often should I re-run a saturation curve?
A saturation curve should be run whenever intensity checks fail, after significant machine maintenance, any life changing event, or at least every year.
What is a saturation curve?
A saturation curve is a standardized method determining shot stream intensity. Almen strips are exposed to a shot stream for varying amounts of time with all other parameters fixed. Adjusting the exposure time can be in time, passes, federates, cycles, etc. The arc height of the almen strips are plotted verses exposure time. A best fit curve is generated through the points. The first point on the line (not necessarily a data point) when time is doubled at which the arc height increases by 10% is called the saturation point and is the shot stream intensity at that location.
Is the saturation point the same as the 100% coverage point?
No. Saturation point is calculated using the saturation curves, and coverage can only be evaluated by shot peening and visually inspecting an actual part. For example coverage will occur much quicker on an aluminum part than on a hardened steel part.
Is my peening intensity the same as the almen arc height at my processing time for my part?
No. The intensity can only be determined by a saturation curve. Since the time to shot peen a part to the proper coverage may take a different amount of time, the arc heights achieved at processing time may be higher or lower than the actual intensity values.
How do I run a saturation curve?
A procedure for saturation curve generation can be seen in the SAE specification SAE J443.
When you have multiple strips on a fixture which all saturate at different times, how to you verify intensity?
Typically, intensity verification occurs at the slowest saturating strip. Development work should be done to help make the strips saturate close to the same time. Most shot peening specs allow for this approach as long as the strips are repeatable (usually within .001"A) to the previous verification run.
What effect does impingement angle have on intensity?
As the impingement angle gets higher (closer to 90 degrees or perpendicular to the surface) intensity increases. The relationship is based on the Sine of the angle of impingement; i.e. a 45 degree impingement angle intensity will be at ~70% of the intensity at 90 degrees.
Can I re-use almen strips?
No. Almen strips can only be re-used for the flapper peening process saturation curve development.
Can I measure shot velocity?
Yes. You can measure shot velocity with the Progressive/Tecnar ShotMeter. See our website for more information on the capabilities of this product.
Is more peening coverage better for my parts?
No. You can actually hurt the fatigue life of a part by over shot peening the surface. Over peening causes excessive cold work and actually cause stress risers and crack initiation sites.
What is 100% vs. 125% coverage?
100% coverage is determined by visual inspection of the surface of a part. 125% coverage is 1.25 times the time to achieve 100% coverage.
Is there a difference between SAE and MIL spec shot peening media?
Yes, the size distribution of the shot peening media is different on the MIL spec than the SAE. AMS-S-13165 outlines the size distribution for MIL spec shot, and SAE J444 outlines the size distribution for SAE cast steel shot, and SAE J442 outlines it for conditioned cut wire peening media.
How do I test the hardness of shot peening media?
Testing the hardness of media is accomplished by mounting media in a epoxy or phenolic metallurgical mount. The specimen is ground approx halfway through the shot diameter. It is polished and micro-hardness tested on either a Knoop or Vickers scale and then converted to Rockwell C scale.
What is better, cast steel or cut wire shot?
Cut wire shot has a more consistent size and density. Because it does not break or crack, it will wear down over time. Typically it has a 10x longer life over cast steel shot. The initial investment in the shot is approx 3x more expensive.
How do I test the size of media? What is a Ro-Tap, and do I need one?
New cast steel shot is tested by performing a sieve analysis on a Ro-Tap. (i.e. SAE J444 or AMS 2431/1,2). A Ro-Tap is a device which shakes and taps test sieves. New cut wire media is typically tested by the measurement and precision weighing a sample. (i.e. SAE J442 or AMS 2431/3,4, and 8). In process media (media being used in a machine) is typically inspected by performing a sieve analysis with a Ro-Tap.
Does the shot have to be round for shot peening?
Yes, non- round media can cause cuts and stress risers in the surface of a part and can create a location for a crack to propagate from.
How can single robotic nozzle be as fast as a multi nozzle machine for processing parts?
A single nozzle robotic machine can be as fast, or even faster than a multi nozzle machine because of target efficiency. The single nozzle can be programmed to precisely strike the surface of the part with very consistent impingement angles and programs can be tuned to prove consistent coverage over the entire surface of the part. This increases target efficiency and reduces wear and tear on a machine caused by overspray.
What does TBC and MG mean, and how do they relate to US standard test sieves?
TBC stands for Tensile Bolting Cloth. MG stands for Market Grade. TBC and MG wire cloth are both used in media screening systems. MG uses thicker wires which help in longevity of screens, but there is less open area on a screen and therefore less media can get through. Talk to a Progressive customer service representative to see what will work best for you.
A US standard test sieve is used for testing and sizing the media. An #80 MG, TBC, and test sieve are not the same and cannot be used interchangeably. See the Sweco screen chart in resources section for more details.
Can you smooth a surface with shot peening?
You can improve the surface finish of a part with shot peening. Many things that affect the ability to do this are initial surface roughness, media diameter, and intensity.
What is NADCAP?
NADCAP stands for the National Aerospace and Defense Contractors Accreditation Program. The goal of the program is to coordinate industry wide standards for special processes. Having industry wide standards makes it easier for contractors to produce parts for several manufacturers without having the burden of going through complicated audits for each manufacturer individually.
