Shot peening can be intimidating and you may not understand the value of a CNC machine over another type. See the answers to some common questions below and don’t hesitate to reach out to us to discuss your project. We’re here to help.
1Why shot peen parts?
Shot peening enhances by replacing tensile stress with a compressive residual stress. Tensile stresses are bad stresses that can fail. Fold a piece of aluminum foil in half, then tear it. It will tear at the tensile stress caused by the fold. Compressive residual stresses are part of a design and therefore reside in the part. Shot peening induces tiny arches or dents in the metal surface thus changing the surface from tensile to compressive. Compression stresses are strong, tensile stresses are weak and vulnerable. Some other well know examples of residual compressive stress designs are the Roman Colosseum and many large manmade Dams, such as Hoover Dam.
2What is residual stress?
Residual stress is the Internal stress distribution residing in the material. These stresses are present even after all external loading forces have been removed. They are a result of the material obtaining equilibrium after it has undergone plastic deformation such as the shot peening process.
3What is the difference in residual stress and applied stress?
Applied stress is generated inside a material due to an external load. Residual stress is present inside the material regardless of loading.
4What is a saturation curve?
A saturation curve is a industry wide standardized method determining shot stream intensity. Intensity is normally called out on the designers part print.
5Is 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.
6How do I run a saturation curve?
Saturation curves are run by shot peening Almen strips. When peening one side the strips warp. That warp is measured on an Almen gauge. That measurement is called an arc height. Four Almen strips are peened at different exposure times. The arc height is then plotted on a chart with arc heights and time values. A curve is then plotted near each arc height. The first point along the curve that when time is doubled and within 10% of the arc height readings is considered intensity. This is the simple explanation. For more detail please see SAE specification J443.
7When you have multiple strips on a fixture which all saturate at different times, how to you verify intensity?
n most cases, intensity verification occurs at the slowest saturating strip. This does not mean the other saturation curves are bad curves. It simply means the intensity took longer on one verification. It is recommended that development work should be done to help make the strips saturate close to the same time.
8How 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.
9What effect does impingement angle have on intensity?
The angle of impingement should be as close to 90 degrees as practical. At 80‐90 degrees the intensity will be at its highest considering all other values are the same. As the angle decreases the intensity will decrease. At 45 degrees intensities can decrease 25%.
10Can I re‐use Almen strips?
No. Flapper peening is an exception.
11What is 100% vs. 200% coverage?
100% coverage is determined by visual inspection of the surface of a part. This can be achieved by peening a part at 50% then 75% and so on until 98% of the part is covered. 98% is considered 100% coverage. 200% coverage is 2t times the time to achieve 100% coverage.
12Will extra peening make a part even better?
No. You can actually hurt the fatigue life of a part by causing stress risers in the part. It is best to simply peen the part at near 100% coverage or the coverage the designer calls for.
13Is there a difference between SAE and MIL spec shot?
Yes, the size distribution of the shot peening media is different on the MIL spec than the SAE. Size distributions are detailed in AMS‐S‐13165 and SAE J444. SAE J442 outlines it for conditioned cut wire peening media.
14Does the shot have to be round for shot peening?
Yes, non‐ round media can cause cuts and stress risers in a part and can create a location for a crack to expand.
15Is cut wire round?
Cut wire starts out in a cylinder shape and is rounded in the manufacturing process. If purchased using the shot peening specifications cut wire should be delivered to the end user in a round shape. Cut wire shot has a more consistent size and density than normal cast shot. Cast shot can break or crack Cut wire simply will wear down over time. Typically it has a 10x longer life over cast steel shot. The initial investment in the shot is approximately 3x more expensive.
16How do I test the size of media?
New cast steel shot is tested by performing a sieve analysis on a Ro‐Tap machine. Ro‐Tap stands for rotating and tapping. New cut wire media is typically tested by the measurement and precision weighing a sample. In process media (media being used in a machine) is typically inspected by performing a sieve analysis with a Ro‐Tap unit.
17How can single robotic nozzle be as fast as a multi nozzle machine for processing parts?
A single nozzle robotic machine is faster than multiple nozzle machines. Since it is programmable and has full nozzle and part motion it will only cover the surface area at near 100%. Multiple nozzles can strike the same place 200, 300 or even 400% before some areas are covered to the required 100% in all areas. Robotic CNC machines also have the capability to peen at optimal angles of impingements therefore creating an even distribution of shot stream intensity on the part surface
18What is NADCAP?
Nadcap (National Aerospace and Defense Contractors Accreditation Program) is an industry‐managed approach to conformity assessment of 'special processes' that brings together technical experts from prime contractors, suppliers, and representatives from the government to work together and establish requirements for approval of suppliers using a standardized approach. Unlike traditional third-party programs, Nadcap approval is granted based upon industry consensus. (Source; Boeing Website).
19If the machine aborts a cycle do I have to start the cycle over?
It depends upon your rules, regulations, requirements etc. Most computer-controlled machines are capable of work resumption at the interrupt point and this can prevent over‐ peening a part. Repeating a complete peening cycle essentially doubles the peening coverage and, depending upon the metal type and alloy, this may be detrimental to the fatigue life.
20Can I use the 1T time value established on the saturation curve as the time value for part coverage?
Determination of coverage or cycle time is an issue which must be addressed on the part to be peened. Complete coverage of your part means complete denting of the intended target, not the Almen strip. The Almen strip use is for intensity determination only and not for coverage.
21What is AMS 2430?
AMS 2430 is an SAE specification for automated shot peening. It is commonly used in most industries world wide.
22What is AMS 2432?
AMS 2432 is an SAE specification for computer monitored shot peening. This specification calls for many AMS 2430 requirements with additional monitoring requirements for process control and documentation
23What is J443?
443 is an SAE specification detailing the procedures for using standard shot peening test strip. This specification details on how to develop a saturation curve to establish true intensity.
24Is 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.