Frequently Asked Questions

Peening intensity is determined by exposing several (minimum of 4) Almen strips to the blast stream for increasingly longer times and plotting the arc-height vs. exposure time. Peening "Intensity" is the value of the arc-height curve at time T1 that is within 10% of the arc-height curve value at twice the time or T2. The requirement 14-18A intensity is met when the arc-height value is within the .014 inch to .018 inch when the "A" thickness strip is used. For more information see SAE document J443.

If you satisfied the conditions listed above then the arc-height value of the curve at time T2 may, or may not, be within the 14-18 tolerance band. As long as the arc-height of the T2 data point is 10% (or less) than the arc-height value of the T1 point then you have determined "intensity". It is VERY common for the T2 data point to actually be above the tolerance band.

Peening for twice the time needed to achieve 100% coverage as determined by examining the actual peened surface (not twice the time for the Almen strip to reach intensity). Peening soft aluminum parts will take much less time than the intensity time of the Almen strip because the aluminum is softer and therefore has larger dimples than the Almen strip. Conversely, hard gears will require much greater peening time since the dimples will be very small compared to the Almen strip. The part's hardness compared to the Almen strip HRc 44-50 range is the key factor in determining coverage time.

Modern shot peening evolved in the automotive industry most likely due to failures of engine valve springs which were prone to failure in service. The immediate cause of the failures was unknown and an off-hand comment that the metal "must be tired" later was designated "fatigue failure". Investigators suspected that the failures, always beginning at the surface of the part which was in tension, might be caused by surface contamination of some sort. Blast cleaning the parts produced a remarkable improvement in life-testing and the investigation focused on improving the surface condition. But, once the procedure was implemented it had to be controlled so that it could be reproduced reliably. It was known that thin metal parts, when blast cleaned, would often distort and this characteristic would be used to develop a test method. A thin strip of steel, from the same material as the valve spring, was subjected to the blast stream and its curvature was noted. This was great news but not very scientific. A numerical value had to be assigned to this test. Another problem was quickly recognized, namely that the strip would continue to curve for longer exposures to the blast stream. Eventually, after an adequate exposure time, the strip rate of curvature would diminish. A plot of the curvature of the strip versus time revealed a curve with an initial high slope that gradually tapered off. There was a "knee" to the curve but the curve seemed to rise indefinitely. The problem now was to assign some numerical value to this "test". What happened next was quite brilliant. Instead of estimating the final value of the curve (the asymptote) the arcing that occurs at the knee of the curve was selected. This point would be determined by applying what is now known as the 10% rule. Guess a point on the curve that might be the knee and note the arc height. Next, note the value of the arc-height at twice the exposure time of the first point. If the second point is exactly 10% greater than the first point then call the first point "intensity of the blast stream". An engineer name John O. Almen developed the test strip which was later named in his honor. He applied for a U. S. Patent in 1942 and the patent was granted in 1944. His original design was superceeded by General Motors in November of 1943 and hence the designation #2 Almen gage. Two changes were made in the gage. The knife edges used to support the strip were replaced with four small balls and the strip was inverted so that the indicator tip would always touch the non-peened side of the strip. The four-ball support recognized the compound curve nature of the strip. Having the indicator tip touch the non-peened surface precluded erroneous reading from nesting the dial indicator tip in the bottom or top of a peening dimple. It also extended the lifetime of the tip since operators tended to slide the strip back and forth searching for a desired reading. Modern Almen gages now have end stops to insure that the reading is taken from the "central portion of the strip" as required.

It depends upon your rules, regulations, requirements etc. Generally you need permission from the "part owner". It would be unusual to substitute glass bead for cast steel shot while it is occasionally permitted to substitute cut wire shot for cast steel shot. Always ask permission and "get it in writing".

It depends upon your rules, regulations, requirements etc. It is common to specify shot that is harder than the part being peened however there are occasions where softer shot does provide some surface compression (dimpling). It wont be as effective as hard shot but it may be adequate for the situation. Less shot will be broken so it represents an economy but it may not perform the same as harder shot, even if the velocity is adjusted so the same intensity is achieved. Always ask permission and "get it in writing".

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. Always ask permission and "get it in writing".

There are several popular ways to determine coverage but they are all based upon visual examination of the part surface, usually with the aid of a 10x to 30x magnification lens. The part to be peened is submitted to the blast stream for a brief period of time and then examined. An estimate of percentage of surface dimpling is made and then the part is exposed to further treatment. This process continues until the surface exhibits a "complete obliteration of the original surface with overlapping dimples". The time to achieve this condition is then declared to be the "complete coverage" or 100% coverage time. For 200% coverage this time would be doubled. An another useful technique for production determination of coverage can be implemented after the visual coverage has been established. A fluorescent liquid tracer, such as Metal Improvement Company "Peen Scan". See SAE document 850708, "Dyescan Tracers As A Quality Control Tool For Coverage Determination In Controlled Shot Peening" for explanation on use of tracers for peening. (Read abstract in the shotpeener.com library).

Media shaker screens vibrate to allow the shot to fall through the screen mesh. The vibration provides a combination of movements that is critical for efficient operation. Once the shaker has been properly adjusted you should monitor its performance. A simple way to do this is with a special decal made by Electronics Inc. called a Screen Check. The decal has a row of circles, each one larger than the previous. As the screen vibrates all of the circles will vibrate. The optical illusion blurs the images. The smallest circle will appear as two distinct circles separated by some distance. The largest circle appears as a blur. One of the circles wil be moving just enough to appear to be a "figure 8". It may be verical or slanted, depending upon the vibration of the screens. Monitor the decal from time to time. If the "figure 8" is made by a larger or smaller circle then the shaker is not vibrating at the same rate and may need adjustment.

Set the Almen gage to zero by placing a flat block onto the gage (press the "zero" button if necessary to achieve zero). Measure one side and then the other of the Almen strip. If either side measures beyond the allowable tolerance then discard the strip. Common tolerances are: 0.015" (grade A3 and A2), 0.001" (grade A1) and 0.0005" (grade A1-S).

Confusion over separation of the concepts "Intensity" and "Coverage" (or "Cycle Time") is often encountered as a problem in the shot peening industry. The Almen method involves only the concept of peening intensity. 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 is correctly employed ONLY for intensity determination and not for coverage. See Discussion on Intensity and Coverage article in the shotpeener.com library.

We all know that there are three strips used to qualify the intensity of a shot peening machine, "A", "N" and "C".
But why isn't there a "B" strip? Read Charles Barrett's explanation.

The term “Flooded Wheel” is used to describe a particular condition when the supply of media to the wheel is greater than the wheel can dispense. The media will then accumulate in the feed spout and in many cases further up the feed line. The flow rate is actually reduced during this condition because the media cannot enter the wheel at a good velocity and flow rate.

 

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