For the shot peening process, many people are confused with two concepts, namely "strength" and "coverage" (shot time). To this end, the US SAE Committee (Fatigue Strength Design and Evaluation Branch) decided to eliminate the reference to “coverage†in SAE J443 “Operation Procedures for Using Standard Enhanced Test Strips†and replace it with new ones since January 2003. Defined terms SAE J2277 - "Enhanced Coverage".
First, let's take a look at the problem and then discuss the solution. Whether it is shot peening or shot peening, the purpose is to introduce a compressive stress layer on the surface of parts that are under long-term alternating stress to form a protective residual compressive stress layer on the surface layer, thereby prolonging the fatigue life or load of the part. ability. The depth of the effective compressive stress layer depends on the strength of the reinforcement. The strength is directly related to the size, hardness and jet velocity of the pellet media used. In order to achieve the desired reinforcement results, ensure that all surfaces of the part or key areas of stress concentration are adequately, appropriately and accurately shot peened/blasted to obtain a uniform, high quality compressive stress layer on the part surface. Another concept is "coverage." In order to achieve the desired enhanced results, we must accomplish two things: determine (or control) intensity and determine (or control) coverage. After we understand the strength of reinforcement, we will explain the enhanced coverage or determine the correct strengthening time.
At present, there is no “strength measuring instrument†that can be used directly to measure the strengthening strength, so the Almen test piece must also be used to determine the strength value. Through the jet of the high-speed pellet flow, numerous small pits are struck on the standard Almen test piece, and the test piece is bent. If more impact is received, the test piece will continue to bend. Until a certain degree, the peening time is prolonged, and the curvature of the test piece is not changed. At this time, the relationship between the bending rate of the test piece and the proportional increase of the shot peening time disappears. The intensity of the strengthening can be determined by plotting the saturation of the Almen test piece (shown as the height of the saturation curve) versus the shot peening time. This chart is called "Almen Intensity Saturation Curve" or simply "Saturation Curve". This curve leads to the 10% theory elaborated by Almen, "Strength is the first occurrence of the shot peening time on the saturation curve is doubled, and the strength value does not increase by more than 10% of the bending height point. The strengthening strength is determined by The properties of the pellets used and the operation of the shot peening are determined. It should be noted here that the Almen test method is only applicable to the concept of strength.
Looking at a separate pit on the Almen test piece, you will find that a 010 inch "A" (or 10A) strength pellet will strike a pit of about 0.009 inches in diameter. A large number of pits of this size are gathered on the Almen test piece, causing the test piece to bend. What happens if you use the same size of pellets to hit a different material than the Almen test strip? The Almen test piece material is a cold rolled steel as defined by SAE 1070 with a hardness value between 44 and 50 HRC. If handling parts with high hardness, such as carburized gears with a hardness of 60 HRC, the pellets will be much smaller at the same strength. Conversely, if a softer part, such as an aluminum part with a hardness of 80 HRB, is processed, the pits formed at the same strength are much larger. Therefore, for a gear with a high hardness to achieve full coverage, the required pits are more than the aluminum parts with a small hardness, so the strengthening treatment time should also be longer.
The conclusion is that the Almen test piece can only be used to accurately measure the intensity value. For coverage, it is the extent to which the surface of the part is covered by the pit. The appropriate shot peening time depends on the preset coverage.
For more information, please contact Will Bellett (Group) Ms. Qian Jing 021-54659896
Axial Expansion Joint,Expansion Joint Stainless Steel,Axial Movement Expansion Joint,Tube Connector Expansion Joint
Hangzhou Ehase-Flex Co.,Ltd. , https://www.ehasetech.com