Measurement of the roughness of the workpiece surface after shot blasting

　　The ratio of the area occupied by the surface steel shot pit (hemp pit) to the total area after the steel shot of the shot blasting machine is strengthened is called the surface coverage (roughness). Only when the workpiece meets the specified shot blasting strength and surface coverage requirements, can it have high resistance to fatigue damage and stress corrosion damage resistance. Steel shot shot peening parts usually require their surface coverage to reach and exceed 100%.

　　Steps for obtaining data on the surface coverage of steel shot projected workpieces:

　　1) Polish the arc height test piece with metallographic sandpaper (grind until Na3 sandpaper).

　　2) Shot blast the polished surface of the test piece under the specified steel shot blasting process parameters.

　　3) Observe the shot blasting surface of the test piece under a 50X metallographic microscope.

　　4) Put the translucent paper on the ground glass of the microscope, and trace the outline of the pit with a pencil.

　　5) Use a planimeter to measure the area occupied by the pits (if the pits occupy a larger area, it is easier to measure the smooth area).

　　6) Calculate the coverage rate of pits according to the calculation formula.

　　Since the shot coverage is calculated based on the measurement data of the pit area, the accurate coverage value can only be obtained when the pit area of the workpiece can be completely distinguished. However, in actual measurement, the accurate measurement value can be 98%, and it is unreliable if it exceeds this value, so 98% is usually used as the limit value of coverage measurement.

　　The test results indicate that when the coverage rate of steel shot shot reaches 30%, the fatigue strength of the workpiece increases obviously; as the fatigue strength increases, the fatigue strength continues to increase, but after it exceeds 80%, the increase rate of the fatigue strength gradually slows down.

　　If the required coverage rate reaches 98%, in the case of high production volume, it should be considered to shorten the shot strengthening time as much as possible. However, it must be pointed out that in order to improve the fatigue performance and stress corrosion resistance of the workpiece, the coverage of the surface of the part should generally reach or exceed 100%.