Current destructive techniques for a complex automotive part may take as long as 30 minutes. Generally, UMA measurements may be made for hardness depths greater than 1.0 mm. Though presently perfected for measuring case depth in automotive axles, the technology has much wider potential application. It can also reduce inspection costs, eliminate the scrap created by destructive tests, and potentially perform a 100 percent inspection. The UMA system includes a tank containing water in which a steel shaft is submerged and rotated.
Water is required as a transmission medium to couple sound into the part.
An ultrasonic sensor positioned over the part directs sound waves at the part. The transmitted wave passes through crystalline structures within the steel, which scatters numerous small waves back toward the sensor. The microstructure within the hardened layer scatters low-amplitude waves and the softer underlying microstructure scatters higher amplitude waves. Temporal shifts of the transition-zone response correspond to proportional changes in hardness depth. Figure 1 below illustrates the process, and shows a typical ultrasonic response from hardened steel axle shafts.