Hardened Depth Measurement Technique by Using Sound Velocity for Work Rolls of Cold Strip Mill

Abstract

In order to measure the hardened depth of quenched work rolls non-destructively, a technique using sound velocity distribution in the rolls was studied.
The sound velocity can be measured by ultrasonic transducers, which are set on a roll surface so that the ultrasonic sound travels on a polygonal beam path in the roll. The velocity is average one in a certain range of depth.
The sound velocity at each depth can be calculated using the average velocity on the transfering route, and then the sound velocity distribution from outer surface to internal portion is obtained.
It is well known that there is a linear relationship between sound velocity and hardness. The sound velocity and hardness of the rolls are influenced by quenching temperature and residual stress. Therefore, if we want to estimate the hardened depth of a roll, we have to know the relationship between sound velocity and hardness of a test block cut out from a roll at first as an original data. And then, the hardened depth of a roll can be estimated using the relationship of the test block.
The accuracy of the estimation of hardened depth is within +/-3mm.