• The hardening cast layer has coarse grain as large as 0.1 mm;

• The cylindrical surface of roller results in serious ultrasonic beam dispersion;

• The connecting interface has a very small change of reflected impedance;

• This testing must have wide range from 10 mm to 55 mm.

To overcome these testing difficulties, we study testing methods and make a kind of double crystal transducer. Four aspects are discussed; ultrasonic backscattering is a strong tool to test the hardening depth and connecting quality of double-chilled roller. PZT compound materials crystal ( c _l = 12 x 10⁶ rayl ) and P_bN_bO₆ crystal are suitable for testing transducer to get highly sensitivity and resolution. Adjusting the angle of both plane crystals can gather transmitting ultrasonic beam. Plexiglass delay wedge can improve the coupling condition and the testing ability of near surface.