Abstract: Briefed in the essay are the characteristics of mid-frequency quench bending process as employed for manufacturing steel elbow, focusing on an analysis on current penetration depth.It is pointed out that during hot-bending of the heavy-wall elbow, the characteristics of the mid-frequency quenching process as used shall eventually cause the differences between heating temperatures on and cooling velocities of the outer and inner surfaces, and accordingly bring about the differences of microstructures and properties of the elbow.A serial of tests demonstrate that different microstructure characteristics show separately in the inner, middle and outer areas of the X80 heavy-wall elbow wall, which determine the property differences of the material after being finally heat treated.Moreover relevant opinions about the future development of the elbow hot-bending process is put forward.