Heat treatment, shaft, gear, quenching and annealing stainless steel products and put quenching as an example. The shallower of the quenching layer.The higher of the frequency.The deeper of the quenching layer.The lower of the frequency.Such as: hardening layer 0.2-0.8mm applicable UHF 100-250 KHz, high-frequency 1.0-1.5mm applicable 40-50 Khz high-frequency, super-audio 1.5-2.0mm applicable VHF 20-25 Khz 2.0-3.0mm applicable VHF 8-20 KHz, IF 3.0-5.0mm applicable 4-8 KHz IF 5.0-8.0mm applicable 2.5-4 Khz IF