Abstract: A batch of gears after production and processing, quenching and low temperature tempering cracked during storage in the warehouse. The causes of gear cracking were analyzed by means of ultrasonic flaw detection, metallographic examination, chemical composition analysis, hardness test, fracture morphology observation, end quenching test and supplementary heat treatment. The results show that the high strength of the cracked gear leads to high hydrogen embrittlement sensitivity, and the hydrogen induced delayed crack at the sub-surface is the direct cause of rapid brittle cracking of the gear. The root cause of the gear cracking is that the hardening capacity and hardenability of the cracked gear are too high, which exceed the hardenability requirements of BS EN 10084-2008 for 18CrNiMo7-6+HH steel. Thus, the cracking can be effectively avoided by reducing the strength and hardness of the quenched and tempered gear through adjusting the quenching medium and reducing the cooling rate of the high temperature zone during quenching.

Key words: 18CrNiMo7-6 steel, gear, hydrogen embrittlement, hardenability, cracking