The low surface hardness of carburized gear will reduce the wear resistance and fatigue resistance of gear, and have an adverse impact on the friction and wear resistance of gear surface.
1. Cause analysis
(1) Surface decarburization and decarburization layer in metallographic examination are caused by poor protection during normalizing or quenching after carburization.
(2) The cooling rate is too low. Under the microscope, the surface structure is not martensite, but sorbite. During metallographic observation, acicular martensite has obvious corrosion resistance, while sorbite is dark (easy to corrosion). There is a big difference in hardness detected by microhardness tester.
(3) The high carburizing temperature and quenching temperature of gears cause too much residual austenite on the surface after quenching.
(4) Poor hardenability of gear materials and insufficient cooling capacity of quenching cooling medium.
(5) The tempering temperature after quenching is too high and the holding time is too long.
2. Preventive measures
(1) Proper carburization treatment shall be taken for the gears that have caused low carbon content on the gear surface.
(2) Select materials with appropriate hardenability and cooling medium with appropriate cooling capacity for quenching cooling.
(3) Take measures in advance to reduce the amount of residual austenite after quenching. The carburized gear containing too much residual austenite is tempered at 650 ~ 670 ℃ for more than 3H to precipitate part of the alloy carbide, so as to reduce the stability of austenite during reheating quenching and promote the transformation of austenite to martensite.
(4) Gear carburizing cooling or reheating quenching shall be carried out in a protective atmosphere. For gears that have undergone oxidation, the oxide scale shall be removed, and quenching shall be carried out after surface carburization.
(5) If the surface hardness of the gear is low and the tempering temperature is too high, it shall be re quenched and tempered at an appropriate temperature.