Failure Analysis Of A Transmission Gear And The Studies On Laser Modification Of Gear Steel Surface

With the rapid development of automobile industry,automobile gearbox manufacturing get more and more attention from the manufacturers.Since transmission gear is the main part of a gearbox,the failure of gear caused by frequently occurred pitting and spalling has aroused people’s concern gradually.The topic of this article from the Qingshan gearbox production company.Firstly,a failure analysis of the failed transmission gear from Qingshan company was executed to evaluate its failure cause which include appearance inspection,chemical analysis,micro hardness test and microscopic inspection.Then,laser surface modification was carried out on the 20 CrMnTi and 20MnCr5 gear steel with laser welding machine and laser metal sintering equipment.This process include single-channel laser quenching,laser multi-pass lap quenching,laser cladding 18Ni300 iron-based powder.The Comprehensive performance of different samples after laser processing were assessed by optical microscope,electron microscope,micro-hardness meter,pin plate abrasion tester and electrochemical workstation.The results indicated that the carburized layer of the gear exist globular and chain inclusions.Due to the presence of inclusions,stress concentration formed at the inclusion and the steel substrate interface.The stress concentrated at the area around the tooth profile,which led to the inclusions separate from steel substrate,and then crack initiated in contact surface and developed in subsurface stratum.Fatigue cracks in these areas led to the helical gear fatigue failure occurred eventually.This process agreed with the numerical simulation.The Comprehensive performance include microstructure,mechanical properties,wear resistance,corrosion resistance of different samples after laser processing were greatly strengthen and improved.The 10 th sample(spot size is 100 μm,the scanning interval is 80 μm,the laser power is 75 W,scanning speed is 350 mm/s)of 20 CrMnTi gear steel obtained the excellent performance by laser multi-pass lap quenching,the value of surface hardness up to 740 HV,beyond the original material 546.3 HV,beyond the material after carburizing and quenching 86.6 HV,the quenching layer thickness is about 50 μm,and the wear resistance and corrosion resistance is superior to the carburizing and quenching,laser single-channel quenching samples.Besides,the 18 th sample(spot size is 100 μm,the scanning interval is 80 μm,the laser power is 90 W,scanning speed is 350 mm/s)of 20 CrMnTi gear also obtained the excellent performance by laser cladding 18Ni300 iron-based powder,the highest micro-hardness of cladding layer up to 620 HV,the cladding metal layer thickness is about 120 μm,transition layer thickness is about 20 microns thick,abrasion resistance and corrosion resistance is superior to the samples after laser hardening processing.