The material of the large gear does not conform to the alloying element composition of the 20CrNi2MoA steel grade (especially the content of Ni is insufficient), and it is more in line with the alloying element composition of the 20CrMo steel. The detected fracture material composition is compared with the alloy element composition of the standard steel grade, the Rockwell hardness test result of the tooth surface and the core/HRC, the Rockwell hardness of the gear core is about 36HRC, and the Rockwell hardness of the tooth surface is about 56HRC. Compared with the large gears that are subjected to heavy loads, the hardness of the tooth surface is low and should be around 60HRC. Failure causes There are two main failure modes of the large gear: broken teeth and flank pits. The main cause of the pits on the tooth surface of the large gear is that the hardness of the tooth surface is low and the load is too large, resulting in insufficient contact strength of the gear. Under the action of alternating contact stress, the gear first produces very small micro cracks on the tooth surface. The wear is called pitting, and the pitting of the pitting continues to develop, gradually causing a shallow exfoliation zone along the pitch circle, and the spalling is further expanded, eventually causing a large piece of the tooth surface to peel off and a pit. The main reason for the large gear breaking is that the load is too large, the flank of the tooth surface is generated, and the installation or machining accuracy is poor (such as the axis deflection and the gear gap is improper), causing the gear to be off-loaded away from the end of the coupling, and the bending strength of the gear is insufficient. The meshing condition of the gear is sharply deteriorated, microcracks are generated on the tooth surface, and under the cyclic stress, fatigue expansion occurs, eventually leading to broken teeth.
From the perspective of the large gear material, 20CrNi2MoA or better gear steel material is used. From the perspective of heat treatment process, the tooth surface of the large gear is subjected to carburizing and quenching and tempering heat treatment to ensure that the hardness of all tooth surfaces is not less than 58HRC, and the remaining thickness of the hardened layer processing shall not be less than 1.5mm; The contact strength and bending strength of the gear have a larger safety factor. Improve the lubrication state of the reducer. After a long period of downtime, it is best to refuel or refuel the pump to cycle during the restart to improve the lubrication state at startup. The macroscopic topography of the fatigue or stress corrosion fracture. There are band-like streaks on the fracture surface of the broken teeth, which is the main evidence of fatigue fracture - fatigue striation, which can be determined that the fracture property of the broken teeth is fatigue fracture. Material Testing In order to determine whether the material of the large gear belongs to 20CrNi2MoA gear steel, two broken teeth are selected, and a university research institute is entrusted to use a direct reading spectrometer to detect the alloying element content of the material. The outer coating of the buried steel pipeline is the barrier to prevent corrosion damage.
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