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The tooth surface of the large gear is subjected to carburizing and quenching and tempering heat treatment
Large gear material does not meet the 20CrNi2MoA steel alloying element composition (especially the Ni element content is not enough), and more in line with 20CrMo steel alloying element composition. Compared with the standard alloy steel, the Rockwell hardness test results of the tooth surface and the core / HRC, the Rockwell hardness of the gear core is about 36HRC, the Rockwell hardness of the tooth surface is about 56HRC , Relative to bear the heavy gears, the tooth surface hardness is low, should be around 60HRC. Causes of failure The main failure modes of large gears are two kinds: broken teeth and tooth surface pit. The main reason for the large tooth surface to produce the pits is the low hardness of the tooth surface and the excessive load, resulting in insufficient contact strength of the gear. Under the effect of the alternating contact stress, the gear firstly produces tiny microscopic cracks on the tooth surface and Wear and tear, called pitting, pitting off to continue the development of pitting, gradually formed along the pitch of a shallow stripping, peeling further expansion, resulting in a larger tooth flake off, there Ma Hang. The main reason for large gear breakage is excessive load, tooth surface pits and poor installation or machining accuracy (such as axis deflection, gear gap improper), resulting in gears away from the coupling side of the partial load, the gear bending strength , The meshing state of the gears deteriorates sharply, micro cracks are generated on the tooth surface, and fatigue expansion occurs under periodic stress, which eventually leads to broken teeth.
From the perspective of large gear material, the use of 20CrNi2MoA or better gear steel material. From the perspective of heat treatment process, the gear tooth surface after carburizing and quenching and tempering heat treatment to ensure that all the tooth surface hardness of not less than 58HRC, and hardening layer processing residual thickness of not less than 1.5mm; re-design check large Gear contact strength and bending strength, the safety factor to take a large number. To improve the lubrication of the gear unit, after a long shutdown, restart the best wheel or install the filling pump cycle, to improve the lubrication state at start. And the macroscopic topographic features of fatigue fracture or stress corrosion fracture. There are stripes on the fracture surfaces of the broken teeth, which is the most important evidence of fatigue fracture - fatigue striation, which can be determined fracture of the fracture nature of the fatigue fracture. Material testing In order to determine whether the material belongs to the large gear gear steel 20CrNi2MoA, select two broken teeth, the use of direct-reading spectrometer to detect the alloying elements of its material.