The important parameters to ensure the load-bearing capacity of low-speed and heavy-duty hard face gear are: enough central strength and enough carburized layer depth, insufficient carburized layer depth, increasing the shear stress / shear strength ratio in the transition zone, easy to cause fatigue cracks in the transition zone, too deep carburized layer, which will reduce the bending strength, and even bring adverse effects on the contact strength.
For the selection of carburized layer depth, abroad has been quoting (0.1-0.2) modulus recommended by Swiss MAG company, and (0.15-0.2) modulus recommended by West German Benz company. It is not very comprehensive to determine the carburized layer of large heavy duty gear by the relationship between gear modulus size. The depth of carburized layer of heavy-duty gear depends on the load, and is related to the geometric parameters such as the modulus of gear and the diameter of indexing circle. Dr. Ishida of Japan’s Jinggang University proposed that: t ≥ 3.15b, where T – the total depth of carburized layer, mm; B – half of the second wheel contact width, mm. It is also suggested that two times of the maximum shear stress depth should be used as the effective hardening depth, which can be used for reference.
The depth of deep carburizing for heavy duty gears is generally calculated by the effective hardening depth. According to the current standard of Japan, the effective hardening layer is defined as hv550 (RC52), while the United States, the Soviet Union and Western European countries still define hv513 (rc50).
The effective hardening depth of carburizing depends on the depth of carburizing layer, the hardenability of material and the cooling rate during quenching, so we should pay special attention to the interaction of these three factors.
The hardness of the center depends on the hardenability of the material, which is actually the problem of gear material selection. For the contact fatigue strength, the higher the core strength is, the better. For the bending strength, the best bending strength is corresponding to the tensile strength of 120-130 kgf / mm2. Benz company recommends hrc35-43 as the core hardness, Fiat company recommends hrc33-40 as the core hardness, which is mainly for small and medium gears, while Japan’s large gears adopt hrc30-40 as the core hardness.
The carbon concentration on Carburized surface and the morphology, size and quantity of carbides have obvious effects on the wear resistance, bending fatigue strength, contact fatigue strength and strength and toughness of gears. Increasing the carbon concentration on the carburized surface is beneficial to improving the hardness of gear surface after quenching. If the surface carbon concentration is too high, the gear is easy to produce grinding cracks. It is generally expected that the surface carbon concentration should be controlled within 0.85-1.05%.
For large low-speed and heavy-duty gears, the anti-wear performance of tooth surface is not a particularly important index, and the requirement of tooth surface hardness is also low, generally around HRC55. Practice has proved that the higher the carbon concentration in the carburized surface, the more difficult it is to control the shape and size of carbides, especially when the carburized layer is required to be deep. In this way, the surface carbon concentration is too high, which has adverse effects on the contact strength and bending fatigue strength of gears. Therefore, it is more appropriate to control the surface carbon concentration of deep carburizing of large low-speed and heavy-duty gears at 0.75-0.90%. The surface carbon concentration of deep carburizing of Japanese heavy duty gears is controlled at about 0.80%.
Recently, the alternating impact test of carburized parts has been carried out. It is found that the maximum value of the relationship between the impact times and the carbon content of the surface layer appears in the range of 0.7-0.9% C.
The size and morphology of surface carbide have influence on the structure uniformity and gear deformation during quenching, and also on the strength and toughness, contact fatigue and bending fatigue properties of gears in service.
In high carbon steel, the carbides larger than 1 μ m are the source of crack formation during stress concentration and fracture, while the carbides smaller than 0.5 μ m have little effect. There are a lot of research results about the effect of carbide quantity and morphology on the life of machine parts. In order to reduce the stress concentration around the carbides, the closer the particles are to the sphere, the better. At the same time, it can also reduce the residual stress near the carbides during quenching.
In addition, the transition zone of carburized layer should be gentle, and the core grain size should not be lower than grade 6 after long time deep carburizing. Finally, the deformation of carburized gear should be reduced as far as possible when it is burnt, so as to prevent the uneven carburized layer caused by excessive eccentric cutting during finishing. During quenching, the residual compressive stress on the tooth surface should be as high as possible.
It is very important to establish a carburizing inspection standard for large heavy duty gears, including hardness, depth of hardened layer, surface carbon concentration, carburized layer structure, surface carbide morphology and uniformity, central grain size, metallographic structure after quenching and tempering, etc., with metallographic rating picture, acceptance method and acceptance procedure attached.