1. Defect content
Including unreasonable distribution of hardened layer, heat treatment defects, surface hardness too high or too low heat treatment defects, core hardness too high or too low heat treatment defects and other factors. The unreasonable distribution of the hardened layer and the defects of heat treatment mainly cause the early failure of the gear due to the damage caused by the insufficient material strength lower than the load stress. The hardness is too low, heat treatment defects, wear resistance, contact stress resistance and core strength can not meet the requirements; If the hardness is too high and there are heat treatment defects, the deformation capacity is very low, and there is a risk of brittle failure such as tooth collapse.
2. Causes of defects
(1) insufficient hardenability of processing materials
As shown in figure (c), the hardenability of material a is better than that of material E. therefore, after the same carburizing and quenching, material a meets the requirements, and material E has failure points in the shaded area shown in the figure. Similarly, low carbon content or insufficient alloy content of some materials will affect their hardenability. Insufficient carbon content of original materials will directly lead to insufficient core hardness and heat treatment defects.
(2) insufficient cooling capacity of quenching coolant or cooling equipment
For the same material, if the cooling capacity of Quenchant is insufficient, as shown in figure (c), the strength curve of material a after normal quenching may fall to the position of material e, resulting in damage below the load stress point, and may make the surface hardness, core hardness and heat treatment defects fail to meet the requirements.
(3) insufficient carburizing holding time
Insufficient carburizing holding time mainly leads to unreasonable hardening layer, and CC ‘carburizing zone with steep gradient shown in figure (b) appears, resulting in damage at c’a’.
(4) improper carburizing temperature or uneven temperature distribution of effective heating zone of carburizing furnace
The carburizing temperature is too high, resulting in too high carbon concentration in the surface layer, and if there are too many residual austenite on the surface layer after quenching, resulting in insufficient surface hardness, heat treatment defects, similar to those shown in figure (d); if there are carbides, resulting in high surface hardness, heat treatment defects will appear as shown in figure (b); If the carburizing temperature is too low, the surface carburization will be insufficient, and defects similar to those shown in figure (d) will appear. After quenching, there will be heat treatment defects due to insufficient C% of the surface and insufficient martensite hardness.
(5) poor adjustment of carbon concentration in carburizing period and diffusion period and unreasonable quenching temperature or tempering temperature
If the carbon concentration is adjusted too high or too low, the defects are similar to the effect of carburizing temperature; When the quenching temperature is too low, the core of the workpiece can not be fully austenitized or non martensitic transformation occurs during quenching and cooling. It is found that ferrite exists in the quenching structure of the core through metallographic examination; High quenching temperature leads to grain growth and coarsening; If the tempering temperature is too high and the tempering time is too long, the hardness of the surface and core after tempering is low, and there are heat treatment defects. On the contrary, it will lead to insufficient tempering.
3. Preventive measures
(1) the hardenability of materials shall be checked before entering the furnace, the carbon content and alloy content of materials shall be clarified by using the carbon determinator and spectrum analyzer, and the carburizing process shall be determined according to the characteristics of materials. Unqualified materials shall be discarded.
(2) analyze the cooling capacity of quenching medium regularly and supplement or replace it in time; In addition, the fuel injection system shall be regularly inspected and maintained to ensure the cooling capacity of the cooling system.
(3) properly combine the carburizing temperature and time, quenching temperature and time, adjust the carburizing atmosphere and tempering process on the basis of knowing the material and quenching medium, and ensure the austenitization of the core; Since the rear axle driven wheel needs to be manually clamped to the press for quenching after being discharged from the furnace, in order to avoid the temperature reduction after being discharged from the furnace, the temperature of the quenching furnace can be appropriately increased by 20 ~ 30 ℃, and the driving wheel directly enters the quenching oil tank for quenching from the quenching furnace, so this measure is not necessary. On the other hand, it is necessary to prevent grain coarsening due to too high temperature and too long time; The control of carbon concentration should be based on the sensitivity of different materials to carbon to determine the carbon concentration in the process of carburization and diffusion. For example, the commonly used 20crm NTI and 20m NV B carburizing materials, the former material has a fast carburizing speed, and the carbon concentration can be controlled at 1.0 ~ 1.15%, while the 20m NV B carburizing speed is slow, The carbon concentration needs to be increased to 1.15 ~ 1.25% to achieve the same effect as the former material.
(4) for materials with insufficient total hardening layer, supplementary carburizing shall be carried out. If the total layer depth is sufficient and the distribution of carburizing gradient curve is unreasonable, rework and quenching shall be carried out, and the workpiece shall be heated to the diffusion temperature again for thermal insulation to rationalize the distribution of carburizing hardening layer.