Analysis of abnormal components in the mixing of auto parts reduction gears
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1 Test method 1.20CrMnTi steel chemical composition (see 1)
120CrMnTi chemical composition 1.220CrMnTi steel carbonitriding process carbonitriding is treated in the RJJ-105-9T well type carburizing furnace according to the process shown, the osmotic medium is kerosene and ammonia.
However, in the production, due to improper operation, or understanding of the production process, the 20CrMnTi steel automobile gear member appeared black tissue during the carbonitriding process. The appearance of the black tissue greatly affected the gear. The performance of the work.
MnSiCrTi0.80～1.100.17～0.371.00～1.300.04～0.10C0.17～0.23 Component mass fraction Exhaust diffusion strong permeation (840°C) 210 Oil quenching time/hNH3: 0.3～0.4m3/h Kerosene: 130 ~140 drops / min temperature 2 test analysis 2.1 tissue analysis After testing, we found that according to the different organizational forms, the black tissue we found can be divided into three categories: black phase, black belt and black net tissue. The black phase structure appears as a dot on the surface of the gear member, and its black point is the microscopic morphology of the surface pore; the black belt surface is a very thin oxide layer, and the inside is the toughite and bainite non-martensitic The structure contains alloy oxides, carbonitrides and a small amount of martensite and retained austenite; the black mesh is mainly a torsite network and a bainite network. Tests show that the general black belt is about 0.005~0.02mm on the surface of the workpiece, the black surface of the tooth surface is more than 0.03mm, and the deep one is up to 0.06mm, even throughout the entire layer.
The appearance of black tissue causes the microhardness of 20CrMnTi steel automobile gear to decrease significantly, thus reducing the wear resistance and fatigue strength of the gear. At the same time, the presence of black band or black mesh structure on the surface of the gear makes the fatigue strength of the workpiece ( Contact fatigue strength and bending fatigue strength) decreased significantly. Tests show that the contact fatigue strength decreased from the normal value of 2560MPa to 1440MPa.
2.2 Reasons for the formation of black tissue The reason why carbon-nitrogen co-infiltration of 20CrMnTi steel automobile gears produces black tissue is mainly caused by the following factors: 1) Insufficient cooling rate is an important cause of black tissue, and carbon and nitrogen elements have penetrated into the gear surface. It can form a metallurgical bond with the alloy structure and isolate it into a black tissue.
2) The carbon and nitrogen content of the co-infiltration medium has an important influence on the generation of black tissue. The higher the nitrogen content of the gear surface, the easier it is to form black tissue defects.
3) If the co-infiltration temperature is too low, the kerosene cracking is incomplete, and the slow recovery of the furnace gas will make the alloying elements in the steel easy to oxidize; when the co-infiltration temperature is lower than the critical point Acm of the steel, if the furnace is slowly released, it will easily produce black tissue; if the temperature is too low, the temperature will be too low. Nitrogen and carbon aggregate on the surface of the workpiece, and it is easy to form a densely distributed large amount of carbonitride black tissue.
4) The oxidation of Si, Mn and Cr in the alloying elements is the most important, and the formation of black structure is promoted. The precipitation of Mn along the grain boundary reduces the stability of austenite and makes the formation of black tissue easier.
3 Preventive measures In the process of drop-controlled carbonitriding, the carbon and nitrogen contents of the gear layer can be controlled by controlling the carbon potential and nitrogen potential of the furnace gas; the quenching cooling rate is increased, and the workpiece deformation requirements are met. Using a slightly lower temperature quenching oil and a larger oil flow rate, it is possible to reduce and prevent the gear from producing black tissue.
According to the actual production, the 20CrMnTi steel automobile gear carbonitriding process is improved as shown in the figure.
The main features of the process are as follows:
1) Dilute methanol before the large exhaust gas, and increase the amount of ammonia as the temperature of the workpiece increases.
2) Control the percentage of CO2 content and ammonia inflow in the furnace atmosphere, so that the furnace gas obtains a stable carbon potential and nitrogen potential, so that the carbon content and nitrogen content of the gear surface are within the safe content range below the critical value of severe black tissue formation.
3) Carbonitriding treatment Under the condition of ensuring the martensite and retained austenite structure, the second-stage co-extrusion quenching temperature is higher, and the 20CrMnTi steel can take 840 °C.
Temperature / °C860 ± 10 air cooling time / min840 ± 10 hot oil cooling 6030306015100 ~ 140
4) When quenching the gear, on the basis of ensuring the deformation requirements of the workpiece, try to use a larger cooling rate. This is important to reduce and prevent the appearance of black tissue in carbonitrided workpieces.
4 Conclusions 1) The black structure of 20CrMnTi steel in the drip-type carbonitrided layer is actually a small pore and a bainite structure distributed along the grain boundary.
2) The distribution of black nets along the grain boundaries is due to the bainite and granular bainite distributed along the grain boundaries. Controlling the water content of the drip into the medium, such as accelerating the exhaust gas, and increasing the nitrogen potential in the furnace at the end of the co-infiltration can reduce the bainite structure.
3) By controlling the carbon potential and nitrogen potential of the furnace gas, it is possible to control the carbon and nitrogen content of the gear layer; increase the quenching cooling rate, use a slightly lower temperature quenching oil and a larger oil flow rate, etc., which can reduce and prevent the black tissue from being generated by the gear. .