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Control of retained austenite in tooth angle by vacuum carburizing heat treatment

Retained austenite (Ar) refers to austenite that does not undergo martensitic transformation under a certain degree of undercooling. The amount of retained austenite, the range of martensitic transformation start temperature (Ms ) and end temperature (Mf ) relative to room temperature, the chemical composition of the steel, the austenitizing temperature and time, and the degree of austenite stabilization are all closely related. relation. The higher the carbon content in the steel, the more retained austenite after quenching; the increase in the austenitizing temperature will further dissolve carbon and alloying elements into the austenite, reduce the temperature at which the martensitic transformation begins, and make the residual austenite The amount of austenite increases.

The conventional controllable gas carburizing and quenching heat treatment method is easy to produce internal oxidation and non-martensitic structure (ferrite formed by surface decarburization, troostite and bainite formed by surface layer along grain boundaries, etc.), which will reduce the surface hardness, Wear resistance and fatigue limit. The use of low-pressure vacuum carburizing and high-pressure gas quenching technology has the following characteristics.

a.Using high temperature and low pressure vacuum carburizing, the surface activity of the parts is high, the carburizing speed is fast, and the construction period is short;

b.During the low-pressure vacuum carburizing heat treatment process, the parts are always in a vacuum state, and there is no oxygen atmosphere, so there will be no internal oxidation, surface non-martensite and other defect structures, and will not lead to surface alloying elements depletion and quenching Problems such as reduced permeability can significantly reduce the early failure of the surface of the parts and improve the service life of the parts.

c.Environmental protection, will not discharge toxic and harmful substances. The main reduction gear of this transmission adopts a low-pressure vacuum production line for trial production of parts.

The material of this part is SAE5120H-M, low carbon Mn-Cr gear steel. The gear teeth are relatively sharp. During the carburizing process, the concentration of carbon atoms at the top of the tooth is high, and the carburizing layer is also deep. ingots and carbides. After tempering, the metallographic structure mainly detects residual austenite and carbide content.

The general controllable atmosphere carburizing (carbonitriding) heat treatment process mainly includes preheating, heating, strong infiltration, diffusion, cooling, heat preservation, quenching, low temperature tempering and other main processes.

The low-pressure vacuum carburizing heat treatment adopts multiple strong infiltration (passing into the carburizing medium) and diffusion (passing in protective gas, such as N2), and finally enters the centralized diffusion. The austenite and carbon are dissolved and saturated, and the dissolved carbon is diffused to the inside to reach the target value during the diffusion period. By adjusting the ratio of carburizing and diffusion time, the purpose of controlling the surface carbon concentration and the depth of the carburized layer is achieved.

The parameters that need to be input in the low-pressure vacuum carburizing equipment simulation program include carburizing temperature, enrichment rate of the medium on the workpiece surface, maximum saturated carbon concentration (Cmax) on the surface after strong infiltration, carbon concentration on the surface of the workpiece after diffusion (Cmin), and final workpiece surface. The carbon concentration (Cfinal) that needs to be achieved, the depth of the infiltration layer, etc.

The enrichment rate refers to the amount of activated carbon adsorbed per unit time and unit area, which is the embodiment of the material’s carbon absorption capacity. The enrichment rate increases with the increase of carburizing temperature. The higher the temperature, the higher the carburizing efficiency. However, the higher the temperature, the better. According to experience, the carburizing temperature of the main reduction gear is set to 940 °C; the set value of Cmax is determined to be 1.30 in combination with the Fe-C phase diagram. Combined with the formula, the flow rate of the carburizing medium can be obtained according to the surface area of the parts. The surface area of the main reduction gear is 106 865 mm2. When the furnace is full of 64 pieces of heat, the calculated flow rate is 2 105 Nl/h, which is generally -5% depending on the furnace load. Floating between ~10%. After inputting the values of the simulation parameters, the simulation process is obtained. The number of carburizing cycles is 14, which correspond to the intensive infiltration time and the diffusion time in turn. Acetylene is used as the carburizing medium gas, and the high pressure N2 quenching is performed directly after the carburization.

The adjustment and ratio determination of strong infiltration and diffusion time are particularly critical. Insufficient diffusion will lead to high carbon concentration in the infiltrated layer, resulting in the formation of retained austenite and carbides; too long diffusion time will reduce the surface carbon concentration, affect the surface hardness, and reduce gears. Abrasion resistance.

a.Optimizing parameters such as carburizing temperature, enrichment rate and maximum surface carbon concentration can obtain a more reasonable carburizing simulation process;

b.Using the principle of mini carburizing, the content of retained austenite at the tip of the tooth can be effectively reduced by adjusting the time and ratio of strong infiltration and diffusion. Experiments show that the content of retained austenite in the tooth angle is improved from grade 7 to grade 3.

c.Continuously optimizing the flow rate of the carburizing medium can assist in the control of the metallographic structure of the tooth angle;

d.Adjusting the strong infiltration, diffusion time and their ratios, and selecting an appropriate carburizing cycle number can effectively control the depth of the effective hardened layer.