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Carbonitriding of 12CrNi3A Steel Parts

Carbonitriding is a chemical heat treatment method in which the carbon and nitrogen atoms decomposed by the active medium penetrate into the surface of the steel in the austenitic state to improve the surface hardness and wear resistance. Carbonitriding is essentially a chemical heat treatment process that mainly carburizes and infiltrates nitrogen. Compared with the nitriding process, it solves the problem of high hardness of the nitriding layer but shallower nitriding layer to a certain extent. The infiltration of nitrogen atoms can reduce the co-infiltration temperature and reduce the distortion of the workpiece. Carbonitriding can also improve the solid solution strengthening effect. For gears, it can improve the surface hardness and wear resistance, so that the surface has better fatigue performance and resistance. meshing performance.

The carbonitriding process has been widely used in the surface strengthening treatment of mechanical parts such as gears and bearings. The dispersed carbonitride and uniformly distributed hardened structure in the co-infiltration layer can effectively strengthen the surface. However, in actual production, black microstructures such as dots and bands are often produced in the high-hardness carbonitriding layer, which reduces the performance of the co-infiltrated parts and shortens the service life. This paper analyzes the reasons and mechanism of black microstructure in the carbonitriding layer, discusses the corresponding solutions, optimizes the carbonitriding process, and provides a reference for improving the performance of the carbonitriding parts.

1 Test materials and methods

1.1 Test material

The test material is a certain type of 12CrNi3A steel bearing seat, which is processed from forging billets. Material composition (mass fraction): 0.12% C, 0.89% Cr, 2.80% Ni, 0.32% Mn, 0.24% Si, 0.001% S, 0.006% P.

1.2 Test method

The required carbonitriding depth of the bearing seat is 0.55~0.65 mm, the surface hardness is 82~85 HRA, and the surface hardness of the non-carbonitriding part is 31~44.5 HRC. According to these technical requirements, the carbonitriding temperature used is 840 ℃. After the co-infiltration, the parts are directly vacuum oil quenched, and then cold-treated at -60 ~ -80 ℃ and tempered at 150 ℃. The equipment is a vacuum carburizing furnace, and the infiltrating agent is kerosene plus ammonia.

2 Test results and analysis

2.1 The black tissue of the seepage layer and its analysis

The cross-sectional morphology of the carbonitriding layer of the 12CrNi3A steel bearing seat is shown in the figure below. It can be seen from the figure that a small amount of black dots and light black mesh structures can be observed on the surface of the uncorroded sample, while black dots and mesh abnormal structures can be clearly seen after corrosion, which is It is consistent with the abnormal black microstructure produced in the carbonitriding process of other alloy steels. Relevant studies have shown that the abnormal black point-like structure that can be observed without corrosion is generally a hole caused by the precipitation of gas molecules. The carbonitriding temperature is generally 800~880 °C, and the workpiece is in a stable austenite state at this temperature, so the formed nitrogen molecules are not easy to precipitate from the austenite. However, in the cooling process at the end of the carbonitriding process, the supersaturated solid solution and nitrides decompose, and nitrogen molecules gradually precipitate and separate from the surface, resulting in defects such as pores. The black network structure observed after the corrosion of the sample is mainly related to the alloy oxides and the non-martensitic structure formed after vacuum quenching. The Cr element in the 12CrNi3A steel can improve the hardenability of the material, but it also aggravates the oxidation tendency of the steel. When the oxygen potential of the furnace atmosphere is high, the alloying elements will form oxides with oxygen on the surface grain boundary, that is, the distribution along the grain boundary is formed. of black reticular tissue. Secondly, the enrichment of alloying elements at the grain boundaries will cause the depletion of alloying elements in the austenite, which reduces the stability of the austenite. During the vacuum quenching process, it is easy to form a non-martensitic structure, which turns black after corrosion.

2.2 Elimination and prevention of black tissue

The existence of black tissue will reduce the surface hardness and wear resistance, especially for gear and bearing parts, it will reduce the bending and torsional fatigue resistance of the core, and greatly shorten the service life of the parts. Therefore, for carbonitriding parts with black defect structure, seeking to eliminate the black structure is an effective way to avoid product scrap.

The black structure produced in the carbonitriding process is mainly related to the precipitation of nitrogen molecules and alloy oxides. The pores remaining after the precipitation of gas molecules are difficult to be eliminated by microstructure transformation, and the stable alloy oxides are difficult to be melted by conventional heat treatment methods. Normalizing, annealing and re-quenching cannot eliminate the black structure in the infiltrated layer.

3 Prevention of Black Tissue

It can be seen from the above analysis that 12CrNi3A steel is prone to black microstructure defects during the carbonitriding process, which are mainly related to the precipitation of nitrogen molecules and alloy oxides. In the process of carbonitriding in the ordinary vacuum carburizing furnace, the high oxygen content and nitrogen potential in the furnace are the direct reasons for the abnormal structure. Therefore, degassing before carbonitriding and ammonia flow control during co-nitriding are critical.

Using a vacuum carburizing furnace, carbonitriding the bearing seat is carried out according to the following process: the temperature is 840 ° C, the high-purity nitrogen exhaust is used for 15 ~ 20 in, the temperature is raised to 650 ° C, and 150 drops/min of kerosene is added, and the ammonia gas flow rate is 0.10 ~0.13 m/h. Strictly control the exhaust gas and ammonia flow during the carbonitriding process, the parts are directly vacuum oil quenched after carbonitriding, and there is no black structure in the microstructure of the infiltrated layer. The depth of the infiltrated layer meets the requirements, but the surface hardness is low and unqualified, indicating that controlling the flow of ammonia gas can effectively avoid the formation of black tissue, but it is difficult to meet the requirements for parts with high surface hardness requirements. Although increasing the carbon potential in the furnace helps to improve the surface hardness, due to the short carbonitriding time, it is easy to cause carbon or ultra-deep layers on the surface of the parts. Therefore, it is necessary to optimize the co-infiltration process, that is, use controlled atmosphere equipment to carry out carbonitriding. Co-osmosis.

4 Conclusion

(1) 12CrNi3A steel parts are easy to form black structure in the carbonitriding layer, which is mainly related to the precipitation of nitrogen molecules and alloy oxides.

(2) The black structure formed in the carbonitriding process of 12CrNi3A steel parts cannot be eliminated by heat treatment such as normalizing.

(3) When a common vacuum carburizing furnace is used for carbonitriding, the exhaust gas before the infiltration and the control of the ammonia flow can effectively avoid the formation of the black structure of the infiltrating layer.

(4) For carbonitriding parts with a surface hardness requirement of ≥82 HRA, measures such as increasing the carbon potential of the furnace atmosphere and using a controlled atmosphere furnace can be used to meet the surface hardness requirements.

Selection of heat treatment equipment: RVN series vacuum nitriding furnace and RVC series vacuum carburizing furnace produced by SIMUWU are excellent choices for completing such processes. Its process performance can fully meet the needs of such hot processing, with good temperature control accuracy, temperature uniformity and gas quenching uniformity, and the production is convenient and fast.