Induction heat treatment process for piston pin carburizing

1. Process and equipment. The carburized steel piston pin is quenched and tempered at low temperature after carburization. Piston pins with higher performance requirements are quenched and tempered twice. The purpose of the first quenching is to eliminate the reticular cementite in the carburized layer and refine the core structure; the second quenching is to refine the carburized layer. Structure and make the infiltration layer obtain high hardness. Piston pins with higher alloy element content need to be cryogenically treated after carburizing and quenching to reduce the amount of retained austenite in the carburized layer. Especially piston pins that require dimensional stability require cryogenic treatment to control retained austenite. quantity. Automobile and tractor piston pin manufacturers generally use 20Cr, 20CrMo steel cold extrusion molding and double-sided carburizing treatment. The depth of the carburizing layer is generally determined based on the wall thickness of the piston pin. The technical requirements in the current relevant standards are not accurate enough. It should be adapted to the technical level of precise control of the carburizing process and further optimize the design, materials, and processes. Combination can reduce costs and improve efficiency while ensuring the service performance of parts. The surface hardness of the piston pin after carburizing heat treatment is 58-64HRC, the hardness difference on the same pin is not greater than 3HRC, and the core hardness is 24-45HRC. The surface microstructure is fine martensite, a small amount of massive carbides are allowed, coarse lumps or continuous network carbides are not allowed, and the carbides grade 1-4 are qualified. The microstructure of the core is lath-shaped martensite and ferrite, and no large pieces of ferrite are allowed. After heat treatment, the inner hole is generally no longer grinded, and the carbon-poor layer should be controlled to be less than 0.03mm.

(1) Carburizing process and equipment. Well-type carburizing furnaces are often used. Drip-type gas carburizing agents generally use no and methanol. Endothermic atmosphere carburizing often uses endothermic atmosphere as carrier gas. Propane or methane is rich. Transform into gas.

(2) Heat treatment process and equipment after carburization. After carburization, it is generally cooled in a cooling well with a water cooling jacket, and methanol is sent into the cooling well for protection to avoid decarburization. In small batch production situations, salt bath furnaces can be used for reheating and quenching equipment. The quenching cooling medium is PZ-2A rapid quenching oil. The quenching hardness is high (>58HRC) and uniform, without quenching soft spots. Use an oil bath furnace for tempering, the tempering temperature is 160-190 degrees Celsius, and the tempering time is 3 hours. In mass production situations, mesh belt furnaces are used as reheating and quenching equipment.

2. Carbide control of the carburized layer of chromium steel piston pin. If the carburization of chromium steel piston pin is not properly controlled, coarse carbide blocks will easily form on the surface; when the amount of carbon on the surface exceeds the eutectoid composition, the cooling rate after carburization will decrease and network carbides will be formed. Coarse massive carbides and network carbides weaken the connection between the infiltration layer and the metal matrix, easily cause stress concentration, and make the infiltration layer brittle, and should be strictly controlled. At present, carbon potential control has been basically popularized, and the application of computer technology in controlling the carburizing process is also becoming more and more perfect. In order to prevent the above carbide defects, production plants that lack carbon potential control methods should accelerate the realization of carbon potential holes and comprehensively optimize carburizing. process (such as carburizing, diffusion temperature, furnace atmosphere and time control, cooling control after carburizing, etc.) to obtain appropriate surface carbon content and carbon concentration distribution, appropriate carbon layer and core microstructure, for carburizing Prepare for post-carbon heat treatment.

3. The impact of hard inner hole carburizing on the fatigue life of the piston pin has a higher fatigue life. Use surveys show that double-sided carburizing not only improves the fatigue life of the piston pin, but can also save a lot of raw materials, simplify the process, and reduce production costs. . Therefore, factories that mass-produce piston pins generally use the double-sided carburizing and hardening process. Only certain piston pins with small production batches and limited manufacturing processes still use single-sided carburizing.

4. Compared with the slow cooling and reheating quenching process after carburizing, the main features of this process are: (1) Due to the microalloying effect of rare earths, mass-distributed carbides are formed in the carburizing layer, making the carbon in the surrounding matrix At a lower concentration, lath-like or fine needle-like martensite is formed after direct quenching. Higher carbon martensite has better toughness and may contribute to fatigue life. (2) Decarburization and carbon depletion of the inner hole surface of the piston pin will reduce its reliability. Reheating and quenching can easily cause decarburization of the inner hole surface, while direct quenching can effectively prevent or reduce decarburization of the inner hole surface.