1. Processing route
The machining routes corresponding to different forms of raw materials are different. At present, the forms of raw materials suitable for manufacturing piston pins generally include hot-rolled round steel, cold-rolled round steel, hot-rolled seamless steel pipe, cold-rolled seamless steel pipe, etc. Therefore, the following different processing routes can be prepared according to the production of piston pins with different raw materials:
1) Hot rolled round steel → rough turning outer circle → drilling inner hole → carburizing → quenching and tempering → grinding → finished product;
2) Cold rolled round steel → drilling inner hole → carburizing → quenching and tempering → grinding → finished product;
3) Hot rolled seamless steel pipe → rough turning outer circle → carburizing → quenching and tempering → grinding → finished product;
4) Cold drawn seamless steel pipe → blanking → carburizing, quenching and tempering → grinding → finished product.
2. Quality control of carburized layer of piston pin (20Cr)
The quality of carburizing layer is mainly to control the size of carbides. Generally, the surface of chromium containing steel is easy to form coarse massive carbides during carburizing, and its formation conditions depend on the carbon content of the surface layer of carburizing layer, or the carbon concentration during carburizing, which has nothing to do with the subsequent cooling mode of carburizing. This carbide can not be eliminated even by direct quenching after carburizing, so it can not be eliminated by normalizing after carburizing. This coarse block carbide weakens the combination of carburized layer and matrix structure, and is easy to cause stress concentration and make carburized layer brittle. Therefore, the solution to this problem: first, limit the carbide to the grinding amount and increase the grinding allowance so as to remove it in the subsequent grinding. Second, the carburizing potential is controlled by microcomputer controlled atmosphere carburizing furnace. The maximum furnace gas carbon potential is 1.25%, the maximum surface carbon potential is 1.15%, the final surface carbon concentration is 0.85%, and the diffusion coefficient is 1.60. At the same time, shorten the carburizing time and prolong the diffusion time to reduce the surface carbon concentration, which can effectively control and reduce the depth of massive carbide.
3. Twice quenching process of piston pin (20Cr)
Improving the quality and service life of automobile piston pin depends on reasonable heat treatment process. The heat treatment process of piston pin is mainly carburizing and quenching process, Carburized steel shall be selected (20Cr) after carburizing, it is generally quenched and tempered at low temperature. However, for piston pin with high performance requirements, the heat treatment process of twice quenching and tempering can be adopted, because the piston pin is carburized at high temperature for a long time, the grain size is significantly coarsened, and the surface layer is easy to form network carburized body. Such poor structure will reduce the strength, wear resistance and brittleness of piston pin The increase, especially the decrease of impact toughness, can be eliminated by twice quenching.
When the first quenched automobile parts are heated above AC3, three-phase recrystallization occurs above this temperature, the core is refined after cooling, and the surface network cementite is dissolved in austenite, so as to improve the strength of the piston pin core. After the second quenching, the automobile parts are heated to a temperature above ACCM. Because the first heating fully austenitizes the core structure to avoid insoluble ferrite, but the high carbon part of the surface layer inevitably tends to overheat and the structure is coarse. In order to improve the surface structure, another heating quenching for hypereutectoid steel is carried out, and the second heating temperature is lower than the previous temperature, Then the surface structure is no longer overheated. As for the core structure, the second heating is reheating in the quenched martensite state, which refines the carburized layer structure, improves the surface hardness of the piston pin and enhances the surface wear resistance. For the piston pin with high alloy element content, cold treatment should be carried out after carburizing and quenching, so as to reduce the residual austenite content in the carburized layer and improve the uniformity and stability of the surface hardness of the piston pin. Twice quenching and tempering process.
4. Effect of Carburization on fatigue life of piston pin inner hole
According to the test results of optical elastic stress analysis, there is a large plane tensile stress between point C and point D in the inner hole of piston pin. In many cases, in order to improve the bending and contact fatigue strength of parts, the residual compressive stress is often produced on the surface of parts by carburizing quenching and low-temperature tempering. Therefore, for the piston pin whose inner hole surface is not carburized and hardened, because there is no residual compressive stress on the inner hole surface of the piston pin, fatigue damage is easy to occur at the point between point C and point D of the inner hole, resulting in the failure of the parts. For the piston pin with carburized and hardened inner hole surface, due to the residual compressive stress on the inner surface of the piston pin, the possibility of fatigue cracks on the inner hole surface is reduced, and the fatigue cracks of parts often occur on the outer surface first. Therefore, carburized and quenched inner surface of the piston pin will greatly improve the fatigue life of the piston pin.
Comparing the fatigue life of the piston pin hardened by carburizing in the inner hole with that hardened by carburizing in the inner hole, the fatigue life of the piston pin hardened by carburizing in the inner hole is significantly improved, the process is simple and the production cost is reduced.