Casting technology of ductile iron piston ring
The piston ring plays the role of sealing, heat transfer and support in the engine, so the piston ring is required to have good wear resistance, elasticity and thermal stability, especially for a ring. The development and application of ductile iron piston ring is a qualitative leap in the history of piston ring manufacturing. How to manufacture ductile iron piston rings stably is a difficult problem facing the piston ring industry. In order to solve this problem, the factory from GOETZEA, Germany, the introduction of single double cast ductile iron piston rings all the technology and related equipment.
The guiding idea of the design of single double piece casting is to milling the center of the casting blank and the upper and lower surfaces which are easy to produce pinholes and slag inclusion defects, so as to obtain excellent nodular cast iron piston rings. This casting method has been widely used abroad, and has rich production experience, many domestic manufacturers have also accepted and started trial production, but it is not very successful. The main reason is that the raw material problems and the production process of ductile iron piston rings are too complex, it is difficult to ensure the matrix structure of ductile iron piston rings, and can not guarantee the qualified rate of castings. Although the factory has introduced all the casting hardware and software, but there are still many problems in the supply of raw materials, production technology and other aspects, the initial stage of trial production often appears the spheroidization rate is not high, the ball number pass rate can not meet the requirements. After trial production and exploration, basically master the production process suitable for our factory, more comprehensive solution to the technical problems of ductile iron piston ring casting, thus ensuring the qualified rate of ductile iron piston ring castings. The following is a brief introduction of how the factory is trying to produce ductile iron piston rings.
1. Process feasibility analysis
German experts have repeatedly stressed that "the key to the production of ductile iron piston rings is raw materials", there is no qualified raw materials is difficult to produce good ductile iron piston rings. They emphasize the use of high purity pig iron or synthetic pig iron, which is divided into: 38% ~ 43% C, 02% ~ 04% SI, 04% ~ 015% Mn, P< 045%, S< 01%, and other elements shall not exceed 03% [1]. There is no such new pig iron resources in China, and the pig iron components that may be supplied in China are: 38% ~ 43% C, 08% ~ 13% Si, 02% ~ 05% Mn, P< 07%, S< 03%. This pig iron affects the key elements Mn, P and S of spheroidization treatment, which brings great unfavorable factors to the production of ductile iron piston rings. At present, it is difficult to find raw material sources or synthetic pig iron that meet the requirements in China, and it is unrealistic to introduce raw materials. Therefore, in the selection of raw materials, should be as far as possible in line with foreign requirements, the test selected pig iron composition: 40% ~ 42% C, 0 80% ~ 09% Si, 0 20% ~ 04% Mn, 0 04% ~ 06%P, S< 026%. This pig iron has the following characteristics compared with the pig iron required by foreign countries: ① The silicon content is ultra-high 0.4%-06%. Due to the use of intermediate frequency furnace smelting, although it will limit the recovery and utilization of the return charge, it will not pose too much threat. ② The manganese content is 0 14% ~ 0 2%. High manganese content will affect the control of the cast structure, which can be solved by the subsequent ferrite annealing treatment. But too much cementite will affect the qualification rate of spheroidization and the size control before and after heat treatment. ③ Phosphorus content is acceptable. Even if the phosphorus content of the new pig iron used later is slightly higher (< 07%), it is not enough to affect the metallographic structure and mechanical properties. If the phosphorus content is too high, it will lead to the phosphorus content of the casting >0 1%, which will produce phosphorus eutectic at the grain boundary, reducing the strength, toughness and plasticity. ④ The sulfur content is over 016%. Higher sulfur content will directly consume the amount of nodulating agent, especially the effect of dissolved sulfur on graphite nodularization during solidification, which will directly affect the effect of nodulating treatment, reduce mechanical properties, and increase casting defects (slag inclusion, subskin pores, etc.).
The use of carbon and sulfur analyzer for the timely grasp of the sulfur content of the original iron liquid, the correct control of the amount of nodulator to create conditions, and the use of M inoculant also for the production of ductile iron piston rings played an auxiliary role. After the above process analysis, the use of domestic raw materials can be produced in line with foreign requirements of ductile iron piston rings.
2. Manufacturability test and analysis
The pig iron of 40% to 100% of the above components and the return charge of less than 60% are melted in a 150kg medium frequency furnace. Strengthen the operation process to ensure that the nodulization temperature reaches 1500 ~ 1550℃, and the time from the molten iron to the completion of pouring is controlled within 10min to ensure the nodulization effect.
2.1 Spheroidizing treatment
Use a customized nodulizer with the following components: 38% ~ 48%Si, 50% ~ 65% Mg, RE≤ 10%, Al≤ 10%. Size: 5 ~ 40mm. Compared with the rare-earth ferrosilicon magnesium alloy or rare-earth ferromagnesia calcium alloy nodulators commonly used in China, the magnesium content of this nodulator is lower than the former, but higher than the latter. The nodularization reaction is relatively peaceful, and under the premise of ensuring the nodularization effect, the addition amount of nodulating agent can be better controlled, the absorption rate of nodulating agent can be ensured, and the drop of liquid iron temperature can be better controlled. In addition, the content of rare earth elements used in the nodulating agent is very low, so that the residual rare earth amount in the liquid iron after nodulating treatment is not too high, and the formation of cementite is reduced. The residual trace rare earth elements in liquid iron can not only inhibit the effect of nodulating interference elements, but also increase the nucleation rate during inoculation.
Through a series of tests, it is found that the different V/O ratio (the ratio of section area to circumference) of the piston ring requires different amounts of spheroidizing agent; The amount of nodulator required for different sulfur content of primary iron is also different. The amount of spheroidizing agent added is the sum of the amount of burning loss, the amount of magnesium consumed by sulfur removal and the amount of residual magnesium.
Since the raw iron liquid temperature, nodulization treatment method, residence time will affect the amount of magnesium loss, in order to correctly control the amount of nodulization agent, to control the factors that change greatly in the production, the nodulization temperature should be controlled at 1500 ~ 1550℃(the best in 1530 ~ 1550℃), the nodulization agent is added by the pit bottom punching method. The pouring time after spheroidizing treatment should not exceed 8min(the best control is within 6min). Under the condition that the operation cycle and process are basically unchanged, it is considered that the amount of magnesium burning is basically the same.
Due to the high sulfur content of the original iron liquid, it is impossible to meet the requirements of <0 018%, and the amount of nodular agent should be taken into account the magnesium consumption of sulfur removal. The rapid determination of carbon and sulfur analyzer can provide the sulfur content of the original iron liquid in time, the sulfur content of the original iron liquid in the test is 0 02% ~ 0 03%, and the sulfur removal magnesium consumption fluctuates up and down in 0 0076%, which is acceptable in the process.
The amount of spheroidizing agent directly affects the amount of residual magnesium. If the amount of nodulating agent is too much, it will cause excessive residual magnesium, although it does not affect the nodulating pass rate (the size of the ball is still normal), but it will increase the shrinkage and cause brittleness. At the same time, due to the high manganese content of the original iron liquid, the addition of too much spheroidizing agent, it is easy to appear carbide, and the white hole tendency is serious, which leads to the decline of the ball number pass rate. If the amount of nodulating agent is too small, it will lead to insufficient residual magnesium, affecting the qualified rate of nodularization, and the qualified rate of ball number will also be reduced. Table 1 shows the results under normal conditions and is described as follows: (1) Wormlike graphite appears due to insufficient residual magnesium content at the dosage of 1 4% spheroidizing agent; When the amount of 1 8% is added, due to the excessive amount of residual magnesium, carbides appear, resulting in a decrease in the number of balls, but the size of the two balls still meets the requirements; ③ Spheroidization qualification rate of non-spheroidization rate.
