As one of the main accessories of the internal combustion engine, the piston, piston ring and cylinder liner together form the cylinder assembly, which converts the pressure generated by fuel combustion in the cylinder into mechanical energy for work through the crank connecting rod mechanism; And with the rapid development of technologies such as high pressure common rail, turbocharging and direct injection in cylinder of internal combustion engines and the application of national VI emission standard internal combustion engines, the cylinder explosion pressure exceeds 22MPa, and the gas temperature exceeds 400 ℃. The working environment of high temperature and high pressure puts forward more stringent requirements for internal combustion engines, and the improvement of the overall performance of internal combustion engines mainly depends on the improvement of key components. As the heart part of the internal combustion engine, the piston plays a vital role in the whole engine working process and exhaust emissions. In order to improve the rigidity and high temperature performance of the piston, generally higher strength steel is used to replace the traditional aluminum alloy, and the welded structure is used to replace the integral structure to prepare the piston. The piston made of high-strength steel by welding has been displayed as a key product in the 2012 Hanover International Commercial Auto Show.
At present, most of the national VI pistons sold on the market are made of split forged steel, and then steel pistons are manufactured by welding; At present, steel pistons have been widely used in high-power internal combustion engines; However, we have to see that the current mainstream national VI steel piston 38MnVS6Ti non quenched and tempered steel and 42CrMo4 quenched and tempered steel piston raw material costs are high, and in the production and processing process, there are difficulties in machining, resulting in a sharp rise in processing costs (the consumption of processing tools), while the processing speed is not guaranteed. Therefore, a new material with good processability, low cost and mechanical properties meeting the requirements of high-performance national VI piston and its preparation method are urgently needed; Spheroidal graphite cast iron refers to the cast iron in which the molten iron is spheroidized to make most or all of the graphite spheroidal. The mechanical properties of the spheroidal graphite cast iron after heat treatment are close to that of steel. Many steel castings and forged steel castings are replaced by nodular cast iron.
In the current internal combustion engine market, some enterprises have begun to research and produce a new generation of ball cast iron pistons, but they basically adopt the integral casting method, generally adopting the investment casting method; For example, Komatsu Manufacturing Institute of Japan successfully developed the ductile iron piston for S6D125 engine with a cylinder diameter of 125mm in early years and put it into practical application; After that, ductile iron pistons for engines with cylinder diameters of 140 mm and 170 mm were successfully developed, which gradually expanded the application scope of ductile iron pistons; In addition, Hino Company in Japan is also using ductile iron to produce pistons, but also using the integral casting method. There are many problems in the production of ball cast iron piston by integral casting method, such as difficult process and high cost; Therefore, technicians have studied a new type of split type ball ground cast iron piston. For example, QT600-7 ductile iron is used as the material of split type piston, the piston head and piston skirt are respectively manufactured, and then solid welding (such as friction welding) is carried out to form an integral piston, which can greatly reduce the material cost and the machining cost.