Study Of Hardenable Cast Iron For Engine Tappets

The name, so called hadenable cast iron, comes from American English. It is referred to a kind of mottled iron, an intermediate iron between white iron and grey iron. A microstructure after being quenched is requested to consist of needle-like carbide and graphite in flakes in a matrix of martensite.When two mechanical parts moves against each other rapidly in a circumstance of lubricating oil, the oil at high speed brushes the surfaces of the parts and makes the surfaces erosion worn. The soft phases in microstructure are first worn away. Pits and furrows are made, which absorb and store oil like oil pools. Graphite also has the performance of absorbing oil. Pits, furrows and graphite make the oil films well continuous sticking on the surfaces of the parts.Obviously hardenable cast iron possesses a good performance of wear-resistance and is widely used to produce engine lifters and camshafts.The formation of carbide conflicts with that of graphite, especially, those in flakes. It is easy to obtain merely either needle-like carbide or graphite. But it is difficult to get a mixture of the two phases of carbide and graphite in flakes in one microstructure, especially, in the case that certain percentage and configuration of the two phases are requested simultaneously. In the commercial producing practice the producers are facing two embarrassing things as following:Either the obtained carbide is excessive with graphite in nodules, or the graphite in flakes with insufficient carbide. None of several companies at home producing hardenable cast iron lifters has solved this technological problem. The microstructure of the products is far from the standards with high rate of rejects. In addition some of them have been bankrupted. It is of great importance to solve this problem obtaining a stable microstructure of needle-like carbide and flake graphite of type A or E, or a mixture of type A and E.Nucleation and its growth during solidification are the universal law for crystallization. Study on nucleations and their growths of carbide and graphite becomes an essential method for stably obtaining carbide and graphite. It proves by experiments:The addition of carbide-forming elements Mo,Cr and Mn enhances the ability of nucleation of carbide and stabilizes it.Strong pregnancy processing promotes the nucleation ability of graphite making it grow easily in flakes.Primary austenite isolates the growths of carbide and graphite preventing them from conflicting. The coexistence and comparative amount of the carbide and graphite are guaranteed.Rapid cooling makes primary austenite play a stronger role in isolating. The coexistence is made more stable. Rapid cooling also makes every phase in the microstructure finer and better distributed.After a long and hard journey of experimentation, the key technological parameters have been searched out and mastered. The successful rate of casting, in respect of metallographic microstructure, has reached to 100%. The metallographic structure has reached or exceeded American standard and the standard of Beijing Jeep Corporation.