Study On Cylinder Head Made Of Vermicular Cast Iron

As a key component of engine, the head of cylinder is a typical complex casting with thin wall. During the engine operation, the head of cylinder suffers from circulate shock of high temperature (gas temperature reaches above 1100℃ in a short time) and high gas pressure. Therefore, the head of cylinder must have a high quality. For the component of the head of cylinder, it is required to have not only high strength, dense materials, good uniformity, but also good casting formability. It is very difficult to produce the heads of cylinders with simple technical process. In the past years, grey casting iron with low alloy was applied to produce component. For the sake of strength enhancing limit (generally, < 400MPa) and poor casting formability after enhancing strength, grey casting iron is gradually replaced by vermicular graphite cast iron with high mechanical properties and good casting formability.Vermicular graphite cast iron with high strength, good dense materials, good thermal-fatigue resistance and strong wear resistance, better casting formability than that of nodular cast iron and similar to that of grey cast iron, is suitable to produce complex casting with thin wall. So, vermicular graphite cast iron is widely used abroad in recent years. Particularly, the application in making body and head ofcylinders has made great progress. In our country, it is increasingly attracting interests of researchers to produce the body and head of cylinder with vermicular graphite cast iron. Especially, pearlite matrix vermicular graphite cast iron is used to produce the head of cylinder. The weight of casting must be decreased in order to increase power of per unit weight of the engine and decrease amount of gasoline consumption. It is crucial for enhancing market competition to produce high strength casting with thin wall with vermicular graphite cast iron. So we must develop quickly in applying vermicular graphite cast iron on the body and head of engine cylinder.Vermicularizing process is crucial and difficult during producing the head of cylinder of cast iron. The paper designed three kinds of rare earth-based, Mg-based and Ca-based vermiculariser, as showin in table1):Table 1 Experimental schemeMedium-frequency induction furnace of lOOKw was used for melting iron .Molten iron's weight of every furnace was 0.5 kilogram, the temperature taking out was between 1420°C and 1460"C.We designed object component of vermicular graphite cast iron in order to obtain pearlite structure and good casting formability:C: 3.5-4.0%; Si: 2.0-3.0%; Mn: 0.6-1.0%O: 0.2-0.5% Cu: 0.4-0.9%; P <0.08%S: Before treatment:0.04~O.07%; After treatment: 0.015-0.03%Analysis of experimental results: 1: Structural analysis of experimental component:As variety of vermiculariser addition, graphite changed from slice to vermicular shape and spherical shape. There were two experimental component reaching to above 80% in vermicular percentage: (S-5) Graphite form was well vermicular shape, where there was a little of spherical graphite. Its vermicular percentage reached above 80%. There was a little of ferrite around graphite and based structure was pearlite.(S-8) All of graphite was almost vermicular, vermicular percentage of which reaches above 90%. Base structure was pearlite, where there was a little of ferrite. 2: Hardness of experimental component:Mg alloy with rare earth and Ti-Fe alloy vermiculariser was obviously better whether considered from the effect of thickness or its amount of addition on hardness. Ca alloy with rare earth and Mg alloy with rare earth vermiculariser was best and rare earth vermiculariser was worst. 3: Tensile strength of experimental component:The tensile strength of the specimens with addition of rare earth, Mg-Ti alloy and Ca alloy with rare earth and Mg alloy with rare earth vermiculariser gradually decreased on the condition of either different or same amount of addition.In the paper, Shape modulus and ratio of length and width of vermicular graphite were investigated in order to describe vermicular extent and shape character of graphite. And we analyzed their effects on mechanical properties.We had a quenching test and analyzed iron liquid, which was chosenrigorously and treated by vermiculariser, so that we can probe into mechanism of vermicular graphite.Based on the experiment and analysis, the conclusion can be drawn as follow:1. During the test, we could obtain well pearlite matrix microstructure and reduce exiting of the white spot. There were two vermiculariser of more than 80% worm-like graphite: the first vermiculariser was Mg alloy with rare earth and Ca alloy with rare earth; the other was Mg alloy with rare earth and Ti-Fe alloy. And Mg alloy with rare earth and Ti-Fe alloy had the best effect, the highest vermicular percentage, and the little dreg during handling with simple operation.2. The ratio of worm-like graphite' length and width directly affected mechanical properties of vermicular graphite cast iron. On the circumstance of little different width, the ratio of length and width is low and mechanism property is good. On the contrary the mechanism properties is bad.3. The graphite separated out during the primary crystals of vermicular graphite cast iron with spherical shape, which was formed in theprophase of eutectic solidification. The worm-like graphite came out of the spherical graphite. Most of aberrance took place in the prophase of eutectic solidification. The worm-like graphite coming out of vermicular graphite cast iron was formed by sphere going on spherical growth. It can have more amount of the worm-like graphite of vermicular graphite cast iron by increasing remains of Mg and Re and cooling speed.4. The aberration of worm-like graphite and growth of main body took place under the condition of liquid contacting. There are two contacted form between worm-like graphite growth tip and liquid. One was the hollow slot between ahead austenite in solid and liquid interface and worm-like graphite grown tip. The other was liquid channel among austenite. During growth of worm-like graphite, two forms could transform each other. It could prolong time of coexistent growth by reducing speed of solidification, which widened main body and refined eutectic colony and increased vermicular percentage.According to primary experimental results, we tried to produce the head of cylinder by putting the Mg-Ti alloy into vermicular graphite cast iron and we have obtained good results. The results of experiment are shown in table 2 and table 3.Table 2 Chemical compositions of the head of cylinder made of vermicular graphite cast ironSerial number of packageChemical component ( %)RemarkCSiMnPSCuCrReMgTi13.692.430.770.0510.0230.7830.3870.0080.0240.14223.642.510.8190.0470.0240.6920.3540.0080.0200.1433.602.310.7150.0420.0210.7440.3600.0090.0210.14443.662.330.8430.0440.0220.7460.3900.0070.0210.1395-----------63.612.440.7160.0410.0190.5780.2530.0090.0240.13573.722.410.6870.040.0180.5220.2550.0070.0220.14183.952.430.7170.0410.0260.4850.2480.0070.0200.13293.502.220.6510.0410.0150.5580.3230.010.0280.150103.712.440.7790.0470.0200.5820.3130.010.0250.148113.782.440.7910.0470.0260.5870.3100.0090.0240.134123.612.440.770.0420.0190.580.3330.0080.0220.131...