| Gray cast iron is the most frequent material using in cylinder liner production which is one of the critical parts of engine. But the thermal fatigue strength, hardness and wear-resisting properties of gray cast iron could not meet requirements for light weight, high speed and high power with the development of engine. Vermicular cast iron has been used in cylinder cap and cylinder block of engine because of its excellent thermal fatigue strength and wear-resisting property, but not in cylinder liner product yet.The effect of vermicular agent, inoculator, alloy element, sulfur content and carbon equivalent on graphite morphology and matrix of vermicular cast iron were investigated systematically, using optical microstructure (OM), scanning electric microscopy (SEM), energy spectrum analysis, micro-hardness, tension and rapid-wear test.The results indicate that good graphite form and matrix with fewer impurities could be obtained by adding proper content of vermicular agent and using post inoculation method. When post inoculant is insufficient, the matrix has a great chilling tendency; and excessive post inoculant leads to more spheroidal graphite and more ferrite in the matrix. The best nucleating technology is 0.8%Si-Fe alloy and 0.2%Si-Sr alloy. Flake graphite are generated in cylinder liner when the amount of vermicular alloy is small; with the amount of vermicular agent increasing, graphite morphology changes into vermicular and there are lots of spheroidal graphite when the amount is excessive, because of the over limit value of RE残and Mg残in the liquid iron.The effect of vermicular treatment is obtained when RE/Mg value is within 9-10 range in vermicular agent. The proper adding amount would also be broadened (8%-9%) by adjusting RE/Mg value of vermicular agent. This method makes vermicular control easier, and reduces the difficulty of mass production. The proper contents of RE残and Mg残are 0.050～0.065 and 0.005～0.010, respectively. More vermicular agent is needed when liquid iron with 0.03% S and the steady vermicular treatment is more difficult to achieve than that of liquid iron with 0.015% S.Vermicular cast iron with high content of pearlite matrix could be obtained by adding Cu,Ti,Cr,Mo alloy elements, adjusting carbon equivalent to 4.3～4.6 and adopting proper heat treatment.The tensile strength of vermicular cast iron with pearlite matrix is 459.2 MPa, which is 50% higher than that of gray cast iron. The mechanical property of vermicular cast iron could be improved after normalizing treatment, owing to grain size, distribution and shape of pearlite change better. The hardness of different zone in cylinder liner changes a little:the hardness of ectotheca is 245HBW, center and inwall are 236 and 239HBW, respectively. Relative wear resistance of cylinder liner with pearlite matrix and piston ring is about 40% higher than that of gray cast iron.After all, we could obtain vermicular cast iron cylinder liner with vermicular graphite form and high pearlite rate in matrix. The optimized centrifugal casting technologies are as follows:chemical composition (C3.5%～3.7%,Si2.3%～2.5%,Mn0.4%～0.5%,P<0.04%,S≤0.015%,Cr0.15%～0.2%,Cu0.9%～1.0%,Ti0.05%～0.06%,Mo0.3%～0.4%,CE4.3-4.6); pouring temperature:1500～1550℃; adding high-quality vermicular alloy (RE/Mg=9～10); post inoculation (0.8%Si-Fe alloy+0.2%Si-Sr alloy); heat treatment technology:1050℃,4h. |
PDF Full Text Request