Effect Of The Amount Of Rare Earth Vermicular Agents On The Microstructure And Properties Of Compacted Graphite Cast Iron

Rapid development in economy requires high-performance diesel engines with great power-mass ratio, which demands for a noval material with higher strength, dimensional accuracy, and thermal fatigue resistance for cylinder head block. Compacted graphite cast iron, with excellent comprehensive mechanical properties, is coming into sight of designer of engine in recent years. However, there are still many technological problems, sepesially controlling of vermicularizing process, to be solved. As a result, the quality of cylinder head block from compacted graphite cast iron failed to meet the standard, GB/T 26656-2011. It is obvious that vermicularizing process of graphite in melt strongly depends on kinds and amount of vermicular agent, pouring process, and operation conditions. Sepecially, vermicular agent is one of the most noticeable faters. So, it is very important to investigate the relationship between amount of vermicular agent, variation of graphite morphology and properties of casting.In this work, a varying amount of vermicular agents, QRMg8RE5 or QRMg7HRE2, were introduced into melt under the conditions of constant total amount of silicon in cast iron, respectively. And the effects of vermicular agents amount on tensile strength, elongation, hardness, impact toughness of cast iron and the graphite morphology were investigated.The results show that with the increase of the amounts of vermicular agents, the graphite morphology gradually transformed from the flake to vermicular, even to spherical. The precent of vermicular graphite in cast iron treated with 0.8wt% QRMg8RE5 reaches 80%~85%, and a 3~5 ratio of length to width, belong to the types II in ASTM. While the graphite of cast iron treated with 0.8~0.9wt% QRMg7HRE2, a great majority of the graphite is vermicular. The percent of graphite vermiculation in cast iron treated with 0.8wt% QRMg7HRE2 reaches 85%~90%, and vermicular graphite with smaller length and thinner thickness, is quite similar to the flake. The percent of vermiculation in cast iron treated with 0.9wt% QRMg7HRE2 reaches 80%~85%, besides, the size of graphite increases. The graphite in cast iron treated with less than 0.7wt% vermicular agents mainly growths in the form of flake. And in excess of 0.9wt%, the graphite in cast iron crystallizes mainly a spheroid.There is similar effects of the amount of QRMg8RE5 and QRMg7HRE2 in the melt on tensile strength, elongation, hardness and impact toughness of cast iron. The melt treated with the two kind of agents of 0.6~1.0wt%, separately, the properties above all increase with increase of vermicular agents. There is an increase in hardness of cast iron with 1.1wt% QRMg7HRE2, meanwhile a fall in tensile strength, elongation and impact toughness. The tensile strength, elongation, hardness and impact toughness of cast iron treated with 0.8wt% QRMg8RE5 reach 340.69 MPa, 1.93%, 257 HBW, and 12.98J/cm2, respectively. The cast iron treated with 0.9wt% QRMg7HRE2 possesses excellent performance, a tensile strength of up to 435.33 MPa, elongation of 1.9%, hardness of 256 HBW, and impact toughness of 17.39J/cm2.In addition, it is also found that the tensile strength, elongation and impact toughness of cast iron treated separately with 0.8 wt% QRMg8RE5 or QRMg7HRE2 increase with amount of silicon increasing from 2.1wt% to 2.3wt%, under the condition of a constant total amount of silicon in cast iron.