The Microstructures, Mechanical Properties And Fracture Characteristics Of High Strength Synthetic Gray Cast Iron

Gray cast-iron usually is prepared with recycled casting and pig iron as the mainly raw metallic materials, and introduced some scrap steel to decrease the carbon content in the iron melt and even added some alloy elements to adopt special applications. However, the cost of the casting increases because of the increasing of the price of the pig iron. Actually, fabricating synthetic cast iron that uses scrap steel and furnace carbon pickup and external furnace inoculation technologies can produce high quality castings. To adopt this technology is because the price of scrap steel is lower than that of pig iron to be used by conventional casting technology. Compared with the casting produced by conventional casting technology, the synthetic technology has some advantages, such as:lower cost, better mechanical properties, etc. In this study, we produced synthetic gray cast irons with different quantity of scrap steel and studied their microstructures, mechanical properties and fracture characteristics. During testing, we used an intermediate frequency induction furnace with 500kg melting capacity to melt alloys, resin bonded sand moulding method to prepare specimen. The tapping temperature and pouring temperature were 1480-1500℃and 1350℃, respectively during testing. HT300 high strength gray cast irons were prepared by using synthetic and conventional technologies. The compositions of iron melt are:2.80%-3.00%C,1.20%-1.40%Si,0.7%-0.9%Mn,0.06%-0.08%S,0.5%-0.6%Cu. And the addition content of scrap steel was changed from 35%to 60%and to 70%for realizing three different runs during testing. The influence of different addition contents of scrap steel on graphite, eutectic cell structure, primary austenite dendrite morphology, tensile properties and fracture mechanism was analyzed. The main conclusion is as follows:1 Compared with gray cast iron prepared by conventional technology, the synthetic gray cast iron has higher eutectic degree, relative strength and maturity, and lower relative hardness and hardenability, which resulted in higher quality index. While high strength, low hardness, good processing properties and sensitivity of wall thickness are benefit in improving the mechanical properties of casting.2 The size of grain and distance of pearlite lamellar structure in gray cast iron decreased with the increase of addition content of scrap steel. And the quantity of eutectic also increased from NO.4 to NO.1. Undercooled graphite was decreased. A type of graphite increased, and the length decreased from NO.4 to NO.5. With the increase of quantity of primary austenite dendrite in gray cast iron, the dendrite refined and its pattern became well-developed, and the spacing between the secondary dendrite arms decreased, which is propitious to a developed, consecutive, complex rectangle frame construction. Accordingly, mechanical properties had been improved remarkably.3 Gray cast iron prepared from conventional technology showed a high brittleness, and the fracture mechanism was due to typical cleavage fracture. From microstructure, some plastic areas were observed which indicated synthetic gray cast iron possess definite local "toughness", and the fractural mechanism was due to intergranular and quasi-cleavage cracks.4 The sample was corroded by the two erodents could widen the boundary of eutectic cell. The mixture of boracic acid and sulfuric acid could be used as the special erodent to show the microstructure of primary austenite dendrite.