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Ductile iron pieces of sand mold casting process and common defects

by:XEX     2021-03-29
Ductile cast iron with its excellent performance, in use can replace expensive sometimes cast steel and forging steel, widely used in mechanical manufacturing industry. Metallurgical industry in the past has been that nodular cast iron was invented by the British in 1947. Some western scholars even claimed that, without modern technology, invention of ductile cast iron is unthinkable. In 1981, ductile iron experts using modern scientific methods in our country, the study of excavated ancient jun 513 iron, our country has been through a lot of data to conclude that han dynasty spheroidal graphite cast iron. Read the paper on the 18th world conference on scientific and technological development, stir the casting and scientific and technological development. History of metallurgy experts in 1987 after verification that the ancient Chinese have been groped to made of cast iron diffusion technique and the regularities of the ductile iron it to the world history of metallurgy to staging generation is of great significance. It will cast iron often quenching and tempering treatment at low temperature. The process is: all kinds of casting heated to 860 - All the temperature of 900 ℃, insulation to the original matrix again after austenitizing cooling achieve quenching in oil or molten salt, after 250 - 350 ℃ heat insulating tempering, the original matrix into tempered martensite and retained austenite, the original spheroidal graphite morphology is constant. After processing of castings with high hardness and a certain toughness, retained the lubrication performance of the graphite, to improve abrasion resistance. Is through a special process formula of precoated sand technology to produce excellent high temperature performance ( High strength under high temperature, heat time is long, small thermal expansion amount, low gas evolution) And a new type of coated sand casting performance. Especially suitable for complicated thin-walled precision of precoated sand cast iron parts ( Such as the automobile engine cylinder block, cylinder head, etc. ) As well as the high demand of steel ( Such as container Angle and the train brake slow; The machine shell, etc. ) Production, which can effectively eliminate the adhering sand, casting defects such as deformation, cracking and porosity. Can make the casting surface is bright and clean, size accurate, reduce or do not need mechanical processing, can shorten the production cycle, saving metal material and reduce the cost; And mold available automatic machinery made ( Shoot core machine must be used) And don't need skilled workers; Especially suitable for batch and mass production of various kinds of metal small to medium sized castings. Sand casting casting defects are: cold shut, misrun, porosity, adhering sand, sand inclusion, sand holes, heaving sand, etc. Cold insulation and misrun liquid metal filling ability is insufficient, or filling conditions is poorer, before the cavity is filled, stopping the flow of liquid metal, will make the casting produces misrun and cold insulation defects. Misrun, can make the casting can't get a complete shape; Cold insulation, castings, though a complete shape can be obtained, but due to being incomplete fusion seam, the mechanical properties of castings was badly damaged. To prevent the misrun and cold shut: increasing pouring temperature and casting speed. Porosity gas liquid metal in the crust before failure to escape, generated by the hole defect in the castings. Porosity of the lining is smooth, bright or with mild oxidation color. After the appearance of porosity in the casting, will reduce the effective bearing area, and can cause stress concentration around the hole and reduce impact resistance and fatigue resistance of the casting. Porosity also decreases the density of the casting, some requirements under hydrostatic test casting scrap. In addition, corrosion resistance and heat resistance and porosity of castings has bad influence. Prevent the formation of pores, reduce the gas content in the liquid metal, increase the permeability of sand mold, as well as in the high places of the cavity additional gas riser, etc. Stick adhesive on the surface has a layer of sand casting is difficult to remove sand called the adhering sand. Adhering sand both affect the casting appearance, and increase casting cleaning and cutting the amount of work, even affect the service life of the machine. Such as tooth surface is easily damaged when adhering sand casting, pump or engine if the machine parts such as adhering sand, will affect the fuel oil, gas, lubricating oil and cooling water, such as fluid flow, and defiled and wear the whole machine. Prevent adhering sand: join the pulverized coal in the sand, and the mold surface besmear brushs paint adhering sand, etc. Sand inclusion in the groove and scar tissue defects of the casting surface, the casting parts with wet type casting large and thick plate when easy to produce. Casting parts are mostly produced in sand and sand table face contact, cavity on the surface of the metal in the effect of radiant heat, easy to hog and warp, when the cock of the sand by the metal flow during continuously scouring may rupture, remain in place or being into the other parts. On the casting surface, the greater the sand volume expansion, the greater the formation sand inclusion of bias. Sand holes in the casting internal or surface dominates sand hole defect. Heaving sand under the action of the pressure of liquid metal casting, mold wall type of mobile, local swell of casting defects. , sand mold strength should be increased in order to prevent the expansion of sand, sand box stiffness, increase the tank of the closet or fastening forces, and appropriately reduce the pouring temperature, the metal liquid surface crusting, early liquid metal in order to reduce the pressure of mold.
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