Use of high-strength cast iron for the production of pipe fitting parts

Abstract

High-strength cast iron with spheroidal graphite is one of the promising constructional materials. In the plants of the industry high-strength iron is used for the production of critical castings forthe parts of paper making machines, polymer equipment, and industrial pipe fittings. In contrast to gray (GOST 1412-79) and malleable irons and carbon (GOST 1050-74) and low-alloy steels high-strength iron combines high production, mechanical and service properties.For example, in high-strength iron in comparison with 20 steel the yield strength is 1.2-1.8 times higher, the stress-rupture strength at high temperatures 1.4 times higher, the creep limit at 370-450~ 1.3-2.4 times higher, and the cyclic toughness 2-3 times higher. The fatigue strengths of high-strength iron and 20 steel are the same. In addition, in comparison with molten 20 steel high-strength iron has a lower heating temperature of the molten metal, which decreases the electric power consumption for melting, a higher fluidity, which makes it possible to cast thin-walled parts of complex configuration from it, and a reduced tendency toward volumetric shrinkage, which increases the output of acceptable castings of high-strength iron by 30-40%. In addition, high-strength iron has higher machinability, which makes it possible to increase the cutting speed by 25% in comparison with steel castings, and 18-20% lower part weight with similar geometric dimensions as the result of thedifference in specific gravities of the materials. With proper selection of the chemical composition and heat treat cycles high-strength iron parts have higher wear and corrosion resistances.For the purpose of use of high-strength irons for castings of industrial pipe fitting parts produced by the P. Montin Baku Oil Field Machinery Plant, the All-Union Scientific-Research Institute for Fitting Technology has conducted investigations of the wear and corrosion resistances of high-strength irons with different metallic bases. The corrosion resistance tests of high-strength iron specimens were made in various media for 520 h. The results of these tests are shown in Table i. As may be seen, in weakly aggressive media high-strength irons have quite high corrosion resistance corresponding to a rating of 4 (GOST 13819-69).Specimens of high-strength irons were tested in a rubbing pair with surface-hardened 20Kh13 steel. The results of these tests are shown in Table 2. As may be seen, the wear resistance of the high-strength irons is greater than the wear resistance of 20Kh13 steel. This may be explained by the fact that the spheroidal graphite precipitated on the contact surface of the iron is a solid lubricant. With an increase in hardness of the high-strength iron its wear decreases but with an increase in specific pressure on the contact surface it increases. In water the wear resistance of the high-strength irons increases, which m~y be explained by the formation on the rubbing surface of strong oxide films with an increased hardness. The high-strength irons have the greatest wear resist...