Machinery's Handbook, 31st Edition
1482 MATERIALS FOR IRON AND STEEL CASTING ductility. Its toughness is intermediate between that of cast iron and steel, and its shock re- sistance is comparable to ordinary grades of mild carbon steel. Melting point and fluidity are similar to those of the high-carbon cast irons. It exhibits good pressure tightness under high stress and can be welded and brazed. It can be softened by annealing or hardened by normalizing and air cooling or oil quenching and drawing. Five grades of this iron are specified in ASTM A536-84 (R2014), “Standard Speci - fication for Ductile Iron Castings.” The grades and their corresponding matrix micro - structures and heat treatments are as follows: Grade 60-40-18, ferritic, may be annealed; Grade 65-45-12, mostly ferritic, as-cast or annealed; Grade 80-55-06, ferritic/pearlitic, as-cast; Grade 100-70-03, mostly pearlitic, may be normalized; Grade 120-90-02, mar- tensitic, oil quenched and tempered. The grade nomenclature identifies the minimum tensile strength, on percent yield strength, and percent elongation in 2 inches. Thus, Grade 60-40-18 has a minimum tensile strength of 60,000 psi, a minimum 0.2 percent yield strength of 40,000 psi, and minimum elongation in 2 inches of 18 percent. Several other types are commercially available to meet specific needs. The common grades of ductile iron can also be specified by just the Brinell Hardness Number, although the appropriate microstructure for the indicated hardness is also a requirement. This method is used in SAE Specification J434C for automotive castings and similar applications. Other spec - ifications not only specify tensile properties but also have limitations in composition. Austenitic types with high nickel content, high corrosion resistance, and good strength at elevated temperatures, are specified in ASTM A439-18. Ductile cast iron can be cast in molds containing metal chills if wear-resisting surfaces are desired. Hard carbide areas will form in a manner similar to the forming of areas of chilled cast iron in gray iron castings. Surface hardening by flame or induction methods is also feasible. Ductile cast iron can be machined with the same ease as gray cast iron. It finds use as crankshafts, pistons, and cylinder heads in the automotive industry; forging hammer anvils, cylinders, guides, and control levers in the heavy machinery field; and wrenches, clamp frames, faceplates, chuck bodies, and dies for forming metals in the tool and die field. The production of ductile iron castings involves complex metallurgy, the use of special melting stock, and close process control. The majority of applications of ductile iron have been made to utilize its excellent mechanical properties in combination with the castability, machinability, and corrosion resistance of gray iron. Steel Castings.— Steel castings are especially adapted for machine parts that must with stand shocks or heavy loads. They are stronger than wrought iron, cast iron, or malleable iron, and are very tough. The steel used for making steel castings may be produced either by the open-hearth, electric arc, side-blow converter, or electric induction methods. The raw materials used are steel scrap, pig iron, and iron ore, the materials and their pro portions varying according to the process and the type of furnace used. The open-hearth method is used when large tonnages are continually required, while a small electric fur- nace might be used for steels of widely differing analyses, which are required in small lot production. The high frequency induction furnace is used for small quantity production of expensive steels of special composition such as high-alloy steels. Steel castings are used for such parts as hydroelectric turbine wheels, forging presses, gears, railroad car frames, valve bodies, pump casings, mining machinery, marine equipment, engine casings, etc. Steel castings can generally be made from any of the many types of carbon and alloy steels produced in wrought form and respond similarly to heat treatment; they also do not exhibit directionality effects that are typical of wrought steel. Steel castings are classified into two general groups: carbon steel and alloy steel. Carbon Steel Castings.— Carbon steel castings may be designated as low-carbon me- dium-carbon, and high-carbon. Low-carbon steel castings have a carbon content of less than 0.20 percent (most are produced in the 0.16 to 0.19 percent range). Other elements present are: manganese, 0.50 to 0.85 percent; silicon, 0.25 to 0.70 percent; phospho- rus, 0.05 percent max.; and sulfur, 0.06 percent max. Their tensile strengths (annealed
Copyright 2020, Industrial Press, Inc.
ebooks.industrialpress.com
Made with FlippingBook - Share PDF online