AWS - Chapter 8 - Cast Irons

Organization: AWS
Publication Date: 1 January 2010
Page Count: 50
scope:

The term cast iron applies to a wide range of ironbase castings with chemical compositions within the following limits: carbon, 1.7% to 4.5%; silicon, 0.5% to 3.0%; manganese, 0.2% to 1.3%; phosphorus, 0.8% maximum; and sulfur, 0.2% maximum. Molybdenum, nickel, chromium, tin, antimony, vanadium, copper, and other elements may be added to enhance specific properties of the cast iron. These elements may be present individually or in combination.1, 2 The weldability of cast irons is relatively poor compared to low-carbon steels. However, consideration of the many favorable properties of cast irons and low production costs leads to the selection of cast irons for numerous applications in foundries, automotive and machine manufacturing, and many other industries. Welding is the preferred process in applications such as the following: 1. Fabrication of simple cast shapes into more complex welded assemblies, which often is more economical than forming the complex shape as a single casting; 2. Fabricating special connections such as shaft-toyoke connections; 3. Joining cast irons to other metals and other wrought shapes; 4. Repair of discontinuities in new castings to meet quality or service requirements or to correct fabrication errors, such as incorrect machining and misaligned holes; 5. Salvaging new castings that have foundry casting discontinuities such as porosity, sand inclusions, cold shuts, shifts, and washouts; 6. Repair of damaged or worn castings that have been in service; and 7. Applying surfaces to build up dimensions, change the metallurgical properties of the casting, or to protect the weldment from corrosion or wear. The fabrication of complex cast iron assemblies by welding continues to be a growing industry, as engineers and metallurgists increase the level of serviceability provided by welds made in cast irons. Improved welding products, such as flux cored arc welding wire, gas metal arc welding wire, and gas tungsten arc welding rods have enhanced the role of welding in cast iron fabrication and repair. Although iron has been used in various forms over two thousand years, iron castings in the forms currently used evolved in Europe in the 15th and 16th centuries. The microstructure of medieval castings basically was gray iron with areas of white iron and nonmetallic inclusions. Over time, refinements in foundry practices and melt control resulted in a more uniform microstructure. Malleabilizing, a method of increasing the ductility of cast white iron by heat treatment, was developed in the early 1700s. About a century later, blackheart malleable iron was developed. Ductile (spheroidal graphite) iron was developed during the 1940s. Ductility was improved by spherodizing the graphite and adding magnesium or rare earth elements, or both. The most recently developed form of cast iron is compacted graphite, a hybrid material with properties ranging between those of gray and ductile iron. The earliest use of cast iron as an engineering material was for low-pressure steam engines, followed by use in frames of large machines and machine components that function at low-impact and slow rates of speed. Because of the vibration-damping characteristics of cast iron, it is a preferred material for applications such as machine bases and large crankshafts. The general properties, metallurgical considerations, and the weldability of the various types of cast iron are discussed in this chapter. The arc welding processes and filler metals suitable for the welding and surfacing of cast irons are discussed. Information on other joining processes, such as brazing, braze welding, and oxyfuel gas welding, and a section on surfacing processes is included. The Applications section of the chapter presents several cast-iron welding projects and case histories with the details of welding procedures discussed in the chapter. A notable application is described: the repair of the cast iron dome of the United States Capitol building, shown on the title page of this chapter. The Safe Practices section at the end of this chapter is brief, deferring to Chapter 17 of Welding Handbook Volume 1, in which a comprehensive discussion of safe practices for the welding industry is presented. It is intended to apply to the entire 9th edition of the Welding Handbook

abstract:

Cast Irons

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