Metal Shaping Processes. Vukota Boljanovic
Casting alloys can be classified in two basic categories: ferrous and nonferrous. Ferrous casting alloys are subdivided into cast iron (gray iron, ductile iron, high-alloy white irons, malleable iron, compacted graphite alloy, and high-alloy graphitic irons) and cast steel (plain carbon steels, low-alloy steels, high-alloy steels, and cast alnico alloys).
Cast iron usually refers to gray cast iron but identifies a large group of ferrous alloys which solidify with a eutectic. In these ferrous alloys pure iron accounts for more than 93%, while the main alloying elements are carbon and silicon. The amount of carbon in cast irons is in the range of 2.11 to 4%, as ferrous alloys with less are denoted “carbon steel” by definition. Cast irons contain appreciable amounts of silicon, normally 1 to 3%, whose composition makes it highly suitable as a casting metal. Cast iron’s melting temperature is between 1150 to 1200°C (2100 to 2192°F), about 300°C (572°F) lower than the melting point of pure iron.
Cast iron tends to be brittle unless the name of the particular alloy suggests otherwise. The color of a fracture surface can be used to identify an alloy; carbide impurities allow cracks to pass straight through, resulting in a smooth, white surface, while graphite flakes deflect a passing crack and initiate countless new cracks as the material breaks, resulting in a rough surface that appears gray.
With their low melting point, good fluidity, castability, excellent machinability, and wear resistance, cast irons have become an engineering material with a wide range of applications, including pipes, machine parts, and car parts.
Cast iron is made by remelting pig iron, often along with substantial quantities of scrap iron and scrap steel, and taking various steps to remove undesirable contaminants such as phosphorus and sulphur.
a) Gray Cast Iron
Gray iron is one of the most easily cast of all metals in a foundry. It has the lowest pouring temperature of the ferrous metals, a characteristic that is reflected in its high fluidity and its ability to be cast into intricate shapes. Gray iron usually contains the following: total carbon, 2.75 to 4.00%; silicon, 0.75 to 3.00%; manganese, 0.25 to 1.50% percent; sulfur, 0.02 to 0.20%; phosphorus, 0.02 to 0.75%. One or more of the following alloying elements may be present in varying amounts: molybdenum, copper, nickel, vanadium, titanium, tin, antimony, and chromium, depending on the desired microstructure. As a result of a peculiarity during the final stages of solidification, the metal has very low and, in some cases, no liquid-to-solid shrinkage, so that sound castings are readily obtainable. For the majority of applications, gray iron is used in its as-cast condition, thus simplifying production. Gray iron has excellent machining qualities, producing easily disposed of chips and yielding a surface with excellent wear characteristics. The resistance of gray iron to scoring and galling with proper matrix and graphite structure is universally recognized. Figure 3.9 shows the form of graphite found in gray iron.
Fig. 3.9 Graphite form in gray iron.
Several molding processes are used to produce gray iron castings. Some of these have a marked influence on the structure and properties of the resulting casting. The selection of a particular process depends on a number of factors, and the design of the casting has much to do with it. Processes using sand as the mold medium have a somewhat similar effect on the rate of solidification of the casting, while the permanent mold process has a very marked effect on structure and properties. Products made from gray cast iron include automotive engine blocks and heads, motor housings, and machine-tool bases.
b) Ductile Iron
Ductile iron, also called ductile cast iron or nodular cast iron, is a cast iron that has been treated while molten with an element such as magnesium or cerium to induce the formation of free graphite as nodules or spherulites, which imparts a measurable degree of ductility to the cast metal. While most varieties of cast iron are brittle, ductile iron is quite easy to form with out braking, as the name implies. The main effects of the chemical composition of nodular (ductile) iron are similar to those described for gray iron, with quantitative differences in the extent of these effects and qualitative differences in the influence on graphite morphology. Fig. 3.10 illustrates graphite form in ductile iron structure.
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.
Fig. 3.10 Graphite form in ductile iron structure.
c) White Cast Iron
White cast iron is unique in that it is the only member of the cast iron family in which carbon is present only as carbide. Due to the absence of graphite, it has a light appearance. The presence of different carbides, depending on the alloy content, makes white cast irons extremely hard and abrasion resistant, but very brittle. When fractured, the surface has a white crystalline appearance that gives the iron its name. An improved form of white cast iron is called chilled cast iron.
When a localized area of a gray cast iron is cooled very rapidly from the melt, cast iron is formed at the place that has been cooled. This type of white cast iron is called chilled iron. Adjusting the carbon composition of the white cast iron can produce a chilled iron casting, so that the normal cooling rate at the surface is just great enough to produce white cast iron while the slower cooling rate below the surface will produce gray iron. The depth of chill decreases and the hardness of the chilled zone increases with increasing carbon content.
Chromium is used in small amounts to control chill depth. Because of the formation of chromium carbides, chromium is used in amounts of 1 to 4% in chilled iron to increase hardness and improve abrasion resistance. These properties make white cast iron suitable for applications where wear resistance is required. Railway brake shoes are an example.
d) Malleable Iron
Malleable irons are cast white that is, their as-cast structure consists of metastable carbide in a pearlitic matrix. If cast iron is cooled rapidly, the graphite flakes needed for gray cast iron do not get a chance to form. Instead, white cast iron forms. This white cast iron is reheated to about 900 to 930°C (1650 to 1700°F) for long periods of time (50 hours), in the presence of materials containing oxygen, such as iron oxide; after that time it is slowly (60 hours) cooled. Upon cooling, the combined carbon further decomposes to small compact particles of graphite (instead of flake like graphite as seen in gray cast iron). If the cooling is very slow, more free carbon is released. This free carbon is referred to as temper carbon, and the process is called malleableizing. The new microstructures can possess significant ductility and good shock resistance properties. Fig. 3.11 shows the microstructure of malleable iron.
Fig. 3.11 Graphite form in malleable iron structure.
Malleable cast iron is used for connecting rods and universal joint yokes, transmission gears, differential cases and certain gears, compressor crankshafts and hubs, flanges, pipefittings, and valve parts for railroad, marine, and other heavy duty applications.
e) Compacted Graphite Iron
In 1949 a now well known material called ductile iron was patented. At the same time that ductile iron was patented, a lesser known material called compacted graphite iron (CGI) was also patented, although it was only considered a curiosity at the