Metal Shaping Processes. Vukota Boljanovic
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The application of casting simulation has been most beneficial toward avoidance of creating shrinkage scrap, improving cast metal yield, optimizing gating system design, optimizing mold filling, and finding the thermal fatigue life in permanent molds.
Many major foundries are using software based on powerful Finite Element analyses such as ProCAST, which covers a wide range of casting processes and alloys. However, several another commercial software programs are now available for modeling casting processes, such as Magma SOFT, Solidia, and AFS solid.
In the early centuries of casting, bronze and brass were preferred over cast iron as foundry materials. Iron was more difficult to cast due to its higher melting temperatures and workers lack of knowledge about its metallurgy. In the second part of the 16th century, the first cannons were cast from iron and after that a large demand for cast iron began to grow, first for military items and later for commercial products. Today, most commercial castings are made of alloys rather than pure metals. Pure metals like zinc, lead, tin, and copper are really good metals for casting but are used only for art, not for commercial products.
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