Steel Alloys S
teel castings are used in a variety of end-use applica- tions that require heavy-duty components. The castings are used for components
of railroad cars, pumps, valves, heavy trucks, construction and mining equip- ment, and power generation units. A good steel casting application requires strength and utilizes the flexible geome- try inherent in the metalcasting process. Steel castings offer mechanical prop- erties over a wide range of operating temperatures and environments, some of which can be corrosive. Further, cast steel will generally offers advantages of cost, weight and dimensional accuracy over wrought steel, which has to be welded to use multiple pieces to pro- duce parts of the same configuration.
Properties The chemical composition and
microstructure of a steel casting deter- mine its mechanical properties, along with the suitability for a given service environment, which can include high or low service temperatures or corrosive environments. Heat treatment also can be used to change the microstructure and provide a wider range of mechani- cal properties than is available as-cast. The addition of alloying elements
will improve the desired properties, but may reduce other properties. For example, alloying for greater strength
Effect on Steel Properties
the expected designed life of the part, how failure may occur, and which alloy can address the design criteria. Alloying elements are added to improve perfor- mance, but will also add to cost; therefore, the best practice is starting with a carbon steel and building from there. Cast and wrought materials dif-
fer in their structural direction. The structure of sections of rolled steel is elongated in the direction of rolling. Strength and ductility are improved in that direction but reduced across the rolling direction. Cold rolling steel also can strengthen the steel, but it reduces ductility and toughness. Contrarily, the lack of a rolling direction in steel cast- ings gives them uniform properties in all directions. Cast steel grades achieve the same tradeoff through alloying and heat treating. Therefore, casting grades with similar mechanical properties to wrought grades are designated with a different name (ASTM A216 grade [wrought carbon with grade B] is the cast counterpart to wrought 1020). ASTM A915 and A958 use grade names similar to their wrought counterparts.
This steel casting keeps a 25,000-lb. min- ing dipper door from opening and closing too fast.
generally will sacrifice some ductility (Table 1). When selecting a steel alloy, it is es- sential to know the service environment,
Table 1. Effects of Common Alloying Elements on Steel Properties Element
Carbon (C)
Manganese (Mn) Silicon (Si)
Nickel (Ni) Chromium (Cr)
Molybdenum (Mo) Vanadium (V) Tungsten (W) Aluminum (Al) Titanium (Ti) Zirconium (Zi) Oxygen (O) Nitrogen (N) Hydrogen (H) Phosphorus (P) Sulfur (S)
Increases strength but decreases toughness and weldability (most common and important)
Similar, though lesser, effect as carbon
Similar to carbon but with a lesser effect than manganese (important for castability)
Improves toughness Improves oxidation resistance
Improves hardenability and high temperature strength Improves high temperature strength Improves high temperature strength
Reduces the oxygen or nitrogen in the molten steel Reduces the oxygen or nitrogen in the molten steel Reduces the oxygen or nitrogen in the molten steel Negative effect by forming gas porosity Negative effect by forming gas porosity In high quantities, results in poor ductility
Can increase strength but drastically reduces toughness and ductility Reduces toughness and ductility
22 METAL CASTING DESIGN & PURCHASING
Steel Types Carbon Steel—Steel in which car-
bon is the principal alloying element. Commonly referred to as steel, straight carbon steel, plain carbon steel or AISI- SAE 1000 series steel. Low Alloy Steel—Steel limited to a
total alloy content of 8%. It can be heat treated to higher levels of hardness and strength (Table 2). These steels are the least expensive to manufacture and ac- count for the largest tonnage of steel castings produced. High Alloy Steel—Steel alloys with
more than 8% alloy content and supe- rior corrosion, heat and wear resistance. The two primary alloying elements of these steel are chromium and nickel. Manganese Steel—A low strength,
high-ductility, wear-resistant material used in demanding applications to avoid unexpected fractures. It also can be work-hardened from an initial hardness of 240 BHN to more than 500 BHN. Corrosion-Resistant Steels—Also
commonly referred to as stainless steels, this group contains a minimum of 12% chromium or chromium and nickel. Ap- plications include chemical processing and power generation equipment located
2011 CASTING SOURCE DIRECTORY
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