S
Steel Alloys
teel castings are used in a variety of end-use ap- plications that require heavy-duty components. The castings are used for
components of railroad cars, pumps, valves, heavy trucks, construction and mining equipment, and power genera- tion units. A good steel casting ap- plication requires strength and utilizes the flexible geometry inherent in the metalcasting process. Steel castings offer mechanical properties over a wide range of operating temperatures and environments, some of which can be corrosive. Further, cast steel will generally offer advantages of cost, weight and dimensional accuracy over wrought steel, which has to be welded to use multiple pieces to produce parts of the same configuration.
Properties The chemical composition and mi-
crostructure of a steel casting deter- mines its mechanical properties, along with the suitability for a given service environment, which can include high or low service temperatures or corrosive environments. Heat treat- ment also can be used to change the microstructure and provide a wider range of mechanical properties than is available as-cast. The addition of alloying elements
will improve the desired properties, but may reduce other properties. For
sential to know the service environment, the expected designed life of the part, how failure could 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-
This steel casting keeps a 25,000-lb. mining dipper door from opening and closing too fast.
example, alloying for greater strength generally will sacrifice some ductility (Table 1). When selecting a steel alloy, it is es-
Table 1. Effects of Common Alloying Elements on Steel Properties Element
Effect on Steel Properties 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 com- mon 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
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. Con- trarily, the lack of a rolling direction in steel castings 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.
Steel Types Carbon Steel—Steel in which
carbon is the principal alloying ele- ment. 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 hard- ness and strength (Table 2). These steels are the least expensive to manu- facture and account for the largest
tonnage of steel castings produced. High Alloy Steel—Steel alloys with
more than 8% alloy content and superior corrosion, heat and wear resistance. T e 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
2016 CASTING SOURCE DIRECTORY
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