Steel Alloys David Poweleit & Raymond Monroe, Steel Founders’ Society of America, Crystal Lake, Illinois S
teel castings are used in a variety of end-use applications that re- quire heavy-duty components. The castings are used in parts for railroad cars, pumps and
valves, heavy trucks, construction and mining equipment, and power generation equipment. A good steel casting applica- tion requires strength and utilizes the fl exible geometry inherent in the metal- casting process. Steel castings offer high mechanical
properties over a wide range of operating temperatures. Further, cast steel offers the mechanical properties of wrought steel and can be welded to produce multi-piece parts, as well as large structures.
Design Infl uence The metalcasting process offers free-
dom of geometry, allowing design to play a key role in the mechanical performance of steel castings. Sections of a cast part subject to high stress can be enhanced, while low-stress regions can be reduced. This fl exibility can help cast a part with optimum performance and reduced weight, both of which minimize cost. It is feasible to cast any geometry, but doing so may increase cost. In the preliminary design stages, it can
be advantageous to work with a metalcast- ing facility. Metalcasters have the technical expertise to assist with the casting design and material selection. To develop a good casting,
first reduce the number of isolated heavy sections. Junc- tions within a casting should be designed not to add mass. When working with metalcasters, datum points should be stated, and machine stock should be added to required locations. Section thickness in a casting should be changed through smooth, easy transitions, which can be achieved by adding taper and large radii. Draft should be added to the design dimen- sions, but metal thickness must be maintained. The amount of draft recommended under nor- mal conditions is 1.5 degrees. Further, reducing undercuts and internal geometry helps minimize cost. The metalcasting
This steel cam design used the freedom of the casting process to apply metal only where needed.
facility and customer also should agree on tolerances because specifying as-cast tolerances is important in minimizing cost. Other post-processing details, such as machining and how the part will be held in a fi xture, also infl uence the fi nal cost of the part.
Material Infl uence When selecting a steel alloy , it is im-
portant fi rst to know the required proper- ties. The chemical composition and mi- crostructure of a steel casting determine its mechanical properties. Heat treatment can change microstructure and provide a wide range of mechanical properties. Steel with high hardenability will have more uniform hardness in thicker sections than
Effect on Steel Properties Carbon (C)
Manganese (Mn) Silicon (Si)
Nickel (Ni) Chromium (Cr)
steel with low hardenability. In general, adding alloying elements
improves some properties but increases cost and may reduce other properties. However, most elements will increase the hardenability of steel. Carbon should be kept as low as pos-
sible to maximize weldability. Minimizing alloying elements to safely meet the per- formance requirements of the component will reduce cost. Here, metalcasting fi rms can provide assistance with material selection to ensure that the appropriate properties are purchased. The effects of common alloying elements on steel properties are given in Table 1.
Performance Conditions
Table 1. Effects of Common Alloying Elements on Steel Properties Element
Similar, though lesser, effect as carbon Improves toughness Improves oxidation resistance
Improves high temperature strength Improves high temperature strength
Increases strength but decreases toughness and weldability (most common and important)
Similar to carbon but with a lesser effect than manganese (important for castability)
Molybdenum (Mo) Improves hardenability and high temperature strength Vanadium (V) Tungsten (W) Aluminum (Al) Titanium (Ti) Zirconium (Zi) Oxygen (O) Nitrogen (N) Hydrogen (H) Phosphorus (P)
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
Sulfur (S) 20 Metal Casting Design & PurChasing Reduces toughness and ductility
Design requirements are typical- ly determined in terms of strength or maximum stress. The design is com- monly constrained by modulus, fatigue, tough- ness or ductility. Increas- ing the strength of steel normally reduces the duc- tility, toughness and weld- ability. Therefore, it often is more desirable in steel casting design to use a low-strength grade and increase the section size or modify the shape. The de- sign freedom makes casting an attractive way to obtain the best material perfor- mance, as well as the need- ed component stiffness and strength. When designing a part, it is important to understand the limits of
2010 Casting sourCe DireCtory
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