Alloy Steel Castings' Flexibility And Other Performance
Carbon and low alloy steel castings are used in parts for railroad cars, pumps and valves, heavy trucks, construction and mining equipment, and power generation equipment. Good applications utilize the performance of steel with the flexible geometry of a casting. Alloy Steel Castings offer high mechanical properties over a wide range of operating temperatures. Cast steel offers the mechanical properties of wrought steel and can be welded.
A casting part design plays a key role in mechanical performance. Sections of a cast part subject to higher stress can be beefed up while low stress regions can be reduced. This flexibility facilitates a part with optimum performance and reduces weight; both minimize cost. One way to ensure the best casting design is to work with a foundry on design. A foundry will have the technical expertise to partner on the casting the design and material selection.
There are several rules of thumb to the development of a good casting. First, reduce the number of isolated heavy sections and have smooth flowing transitions. It is feasible to cast any geometry but this may increase cost. Junctions within a casting should be designed not to add mass. Changing section thickness in a casting should be through smooth, easy transitions, adding taper and large radii help. Reducing undercuts and internal geometry help to minimize cost. The foundry and customer should agree upon tolerances.
Specifying as-cast tolerances is also important in minimizing cost. Other post-processing such as machining and how the part will be held in a fixture also influence the final cost of the part. Datum points should be stated and machine stock should be added to required locations. Draft is the amount of taper or the angle, which must be allowed on all vertical faces of a casting tool to permit its removal from the mold without tearing the mold walls. Draft should be added to the design dimensions but metal thickness must be maintained. The amount of draft recommended under normal conditions is about 1.5 degrees. Design resources and tutorials are available at www.sfsa.org.
The chemical composition and microstructure of a steel casting determine its mechanical properties. Heat treatment can change microstructure and provide a wide range of mechanical properties. The response to heat treatment for a given section is hardenability. A steel with a high hardenability will have uniform hardness in thicker sections, than ones with low hardenability.