STEEL SELECTION IN ACTION
Suppose a designer needed a steel casting with a 3-in. cylindri- cal section requiring a surface hardness of 28-34 on the Rockwell C scale and a minimum tensile strength of 120 ksi at the half-radius point (0.75 in. below the surface). Would a metalcasting facility’s suggestion of AISI/SAE 8630 steel fulfi ll the requirements? A tensile strength of 120 ksi approximately correlates with 26 Rock-
well C. Using the minimum carbon for the 8630 specifi cation, which is 0.27%, the maximum hardness attainable is about 47 Rockwell C. This is adequate hardness to allow for a tempering temperature high enough to provide good ductility. The quenching media in this example is water with strong agita- tion with an H-value (used to describe the severity of quench, ranging from 0.2-5.0) of about 1.5. In order to determine if 8630 steel would provide the necessary 120 ksi at the half-radius section, consider the equivalent cooling rates. For a 3-in. diameter bar, the hardness at the half radius is equivalent to 0.56 in. on the end-quench test. From the 8630 hardenability bands, the minimum hardness is about 36.5 Rock- well C in the as-quenched condition. Even after tempering at 1,200F (649C), 8630 steel would still have suffi cient hardness at a distance of 0.56 in. from the end of a test bar. This indicates the 8630 steel would be more than adequate to meet the requirements.
treatment for a particular application, the design engineer must be clear on which properties are required. If the required properties and section size are known, the steel alloy composi- tion’s response to heat treatment can be evaluated using hardenability. T e minimum ideal critical diameter (DI) is a single number often used to de- scribe the hardenability of an alloy composition. Casting geometry and section
size is important in determining the eff ectiveness of the heat treatment and hardenability. If the casting under consideration is more plate-shaped than spherical or cylindrical, the DI needs to be larger because plates cool more slowly than cylinders. T is is especially important since more applications for castings will approximate plates than cylinders. T e hardenability requirement for a plate section can be estimated by multiplying the calculated DI by 1.5. Tempering curves are useful in approxi- mating hardness at various locations after quenching and tempering. In carbon and low alloy steels,
chemical composition largely deter- mines hardenability, which in turn dictates the mechanical properties of steel. Carbon is one element that is present in every steel type, and its ef- fect on hardenability must be consid- ered. Increasing carbon increases hard- enability. When selecting an alloy’s composition, start with carbon, which should be as low as possible while still meeting the established objectives. T e higher the carbon, the more prone the steel will be to quench cracking and welding diffi culties. It also has been determined that moderate amounts of several alloying elements are more eff ective in attaining a desired harden- ability than large amounts of one or two elements.
@ 32 | METAL CASTING DESIGN & PURCHASING | Jul/Aug 2012
ONLINE RESOURCE
For a free download of the Steel Founders’ Society of America’s “Steel Castings Handbook,” visit
http://www.sfsa.org/sfsa/pubs/ hbk/s2.pdf.
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