iron, which has the lowest strength and highest ductility. Alloyed iron—This classifi cation in-
cludes gray irons, ductile irons and white irons that have more than 3% alloying elements (nickel, chromium, molybde- num, silicon or copper). Malleable irons are not heavily alloyed because many of the alloying elements interfere with the graphite-forming process that occurs dur- ing heat treatment.
Design Considerations Strength and hardness are sensitive to
section thickness, specifi cally in gray iron. In thin sections, the material can be hard and diffi cult to machine. In heavy sections, its strength is reduced signifi cantly. Because the modulus of elasticity for gray iron is higher in compression than it is in tension, the use of standard structural formulas results in a conservative design. Ductile iron has the ability to be used as-cast and without heat treatments or other further refi ning. It can be cast in a wide range of casting sizes and section thicknesses; however, thinner sections may require annealing to obtain high
ductility. Alloy additions may be needed to obtain the higher-strength grades in heavy sections. CGI has benefi ts in tensile strength, stiff-
ness, fatigue behavior and strength-to-weight ratios for moderately thin and medium sec- tion castings compared to other irons. White iron is unique in that it is the
only member of the cast iron family in which carbon is present only as a car- bide. The presence of different carbides, depending on the alloy content, makes white iron hard and abrasion-resistant but also very brittle. Malleable iron is ideal for thin-sec-
tioned components that require ductility. Ferritic malleable iron is produced to a lower strength range than pearlitic mal- leable iron but with higher ductility. It is the most machinable of cast irons, and it can be die-strengthened or coined to bring key dimensions to close tolerance limits. However, ductile iron is replac- ing malleable iron in many applications because the engineering properties of ductile iron are almost identical to that of malleable iron, and ductile iron does not require extensive heat treatment to precipitate graphite.
METAL Online Resource
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www.metalcastingdesign.com to read more articles on iron castings.
Cast in white iron, this static mixer tube is highly abrasion resistant, so it can with- stand the high velocity mixing and cleaning of coal powder.
20 METAL CASTING DESIGN & PURCHASING 2011 CASTING SOURCE DIRECTORY
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