9 T
TIPS FOR
GRAY IRON
DESIGNING WITH
MCDP STAFF REPORT
he success of a casting can be determined before the metal is poured, a design is fi nalized or a pen even touches paper. From the design of the casting, to the metal picked, the success of any cast part is the balance between applied stress (design) and resistance to
stress (metallurgy). Knowledge of the casting process and un- derstanding the limitations and liberties metalcasting facilities have is essential to creating an effi cient part. Before starting any project, match the design of the casting with the material. Gray iron is one of the most versatile and widely cast metals. Consider the strengths and weaknesses of gray iron before selecting it to design an eff ective casting: • Gray iron has a large percentage of carbon in the form of graphite, which aids in the production of economical sound castings in complex shapes and adds dimensional stability under diff erential heating.
• T e material has exceptional damping capacity (the abil- ity to absorb vibration and noise).
• Gray iron is considered the most castable of all ferrous metals, so it is possible to cast and form complicated shapes that cannot be achieved with other metals.
• If an application requires high tensile strength, gray iron may not be the best material to use. Its maximum is 60,000 psi. Other ferrous
34 | METAL CASTING DESIGN & PURCHASING | Sept/Oct 2011 metals can reach more than 150,000 psi.
• Gray iron is ideal for applications that require more compression strength than tensile strength. It has 2.5 to four times more compression than tensile strength, while other metals typically have equal amounts.
• T e shear and torsional strength of gray iron are greater than its tensile strength, while ductile iron typically has less shear and torsional strength than tensile strength.
• Gray iron has relatively poor weldability (i.e. it does not have the ability to be plastically deformed).
• Gray iron is classed as a “brittle” metal because of its low plastic deformation, so designers should avoid stress and strain concentrated in one area.
• Gray iron is sensitive to stress concentration under static stress. However, compared to other metals, gray iron has a relative insensitivity to stress concentration when exposed to fatigue stress, making streamlined design for static loading more important than for fatigue resistance.
• A unique attribute of gray iron is that it varies in moduli of elasticity, or stiff ness, at diff erent tensile strengths. T e elastic modulus increases from an aver- age of 12 million psi at 20,000 psi tensile strength to 20 million psi at 50,000 psi tensile strength.
• T e mechanical properties of gray iron vary with sec- tion size to a greater degree than other metals, and the variation occurs more commonly when using gray iron as-cast, without heat treatments.
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