iron can be produced consistently by simply selecting the proper melt- ing stock. Iron castings in the higher strength grades require closer control of their processing and composition. Alloys can be added to gray iron to increase its strength and hardness in the as-cast condition. T e metal also can be heat treated to a low hardness to increase machinability, but this decreases its strength. An important infl uence on gray
iron properties is the eff ective sec- tion thickness in which it is cast. T e thicker the metal and the more compact the casting, the slower the liquid metal will solidify and cool in the mold. In gray iron, slower solidi- fi cation of the casting will produce a lower hardness. Alternately, iron that is cast with thin sections will solidify more rapidly. A casting with varying section thick- nesses exhibits diff erences in hardness between the thick and thin sections. T ese casting structure variations produce dif- ferences in mechanical properties.
Ductile Iron Ductile iron, also
referred to as nodular iron or spheroidal graphite iron, was patented in 1948. After a decade of intensive development in the 1950s, ductile iron had a phenom- enal increase in use as an engineering material during the 1960s, and the rapid increase in commercial ap- plication continues today.
“
Table 3. Property Comparisons for Gray Iron Classes Property
Class 25 (as-cast)
Brinell Hardness Tensile Strength
Modulus of Elasticity Tensile Poisson’s Ratio
Compression-to-Tensile Strength Ratio
187
Class 30 (as-cast)
207
Ductile iron can be a low-cost alternative to wrought steel products because of its elevated strength-to-weight ratio, castability and machinability.”
An unusual combination of
properties is obtained in ductile iron because the graphite occurs as spher- oids rather than as individual fl akes as in gray iron. T is mode of solidi- fi cation is obtained by adding a very small but specifi c amount of magne- sium to molten iron. T e allowable content of minor elements that can interfere with the graphite spheroid formation in the base iron is severely restricted. T e added magnesium reacts with the sulfur and oxygen in the molten iron and changes the way the graphite is formed.
Ductile iron can be a low-cost
Ductile iron features graphite spheroids created when added magnesium reacts with the sulfur and oxygen in the molten iron.
alternative to wrought steel products because of its elevated strength- to-weight ratio, castability and machinability. Many products that were produced from machined or fabricated steel in the past are cast in ductile iron today. Ductile iron ex- hibits a linear stress-strain relation, a considerable range of yield strengths and ductility. Castings are made in a wide range of sizes with thin or thick sections. Ductile iron castings are used in a number of automotive ap- plications, including crankshafts, camshafts, exhaust manifolds, pis- ton rings and cylinder liners. In the earth- moving, mining and agricultural industries, ductile iron castings are used for couplings, hydraulic valves, sprock- ets wheels, track and backhoe components, structural brackets and rollers. General applica- tions include hydraulic cylinders, machine frames, mill rolls, tunnel segments, rubber molds and mandrels. T e common grades of ductile iron diff er
Class 30 (annealed)
109
Class 35 (as-cast)
212
Class 40 (as-cast)
235
29.9 ksi (206 MPa) 33.7 ksi (232 MPa) 20.6 ksi (142 MPa) 34.8 ksi (240 MPa) 41.9 ksi (289 MPa) 16.6 Msi (114 GPa) 17.0 Msi (117 GPa) 14.5 Msi (100 GPa) 18.0 Msi (124 GPa) 18.2 Msi (126 GPa) 0.29
Compression Poisson’s Ratio 0.27 3.68
0.19 0.28 3.84
0.21 0.26 4.05
0.22 0.28 3.63
0.24 0.23 3.71
Sept/Oct 2014 | METAL CASTING DESIGN & PURCHASING | 41
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