As with most grades of cast iron, ductile irons display: · Good hardness and good wear resistance · Good corrosion resistance · Have tensile and yields strengths that vary widely across the various grades. · Have compressive strengths that can be utilised more widely (than tensile strengths), with values tending to be about twice the tensile strength. · Impact strengths are better than grey irons, with lower grades approaching values common for mild steel. · Fatigue strengths are approximately 40 to 50% of tensile strengths. · Electrical resistivities are significantly lower compared to grey irons · Corrosion resistance is similar to grey iron · Machinability is dependent on hardness, with ferritic grades machining better. Heat Treatments Ductile irons can be heat treated similarly to grey irons, i.e. they can be: · Normalised – resulting in increased strength · Annealed – to relieve internal stresses and aid machining · Stress relieved – to remove internal stresses from uneven cooling and other effects · Quench hardened – to produce a stronger, harder, more wear resistant material. Pearlitic matrix ductile irons produce the best results for quench hardening. Prime candidates for this operation include the grades 80-55-06 and 100-70-03. Heat treatments are often integral in the production process Typically: · Grade 60-40-18 – is annealed · Grade 65-45-12 – is used as-cast · Grade 80-55-06 - is used as-cast · Grade 100-70-03 – is normalised · Grade 120-90-02 – is quenched and tempered |
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