Also referred to as spheroidal or nodular iron – is a group of irons that exhibit high strength, flexibility, durability, and elasticity due to their unique microstructure. Cast ductile iron normally contains over 3 percent carbon; it can be bent, twisted or deformed without fracturing. Its mechanical properties are similar to steel, and far exceed those of standard cast irons.
The graphite and carbon contained in cast iron occur as flakes; cast iron exhibits positive compressive load capabilities, but tensile loading above its natural tensile strength can cause cracks to form and propagate rapidly out from stress points within the flaked graphite microstructure. As a result, cast iron has virtually no elongation. It is a brittle material, and therefore limited in its use in tensile and shock loading applications.
The graphite within ductile iron occurs as spheroids, so it is sometimes referred to as spheroidal graphite. Similarly, the term nodular iron stems from the fact that the carbon contained in ductile iron is held in the form of nodules. This unique microstructure causes ductile iron to withstand bending and shock loading far better than traditional cast iron.
Advantages of Ductile Iron
- Easily cast and machined
- Excellent strength to weight ratio
- Lower cost than steel
- Superior castability and machinability
- An exceptional combination of toughness, low-cost manufacturing, and reliability.
Stainless steel is characterised by its resistance to corrosion however given the right conditions, and adequate time, stainless steels will rust eventually. They will, however, take significantly longer to rust than other iron-based metals.
The corrosion resistance of stainless steel results from its chromium content: all stainless steels contain at least 10.5 percent chromium, but may contain significantly more depending on the application. Higher chromium content results in increased corrosion resistance.
When stainless steel is exposed to the atmosphere, the chromium combines with oxygen to form a thin, stable passivation layer of Chromium(III) Oxide (Cr2O3). The passivation layer protects the interior steel from oxidization, and quickly reforms if the surface is scratched.
The passivation layer that forms over stainless steel is different than plating. Other metals are commonly plated with white metals, including chromium and nickel, for surface protection. In those cases, the benefits of the coating are lost once a scratch penetrates the plating. Because stainless steel is alloyed with chromium, and not just surface plated, the passivation layer will reform no matter how deeply it is scratched.