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Pentlandite in pyrrhotite, ore specimen from the Sudbury Basin (field of view 3.4 cm). Pentlandite is an iron–nickel sulfide with the chemical formula (Fe,Ni) 9 S 8.Pentlandite has a narrow variation range in nickel to iron ratios (Ni:Fe), but it is usually described as 1:1.
The steel is then quenched so that the austenite is transformed into martensite, and the ferrite remains on cooling. The steel is then subjected to a temper cycle to allow some level of marten-site decomposition. By controlling the amount of martensite in the steel, as well as the degree of temper, the strength level can be controlled.
Steel is made from iron and carbon. Cast iron is a hard, brittle material that is difficult to work, whereas steel is malleable, relatively easily formed and versatile. On its own, iron is not strong, but a low concentration of carbon – less than 1 percent, depending on the kind of steel – gives steel strength and other important properties.
Some hypereutectoid pearlitic steel wires, when cold wire drawn to true (logarithmic) strains above 5, can even show a maximal tensile strength above 6 GPa (870 ksi). [2] Although pearlite is used in many engineering applications, the origin of its extreme strength is not well understood.
Iron alloys are most broadly divided by their carbon content: cast iron has 2–4% carbon impurities; wrought iron oxidizes away most of its carbon, to less than 0.1%. The much more valuable steel has a delicately intermediate carbon fraction, and its material properties range according to the carbon percentage: high carbon steel is stronger but more brittle than low carbon steel.
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The carbon content of steel is between 0.02% and 2.14% by weight for plain carbon steel (iron-carbon alloys). Too little carbon content leaves (pure) iron quite soft, ductile, and weak. Carbon contents higher than those of steel make a brittle alloy commonly called pig iron.
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