Carbon plays an important role in heat treatment, because it expands the temperature range of austenite stability. However, it can be retained to room temperature by alloy additions such as nickel or manganese. Austenite in iron-carbon alloys is generally only present above the critical eutectoid temperature (723☌), and below 1500☌, depending on carbon content. Austenite, also known as gamma-phase iron (γ-Fe), is a non-magnetic face-centered cubic structure phase of iron. This is because of the configuration of the iron lattice which forms a BCC crystal structure. The primary phase of low-carbon or mild steel and most cast irons at room temperature is ferromagnetic α-Fe. α-ferrite can only dissolve up to 0.02 percent of carbon at 727☌. Ferrite or α-ferrite is a body-centered cubic structure phase of iron which exists below temperatures of 912☌ for low concentrations of carbon in iron. For steels, the stable equilibrium phases include: Heat treatment of steels requires an understanding of both the equilibrium phases and the metastable phases that occur during heating and/or cooling. Source: Läpple, Volker – Wärmebehandlung des Stahls Grundlagen. The percentage of carbon determines the type of the ferrous alloy: iron, steel or cast iron. The percentage of carbon present and the temperature define the phase of the iron carbon alloy and therefore its physical characteristics and mechanical properties.
Other Common Phases in Steels and Irons In the figure, there is the iron–iron carbide (Fe–Fe3C) phase diagram. These steels are relatively inexpensive and are produced in large tonnages. Their applications include beams for bridges and high-rise buildings, plates for ships, and reinforcing bars for roadways. The most common structural steels produced have a mixed ferrite-pearlite microstructure. The above assumes that the cooling process is very slow, allowing enough time for the carbon to migrate. No large inclusions of cementite will form at the boundaries in hypoeuctoid steel. In a hypoeutectoid composition, that means when low-carbon steels (with up to 0.30% C) or medium-carbon steels (with 0.30 to 0.60% C) are slowly cooled from the austenitic phase, some low-carbon α-ferrite forms first along austenite grain boundaries) until the remaining composition rises to 0.8% of carbon, at which point the pearlite structure will form.
In a hypereutectoid composition (greater than 0.8% carbon), the carbon will first precipitate out as large inclusions of cementite at the austenite grain boundaries until the percentage of carbon in the grains has decreased to the eutectoid composition (0.8% carbon), at which point the pearlite structure forms. A ferrite phase has a much lower carbon content, and cementite has a much higher carbon concentration. This distinctive microstructure of steel is called pearlite. If steel (austenite) with a eutectoid composition of approximately 0.77% C is slowly cooled below 727 ☌, the ferrite and cementite phase separate almost simultaneously to produce a microstructure with distinctive platelets. It is named for its resemblance to mother of pearl. In metallurgy, pearlite is a layered metallic structure of two-phases, which compose of alternating layers of ferrite (87.5 wt%) and cementite (12.5 wt%) that occurs in some steels and cast irons.
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