Pig iron

- Pig iron is an iron-carbon alloy containing more than 2.14 % carbon (maximum carbon solubility in austenite on the phase diagram). Carbon may be present in the form of cementite and graphite in pig iron. The following types of cast iron are identified depending on the graphite form and portion of cementite: white, grey, malleable and ductile iron. Pig iron contains unavoidable impurities (Si, Mn, S, P) and, in some cases, also alloying elements (Cr, Ni, V, Al, etc.). Generally, pig iron is brittle.

Output

The world production of pig iron in 2011 set a new record and reached 1.083 billion tons representing an increase of 5% over the previous year (1.031 billion tons). The Asian region produced 804.78 million tons in the year, the EC 94.134 million tons, North America 42.478 million tons and the CIS countries 80.177 million tons. All pig iron producing countries increased their output in 2011.

Top 10 pig iron producing countries

Kinds of cast iron

White cast iron

There are no visible graphite inclusions in the structure of white cast iron and only a small quantity of graphite (0.03-0.30 %) can be detected by sensitive chemical methods or high-power micrography. The metal matrix of white cast iron consists of cementite eutectic, secondary and eutectoid cementite, while the matrix of alloyed white cast iron consists of complex carbides and alloyed ferrite. White iron castings demonstrate wear resistance, relative heat resistance and resistance to corrosion. Any zones of a casting cross section that differ from white cast iron in structure impair the above properties. The strength of white cast iron decreases with increasing carbon and, therefore, carbide content. And the higher is carbide quantity in the structure of white cast iron and, consequently, the higher is carbon content, the higher is hardness of white cast iron.

The highest hardness of white cast iron is ensured by a martensitic structure of the matrix. Carbides coagulation greatly reduces the hardness of cast iron. Impurities dissolved in iron carbides result in formation of complex carbides, and in this case hardness of carbides and white cast iron increases. By the intensity of their influence on the hardness of white cast iron, primary and alloying elements are arranged in the following order, starting with carbon that determines the amount of carbides and increases the hardness of cast iron more intensively than other elements: C — Ni — P — Mn — Cr — Mo — V — Si — Al — Cu — Ti — S.

Effect of nickel and manganese and, in some measure, chromium and molybdenum is determined by their influence on formation of martensitic-carbide structure, and amounts of these elements corresponding to the carbon content in cast iron ensure maximum hardness of white cast iron.

Cast iron containing 0.7-1.8 % boron has an extra-high hardness HB 800-850. White cast iron is a very valuable material for components operating under wear-causing conditions, at very high specific pressures and mostly without lubrication. There is no direct correlation between wear resistance and hardness; hardness does not determine wear resistance, but should be taken into consideration along with the cast iron structure. The best wear resistance is shown by white cast iron with a fine structure of its matrix containing carbides, phosphides, etc. in the form of small isolated inclusions uniformly distributed within it or in the form of a fine network.

Special properties of alloyed cast iron — its corrosion, heat and electrical resistance — are also attributed to the matrix structure. Depending on the composition and concentration of alloying elements, the matrix of alloyed white cast iron can be carbide-austenite or carbide-pearlite and can also contain alloyed ferrite.

The main alloying element for this purpose is chromium that combines with carbon to form chromium carbides and complex chromium-iron carbides.

Solid solutions of these carbides have a high electrode potential close to that of the second structural component of the matrix, chromium ferrite. Additionally, protective oxide films are formed to ensure an increased corrosion resistance of high-chromium white cast iron. Thermal stability of carbides is considerably increased in the presence of chromium due to a significant slowdown in diffusion processes in multialloyed iron. Depending on structure, these specific characteristics of alloyed white cast iron identify its scope of use as magnetic stainless cast iron or cast iron with a high electrical resistance.

Grey cast iron

This iron-carbon alloy contains crab, flake or vermicular graphite. A special kind (grade group) of grey cast iron is represented by ductile cast iron with globular graphite modified by magnesium (Mg), cerium (Ce) or other elements. Cast iron may have a ferrite, ferrite-pearlite or pearlite structure depending on the cooling rate after solidification (and consequently, on cast sizes too). As the cooling rate increases, the amount of pearlite and, hence, the strength of cast iron increases too, but the ductility decreases. A grade of cast iron with an optimized set of properties is to be chosen for each area of application.

Grey cast iron is designated by letter symbols СЧ (SCh) followed by the minimum guaranteed value of tensile strength in kg/mm2, e. g. СЧ30 (SCh30). Ductile cast irons are produced under the designation BЧ (VCh) immediately followed by the strength value, and then by the elongation value in % separated by the dash, e. g. ВЧ60-2 (VCh60-2). Grey iron is the main foundry material characterized by high casting properties (low temperature of crystallization, good castability and low shrinkage). It is widely used in machinery to cast beds of machines and mechanisms as well as pistons and cylinders. High brittleness inherent in grey cast irons and attributable to graphite present in their structure precludes their use for parts subjected mostly to tensile or bending stresses, and cast irons are used only to withstand compressive stresses. In addition to carbon, grey cast iron always contains other elements, especially silicon facilitating graphite formation. Most grades of grey cast iron contain up to 1.4 % carbon, provided that total content of silicon and carbon is higher than 2%.

Malleable cast iron

Malleable cast iron is a conventional name of mild and tough metal produced from white cast iron by casting and subsequent heat treatment. Use of a long-term annealing results in cementite decomposition and graphite formation, or graphitization, and such a treatment is therefore called graphitizing annealing.

Like grey cast iron, malleable cast iron has a semisteel matrix and contains carbon in the form of graphite, but graphite inclusions in malleable iron differ from those in ordinary grey iron. The difference is that the graphite inclusions in malleable cast iron are arranged like flakes formed after annealing and isolated from each other, which results in a less disjunct metal matrix and a certain toughness and malleability of cast iron. Graphite in malleable iron is often called annealing carbon because of its flaky form and method of its formation (annealing).

White cast iron giving a malleable grade after annealing is hypoeutectic by its phase composition and has a structure of ledeburite + (secondary) cementite + pearlite. Cementite in ledeburite, secondary cementite and eutectoid cementite in pearlite should be decomposed during the annealing in order to obtain the structure of ferrite + annealing carbon. Decomposition of cementite in ledeburite and partial decomposition of secondary cementite occur during the first stage of graphitization at a temperature above the critical value (950 1,000 °C), while decomposition of eutectoid cementite occurs in the second stage of graphitization during thermal retardation at a temperature below the critical value (740 720 °C) or during slow cooling in the range of critical temperatures (760-720 ° C).

Ductile cast iron (SG iron)

This grade contains spheroidal graphite inclusions. Spheroidal graphite has a lower surface-to-volume ratio, which determines the maximum continuity of the metal matrix and hence maximum strength of iron. The metal matrix of nodular (spheroidal) graphite cast iron has the same structure as that of usual grey cast iron, that is, iron grades can be produced with the following structures: ferrite + spheroidal graphite (ferritic ductile iron), ferrite + pearlite + spheroidal graphite (ferritic-pearlitic ductile iron) or pearlite + spheroidal graphite (pearlitic ductile iron) depending on the chemical composition of cast iron and cooling rate (casting wall thickness). This grade is the most commonly used to manufacture heavy-duty components in mechanical engineering as well as to produce high-strength pipes (for water, sewerage, gas and oil pipe lines). The components and pipes of ductile cast iron have high strength, durability and high performance.

Mottled cast iron

Carbon in mottled cast iron is partially present in the form of cementite (more than 0.8 %). The structural components of such cast iron are pearlite, ledeburite and flake graphite.

Classification

According to carbon content, grey cast iron is said to be hypoeutectic (2.14-4.3 % carbon), eutectic (4.3 %) or hypereutectic (4.3-6.67 %). The alloy composition affects its structure. Depending on carbon state and content, the following kinds of cast iron are distinguished: white and grey iron (subject to the fracture colour resulting from the carbon structure in iron — in the form of iron carbide or free graphite), spheroidal graphite ductile iron, malleable iron, vermicular graphite iron. Carbon is present in white cast iron in the form of cementite and in grey cast iron mostly in the form of graphite.

Types of iron are designated in the industrial sector as follows:

• conversion pig iron — П1, П2 (P1, P2)
• conversion pig iron for casts (conversion - foundry) — ПЛ1, ПЛ2 (PL1, PL2)
• conversion phosphoric pig iron — ПФ1, ПФ2, ПФ3 (PF1, PF2, PF3)
• high-quality conversion pig iron— ПВК1, ПВК2, ПВК3 (PVK1, PVK2, PVK3)
• flake graphite cast iron — СЧ (SCh) (succeeding figures are the minimum guaranteed value of ultimate tensile strength in kg/mm2)
• antifriction cast iron
• antifriction grey iron — АЧС (AChS)
• antifriction ductile cast iron — АЧВ (AChV)
• antifriction malleable — АЧК (AChK)
• spheroidal graphite ductile iron for casts — BЧ (VCh) (figures following letter symbols ВЧ are the strength value in kg/mm2 and elongation value in %)
• special alloyed iron — Ч (Ch)

Classification of alloyed irons:

• heat-resistant chromic cast irons
• corrosion-resistant cast irons
• abrasion-resistant cast irons
• others