Classification of Steels (Fe-C alloys) and Cast irons
Ferrous means alloys made of mainly Iron (Fe) and Steels are made of mainly Fe-C and alloy steels contain other elements such as Cr, V, Mo, Ni, Mn, Cu, Si, W, etc. The alloying elements in steels (e.g., Cr, V, W, and Mo) combine with the carbon to form very hard and wear-resistant carbides and to protect the matrix (Fe) from wear and tear in difficult applications such as in cutting tools and corrosive, oxidative environments as in pressure vessels.
Steels are classified in terms of their carbon content and presence of other alloying elements as follows:
Low alloy: Low-carbon (Plain and High strength) steels,
Medium-carbon (Plain and Heat treatable) steels,
High-carbon (Plain) steels,
High alloy (tool and stainless) steels
 
Low Carbon Steels Properties: nonresponsive to heat treatments; relatively soft and weak; machinable and weldable. Typical applications: automobile bodies, structural shapes, pipelines, buildings, bridges, and tin cans.
Medium Carbon Steels Properties: heat treatable, relatively large combinations of mechanical characteristics. Typical applications: railway wheels and tracks, gears, crankshafts, and machine parts.
High Carbon Steels Properties: hard, strong, and relatively brittle. Typical applications: chisels, hammers, knives, and hacksaw blades.
High Alloy Steels (Stainless and Tool) Properties: hard and wear resistant; resistant to corrosion in a large variety of environments. Typical applications: cutting tools, drills, cutlery, food processing, and surgical tools.
 
CAST IRONS: Types, Composition, Heat-treatment, and Microstrctures
Carbon (C) and silicon (Si) are the main alloying elements, from 2.1 to 4 wt%C and 1 to 3 wt%Si,
remainder Fe. Technically cast irons are ternary Fe-C-Si alloys, but since the compositions of most
cast irons are around the eutectic point (at 4.3 wt% C, 1147°C, L ↔ γ + Fe3C)  of the iron-carbon
system, the melting temperatures closely correlate, usually ranging from 1150 to 1200°C, which is
about 300°C lower than the melting point of pure iron.
Cast iron tends to be brittle, except for  malleable cast irons. With its relatively low melting point, good fluidity, castability, excellent machinability, resistance to deformation, and wear resistance, cast irons have become an engineering material with a wide range of applications, and are used in pipes, machines, and automotive industry parts, such as cylinder heads (declining usage), cylinder blocks,  and gearbox cases (declining usage). It is resistant to destruction and weakening by oxidation (rust).
The flakes of graphite have good damping characteristics and good machinability (because the graphite acts as a chip-breaker and lubricates the cutting tools). In applications involving wear, the graphite is beneficial because it helps retain lubricants. However, the flakes of graphite also are stress concentrators, leading to poor toughness. The recommended applied tensile stress is therefore only a quarter of its actual ultimate tensile strength.
The colour of a fractured surface can be used to identify the type of cast iron. White cast iron  is named after its white surface when fractured, due to its carbide (white cementite mainly at grain boundaries) which allow cracks to pass straight through. Grey cast iron  is named after its grey fractured surface, which occurs because the graphitic flakes deflect a passing crack and initiate countless new cracks as the material breaks.
Alloying elements in Cast irons
Cast iron's properties are changed by adding various alloying elements. Next to  carbon, silicon is the most important element because it forces carbon out of solution. Instead the carbon forms   graphite which results in a softer iron, reduces shrinkage, lowers strength, and decreases density. Sulfur,  when added, forms iron sulfide,  which prevents the formation of graphite and increases hardness.  The problem with sulfur is that it makes molten cast iron sluggish, which causes short run defects. To counter the effects of sulfur, manganese is added because the two form into manganese sulfide instead of iron sulfide. The manganese sulfide is lighter than the melt so it tends to float out of the melt and into the slag.  The amount of manganese required to neutralize sulfur is 1.7×sulfur content+0.3wt%. If more than this amount of manganese is added, then manganese carbide forms, which increases hardness and chilling, except in grey iron, where up to 1% of manganese increases strength and density.
Nickel is one of the most common elements because it refines the pearlite and graphite structure, improves toughness, and provides uniform hardness for different section thicknesses.   Chromium is added in small amounts to the ladle to reduce free graphite, produce chill, and because it is a powerful carbide stabilizer; nickel is often added in conjunction. A small amount of   tin can be added as a substitute for 0.5% chromium. Copper is added in the ladle or in the furnace, on the order of 0.5 to 2.5wt%, to decrease chill, refine graphite, and increase fluidity. Molybdenum is added on the order of 0.3 to 1wt% to increase chill and refine the graphite and pearlite structure; it is often added in conjunction with nickel, copper, and chromium to form high strength irons. Titanium is added as a degasser and deoxidizer, but it also increases fluidity. 0.15 to 0.5wt%  vanadium are added to cast iron to stabilize cementite, increase hardness, and increase resistance to wear and heat. 0.1 to 0.3wt% zirconium helps to form graphite, deoxidize, and increase fluidity.
In malleable iron melts, bismuth is added, on the scale of 0.002 to 0.01wt%, to increase how much silicon can be added. In white iron,  boron is added to aid in the production of malleable iron; it also reduces the coarsening effect of bismuth.
Grey cast iron
Grey cast iron is characterized by its graphitic microstructure, which causes fractures of the material to have a grey appearance. It is the most commonly used cast iron and the most widely used cast material based on weight. Most cast irons have a chemical composition of 2.5 to 4.0wt% carbon, 1 to 3wt% silicon, and the remainder is iron. Grey cast iron has less  tensile strength and shock resistance than steel, however its compressive strength is comparable to low and medium carbon steel.
White cast iron
With a lower silicon content and faster cooling, the carbon in white cast iron precipitates out of the
melt as the metastable phase cementite, Fe3C, rather than graphite. The cementite which precipitates from the melt forms as relatively large particles, usually in a eutectic mixture, where the other phase is austenite (which on cooling might transform to martensite). These eutectic carbides are much too large to provide precipitation hardening (as in some steels, where cementite precipitates might inhibit plastic deformation by impeding the movement of  dislocations through the ferrite matrix). Rather, they increase the bulk hardness of the cast iron simply by virtue of their own very high hardness and their substantial volume fraction, such that the bulk hardness can be approximated by a rule of mixtures. In any case, they offer  hardness at the expense of  toughness. Since carbide makes up a large fraction of the material, white cast iron could reasonably be classified as a  cermet. White iron is too brittle for use in many structural components, but with good hardness and abrasion resistance and relatively low cost, it finds use in such applications as the wear surfaces (impeller and volute) of  slurry pumps,  shell liners and lifter bars in ball mills and autogenous grinding mills,  balls and rings in coal pulverisers, and the teeth of a backhoe's digging bucket (although cast medium-carbon martensitic steel is more common for this application).
It is difficult to cool thick castings fast enough to solidify the melt as white cast iron all the way through. However, rapid cooling can be used to solidify a shell of white cast iron, after which the remainder cools more slowly to form a core of grey cast iron. The resulting casting, called a chilled casting , has the benefits of a hard surface and a somewhat tougher interior.
High-chromium white iron alloys allow massive castings (for example, a 10-tonne impeller) to be sand cast, i.e., a high cooling rate is not required, as well as providing impressive abrasion resistance.
Malleable cast iron
Malleable iron  starts as a white iron casting that is then heat treated at about 900°C. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack or phonon. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by grey cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable iron, since it is made from white cast iron.
Ductile cast iron
 A more recent development is nodular   or ductile cast iron. Tiny amounts of   magnesium or cerium added to these alloys slow down the growth of graphite precipitates by bonding to the edges of the graphite planes. Along with careful control of other elements and timing, this allows the carbon to separate as spheroidal particles as the material solidifies. The properties are similar to malleable iron, but parts can be cast with larger sections.
Name 
Nominal
composition
Engine cylinder
Axle bearings,
track wheels,
Gears,
camshafts, 
crankshafts 
applications 
Ni-resist
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Fe-C Phase diagram showing steel and cast iron with related microstructures and the eutectic and eutectoid reactions.
Comparison between gray and malleable cast irons:
(a) With respect to composition and heat treatment:
Gray iron--2.5 to 4.0 wt% C and 1.0 to 3.0 wt% Si. For most gray irons there is no heat treatment after solidification.
Malleable iron--2.5 to 4.0 wt% C and less than 1.0 wt% Si. White iron is heated in a
nonoxidizing atmosphere and at a temperature between 800 and 900 C for an extended time period.
(b) With respect to microstructure:
Gray iron--Graphite flakes are embedded in a ferrite or pearlite matrix.
Malleable iron--Graphite clusters are embedded in a ferrite or pearlite matrix. (c) With respect to mechanical characteristics:
Gray iron--Relatively weak and brittle in tension; good capacity for damping vibrations.
Malleable iron--Moderate strength and ductility.
Comparison between white and nodular cast irons:
 
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Nodular cast iron--2.5 to 4.0 wt% C, 1.0 to 3.0 wt% Si, and a small amount of Mg or
Ce. A heat treatment at about 700 C may be necessary to produce a ferritic matrix.
(b) With regard to microstructure:
White iron--There are regions of cementite interspersed within pearlite.
Nodular cast iron--Nodules of graphite are embedded in a ferrite or pearlite matrix.
(c) With respect to mechanical characteristics:
White iron--Extremely hard and brittle.
Nodular cast iron--Moderate strength and ductility.
Grey Cast Iron (Flake Graphite)
Grey cast iron, Fe-3.2C-2.5Si (by wt%), containing graphite flakes in a pearlitic matrix; Etchant: 2% NITAL. 
Spheroidal Graphite in Nodular Cast Iron The chemical composition of the cast iron is similar to that of the grey cast iron but with 0.05 wt% of magnesium. All samples are etched using 2% nital.
 
Heat Treated Spheroidal Graphite (nodular) Cast Iron
Spheroidal graphite cast iron usually has a pearlitic matrix. However, annealing causes the carbon in the pearlite to precipitate on to the existing graphite or to form further small graphite particles, leaving behind a ferritic matrix. This gives the iron even greater ductility.  All samples are etched using 2% nital.
Graphite nodules in a ferritic matrix. Some carbon deposited during annealing is also visible. Etchant: 2% NITAL.
Austempered Ductile Cast Iron
The chemical composition of the cast iron is Fe-3.52C-2.51Si-0.49Mn-0.15Mo-0.31Cu wt%.  All samples are etched using 2% nital.
 
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The above images are of austempered ductile iron automobile components, provided by the Institute of Cast Metals Engineers.  In order to avoid distortion, the crankshaft for the TVR sportscar is rough-machined after casting, heat-treated to produce the bainitic microstructure, and then properly machined. It is reported to have excellent fatigue properties; its damping characteristics due to graphite reduce engine noise.
The Ford Mustang suspension arm was made from austempered ductile iron in order to reduce weight, noise and cost. It was designed using finite element modelling to optimise strength and stiffness. Aluminium alloys were considered but rejected because the component would then occupy a much larger space because of their lower strength.
The truck trailer suspension arm was originally made from welded steel, for use on transportation across the rugged Australian Outback. These failed at the welds and were associated with distortions which led to accelerated deterioration of the tyres. The suspension made from the cast austempered ductile iron has proved to be much more robust.
Wear-Resistant High-Chromium Cast Iron
This cast iron is used in circumstances where a very high wear resistance is desirable. For example, during the violent crushing of rocks and minerals. It contains a combination of very strong carbide-forming alloying elements. Its chemical composition is, therefore, Fe-2.6C- 17Cr-2Mo-2Ni wt%. All samples are etched using Villela's reagent, which is a mixture of picric acid, hydrochloric acid and ethanol. 
The white phase is a chromium-rich
carbide known as M7C3. The matrix
consists of dendrites of austenite,
some of which may have transformed
into martensite. There may also be
relatively small quantities of other
alloy carbides.
consists of dendrites of austenite,
some of which may have transformed
into martensite. There may also be
relatively small quantities of other
alloy carbides. 
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Note that the principal difference between wrought products (those which are cold or hot worked/forged/rolled/drawn/extruded) and cast alloys is as follows: wrought alloys are ductile enough so as to be hot or cold worked during fabrication, whereas cast alloys are brittle to the degree that shaping by deformation is not possible and they must be fabricated by casting.
Note also that Gray iron is weak and brittle in tension because the tips of the graphite flakes act as points of stress concentration.
It is possible to produce cast irons that consist of a martensite matrix in which graphite is embedded in either flake, nodule, or rosette form. For graphite flakes, gray cast iron is formed which is then heated to a temperature at which the ferrite transforms to austenite; the austenite is then rapidly quenched, which transforms to martensite. For graphite nodules and rosettes, nodular and malleable cast irons are first formed which are then austenitized and rapidly quenched. Note also that it is not possible to produce malleable iron in pieces having large cross- sectional dimensions. White cast iron is the precursor of malleable iron, and a rapid cooling rate is necessary for the formation of white iron, which may not be accomplished at interior regions of thick cross sections.
 
Titanium Alloys
Distinctive features: relatively low density, high melting temperatures, and high strengths are possible. Limitation: because of chemical reactivity with other materials at elevated temperatures, these alloys are expensive to refine.
 Applications: aircraft structures, space vehicles, and in chemical and petroleum industries. Refractory Metals Distinctive features: extremely high melting temperatures; large elastic moduli, hardnesses, and strengths. Limitation: some experience rapid oxidation at elevated temperatures.
 Applications: extrusion dies, structural parts in space vehicles, incandescent light filaments, x-ray tubes, and welding electrodes. Superalloys Distinctive features: able to withstand high temperatures and oxidizing atmospheres for long time periods.
 Applications: aircraft turbines, nuclear reactors, and petrochemical equipment. Noble Metals Distinctive features: highly resistant to oxidation, especially at elevated temperatures; soft and ductile. Limitation: expensive.
 Applications: jewelry, dental restoration materials, coins, catalysts, and thermocouples.