History of Steelmaking

Steel was known in antiquity, and may have been produced by managing bloomeries — iron-smelting facilities — so that the bloom contained carbon. Steel is mentioned in the Bible: Jeremiah 15:12 of the Authorized King James Version, it reads: "Shall iron break the northern iron and the steel?". However, it seems the Hebrews had no word for "steel" but used instead אסטמא (istoma).

The earliest known production of steel is a piece of ironware excavated from an archaeological site in Anatolia and is about 4,000 years old. Other ancient steel comes from East Africa, dating back to 1400 BC. In the 4th century BC steel weapons like the Falcata were produced in the Iberian Peninsula, while Noric steel was used by the Roman military. The Chinese of the Warring States (403–221 BC) had quench-hardened steel, while Chinese of the Han Dynasty (202 BC – 220 AD) created steel by melting together wrought iron with cast iron, gaining an ultimate product of a carbon-intermediate steel by the 1st century AD.

Evidence of the earliest production of high carbon steel in the Indian Subcontinent was found in Samanalawewa area in Sri Lanka. Wootz steel was produced in India by about 300 BC. Along with their original methods of forging steel, the Chinese had also adopted the production methods of creating Wootz steel, an idea imported into China from India by the 5th century AD. This early steel-making method in Sri Lanka employed the unique use of a wind furnace, blown by the monsoon winds and producing almost pure steel. Also known as Damascus steel, wootz is famous for its durability and ability to hold an edge.

It was originally created from a number of different materials including various trace elements. It was essentially a complicated alloy with iron as its main component. Recent studies have suggested that carbon nanotubes were included in its structure, which might explain some of its legendary qualities, though given the technology available at that time, they were produced by chance rather than by design. Natural wind was used where the soil containing iron was heated up with the use of wood. The ancient Sinhalese managed to extract a ton of steel for every 2 tons of soil, a remarkable feat at the time. One such furnace was found in Samanalawewa and archaeologists were able to produce steel as the ancients did long ago.

Crucible steel, formed by slowly heating and cooling pure iron and carbon (typically in the form of charcoal) in a crucible, was produced in Merv by the 9th to 10th century AD. In the 11th century, there is evidence of the production of steel in Song China using two techniques: a "berganesque" method that produced inferior, inhomogeneous steel and a precursor to the modern Bessemer process that utilized partial decarbonization via repeated forging under a cold blast.

Reference : Wikipedia

Microstuctures of Steel

Austenitic

Martensite

Ferrite

Bainite

What is Steel?

Steel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, andtungsten. Carbon and other elements act as a hardening agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but is also less ductile.

Alloys with a higher carbon content are known as cast iron because of their lower melting point and castability. Steel is also distinguished from wrought iron, which can contain a small amount of carbon, but it is included in the form of slag inclusions. Two distinguishing factors are steel's increased rust resistance and better weldability.

Though steel had been produced by various inefficient methods long before the Renaissance, its use became more common after more efficient production methods were devised in the 17th century. With the invention of the Bessemer process in the mid-19th century, steel became an inexpensive mass-produced material. Further refinements in the process, such as basic oxygen steelmaking, further lowered the cost of production while increasing the quality of the metal.

Today, steel is one of the most common materials in the world, with more than 1300 million tons produced annually. It is a major component in buildings, infrastructure, tools, ships, automobiles, machines, appliances, and weapons. Modern steel is generally identified by various grades of steel defined by various standards organizations.

Martensite Formation

Rapid quenching of austenite to room temperature often results in the formation of martensite, a very hard structure in which the carbon, formerly in solid solution in the austenite, remains in solution in the new phase. Unlike ferrite or pearlite, martensite forms by a sudden shear process in the austenite lattice which is not normally accompanied by atomic diffusion.

Ideally, the martensite reaction is a diffusionless shear transformation, highly crystallographic in character, which leads to a characteristic lath or lenticular microstructure. The martensite reaction in steels is the best known of a large group of transformations in alloys in which the transformation occurs by shear without change in chemical composition. The generic name of martensitic transformation describes all such reactions.

The martensite reaction in steels normally occurs athermally, i.e. during cooling in a temperature range which can be precisely defined for a particular steel. The reaction begins at a martensitic start temperature Ms which can vary over a wide temperature range from as high as 500°C to well below room temperature, depending on the concentration of γ-stabilizing alloying elements in the steel.

Once the Ms is reached, further transformation takes place during cooling until the reaction ceases at the Mf temperature. Larger volume fractions of austenite are retained in some highly alloyed steels, where the Mf temperature is well below room temperature.

Each grain of austenite transforms by the sudden formation of thin plates or laths of martensite of striking crystallographic character. The laths have a well-defined habit plane and they normally occur on several variants of this plane within each grain. The habit plane is not constant, but changes as the carbon content is increased.

Martensite is a supersaturated solid solution of carbon in iron which has a body-centred tetragonal structure, a distorted form of bcc iron. It is interesting to note that carbon in interstitial solid solution expands the fcc iron lattice uniformly, but with bcc iron the expansion is nonsymmetrical giving rise to tetragonal distortion.

Assuming that the fcc-bcc tetragonal transformation occurs in a diffusionless way, there will be no opportunity for carbon atoms to move, so those interstitial sites already occupied by carbon will be favored. Since only the z sites are common to both the fcc and bcc lattices, on transformation there are more carbon atoms at these sites causing the z-axis to expand, and the non-regular the martensite, as well as the shape deformation for a number of martensitic transformations including ferrous martensites. It is, however, necessary to have accurate data, so that the habit planes of individual martensite plates can be directly associated with a specific orientation relationship of the plate with the adjacent matrix.

The lower carbon (<0.5% C) martensites on the whole exhibit only dislocations. At higher carbon levels very fine twins (5-10 nm wide) commonly occur. The evidence suggests that deformation by dislocations and by twinning are alternative methods by which the lattice invariant deformation occurs. From general knowledge of the two deformation processes, the critical resolved shear stress for twinning is always much higher than that for slip on the usual slip plane. This applies to numerous alloys of different crystal structure.

Thus it might be expected that those factors, which raise the yield stress of the austenite, and martensite, will increase the likelihood of twinning. The important variables are: 1. carbon concentration; 2. alloying element concentration; 3. temperature of transformation; 4. strain rate. The yield stress of both austenite and martensite increases with carbon content, so it would normally be expected that twinning would, therefore, be encouraged. Likewise, an increase in the substitutional solute concentration raises the strength and should also increase the incidence of twinning, even in the absence of carbon, which would account for the twins observed in martensite in high concentration binary alloys such as Fe-32%Ni. A decrease in transformation temperature, i.e. reduction in Ms, should also help the formation of twins, and one would particularly expect this in alloys transformed, for example, well below room temperature.

It should also be noted that carbon concentration and alloying element concentration should assist by lowering Ms. As martensite forms over a range of temperatures, it might be expected in some steels that the first formed plates would be free of twins whereas the plates formed nearer to Ms would more likely be twinned. However, often plates have a mid-rib along which twinning occurs, the outer regions of the plate being twin-free. This could possibly take place when the Ms is below room temperature leading to twinned plates which might then grow further on resting at room temperature.