In this article we will discuss about:- 1. Meaning of Steel 2. Elements of Steel 3. Necessity of Manufacturing 4. Ingots 5. Phases.
Meaning of Steel:
Steel is an alloy of iron and iron carbide. It is initially cast into a malleable form, and then it can be changed in shape by forging, rolling or other mechanical processes. The essential difference between cast iron and steel is that steel never contains graphite or free carbon. Carbon exists in very small quantity in ferrite and majority in cementite.
Besides carbon, steel contains many chemical elements which are added into iron to form steels of different kinds having different physical properties. The wide range of hardness that is possible in steel is a consequence of the carbon present; the manner in which it is associated with the iron; and the aggregation of the resulting phase. A phase is related to any homogeneous mechanically separable constituent of an alloy; and in steel there are three important phases; Ferrite, Cementite and Austenite.
Elements of Steel:
The other elements which are added are given below:
i. Carbon Steels:
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The following are the constituent elements which form steel:
Iron … Over 90% in most steels.
Carbon … The element which mainly determines the physical properties of steel.
Manganese … Very essential for steels of all types.
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Phosphorus, sulphur and silicon are present in varying quantities. The proper adjustments and various combinations of the above elements give a wide range of properties.
ii. For Alloy Steels and Stainless Steels:
Many other elements are added in the above basic elements in order to widen the range of physical, chemical and magnetic properties.
Various combinations of the following alloying elements are used:
The elements which are added for special purposes are:
Rare earth elements of atomic numbers 57 to 61 are used for making stainless steels.
Selenium or tellurium’s are also used in order to replace or augment sulphur.
Alloys having 50% or more iron are categorised as steel, and below 50% these are categorised as nonferrous alloys.
The Necessity of Manufacturing Steel:
Pig iron contains lot of impurities due to which it cannot be used for machine tools and industrial construction. So pig iron is refined and mixed with desired elements in a definite proportion to make steel. The impurities are removed by oxidation using a fluxing stage. When the process of refining involves only oxidation of carbon, manganese and silicon, the process is known as acid.
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When oxidation is supplemented by the use of lime or other strong base which removes phosphorus, sulphur and silicon, the process is called base. It is produced from pig iron by the removal of impurities in either an open-hearth furnace, a Bessemer converter or an electric furnace, or by the spray technique.
Steel Ingots:
After manufacture of steel, it is tapped into a ladle and poured into iron ingot moulds. A great care is required at this stage because many defects in rolled steel products are likely to be introduced by incorrect ingot practice. In order to facilitate withdrawal of ingot, the mould half has to be made tapered.
When mould is placed with big end up, then shrinkage cavity is formed at top and only small portion has to be discarded. When mould with big end down is used then primary and secondary piping effects are observed at top and the portion to be discarded is excessive. Therefore, former arrangement is commonly used.
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When molten metal is poured from top, splashing and turbulence are worst. This effect can be minimised by making the molten metal to enter from the bottom.
i. Killed Steel:
High quality machine and tool steels are de-oxidised in the ladle with silicon and aluminium to such an extent that there is no gas evolution in the steel upon solidification and they lie quietly in the mould as it cools. Such steels are called killed steels. Usually a cavity is found in the upper portion of the ingot because of the cooling to steel in the mould and shrinkage of steel on solidifying.
The minimise this condition, a large-end-up mould is used together with a refractory ‘hot-top’ which supplies molten metal to the main body of the ingot while solidification proceeds. Killed steel is characterised by relatively uniform chemical composition and properties. Sheets and strips made of killed steel have excellent forming and drawing qualities.
ii. Semi-Killed Steel:
In order to reduce the cost of hot tops and large percentage of metal discard when making mild steel for structural purposes, the steel is not fully de-oxidised. This results in blow holes in the steel on solidification.
The presence of these blow-holes minimises piping by distributing small voids throughout the ingot instead of having one large pipe in the upper centre of the ingot. If not exposed at the surface of the ingot, these blow-holes weld together during rolling. Steel de-oxidised in this manner is called semi-killed steel. It is suitable for drawing operation (except severe drawing).
iii. Rimmed Steel:
If the steel is de-oxidised to still less extent in the ladle, a reaction takes place during solidification in which the oxygen and carbon in the steel form carbon monoxide which is freely evolved from the ingot at the outer rim of the ingot.
The intensity of this reaction affects the ingot structure greatly. If the reaction is allowed to go to completion, the product is called rimmed steel. It is also known as drawing quality steel.
iv. Capped Steel:
If the above reaction is stopped after a short while by preventing, in a mechanical manner, further evolution of gas from the top of the ingot, the steel is called capped steel. The gas evolution results in an outer skin on the ingot which is clean and very low in carbon.
In capped steel, the skin is thinner and there is less segregation or concentration of impurities than in rimmed steel. The presence of this nearly pure iron skin enables the production of an excellent surface finish on the rolled product, and therefore sheet and strip are made nearly exclusively from rimmed or capped steel.
The defects which are likely to occur in steel ingots may be classified into two categories:
(a) Those which can’t be remedied to any large extent (pipe, segregation, and inclusion),
(b) Those which can be removed by chopping the surface with air hammers (seams, laps and scabs).
Segregation is the concentration of impurities which occurs in all steels upon solidification. It can be minimised by proper mould design and low pouring temperatures.
Non-metallic inclusions consist of sulphides, silicates etc., and are found to some extent in steels and are introduced in the refining and deoxidation of the steel.
If the surface of the mould is rough or if it contains cracks or cavities, then these interfere with the normal contraction of the ingot, thus resulting in transverse cracks in the ingot. Cracks thus produced have their surfaces oxidised, and when the ingot is rolled out, these defects are elongated in the direction of rolling and are called seams. Improper pouring conditions such as splashing of steel in the moulds forms scabs.
When rolling with grooved rolls which are not properly designed or set up, fins are liable to result from the flow of metal between the flat bodies of the rolls. If the fin is thin and wide, if will be folded over when the steel passes through the next set of rolls and will form a lap.
Phases of Steel:
i. Austenite:
Generally austenite is not present in plain or low alloy steels. The presence of austenite is ensured only when the transformation from y form to a form is completely suppressed but this is not possible in plain or low alloy steels even if they are very rapidly quenched.
However, in alloy steels containing manganese and nickel, there is always a high percentage of austenite, as the presence of these elements helps in suppressing the transformation from ϒ to α form. It has been noted that steels containing high percentages of these elements have got appreciable percentage of austenite at room temperature even if the rate of their cooling is quite slow.
Austenite is soft and ductile but at high temperatures it is stronger and less ductile than ferritic steel. Austenite is non-magnetic, more dense than ferrite and has got higher electrical resistance and thermal coefficient of expansion than ferrite.
When austenite is chilled in liquid it is converted into martensite.
ii. Martensite:
It is formed in carbon steels by fast and continuous cooling of austenite to temperatures 205 to 315°C or even lower than that.
Martensite has tetragonal crystal structure. Hardness of martensite varies from 500 to 1000 Brinell depending upon the carbon content and fineness of the structure.
iii. Pearlite:
It is composed of alternate layers of ferrite and cementite in the ratio of 87 to 13 by weight. The formation of pearlite takes place by the slow cooling of austenite along the line PSK in Fig. 1.13. Pearlite is the eutectoid structure of two phases in iron carbon alloys.
iv. Spheroidite:
It is produced by slow cooling of hypereutectoid austenite (steel containing more than 0.83% carbon) or by reheating (tempering) martensite in the range 650 to 705°C.
Spheroidite is the structure in which cementite takes the form of rounded particles or spheroids instead of plates.
Spheroidite is softer and more ductile than pearlite but not so much machinable as pearlite.
v. Troostite:
If martensite is reheated (tempered) in case of plain steels between temperatures 205 to 395°C, troostite is formed. Troostite is softer and more ductile than spheroidite. Troostite consists of submicroscopic particles of cementite in ferrite.
vi. Sorbite:
If troostite present in plain carbon steel is heated to temperature range of 595 to 395°C, it changes into a structure called sorbite. In sorbite, cementite is in granular form. Sorbite is softer and more ductile than troostite.
Role of Carbon in Steels:
Carbon is the most important alloying element in steel. It has profound effect not only on the properties of the steel but also on the way in which these can be altered by heat treatment.
Iron and carbon combine together to form a compound, iron carbide (Fe3C) in which one carbon atom is bonded to three iron atoms. In the structure of iron carbon alloys, the carbide is usually called cementite.
The amount of cementite present in an iron-carbon alloy depends on the carbon content. If carbon is not present (or present in small traces), the micro-structure consists of uniform grains of ferrite (pure B.C.C. iron with a very small carbon in solution) which is soft and ductile due to absence of cementite.
Cementite consists of 6.67% carbon and it is very hard structure with virtually no ductility and no commercial use. Table 1.1 shows how the ductility of steel varies with increase in carbon content (i.e. proportion of cementite in the microstructure).
If manganese is also present, then elongation value will be still lower than the values listed in above table for plain carbon steel.
How the properties of carbon steel change with increase in carbon content would be appreciated from following examples:
i. Steel with 0.08% Carbon:
It has good ductility with reasonably low yield stress. It can be pressed accurately into shape, especially around sharp corners. Suitable for car body panels with 0.3% manganese.
ii. Steel with 0.18% Carbon:
It has good impact strength at sub-zero temperatures (to avoid catastrophic brittle fractures) and is also weldable. Added with 0.8% Mn and 0.1% Si, it is used for ship’s hulls and can withstand the stresses experienced in service in heavy seas.
iii. Steel with 0.4% Carbon:
Has good strength in bending and torsion. Surface layer can be hardened to improve resistance to wear. With 0.8% Mn and 0.1% Si, it is used for axle shafts.
iv. Steel with 1.0% Carbon:
It is capable of being rolled into rods. It has good ductility and coils can be formed. With 0.6% Mn and 0.3% Si, it is used for helical springs.
Steels normally contain 0.1% to 2.2% carbon and cast iron contains 2.4% to 4.2% carbon. Steels with 0.1% to 0.8% carbon are used for general engineering and with 0.9% to 1.2% carbon are used for wear resistance purposes.
v. Strength Vs. Carbon Content in Steels:
With very low carbon contents, the metal consists of ferrite grains having tensile strength of 250 N/mm2. At 0.4% carbon, just over half of the area of microstructure consists of pearlite and the strength is around 540 N/mm2. With 0.8% carbon, the microstructure is completely pearlitic and the maximum strength of 850 N/mm2 is attained.
Beyond 0.8% and upto 1.2% carbon, the steel continues to have a pearlitic structure but cementite forms a network at the grain boundaries. Since it is not an integral component of the pearlite, it is referred to as free cementite.
This network of a hard brittle compound at the grain boundaries is associated with an increase in the hardness, but it causes a marked deterioration of the ductility and this is undesirable for most commercial purposes. The strength in this region more or less remains constant, but may fall little bit.
From above it would be obvious that there is some relationship between the presence of pearlite and the increase in the strength of the steel. (Refer Fig. 1.12). Pearlite has a laminated structure. It consists of alternate layers of ferrite and cementite. As carbon content increases, the amount of pearlite produced also increases, and it increases the strength.
This pearlite is an important constituent in the structure of a steel and it is better to understand how this constituent is formed and controlled.
It is observed that with increase in carbon content, the hardness, tensile strength and yield strength increase; but impact strength, reduction of area and elongation are reduced.