Steel is a very popular construction and engineering material. Steel is a strong, hard, bluish-gray metal alloy of iron and is one of the most widely used materials around the world. Different Types of Steel are manufactured from carbon content from 0.2 to 2.1 percent (by weight), depending on classification according to composition and their physical properties.
The major element in steel is carbon but other alloying elements i.e. tungsten, chromium, vanadium and, magnesium and a small amount of sulfur, silicon, phosphorus, and oxygen are also present.
Steel is mainly used for structural and fabricating material. In steel manufacturing, carbon and other alloying material are used for creating hardening and preventing dislocations in the iron atom.
The important properties of steel like ductility, hardness and tensile strength of steel depend on the amount of alloying materials and their form of presence in iron. An increasing the amount of carbon makes the steel harden and strong, but less ductile. Steel is a versatile, strong and resilient material.
The biggest advantage is that steel can be recycled over and over again without losing its properties. Steel is one of the essential materials in construction, infrastructure, and bridges as well as used in household appliances. Steel industries are playing a vital role in economic development.
Types of Steel According to Composition
The main commercial classification of steel is as under,
1. Carbon steel
2. Alloy steel
1. Carbon Steel
The carbon steel is generally contested less than 1 % carbon and traces of manganese, sulfur, silicon, phosphorus . The properties and characteristics of carbon steel mainly depend on the carbon content in steel and there is a minor influence on this type of carbon due to the alloying and residual materials. Plain carbon steel is further classified into four categories.
Following are the types of carbon steel,
1) Low carbon steel
2) Medium carbon steel
3) High carbon steel
4) Very high carbon steel
Low Carbon Steel
In low carbon steel types of steel, carbon % is limited to 0.30 and is the most commonly used grade. These types of steel can be machined and welded easily and also has the ductile property higher than high carbon steel.
Medium Carbon Steel
In medium carbon steel, the amount of carbon is kept between 0.30 to 0.45 percent carbons. As there is increase in the amount of carbon content there is an increase in hardness and tensile strength and a decrease in ductility. But, because of higher carbon content, its machining and welding are difficult than low carbon steel due to increased content of carbon.
High Carbon Steel
In high carbon steel, the amount of carbon content is between 0.45 to 0.75 percent. So, these types of steel face the challenge of welding and machining this type of steel. So, for any type of moulding work heating is necessary to produce acceptable welds and is also used to control the mechanical properties of steel after welding.
Very high Carbon Steel
In very high carbon steel, the amount of carbon content is up to 1.50 percent. Because of high carbon in the steel, it requires heat before, during and after welding to control its mechanical properties. So, its major application in the production of hard steel products such as metal cutting tools and truck springs.
Designation System of Carbon Steel
A four-digit designation system has introduced by the American Iron and Steel Institute (AISI) together with the Society of Automotive Engineers (SAE). Also as per ASTM a240
1st letter of digit indicate that either it is a carbon steel or alloy steel.
1 indicate represents carbon steel and 2-9 are for alloy steel.
2nd digit indicates the modification of steel:
0 – Plain carbon, non-modified
1 – Resulfurized
2 – Resulfurized and rephosphorized
5 – Non-resulfurized, Mn over 1.0%
Last two digits:
Last 2 digits show the carbon concentration in 0.01%.
SAE 1045: In which 1 represents the plain carbon (non-modified) steel and contains 0.45% carbon in this type of steel.
2. Alloy Steel
Alloy steel is the type of carbon steel in which one or more elements other than carbon have been intentionally added, to produce a desired physical property or characteristic. Generally, molybdenum, manganese, nickel, silicon, boron, chromium, boron and vanadium are added as external elements. There are two types of alloy steel
Following are the types of alloy steel,
1. Low alloy steel
2. High alloy steel
1. Low Alloy Steel
In low alloy steel, the carbon content is generally kept 0.25 % and often 0.15 % for specially welding application. There are some alloying elements like manganese, nickel, chromium, molybdenum, silicon, vanadium and boron and the less common alloying elements are aluminum, cobalt, copper, titanium, tungsten, tin and zirconium.
Most popularly low alloy steel is used to achieve better hardenability and is increased corrosion resistance in certain environments. There is one drawback is that the low alloy steels are difficult to weld. If we lower the carbon content up to 0.10 percent along with other alloying materials increase the strength of the material.
2. High Alloy Steel:
Generally, steel having other elements more than 8% by weight of total other than carbon and iron is classified as high alloy steel. High alloy steel essentially has the two chemical elements and the properties of this type of steel depend on the percentage of the chemical element present in 5 it.
Its major advantage, it offers high corrosion resistance with high reliability. These types of high carbon steel are largely used in uclear power plants, heat exchangers, centrifugal separators, driers, pipelines, couplings, valves, bolts, salt manufacturing, exhaust gas desulfurizes, petrochemical, pharmaceutical and semiconductor cleaning equipment.
Designation System of Alloy Steel
The four-digit-designation system for alloy steel has developed by American Iron and Steel Institute (AISI) together with Society of Automotive Engineers (SAE).
As per four-digit classification SAE-AISI system:
The first digit shows the class of alloy steel:
2- Nickel steels
3- Nickel-chromium steels
4- Molybdenum steels
5- Chromium steels
6- Chromium-vanadium steels
7- Tungsten-chromium steels
9- Silicon-manganese steels
The second digit indicates the concentration of the major element in percents. if the 2nd element is 1 or 2 it means 1% and 2%.
Last two digits:
The last two digits indicate carbon concentration by 0.01%.
It shows the alloy of Chromium-vanadium steel, containing 2% of chromium and0.30% of carbon.
Types of Steel According to Application
As per application of steel is classified as under,
1. Stainless steel
2. Tool and die steel
1. Stainless steel
Stainless steel is a type of steel that comes with a minimum of 10% chromium content. This steel is more resistant to stains, corrosion, and rust than ordinary steel. It was invented or made in 1913 by Harry Brearley of Sheffield, England, but the discovery was not announced to the world until 1915.
It is called as corrosion resistance steel when the alloy type and grade are not detailed, particularly in the aviation industry. It mainly utilized where the properties of steel, as well as the resistance to corrosion, are required. Stainless steel has almost 150 grades out of which 15 are most commonly used.
This type of steel mainly used in watchbands,watches, handgun models, pistol , storage tanks , tankers , table cutlery, jewelry, food processing plant, surgical instruments as well as in the aviation industry.
Designation system of Alloy Steel
AISI has introduced a three-digit system for the stainless steels:
2XX series – chromium-nickel-manganese austenitic stainless steels
3XX series – chromium-nickel austenitic stainless steels
4XX series – ferritic stainless steels and chromium martensitic stainless steels
5XX series – low chromium martensitic stainless steels
2. Tool and die steel
This is very high carbon steels (either carbon or alloy) possessing high hardness, strength and wear resistance. Tools and die steel generally has a carbon content between 0.7% to 1.5%, tool steels are manufactured under carefully controlled conditions to produce the required quality of steel. Tool steels are heat treatable.
To increase hardness of steel tool alloying elements forming hard and stable carbides are added to the composition. It is used to shape other metals by cutting, forming, machining and die casting.
Tool and die steel is mainly utilized for making chisels, forging dies, hummers, drills, cutters, shear blades, drills, and razors.
Tool and die steels can be divided based on their use, mechanical properties, composition and method of heat treatment. There are various grades of tools and die steel available for different applications.
Selection of grade mainly depends on whether a keen cutting edge is necessary or not, as in stamping dies, or whether the tool has to withstand impact loading and service conditions encountered with such hand tools as axes, pickaxes, and quarrying implements or not.
Designation System of Tool Steel
One-letter system in combination with a number for tool steel system has developed by American Iron and Steel Institute (AISI) together with Society of Automotive Engineers (SAE).
The letter means;
W- Water hardened plain carbon tool steel
O- Oil hardening cold work alloy steel
A- Air hardening cold work alloy steel
D- Diffused hardening cold work alloy steel
S- Shock resistance low carbon tool steel
T- High speed tungsten tool steel
M- High speed molybdenum tool steel
H- Hot work tool steel
P- Plastic mold tool steel
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