Steel is a versatile metal, hence there are approximately 3,500 types of steel available, according to the World Steel Association (WSA). Depending on the production method, their composition and manufacturing process, grading of steel is done. Different standards are used by American Iron and Steel Institute (AISI), Society of Automotive Engineers (SAE) and American Society of Mechanical Engineers (ASME) to classify and grade steel.
What Is Steel
When is an alloy classified as steel. All steel types are iron alloys with the addition of different elements. They are mainly composed of iron and carbon (< 2%), manganese (1%) and small quantities of other elements. It is manufactured using iron ore, scrap steel, coal, and limestone. All steel can be recycled an infinite number of times, and about 37% of all new steel is recycled steel. It is the most commonly used metal in the world.
The multiple combinations of elements and processes used to make different types of steel gives rise to an almost infinite number of possible steel grades. Incidentally, more than 70% of the current steel grades have been developed in the last 20 years and many more will be developed in the future.
Different Types of Steel
Almost all grades of steel typically fit into the following categories, based on composition, as specified by AISI:
Carbon Steel
These are steel alloys that rely primarily on the element Carbon to influence their specific properties. They may have other elements present, but only in miniscule amounts. They can be classified as:
- Low carbon: 0.01-0.3%
- Medium carbon: 0.3-0.6%
- High carbon: 0.6-1.4%
As Carbon content increases, the alloy will
- decrease in ductility (the ability to stretch without breaking)
- decrease in weldability
- increase in strength and hardness
In other words, higher carbon alloys are more likely to break when stretched, but less likely to break under tension. They are also more resistant to deformation such as scratching and denting. In general, they are more difficult to work with and more resistant to welding processes.
Stainless Steel
Steel that contains at least 10.5% chromium in their composition are called stainless steel. The primary purpose of this addition is for maximum corrosion resistance while still retaining its strength. So, stainless steel is much less likely to oxidize or rust. Stainless steel can be classified as:
Austenitic - Not heat treatable (unable to be strengthened through heat treatment) and non-magnetic. This stainless steel type typically contains high amounts of nickel and about 18% chromium. It’s commonly used for kitchen utensils or food processing equipment.
Ferritic - Not heat treatable and magnetic. Ferritic stainless steel generally has lower carbon content, contains about 12-17% chromium plus miniscule amounts of nickel. It may also contain other elements such as aluminum, molybdenum, and titanium. It’s commonly used in the automotive industry and industrial equipment.
Martensitic - Heat treatable and magnetic. This alloy has high carbon content, about 11-17% and a small amount of nickel. It is commonly used for surgical and dental equipment or utilized in knife-making.
Manganese Steel
Manganese steel contains 12-14% manganese. Well known for its high impact strength and resistance to abrasion in its hardened state, the steel is often described as the ultimate in hardening steel.
Because of its self hardening properties, manganese steel has been used in the following industries for decades.
Mining – Cement mixers, rock crushers, crawlers tractors and shovel buckets
Rail – Switches and crossings
Manganese steel is used in the window bars in prisons as it makes life difficult for potential escapees. The steel is also used in safes, bullet proof cabinets and anti-drill plates.
While all steels and low-alloy steels rust in moist atmospheres, an increase in manganese in steel has a positive effect on corrosion resistance, partly due to the adsorption of manganese ions.
Nickel Alloy
Nickel alloy is the most important among all the steel alloys due to its heavy usage everywhere. It is formed when some nickel along with some carbon is added to the steel. It contains 3.5% nickel and 0.35% carbon in its composition. The addition of nickel to structural steel results in an increase of strength with very little decrease in ductility.
The addition of Nickel to steel does have an effect on the steel as mentioned below.
Toughness
The nickel component increases the toughness of steel, hence the property of steel to resist fracture due to high impact increases. This property is good for making parts that are subjected to shock and impact load.
Distortion
The addition of nickel in steel decreases the distortion.
Critical Temperature
The critical temperature of steel decreases when nickel is added to it, so the steel becomes better for any type of heat treatment.
Strength
Steel strength increases because of nickel in it. Strength is an ability of a material to resist external forces without breaking. Hence, steel can resist external forces much better.
Corrosion
The use of nickel increases corrosion in steel.
Abrasive Resistance
The abrasive resistance of steel increases when nickel is added to it as an alloy.
Elastic Limit
Nickel increases the elasticity of steel.
Chromium Steel
Chromium molybdenum steel – also called chrome moly – is a kind of low alloy steel used in many applications and industries. The two key alloying elements are molybdenum (Mo) and chromium (Cr). This alloy is used in construction, energy, oil & gas, and automotive.
For long, Mo has been an important element used to produce steel capable of withstanding temperatures up to 530 °C. This is because Mo slows the coagulation and coarsening of carbides during high-temperature use. Hence it is suitable in power generation and petrochemical plants.
However, increasing Mo content of the steel beyond a certain point actually decreases the ductility. A solution was discovered by adding chromium along with molybdenum. This gave the steel a number of advantages not found in Mo-based alloys. This added strength and corrosion resistance explains the fact that CrMo steel is used when the strength provided by mild carbon steel is not enough. These benefits provide chrome moly added reliability, which is why it is used in so many different applications.