Steel Variants – And the Industries That Use Them: Steel is one of the oldest and most widely used commodities in existence. Its uses span from home appliances and furniture to industrial machinery. This incredible metal is an alloy of iron and carbon with a few other things – called impurities – thrown in.
Steel is graded into multiple variants or types depending on the proportion in which the components are used and the potential use of the end product. According to the World Steel Association (WSA), there are more than 3000 variants of steel.
There are Four Broad Categories of Steel:
It is quite difficult to classify a metal such as steel because of the sheer number of criteria for classification. Some of these criteria include:
The American Iron and Steel Institute (AISI) therefore states four broad categories of steel. Each broad category has several sub and sub-sub categories. The four broad categories include:
Before we venture to explain these classifications, it is essential to understand that steel is primarily made from mined iron ore combined with carbon. Several other alloying materials are also used, but iron ore and carbon are the two main components. The composition is what differentiates the four main categories.
Carbon steel is made only from iron and carbon with no other substances. This type of steel accounts for 90% of the world’s steel production. Some other metal or mineral – such as nickel, chromium or manganese – may be used for hardening. Carbon steel, therefore, has many variants depending on the proportion of other substances used in production. Sub-categories of carbon steel are differentiated on the amount of carbon and other alloying substances used in making the steel. It may also be differentiated on the finishing technique used, the shape of the end product or the uses that the product would be put to.
Alloy steel is made up of iron ore mixed with various chemicals such as aluminium, copper, chromium, silicon, nickel, manganese and titanium. The proportion of chemicals varies depending on the desired properties, which in turn depends on where the steel would be used. These variations form the sub-classification of alloy steel. The four most commonly used varieties are:
Grade 4140 – Chromium Molybdenum Steel
Grade 4340 – Nickel-Chromium-Molybdenum Steel
Grade 6150 – Chromium-Vanadium Steel
Grade 8620 – HSLA-Nickel-Chromium-Molybdenum Steel
Alloy steel is used for industrial purposes such as making pipes, auto-parts and electronic machinery.
Stainless steel – named for its property of resisting corrosion, heat and stains – is made up of iron ore mixed with 11-20% chromium, which is the principal alloy, and some nickel. Chromium gives stainless steel its corrosion resistance, and, depending on the amount of chromium used, steel can be magnetic or non-magnetic. The three broad categories of stainless steel are:
Austenitic Steel: Non-magnetic-heat-treatable steel which usually contains a high proportion of chromium and 5-10% nickel. This steel is what we see in our kitchen – it is what home appliances and utensils are made of.
Ferritic Steel: Ferritic steel also contains a high amount of chromium – about 12-17% – and traces of nickel and carbon along with a few other alloys. While Austenitic steel is hardened by heat, Ferritic steel is hardened by cold-pressing. Cheaper than Austenitic steel, Ferritic steel finds application for industrial purposes.
Martensitic Steel: This contains 11-17% chromium and traces of nickel as well as carbon. This type of steel is used for making knives and other cutting tools as well as in dental equipment.
As the name suggests, this is steel used to make tools. Tungsten is the primary alloy used in making tool steel. It is used in combination with molybdenum, cobalt and vanadium. Varying quantities of these substances are used to make steel more or less resistant depending on the purpose or use of the steel.
Other Classifications of Steel
Steel products are often classified based on their shape or use. Tubular steel, flat steel, coils and roll steel are some such classifications. Another classification is based on the finishing applied to the finished product – galvanised, coated, cold-rolled or hot-rolled, among others.