Although steel consists mostly of iron and carbon, the World Steel Association reports that more than 3,500 grades of steel are in existence, each featuring unique chemical, physical and environmental properties. The amount of carbon, additional alloying elements, and level of impurities contained determine the properties of each grade. Even though the most commonly used grades of steel contain between 0.1 and 0.25 percent carbon, some can have a carbon content of up to 1.5 percent. All grades of steel contain elements like manganese, phosphorus, and sulfur. While the last two are known to have deleterious effects on the strength and durability of steel, manganese has significant benefits.
Steel is produced in various forms, each featuring unique properties that may have been manipulated to fit specific applications. These properties are the basis of every grading system used to distinguish one form of steel from the other. According to AISI, steels can be grouped into the following categories depending on their chemical compositions or metal alloy contents:
• Carbon steels
• Alloy steels
• Tool steels
• Stainless steels
Carbon steels contain iron, carbon, and trace amounts of other alloying elements. Carbon is, however, the main alloying constituent of carbon steels, which account for approximately 90 percent of all steel productions. Because it has small enough atoms, carbon can travel through iron’s crystal lattice, filling the gaps between the metallic atoms and distorting the metal’s lattice slightly. As a result of this, the metal becomes much stronger and a lot more rigid. Carbon is the hardest element, which is why carbon steel products are also very hard. Varying the percentage of carbon produces steel with different qualities. However, higher carbon content often translates to stronger yet more brittle steel. Depending on the level of carbon contained, carbon steels can be classified into the following groups:
• Mild or low carbon steels that contain up to 0.32 percent carbon
• Medium carbon steels, containing 0.30-0.59 percent carbon
• High carbon steels, known to contain over 0.6-0.99 percent carbon
• Ultra-high carbon steels that usually contain about 1.0–2.0 percent carbon
Also known as wrought iron, low carbon steel is the commonest and most cost effective form. It is easy to work, making it suitable for decorative products like lamp posts and fencing. As one of the stronger variants, medium carbon steel is often used to forge large structural applications and automotive components while high-carbon steel is mostly used for high-strength wires and springs. Also known as cast iron, ultra-high carbon steel is the hardest form of them all and often used for knives, axles, punches and other special purposes. Since carbon steels do not contain chromium, they tend to corrode faster than almost every other type of steel.
Depending on the variant, carbon steels are:
• Extremely hard
• Resistant to wear
• Offer exceptional machinability when treated with sulfur
• Can be tempered to great hardness
• Poor rust resistance
• Relatively low tensile strength
• Some types suffer from yield-point runout and can, therefore, require high levels of maintenance.
The name comes from the fact that steels in this group contain a small percentage of other metals besides iron. Alloy steels contain common alloy metals in varying proportions, which makes this type of steel suitable for specific applications. These alloy metals include aluminum, manganese, nickel, titanium, silicon, copper, and chromium, the addition of which produces properties that are different from those found in regular carbon steels. When added, alloying elements can change properties like strength, ductility, formability, hardenability, and ability to resist corrosion. For instance, stainless steel is produced by adding chromium and nickel whereas the addition of aluminum results in a more uniform appearance. On the other hand, the addition of manganese is known to make steel extremely hard and strong. Alloy steels can have diverse mechanical properties due to the broad range of compositions possible.
Depending on the variant, alloy steels are:
• Widely available
• Easy to process
• Resistant to corrosion
• Responsive to heat treatments
• The hardness has a negative impact on workability
• Can be allergic, especially if it contains nickel
Because of the properties mentioned above, alloys steels serve a broad range of applications including the manufacture of pipelines, transformers, auto parts, electric motors and power generators.
This type of steel is alloyed at very high temperatures and often contains hard metals like tungsten, cobalt, molybdenum and vanadium. Since they are not only heat resistant but also durable, tool steels are often used for cutting and drilling equipment. Even so, there are various types of tools steels, each containing varying quantities of different alloy metals. As a result, each type of tool steel offers a different level of heat resistance and durability.
• Extremely durable
• Highly heat resistant
• Limited application
• Difficult to manipulate
Although stainless steels comprise of several metal alloys, they usually contain 10-20 percent chromium, making it the primary alloying element. Compared to the other forms of steel, stainless steels are approximately 200 times more resistant to rusting, especially the types that contain at least 11 percent chromium. As a result, stainless steel is highly valued for its ability to resist corrosion. Based on their crystalline structure, stainless steels fall into one of the following categories:
1. Austenitic steels
Although austenitic steels contain trace amounts of nickel (eight percent) and carbon (0.8 percent), they are high in chromium. In general, austenitic steels have a chromium content of about 18 percent. With applications like the manufacture of pipes, kitchen utensils, and food processing equipment, austenitic steels are the most commonly used type of stainless steel. Even though austenitic steels are not responsive to heat treatments, they are valued for their non-magnetic properties.
2. Ferritic steels
Apart from containing trace amounts of nickel, less than 0.1 percent carbon, and about 12-17 percent chromium, ferritic steels usually contain alloy metals like molybdenum, aluminum or titanium in small quantities. Ferritic steels are magnetic, tough, and very strong. However, cold working can be used to strengthen them further. Unfortunately, they are not responsive to heat treatment, meaning no heating technique can be used to harden them.
3. Martensitic steels
In addition to containing moderate amounts of carbon (about 1.2 percent) and nickel (less than 0.4 percent), martensitic steels contain 11-17 percent chromium. Aside from having magnetic properties, martensitic steels are also responsive to heat treatments. This type is mainly used to make dental and surgical equipment, blades, knives, and several other cutting tools.
Stainless steels have the ability to withstand most of the wear and tear caused by everyday use, making them highly durable. Additionally, an invisible layer of chromium serves to prevent oxidation, making stainless steels resistant to scratches and corrosion.
Stainless steels are generally:
• Low maintenance
• Highly resistant to scratches and corrosion
• Easy to dent
• The inclusion of nickel raises allergic concerns
• The hardness means limited workability
As evidenced by the properties offered by each of the forms, steel has qualities that can meet a broad range of applications. In fact, steel serves an array of construction, appliance, energy transport, and packaging purposes in today's world.
Jessica Kane is a professional blogger who writes for Federal Steel Supply, Inc., a leading steel tubing suppliers of carbon, alloy and stainless steel pipe, tubes, fittings and flanges.