Broad Categorisation of Steels

Different types of steel are produced according to the mechanical and physical properties required for their application. Various grading systems are used to distinguish steels based on these properties, which include density, elasticity, melting point, thermal conductivity, strength, and hardness (among others). To make different steels, manufacturers vary the type and quantity of alloy metals, the production process, and the manner in which the steels are worked to produce particular products. According to the American Iron and Steel Institute (AISI), steels can be broadly categorized into four groups based on their chemical compositions:

• Carbon Steels
• Alloy Steels
• Stainless Steels
• Tool Steels

Where does Spring Steel fit in?

This is the first point of confusion for many. Spring Steels occur across all these categories. For example, SAE1070 is a Carbon Spring Steel, also called a ow Alloy Carbon Spring Steek, whilst SAE6150 is an Alloy Spring Steel, and then we have INCONEL-x750, a stainless steel, not strictly a Spring Steel, but in certain conditions of high ambient heat and flux, the best grade of material to use.

Properties of Carbon Steels

Carbon steels are alloys made from a combination of iron and carbon. By varying the percentage of carbon, it is possible to produce steel with a variety of different qualities. In general, the higher the carbon level the stronger and more brittle the steel. Low carbon steel is sometimes called "wrought iron." It is easy to work and may be used for decorative products such as fencing or lamp posts. Medium carbon steel is very strong and is often used for large structures such as bridges. High carbon steel is used mainly for wires. The ultra-high carbon steel also called "cast iron" is used for pots and other items. Cast iron is very hard steel, but it is also quite brittle.

Properties of Alloy Steels

Alloy steels are so named because they are made with a small percentage of one or more metals besides iron. The addition of alloys changes the properties of steels. For example, steel made from iron, chromium, and nickel produces stainless steel. The addition of aluminum can make steel more uniform in appearance. Steel with added manganese becomes exceptionally hard and strong.

Properties of Stainless Steels

Stainless steels contain between 10 to 20% chromium, making the steel extremely resistant to corrosion (rusting). When steel contains over 11% chromium, it is about 200 times more resistant to corrosion as steels that do not contain chromium. There are three groups of stainless steels:

• Austenitic steels, which are very high in chromium, also contain small amounts of nickel and carbon. These are very commonly used for food processing and piping. They are valued, in part, because they are non-magnetic.
• Ferritic steels contain about 15% chromium but only trace amounts of carbon and metal alloys such as molybdenum, aluminum, or titanium. These steels are magnetic, very hard and strong, and can be strengthened further by cold working.
• Martensitic steels contain moderate amounts of chromium, nickel, and carbon, They are magnetic and heat-treatable. Martensitic steels are often used for cutting tools such as knives and surgical equipment.

Properties of Tool Steels

Tool steels are durable, heat resistant metals containing tungsten, molybdenum, cobalt, and vanadium. They are used, not surprisingly, to make tools such as drills. There are a variety of different types of tools steels, containing varying amounts of different alloy metals.