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Steel (Carbon, Stainless)
METALS
These are the most common metals and can be found in many industrial and domestic applications. The specific properties of each type are determined by the carbon content and alloys. There are four main groups of steels: carbon steels; stainless steels; low alloy steels; and tool steels. Prices range according to the type of steel. Carbon steels are the least expensive, followed by low alloy steels and stainless steels. Tools steels are the most expensive.
Properties
Carbon Steels
Carbon steels have a low, medium or high carbon content, ranging from approximately 0.2–2%. Higher carbon content produces a harder, less ductile and more brittle material. Mild steel (plain carbon steel) is a term that covers a range of carbon steels up to 0.25% carbon content. They are distinguished by ease of solid state forming and welding. Carbon steels are prone to oxidisation and corrosion, so are protected with a coating of some form.
Low carbon steels are relatively ductile, malleable and easy to shape. In contrast, high carbon steels are hard and as a consequence they are both resistant to abrasion and more brittle. Medium carbon steels have levels of carbon and alloys that are ideal for hardening by heat treatment.
Steels are classified as ferrous metals because they contain iron. Certain ferrous metals are prone to corrosion. In the presence of oxygen and water the iron reacts to form a layer of iron oxide, commonly known as rust. This is a degradation process that gradually consumes metal that contains iron from the outside inwards. Protective coatings are applied and must be maintained to prevent corrosion.
Low Alloy Steels
Low alloy steels are made up of iron, carbon and up to approximately 10% of other metals, such as chromium, molybdenum and nickel. The additional alloys are used to improve certain properties of the steel such as resistance to corrosion, formability and toughness. Certain high performance grades of these materials are also referred to as high strength low alloy steels (HSLA), which are commonly used in bike locks and other similarly demanding applications.
Cor-ten is a low allow steel that eliminates the need for a protective coating. The material develops a protective oxidised layer (rust) that prevents further corrosion of the metal.
Stainless Steels
Stainless steels are a group of alloy steels that contain iron, less than 1% carbon, 10% chromium or more and other alloys. The high levels of chromium result in very good resistance to corrosion. There are four main types, which are austenitic, ferritic, martensitic and precipitation hardening. Austenitic grades are ductile, strong and non-magnetic; ferritic grades are less strong, magnetic and generally used for indoor and decorative applications; martensitic grades are the hardest but least resistant to corrosion; and precipitation hardening are high strength and have moderate resistance to corrosion.
Stainless steels are less prone to corrosion than other grades due to the high chromium content. Electropolishing enriches the layer of the chromium on the surface of the metal, which improves brightness and resistance to corrosion.
Tool Steels
Tool steels are so called because they are used for cutting tools and dies. The carbon and alloy content make them very hard, tough and resistant to abrasion.
Processing
Steel alloys are shaped by panel beating, metal spinning, metal stamping, deep drawing, superforming, forging, tube and section bending, casting, press braking and extrusion.
Relevant Applications Bike Frames
Carbon steels are still used for mass-produced bike frames in the low to medium price range. Low alloy steels (in particular chromium molybdenum and manganese molybdenum) have superior properties, such as improved strength to weight, resistance to corrosion and resilience, and are used in the construction of high performance bike frames.
Bike Locks
HSLA steels are strong, resilient and durable. Bike lock manufacturers utilise these properties in both the chains and locks. The properties of these metals are further improved by heat treatment (often referred to as hardening), which greatly improves the steel's resistance to shearing and breaking on impact. This is the process of heating up the steel and cooling it at different rates to form different microstructures. Typical alloying elements include boron, manganese, chromium and molybdenum.
Bike Furniture
The majority of metal street furniture is manufactured from carbon steel, because it is inexpensive, versatile and durable. It is essential that a suitable protective coating is applied and maintained to ensure the longevity of parts.
