Steel, a term that actually describes an entire family of metal alloys, is a versatile and common type of metal with a wide variety of applications and uses. There are many grades but most types of steel fall into two broad categories, carbon steels and stainless steels. Though they have the same basic composition of iron and carbon, steel types tend to have a variety of alloying elements. Carbon steel tends to have under 10.5% chromium content, but steel must be at least 10.5% chromium to be considered stainless. These differences give each type of steel its respective properties.
Stainless Steel
Stainless steel refers to a type of steel which is defined by the addition of chromium, and some other alloying elements such as nickel. It is sometimes called inox steel as it is designed to protect against oxidisation and so is ‘inoxidable.’ When exposed to oxygen, iron oxidises, making it rust, however chromium can be exposed to oxygen without undergoing this process. Stainless steel is therefore given a protective layer of chromium to create a barrier between environmental oxygen and the metal’s iron content. This allows it to resist corrosion or rust and makes it ‘stainless.’
Types of Stainless Steel
Different chromium levels of stainless steel will give it different properties, with a lower chromium content generally producing a cheaper but less durable steel. There are various types of stainless steel, which include:
- Austenitic, the most widely used type of stainless steel, with low yield strength but strong corrosion and heat resistance, commonly used in housewares, industrial piping and vessels, construction, and architectural facades – this is the largest family of stainless steel and makes up about two thirds of all stainless steel production
- Ferritic, a form of steel generally with no nickel, often possessing better corrosion, heat, and cracking resistance than more common types, and frequently used in washing machines, boilers and indoor architecture
- Martensitic, which tends to be magnetic and less corrosion-resistant than other stainless steels due to its low chromium content – these matels are very hard and strong and are used to make knives and turbine blades
- Duplex, a composite of austenitic and ferritic steels, making it both strong and flexible, with twice the yield strength of austenitic stainless steel, used in the paper, pulp, shipbuilding, and petrochemical industries
- Precipitation, with the corrosion resistance of austenitic metals, but can be hardened to higher strengths, and so can be made to be extremely strong when other elements like aluminium, copper and niobium are added
Advantages
- Corrosion resistant properties
- High and low temperature resistant
- Exists in a large variety of types
- Strong and highly durable
- Low maintenance and easily cleaned
- Long lasting, with a relatively low cost over the course of its lifecycle
- Can be given a particular finish if attractive cosmetic appearance is desired, and does not tarnish easily
- Environmentally friendly and recyclable
Disadvantages
- High cost especially at the initial expense
- Can be a difficult metal to handle, especially without the highest technology machines and techniques
- Can often result in costly waste and re-work
Applications
Stainless steel has a wide range of uses and industrial applications, depending on the type of steel used. Stainless steel was first used in the automotive industry by Ford in the 1930s by Ford, and has since been used in cars for exhaust systems, grills, trims, and structural components. This extends to aircraft construction, where it is used in aeroplane frames, jet engines, and landing gear. Its resistance to corrosion, low maintenance, and how easy it is to clean makes it useful for transporting and interacting with chemicals, and it is often used in clean and sterile environments. Therefore, medical technology is also a fairly common use.
The strength, resistance, and flexibility of certain stainless steels make it a common feature in architecture, a property helped by its aesthetic features and attractive finish as well. For example, stainless steel is frequently used in the Eurostar Terminal in London and the Helix Bridge in Singapore.
One of the most common everyday uses of stainless steel is in food and the catering industry, where it is used to manufacture cookware, cutlery, kitchen accessories, and appliances. Utensils like knives are made from the less ductile grades of stainless steel, whilst the more ductile grades are used for grills, ovens, saucepans, and sinks.
Carbon Steel
Carbon steel, on the other hand, has far lower chromium levels, and is instead a carbon-iron alloy with only a few other materials, if any, as part of its makeup. As a result, it can be referred to as ‘low-alloy’ steel. Both stainless steel and carbon steel feature this basic composition, but carbon steel can be defined by its carbon content. This is normally around 2-2.5%, however this often varies. Though susceptible to rust unlike stainless steel, carbon steel is often cheaper and has its own varying mechanical properties based on carbon content.
Low-carbon steels are weaker and softer, but can be machined and welded easily; while high-carbon steel is stronger, but significantly harder to process.
Types of Carbon Steel
The defining element of any carbon steel alloy is its carbon content, and so this is how the various types of carbon steel are ordered and categorised.
- Low-carbon steel: the most widely used form of carbon steel, with a carbon content of less than 0.25% - they are usually relatively weaker and softer but more easily welded and ductile, and so are often used to machine and weld for a low cost
- Medium-carbon steel: with a carbon content of 0.25-0.6%, and a manganese content of 0.6-1.65%, these metals can be improved via heat treatment, though this can only be performed on very thin sections unless additional alloying elements are added – these steels are stronger but less ductile than lower-carbon steels
- High-carbon steel: this is the hardest and toughest carbon steel, with the lowest ductility, very wear-resistant and almost always hardened and tempered – they tend to have a carbon content from 0.6-1.25% and a manganese of 0.3-0.9%
Advantages
- Increased strength
- Less expensive than stainless steels
- Wear-resistant
- Durable and shock resistant
- Safe to handle and work with compared to other metals
- Environmentally-friendly and easy to recycle
Disadvantages
- Strong and sometimes difficult to work with
- Brittle, not easily bent or moulded
- More susceptible to rust and corrosion
- Less attractive appearance, cannot achieve the finish of stainless steel
Applications
The various types of carbon steel are applicable to a wide range of industries and sectors. Low-carbon steels can be used in automobile body components, pipes, construction and bridge components, and food cans. Medium-carbon steel is useful for railway tracks, train wheels, crankshafts, and gears and machinery parts requiring its higher strength and toughness, and similarly high-carbon steel is used in cutting tools, springs high strength wire and dies for its hardness.
Can Stainless Steel Weld to Carbon Steel?
Though it is technically possible to weld these different types of metals together, the popular consensus is that to do so is not advisable. The welding of carbon and stainless steel is not typically performed as they have differing levels of electrical conductivity. Carbon steel is normally preheated during resistance welding because it is more electrically conductive and doesn’t heat up as quickly as stainless steel. Therefore, reaching the right weld temperature is often extremely difficult. Therefore, it is generally advised that steels of similar types be welded together rather than mixing and matching.