What is stainless steel?

 
     
 Steel is an alloy of iron and carbon which resists corrosion and rust 

Steels are a family of alloys based on iron and carbon. Stainless steels also contains chromium, and are a subgroup of steels, rather than a single material.

 

Standard steel rusts easily in moisture, and stains if exposed to acids, so stainless steel was developed to stain 'less' and resist rust and corrosion. 

 
     
    Wonkee Donkee says "An alloy is defined as a metal made by combining a metal element with one or more other elements. Commonly two or more metal elements are combined. It is also the name of the fancy wheels on some cars!" 
     
 There are over 150 different grades of stainless steels available  

There are currently over 150 different grades of stainless steel, with about 15 being the most commonly used. Austenitic stainless steels are the most widely used, with 304 and 316 stainless steels being two of the most popular grades.

 
     
   

Composition of stainless steel

 
 Steel has many uses and is a common material 

The addition of chromium to a low carbon steel changes its properties. Usually a minimum of 10.5% chromium needs to be present for steel to be classed as stainless, with the cutlery industry using a standard of 13%. Also, there needs to be less than 1.2% carbon present.

 
     
 Stainless steel forms a passive layer which protects it 

This composition is necessary for stainless steel to be able to build up a 'self-healing' layer on its surface, called the 'passive' layer. A material is classed as 'passive' when it is less affected by environmental factors. This passive layer is chrome oxide, and it protects the steel beneath it. If stainless steel is scratched, the passive layer can reform, as long as it has exposure to oxygen.

 
     
 Other elements can be added to steel, as well as chromium, these include molybdenum, nickel and silicon 

The properties of stainless steel can be altered by the addition of other elements. Even trace amounts of some elements drastically alter the alloy's corrosion resistance. 

 

Elements which can be added to stainless steels include vanadium, nickel, niobium, or molybdenum. The most corrosion resistant and usually most expensive, usually have high amounts of chromium, molybdenum and nickel.

 
     
   

A brief history of stainless steel

 
 The iron pillar from Delhi is a famous iron pillar which has resisted rust for about 1,600 years  

Some rust-resistant alloys have been found which date back to ancient times. A famous example is the Iron pillar from Delhi, dating back to around the 4th/5th Century AD. This 7.3-metre tall pillar was forged out of wrought iron, yet it's resisted corrosion for about 1,600 years. Although no chromium is present in the iron, it's believed its corrosion resistance is due to higher levels of phosphorus.

It wasn't until around 1820 that scientists noted that irons alloyed with chromium were more resistant to corrosion from certain acids. Then, in 1875, a crucial development was made in France, when the importance of the low carbon content was detailed.

 
     
 Other elements can be added to steel, as well as chromium, these include molybdenum, nickel and silicon 

Arguably, the first stainless steel was actually created accidentally, by Harry Brearley of Sheffield, in 1913. Brearley was trying to find erosion-resistant steel for making gun barrels. He discovered a steel with 12.8% chromium and 0.24% carbon resisted corrosion from acids and went on to produce the first cutlery in this stainless material. Although other scientists in different countries were also making similar discoveries around this time, Brearley was the first to use the name 'stainless', which has stuck ever since.

 
     
   

What are the different types of stainless steel?

 
 Other elements can be added to steel, as well as chromium, these include molybdenum, nickel and silicon 

Adding different elements to stainless steel, as well as the chromium, change its microstructure. The microstructure of stainless steels are used to group them into families and each grade is developed within these families.

 

These are the main groups:

 
     
 Austenitic stainless steels can be used in low temperature applications  

Austenitic stainless steels

These are the most commonly used, making up about 70% of the world's stainless steel usage. They are generally suited to use in low temperatures, including cryogenic applications, unlike many other steels which become brittle at reduced temperatures.

 
     
    Wonkee Donkee says "Cryogenics is a branch of engineering and physics which studies how materials behave at extremely low temperatures, below -150°C. I’m shivering just thinking about it!" 
     
 Steel has many uses and is a common material 

They can be work hardened with cold working and are the easiest stainless steels to form, although they can't be hardened with heat treatment. Elements, such as molybdenum, can be added to increase the corrosion resistance and make them suitable for use in high-temperature applications. 

 
     
 ferritic stainless steels are magnetic 

Ferritic stainless steels

These stainless steels contain little or no nickel, consisting mainly of iron and chromium. They have the advantage of being corrosion-resistant at high temperatures, but generally less resistant to corrosion than austenitic steels, and less ductile. They can't be hardened with heat treatment and are magnetic. 

 
     
 Martensitic stainless steels are able to be hardened with heat treatment 

Martensitic stainless steels

These contain higher levels of carbon than other stainless steels, which means that they can be hardened with heat treatment, like alloy steels. However, they can't be cold worked and have poor weldability. They maintain moderate corrosion resistance and are magnetic.

 
     
 Duplex stainless steels can be used in applications such as offshore oil and gas rigs  

Duplex stainless steels

These steels have two microstructure phases, austenite and ferrite, and combine the properties of both those stainless steel families. They are stronger than ferritic and austenitic steels and also more resistant to chloride corrosion. These are usually found in industrial applications such as chemical plants and offshore gas and oil installations.

 
     
   

What are the properties of stainless steel?

 
 Stainless steel is incredibly versatile 

Stainless steels are particularly versatile, and can maintain the strength of standard steel, with the added advantage of corrosion and oxidation resistance. 

 
     
 Advantages and disadvantages of stainless steel 

ADVANTAGES

 

DISADVANTAGES

 
    
  • Attractive appearance

  • Strong but fairly ductile

  • Resists corrosion, oxidation and abrasions, without the need of an extra finish or coating

  • They can be produced to be used in high or low-temperature applications

  • 100% recyclable

   
  • More expensive to produce than standard steels

  • Often harder to work and form than standard steels

  • Harder to weld than standard steel

  • Apart from some austenitic stainless steels, they are not as tough as standard steel

 
   
   

Uses of stainless steel

 
 Stainless steel is widely used in many applications and industries 

Stainless steels are used extensively, in many applications and for many different industries. Here are just a few common uses for them:

 
     
 Stainless steels are a common material used to make tools 

DIY industry

Many tools are made from stainless steel, and it's a fairly popular choice due to its strength, durability and resistance to corrosion. As a result, stainless steel tools may be more expensive than some standard steel tools. 

 

Note: Stainless steel may not be as strong as some steel, especially if it has been hardened through heat treatment. 

 
     
 Many different types of tools can be made from stainless steels 

Examples of tools which are often made of stainless steels include blades of various cutting tools such as knives or shears, pliers, scrapers, multi-tools, and many measuring instruments, such as gauges, callipers, or rulers.

 

You will often find that stainless steel is used for tools that will be continually subjected to moisture throughout their working life, either from the materials they come into contact with or as a result of being used outdoors or in humid conditions, where corrosion is a potential risk. 

 
     
 Stainless steel is used in many applications throughout the home. 

Domestic 

You will find stainless steel items in many places around the home. Such items include jewellery, cutlery, knives, cookware, sinks, appliances such as cookers, freezers, and washing machine drums, as well as things like knives and razor blades. 

 
     
 Surgical and medical instruments are often made from stainless steels 

Medical and surgical equipment

Stainless steels are hygienic, can be easily serialised, and won't corrode, so they are popular materials for medical and surgical equipment.

 

Stainless steel is used for making a huge range of medical instruments and also operating tables and some surgical implants.

 
     
 Stainless steels are used in many applications for lots of different transport vehicles. 

Transport

Stainless steels are used across a range of different transport vehicles. This includes exhausts and trims on cars, tankers, aircrafts, road tankers, ship containers and tankers, refuse vehicles and rail carriers. 

 
     
 The gateway arch in St. Louis is made from stainless steels 

Architecture and construction 

Stainless steels have been commonly used in construction for decades, for things such as door and window fittings, cladding, handrails, structural sections, and reinforcement bars. They are now also used on a larger scale, particularly with high impact buildings.

 
     
 Many large building use stainless steel in their construction 

Some famous stainless steel structures include the Gateway Arch in St. Louis, the top of the Chrysler building, the Eurostar tunnel in London, the Petronas towers in Kuala Lumpur, the Cala Galdana Bridge in Spain, and the Walt Disney concert hall, by Frank Gehry, in Los Angeles.

 
     
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