Stainless steel is one of the most widely used materials on this planet, and it comes in a multitude of different grades and make ups. It is key to know the differences between them, and what this means for the steels various forms of applications across numerous industries worldwide.
Sometimes called inox, or inox steel, stainless steel is a steel alloy that is composed of a minimum of 10.5 per cent chromium. Steel is made into an alloy in this way to increase its corrosion resistance, rust resistance, and well – stain resistance thus its more common name.
There are austenitic and ferritic grades of stainless steel. These varieties offer different constructions within the steel. All stainless steels are made up of a combination of oxidation, acids, and bases. Ferritic types are magnetic whereas austenitic are not. Austenitic has austenite as its primary phase (face centred cubic crystal) and it makes up about 70 per cent of all stainless steel production. These are alloys containing chromium and nickel, and sometimes molybdenum and nitrogen. Superaustenitic stainless steel contains a high molybdenum content. Ferritic steels are high chromium, magnetic stainless steels that have a low carbon content – less than 0.2 per cent. This means they have better engineering properties and are also generally less expensive.
There is also a third type called duplex stainless steels. These have a mixed microstructure of austenite and ferrite, the aim usually being to produce a 50/50 mix. These duplex grades are characterized into groups based on their alloy content and corrosion resistance. You can get lean, standard, super, and hyper duplex stainless steels – all with their own individual formulaic make ups.
Choosing the right construction of stainless steel depends on its intended usage. All stainless steels have a good level of corrosion resistance, which alongside its strength, is one of the main reasons the material is in such common usage. The material is versatile, and compared to other substances available, it requires relatively little maintenance, has a long shelf life, and can take enormous loads.
The ‘stainless’ part of stainless steel is used for its heat and corrosion resistance. The nature of the environment must be known before selecting which grade to use, to ensure that the composition of materials is correct and up to standard for the context it will be used in. Mechanical properties must also be taken into consideration, that is, the strength of the stainless steel at a variety of temperatures. Its fabrication operation and how the steel is compiled should also be taken into account – as stainless steel of different grades can be forged, machine made, formed, welded, stamped, or rolled; each process bringing different strengths and weaknesses to the resultant product. These different properties are all affected by the chemical composition of the stainless steel. Stainless steel can be made from different percentages of the following materials, and the DNA of it alters its suitability in different environments and settings:
- Chromium: this helps with corrosion, scaling resistance, tensile strength and reduces wear
- Manganese: this improves performance in heat, can affect strength, acts as a stabiliser, and can be a partial replacement for nickel
- Molybdenum: good for high temperatures and corrosion resistance in acidic and salty environments
- Nickle: this also stabilises the austenitic structure, increased strength in high temperatures, and is a strong material for resisting corrosion in industrial and marine settings
- Silicon: this forms a tight initial scale that helps the end product withstand cyclic temperature changes, it is also good for hardness
- Sulphur Phosphorous and Selenium: these increase machinability and decreases transverse tensile strength
- Titanium: this prevents corrosion through acting as a stabiliser, it helps produce a diner grain, and reduces stretch.
Obviously all of these composite elements are priced differently, and so the make up or grade chosen may also be determined by economic constraints as well as the constraints of the environment. Manganese for instance is quite low cost, and grades with a higher manganese content are often cheaper.
Stainless steel grades are denoted by numbers. Type 201 for example refers to a low nickel, austenitic chromium-nickel-manganese steel that was developed originally to conserve reserves of nickel. SAE International is one of the standards organisations for steel grading, AISI is another. All grading bodies and systems speak a universal language which means the resultant numbering can relate to one another. For example carbon steels and alloy steels usually have a four digit number. The numbers relate to the alloy components and is a quicker way of being assured of the composites you are getting. The first number relates to the dominant alloy element, the second the secondary, the third the percentage of carbon present and the forth the weight. There are numerous combinations and so a great number of different options and types to consider when picking a grade of stainless steel. Wikipedia provides a great summary.
Stainless steel is popular not only due to its multi-functionality and suitability from heavy industry to jewellery, marine use to kitchenware, but also because as a material it is 100 per cent recyclable – whatever grade and composition it is. The material also lends itself to an impressive number of finishes that means you can achieve a variety of looks from a number of grades of stainless steel which is a highly attractive quality.