Stainless steel is the general name for a family of steels that are corrosion-resistant and contain a minimum of 10.5% chromium. The effect of this minimum level creates a chromium oxide layer on the surface, which is a self-healing oxygen barrier that stops further oxidation. Below 10.5% chromium, the oxide film is of insufficient durability to be self-healing.
Stainless steels are widely used in most industries, including construction, marine, aerospace, automotive, and consumer goods. The material delivers high strength, durability, and resistance to corrosion, making it ideal for many cosmetic and open-environment components. Stainless steel is supplied in grades classed by their alloy content. The grade selected for an application depends on factors such as the environment in which it will be used, the required strength and durability, and the desired aesthetic standard. This article will define: What is stainless steel?, its composition, types, grades, properties, and applications.
What Is Stainless Steel?
Stainless steel is primarily made from medium and low-carbon steel. They are alloyed with a range of metals to alter the resulting properties. For example, chromium and nickel lend corrosion resistance and hardness. Other metals enhance malleability, toughness, ductility, tensile and shear strength, and other properties. Stainless steels are generally high strength, with some examples being among the highest tensile and yield strengths of all metals. They are also generally corrosion-resistant, with exceptional performance in particular environments according to grade/alloy type. Additionally, stainless steels are capable of high- and low-temperature performance, from cryogenic temperatures to 2,000 °C, depending on grade.
Depending on the definition, there are at least 60 and possibly over 100 distinct grades of stainless steel that continue to be developed for general and specific purposes. The study of stainless steel is among the most active areas in metallurgical research. These grades are covered by various national and international standards which are progressively becoming unified and generally define close or identical alloys/properties.
The History of Stainless Steel
The history of stainless steel began in 1912, when Harry Brearley, an English metallurgist leading the Brown Firth Laboratories in Sheffield, was researching hard-wearing steels for gun barrel improvements. Some of his experimental alloys were observed to be highly resistant to corrosion, and he saw and understood the wider implications of his work. He began experimenting with different compositions of steel and found that it was the addition of chromium to the alloy that created the biggest influence on the resulting resistance to corrosion.
Brearley patented his new alloy under the name “rustless steel” (later modified to stainless steel) in 1915. The research lab owners were Sheffield steelmakers, whose primary reputation was in weapons and table cutlery. It was in this market that the first application for stainless steel was developed, making good use of the strengths of the new alloys. During World War I, stainless steel was also used in military applications such as aircraft engines and gun parts.
Stainless steel is now a primary material in most industrial sectors, including appliances, medical equipment, automotive parts, and construction materials. Its resistance to corrosion, as well as its strength and durability, increasingly make it a popular choice for many industries.
How Stainless Steel is Made
Stainless steels are made of alloying elements such as: chromium, nickel, molybdenum, and titanium to achieve specific properties like improved corrosion resistance, higher tensile strength, or improved ductility/malleability. The exact composition of stainless steel is a wide-ranging topic, with small changes in constituent metals potentially making significant adjustments in properties.
Stainless steel is manufactured by a range of smelting methods that are typical of most metallurgical processing. Three of the common processes are listed below:
- An electric arc furnace uses a high current flowing between two carbon electrodes to heat the raw materials. Alloying elements of scrap and pre-made steel, chromium, etc. are added to the furnace crucible, along with iron ore and the necessary additions to refining this component, to create the stainless steel alloy. Melting and refinement occur in the furnace and the impurities float off as slag/clinker.
- Blast furnaces melt iron ore mixed with coke in the crucible, blowing oxygen through the molten iron to remove impurities that float off as slag. Once the ore has been purified, chromium and other alloying elements are added to the molten metal to create the stainless steel alloy.
- Vacuum induction melting and vacuum arc melting produce very high-purity metals, from already refined source materials (i.e., no ore is added). This process allows for more precision in the control of the composition of stainless steel alloys.
Once the stainless steel has been melted and refined, it will generally be rough cast into billets. These billets are then formed into various shapes and sizes using a range of manufacturing processes, including:
- The casting of stainless steel involves molten metal being poured into a cast cavity, to which it conforms and the cast retains the cavity shape when cooled.
- Rolling of stainless steel sees the raw billets pass through a series of precise pinch rollers, to reduce their thickness and shape the steel into sheets or other forms. This process can be performed hot or cold, to produce a variety of final materials of diverse strengths and crystalline structures.
- The forging of stainless steel requires it to be heated and then shaped by hammering or pressing it into the desired form.
- Machining stainless steel enables it to be cut and shaped using various machines, such as lathes and mills.
After the stainless steel has been formed into its final shape, it may undergo additional processing to improve its properties, such as heat treatment, polishing, or coating.
Characteristics of Stainless Steel
Some of the characteristics of stainless steel include:
- Generally highly resistant to corrosion.
- Strong and durable—resistant to bending, cracking, and breaking.
- Non-porous and non-reactive—easy to clean and autoclave.
- Can achieve a range of quality finishes.
- Can withstand high temperatures without degrading.
- Suitable for use at cryogenic temperatures.
- Sustainable materials, in that they are 100% recyclable without loss or degradation.
Color and Appearance of Stainless Steel
Stainless steels are naturally a uniform silver in color, although various companies offer proprietary “stainless coloring” processes that are analogous to anodizing. A range of bronze to golden colors can be achieved by heating stainless steel in an oven to around 700 °C in an oxygen atmosphere. This will result in the formation of iron oxides in the surface film, which will stain with shades of yellow, gold, and brown depending on temperature and time. Stainless steel will also develop hues of blue when heated to 250 °C in air, also by an oxidation process.
Stainless steel parts are often polished to a mirror finish by electropolishing. This results in a reflective and generally silver color, but can be colored as described above. Stainless steel components can also have various types and levels of texturing added—from sandblasting to shot peening, brush finishing to photo etching.
A brushed or linished finish is often used on street furniture and building cladding, to reduce the appearance degradation that results from scratches. Figure 1 below shows stainless steel parts:
