What is steel?
Steel is an alloy of iron. Since the primary element used in the alloying process, other than iron itself, is carbon, steel is defined by its carbon content. Steel can have anywhere between 0.002% and 2.1% carbon by weight, but always contains smaller amounts of the elements manganese, phosphorus, sulphur, silicon, and traces of oxygen, nitrogen and aluminium in addition to carbon and iron. While these elements are required for an alloy to be accurately referred to as 'steel', others such as manganese, nickel, chromium, molybdenum, boron, titanium, vanadium and niobium can be added to modify the characteristics of the alloy. This article will focus on stainless steel, which is formed by creating a steel alloy containing a minimum of 10.5% chromium by mass.
How does stainless steel differ from regular steel?
Under conditions of high moisture and high salinity, the iron found in steel will oxidise to form iron oxide. A layer of iron oxide on the surface of the steel is able to increase the rate of its own formation, thus encouraging more corrosion to occur, and causing the structure of the steel to be compromised. Stainless steel contains a sufficient proportion of chromium to facilitate the formation of a layer of chromium oxide on the surface of the material. This layer of chromium oxide does not catalyse the formation of more chromium oxide, and acts to protect the iron within the structure from oxidation, a property referred to as 'passivation'. This property of stainless steel makes it incredibly corrosion resistant, although the lustre of the material can be lost under certain conditions, so it is not truly 'stainless'.
Grades of stainless steel
Since steel can be alloyed with an infinite number of combinations of alloying elements, a classification system has been developed to facilitate easy identification of the compounds used. Stainless steel has its own list of classifications, each of which will be explained in outline below. Note that some of the types of stainless steel are described as 'austenitic', meaning that they are composed of the gamma allotrope of iron:
Type 102 - an austenitic, general purpose stainless steel widely used in furniture.
200 Series - austenitic chromium-nickel-manganese alloy series used for general purpose metalwork.
300 Series - austenitic chromium-nickel alloy series. Contains the most popular grade of stainless steel, grade 304. This series of stainless steel is used for food utensils and surgical scrub sinks and instruments.
400 Series - ferritic and martensitic chromium alloy series. This series contains some of the cheapest stainless steel, used for car exhausts, some higher grade cutlery steel, and some of the hardest stainless steel, used for replica swords and knives.
500 Series - heat-resistant chromium alloy series.
600 Series - martensitic alloy series formed by precipitation hardening.
Type 2205 - a duplex (both ferric and austenitic) alloy with excellent strength and corrosion resistance.
Type 2304 - an alloy similar to type 2205 in all respects except for a lower molybdenum content, leading to reduced pitted corrosion resistance.
304 Grade stainless steel
The most common type of stainless steel in use is grade 304. This grade has an 18-20% chromium content and a 8-10.5% nickel content by mass, although 304L exists as a low-carbon alternative to 304, and 304Cu and 304N versions, enriched in copper and nitrogen respectively, also exist. This type of steel is also often referred to as A2 stainless steel. Grade 304 offers an impressive strength, as well as incredible corrosion resistance, lending it to applications in which contact with food, chemicals or fresh water must be tolerated. Type 304 stainless steel also has properties which make it well suited to welding and machining applications, thereby making it an excellent material for use in making machined parts.
316 Grade stainless steel
The second most common type of stainless steel is 316. The physical and mechanical properties are very similar to 304 stainless steel, but the key difference is that 316 stainless steel incorporates about 2-3 percent molybdenum. Molybdenum has the ability to protect against chlorides and other industrial solvents, which gives the steel a stronger corrosion resistance.
Our products are made in 304 grade stainless steel as standard, but 316 is available for certain products. If you have a query, please contact us here.