Stainless steel is a metal alloy, made up of steel mixed with elements such as chromium, nickel, molybdenum, silicon, aluminum, carbon, etc. It is universally preferred for making kitchen utensils, because it does not affect the flavor of food and is easy to clean. Minimal maintenance and possible total recycling of stainless steel utensils also contribute to their popularity. It is called stainless as it is highly resistant to stains, i.e., rusting.
Pure iron (Fe), mixed with carbon to produce 'steel', is the main element of stainless steel. Chromium is added to steel to make it resistant to rust. Pure iron is prone to rusting, which is due to its reaction with oxygen in the presence of water (moisture), whereas iron's reaction with oxygen is corrosive and eats away at the iron, chromium reacts with oxygen in such a way that only the outer surfaces become oxidized and further oxidation is prevented. Chromium forms a passive layer of chromium oxide, which prevents mechanical and chemical damage, over steel.
The other minor constituents of steel are nickel, nitrogen, molybdenum and aluminum. Small contents of nickel increase the corrosion resistance further, and protect it from rough usage and harsh environmental conditions. Pitting or scarring is avoided by adding molybdenum to steel.
Chemical properties and structure of stainless steel are improved using other alloys. Titanium, vanadium and copper are the alloys which make it more suitable for specific uses, and are used only when the buyer demands the specialized type of steel.
Classes of Stainless Steel
The five classes of stainless steel are austenitic, martensitic, duplex, ferritic and precipitation hardening. The chemical compositions of these five classes are as follows:
This steel is called austenitic because it is made from austenitizing elements. Austenitization changes the crystal structure of iron or iron-based material, such as steel, from ferric to austenitic. Since the basic iron-based structure is changed, austenitic steel is not magnetic.
Iron, chromium, nickel and molybdenum are the basic constituents of this type of steel. This steel has high ductility and relatively high tensile strength. Approximately, 16-20% chromium and about 10% nickel are the typical contents of this steel. This is the most widely used steel, with over 70% of the total stainless steel produced being austenitic.
Applications of this type of steel can be found in:
- Petrochemical industries
- Food processing industries
- Kitchen sinks
- Chemical plants
This steel is called martensitic as it possesses a martensitic crystal structure in hardened condition. Chromium and carbon are its main contents. It is less resistant to corrosion, and contains around 18% chromium and up to 1% carbon. The high carbon content (other stainless steels only have about 0.1%, or even less, of carbon) makes this steel extremely strong but slightly brittle. Martensitic steel is magnetic. It is used in:
- Surgical instruments
- Knives and blades
- Shafts and spindles
This type of steel is ferromagnetic in nature, has relatively good ductility and is usually used to make kitchen utensils. Approximately 10-27% chromium is present along with iron. Ferritic steel rarely contains nickel, but lead or titanium may be present. Because of the low chromium and nickel content, resistance to rust is reduced. As a result, this type of steel is usually a bit cheaper. Its applications are in:
- Automotive exhausts
- Automotive trims
- Computer floppy disk hubs
This type of steel is used in chloride and sulfide environment and is the least corrosive. It is a mixture of ferritic and austenitic steels (hence the name), usually in 50-50 or 60/40 ratio. Primary constituents are chromium and molybdenum. Nickel may be present in a small quantity or may be absent. Its applications are found in:
- Oil and gas explorations and off-shore rigs
- Chemical processing, transport and storage
- Pulp and paper manufacturing
Precipitation Hardening Steel
It is made from heat-treated martensitic or austenitic stainless steel and is stronger than either. It also has a higher resistance to rust. Chromium and nickel are the primary components of this type of steel. Because of its strength, this type of steel is used in:
- Turbine blades
- Aviation engine components
- Nuclear waste container
The basic requirement from stainless steel is corrosion-resistance. The factors affecting this property are the chemical composition of the corrosive medium, percentage of the components used, temperature variations, oxygen and moisture content and aeration of the medium. Thus, slight variations in chemical compositions can be used to make a variety of stainless steel, which, in addition to being an excellent engineering material, is 100% recyclable and can therefore said to be environment-friendly.