Since normal unalloyed and low alloyed steels are not resistant to corrosive effects, it is normally necessary to use stainless steel in such applications. Stainless steels have perfect corrosion resistance, have specific types with various mechanical properties, have the ability to be used at low and high temperatures, and are easily malleable and aesthetic, etc.
Stainless steels is a steel family that contains at least 11% chromium. The factor that provides these steels with high corrosion resistance is that they contain specific elements such as Cr, Si and Al with oxygen content to create a dense, ductile, very thin and transparent oxide layer that sticks on the surface. Thanks to this very thin amorphous layer, stainless steels are passive against chemical reactions and gain strength against corrosion in non-reducing environments. Suck oxide layer is formed in mediums with oxygen, and even if it deteriorates with external effects (wear, cut or machining, etc.) it repairs itself and regains its previous characteristics.
The carbon content available in stainless steels can range from 0.02% to 1%. Low carbon content is more typical, while high concentrations are found in martensitic steels. This is because, in the case of such stainless steels, chromium carbide is formed in the presence of carbon and normally deposits on the grain borders as chromium carbide; thus, the amount of chromium dissolved in the frame can fall below the limit of 12% resulting in disappearance of the material. Therefore, with the increased carbon percentage in the steel composition:
- Chromium content should be increased or
- Elements with higher carbide forming tendency than chromium should be added and chromium carbide should be prevented from forming to decrease the dissolved chromium in the frame. (Stabilization)
Superiority of Stainless Steel
All stainless steels have high corrosion resistance. Low-allow types of steel are more resistant to atmospheric corrosion, while high-alloy types are more resistant to acid, alkali solutions and mediums that contain chloride. These steels can also be used at high temperature and pressures.
High and Low Temperatures
In certain types of stainless steel, no important decrease in mechanical strength of the material and descaling are observed even at high temperatures. While some other types never get brittle at very low temperature and keep their firmness.
Ease of Production
Nearly all types of stainless steels can be easily given form by cutting, welding, hot and cold forming and machining processes.
The majority of stainless steels are hardened by cold forming and thanks to the increased strength, significant decreases in part weight and cost may be achieved by decreasing material thickness in designs. In some types, it is possible to give a very high strength to the material through heat treatments.
The fact that stainless steels can be cleaned easily makes these materials available for use widely in hospitals, kitchens, food and drug industries.
Since stainless steels are durable and easy to maintain materials, they are economic when considered the entire lifetime of the manufactured part.
Types of Stainless Steel
Alloys with different properties are obtained by changing the chemical composition of stainless steels. Corrosion resistance can be increased by increasing the chromium content or adding alloy elements such as nickel or molybdenum. Other than these, positive effects can be obtained by alloying with certain elements such as copper, titanium, aluminum, silicon, niobium, nitrogen, sulphur and selenium. In this way, machine designers and manufacturers would have the chance to select the most suitable stainless steel for various uses. The most important alloy elements that determine the inner structure in stainless steels are, in order of importance, chromium, nickel, molybdenum and manganese. Especially chromium and nickel would be effective in determining whether the inner structure is ferritic or austenitic.
Stainless steels are gathered in 3 main groups:
This classification is done by the inner structure of the materials. The most widely used ones among these groups are austenitic and ferritic steels with a rate of 95% among all stainless steels.
Low-alloy and unalloyed steels rust fast or sustain chemical corrosion depending on the medium. Using paint materials or additional measures to prevent damage and rust may not provide the desired protection. Therefore, selecting the appropriate type of stainless steel is important
Elements such as Cr, Si and Al contained in the steel combine with oxygen to form a very thin, dense and adhesive oxide film layer. This layer prevents rust formation. For a material to be defined as rust resistant, the material loss for each square meter must be less than 2,4 g.
The rusting process does not relate to the alloy status of the steel only. The machining process features of the components are also significant with respect to rusting. For example, the rusting rate of a component with precisely-ground surface is lower than that of a component with poor surface quality.
Classification of Stainless Steels
- Chromium steels
- Chromium-nickel steels
- Chromium-manganese steels
- Multi-phase steels
Corrosion resistance of chromium steels mainly depends on chromium content. Generally they contain 13-17% chromium.
Chromium-nickel steels contain 13-21% chromium, 5-20% nickel, and 0.02-0.16% carbon. Their corrosion resistance increases with the addition of nickel.
Chromium-manganese steels can be used in special cases in parallel with chromium-nickel steels. X 12 MnCr18 12 steel can be used at temperatures down to -180 °C.