What is Stainless Steel?
In metallurgy, stainless steel is described as an iron-carbon alloy containing a minimum of 10.5% chromium. The chromium element is the basic component that protects the steel against corrosion as can be seen in the below figure. It takes its name from the fact that these steels do not stain, corrode and rust like other steels.
This material is also referred to corrosion resistant steel, especially in the aerospace industry, without detailing alloy types and grades. Today, it is very easy to reach stainless steels in many different and easily accessible qualities and surface properties that work without any problems in harsh environmental conditions, where it is applied throughout the lifespan of the product. Even in our normal daily life, we see that these products are used widely from cutlery to watches.
Stainless steel has a high resistance to corrosion and oxidation in many natural and man-made environments. But,ıt is critical that the right grade and type of stainless steel is chosen for each specific application. The first step in quality selection begins with a detailed analysis and definition of all possible and existing working conditions that stainless steel will be exposed to during the design phase.
High oxidation resistance at normal room temperature and weather conditions is achieved by adding chromium up to a minimum of 13% (by weight), and up to 30% in very harsh and difficult environmental conditions. When the chromium element in stainless steel is exposed to oxygen (namely, oxygen available in the normal atmosphere), it immediately forms a chromium-oxide (Cr2O3) passivation layer. This layer is too thin to be seen with the naked eye and protects the metal covered byit by completely preventing the penetration of water or any gas, oxygen to the metal (stainless steel) that makes up the product. Furthermore, if this layer is torn, scrapped or scratched for any reason, an opening occurs again when the layer refreshes very quickly. This phenomenon is called passivation and is also observed in some other metals such as titanium.
Like molybdenum and vanadium elements used in other low ratios, nickel element also contributes to the passivation property.
Resistance of stainless steel to corrosion and staining makes it an ideal and indispensable material in a wide range of commercial applications, due to its low maintenance cost, lower cost compared to others, and attractive appearance. Although there are over 150 stainless steel grades in total, 15 of them are widely used and well-known in the market.
Like other related steels,stainless steels are produced by cold and hot rolling methods in many forms such as flat products, plates, bars, wires, pipes, long-shaped products and castings. It is used widely used in the food industry, medicine, surgical equipment, industrial equipment, automotive, white goods, structures and building elements, buildings, etc. One of the areas, where stainless steels are used, includes products such as jewelry and watches that we never leave with us in our daily lives. The most common grade used in jewelry is 316L. Unlike silver, stainless steel does not oxidize and tarnish over time. Furthermore, since the density of stainless steel is slightly lighter than silver, it provides convenience to designers.
Stainless steel is 100% recyclable. 60% of the used stainless steel is produced by re-processingany recycled materials such as stainless steels from end-of-life products and scraps from the production processes.
Stainless Steel Types
There are different types of stainless steel. For example, when nickel is added, the austenitic microstructure of iron becomes stable. This crystalline structure makes the steel non-magnetic, less brittle at low temperatures. The amount of carbon it contains is increased for higher hardness and strength. With heat treatment, these steels can be used in many products such as razors, knives and cutting tips. Manganese is also found in many steels in varying proportions and helps to maintain the austenitic structure provided by nickel at lower costs.
Stainless steels are classified into five groups according to their crystalline microstructure:
1. Austenitic Stainless Steels
2. Ferritic Stainless Steels
3. Duplex Stainless Steels
4. Martensitic Stainless Steels
5. Precipitation Hardened Stainless Steels (PH)
300 series or austenitic stainless steels account for approximately 60% of the world's total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of 16% chromium and sufficient amounts of nickel and/or manganese to stabilize the austenitic structure from very low temperatures to the melting point.
The most well-known type is 18/8 (304grade) stainless steel, which contains 18% chromium and 8% nickel. Steels known as "superaustenitic" stainless steels such as AL-6XN and 254SMO show a very effective resistance to chloride nucleation and crevice corrosion due to the high molybdenum (>6%) and nitrogen additions contained in them, and the high stress corrosion resistance shown by high nickel.
The high alloy content of "superaustenitics"' causes their costs to be very high. For this reason, it must be noted that, although it is not exactly the same, a similar performance can be obtained from ferritic or duplex stainless steel groups at lower cost. The most widely known austenitic grades are 304 and 316.
Austenitic stainless steels are non-magnetic and non-heat treatable, have high ductility, can be hardened by rolling, and have excellent corrosion resistance, machinability, and weldability. Their structure is FCC.
Ferritic stainless steels are generally a group of stainless steels that do not contain nickel but contain high chromium (between 10.5% and 30%), carbide-forming elements such as molybdenum, titanium vanadium, and alloying elements that stabilize the ferritic structure.
In general, the high chromium content contained in them gives ferritics a very high corrosion resistance. Ferritic stainless steels, which have mechanical and physical properties close to those of their close relatives, carbon steels, are magnetic unlike austenitics, cannot be heat treated due to their low carbon content and can be easily rolled. The only heat treatment that can be applied to this type of steel is annealing.
Recently, the extreme price increase and change in alloying elements, especially nickel, has accelerated the development of ferritics, and new ferritic grades, which are corrosion resistant as well as austenitics with low cost, have been developed with a wide range of use and much lower cost.
The corrosion performance of these steels, which generally contain equal proportions of ferrite and austenite in their microstructure, differs according to the alloys they contain.
Although duplex stainless steels have a higher strength than austenitic stainless steels, they have a better resistance to localized corrosion, especially to nucleation, crevice and stress corrosion, than austenitics. Duplex grades are also more durable due to their high content of chromium between 19% and 28%, molybdenum up to 5%, and nickel at lower rates than austenitics.
The most important limiting feature of Duplex stainless steels is their brittleness at high temperatures and very low temperatures. Duplex steels become brittle and need annealing, especially if they are worked above 300°C and below -50°C even for a short time. The most widely known duplex stainless steel grade is 2205 grade. Their structure is BCC for ferritic parts and FCC for austenitic parts.
However, due to the extra carbon addition contained in it, it can be hardened by heat treatment like carbon steels and its strength can be increased. Basic alloying elements include 12% to 15% chromium, 0.2% to 1.0% molybdenum, and 0.1% to 1.2% carbon. Except for a few martensitic grades, it does not contain nickel.
Martensitic stainless steels, an example of a microstructure shown below, are magnetic. Depending on the increasing carbon ratio, their hardenability and strength increase, while their toughness and ductility decrease. Due to their high carbon content and other alloying elements, they can be hardened by heat treatment up to 60 HRC. After heat treatment, which is called tempering or tempering, after stress relieving, the optimum corrosion resistance is reached.
Martensitic grades have a slightly lower corrosion resistance when compared to ferritic and austenitic grades. It has high machinability and formability properties. Depending on the alloying elements contained in them and their ratios, there may be a small amount of residual-austenite structure in their structures.
Martensitic steels can be applied very successfully, especially in areas where strength and resistance to mechanical wear are desired along with resistance to corrosion. It is also used as tool steel. The scope of application is very wide. Their structure is BCT.
Martensitic stainless steels are similar to low alloy - high strength steels or carbon steels, with structures similar to ferritic steels..
Precipitation-hardened stainless steels, also called “age-hardened stainless steels,” are a type of stainless steel that mainly contains chromium and nickel, and combines the properties of both in its structure, between martensitic and austenitic grades.
Like martensitic stainless steels, they can gain high strength by heat treatment, and they also have corrosion resistance like austenitic grades. Hardening is achieved by the addition of one or more alloying elements such as copper, aluminum, titanium, niobium and molybdenum. The most widely known grade in this group is 17-4 PH. This grade is also known as 630. This quality, which takes its name from its 17% chromium and 4% nickel content, also contains 4% copper and 0.3% niobium.
An advantage of precipitation hardened stainless steels is that these materials are also available in “treated solution” conditions, ready to be worked and machined mechanically. After mechanical processing or manufacturing, the strength of the steel can be increased as desired by very simply applying a low temperature heat treatment. Since this process is done at low temperature, temperature-related distortions or distortions do not occur in the material produced or applied.
Precipitation hardened stainless steels are divided into three subgroups: Martensitic PH, semi-austenitic PH, and austenitic PH.
Depending on the alloy ratio, PH stainless steels can have as much corrosion resistance as the austenitic 304 grade. In annealed form, corrosion resistance is very low. Therefore, it should not be used before heat treatment. Their structures can also be BCT or FCC or both, depending on their subgroups.
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