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What is Stainless Steel?

       In metallurgy, stainless steel is defined as an iron-carbon alloy containing a minimum of 10.5% chromium element. The element chromium is the main component that protects steel against rust. Its name comes from the fact that these steels do not stain, corrode or rust like other steels.

    Nowadays, it is very easy to obtain stainless steels in many different and easily accessible qualities and surface properties that work without problems in the harsh environmental conditions in which the product is applied throughout its life.

    Stainless steel has a high resistance to corrosion and oxidation in many natural and man-made environments. However, choosing the right grade and type of stainless steel for each specific application is of utmost importance.

    The first step in quality selection begins with a detailed analysis and definition of all possible and existing operating conditions to which stainless steel will be exposed during the design phase. High oxidation resistance at normal room temperature and weather conditions, minimum 13% (by weight), very harsh and harsh environments conditions, it is achieved by adding up to 30% chromium.

    When the chromium element in stainless steel is exposed to oxygen (meaning the oxygen found 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 it covers by completely preventing the penetration of water or any gas or oxygen into the metal that forms the product (stainless steel). Additionally, if this layer is torn, opened or scratched for any reason, the gap that will occur will be formed again by the layer renewing itself very quickly. This phenomenon is called passivation and is also seen in some other metals such as titanium. The nickel element contributes to the passivation feature, as do other elements such as molybdenum and vanadium, which are used in low amounts.

    Stainless steel’s resistance to corrosion and staining makes it an ideal and indispensable material in a wide range of commercial applications, as it has low maintenance costs, is cheaper compared to others and has an eye-catching appearance. Although there are over 150 stainless steel grades in total, 15 of them are the most used and well-known stainless steels in the market.

    Stainless steels, like their other steel relatives, are widely used in many areas as flat products, plates, rods, wires, pipes and shaped long products.

    Stainless steel does not oxidize or darken over time like silver. In addition, the density of stainless steel is slightly lighter than silver, making it easier for designers.

    Stainless steel is 100% recyclable. 60% of the stainless steel used is produced by reusing recycled materials such as stainless steel from end-of-life products and scraps from production processes.

    The resistance of iron-chromium alloys to corrosion was first noticed by the French metallurgist Pierre Berthier in 1821. Since the technology at that time was not sufficient to process chrome and iron as today, it could not be put into practical use.

    In the 1890s, German Hans Goldschmidt discovered the aluminothermic method, which paved the way for carbon-free chromium production. Since this date, many studies have been carried out on stainless steel.

    The beginnings of modern stainless steels began in 1913, when they were discovered by British metallurgist Harry Brearly while he was doing research to improve rifle barrels. This was the starting point of stainless steel. In subsequent studies, austenitic stainless steel (304), described as 18-8, was introduced to the market in the 1920s and great success was achieved in its applications. Since the 1930s, stainless steel has taken an indispensable place in industry and in our daily lives. Continuing these developments with other alloying elements, researchers over time developed the stainless steel qualities we know today.

  Stainless Steel Types

    There are different types of stainless steel. For example, when nickel is added, the austenitic microstructure of iron becomes stable. This crystal 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 treatments, these steels can be used in many products such as razors, knives and cutting tips.

    Manganese is also found in different proportions in many steels and helps preserve the austenitic structure given by nickel at lower costs.

    Stainless steels are classified into five groups according to their crystal microstructure:

  1. Austenitic Stainless Steels
  2. Ferritic Stainless Steels
  3. Martensitic Stainless Steels
  4. Duplex Stainless steels
  5. Precipitation Hardened Stainless Steels
  1. Austenitic Stainless Steels:

    300 series or austenitic stainless steels constitute 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 temperature. The most well-known type is 18/10 (304 quality) stainless steel, which contains 18% chromium and 8% nickel.

    Steels known as “superustenitic” stainless steels, such as AL-6XN and 254SMO, show a very effective chloride nucleation and crevice corrosion resistance thanks to the high molybdenum (>6%) and nitrogen additions they contain and the high stress corrosion resistance shown by high nickel.

    The high alloy content of “superustenitic” materials causes their costs to increase significantly. For this reason, it should not be forgotten that similar, although not exactly the same, performance can be obtained from ferritic or duplex stainless steel groups at a lower cost.

    The most commonly known austenitic grades are 304 and 316.

    Austenitic stainless steels are non-magnetic and cannot be subjected to heat treatment, they have high ductility, can be hardened by rolling, and have excellent corrosion resistance, machinability and weldability. Their structure is FCC.

2. Ferritic Stainless Steels:

     Ferritic stainless steels are a group of stainless steels that generally do not contain nickel, contain high chromium (between 10.5% and 30%), and contain carbide-forming and alloying elements that stabilize the ferritic structure, such as molybdenum and titanium vanadium.

    The high chromium content they generally contain 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 austenitic ones, cannot be subjected to heat treatment due to their low carbon content, and can be rolled easily.

    The only heat treatment that can be applied to such steels is annealing. Recently, the extreme price increase and variability in alloying elements, especially nickel, has accelerated the development of ferritics, and new ferritic grades have been developed that are as corrosion resistant as austenitics at low cost, have a wide usage area and are much lower in cost.

    The most commonly known ferritic grades are 430 and 442. The structure of ferritics is BCC.

3. Martensitic Stainless Steels:

    Martensitic stainless steels are similar to low alloy – high strength steels or carbon steels, with their structure similar to ferritic steels. However, due to the addition of extra carbon, they can be hardened and their strength increased by heat treatment, like carbon steels.

    The main alloying elements are: 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 are magnetic. Due to the increasing carbon content, their hardenability and strength increase, while their toughness and ductility decrease.

    Due to the high carbon content and other alloying elements, they can be hardened by heat treatment up to 60 HRC. After post-heat treatment stress relief, called tempering or tempering, the optimal corrosion resistance is achieved.

    Compared to ferritic and austenitic grades, the corrosion resistance feature of martensitic grades is slightly lower. It has high processability and formability properties. Depending on the alloying elements they contain and their ratios, there may be a small amount of residual austenite structure in their structure.

    Martensitic steels can be applied very successfully, especially in areas where strength and resistance to mechanical wear are required along with resistance to corrosion. It is also used as tool steel. Its application area is very wide. Their structure is BCT.

4. Duplex Stainless Steels:

    The anti-corrosion performance of these steels, which generally contain equal proportions of ferrite and austenite in their microstructures.

They differ depending on the alloys they contain.

    Although duplex stainless steels have higher strength than austenitic stainless steels, they have better resistance to local corrosion, especially nucleation, crack and stress corrosion, than austenitic ones.

    Duplex grades are also more durable than austenitics, thanks to their high chromium content, which is between 19 and 28%, molybdenum, which is up to 5%, and nickel content, which is lower than austenitics.

    The most important limiting feature of duplex stainless steels is that they become brittle at high temperatures and very low temperatures. Especially if worked at temperatures above 300 °C and below -50 °C, even for a short time, duplex steels become brittle and re-annealing is required.

The most commonly known duplex stainless steel grade is 2205 grade. Their structures are BCC for ferritic parts and FCC for austenitic parts.

5. Precipitation Hardened Stainless Steels (PH):

    Precipitation-hardened stainless steels, also called “age-hardened stainless steels”, are a type of stainless steel that basically contains chromium and nickel and combines the properties of both martensitic and austenitic grades in its structure.

    While they can gain high strength through heat treatment, like martensitic stainless steels, 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 commonly known quality 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.

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10 Differences Between Aluminum And Stainless Steel

Aluminum and Stainless Steel might look similar, but they are actually quite different. Keep these 10 differences in mind when deciding which type of metal to use in your next project:

  1. Strength to weight ratio. Aluminum is typically not as strong as steel, but it is also almost one third of the weight. This is the main reason why aircraft are made from Aluminum.
  2. Corrosion. Stainless steel is made up of iron, chromium, nickel, manganese and copper.  The chromium is added as an agent to provide corrosion resistance.  Also, because it is non-porous the resistance to corrosion is increased. Aluminum has a high oxidation and corrosion resistance mainly due to its passivation layer.  When aluminum is oxidized, its surface will turn white and will sometimes pit. In some extreme acidic or base environments, Aluminum may corrode rapidly with catastrophic results.
  3. Thermal Conductivity. Aluminum has a much better thermal conductivity (conductor of heat) than stainless steel. One of the main reasons it is used for car radiators and air conditioning units.
  4. Cost. Aluminum is typically cheaper than stainless steel.
  5. Workability. Aluminum is fairly soft and easier to cut and form. Due to its resistance to wear and abrasion, Stainless can be difficult to work with. Stainless steels are harder and are especially harder to form than aluminum.
  6. Welding. Stainless is relatively easy to weld, while Aluminum can be difficult.
  7. Thermal properties. Stainless can be used at much higher temperatures than Aluminum which can become very soft above about 400 degrees.
  8. Electrical Conductivity. Stainless steel is a really poor conductor compared to most metals. Aluminum is a very good conductor of electricity. Due to its high conductance, light weight, and corrosion resistance, high-voltage overhead power lines are generally made of aluminum.
  9. Strength. Stainless steel is stronger than Aluminum (provided weight is not a consideration).
  10. Effect on Foods. Stainless steel is less reactive with foods.  Aluminum can react to foods which may affect color and flavor.