STAINLESS STEEL METALLURGY
- Ferritic stainless steels have very low carbon content (< 0.1%) and therefore have a ferritic structure. Their resistance to corrosion increases according to their chromium content, which can range from 12% to more than 25%. These steels are magnetic, and when stabilised (addition of titanium, niobium and zirconium) they are weldable. However, their structure limits their mechanical properties (strength and hardness inparticular).
Example of grade: X6Cr17 (AISI: 430).
- Martensitic stainless steels have sufficient carbon content (> 0.08% and up to 1.2%). They are composed of 12-18% chromium and generally have lower corrosion resistance than other classes of stainless steel due to their martensitic structure. This structure is obtained by heat treatment and these stainless steels behave similarly to conventional treated steels. They are therefore magnetic and are used where high mechanical characteristics are required.
Example of grade: X20Cr13 (AISI: 420).
- Austenitic stainless steels are the most commonly used. They have excellent corrosion resistance and high ductility. Their chromium content is 16-20% and they have fairly high nickel content, usually 8-10%. It is this nickel content that gives stainless steel its austenitic structure. Other elements can be added and/or the carbon content reduced in order to improve corrosion resistance. This structure makes these stainless steels non-magnetic. Their mechanical properties are influenced by cold treatment (hot treatment is not possible).
Example of grade: X5CrNi18-10 (AISI: 304) or X2CrNiMo17-12-2 (AISI: 316L).
- Precipitation-hardening stainless steels are grades consisting of several additive elements in addition to 13-17%, chromium content, including copper, aluminum, molybdenum and niobium. The mechanical properties of these stainless steels are improved by undergoing heat treatment to precipitate intermetallic compounds. Very often, the grades are martensitic matrix grades.
Example of a grade: W8CrNiMoAl15-7-2 (AISI: 630(17-4PH)).
- Austenitic-ferritic stainless steels (commonly known as duplex), have a structure with approximately equal ferritic and austenitic parts. The aim is to obtain superior mechanical properties to purely ferritic or austenitic stainless steels. Their chromium content is high (> 20%) and they are characterised by the use of nitrogen as an additive element, which promotes structural hardening and increases toughness.
Example of grade: X2CrNiMoN22-5-3 (AISI: 2205).
A succession of operations is required to inspect surfaces, each of which is as important as the next, regardless of the material. These steps are in the following order:
- Removal of the product to be examined (if necessary), called “CUTTING”.
- Standardisation of the geometry of the sample taken (if necessary), called “MOUNTING”.
- Improvement of the surface condition of this sample, called “POLISHING”.
- Sample characterisation: to reveal the microstructure of the sample by an etching reagent (if necessary) called “METALLOGRAPHIC ETCHING” and microscopic observation (optical or electronic).
=> Each of these steps must be carried out rigorously, otherwise the following steps will not be possible.
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