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What is the difference between martensitic stainless steel and austenitic stainless steel?

In mechanical design, we often use austenitic and martensitic stainless steels because of their good physical and mechanical properties.

For example, the commonly used austenitic stainless steel AISI303 and AISI304 have an elastic modulus of about 200Gpa and a yield strength of 190Mpa-230Mpa.

The commonly used martensitic stainless steels AISI420 and AISI440C have an elastic modulus of 215Gpa. After 420 quenching and tempering heat treatment, the yield strength can reach 345Mpa-1420Mpa, and after 440C heat treatment, the yield strength can even reach 1900Mpa.
Austenitic stainless steel is based on low carbon steel, adding 17%-25% chromium element and 8%-29% nickel element, such as typical 18-8 type austenitic stainless steel, that is, chromium ≥ 18 %, alloy steel with nickel ≥ 8%.

The addition of nickel makes the steel a single-phase austenite structure at room temperature, which reduces the number of micro-batteries formed inside the metal due to different structures, thereby improving the ability to resist electrochemical corrosion.
At the same time, the addition of chromium element increases the electrode potential of the substrate, and forms a dense oxide film Cr2O3 on the surface of the steel, so that the steel is not easy to rust in a certain medium, so it is called austenitic stainless steel.

Similarly, in carbon steel with a carbon content of 0.1%-1%, adding 12%-18% chromium and air cooling can form martensitic stainless steel.
Because of the single alloying element, martensitic stainless steel has good corrosion resistance only in non-oxidizing medium, such as atmosphere and water vapor, but in non-oxidizing medium, such as hydrochloric acid solution, the corrosion resistance becomes very low.

Therefore, the corrosion resistance of austenitic stainless steel is higher than that of martensitic stainless steel. If there is a requirement for corrosion resistance, it is best to choose austenitic stainless steel.
In addition, after cold working of austenitic stainless steel, the structure will also be transformed to martensite. The greater the cold working deformation, the more martensite transformation, and the greater the magnetic properties of the steel.

For the application of stainless steel, we still use 303 and 304 the most, but because 304 is less machinable than 303, and because 304 sticks to the tool, we use 303 more often.

In addition, our sheet metal parts are generally bent with 304 steel plates, and the most used thicknesses are from 1mm, 1.5mm, 2mm and 3mm. Of course, sometimes when it is only used for covering, it is also bent with an aluminum plate and treated with a blackened surface to prevent rust.

There are also many applications of 420, because sometimes the workpiece is too large, and the raw materials of 303 and 304 are not so big, so they are replaced by 420 processing, but they must be treated with surface treatment, such as galvanizing and chrome plating, to prevent rusting.

420 and 440C have high yield strength after quenching and tempering (quenching and tempering at a high temperature of 500-650 degrees), so they are often used in designs with high strength requirements.

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