Stainless steel

Stainless steels can be divided into four large families, each with its own characteristics.

• Austenitic stainless steels

• Martensitic stainless steels

• Ferritic stainless steels

• Austeno-ferritic stainless steels also called "Duplex".

1. Austenitic stainless steels (grades A1 to A5)

These are by far the best known and most widespread stainless steels: In addition to a minimum chromium content of around 17% nickel (generally 7% and more) and possible additives molybdenum, titanium, niobium, . ..

• In order to reduce the susceptibility to work hardening, copper can be added to steel grades A1 to A5.

• Their tensile mechanical properties are generally modest, but strain hardening can significantly improve certain grades.

On the other hand, their lack of brittleness at low temperature makes them very suitable for cryogenic applications.

• Their corrosion resistance increases with the chromium and molybdenum content.

Their resistance to oxidation increases with their chromium content: the 18% chromium standards last - in an oxidizing atmosphere without sulfur - up to around 800°C. Beyond that, you have to move on to so-called “refractory” shades, which are significantly more bound.

The introduction of stabilizing elements such as titanium or niobium makes it possible to avoid intergranular corrosion, in particular on the welds, and increases the mechanical resistance at high temperature.TInREs is.

Class A1 steels

Grade A1 steels are specially designed for machining. They belong to this group because of the high sulfur content

steels less resistant to corrosion than steels with normal sulfur content.

Class A2 steels

Class A2 steels are the most commonly used stainless steels. They are used for

Kitchen utensils, chemical industry equipment, screws... Steels in this group are not suitable for use in non-oxidizing acids and chlorinated substances such as swimming pools and sea water.

Class A3 steels

A3 steels are stabilized stainless steels with the properties of A2 steels.

A4 grade steels

A4 steels alloyed with molybdenum are "acid resistant" and offer better resistance to corrosion. A4 is widely used in the cellulosic industry as this grade of steel was developed to withstand boiling sulfuric acid (hence the name 'acid proof'), it is also suitable to some extent for chlorine environments. A4 is also frequently used in the food industry and in shipbuilding.

Class A5 steels

A5 steels are "acid resistant" stabilized steels with properties similar to A4 steels.

2-martensitic (classes C1 to C4)

These steels generally contain 12 to 19% chromium, their carbon content varies from 0.08%

• 1.2%; They may contain nickel and molybdenum as well as certain complementary elements such as copper, titanium or vanadium. they are generally supplied in the annealed condition; it is obviously recommended to use them in the quenched and quenched and tempered state, just like alloy steels for mechanics, because they represent the best compromise between mechanical properties and resistance to corrosion. They are interesting for hot applications when the operating temperature does not exceed 650°C (power generation turbines).

In practice, they are used

*Either after quenching and stress relieving at approximately 200°C, which allows maximum mechanical strength to be maintained,

*Or after quenching and tempering between 550 and 700°C, ensuring a better compromise between durability - toughness - corrosion resistance.

These steels make it possible to combine interesting corrosion resistance with mechanical properties equivalent to those of high quality alloy steels. They can be hardened for better strength and are magnetic.

Class C1 steels

C1 steels have limited resistance to corrosion. They are used in turbines, pumps and cutlery.

Class C3 steels

C3 steels have limited corrosion resistance, although better than C1 steels. They are used in pumps and valves.

Class C4 steels

C4 steels have limited resistance to corrosion. They are intended for machining and are otherwise similar to C1 steels.

3-Ferritique (Nuance F1) :

These are iron-chromium or iron-molybdenum alloys whose chromium content varies between 10.5% and 28% and whose carbon content does not exceed 0.08%. These steels generally do not contain nickel.

• Other additional elements - such as Ti, Nb or Zr - can be introduced to improve certain properties such as weldability, corrosion resistance or cold workability

• Ferritic steels with a high chromium content (>20%) are mainly used for their remarkable resistance to corrosion (superferritic) and to hot oxidation

• Certain grades alloyed with molybdenum and/or titanium have corrosion resistance comparable to austenitic standards

• These steels cannot be hardened and are used in the annealed condition, they are very sensitive to grain coarsening at high temperatures, but can be used in oxidizing atmospheres (sometimes higher) up to about 800°C due to the absence of nickel - often more resistant to sulphurous atmospheres than austenitic steels.

• Their low temperature fragility makes them unsuitable for cryogenic applications.

• Contrary to popular belief, the fact that this family of steels is magnetic is in no way

case correlated with poor corrosion resistance | Some grades have comparable or even better properties in this area than the most common austenitic steels.

Class F1 steels

F1 steels cannot and should not normally be work hardened, in some cases F1 steels are magnetic. The group of F1 steels is generally used for simple devices, with the exception of "superferritics", whose percentage of C il N is very low. F1 steels can advantageously replace A2 and A3 steels and can be used in highly chlorinated environments.