Chromium, as the main element in casting stainless steel, normally has a content of 11 to 12% of Chromium (Cr), since it is the element that forms an extremely thin, continuous and stable film on the surface. This film renders the surface inert to chemical reactions. This is the fundamental characteristic of resistance to corrosion of stainless steel. Chromium is a ferrite former

The second important element in stainless steel is Nickel (Ni), whose purpose is to form austenite, therefore increasing resistance to high and cryogenic temperatures, increasing corrosion resistance and ductility.

The third element is Molybdenum (Mo) which favours the formation of ferrite. It further improves resistance to high temperatures and corrosion resistance.

 

 The fourth critical element is Carbon (C), which is a strong austenite former, improves mechanical properties but can cause intergranular corrosion due to carbides that form with chromium. In stainless steel, this element is intended to be very low and controlled since, as its purpose indicates, a high content can make machining difficult and increase the risk of corrosion.

As a fifth element, Manganese (Mn), which stabilizes the austenite at room temperature but forms ferrite at high temperatures. Inhibits hot cracking.

Finally, as the sixth important element, Titanium (Ti), favours the formation of ferrite, acts as a grain refiner, since it combines with carbon, reducing susceptibility to intergranular corrosion.

There are residual elements or in the much smaller quantity that are part of the alloy are Niobium (Nb), Phosphorus (P), Sulfur (S), Selenium (Se), Silicon (Si), Nitrogen (N); each one of them has a different influence on the physical, chemical and mechanical characteristics of stainless steels.

From these elements and their different contents, the families of stainless steels emerge that will be selected based on the following criteria.

1. Resistance to corrosion and oxidation at high temperatures,
2. Mechanical properties
3. Product transformation characteristics
4. Total cost

Stainless steel families

 

FERRITIC STAINLESS STEELS

Known as AISI: 430,409,434. Chromium stainless steel. They have a chromium content that varies from 12 to 18% Cr. But a low content of carbon (C)> 0.2%

Properties

• Moderate to good corrosion resistance, which increases with chromium content and in some molybdenum alloys.
• Magnetic, non-hardenable and always used in annealed conditions.
• Their weldability is poor so weld joints are generally limited to thin gauges. In the case of 409, titanium is added as a stabilizer, improving its weldability.

 

AUSTENITIC STAINLESS STEELS

Known as AISI 304,304L, 321,316,316L, 316Ti and 317L. By adding nickel (Ni) to stainless steel, the crystalline structure is transformed into austenite, hence the term Austenitic steel. The chemical composition of this type of stainless steel is 18% Chromium and 8% Nickel (18/8). If it is required to increase the resistance to corrosion, add 2% to 3% of molybdenum (Mo), (8/18/3). Carbon content is low 0.08% C max. Although there is also an additional rating "low carbon" or "L" (0.03% C max.).

Properties :

• Excellent corrosion resistance
• Excellent for hygiene and cleaning
• Excellent weldability
• They are used in the annealed condition, in which they are non-magnetic
• They can be functional in extreme temperatures, low temperatures (cryogenic) preventing embrittlement and high temperatures (up to 925 ° C)

 

HIGH TEMPERATURE RESISTANT AUSTENITIC STAINLESS STEELS

Known as AISI (*): 309,310. This type of stainless steel is designed to operate at high temperatures (more than 925 ° C). The basic content of Chromium increases to 24% Cr and Nickel varies from 14 to 22% Ni.

Properties:

• Resistance to oxidation at high temperatures  
• Applications at temperatures from 950 ° C to 1100 ° C

 

SUPER-FERRITIC STAINLESS STEELS

Known as AISI (*): 444 This family of super ferritic stainless steels were developed to overcome pitting corrosion and stress fracture corrosion, to which austenitic steels are susceptible. These stainless steels simply with Chromium and low Carbon "L", have a chemical ageing of 18% Cr and 2% Mo or 26% Cr and 1% Mo.

Properties

• The same as the 400 series stainless steel but they are more resistant to pitting and stress fracture corrosion.

 

STAINLESS STEEL DUPLEX

Known as AISI (*): 329. Chromium content varies from 18% to 26%. Nickel from 4.5% to 6%. Molybdenum in the range of 2.5% to 3%.

Properties:

• Austenite and ferrite have been mixed, that is, they present a DUPLEX crystalline structure that gives high resistance to stress fracture corrosion in environments with chloride ions CL
• An increased instability due to the high content of Chromium and Molybdenum.
• Improved weldability. It has high tensile strength and a high elastic limit.