Austenitic, ferritic and martensitic stainless steels. What is the most suitable stainless steel to high or low temperatures?
High and low temperature. Which types of stainless steel is the best?
The type of stainless steel is primarily chosen depending on its end-use. The various types of steel differ for many reasons; in this case, depening on the usage for high temperature use, and vice versa, for low temperature use.
Martensitic stainless steels
Chromium is almost the only characterizing element contained in martensitic types, from the lowest level that ensures the stainlessness of the material (10%) up to a maximum of 18%. As a result, they are the less expensive type of stainless steels, but they are also less resistant to corrosion than other types of steel.
Furthermore, they are not suitable to use at a low ambient temperature, since for most of them the transition temperature is above 0 °C. Already at this temperature, they become frail, and at below zero they lose all their toughness and ductility.
For use in high ambient temperatures, these types of stainless steel have a break load and tensile stress higher than ferritic and austenitic steel, up to a temperature of about 500 °C.
Ferritic stainless steels
On average, ferritic stainless steel has an alloying element content higher than martenisitic steel. Therefore, their price is higher, the more their corrosion and heat-oxidation resistance is.
As for low temperatures, the performance of ferritic types is similar to martensitic types. Therefore, these are not recommended to use below 0 °C.
For temperature higher than ambient temperature, this type of stainless steel cannot be used between 400 °C and 550 °C, because of its fragility to 475 °C. Generally, ferritic stainless steel more than 550 °C are preferable to the martensitic ones for a better performance both in the creep-cold flow and in the heat-oxidation.
Autenitic stainless steels
The austenitic stainless steel have two characteristic elements: the Chromium and Nickel. (e.i. AISI 304 and AISI 316)
The austenitic stainless steel are unequalled for use at low temperatures, even close to absolute zero. Exposure to low temperatures does not change the mechanical characteristics at the ambient temperature.
Also for rapid tensile test, almost all the austenitic types to 600 °C and beyond have a mechanical strength greater than ferritic and martensitic types.
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