Ferritic Stainless Steels
Introduction
Stainless steel is the name given to a family of corrosion and heat resistant steels containing a minimum of 10.5% chromium. Just as there is a range of structural and engineering carbon steels meeting different requirements of strength, weldability and toughness, so there is a wide range of stainless steels with progressively higher levels of corrosion resistance and strength. This results from the controlled addition of alloying elements, each offering specific attributes in respect of strength and ability to resist different environments. The available grades of stainless steel can be classified into five basic families: ferritic, martensitic, austenitic, duplex and precipitation hardenable.
Ferritic stainless steels
Ferritic stainless steels have a “body-centred-cubic”, (bcc), crystal structure, which is the same as that of pure iron at room temperature.
The main alloying element is chromium, with contents typically between 11 and 17%, although a higher chromium content of about 29% is found in one specialised grade. Carbon is kept low which results in these steels having limited strength. They are not hardenable by heat treatment and have annealed yield strengths in the range of 275 to 350 MPa.
The ferritics are usually lower in cost compared with the austenitic steels due to the absence of nickel. They have often been thought of as also having lower corrosion resistance. However, stabilised grades such as 1.4509 and 1.4521 are broadly similar in corrosion resistance to 1.4301, (304), and 1.4401, (316). The main disadvantages of the ferritics are:
- Limited toughness – Not acceptable for sub zero temperatures
- Formability – Good for deep drawing, but not stretch forming due to lower ductility
- Weldability – Rapid grain growth in thick sections, (greater than about 3mm), leads to poor weld toughness in comparison to the austenitics.
They are however a “soft ” ferro-magnetic group and so do have some special uses, for example as solenoid cores.
More details on these grades is available in the article Related ferritic stainless steel grades.
A detailed booklet on the Ferritic Stainless Steels can be downloaded from here.
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