Martensitic 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.
Martensitic Stainless Steels
Martensitic stainless steels are similar to low alloy or carbon steels. In the annealed condition they have a structure similar to the ferritics, but when hardened they have a ‘body-centred tetragonal’, (bct), crystal lattice, rather than a body centred cubic, (bcc), lattice. Due to the deliberate addition of carbon, they can be hardened and strengthened by heat treatment, in a similar way to many carbon/carbon alloy steels. They are classed as a “hard” ferro-magnetic group. The main alloying element is chromium, with a typical content of 12-15%.
In the annealed condition, (where they do have a body centred cubic , (bcc), lattice structure), they have tensile yield strengths of about 275 MPa., and so they are usually machined, cold formed, or cold worked in this condition. The strength obtained by heat treatment depends on the carbon content of the alloy. Increasing the carbon content increases the strength and hardness potential, but decreases ductility and toughness. The higher carbon grades are capable of being heat treated to hardnesses of 60 HRC.
Optimum corrosion resistance is attained in the heat-treated, i.e. hardened and tempered, condition. Tempering after hardening, whilst softening the steels somewhat, also restores a degree of ductility. Martensitic grades have been developed with nitrogen and nickel additions, but with lower carbon levels than the traditional grades. These steels have improved toughness, weldability and corrosion resistance
Examples of martensitic grades are, (420S45), 1.4028, 431, (1.4057), as traditional carbon hardenable grades, and 248SV, (1.4418), as one of the low carbon / nitrogen grades.
More details on these grades is available in the article Related martensitic and precipitation hardening stainless steel grades.
A longer article on martensitic stainless steels can be downloaded here.
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