Martensitic stainless steel

Martensitic stainless steel is a type of stainless steel alloy that has a martensite crystal structure. It can be hardened and tempered through aging and heat treatment. The other main types of stainless steel are austenitic, ferritic, duplex, and precipitation hardened.

History
In 1912, Harry Brearley of the Brown-Firth research laboratory in Sheffield, England, while seeking a corrosion-resistant alloy for gun barrels, discovered and subsequently industrialized a martensitic stainless steel alloy. The discovery was announced two years later in a January 1915 newspaper article in The New York Times. Brearly applied for a U.S. patent during 1915. This was later marketed under the "Staybrite" brand by Firth Vickers in England and was used for the new entrance canopy for the Savoy Hotel in 1929 in London.

The characteristic body-centered tetragonal martensite microstructure was first observed by German microscopist Adolf Martens around 1890. In 1912, Elwood Haynes applied for a U.S. patent on a martensitic stainless steel alloy. This patent was not granted until 1919.

Overview
Martensitic stainless steels can be high- or low-carbon steels built around the composition of iron, 12% up to 17% chromium, carbon from 0.10% (Type 410) up to 1.2% (Type 440C):
 * Up to about 0.4%C they are used mostly for their mechanical properties in applications such as pumps, valves, and shafts.
 * Above 0.4%C they are used mostly for their wear resistance, such as in cutlery surgical blades, plastic injection molds, and nozzles.

They may contain some Ni (Type 431) which allows a higher Cr and/or Mo content, thereby improving corrosion resistance and as the carbon content is also lower, the toughness is improved. Grade EN 1.4313 (CA6NM) with a low C, 13%Cr and 4%Ni offers good mechanical properties, good castability, and good weldability. It is used for nearly all the hydroelectric turbines in the world, including those of the huge "Three Gorges" dam in China.

Additions of B, Co, Nb, Ti improve the high temperature properties, particularly creep resistance. This is used for heat exchangers in steam turbines.

A specific grade is Type 630 (also called 17-4 PH) which is martensitic and hardens by precipitation at 475 C.

Chemical compositions
There are many proprietary grades not listed in the standards, particularly for cutlery.

Mechanical Properties
They are hardenable by heat treatment (specifically by quenching and stress relieving, or by quenching and tempering (referred to as QT). The alloy composition, and the high cooling rate of quenching enable the formation of martensite.  Untempered martensite is low in toughness and therefore brittle.Tempered martensite gives steel good hardness and high toughness as can be seen below; used largely for medical tools (scalpels, razors and internal clamps).  In the heat treatment column, QT refers to Quenched and Tempered, P refers to Precipitation hardened

Processing
When formability, softness, etc. are required in fabrication, steel having 0.12% maximum carbon is often used in soft condition. With increasing carbon, it is possible by hardening and tempering to obtain tensile strength in the range of 600 to 900 MPa, combined with reasonable toughness and ductility. In this condition, these steels find many useful general applications where mild corrosion resistance is required. Also, with the higher carbon range in the hardened and lightly tempered condition, tensile strength of about 1600 MPa may be developed with lowered ductility.

A common example of a Martensitic stainless steel is X46Cr13.

Martensitic stainless steel can be nondestructively tested using the magnetic particle inspection method, unlike austenitic stainless steel.

Applications
Martensitic stainless steels, depending upon their carbon content are often used for their corrosion resistance and high strength in pumps, valves, and boat shafts.

They are also used for their wear resistance in, cutlery, medical tools (scalpels, razors and internal clamps), ball bearings, razor blades, injection molds for polymers, and brake disks for bicycles and motorbikes.