Martensitic stainless steel


Stainless steels may be classified by their crystalline structure into four main types: austenitic, ferritic, martensitic, and duplex. Martensitic stainless steel is a specific type of stainless steel alloy that can be hardened and tempered through multiple ways of aging/heat treatment.

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% up to 1.2% :
They may contain some Ni 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 with a low C, 13%Cr and 4%Ni offers good mechanical properties, good castability, good weldability and good resistance to cavitation. 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.
A specific grade is Type 630 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. 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 see below; used largely for medical tools.
Mininmum Yield stress, MPaTensile strength, MPaMinimum Elongation, %Heat treatment
1.4006450650 - 85015QT650
1.4021600650 - 85012QT800
1.4122550750 - 95012QT750
1.4057700900 - 105012QT900
1.4418700840 - 110016QT900
1.4542790960 - 116012P960

IN the heat treatment column, QT refers to Quenched and Tempered, P refers to Precipitation hardened

Physical properties

Processing

When formability, softness, etc. are required in fabrication, steel having 0.12 per cent 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 N/mm2, 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 N/mm2 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 can be seen as

- corrosion resistant engineering steels (see above table on mechanical properties) used in a variety of mechanical engineering applications

pumps
valves
boat shafts

-wear resistant and corrosion resistant applications

cutlery
medical tools
bearings
razor blades
injection molds for polymers
brake disks for bikes & motorbikes