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.^[1][2][3][4]

Tweezers made of 410 martensitic stainless steel

HistoryEdit

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.^[5] 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.^[6]

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.^[7]

OverviewEdit

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):^[8]

• Up to about 0.4%C they are used mostly for their mechanical properties ( pumps, valves, shafts ..).
• Above, 0.4% they are used mostly for their wear resistance (cutlery surgical blades, plastic injection molds, 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, 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 (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 compositionsEdit

Chemical composition of a few common martensitic stainless steel grades from EN 10088-1 (2005) standard

Chemical composition (main alloying elements) in wt%
EN Steel designation	EN Number	AISI Number
Number			C	Cr		Mo	Others	Remarks
X12Cr13	1.4006	410	0.12	12.5		—	—	Base grade, used as stainless engineering steel
X20Cr13	1.4021	420	0.20	13.0		—	—	Base grade, used as stainless engineering steel
X50CrMoV15	1.4116	-	0.50	14.5		0.65	V: 0.15	Used chiefly for professional knives
X14CrMoS17	1.4104	430F	0.14	16.5		0.40	S: 0.25	Sulphur improves machinability
X39CrMo17-1	1.4122	-	0.40	16.5		1.10	—	Used chiefly for professional knives
X105CrMo17	1.4125	440C	1.10	17.0		0.60	—	Tool steel grade (440C), high wear resistance
X17CrNi16-2	1.4057	431	0.17	16.0		—	Ni: 2.00	Ni replaces some C for higher ductility & toughness
X4CrNiMo16-5-1	1.4418	-	≤ 0.06	16.0		1.10	Ni: 2.00	Highest corrosion resistance of martensitics
X5CrNiCuNb16-4	1.4542	630 (17/4PH)	≤ 0.07	16.0		-	Ni: 4.00 Cu: 4.00 Nb: 5xC to 0.45	Precipitation hardening grade High strength. Used in aerospace

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

Mechanical PropertiesEdit

They are hardenable by heat treatment (specifically by quenching and stress relieving, or by quenching and tempering (referred to as QT).^[9][10] 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 (scalpels, razors and internal clamps).^[11]

Mechanical properties of a few common martensitic stainless steel grades according to EN 10088-3 Standard

			Mininmum Yield stress, MPa	Tensile strength, MPa	Minimum Elongation, %	Heat treatment
	1.4006		450	650 - 850	15	QT650
	1.4021		600	650 - 850	12	QT800
	1.4122		550	750 - 950	12	QT750
	1.4057		700	900 - 1050	12	QT900
	1.4418		700	840 - 1100	16	QT900
	1.4542		790	960 - 1160	12	P960

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

Physical propertiesEdit

Physical properties of a few common martensitic stainless steels from EN 10088-1 (2005) standard

EN Designation	EN	AISI	Young’s Modulus at 20 °C, Gpa	Mean coefficient of thermal expansion between 20 and 100 °C 10−6K−1.	Thermal Conductivity at 20 °C W.m−1K−1	Specific Thermal capacity at 20 °C J.Kg−1.K−1	Electrical resitivity 10−6Ω.m
X12Cr13	1.4006	410	215	10.5	30	460	0.60
X20Cr13	1.4021	420	215	10.5	30	460	0.65
X50CrMoV15	1.4116		215	10.5	30	460	0.65
X39CrMo17-1	1.4122		215	10.4	15	430	0.80
X105CrMo17	1.4125	440C	215	10.4	15	430	0.80
X17CrNi16-2	1.4057	431	215	10.0	25	460	0.70
X3CrNiMo13-4	1.4313		200	10.5	25	430	0.60
X4CrNiMo16-5-1	1.4418		195	10.3	30	430	0.80
X5CrNiCuNb16-4	1.4542	630	200	10.9	30	500	0.71

ProcessingEdit

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 ^[4]Edit

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 applicationsEdit

pumps

valves

boat shafts

-wear resistant and corrosion resistant applicationsEdit

cutlery

medical tools (scalpels, razors and internal clamps)^[11]

bearings (ball bearings)

razor blades

injection molds for polymers

brake disks for bikes & motorbikes

This article uses material from the Wikipedia article  Metasyntactic variable, which is released under the  Creative Commons Attribution-ShareAlike 3.0 Unported License.