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. [1] [2] [3] [4] The other main types of stainless steel are austenitic , ferritic , duplex , and precipitation hardened . [5]
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 . [6] 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. [7]
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. [8]
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): [9]
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 (887 °F).
| 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.
Martensitic stainless alloys are hardenable by heat treatment, specifically by quenching and stress relieving, or by quenching and tempering (referred to as QT). [10] [11] 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, and is largely used for medical surgical instruments, such as scalpels, razors, and internal clamps. [12]
| EN | Mininmum Yield stress | Tensile strength | Minimum Elongation, % | Heat treatment |
|---|---|---|---|---|
| 1.4006 | 450 MPa (65 ksi) | 650–850 MPa (94–123 ksi) | 15 | QT650 |
| 1.4021 | 600 MPa (87 ksi) | 650–850 MPa (94–123 ksi) | 12 | QT800 |
| 1.4122 | 550 MPa (80 ksi) | 750–950 MPa (109–138 ksi) | 12 | QT750 |
| 1.4057 | 700 MPa (100 ksi) | 900–1,050 MPa (131–152 ksi) | 12 | QT900 |
| 1.4418 | 700 MPa (100 ksi) | 840–1,100 MPa (122–160 ksi) | 16 | QT900 |
| 1.4542 | 790 MPa (115 ksi) | 960–1,160 MPa (139–168 ksi) | 12 | P960 |
In the heat treatment column, QT refers to Quenched and Tempered, P refers to Precipitation hardened
| EN Designation | EN | AISI | Young's Modulus at 20 °C (68 °F), Gpa | Mean coefficient of thermal expansion between 20 and 100 °C (68 and 212 °F) 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 GPa (31.2×106 psi) | 10.5 | 30 | 460 | 0.60 |
| X20Cr13 | 1.4021 | 420 | 215 GPa (31.2×106 psi) | 10.5 | 30 | 460 | 0.65 |
| X50CrMoV15 | 1.4116 | 420MoV | 215 GPa (31.2×106 psi) | 10.5 | 30 | 460 | 0.65 |
| X39CrMo17-1 | 1.4122 | 215 GPa (31.2×106 psi) | 10.4 | 15 | 430 | 0.80 | |
| X105CrMo17 | 1.4125 | 440C | 215 GPa (31.2×106 psi) | 10.4 | 15 | 430 | 0.80 |
| X17CrNi16-2 | 1.4057 | 431 | 215 GPa (31.2×106 psi) | 10.0 | 25 | 460 | 0.70 |
| X3CrNiMo13-4 | 1.4313 | 200 GPa (29×106 psi) | 10.5 | 25 | 430 | 0.60 | |
| X4CrNiMo16-5-1 | 1.4418 | 195 GPa (28.3×106 psi) | 10.3 | 30 | 430 | 0.80 | |
| X5CrNiCuNb16-4 | 1.4542 | 630 | 200 GPa (29×106 psi) | 10.9 | 30 | 500 | 0.71 |
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 (87 to 131 ksi), 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 1,600 MPa (230 ksi) 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.
Martensitic stainless steels, depending upon their carbon content are often used for their corrosion resistance and high strength in pumps, valves, and boat shafts. [4]
They are also used for their wear resistance in, cutlery, medical tools (scalpels, razors and internal clamps), [12] ball bearings, razor blades, injection molds for polymers, and brake disks for bicycles and motorbikes.
Stainless steel, also known as inox, corrosion-resistant steel (CRES), and rustless steel, is an alloy of iron that is resistant to rusting and corrosion. It contains iron with chromium and other elements such as molybdenum, carbon, nickel and nitrogen depending on its specific use and cost. Stainless steel's resistance to corrosion results from the 10.5%, or more, chromium content which forms a passive film that can protect the material and self-heal in the presence of oxygen.
Heat treating is a group of industrial, thermal and metalworking processes used to alter the physical, and sometimes chemical, properties of a material. The most common application is metallurgical. Heat treatments are also used in the manufacture of many other materials, such as glass. Heat treatment involves the use of heating or chilling, normally to extreme temperatures, to achieve the desired result such as hardening or softening of a material. Heat treatment techniques include annealing, case hardening, precipitation strengthening, tempering, carburizing, normalizing and quenching. Although the term heat treatment applies only to processes where the heating and cooling are done for the specific purpose of altering properties intentionally, heating and cooling often occur incidentally during other manufacturing processes such as hot forming or welding.
Austenite, also known as gamma-phase iron (γ-Fe), is a metallic, non-magnetic allotrope of iron or a solid solution of iron with an alloying element. In plain-carbon steel, austenite exists above the critical eutectoid temperature of 1000 K (727 °C); other alloys of steel have different eutectoid temperatures. The austenite allotrope is named after Sir William Chandler Roberts-Austen (1843–1902). It exists at room temperature in some stainless steels due to the presence of nickel stabilizing the austenite at lower temperatures.
Carbon steel is a steel with carbon content from about 0.05 up to 2.1 percent by weight. The definition of carbon steel from the American Iron and Steel Institute (AISI) states:
Tool steel is any of various carbon steels and alloy steels that are particularly well-suited to be made into tools and tooling, including cutting tools, dies, hand tools, knives, and others. Their suitability comes from their distinctive hardness, resistance to abrasion and deformation, and their ability to hold a cutting edge at elevated temperatures. As a result, tool steels are suited for use in the shaping of other materials, as for example in cutting, machining, stamping, or forging.
Maraging steels are steels that are known for possessing superior strength and toughness without losing ductility. Aging refers to the extended heat-treatment process. These steels are a special class of very-low-carbon ultra-high-strength steels that derive their strength not from carbon, but from precipitation of intermetallic compounds. The principal alloying element is 15 to 25 wt% nickel. Secondary alloying elements, which include cobalt, molybdenum and titanium, are added to produce intermetallic precipitates. Original development was carried out on 20 and 25 wt% Ni steels to which small additions of aluminium, titanium, and niobium were made; a rise in the price of cobalt in the late 1970s led to the development of cobalt-free maraging steels.
Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical point for a certain period of time, then allowing it to cool in still air. The exact temperature determines the amount of hardness removed, and depends on both the specific composition of the alloy and on the desired properties in the finished product. For instance, very hard tools are often tempered at low temperatures, while springs are tempered at much higher temperatures.
Cryogenic hardening is a cryogenic treatment process where the material is cooled to approximately −185 °C (−301 °F), usually using liquid nitrogen. It can have a profound effect on the mechanical properties of certain steels, provided their composition and prior heat treatment are such that they retain some austenite at room temperature. It is designed to increase the amount of martensite in the steel's crystal structure, increasing its strength and hardness, sometimes at the cost of toughness. Presently this treatment is being used on tool steels, high-carbon, high-chromium steels and in some cases to cemented carbide to obtain excellent wear resistance. Recent research shows that there is precipitation of fine carbides in the matrix during this treatment which imparts very high wear resistance to the steels.
The weldability, also known as joinability, of a material refers to its ability to be welded. Many metals and thermoplastics can be welded, but some are easier to weld than others. A material's weldability is used to determine the welding process and to compare the final weld quality to other materials.
Hardening is a metallurgical metalworking process used to increase the hardness of a metal. The hardness of a metal is directly proportional to the uniaxial yield stress at the location of the imposed strain. A harder metal will have a higher resistance to plastic deformation than a less hard metal.
Austenitic stainless steel is one of the five classes of stainless steel by crystalline structure. Its primary crystalline structure is austenite and it prevents steels from being hardenable by heat treatment and makes them essentially non-magnetic. This structure is achieved by adding enough austenite-stabilizing elements such as nickel, manganese and nitrogen. The Incoloy family of alloys belong to the category of super austenitic stainless steels.
The SAE steel grades system is a standard alloy numbering system for steel grades maintained by SAE International.
Alloy steel is steel that is alloyed with a variety of elements in amounts between 1.0% and 50% by weight, typically to improve its mechanical properties.
AerMet alloy is an ultra-high strength type of martensitic alloy steel. The main alloying elements are cobalt and nickel, but chromium, molybdenum and carbon are also added. Its exceptional properties are hardness, tensile strength, fracture toughness and ductility. Aermet is weldable with no preheating needed. AerMet alloy is not corrosion resistant, so it must be sealed if used in a moist environment. AerMet is a registered trademark of Carpenter Technology Corporation.
Austempering is heat treatment that is applied to ferrous metals, most notably steel and ductile iron. In steel it produces a bainite microstructure whereas in cast irons it produces a structure of acicular ferrite and high carbon, stabilized austenite known as ausferrite. It is primarily used to improve mechanical properties or reduce / eliminate distortion. Austempering is defined by both the process and the resultant microstructure. Typical austempering process parameters applied to an unsuitable material will not result in the formation of bainite or ausferrite and thus the final product will not be called austempered. Both microstructures may also be produced via other methods. For example, they may be produced as-cast or air cooled with the proper alloy content. These materials are also not referred to as austempered.
Microalloyed steel is a type of alloy steel that contains small amounts of alloying elements, including niobium, vanadium, titanium, molybdenum, zirconium, boron, and rare-earth metals. They are used to refine the grain microstructure or facilitate precipitation hardening.
Mangalloy, also called manganese steel or Hadfield steel, is an alloy steel containing an average of around 13% manganese. Mangalloy is known for its high impact strength and resistance to abrasion once in its work-hardened state.
Thermomechanical processing is a metallurgical process that combines mechanical or plastic deformation process like compression or forging, rolling, etc. with thermal processes like heat-treatment, water quenching, heating and cooling at various rates into a single process.
Ferritic stainless steel forms one of the five stainless steel families, the other four being austenitic, martensitic, duplex stainless steels, and precipitation hardened. For example, many of AISI 400-series of stainless steels are ferritic steels. By comparison with austenitic types, these are less hardenable by cold working, less weldable, and should not be used at cryogenic temperatures. Some types, like the 430, have excellent corrosion resistance and are very heat tolerant.