What is martensitic stainless steel
What is martensitic stainless steel?
Martensitic stainless steel is a kind of stainless steel whose mechanical properties can be adjusted by heat treatment. The typical grade is Cr13 type, such as 2Cr13, 3Cr13, 4Cr13, etc. It is mainly used for steam turbine blades, tableware and surgical instruments. According to the difference of chemical composition, martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium nickel steel. According to the different structure and strengthening mechanism, it can also be divided into martensitic stainless steel, martensitic and semi austenitic (or semi martensitic) precipitation hardening stainless steel and maraging stainless steel.
The standard martensitic stainless steels are 410, 414, 416, 416 (Se), 420, 431, 440A, 440b and 440C, which are magnetic; The corrosion resistance of these steels comes from “chromium”, which ranges from 11.5% to 18%. The higher the chromium content is, the higher the carbon content is, so as to ensure the formation of martensite during heat treatment. The above three types of 440 stainless steels are rarely considered as applications requiring welding, and the molten metal with 440 type composition is not easy to obtain.
The improvement of standard martensitic steel contains elements such as nickel, molybdenum, vanadium, etc. it is mainly used to raise the limited allowable working temperature of standard steel to higher than 1100k. When these elements are added, the carbon content also increases. With the increase of carbon content, the problem of avoiding cracking in the heat affected zone of weldment becomes more serious.
Properties of martensitic stainless steel
Martensitic stainless steel can be welded in the state of annealing, hardening and tempering. No matter what the original state of the steel is, a hardened martensitic zone will be formed near the weld bead after welding. The hardness of the heat affected zone mainly depends on the carbon content of the base metal. When the hardness increases, the toughness decreases, and this zone becomes more prone to cracking and cracking Preheating and controlling interlayer temperature are the most effective methods to avoid cracking. In order to obtain the best properties, post weld heat treatment is needed.
Martensitic stainless steel is a kind of stainless steel whose properties can be adjusted by heat treatment (quenching and tempering). Generally speaking, it is a kind of hardenable stainless steel. This characteristic determines that this kind of steel must have two basic conditions: one is that there must be austenite phase zone in the equilibrium phase diagram, which is heated for a long time within the temperature range of this zone to make the carbide solid solution into the steel, and then quenched to form martensite, that is, the chemical composition must be controlled within the temperature range γ or γ+α Second, the chromium content must be more than 10.5% in order to form a corrosion-resistant and oxidation-resistant passive film. According to the difference of alloy elements, it can be divided into martensitic chromium stainless steel and martensitic chromium nickel stainless steel.
The main alloying elements of martensitic chromium stainless steel are iron, chromium and carbon. Figure 1-4 shows the iron rich part of the Fe Cr system phase diagram, which does not exist when Cr is more than 13% γ This kind of alloy is a single-phase ferritic alloy and can not produce martensite under any heat treatment system. Therefore, austenite forming elements must be added to the internal Fe Cr binary alloy. To expand the scope, C and N are effective elements, and the addition of C and n makes the alloy allow higher chromium content. In addition to chromium, C is the most important element in martensitic chromium stainless steel. In fact, martensitic chromium stainless steel is a ternary alloy of iron, chromium and carbon. Of course, there are other elements with which the approximate organization can be determined from the Schaeffler diagram.
Martensitic stainless steel is mainly low carbon or high carbon steel with chromium content in the range of 12% – 18%. There are three types of martensitic stainless steel widely used in various countries:
- 1. Low carbon and medium carbon 13% Cr steel;
- 2. High carbon 18% Cr steel;
- 3. Low carbon 17% Cr steel containing nickel (about 2%).
Martensitic stainless steel has high strength and corrosion resistance, which can be used to make machine parts such as blades of steam turbine (1Cr13), shafts and tie rods of steam equipment (2Cr13), and parts working in corrosive media such as valves and bolts (4Cr13). Steel grades with high carbon content (4Cr13, 9Cr18) are suitable for manufacturing medical instruments, knives, measuring appliances, springs, etc.
Similar to ferritic stainless steel, other alloying elements can be added to martensitic stainless steel to improve other properties:
- 1. Adding 0.07% s or se to improve the machinability, such as 1cr13s or 4cr13se;
- 2. Adding about 1% Mo and 0.1% v can increase the wear resistance and corrosion resistance of 9Cr18 steel;
- 3. Adding about 1mo-1w-0.2v can improve the hot strength of 1Cr13 and 2Cr13 steel.
Martensitic stainless steel can be quenched, tempered and annealed just like modulated steel. The mechanical properties of the steel are similar to that of the modulated steel: when the hardness increases, the tensile strength and yield strength increase, while the elongation, cross-section shrinkage and impact energy decrease.
The corrosion resistance of martensitic stainless steel mainly depends on the chromium content, and the carbon in the steel forms stable chromium carbide with chromium, which indirectly affects the corrosion resistance of the steel. Therefore, in 13% Cr steel, the lower the carbon content is, the higher the corrosion resistance is. But in 1Cr13, 2Cr13, 3Cr13 and 4Cr13, the order of corrosion resistance and strength is just opposite.
When processing products, in order to improve the strength and hardness of martensitic stainless steel products, carbon content will be increased, resulting in the decline of plasticity and corrosion resistance of products. Therefore, the corrosion resistance of martensitic stainless steel products is usually poor.
Classification of martensitic stainless steels
Martensitic stainless steel can be divided into martensitic chromium steel and martensitic chromium nickel steel according to different chemical composition; According to the different structure and strengthening mechanism, it can be divided into martensitic and semi austenitic precipitation hardening stainless steels and maraging stainless steels.
- (1) Martensitic chromium steel. In addition to chromium, steel also contains a certain amount of carbon. The higher the carbon content, the higher the strength, hardness and wear resistance. The normal structure of this kind of steel is martensite, some of which also contain a small amount of austenite, ferrite or pearlite. It is mainly used for manufacturing parts, components, tools and cutting tools with high requirements for strength and hardness but low requirements for corrosion resistance. Typical steel grades are 2Crl3, 4Crl3, 9crl8, etc.
- (2) Martensitic chromium nickel steel. Including martensitic precipitation hardening stainless steel, semi austenitic precipitation hardening stainless steel and maraging stainless steel, are high strength or ultra-high strength stainless steel. This kind of steel has low carbon content (less than 0.10%), and contains nickel. Some brands also contain high molybdenum, copper and other elements. Therefore, this kind of steel has high strength, at the same time, the combination of strength and toughness, corrosion resistance and weldability are better than martensitic chromium steel. Crl7ni2 is the most commonly used low nickel martensitic stainless steel. Martensitic precipitation hardening stainless steel usually contains Al, Ti, Cu and other elements. It precipitates ni3a1, Ni3Ti and other dispersion strengthening phases on the martensitic matrix through precipitation hardening to further improve the strength of the steel, such as crl7ni4cu4; However, the semi austenitic (or semi martensitic) precipitation hardening stainless steel, because the quenched state is still austenite, can still be cold worked, and then strengthened by intermediate treatment, aging treatment and other processes. In this way, the austenite in the martensitic precipitation hardening stainless steel can not be directly transformed into martensite after quenching, The disadvantage that leads to the difficulty of subsequent processing. The commonly used steel grades are 0crl7ni7ai, 0crl5ni7m02a1, etc. This kind of steel has high strength, generally up to 1200 ~ 1400MPa. It is often used to make structural parts that do not require high corrosion resistance but need high strength, such as aircraft skin.
Maraging stainless steel is made by adding more than 10% chromium to the ultra-low carbon maraging steel. It not only retains the good comprehensive properties of maraging steel, but also improves the corrosion resistance. The carbon content of this kind of steel is less than 0.03%, the chromium content is 10% ~ 15%, the nickel content is 6% ~ 11% (or the cobalt content is 10% ~ 20%), and the strengthening elements such as Mo, Ti and Cu are added.
Stainless steel grade grouping
The types of stainless steel can be divided into CR system (400 Series), Cr Ni system (300 Series), Cr Mn Ni system (200 Series) and precipitation hardening system (600 Series).
200 series Cr Ni Mn austenitic stainless steel.
300 series – Cr Ni austenitic stainless steel
Model 301 – good ductility, used for molding products. It can also be hardened rapidly by machining. Good weldability. The wear resistance and fatigue strength are better than 304 stainless steel.
The corrosion resistance of model 302 is the same as that of model 304, and its strength is better due to its relatively high carbon content.
Model 303 – makes it easier to cut than 304 by adding a small amount of sulfur and phosphorus.
Model 304 – general model; 18 / 8 stainless steel. GB grade is 0Cr18Ni9.
Model 309 has better temperature resistance than 304.
Model 316 – the second most widely used steel after 304. It is mainly used in food industry and surgical equipment. Adding molybdenum element makes it obtain a special corrosion-resistant structure. Because of its better resistance to chloride corrosion than 304, it is also used as “marine steel”. SS316 is usually used in nuclear fuel recovery devices. Grade 18 / 10 stainless steel generally meets this application grade.
Model 321 – other properties are similar to 304 except that the addition of titanium reduces the risk of weld corrosion.
400 series – Ferritic and martensitic stainless steels
Model 408 – good heat resistance, weak corrosion resistance, 11% Cr, 8% Ni.
Model 410 – martensite (high strength chromium steel), good wear resistance, poor corrosion resistance.
Model 416 – the addition of sulfur improves the processability of the material.
Model 420 – “cutting tool grade” martensitic steel, similar to Brinell high chromium steel, the earliest stainless steel. It’s also used for surgical knives. It’s very bright.
Model 430 – ferritic stainless steel for decorative purposes, e.g. automotive accessories. Good formability, but poor temperature resistance and corrosion resistance.
Model 440 – high strength cutting tool steel, with slightly higher carbon content, can obtain higher yield strength after proper heat treatment, and its hardness can reach 58hrc, which is among the hardest stainless steels. The most common application example is “razor blade”. There are three common models: 440A, 440b, 440C, and 440f (easy to process).
500 series – heat resistant chromium alloy steel.
600 series – Martensitic precipitation hardening stainless steel.
Model 630 – the most commonly used precipitation hardening stainless steel model, also known as 17-4; 17%Cr，4%Ni。
Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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