Stainless steel definition
A high-alloy steel that resists corrosion in air or chemically corrosive media. Stainless steel has an aesthetically pleasing surface and good corrosion resistance. It does not require surface treatment such as plating to give full surface properties to stainless steel. One of the aspects of steel, commonly referred to as stainless steel. Representative properties include high-alloy steels such as 13 chrome steel and 18-chrome nickel steel.
From the perspective of metallography, because stainless steel contains chromium and the surface forms a very thin chromium film, this film is isolated from the intrusion of oxygen in the steel to resist corrosion.
In order to maintain the corrosion resistance inherent in stainless steel, steel must contain more than 12% chromium.
Type of stainless steel:
Stainless steel can be roughly classified by use, chemical composition and metallographic structure.
The steel of the austenitic system is composed of 18% chromium-8% nickel, and the amount of each element is changed differently, and steel grades for various uses are developed.
Classified by chemical composition:
1. CR series: ferrite series, martensite series
2. CR-NI series: austenite series, anomaly series, precipitation hardening series.
Classification by metallographic organization:
Austenitic stainless steel
2. Ferritic stainless steel
3. Martensitic stainless steel
4. Duplex stainless steel
5. Precipitation hardening stainless steel
Stainless steel marking method
1. Numbering and representation of steel
1. Use the international chemical element symbol and the national symbol to represent the chemical composition, and use the Arabic alphabet to indicate the content of the ingredient: eg: China, Russia 12CrNi3A
2. Use a fixed number of digits to indicate a steel series or number; for example: the United States, Japan, 300 series, 400 series, 200 series;
3. Use the Latin alphabet and the sequence to form the serial number, which only indicates the purpose.
2. China’s numbering rules
1. Use element symbols
2. Use, Chinese Pinyin,
Flat furnace steel: P, boiling steel: F, killed steel: B, A steel: A, T8: special 8,
Coupling steel, spring steel, such as: 20CrMnTi60SiMn, (using a few ten thousandths to indicate C content)
Stainless steel, alloy tool steel (representing C content in parts per thousand), such as: 1Cr18Ni9 one thousandth (ie 0.1% C), stainless C≤0.08% such as 0Cr18Ni9, ultra low carbon C≤0.03% such as 0Cr17Ni13Mo
3. International stainless steel marking method
The American Iron and Steel Institute uses three digits to mark various standard grades of malleable stainless steel. among them:
1. Austenitic stainless steel is marked with numbers in the 200 and 300 series.
2. Ferritic and martensitic stainless steels are represented by numbers in the 400 series. For example, some of the more common austenitic stainless steels are marked with 201, 304, 316, and 310.
3. Ferritic stainless steel is marked with 430 and 446, and martensitic stainless steel is marked with 410, 420 and 440C.
Remember, two-phase (austenitic-ferritic),
4. Stainless steel, precipitation hardened stainless steel and high alloys containing less than 50% iron are usually named after patent names or trademarks.
4. Standard classification and classification
1 national standard GB
2 industry standard YB
3 local standards
4 enterprise standard Q/CB
1 product standard
2 packaging standards
3 method standard
4 basic standards
4-3 Standard level (in three levels):
Y level: international advanced level
Level I: International general level
H level: domestic advanced level
4-4 national standard
GB1220-84 stainless steel bar (I grade)
GB4241-84 stainless welded plate garden (H class)
GB4356-84 Stainless Welding Plate Garden (Class I)
GB1270-80 stainless steel pipe (Class I)
GB12771-91 stainless welded pipe (Y grade)
GB3280-84 stainless cold plate (Class I)
GB4237-84 stainless hot plate (Class I)
GB4239-91 stainless cold belt (Class I)
Stainless steel term
In layman’s terms, stainless steel is a steel that is not easily rusted. In fact, some stainless steels have both rust and acid resistance (corrosion resistance). The stainless steel’s rust and corrosion resistance is due to the formation of a chromium-rich oxide film (passivation film) on its surface. This rust and corrosion resistance are relative. The test shows that the corrosion resistance of steel in the weak medium such as atmosphere and water and the oxidizing medium such as nitric acid increases with the increase of the water content of chromium in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel occurs. Mutations, from rust to rust, from corrosion to corrosion. There are many methods for classifying stainless steel. According to the structure of the structure at room temperature, there are martensite, austenitic, ferrite and duplex stainless steel; according to the main chemical composition, can be basically divided into two systems: chromium stainless steel and chromium nickel stainless steel; There are nitric acid resistant stainless steel, sulfuric acid resistant stainless steel, seawater resistant stainless steel, etc. According to the type of corrosion resistance, it can be divided into pitting resistant stainless steel, stress corrosion resistant stainless steel, intergranular corrosion resistant stainless steel, etc. Magnetic stainless steel, free-cutting stainless steel, low-temperature stainless steel, high-strength stainless steel, etc. Due to its excellent corrosion resistance, formability, compatibility and toughness over a wide temperature range, stainless steel is widely used in heavy industry, light industry, household goods industry and architectural decoration industries. .
Austenitic stainless steel: stainless steel having an austenitic structure at normal temperature. When the steel contains about 18% Cr, 8%-10% Ni, and about 0.1% C, it has a stable austenite structure. Austenitic chrome-nickel stainless steel includes the famous 18Cr-8Ni steel and high-Cr-Ni series steel which is developed by adding Cr, Ni content and adding elements such as Mo, Cu, Si, Nb and Ti. Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but its strength is low, it is impossible to strengthen it by phase transformation, and it can only be strengthened by cold working. If S, Ca, Se, Te and other elements are added, it has good machinability. In addition to the corrosion resistance of oxidizing acid medium, such steel can also resist the corrosion of sulfuric acid, phosphoric acid, formic acid, acetic acid, urea, etc. if it contains elements such as Mo and Cu. If the carbon content of such steel is less than 0.03% or contains Ti or Ni, the intergranular corrosion resistance can be remarkably improved. High silicon austenitic stainless steel concentrated nitric acid has good corrosion resistance. Due to the comprehensive and good comprehensive properties of austenitic stainless steel, it has been widely used in various industries.
Ferritic stainless steel: stainless steel mainly composed of ferrite in use. The chromium content is between 11% and 30%, and has a body-centered cubic crystal structure. These steels generally do not contain nickel, and sometimes contain a small amount of elements such as Mo, Ti, Nb, etc. These steels are characterized by large thermal conductivity, small expansion coefficient, good oxidation resistance and excellent resistance to stress corrosion. Parts that are corroded by water vapor, water, and oxidizing acids. Such steels have the disadvantages of poor plasticity, post-weld plasticity and corrosion resistance, which limit their application. The application of refining technology (AOD or VOD) can greatly reduce the gap elements such as carbon and nitrogen, thus making this type of steel widely used.
Austenitic-ferritic duplex stainless steel: stainless steel with approximately half of the austenite and ferrite structures. In the case of a lower C content, the Cr content is between 18% and 28%, and the Ni content is between 3% and 10%. Some steels also contain alloying elements such as Mo, Cu, Si, Nb, Ti, and N. This type of steel combines the characteristics of austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity, toughness, no room temperature brittleness, resistance to intergranular corrosion and weldability, and iron retention. The stainless steel has a brittleness at 475 ° C and a high thermal conductivity, and is characterized by superplasticity. Compared with austenitic stainless steel, it has high strength and resistance to intergranular corrosion and chloride stress corrosion resistance. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel
Martensitic stainless steel: Stainless steel whose mechanical properties can be adjusted by heat treatment. Generally speaking, it is a kind of hardenable stainless steel. Typical grades are Cr13, such as 2Cr13, 3Cr13, 4Cr13, etc. After fire, the hardness is higher. Different tempering temperatures have different combinations of toughness and are mainly used for steam turbine blades, tableware and surgical instruments. According to the difference in chemical composition, martensitic stainless steel can be divided into martensite chromium steel and martensitic chromium nickel steel. According to the organization and strengthening mechanism, it can be divided into martensitic stainless steel, martensite and semi-austenitic (or semi-martensitic) precipitation hardening stainless steel and maraging stainless steel.
Physical and chemical mechanical properties of stainless steel
The physical properties of stainless steel are mainly expressed in the following aspects:
1. Thermal expansion coefficient: A change in the elemental mass element caused by a change in temperature. The expansion coefficient is the slope of the expansion-temperature curve, the instantaneous expansion coefficient is the slope at a particular temperature, and the average slope between the two specified temperatures is the average coefficient of thermal expansion. The coefficient of expansion can be expressed in terms of volume or length, usually expressed in length.
2. Density: The density of a substance is the mass per unit volume of the substance, and the unit is kg/m3 or 1b/in3.
3. Modulus of elasticity: When the force applied to the ends of the unit length ribs can cause a unit change in the length of the object, the force required per unit area is called the modulus of elasticity. The unit is 1b/in3 or N/m3.
4. Resistivity: The resistance measured between two pairs of cubes of unit length, expressed in Ω?m, μΩ?cm or (disused) Ω/(circular mil.ft).
5. Permeability: The dimensionless coefficient indicates the degree to which a substance is easily magnetized, and is the ratio of the magnetic induction intensity to the magnetic field strength.
6. Melting temperature range: Determine the temperature at which the alloy begins to solidify and solidify.
7. Specific heat: The amount of heat required to change the temperature of a unit mass of material by one degree. In the Inch and CGs systems, the two values are the same as the heat, because the unit of heat (Biu or cal) depends on the unit mass of water rising by 1 degree. The value of specific heat in the International System of Units is different from the English or CGS system because the unit of energy (J) is defined by different definitions. The unit of specific heat is Btu (1b?0F) and J/(kg?k).
8. Thermal conductivity: A measure of the rate at which a substance conducts heat. When a temperature gradient of 1 degree per unit length is established on a unit cross-sectional area material, the thermal conductivity is defined as the amount of heat per unit time, and the unit of thermal conductivity is Btu/(h?ft?0F) or w/(m ?K).
9. Thermal diffusivity: a property that determines the rate of advancement of the internal temperature of a substance. It is the ratio of thermal conductivity to the product of heat and density. The unit of thermal diffusivity is Btu/(h?ft?0F) or w/(m ?k) indicates.
Stainless steel performance and organization
There are currently more than 100 kinds of chemical elements known, and about twenty kinds of chemical elements can be encountered in steel materials commonly used in industry. For the special steel series of stainless steel formed by people’s long-term struggle with corrosion phenomena, there are more than a dozen of the most commonly used elements. In addition to the basic elemental iron that makes up the steel, it has the greatest influence on the performance and microstructure of stainless steel. The elements are: carbon, chromium, nickel, manganese, silicon, molybdenum, titanium, niobium, titanium, manganese, nitrogen, copper, cobalt, and the like. Among these elements, except for carbon, silicon, and nitrogen, are elements of the transition period in the chemical element periodic table.
In fact, industrially applied stainless steels have several or even more than a dozen elements at the same time. When several elements coexist in the continuum of stainless steel, their effects are much more complicated than when they exist alone, because in this In this case, it is necessary to consider not only the role of each element but also the influence of each other, so the organization of stainless steel is determined by the sum of the influences of various elements.
1. The influence and function of various elements on the performance and microstructure of stainless steel
1-1. The role of chromium in stainless steel: There is only one element that determines the nature of stainless steel. This is chromium, and each type of stainless steel contains a certain amount of chromium. To date, there is no stainless steel that does not contain chromium. The reason why chromium has become the main element determining the performance of stainless steel is that after adding chromium as an alloying element to steel, it promotes the contradictory movement inside it to the development of corrosion resistance. This change can be explained in the following ways:
1. Chromium increases the electrode potential of iron-based solid solution
2. Chromium absorbs iron electrons to passivate iron
Passivation is a phenomenon in which the corrosion resistance of metals and alloys is improved due to the inhibition of the anode reaction. There are many theories that constitute the passivation of metals and alloys, mainly thin film theory, adsorption theory and electronic alignment theory.
1-2. Duality of carbon in stainless steel
Carbon is one of the main elements of industrial steel. The properties and organization of steel are largely determined by the content of carbon in steel and its distribution. The effect of carbon in stainless steel is particularly significant. The influence of carbon on the microstructure of stainless steel is mainly manifested in two aspects. On the one hand, carbon is an element that stabilizes austenite, and the degree of action is very large (about 30 times that of nickel). On the other hand, the affinity of carbon and chromium is very high. Large, with chromium – a complex series of carbides. Therefore, from the two aspects of strength and corrosion resistance, the role of carbon in stainless steel is contradictory.
Knowing the law of this influence, we can choose different stainless steels with different carbon content from different requirements.
For example, the most widely used and most inferior stainless steel in the industry – 0Crl3 ~ 4Cr13, the standard chromium content of the five steel grades is 12-14%, which is to consider the carbon and chromium to form chromium carbide. The purpose of the decision is to combine the carbon and chromium into chromium carbide, the chromium content in the solid solution is not lower than the minimum chromium content of 11.7%.
For the five steel grades, the strength and corrosion resistance are also different due to the different carbon content.
Post time: Nov-07-2018