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Basic knowledge of stainless steel

Basic knowledge of stainless steel

Basic knowledge of stainless steel

Stainless steel

In layman’s terms, stainless steel is steel that is not easy to rust. In fact, some stainless steels have both rust resistance and acid resistance (corrosion resistance). The rust resistance and corrosion resistance of stainless steel are due to the formation of a chromium-rich oxide film (passivation film) on its surface. This rust resistance and corrosion resistance are relative. Tests have shown that the corrosion resistance of steel in weak media such as the atmosphere and water and oxidizing media such as nitric acid increases with the increase of the chromium water content in the steel. When the chromium content reaches a certain percentage, the corrosion resistance of the steel occurs. Sudden change, that is, from easy to rust to not easy to rust, from not resistant to corrosion. There are many ways to classify stainless steel. According to the organization structure at room temperature, there are martensitic, austenitic, ferritic and duplex stainless steels; according to the main chemical composition, they can be basically divided into two major systems: chromium stainless steel and chromium nickel stainless steel; divided by purpose 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.; according to functional characteristics, it can be divided into none Magnetic stainless steel, free-cutting stainless steel, low temperature stainless steel, high strength stainless steel, etc. Because stainless steel has excellent corrosion resistance, formability, compatibility, and toughness in a wide temperature range, it has been widely used in heavy industry, light industry, daily necessities industry, and architectural decoration industries. .

According to ISO / TS / 5510-2003 , EN10088-1 : 1995 , ASTMA959-01 , EN10095-01 and other foreign standards, corrosion resistance is the most important feature and the Cr content is at least 10 . . 5 % or containing C largest one . 2 % of steel is stainless steel. The main technical requirement of heat-resistant steel is that they can bear higher than550 ℃The temperature of the gas and its combustion products are affected.

Austenitic steel (Austenitic stainless steel)

Stainless steel with austenitic structure at room temperature. When the steel contains about 18% Cr , 8% to 10% Ni , and about 0.1% C , it has a stable austenite structure. Austenitic chromium-nickel stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed by adding Cr and Ni content and adding Mo , Cu , Si , Nb , Ti and other elements on this basis . Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but its strength is low. It is impossible to strengthen it through phase transformation. It can only be strengthened by cold working. Such as adding S , Ca , Se , Te and other elements, it has good machinability. In addition to resistance to oxidizing acid medium corrosion, this type of steel can also be resistant to corrosion by sulfuric acid, phosphoric acid, formic acid, acetic acid, and urea if it contains elements such as Mo and Cu . If the carbon content in this type of steel is less than 0.03% orTi and Ni can significantly improve its resistance to intergranular corrosion. The high-silicon austenitic stainless steel with concentrated nitric acid has good corrosion resistance. As austenitic stainless steel has comprehensive and good comprehensive properties, it has been widely used in various industries.

The main alloying elements of austenitic steel are Cr and Ni , which have a face-centered cubic lattice of γ and δ phases, and cannot be magnetized. The stability of austenitic steel can be improved by increasing the austenite forming elements C , Ni , Mn , N and Cu . Austenitic stainless steel has good general corrosion resistance. Any heat treatment process will not harden it. However, due to the increase of N content or cold forming, the mechanical properties of steel can be improved. Austenitic steel has good weldability, high low temperature impact toughness, and high safety to prevent brittleness.

Ferritic stainless steel (Ferritic steels)

The stainless steel whose structure is mainly ferrite in use. The chromium content is between 11% and 30% , and it has a body-centered cubic crystal structure. This kind of steel generally does not contain nickel, and sometimes contains a small amount of Mo , Ti , Nb and other elements. This kind of steel has the characteristics of large thermal conductivity, small expansion coefficient, good oxidation resistance, and excellent stress corrosion resistance. It is mostly used to make atmospheric resistance. , Water vapor, water and oxidizing acid corroded parts. This type of steel has disadvantages such as poor plasticity, significantly reduced plasticity and corrosion resistance after welding, which limits its application. The application of out-of-furnace refining technology ( AOD or VOD ) can greatly reduce interstitial elements such as carbon and nitrogen, so this type of steel is widely used.

Cr is a main alloying elements, does not C binding Cr determine the corrosion resistance of steel elements. Ferritic stainless steel having a C amount is limited to 0.08% or less heat treatment does not significantly hardening , work hardening can significantly increase the hardness. Ferritic stainless steel is annealed at a temperature below the temperature at which austenite is formed The annealing temperature depends on the chemical composition, usually the highest is 850 ~950°C.

Austenitic ferritic duplex stainless steel (Austenitic ferriticduplexsteels)

It is a stainless steel with austenite and ferrite structure each accounting for about half. In the case of low C content , the Cr content is 18% to 28% , and the Ni content is 3% to 10% . Some steels also contain alloying elements such as Mo , Cu , Si , Nb , Ti , and N. This type of steel has the characteristics of austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, and significantly improved intergranular corrosion resistance and welding performance, while still maintaining iron Stainless steel475 °CBrittleness, high thermal conductivity, and superplasticity. Compared with austenitic stainless steel, it has high strength and significantly improved resistance to intergranular corrosion and chloride stress corrosion. Duplex stainless steel has excellent pitting corrosion resistance and is also a nickel-saving stainless steel.

Typical duplex stainless steel contains high Cr ( 20% to 26% ), no Mo or less than 4% Mo , and low Ni content , which is about medium. It has obvious duplex structure at room temperature, typical metallographic structure It contains 40% to 60% of austenite in the ferrite matrix They are stronger than austenitic steels. It has exceptionally good stress corrosion resistance, and the addition of N can sufficiently retain the toughness and corrosion resistance that are not annealed after welding .

Martensitic stainless steel (Martensitic steels)

The stainless steel whose mechanical properties can be adjusted by heat treatment is generally a kind of hardenable stainless steel. Typical grades are Cr13 type, such as 2Cr13 , 3Cr13 , 4Cr13, etc. The hardness is higher after pure fire, and different tempering temperatures have different strength and toughness combinations. It is mainly used for steam turbine blades, tableware, and surgical instruments. According to the difference in 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 be divided into martensitic stainless steel, martensitic and semi-austenitic (or semi-martensitic) precipitation hardening stainless steel and maraging stainless steel.

The C content of martensitic steel ranges from 0.08% to 1.00%, or even higher. Through martensite transformation after quenching, the strength can be greatly improved, and tempering is usually required. Usually delivered in the normal annealing (ferrite) or quenched + tempered state. Martensitic stainless steel has a body-centered cubic structure and can be magnetized. When containing S amount> 0.15% when cutting steel , it can be used to withstand high-speed mechanical machining , but is not conducive to corrosion.

Chromium nickel molybdenum stainless steel

Stainless steel with chromium, nickel and molybdenum as the main alloying elements. For example, 00Cr17Ni14Mo2 , 00Cr19Ni13Mo3 , 0Cr18Ni16Mo5, etc. are austenitic stainless steels , and 0Cr26Ni5Mo2 is austenitic-ferritic stainless steel. Cr-Ni-Mo stainless steel, the corrosion resistance to both oxidizing medium , and corrosion resistance to reducing medium; both uniform corrosion , and resistance to localized corrosion. Chromium is the basic element for stainless steel to obtain corrosion resistance. Chromium against general corrosion, pitting and crevice corrosion very favorable, favorable corrosion and stress corrosion resistance against inter-crystalline; and molybdenum against general corrosion Advantageously , very advantageously against pitting and crevice corrosion. The combination of chromium and molybdenum greatly improves the corrosion resistance of stainless steel . Nickel is the most effective element to improve the toughness of stainless steel.

Precipitation hardening steels (Precipitation hardening steels)

Precipitation hardening steel is also called age hardening steel. Such steel having high corrosion resistance while also having high strength level. Steel high strength of the final because at lower temperatures the heat treatment phase intermetallic compound precipitation results from martensite.

Stainless steel grade group

200 Series  Cr  Ni  Manganese Austenitic Stainless Steel

300 series  chromium  nickel austenitic stainless steel

  • Model 301- good ductility, used for forming products. It can also be hardened by mechanical processing. Good weldability. Abrasion resistance and fatigue strength are better than 304 stainless steel.
  • Model 302—The corrosion resistance is the same as that of 304 , and the strength is better due to the relatively high carbon content.
  • Model 303—It is easier to cut than 304 by adding a small amount of sulfur and phosphorus .
  • Model 304 general model; that is 18/8 stainless steel. The old GB grade is 0Cr18Ni9 and the new grade is 06Cr19Ni10 .
  • Model 309- has better temperature resistance than 304 .
  • Model 316 Following 304 after the second of the most widely used of steel, mainly for the food industry and surgical equipment, add molybdenum to obtain a special corrosion-resistant structure. Because it has better resistance to chloride corrosion than 304, it is also used as “Marine steel  is used. SS316 is usually used in nuclear fuel recovery devices. 18/10 grade stainless steel is usually also suitable for this application level.
  • Model 321 except for the addition of titanium to reduce the risk of material weld corrosion, other properties are similar to 304 .

400 Series  ferritic and martensitic stainless steel

  • Model 408 good heat resistance, low corrosion resistance, 11% of of Cr , . 8% of of Ni .
  • Model 409— the cheapest model (British and American), usually used as a car exhaust pipe, is a ferritic stainless steel (chrome steel).
  • Model 410— Martensite (high-strength chromium steel), with good wear resistance and poor corrosion resistance.
  • Model 416— Add sulfur to improve the processing performance of the material.
  • Model 420—” cutting tool grade ” martensitic steel, similar to the earliest stainless steel such as Brinell high chromium steel. It is also used for surgical knives, which can be very bright.
  • Model 430 —Ferritic stainless steel, for decoration, such as car accessories. Good formability, but poor temperature resistance and corrosion resistance.
  • Model 440— High-strength cutting tool steel with slightly higher carbon content. After proper heat treatment, higher yield strength can be obtained. The hardness can reach 58HRC , which is among the hardest stainless steels. The most common application example is the ” razor blade ” . There are three commonly used models:440A, 440B ,440C,and also440F(Easy processing type).

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, usually called 17-4 ; 17%Cr , 4%Ni .

Characteristics and uses of stainless steel


American Standard ( ASTM )

National standard ( GB )

Japanese standard ( SUS )







It has the characteristics of acid and alkali resistance , high density, no bubbles, no pinholes, etc. It is used to produce high-quality materials for various watch cases and watchband bottom covers.

Mainly used to make decorative tubes , industrial tubes , and some shallow stretched products .




Part of the nickel is replaced by Mn and N , thereby obtaining good mechanical properties and corrosion resistance. It is a new type of nickel-saving stainless steel. Its room temperature strength is higher than that of 304 , and it has better oxidation resistance and resistance below 800 degrees. Medium temperature intensity

Mainly used to make decorative tubes , industrial tubes , and some shallow stretched products .




It is of Cr , of Mo and N interval narrower than the element , easy to achieve equilibrium phase ( i.e., two similar half and half ), improves the strength of steel , corrosion resistance and weldability , used for high performance requirements and the need for welding Materials , such as oil and gas pipelines.

Used in heat exchangers and cold showers and devices for oil refining , fertilizers , papermaking , petroleum , chemical industries, etc. that are resistant to seawater and high temperature and concentrated nitric acid.





As a widely used steel, it has good corrosion resistance, heat resistance, low temperature strength and mechanical properties. Good hot workability for stamping, bending, etc., no heat treatment hardening phenomenon (non-magnetic, use temperature-196 ~800 )

Food industry, household products ( class 1 and 2 tableware, kitchen cabinets, indoor pipelines, water heaters, boilers, bathtubs)




Cu should be added for its good formability, especially wire drawing and ageing crack resistance, and it can be formed into products with complex shapes. Its corrosion resistance is the same as 304 steel

Vacuum flasks, kitchen sinks, pots, pots, insulated lunch boxes, door handles, textile processing machines.




As the low C of 304 steel, in the general state, the corrosion resistance of 304 steel similar, but after welding or eliminate stress, its excellent resistance to intergranular corrosion. It can maintain good corrosion resistance without heat treatment, generally used below 400 (non-magnetic, use temperature-196 ~800 )

It is used in outdoor machinery, building materials, heat-resistant parts and parts with difficult heat treatment requirements in the chemical, coal, and petroleum industries that require high grain boundary corrosion resistance





In 304 added steel Ti element to prevent intergranular corrosion. Suitable for430 ~900 Use at temperature.

Medical appliances, building materials, chemistry, agriculture, ship parts, nuclear energy industry, aircraft, pipes, boiler drums, auto parts (windshield wipers, mufflers, molded products)





Mo should be added , so its corrosion resistance, atmospheric corrosion resistance and high temperature strength are particularly good, and it can be used under harsh conditions. Excellent work hardening (non-magnetic)

Equipment, chemistry, dyes, paper making in sea water. Oxalic acid, fertilizer and other production equipment; photography, food industry, coastal area facilities, ropes, CD rods, bolts, nuts




316 steel of low C series, in addition to 316 steel has the same characteristics, excellent resistance to grain boundary corrosion.

In 316 steel applications, products with special requirements for resisting grain boundary corrosion





23Cr-13Ni high-alloy steel stainless steel , excellent corrosion resistance and strength, suitable for working temperature1000 Parts

Exhaust equipment, heat treatment furnace and heat exchanger, etc.




Very good oxidation resistance, corrosion resistance, and high temperature resistance. Because of the higher percentage of chromium and nickel, 310S has much better creep strength, can work continuously at high temperatures, and has good high temperature resistance.


Boilers, exhaust equipment, heat treatment furnaces, heat exchangers, etc.

Physical, chemical and mechanical properties of stainless steel

The physical properties of stainless steel are mainly expressed in the following aspects:

  • ①. Coefficient of thermal expansion: the change in the quality of the material due to temperature changes. The expansion coefficient is the slope of the expansion-temperature curve, the instantaneous expansion coefficient is the slope at a specific temperature, and the average slope between two specified temperatures is the average thermal expansion coefficient. The expansion coefficient can be expressed by volume or length, usually expressed by length.
  • ②. Density: The density of a substance is the mass per unit volume of the substance, and the unit is kg/m 3 or 1b/in 3.
  • ③. Elastic modulus: When applying a force to the two ends of the edge per unit length 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/in 3 or N/m 3.
  • ④. Resistivity: The electrical resistance measured between two opposite faces of a cubic material per unit length, expressed in units of Ω•m , μΩ•cm or ( obsolete) Ω/(circular mil.ft) .
  • ⑤. Permeability: Dimensionless coefficient, indicating the degree to which a substance is easily magnetized, and is the ratio of magnetic induction intensity to magnetic field intensity.
  • ⑥. Melting temperature range: Determine the temperature at which the alloy begins to solidify and when it is solidified.
  • ⑦. Specific heat: The amount of heat required to change the temperature of a unit of material by 1 degree. In the British system and the CGs system, the specific heat values of the two are the same, because the unit of heat ( Biu or cal) depends on the amount of heat required to raise 1 degree per unit mass of water 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 a different definition. 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 , then the thermal conductivity is defined as the heat conducted per unit time, and the unit of thermal conductivity is Btu/(h•ft•0F) Or w/(m •K).

  • ⑨. Thermal diffusivity: It is a performance 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).

Source: China Stainless Steel Pipes Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)

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