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 than
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.
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 ~
Austenitic ferritic duplex stainless steel (Austenitic ferritic＜duplex＞steels)
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 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 .
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.
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 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.
200 Series — Cr - Ni - Manganese Austenitic Stainless Steel
300 series - chromium - nickel austenitic stainless steel
400 Series - ferritic and martensitic stainless steel
500 series - heat - resistant chromium alloy steel.
600 series — Martensitic precipitation hardening stainless steel.
The physical properties of stainless steel are mainly expressed in the following aspects: