Usually, people call alloy steel with chromium content greater than 12% or nickel content greater than 8% stainless steel. This steel has certain corrosion resistance in the atmosphere or in corrosive media, and has high strength at higher temperatures (>450°C). Steel with a chromium content of 16% to 18% is called acid-resistant steel or acid-resistant stainless steel, and is commonly referred to as stainless steel.
When the chromium content in steel reaches more than 12%, in contact with oxidizing media, due to electrochemical action, a chromium-rich passivation film will quickly form on the surface to protect the metal from corrosion; but in non-oxidative corrosion In the medium, it is still not easy to form a strong passivation film. In order to improve the corrosion resistance of steel, usually increase the proportion of chromium or add alloy elements that can promote passivation, adding Ni, Mo, Mn, Cu, Nb, Ti, W, Co, etc., these elements not only improve the resistance of steel Corrosion ability changes the internal structure and physical and mechanical properties of steel at the same time. The content of these alloying elements in steel has different effects on the performance of stainless steel. Some are magnetic, some are non-magnetic, some can be heat treated, and some cannot be heat treated.
Due to the above-mentioned characteristics of stainless steel, it is more and more widely used in aviation, aerospace, chemical, petroleum, construction and food industries and daily life. The contained alloy elements have a great influence on the machinability, and some are even difficult to cut.
2 What types of stainless steel can be divided into?
Austenitic stainless steel Martensitic stainless steel Ferritic stainless steel
According to its composition, stainless steel can be divided into two categories: chromium stainless steel mainly chromium and chromium-nickel stainless steel mainly chromium and nickel.
Stainless steel commonly used in industry is generally classified according to metallographic structure and can be divided into the following five categories:
1. Martensitic stainless steel: chromium content of 12% to 18%, carbon content of 0.1% to 0.5% (sometimes up to 1%), common ones are 1Cr13, 2Cr13, 3Cr13, 4Cr13, 1Cr17Ni2, 9Cr18, 9Cr18MoV, 30Cr13Mo, etc. .
2. Ferritic stainless steel: chromium content of 12% to 30%, the common ones are 0Cr13, 0Cr17Ti, 0Cr13Si4NbRE, 1Cr17, 1Cr17Ti, 1Cr17M02Ti, 1Cr25Ti, 1Cr28, etc.
3. Austenitic stainless steel: the complex content is 12%-25%, the nickel content is 7%-20% (or more than 20%), the most typical representative is 1Cr18Ni9Ti, and the common ones are 00Cr18Ni10, 00Cr18Ni14Mo2Cu2, 0Cr18Ni12Mo2Ti, 0Cr18Ni18Mo2Cu2Ti, 0Cr23Ni28M03Cu3Ti, 1Cr14Mn14Ni, 2Cr13Mn9Ni4, 1Cr18Mn8Ni5N, etc.
4. Austenitic + ferritic stainless steel: similar to austenitic stainless steel, only a certain amount of ferrite is contained in the structure. The common ones are 0Cr21Ni5Ti, 1Cr21Ni5Ti, 1Cr18Mn10Ni5M03N, 0Cr17Mn13Mo2N, 1Cr17Mn9Ni3M03Cu2N, Cr2bNi17M03CuSiN, AlCr18Ni11Si4, etc.
5. Precipitation hardening stainless steel: contains high chromium, nickel and very low carbon, the common ones are 0Cr17Ni4Cu4Nb, 0Cr17Ni7Al, 0Cr15Ni7M02Al, etc.
The first two categories are chromium stainless steel, and the latter three categories are chromium-nickel stainless steel.
3 What are the physical and mechanical properties of stainless steel?
1. Martensitic stainless steel: It can be quenched. After quenching, it has high hardness, strength, wear resistance and good oxidation resistance. Some are magnetic, but the internal stress is large and brittle. After low-temperature tempering, the stress can be eliminated, plasticity can be improved, cutting processing is more difficult, there is an obvious tendency of chip abrasion or bonding, and the tool is easy to wear.
When the carbon content in the steel is less than 0.3%, the structure is not uniform, the adhesion is strong, the built-up edge is prone to be generated during cutting, and the chip breaking is difficult, and the processed surface quality of the workpiece is low. When the carbon content reaches 0.4% to 0.5%, the machinability is better.
After quenching and tempering, martensitic stainless steel can obtain excellent comprehensive mechanical properties, and its machinability is greatly improved compared with the annealed state.
2. Ferritic stainless steel: The structure is stable during heating and cooling, and no phase change occurs. Therefore, heat treatment can not strengthen it. It can only be strengthened by deformation. The performance is brittle and the machinability is generally good. The chips are band-shaped, and the chips are easy to scratch or stick to the cutting edge, thereby increasing the cutting force and the cutting temperature, and at the same time, it may cause the surface of the workpiece to tear.
3. Austenitic stainless steel: Because it contains more nickel (or manganese), the structure does not change when heated, so quenching cannot strengthen it, and its workability can be slightly improved. Through cold work hardening, the strength can be greatly improved. If the aging treatment is carried out, the tensile strength can reach 2550-2740 MPa.
When cutting austenitic stainless steel, the band-shaped chips are continuous, chip breaking is difficult, and work hardening is easy to occur. The hardened layer brings great difficulty to the next cutting, causing sharp wear of the tool and greatly reducing the durability of the tool.
Austenitic stainless steel has excellent mechanical properties, good corrosion resistance, the more prominent is the cold deformation ability, non-magnetic.
4. Austenitic + ferritic stainless steel: intermetallic compounds with extremely high hardness are precipitated, the strength is higher than that of austenitic stainless steel, and its machinability is worse.
5. Precipitation hardening stainless steel: Contains alloying elements such as thallium, aluminum, molybdenum, titanium, etc., which can cause precipitation hardening. They are precipitated during tempering, resulting in precipitation hardening, which gives the steel high strength and hardness. Due to the low carbon content to ensure sufficient chromium content, it has good corrosion resistance.