Class I steel
I represents a type of steel: carbon steel
FeⅠ type (carbon steel): Q235, 20#, 20g, 20R, L210, S205, HP265, etc.
Steel with a carbon content of less than 1.35% (0.1%-1.2%), steel that does not contain other alloying elements except iron, carbon, silicon, manganese, phosphorus, sulfur, and other impurities within limits. The properties of carbon steel mainly depend on the carbon content.
As the carbon content increases, the strength and hardness of the steel increase, and the plasticity, toughness, and weldability decrease. Compared with other steel types, carbon steel is used earliest, has low cost, wide performance range, and the largest usage. It is suitable for media such as water, steam, air, hydrogen, ammonia, nitrogen, and petroleum products with nominal pressure PN≤32.0MPa and temperature of -30-425℃. Commonly used grades include WC1, WCB, ZG25, high-quality steel 20, 25, 30 and low-alloy structural steel 16Mn

Class II steel
Ⅱ stands for Category II steel: low alloy steel
FeⅡ type (low alloy steel): 16MnR, 12Cr1MoV, 20MnMoD, S240, L245, HP295, etc.
Alloy steel with a total alloying element content of less than 5% is called low alloy steel. Low alloy steel is relative to carbon steel. On the basis of carbon steel, in order to improve one or several properties of the steel, one or several alloying elements are intentionally added to the steel. The amount of alloy added exceeds When carbon steel has the normal content of normal production methods, this steel is called alloy steel.
When the total alloy content is less than 5%, it is called low alloy steel. The alloy content between 5-10% is called medium alloy steel; the alloy content greater than 10% is called high alloy steel.

Class III steel
III represents three types of steel: martensitic stainless steel, ferritic stainless steel, chromium-molybdenum steel
Fe III type (martensitic, ferritic stainless steel): 1Cr5Mo, 0Cr13, 1Cr17, 1Cr9Mo, etc.
Standard martensitic stainless steels are 403, 410, 414, 416, 416 (Se), 420, 431, 440A, 440B, and magnetic 440C types; the corrosion resistance of these steels comes from "chromium", which ranges from 11.5 to 18%. Steels with higher chromium content require higher carbon content to ensure the formation of martensite during heat treatment. The above three types of 440 stainless steel are rarely considered for applications that require welding, and the composition of type 440 Filler metal is not easy to obtain.
Ferritic stainless steel (400 series) contains 15% to 30% chromium and has a body-centered cubic crystal structure. This type of steel generally does not contain nickel and sometimes contains a small amount of Mo, Ti, Nb, and other elements. This type 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-resistant, parts corroded by steam, water and oxidizing acids.
The price of ferritic stainless steel is not only relatively low and stable, but also has many unique characteristics and advantages. It has been proven that ferritic stainless steel is an extremely good choice in many application fields where it was originally thought that only austenitic stainless steel (300 series) could be used. An excellent alternative material, ferritic stainless steel does not contain nickel, and the main elements are chromium (>10%) and iron. Chromium is an element that makes stainless steel particularly corrosion-resistant, and its price is relatively stable.


Class IV steel
Ⅳ represents four types of steel: austenitic stainless steel, duplex stainless steel
Fe IV type (austenitic, duplex stainless steel) austenitic stainless steel refers to stainless steel with an austenitic structure at normal temperature. When steel contains approximately 18% Cr, 8%~10% Ni, and approximately 0.1% C, it will have a stable austenite structure.
Austenitic chromium-nickel stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed by increasing the Cr and Ni content and adding Mo, Cu, Si, Nb, Ti, and other elements.
Austenitic stainless steel is non-magnetic and has high toughness and plasticity, but its strength is low. It cannot be strengthened through phase transformation and can only be strengthened through cold working. If elements such as S, Ca, Se, Te, etc. are added, it is easy to cut.
Duplex Stainless Steel (DSS) refers to stainless steel with ferrite and austenite each accounting for about 50%. Generally, the content of the smaller phase needs to be at least 30%. In the case of low C content, the Cr content is 18%~28% and the Ni content is 3%~10%. Some steels also contain alloying elements such as Mo, Cu, Nb, Ti, and N.
This type of steel has the characteristics of both austenitic and ferritic stainless steel. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, at the same time, it also maintains the 475°C brittleness of ferritic stainless steel, 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.

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