What do Class I steel, Class II steel, Class III steel, and Class IV steel mean?

Class I steel-I represents Class I steel: belongs to carbon steel

FeⅠ class (carbon steel): Q235, 20#, 20g, 20R, L210, S205, HP265, etc.

Steel with a carbon content of less than 1.35% (0.1%-1.2%), except for iron, carbon and impurities such as silicon, manganese, phosphorus, sulfur within a limited amount, and no other alloying elements. The performance of carbon steel mainly depends 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 steels, carbon steel was used the earliest, has low cost, a wide range of performance, and the largest amount. It is suitable for media such as water, steam, air, hydrogen, ammonia, nitrogen and petroleum products with a nominal pressure of PN≤32.0MPa and a 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 represents Class II steel: low-alloy steel

FeⅡ (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, one or more alloying elements are intentionally added to the steel in order to improve one or more properties of the steel. When the amount of alloy added exceeds the general content of carbon steel in the normal production method, 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; greater than 10% is called high-alloy steel.

Category III steel – III represents three types of steel: martensitic stainless steel, ferritic stainless steel, chromium-molybdenum steel

FeIII type (martensitic and ferritic stainless steel): 1Cr5Mo, 0Cr13, 1Cr17, 1Cr9Mo, etc.

Standard martensitic stainless steels are: 403, 410, 414, 416, 416(Se), 420, 431, 440A, 440B and 440C, which are magnetic; the corrosion resistance of these steels comes from “chromium”, which ranges from 11.5 to 18%. The higher the chromium content, the higher the carbon content is required to ensure the formation of martensite during heat treatment. The above three types of 440 stainless steel are rarely considered for applications requiring welding, and the filler metal of the 440 composition 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 high thermal conductivity, low expansion coefficient, good oxidation resistance, excellent stress corrosion resistance, etc., and is mostly used to manufacture parts resistant to atmospheric, water vapor, water and oxidizing acid corrosion.

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 in many application fields that were originally believed to be only austenitic stainless steel (300 series), ferritic stainless steel is an extremely 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

FeIV type (austenitic, duplex stainless steel) austenitic stainless steel refers to stainless steel with austenitic structure at room temperature. When the steel contains about 18% Cr, 8%~10% Ni, and about 0.1% C, it has a stable austenitic structure.

Austenitic chromium-nickel stainless steel includes the famous 18Cr-8Ni steel and the high Cr-Ni series steel developed on this basis 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 has low strength. It is impossible to strengthen it through phase transformation. It can only be strengthened through cold working. If elements such as S, Ca, Se, Te are added, it has good machinability.

Duplex stainless steel (DSS) refers to stainless steel with ferrite and austenite accounting for about 50% each, and the content of the lesser phase generally needs to reach 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 steels. Compared with ferrite, it has higher plasticity and toughness, no room temperature brittleness, significantly improved intergranular corrosion resistance and welding performance, while maintaining the 475℃ brittleness and high thermal conductivity of ferritic stainless steel, and has superplasticity and other characteristics.

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 resistance and is also a nickel-saving stainless steel.