Steel classification based on carbon content is fundamental to understanding material properties and applications. Carbon content directly affects the strength, hardness, and ductility of steel, making it a critical factor in material selection for various industrial applications.

• Low Carbon Steel - Generally less than 0.25% carbon content. Examples: 10 and 20 steels. Known for good ductility, toughness, and weldability. Commonly used in automotive body panels, structural shapes, and pipes.
• Medium Carbon Steel - Typically between 0.25% and 0.60% carbon content. Examples: 35 and 45 steels. Offers a balance of strength, ductility, and wear resistance. Often used in machinery parts, gears, and railway components.
• High Carbon Steel - Generally greater than 0.60% carbon content. This type of steel is rarely used in the manufacture of steel pipes due to its brittleness. Applications include cutting tools, springs, and high-strength wires.

In practical applications, there is no absolute distinction between these carbon content classifications, as properties can be modified through heat treatment and alloying elements.

Carbon steel is produced in large quantities and serves a wide range of industrial applications due to its versatility and cost-effectiveness.

Manufacturing and Structural Applications

Carbon steel is typically rolled (forged) into various forms including round, square, and flat profiles, plates, and seamless steel pipes. Its primary uses include:

• Manufacture of general structural and mechanical parts
• Building and construction structures
• Pipes for conveying fluids in various industries

Depending on application requirements, heat treatment (normalizing or quenching and tempering) may be required before use to achieve desired mechanical properties.

Boiler Tubes and High-Pressure Applications

20 High-Pressure Boiler Seamless Steel Pipe (GB5310-2008) is a high-quality seamless steel pipe made of carbon steel, alloy steel, and stainless heat-resistant steel used for the heating surfaces of water-tube boilers operating at high pressures and above.

High-pressure boiler tubes are a specialized category within seamless steel pipes. While their manufacturing method is similar to standard seamless pipes, they must meet stricter requirements for material properties due to their operating conditions:

• Exposure to high temperatures and pressures
• Potential oxidation and corrosion from high-temperature flue gas and steam

These demanding conditions require high-pressure boiler tubes to possess:

• High endurance strength
• Excellent resistance to oxidation and corrosion
• Good structural stability under prolonged exposure to heat

Primary applications of high-pressure boiler tubes include manufacturing superheater tubes, reheater tubes, gas ducts, and main steam pipes for high- and ultra-high-pressure boilers.

Classification of Boiler Tubes

Type	Standard	Applications	Material
Low- and Medium-Pressure Boiler Tubes	GB3087-2008	Superheated steam pipes, boiling water pipes, locomotive boiler pipes, flue pipes	Carbon structural steel
High-Pressure Boiler Tubes	GB5310-2008	Superheater tubes, reheater tubes, main steam pipes for high-pressure boilers	Carbon steel, alloy steel, stainless heat-resistant steel
Structural Seamless Tubes	GB/T8162-1999	General structural and mechanical applications	Carbon steel, alloy steel

Standard Dimensions

According to GB5310-2008 "Seamless Steel Tubes for High-Pressure Boilers":

• Hot-rolled tubes: Outer diameter 22 to 530 mm, wall thickness 20 to 70 mm
• Cold-drawn (cold-rolled) tubes: Outer diameter 10 to 108 mm, wall thickness 2.0 to 13.0 mm

Special-shaped Seamless Steel Pipes

Special-shaped seamless steel pipes refer to seamless steel pipes with cross-sectional shapes other than round. They are categorized as:

Type	Code	Description
Equal Wall Thickness	D	Uniform thickness throughout the cross-section
Unequal Wall Thickness	BD	Varying thickness at different points in cross-section
Variable Diameter	BJ	Diameter changes along the length of the tube

Special-shaped tubes offer significant advantages over round tubes:

• Larger moment of inertia and section modulus
• Greater resistance to bending and torsion
• Weight reduction in structural applications
• Material savings through optimized designs

These properties make special-shaped tubes ideal for various structural components, tools, and mechanical parts where performance and efficiency are critical.

Chemical Composition Testing

Chemical composition analysis is critical for ensuring material compliance with specifications:

• GB3087-2008 (Low and Medium Pressure Boilers): Chemical composition test methods comply with GB222-84 and GB223 "Methods for Chemical Analysis of Steel and Alloys"
• GB5310-2008 (High-Pressure Boilers): Chemical composition test methods comply with GB222-84 and GB223 "Methods for Chemical Analysis of Steel and Alloys"
• Imported Boiler Steel Tubes: Chemical composition inspection follows relevant standards specified in the contract

Weight Calculation Formula

Mechanical Properties

Mechanical properties of boiler tubes are defined in relevant standards and include:

• Tensile strength
• Yield strength
• Elongation
• Hardness
• Impact resistance

These properties ensure that boiler tubes can withstand the demanding conditions of high-pressure and high-temperature environments while maintaining structural integrity over extended operational periods.

20G High Pressure Boiler Tube Chemical Composition

The chemical composition of 20G steel for high-pressure boiler tubes is strictly controlled to ensure performance under demanding conditions: