What are seamless boiler tubes and how are they made?

What are Seamless Boiler Tubes?

Seamless boiler tubes are boiler-specific pipes manufactured using a seamless production process. They are made of rigorously selected carbon steel or alloy steel, and their steel composition must adhere to strict standards (such as ASTM A355, ASTM A213, GB 5310). Due to the absence of weld seams, they possess higher overall strength and reliability, making them suitable for harsh operating conditions involving high temperatures, high pressures, and cyclic stress.

During boiler operation, these pipes are constantly exposed to high-temperature flue gas and high-pressure steam, facing challenges of oxidation, corrosion, and creep. Therefore, seamless boiler tubes must simultaneously possess the following key properties:

● Excellent high-temperature strength and creep endurance strength: They must not undergo excessive plastic deformation or cracking during long-term operation at high temperatures.
● Good oxidation and corrosion resistance: They must resist high-temperature steam oxidation and flue gas side corrosion (such as sulfur corrosion and vanadium corrosion).
● Microstructural stability during long-term service: The internal microstructure remains stable under prolonged high temperatures, preventing rapid performance degradation.

Applications: Seamless boiler tubes are mainly used in key pressure-bearing components of boilers in thermal power plants and industrial boilers, such as superheaters, reheaters, water walls, economizers, headers, and main steam pipelines. They are widely used in the power, chemical, and energy industries.

Classification and Uses of Boiler Tubes:

They are typically classified according to operating conditions (temperature, pressure) and material grade:

1. Classification by Operating Conditions

Medium and Low Pressure Boiler Tubes: Operating temperatures are typically ≤450℃, and pressures are relatively low. Mainly used in the manufacture of water walls, economizers, boiling water tubes, and low-temperature superheaters. Common materials are 20G and SA210-C carbon steel.

High Pressure and Above Boiler Tubes: Operating temperatures are typically >450℃, and pressures are higher. Used in the manufacture of high-temperature superheaters, reheaters, and main steam pipelines. Materials requiring higher thermal strength and structural stability are commonly used, including low-alloy steel (such as 15CrMoG), martensitic heat-resistant steel (such as T/P91), and various austenitic stainless steels (such as TP347H, Super304H).

2. Classification by Material Grade

a. Carbon Steel Boiler Tubes

The most basic boiler tube material, containing no or only trace amounts of alloying elements. Lowest cost, excellent processing and welding performance, but limited high-temperature strength and oxidation resistance. Suitable for low-temperature, low-pressure conditions.

Typical Grades: 20G (GB 5310), SA210-A1/C (ASME) 

Main Applications (≤450℃): Economizers, water-cooled walls, and low-temperature superheaters in industrial and power plant boilers.

b. Low-Alloy Heat-Resistant Steel Boiler Tubes

Based on carbon steel, alloying elements mainly composed of chromium (Cr), molybdenum (Mo), and vanadium (V) are added, significantly improving the material's mid-temperature strength, creep resistance, and certain oxidation resistance. Suitable for medium-temperature and medium-pressure operating conditions, it is the main material for subcritical and lower-parameter boilers.

Typical grades: 15CrMoG, 12Cr1MoVG, T/P22 

Main application areas (450℃ to 580℃): Medium-temperature superheaters, reheaters, headers, main steam pipelines.

c. Martensitic Heat-resistant Steel Boiler Tubes

Achieved through higher chromium content and composite multi-element microalloying (V, Nb, N, etc.), followed by normalizing + high-temperature tempering to obtain a tempered martensitic structure. Its high-temperature creep strength is 2-3 times that of low-alloy steel, and its oxidation resistance is also significantly improved. Suitable for advanced high-temperature and high-pressure units, it is a key material for achieving supercritical and ultra-supercritical parameters.

Typical grades: T/P91, T/P92

Main application areas (580℃ to 620℃): High-temperature superheaters, reheater pipelines, and large-diameter main steam and reheat steam pipelines.

d. Austenitic Stainless Steel Boiler Tubes

With their high chromium and nickel content, possess excellent oxidation and corrosion resistance, making them suitable for the most demanding high-temperature corrosive environments.

Typical grades: TP304H, TP347H, Super304H, HR3C 

Main applications (up to 650℃ and above): Tube banks in the last stage superheater and reheater of ultra-supercritical boilers, where flue gas temperatures are highest and corrosion is most severe.

Core performance indicators: 

The mechanical properties of boiler tubes are crucial indicators for ensuring the final performance of the steel. They depend on the steel's chemical composition and heat treatment system (e.g., normalizing + tempering). Steel pipe standards specify tensile properties (tensile strength, yield strength or yield point, elongation), hardness, and toughness indicators, as well as high and low temperature performance requirements (creep strength, endurance strength), according to different application requirements.

How are Seamless Boiler Tubes Made?

1. Main manufacturing methods

●Hot rolling (extrusion) process: This is the mainstream process. The process is as follows: Round billet → Heating → Piercing → Rolling/Extrusion → Sizing → Cooling → Straightening → Post-processing.
Process characteristics: High production efficiency, applicable to most specifications.

●Cold drawing (rolling) process: Based on hot-rolled tubes. The process is as follows: Billet → Annealing → Pickling → Lubricating layer coating → Multi-pass cold drawing (rolling) → Heat treatment.
Process characteristics: Can obtain tubes with extremely high dimensional accuracy and better surface finish.

2. Key Heat Treatment Processes 

Heat treatment is the core link in improving and stabilizing the performance of seamless steel pipes, especially crucial for alloy steel pipes:

a. Normalizing + Tempering: The most commonly used heat treatment process for high-pressure boiler tubes. Normalizing (heating followed by air cooling) homogenizes the microstructure and refines the grains; tempering is used to eliminate stress, stabilize the microstructure, and obtain the best strength and toughness balance.
b. Quenching + Tempering: Mainly used for martensitic steels (such as P91). Quenching yields high-strength martensite, followed by high-temperature tempering to improve toughness, resulting in a product that combines high strength with sufficient ductility.
c. Annealing: Primarily used to soften pipes, eliminate internal stress, or for work hardening, improving machinability.

3. Finishing and Inspection

After forming heat treatment, each steel pipe undergoes rigorous non-destructive testing (ultrasonic, eddy current, X-ray), hydrostatic testing, mechanical property testing, and metallographic analysis to ensure it meets standard requirements.

How to Choose the Right Boiler Tube Material?

Choosing the right boiler tube material requires considering the following factors:

1. Design Parameters: Operating temperature and pressure are the primary determining factors. Temperature determines the required creep resistance and oxidation resistance level of the material; pressure affects wall thickness design and material strength requirements.

2. Media Environment: Analyze flue gas composition (sulfur, chlorine, vanadium, etc.) and steam quality to assess corrosion and oxidation risks, and select materials with appropriate corrosion resistance accordingly. For highly corrosive environments, high-chromium stainless steel (such as TP347H and HR3C) should be given priority.

3. Standards and Specifications: The project must comply with the national standards (e.g., GB 5310), industry standards, or international standards (e.g., ASME SA213/SA335) specified for the project. These standards detail all technical requirements, including the chemical composition, mechanical properties, and testing methods of the materials.

4. Process and Cost: Consider the weldability, machinability, and life-cycle cost of the materials. While advanced materials may have higher initial costs, they can be more economical due to reduced wall thickness, increased efficiency, or extended lifespan.

Professional Advice: For the selection of materials for critical components, it is strongly recommended to consult a professional materials engineer or collaborate with a qualified supplier to conduct a technical evaluation based on specific design parameters and the operating environment.

Conclusion: 

Seamless boiler tubes are critical pressure-bearing components of boiler systems. Their material selection and manufacturing revolve around high-temperature strength, corrosion resistance, structural stability, and process reliability. Correctly selecting materials and specifications that meet the operating conditions and strictly adhering to relevant standards and testing procedures are fundamental to ensuring the long-term safe and efficient operation of the boiler.

Read more: Boiler Tube Size Chart or Low, Medium and High Pressure Boiler Tube