I. What are high pressure boiler pipe?
High-pressure boiler pipes are seamless steel pipes specifically used to manufacture key components of high-pressure boilers.
They are mainly used to transport steam or hot water in high-temperature, high-pressure environments and are widely used in the power, chemical, and metallurgical industries.
High-pressure boiler pipes must have high strength, good toughness, and corrosion resistance to ensure the safe operation of boilers.
II. Common materials used for high-pressure boiler pipes
| Material | Main Features | Application Scenarios |
|---|---|---|
| 20G | Carbon steel, moderate strength, good toughness, low cost | Medium and low-pressure boilers, working pressure < 9.8 MPa, temperature < 450°C |
| 20MnG, 25MnG | Added manganese element, higher strength | Medium and low-pressure boilers, working pressure < 9.8 MPa |
| 15MoG, 20MoG | Low-alloy steel, better strength and heat resistance | Moderate temperature and pressure conditions |
| 12CrMoG, 15CrMoG | Added chromium and molybdenum, good heat resistance and oxidation resistance | High-pressure boilers, working temperature < 550°C |
| 12Cr1MoVG | High strength, high heat resistance, added vanadium, titanium, etc. | High-pressure and ultra-high-pressure boilers, working temperature < 580°C |
| 12Cr2MoG | High strength, high heat resistance, suitable for higher temperature and pressure environments | Ultra-high-pressure boilers, working temperature < 600°C |
| 07Cr19Ni10 (304L) | Stainless steel, strong corrosion resistance | Chemical, pharmaceutical and other fields, working temperature < 300°C |
| 10Cr18Ni9NbCu3BN | Added nickel, copper, strong corrosion resistance | Corrosive environments, working temperature < 350°C |
| 07Cr25Ni21NbN | Extremely strong corrosion resistance | Extreme corrosive environments, working temperature < 350°C |
| 12Cr2MoWVTiB | Added tungsten, titanium, etc., high strength, good structural stability | Special high-pressure boilers, working temperature < 600°C |
| 10Cr9Mo1VNbN | High strength, high heat resistance, suitable for high-temperature and high-pressure conditions | High-temperature and high-pressure boilers, working temperature < 600°C |
III. Main manufacturing processes for high-pressure boiler pipes
i. Seamless Steel Pipe Manufacturing Process
Core Process: High-pressure boiler pipes are typically manufactured using seamless processes. The main processes include piercing, rolling, and heat treatment.
Key Steps: Solid tube blanks are pierced to form rough tubes. These are then hot-rolled or cold-rolled to achieve dimensional accuracy. Boiler seamless pipes must undergo strict heat treatment (such as normalizing and tempering).
Process Advantages: Seamless carbon steel pipes or alloy steel tubes produced via this process exhibit excellent structural integrity. The absence of welds results in high strength and superior pressure-bearing capacity. This is the primary reason seamless processes are preferred for high-pressure boiler pipes.
ii. Welded Steel pipe Manufacturing Process
Core Process: Some high-pressure boiler pipes are also produced using welded processes. The main processes include steel plate/strip forming, welding (e.g., ERW, SAW), and heat treatment.
Key Steps: The material is first rolled into a tubular shape. It is then welded using methods such as high-frequency resistance welding or submerged arc welding. Post-welding heat treatment is required to eliminate residual stresses.
Process Characteristics: Welded high-pressure boiler tubes offer high production efficiency and lower costs. However, the weld seam is a potential weak point and requires precise control. Compared to seamless boiler pipes, their pressure-bearing capacity is typically slightly lower.
iii. Strict quality control
Inspection methods: The entire production process of high-pressure boiler pipes must be strictly monitored. Non-destructive testing (such as ultrasonic, eddy current, and radiographic testing), hydrostatic testing, and metallographic analysis are employed.
Compliance with standards: Compliance with international standards is critical. Examples include China’s GB, the United States’ ASTM, and Germany’s DIN. For seamless alloy steel pipes, composition and performance testing are even more stringent.
IV. Core application areas of high-pressure boiler pipes
i. Power Industry (Core Applications)
Critical Components: High-pressure boiler pipes are core components of boilers in thermal power plants. They are extensively used in the manufacture of water-cooled walls, superheaters, reheaters, and economizer pipes.
Core Function: Boiler seamless steel pipes (typically alloy steel seamless tubes) withstand high-temperature, high-pressure steam. Their reliability directly impacts power generation efficiency and plant safety. Failure of high-pressure boiler pipes could lead to major accidents.
ii. Petrochemical Industry
Key Equipment: High-pressure boiler pipes are widely used in petrochemical plants, such as heaters (furnace tubes), reactors, heat exchangers, and high-temperature, high-pressure process pipelines.
Stringent Requirements: In corrosive environments (such as sulfur-containing media), the corrosion resistance of seamless carbon steel pipes or alloy steel seamless pipes is critical. High-pressure boiler tubes must withstand high-temperature hydrogen corrosion, sulfidation, and other corrosive effects.
iii. Other Industrial Fields
Widespread Application: High-pressure boiler pipes are also used in steel mills (waste heat boilers), paper mills (alkali recovery boilers), and the food and pharmaceutical industries (steam systems).
Adapting to Requirements: Different industries have specific requirements for boiler seamless steel tubes. For example, food-grade applications require cleanliness, and certain applications require specific alloy steel seamless pipes. High-pressure boiler pipes must meet these general and special requirements.
V. Welding process for high-pressure boiler pipes
i. Welding Method: Ensuring the Quality of the Root of Seamless Steel Pipes
The root welds of high-pressure boiler tubes have extremely high requirements. Full penetration must be achieved. The back of the weld must be flat and free of slag. Recommended combined process:
1) Base layer welding: GTAW (Tungsten Inert Gas Welding)
Manual tungsten inert gas shielded welding is used.
Argon gas shielding achieves an oxidation-free molten pool.
Precise control of penetration depth and weld bead formation.
This is critical for ensuring the reliability of the root weld in seamless carbon steel pipes. This process significantly outperforms conventional welded steel pipes.
2) Filler and Cover Weld: SMAW (Shielded Metal Arc Welding)
The filler and cover layers are welded using manual shielded metal arc welding.
Balances welding efficiency and weld strength.
Layered welding optimizes overall stress resistance.
ii. Welding materials: Matching the characteristics of seamless steel pipes
1) TIG welding wire
Select ER50-6 (φ2.5mm).
Its low-silicon, high-manganese characteristics suppress porosity.
Improves melt pool fluidity.
2) Welding Rods
Select R307 (E5515-B2, φ3.2/4.0mm).
Matches the Cr-Mo-V alloy system of the base material.
Ensures high-temperature performance of the weld. The requirements are higher than those for ordinary carbon steel seamless steel pipes or structural seamless pipes.
3) Precautions
Welding materials must be strictly dried.
R-type electrodes must be dried at 350°C for 1 hour.
Avoid the risk of hydrogen-induced cracking.
iii. Preheating control: Preventing cold cracking in seamless steel pipes
1) Preheating Temperature
Maintained between 200-300°C.
Heated uniformly using electric heating cables or flames.
Coverage area on both sides of the weld ≥100mm.
2) Interlayer Temperature
Maintained at ≥200°C throughout the process.
Monitored in real-time using an infrared thermometer.
3) Wall Thickness Requirements
Seamless steel pipes with wall thicknesses exceeding 20mm must be preheated in sections.
Avoid localized overheating, which can cause grain coarsening.
iv. Key points for welding high-pressure boiler pipes
1) Base Layer TIG Welding
Full-position fixed welding.
The welding torch is perpendicular to the pipe wall.
Argon gas flow rate: 8–12 L/min (purity ≥ 99.99%).
Pulse current is used (base value/peak value = 70%/130%).
Reduce heat input to avoid thermal overload.
The weld seam must be ground to remove the arc pit.
Overlap weld length ≥5 mm.
2) Arc Welding Filler Layer
Use short arc operation (arc length ≤3 mm).
Move the electrode in a crescent shape.
Hold for 0.5–1 seconds on both sides of the groove to ensure fusion.
Control heat input at 15–25 kJ/cm³.
Thoroughly remove interlayer slag during multi-layer, multi-pass welding.
Offset joints should be ≥30 mm.
3) Cover layer formation
Reduce current by 10%-15%.
Move the electrode in a straight line.
Control the excess height to 1-3 mm.
Stop at the edge to prevent undercutting.
Immediately wrap with asbestos cloth after welding for slow cooling.
Avoid rapid cooling, which can cause a hard and brittle structure. This is critical for round seamless pipe joints subjected to high stress.
v. Post-welding heat treatment: Optimizing seamless steel pipe joints
To eliminate residual stress and enhance joint toughness:
1) Stress-relief annealing
Temperature: 720–760°C.
Holding time: 1–2 hours (add 1 hour for every additional 25 mm of wall thickness).
Cool in the furnace to 300°C, then air cool.
2) Hardness verification
The hardness of the weld after heat treatment should be ≤250HB.
Areas exceeding the limit require local tempering treatment.
3) Important warning
Slow cooling is strictly prohibited within the temperature range of 370°C–550°C!
Prevent temper brittleness. This is a special requirement for alloy heat-resistant seamless steel pipes that distinguishes them from ordinary carbon steel seamless pipes.
vi. Quality Inspection: Ensuring the Safety of Seamless Steel Pipes
1) Non-Destructive Testing
100% radiographic testing (RT) combined with ultrasonic testing (UT).
Compliant with ASME BPVC IX standards. These standards are significantly higher than those for ordinary structural seamless pipes or welded steel pipes.
2) Mechanical Property Testing
Samples are subjected to high-temperature tensile testing (550°C).
Impact testing is conducted (KV2 ≥ 41 J).
3) Metallographic Analysis
Inspect the heat-affected zone (HAZ) for the absence of martensitic regions.
Confirm that the weld metal exhibits a uniform sorbitic microstructure. This serves as the microscopic assurance of welding quality for high pressure boiler pipe
