How to Weld Carbon Steel Pipes?

Carbon steel pipes are widely used in oil and gas, chemical, power, building structures, and water pipe systems. Welding quality directly determines the strength, sealing, and service life of the carbon steel pipes and pipeline systems. Welding methods for carbon steel pipes include manual electric arc welding, submerged arc welding, and gas welding. The appropriate welding method needs to be determined based on the specific welding occasion and requirements, while also paying attention to welding quality and safety.

Preparation Work Before Welding

1. Surface Cleaning of Carbon Steel Pipes

First, use methods such as grinding and sandblasting to clean the surface of the pipe to remove oil, scale, and other impurities to ensure smooth welding.

2. Confirmation of Pipe Size and Position

Based on drawings or actual requirements, determine the pipe's diameter, wall thickness, length, and other parameters, and confirm the pipe's position and angle.

3. Inspection of Welding Equipment

Inspect the welding equipment to ensure it is functioning properly, including checking the power supply and grounding.

4. Preparation of Welding Materials

This includes preparing materials such as welding wire, electrodes, and shielding gas, ensuring that the quality of these materials meets standards.

Welding Tools for Carbon Steel Pipes

1. Welding Machine

Select a welding machine suitable for the pipe quality and use a normal power supply.

2. Welding Electrode or Welding Wire

Select appropriate welding materials based on parameters such as pipe diameter and wall thickness.

3. Gas Protection

Ensure that oxygen and moisture are isolated from the welding area, ensuring a sufficient protective atmosphere during welding.

4. Cutting Tools

Tools such as rulers, pliers, and sharpening stones should be carefully inspected before welding carbon steel pipes.

5. Safety Equipment

Select appropriate safety equipment for the environment, including gloves, goggles, and earplugs.

Common Welding Methods for Carbon Steel Pipes

1. Manual Arc Welding (SMAW)

Applicable Scenarios: Suitable for on-site operations or small-batch production, especially suitable for pipes with diameter ≤300mm and wall thickness 4-20mm.

Technical Parameters: Current range 80-200A (refer to AWS D1.1 standard), electrode selection E6010 (root pass) or E7018 (fill/cap pass).

Advantages and disadvantages: Simple equipment, low cost, but low efficiency, and requires high welder skills.

2. Tungsten Inert Gas Welding (GTAW/TIG)

Applicable scenarios: High-precision thin-walled pipes (wall thickness ≤ 6mm) or applications requiring no spatter, such as food-grade pipelines.

Technical parameters: Argon flow rate 8-15L/min, current 60-150A (DC positive polarity), welding wire ER70S-6.

Advantages: Beautiful weld, slag-free, but slow speed and higher cost.

3. Gas Metal Arc Welding (GMAW/MIG)

Applicable scenarios: Automated welding of medium-thick-walled pipes (wall thickness 5-50mm), such as long-distance pipelines.

Technical parameters: CO₂ or mixed gas protection, current 200-400A, voltage 22-32V.

Efficiency comparison: 3-5 times more efficient than manual welding.

4. Submerged Arc Welding (SAW)

Application Scenarios: Submerged arc welding is an automated electric arc welding method suitable for the production of large quantities of pipes.

Technical Parameters: Current 400~600A, Voltage 30~36V (Refer to AWS D1.1).

Efficiency Comparison: Automated electric arc welding uses an automatic feeder that automatically supplies the welding core and current, increasing welding speed several times and improving production efficiency.

Welding Electrodes for Carbon Steel Pipes

1. Commonly used welding electrodes for manual electric arc welding include AWS E6010 and AWS E7018.

AWS E6010 is a DC welding electrode suitable for DC operation, especially for welding carbon steel and low alloy steel in mildly corrosive or heavy alloy environments. AWS E7018 is suitable for DC or AC welding, has good crack resistance and corrosion resistance, and is commonly used for welding high-strength low alloy steel, high-hardness steel, and cooling deformation stress. It is also used in the manufacture of industrial equipment and large structures.

2. Commonly used welding electrodes for gas shielded welding include AWS ER70S-6 and AWS ER80S-D2.

AWS ER70S-6 electrodes are suitable for both carbon dioxide and inert gas shielded gases and can be used to weld all carbon products, low-alloy steels, and corrosion-resistant steels.

AWS ER80S-D2 electrodes are suitable for welding high-strength low-alloy steels such as low-temperature carbon steel pipes and can be used at low temperatures.

Carbon Steel Pipe Welding Process

1. Butt Joint

Align the two ends of the carbon steel pipe using tools such as clamps and positioners for precise welding.

2. Joint Cut

Cut the pipe joint into a V-shape using a cutting machine or blade, while removing all oil, scale, and other contaminants.

3. Weld Filling

Select welding materials suitable for the diameter and wall thickness, perform manual or semi-automatic welding, and fill the weld seam.

4. Welding Completion

After welding, inspection and cleaning are required, including cleaning the weld seam with anhydrous alcohol or organic solvents to check for any defects or gaps.

Technical Requirements for Carbon Steel Pipe Welding Process

1. Control of Welding Current and Voltage

During the welding of carbon steel pipes, it is necessary to strictly control the welding current and voltage to ensure the stability of the welding process and the quality of the weld. Excessive or insufficient current and voltage will lead to a decrease in weld quality and may even cause welding defects.

Manual Arc Welding (SMAW): Current range 80~200A (φ3.2mm electrode), voltage 22~28V.

Submerged Arc Welding (SAW): Current 400~600A, voltage 30~36V (refer to AWS D1.1).

2. Adjustment of Welding Speed

Welding speed is an important factor affecting weld quality. When welding carbon steel pipes, the welding speed should be reasonably adjusted according to factors such as material thickness, welding method, and welding conditions to ensure the uniformity and density of the weld.

Welding Speed: GTAW recommends 5-15 cm/min; excessively high speeds can easily lead to incomplete fusion defects.

3. Control of Welding Deformation

Carbon steel pipes are prone to deformation during welding. Therefore, a series of measures are needed to control welding deformation, such as using fixtures, preheating, and slow cooling. These measures help reduce stress and deformation during welding, improving weld quality and project safety.

Technical Requirements for Carbon Steel Pipe Welds

1. Weld Appearance Quality Requirements

Carbon steel pipe welds should have a smooth, uniform appearance, free from obvious defects such as cracks, slag inclusions, and porosity. The weld reinforcement and width should meet design requirements to ensure weld strength and sealing.

2. Internal Weld Quality Requirements

The internal weld should be free from defects such as incomplete fusion and incomplete penetration. The chemical composition and mechanical properties of the weld metal should match those of the base metal. To ensure the internal quality of the weld, non-destructive testing methods can be used for flaw detection.

Key Control Points in Welding Process

1. Pre-welding Preparation

Groove Design: V-groove angle 60°±5° (for wall thickness > 6mm), blunt edge 1-2mm.

Cleanliness Requirements: Oil and rust must be removed from both sides of the weld within 20mm to prevent porosity.

Preheating Requirements: When carbon equivalent (Ceq) ≥ 0.4% or wall thickness > 25mm, the preheating temperature must reach 100~200℃.

2. Welding Parameter Selection

3. Post-Weld Inspection

Visual Inspection: Weld reinforcement ≤3mm, undercut depth <0.5mm (GB 50236-2011).

Non-destructive Testing: X-ray inspection conforms to ASME B31.3; Class I welds allowable defects ≤2mm.

Hardness Testing: Heat-affected zone hardness must be ≤248HV (to avoid cold cracking).

Pressure Testing: Hydrostatic test pressure is 1.5 times the design pressure, holding time ≥10 minutes (ASME B31.1).

Why Do Carbon Steel Pipes Fail to Weld Properly?

There are generally three main reasons why carbon steel pipes fail to weld properly:

1. Surface Oxide Layer

Rust and oil on the pipe surface act as a barrier, hindering metal fusion.

2. Temperature Imbalance

Excessive current leads to burn-through, while insufficient current fails to penetrate.

3. Improper Operation

Deviations in the welding torch angle or unstable movement speed will affect weld formation.

4. Welding Quality Requirements

When selecting welding rods, the post-weld quality requirements must be considered, such as better vibration damping or higher corrosion resistance.

FAQ

1. Does carbon steel pipe welding require preheating?

Not always, but preheating is mandatory in the following situations:

Wall thickness > 25mm

Carbon equivalent Ceq ≥ 0.40%

Low-temperature working environment

Recommended preheating temperature: 100–200℃, which effectively prevents cold cracking and post-weld cracking.

2. Why do carbon steel pipes crack after welding?

Common causes include: using high-hydrogen welding rods that were not dried; insufficient preheating of thick-walled or high-carbon steel pipes; excessively rapid post-weld cooling; and excessive residual welding stress.

Solutions: Use low-hydrogen welding rods (E7018), control the cooling rate, and perform post-weld heat treatment (PWHT) if necessary.

3. What international standards are commonly used for carbon steel pipe welding?

Common authoritative standards include:

ASME B31.1 / B31.3 (Pressure piping)

AWS D1.1 (Welding of structural steel)

ISO 15614 / ISO 9606

EN 1011 (European Welding Standard) Export projects typically require WPS / PQR / WPQ documentation.

Summary

In conclusion, welding carbon steel pipes is a complex and meticulous process that requires welders with professional skills and experience. By selecting appropriate welding processes, conducting thorough pre-weld preparation, and performing post-weld quality inspections, it is possible to ensure that the weld quality of carbon steel pipes meets standard requirements, thereby improving the overall quality and service life of the project.