Key stages of welding pipe fittings

The key stages of welding pipe fittings involve strict operating procedures and quality control, which directly affect the welding strength, sealing and service life. The following are the main steps and technical points of welding pipe fittings:

I. Preparation stage before welding

1. Material inspection and matching

Check whether the pipe material (carbon steel, stainless steel, alloy steel, etc.), specifications, and wall thickness meet the design requirements.

Ensure that the welding materials (welding rods, welding wires, shielding gases) match the parent material to avoid weld cracks or corrosion due to composition differences.

2. Bevel processing and cleaning

Process the bevel (V-type, U-type, etc.) according to the process requirements, control the bevel angle (usually 30°~60°) and blunt edge thickness (1~3mm).

Thoroughly remove oil, rust, and moisture within 20mm of the bevel and both sides to prevent defects such as pores and slag inclusions.

3. Assembly and fixation

Ensure that the gap between the pipe fittings is uniform (usually 2~4mm), and the misalignment is ≤10% of the wall thickness.

Use a clamp or spot welding to fix, the spot welding length is 5~10mm, and the spacing is 100~300mm to avoid stress concentration caused by forced assembly.

2. Welding implementation stage

1. Preheating (if necessary)

For thick-walled pipes (≥20mm) or easy-to-harden materials such as high carbon steel and alloy steel, preheat to 100~300℃ (depending on the material) to reduce the risk of cold cracking.

2. Welding parameter control

Current/voltage: Adjust according to the diameter of the welding material and the wall thickness of the pipe to avoid excessive current causing burn-through or too small current causing unfusion.

Welding speed: Keep uniform, too fast is prone to incomplete penetration, too slow is prone to overheating and deformation.

Interlayer temperature: Control the layer temperature ≤230℃ (carbon steel) or the process requirement value during multi-layer welding to prevent intergranular corrosion.

3. Key points of welding operation

Base welding: Use small current and short arc welding to ensure root penetration and avoid concave or weld nodules.

Filling welding: fill layer by layer, clean the slag in each weld to avoid slag inclusion.

Cover welding: control the weld excess height (0~3mm) to ensure a smooth transition to the parent material.

4. Selection of welding method

TIG welding: suitable for thin-walled pipes, stainless steel/non-ferrous metals, and high weld purity.

MIG/MAG welding: high efficiency, suitable for batch operation of medium and thick wall pipes.

Manual arc welding (SMAW): strong flexibility, suitable for complex locations and field construction.

III. Post-weld processing stage

1. Weld cleaning

Remove spatter and welding slag, grind the weld surface to smoothness, and avoid stress concentration.

2. Post-heating and heat treatment

Heat the crack-prone materials to 200~350℃ immediately after welding, keep warm and cool slowly to release residual stress.

Important pipelines (such as high-pressure pipelines) need to be stress-relieved annealed or normalized.

3. Quality Inspection

Visual Inspection (VT): Check for surface cracks, undercuts, pores and other defects.

Nondestructive Testing (NDT):

Radiographic Testing (RT): Detect internal pores and lack of fusion.

Ultrasonic Testing (UT): Find deep defects.

Penetrant Testing (PT): Check for surface opening defects.

Pressure Test: Water pressure or air pressure test to verify sealing.

4. Corrosion Protection

Sandblast, paint or plate the weld area to prevent environmental corrosion.

IV. Key Notes

Safety Protection: Wear welding mask and fireproof clothing to ensure good ventilation to avoid electric shock and smoke poisoning.

Environmental Control: Heating and dehumidification measures are required when humidity is >90% or temperature is <-5℃.

Defect Repair: The same part is repaired ≤2 times, and the defects must be completely removed and then re-welded according to the original process.

Summary

The core of welding pipe fittings lies in "careful preparation at the beginning and strict preparation at the end": the preliminary preparation determines 50% of the quality, the welding process requires precise control of parameters, and the post-welding inspection ensures that there is no mistake. Mastering these key stages can greatly improve the safety and reliability of the pipeline system.