Welding a quench and tempered (Q&T) pipe requires careful consideration of the material properties and the impact of welding on those properties. The process must ensure that the welded joint maintains mechanical integrity without compromising the base material’s characteristics. Here are the key requirements and considerations:
1. Pre-Weld Requirements
  1. Material Properties Understanding:
    • Q&T pipes are designed for high strength and toughness. The welding process must avoid degrading these properties, particularly in the heat-affected zone (HAZ).
  2. Welding Procedure Qualification:
    • Develop and qualify a Welding Procedure Specification (WPS) per applicable codes (e.g., ASME Section IX, API 1104, or CSA Z662).
    • Perform procedure qualification tests to ensure that mechanical properties (e.g., yield strength, tensile strength, and toughness) meet the original design requirements.
  3. Preheat Temperature:
    • Preheating is required to reduce the risk of hydrogen-induced cracking (HIC) and control cooling rates.
    • Typical preheat temperatures for Q&T materials range from 150°C to 250°C but depend on the carbon equivalent (CECECE) of the material: CE=C+Mn6+Cr+Mo+V5+Ni+Cu15CE = C + \frac{Mn}{6} + \frac{Cr + Mo + V}{5} + \frac{Ni + Cu}{15}CE=C+6Mn​+5Cr+Mo+V​+15Ni+Cu​
  4. Low Hydrogen Welding Environment:
    • Use low-hydrogen electrodes (e.g., AWS E7018 or equivalent) or processes (e.g., GTAW or GMAW with low-hydrogen shielding gases).
    • Ensure electrodes are properly dried and stored per manufacturer recommendations.
2. Welding Process
  1. Heat Input Control:
    • Control heat input to prevent excessive grain growth or softening in the HAZ, which can compromise strength and toughness.
    • Use the formula to monitor heat input: Heat Input=Voltage (V)×Current (A)Travel Speed (mm/s)\text{Heat Input} = \frac{\text{Voltage (V)} \times \text{Current (A)}}{\text{Travel Speed (mm/s)}}Heat Input=Travel Speed (mm/s)Voltage (V)×Current (A)​
    • Target heat input is typically 1.0 – 2.5 kJ/mm, depending on material and joint design.
  2. Interpass Temperature:
    • Maintain interpass temperatures below a maximum limit (e.g., 250°C) to avoid tempering effects.
  3. Welding Consumables:
    • Select consumables with a tensile strength and toughness matching or exceeding the Q&T material properties.
  4. Weld Sequence:
    • Plan the sequence to minimize residual stresses. For example, symmetrical welding can help balance thermal stresses.
3. Post-Weld Requirements
  1. Post-Weld Heat Treatment (PWHT):
    • PWHT may be required to relieve residual stresses and restore toughness in the HAZ. However:
      • Excessive PWHT temperatures can temper the quenched and tempered microstructure, reducing strength.
      • Carefully follow recommended temperature ranges and durations (e.g., 550°C to 650°C) based on material specifications.
  2. Non-Destructive Testing (NDT):
    • Perform NDT to ensure weld quality, including:
      • Visual Inspection (VT): Assess surface quality.
      • Radiographic (RT) or Ultrasonic Testing (UT): Detect internal flaws.
      • Magnetic Particle Testing (MT): Identify surface and near-surface cracks.
      • Hardness Testing: Verify that the weld and HAZ hardness meet code and design requirements (e.g., < 350 HV for sour service).
  3. Mechanical Testing:
    • Conduct tensile, Charpy V-notch impact, and hardness tests to confirm mechanical properties in the weld, HAZ, and base metal.
4. Special Considerations
  1. Sour Service:
    • For pipelines exposed to H2S, ensure compliance with NACE MR0175/ISO 15156 requirements.
    • Limit hardness in the weld and HAZ to mitigate sulfide stress cracking (SSC).
  2. Weld Overmatching:
    • Use filler materials with a slightly higher strength than the base material to ensure joint integrity.
  3. Repair Welds:
    • Minimize repairs due to the risk of over-tempering the material. If repairs are necessary, follow strict WPS and PWHT guidelines.
  4. Environmental Factors:
    • Avoid welding in humid or wet conditions to minimize hydrogen pickup.
Summary
Welding Q&T pipes requires stringent control of heat input, preheat and interpass temperatures, and PWHT to maintain the pipe’s mechanical properties. Proper material testing, low-hydrogen consumables, and adherence to qualified procedures are essential for ensuring weld integrity.