Why is post weld heat treatment done

Post Heating

In case of alloy steel materials such as Cr-Mo stels, if the post weld heat treatment is not performed immediately after welding, then the weld joint and adjacent portion of pipe, at least 50 mm on either side of pipe, shall be uniformly heated to 300°C. This temperature shall be maintained for half an hour minimum, and then wrapped with mineral wool before allowing it to cool to room temperature.

If the post heating temperature specified in the Welding Specification Chart exceeds 300°C, the same shall be followed. Similarly, if the Welding Specification Chart specifies post-heat time, the same shall be followed. Post weld heat treatment as specified in the Welding Specification Chart shall be carried out later on.

Post Weld Heat Treatment

  1. Post weld heat treatment, wherever required for joints between pipes, pipes and fittings, pipe body and supports shall be carried out as per the welding specification chart, applicable codes   standards and the instructions of the Engineer-in-Charge. In
    this regard procedure qualification to be done before carrying out PWHT in production welds.
  2. The Contractor shall submit for the approval of the Engineer-in-Charge, well before carrying out actual heat treatment, the details of the post weld heat treatment procedure , that he proposes to adopt for each of the materials/assembly/part involved.
  3. Post weld heat treatment shall be done in a furnace or by using an electric resistance or induction-heating equipment, as decided by the Engineer-in-Charge.
  4. While carrying out local post weld heat treatment, technique of application of heat must ensure uniform temperature attainment at all points of the portion being heat­ treated.
  5. Care shall be taken to ensure that width of heated band over which specified post weld heat treatment temperature attained is at least that specified in the relevant applicable standards/codes.
  6. Control of temperature shall be done using microprocessor / computer controlled system.
  7. The desired time-temperature cycle shall be entered into the microprocessor/computer.
  8. Throughout the cycle of heat treatment, the portion outside the heated  band shall be suitably wrapped under insulation so as to avoid any harmful temperature gradient at the exposed surface of pipe. For this purpose temperature at the exposed surface
    should not be allowed to exceed 50% ofthe peak temperature.
  9. The temperature attained by the portion under heat treatment shall be recorded by means of thermo couple pyrometers.
  10. Adequate number of thermo couples should be attached to the pipe directly at equally spaced location along the periphery of the pipe joint.
  11. The minimum number of thermocouples attached per joint shall be 1 up to 6″ dia., 2 up to 10″ dia. and 3 for 12″ dia. and above.
  12. However, the Engineer-in-Charge can increase the required number of thermocouples to be attached if found necessary.
  13. Automatic temperature recorders, which have been suitably calibrated, shall be employed for measuring & recording temperature.
  14. Both, the actual time-temperature graph and the designed time temperature graph shall be available on every chart.
  15. The time-temp graph shall be submitted to Engineer-in-Charge immediately on completion of Stress Relieving Cycle.
  16. The calibration record of each recorder should be submitted
    to the Engineer-in-Charge prior to starting the heat treatment operations and his approval should be obtained.
  17. Manufacturer’s test certificate shall be submitted for the thermo couples materials and record shall be maintained by the Contractor.
  18. Immediately on completion of the Heat Treatment, the Post Weld Heat Treatment charts/records along with the hardness test results on the weld points, wherever  required as per the Welding Specification Chart, shall be submitted to Engineer-in­  Charge for his approval.
  19. Each weld joint shall bear a unique identification number, which shall be maintained in the piping sketch to be prepared by the Contractor.
  20. The weld joint identification number should appear on the corresponding post weld heat treatment charts. The chart containing the identification numbers and piping sketch shall be submitted to the Engineer-in-Charge in suitable folders.

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