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answered Feb 28, 2015 by TPham (5,010 points)

All equipment and piping in carbon and low alloy steel should be subject to a post weld heat treatment (PWHT). PWHT for piping should comply with the requirements of ASME B31.3 and NACE MR0175 / ISO 15156.

In order to avoid the PWHT requirement on a piping system, all the following requirements should be satisfied (this is only applicable to Carbon Steels):

  • Thickness should be below the limit for which the code requires PWHT.
  • Maximum carbon content should be 0.23%.
  • Maximum carbon equivalent should be 0.43%.
  • SMYS should be equal or less than 360 MPa (52 ksi).
  • The weld procedure should be qualified with a maximum allowable hardness in the survey of 248 HV10.
  • The material in the final fabricated form must meet the hardness requirements.
  • Piping must be seamless. Welded pipe can be considered on a case by case basis.

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