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.