Maximum temperature for PWHT

[zirsald]

Dear members,
Although some codes specify minimum and maximum holding temperature for PWHT (ASME B31.1, table 132, holding temperature range), ASME VIII-1, UCS-56 establishes only minimum holding temperature. Could you please let me know what the maximum temperature of PWHT shall be as per ASME VIII-1 for materials with P-number: 1.
Thanks

[wefel]

The temperatures that are specified in ASME VIII-1, table UCS-56 could be lower provided that you increase the holding time as per table UCS-56.1. Regarding maximum PWHT temperature, it shall not exceed the temperature in which the steel starts to turn to austenite (lower critical temperature, please have a look at the attached file) that for steels with P-number: 1 is approximately 725 deg C (ASME B31.1, table 129.3.2). In case that you perform PWHT above this temperature the steel microstructure may change.

For quenching and tempering steels (quenching: the steel is heated above the upper critical temperature and then quickly cooled in order to produce martensite transformation and thus a harder metal. Tempering: heating the quenced steel below the lower critical temperature to improve toughness since the quenching process has produced a brittle crystalline structure) the PWHT temperature shall be kept 30-50 deg C below tempering temperature (usually not more than 600 deg C) so as to maintain quench-temper properties.

Nevertheless, in some specific cases PWHT is required to be performed above the lower critical temperature. In such cases you shall be aware of the microstructure changes of steel and investigate the results (effect on tensile strength, yield strength, hardness, ductility, etc) as well as a separate PQR is required (ASME IX, QW-407.1).

[Logosweld]

The temperatures that are specified in ASME VIII-1, table UCS-56 could be lower provided that you increase the holding time as per table UCS-56.1. Regarding maximum PWHT temperature, it shall not exceed the temperature in which the steel starts to turn to austenite (lower critical temperature, please have a look at the attached file) that for steels with P-number: 1 is approximately 725 deg C (ASME B31.1, table 129.3.2). In case that you perform PWHT above this temperature the steel microstructure may change.

For quenching and tempering steels (quenching: the steel is heated above the upper critical temperature and then quickly cooled in order to produce martensite transformation and thus a harder metal. Tempering: heating the quenced steel below the lower critical temperature to improve toughness since the quenching process has produced a brittle crystalline structure) the PWHT temperature shall be kept 30-50 deg C below tempering temperature (usually not more than 600 deg C) so as to maintain quench-temper properties.

Nevertheless, in some specific cases PWHT is required to be performed above the lower critical temperature. In such cases you shall be aware of the microstructure changes of steel and investigate the results (effect on tensile strength, yield strength, hardness, ductility, etc) as well as a separate PQR is required (ASME IX, QW-407.1).

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