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In this article we shall see the clauses under which Section IX addresses the subject of post weld heat treatment, that is – PWHT in procedure qualification.
PWHT As Per ASME Section IX
Post Weld Heat Treatment is a significant feature of welding, and plays an important role in determining the properties of the weld. PWHT is addressed through QW 407 in Section IX. QW 407.1 and QW 407.4 are specified as essential variables, while QW 407.2 is specified as a supplementary essential variable, for most of the processes under QW 250.
For special processes (hard-facing and corrosion-resistant overlays), QW 407.6 and QW 407.9 are specified as essential variables for most processes.
QW 407.1 is an essential variable for almost all the processes under QW 250. This means that it remains in force at all times regardless of whether toughness is specified or not.
When toughness requirements are specified, QW 407.2 – a supplementary essential variable, also becomes applicable. QW 407.2 has been discussed in some detail in another article, which may be accessed here.
Let us see what QW 407.1 is all about. QW 407.1 says that a new procedure qualification is required for each of the following conditions:
(1) No PWHT.
(2) PWHT below the lower transformation temperature.
(3) PWHT above the upper transformation temperature (e.g., normalizing).
(4) PWHT above the upper transformation temperature followed by heat treatment below the lower transformation temperature (e.g., normalizing or quenching followed by tempering).
(5) PWHT between the upper and lower transformation temperatures.
For carbon steels, the lower transformation temperature is the one when austenite and ferrite structure finishes turning into ferrite and pearlite mixture while cooling (during heating, ferrite & pearlite just starts turning into austenite and ferrite mixture at this temperature).
This temperature is indicated by A1 line on the iron carbon diagram. The A1 line is a flat line at the 723° C temperature point, meaning that for carbon steels the lower transformation temperature falls at 723°C.
Likewise, for carbon steels, the upper transformation temperature is the one when pure austenite starts transforming into austenite and ferrite mixture while cooling (during heating, ferrite and austenite mixture complete their transformation and become pure austenite at this temperature).
This temperature is represented by A3 (for C < 0.8%) and Acm (for C > 0.8%) lines on the iron carbon diagram. The A3 line is an inclined line sloping downwards (as C% increases) falling between 723°C and 910°C. This means that the upper transformation temperature for carbon steels falls between 723° C and 910°C for steels with C < 0.8%.
The exact temperature needs to be determined for a particular composition by looking at the iron-carbon diagram. For a 0.4% carbon steel, it would be in the region of 800°C.
Let us say 723°C and 800°C are the lower transformation (LTT) and upper transformation temperatures (UTT) respectively of a certain steel. The QW 407.1 says that a separate procedure qualification is required if the PWHT is to be done below 723°C, a separate one if PWHT is to be done above 800°, a separate one if it is to be done between 723°C and 800°C, a separate one if it is to be done above 800°C followed by a temperature lower than 723°, and a separate one if there is no PWHT at all.
All the above blah blah is applicable when P-Nos. 1 through 6 and 9 through 15F base metals are involved. For all other base metals, the following conditions apply:
- No PWHT.
- PWHT in a specified temperature range.
This was all about QW 407.1.
Besides QW 407.1, QW 407.4 also is an essential variable pertaining to PWHT, and has to be reckoned in addition to QW 407.1. It becomes applicable when, for ferrous base metals other than P-No. 7, 8 and 45, the procedure qualification coupon receives a PWHT exceeding the upper transformation temperature, and for P-No. 10H base metals, the procedure qualification test coupon receives a solution heat treatment.
When this happens, the maximum qualified base metal thickness is limited to 1.1 times the thickness of test coupon (instead of 2T as permitted in QW 451).
Is It Required To Simulate PWHT Condition Of Production Component?
Another question that frequently dogged me in earlier days is that – does the heat treatment condition of the base material need to be simulated in procedure qualification?
For example, a component made up of base material ‘X’ encounters several rounds of heat treatment during fabrication. Subsequently, the component needs to be welded to another part of base material grade ‘X’ which is fresh (that is, has not undergone any heat treatment till then).
A PQR is to be qualified to support this welding. Do the base materials of grade ‘X’ selected for the PQR need to be given any heat treatment before welding of the coupon, to simulate job condition? Would the answer to this question differ if toughness requirements were a consideration?
The answer to the above question is that Section IX does not mention anywhere that heat treatment condition of the production joint needs to be simulated in the preparation of procedure qualification test coupon.
Hence, for the situation described above, two fresh base plates (or pipes, as applicable) may be chosen for the test coupon, without needing to have any heat treatment done on them prior to the welding. This is the understanding from Section IX.
However, when toughness testing is required – some Codes of construction may require use of base material with the same heat treatment condition. If the Code of construction requires it, that of course overrides the understanding of Section IX, and a heat treatment would be required to be done on the base material prior to doing any welding.
This understanding finds affirmation in the Interpretation IX-89-84 as well, which addresses almost the same question.
So this was about the subject of PWHT in procedure qualification in accordance with ASME Section IX. Would you like to add something? Please do leave your observations in the comments section below.