I will limit myself to discussing ferritic materials {limited to QW-407.1(a)}. Ferritic materials undergoing PWHT typically experience a change in impact properties. The change that occurs with subcritical {below the A1} is a decrease in the upper shelf energy and an increase in the ductile-to-brittle transition temperature {DBTT}. Assuming that the PWHT temperature will be in the range specified by the design Code, Section IX primarily addresses the time at temperature. For a given temperature, the greater the time at the temperature the lower the upper shelf absorbed energy {USE} will be and the higher the DBTT will be.
The temperature range for ferritic materials is the generic range specified in QW-407.1(a) with, as discussed above, at least a minimum value assigned by the design Code. These ranges are the following:
(1) No PWHT – obvious,
(2) Subcritical PWHT, below the lower transformation temperature, ie. T < A1 - this is the predominant PWHT following welding. The change in USE and DBTT will be as above.
(3) PWHT above the upper transformation temperature or normalizing, ie. T > A3 – this PWHT will result in complete change in structure for the HAZ and the weld metal. Mechanical (tensile strength, yield strength, impact ) properties will have to be reestablished. Material test report for base metal is no longer valid. Tensile strength, yield strength, and impact properties for weld metal may be degraded due to lower carbon content in comparison to the base metal. Loss of the as “as cast” structure in the weld metal will affect mechanical properties as well. This is primarily applied to P1 (carbon steel) and P3 (carbon – 1/2Mo).
(4) PWHT above the upper transformation temperature followed by tempering, ie. TPWHT > A3 with subsequent TTemper < A1. Same as #3 above. This applied primarily to low alloy steels.
Because the temperature of PWHT (#2) is considered to be bounded by: minimum - Code required and maximum - A1, time becomes a critical factor. Since impact properties “degrade” with additional time, Section IX requires that the PQR have a PWHT that is at least close to the hold time in production or per QW-407.1 at least 80% of the production time at the PWHT temperature. Since the USE continues to decrease and the DBTT continues to shift to higher temperature, if the impact properties of the weld metal and HAZ are acceptable with a longer time, it is assumed that a production PWHT with a shorter PWHT time will have better impact properties (higher USE & lower DBTT) {than the PQR specimen with a longer time at the PWHT temperature}.
As a result, it is typical when performing PQRs for Section IX, that the test specimen(s) will be given long PWHT times; much longer than the anticipated production PWHT time. A longer PQR PWHT time provides the greatest flexibility for manufacturing, it provides for repair cycles if required, and allows the WPS to cover a greater thickness range.
Hope this helps. If you have additional questions, please post.
John
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of sujay kumar
Sent: Wednesday, January 19, 2011 12:57 AM
To: materials-welding@googlegroups.com
Subject: [MW:9322] QW-407.2
Dear Friends,
i need correct interperation of the Clause of mentioned below.
thanks and regards
QW-407.2
A change in the postweld heat treatment
(see QW-407.1) temperature and time range
The procedure qualification test shall be subjected to
PWHT essentially equivalent to that encountered in the
fabrication of production welds, including at least 80% of
the aggregate times at temperature(s). The PWHT total
time(s) at temperature(s) may be applied in one heating
cycle
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