Page 31

Inspection Trends - January 2011 - Winter

Preheat and Postweld Heat Treatment Welding Procedure Specification (continued) Preheat is utilized when welding carbon steels and high-strength lowalloy steels to control the cooling rates and the microstructure of the completed weld. Sufficient preheat reduces the cooling rate of the weld, heat-affected zone (HAZ), and adjacent base metal to minimize or prevent the formation of hard brittle martensite. Preheat requirements are dependent on the carbon equivalency, base metal thickness, and level of joint restraint. The preheat requirements can be presented as a table in the WPS or the WPS can reference an annex where the preheat requirements can be addressed in more detail. Not all base metals respond to heat treatment in the same manner. Aluminum and austenitic stainless steels do not typically need to be preheated for purposes other than to remove any surface moisture. What works for one base metal does not necessarily work for all base metals. Postweld heat treatment (PWHT) is usually employed to reduce the residual stresses to a tolerable level. It can also be used to reduce the level of diffusible hydrogen in the weld and HAZ when welding steels sensitive to the detrimental effects of hydrogen. The PWHT that works for one family of base metals may not work with a different family of base metals. The level of detail required is specific to the type of heat treatment required. The WPS can provide the details required or it can refer the reader to a separate procedure that describes the PWHT requirements. The WPS must indicate whether PWHT is required or not. Entries such as “NA” or “NR” are not appropriate. The PQR must support the WPS. If the production weld is subject to PWHT, the supporting PQR must be subjected to the same PWHT. If the production weld is used in the “as- Inspection Trends / Winter 2011 35 Shielding Gas WPS: GTA/FCA-P1F6 Process: GTAW FCAW Layer: All All Shielding Gas: SG-A SG-AC-25% Specification: AWS A5.32 AWS A5.32 Flow Rate: 15 to 35 ft3/h 45 ft3/h Nozzle Diameter: #4 through #10 (1⁄4 through 5⁄8 in.) w/ gas lens 1⁄2 in. Root Purge: None None Purge Gas: None NA Specification: NA NA Flow Rate: NA NA Trailing Gas: None NA Trailing Gas Type: None NA Specification: NA NA Flow Rate: NA NA Electrical Characteristics Process: GTAW FCAW Layer: Root Intermediate and cover CV or CC: CC CV Current/Polarity: DCEN DCEP Voltage: 11–12 Refer to chart Amps: 160–175 Refer to chart Tungsten: EWTh-2 Gun: Profax 400A Diameter: 1⁄8 in. Cooling: Gas cooled Electrode Extension: 1⁄2 cup dia. 1⁄2 to 3⁄4 in. Pulsing: No No Preheat/Interpass Temp./PWHT Preheat Temp.: Annex A-1 Max. Interpass Temp.: 500°F PWHT: Required Max. Temp.: 1150°F Time @ Temp.: 1-hour plus 1-hour per in. Max Cooling Rate: 100°F/hour to 500°F then air cool


Inspection Trends - January 2011 - Winter
To see the actual publication please follow the link above