Pre Heat Maintenace

[jessecwi]

Is preheat maintenance an essestial variable when you are not exceding the maximum interpass temperature?

My GTAW P8 WPS states in QW-403 60 degree min preheat temp / 350 degree max interpass temp / preheat maintenance none.

we will be welding 347ss overlay on existing 347ss overlay to build up for machining RTJ flange grooves and are required to preheat at 250 degrees.

would this wps need to be requalified or is this acceptable?

 

[metengr]

Is preheat maintenance an essestial variable when you are not exceding the maximum interpass temperature?

If minimum preheat is specified you need to be at or above it and not exceed the interpass temperature.

You do not need a 250 deg F preheat for joining P-No 8 base materials.

 

[stanweld]

Assuming that this is a repair to corroded overlay, the flange is F22 alloy steel and the remaining overlay thickness is quite thin, a 250 F preheat may be warranted.

 

[gtaw]

The minimum interpass temperature is often defined as being no less than the minimum preheat temperature.

Typically preheat is necessary to reduce the potential of initiating cracks due to diffusible hydrogen that is introduced into the weld. Hydrogen cracking is typically associated most with carbon and high strength low alloy steels as well as martensitic and ferritic stainless steels where there is a potential for increased hardenability when the base metal is subjected to high cooling rates. The susceptibility to hydrogen assisted cracking is closely related to the carbon equivalency of the ferrous alloy that is being welded.

Maximum interpass temperature is usually specified when welding steel alloys that must meet notch toughness requirements. Slow cooling, while it promotes good ductility, can reduce notch toughness due to grain coarsening. The limitation on interpass temperature reduces the time that the HAZ is exposed to temperatures where gain coarsening can occur. Another situation where interpass temperature is limited is when welding austenitic stainless steels to reduce the potential for sensitization. One other example of when it is prudent to limit interpass temperatures is when welding precipitation hardenable base metals where the base metal can be overaged if it is cooled too slowly from the welding temperatures.


Best regards - Al

 

[metengr]

would this wps need to be requalified or is this acceptable?

The WPS is acceptable for this repair provided you are welding on P-No 8 base material. Preheat can always be increased without procedure re-qualification. It is the drop in preheat that is a concern.

 

[jessecwi]

Thanks for the info, also the only reason the preheat is required on this particular weld application is because the flanges are 2 1/4 chrome overlayed with 347,we are just building up the original 347 overlay prior to refacing.

 

[moseley]

According to QW-256 (Variable table in ASME IX), GTAW does not have any documented variable that apply. For instance with SMAW welding Preheat maintenance is a Non Essential Variable; and it only applies when PWHT is required:
Example:
SMAW NEV QW-406.2 (per QW-253):
"A change in the maintenace or reduction of preheat upon completions of welding PRIOR to any required postweld heat treatment".

 

[Kiwi2671]

Jesse,
IMHO the WPS does not need to be re-qualified but it does need to be revised.
QW 256.1 GTAW Corrosion Resistant Overlay lists QW 406.4 as an essential variable.
QW 406.4 states " The minimum temperature for welding shall be shown in the WPS"
Your WPS states minimum temp for welding of 60 degrees and yet production requirements are for 250 degrees minimum.
Regards,
Kiwi

 

[metengr]

You do not necessarily need to use a WPS qualified specifically for CRO to repair a CRO. This is a weld repair to an original weld overlay. In this case a groove weld WPS using correct base metals and applicable welding process can be used in lieu of .

Regarding production welding requirements, you can place a specific note on the WPS to require a higher preheat or use a job traveler. If the WPS was qualified at 60 deg F, this is the minimum preheat for the WPS.