My reply was based solely on the ASME Section IX code requirements. I
agree with you on the fact that many factors need to be included if
(1) you are going to deviate from Code requirements, (2) have
experienced a cold cracking problem, or (3) are dealing with a
material with which you are unfamiliar. You failed to include in your
list the degree of restraint - more highly restrained joints require
greater preheat.
From my experience, the one common material (ASME Spec.) used in
building routine carbon steel presure vessels, SA-105, which has a
maximum specified carbon content of 0.35%, can easily exceed a carbon
equivalent of 0.43. Thankfully, cold cracking has become a very rare
occurrence, at least in our shops, because most of the filler metals/
processes have reasonably low diffusible hydrogen contents (SMAW,
FCAW, SAW).
Not to say that we don't have our share of welding problems, but cold
cracking is not one of them.
J
On Mar 27, 2:01 am, "Sayee Raghunathan" <Sayee.Raghunat...@twi.co.uk>
wrote:
> On a code basis, the explanation given by others limiting the decrease of preheat on WPS to 55Deg C below that of the PQR is correct, and I agree that the preheat shall be the minimum interpass temperature recorded for the FCAW process-55Deg C, purely from a perspective of application/interpretation of ASME Section IX code requirements.
>
> For a more comprehensive answer, the preheat is determined by the combined thickness, the carbon equivalent of the material(s), the level of diffusible hydrogen in the weld metal and the heat input or arc energy of the process. These factors either affect the cooling rate(s) or the hardenability of the material and the susceptibility to hydrogen induced cracking. Higher the hardness in the HAZ and higher the diffusible hydrogen content in the wm, greater is the susceptibility to cracking. The method of calculating the preheat temperatures is explained very clearly in EN 1011-2, which can be downloaded from the BSI website.
>
> Therefore in my personal opinion, I would be looking at which has the lower susceptibility to cracking, the FCAW process with potentially high weld metal hydrogen, 24Deg preheat or with 32Deg preheat and compare with the TIG process with diffusible hydrogen levels <5ml/100gm weld metal, 24Deg preheat and lower arc energy(?). In my personal opinion the change in preheat temperature will hardly be critical for welding C-Mn steels with less than 0.43CEQ (IIW) and thicknesses lower than typically 25mm. I mention this thickness and CEQ limit because from the level of preheat specified by you, I would assume this is the case.
>
> Regards
>
> Sayee
>
>
>
> -----Original Message-----
> From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of pieper-...@kpnmail.nl
> Sent: 26 March 2009 21:09
> To: Materials & Welding
> Subject: [MW:1812] Re: Pre heat requirement for split welding procedures from combined PQR.
>
> Dear J,
>
> Your explanation is correct regarding the problem with hydrogen cracks but you call them HIC cracks and in my opinion you only get typical HIC cracks (better known as stepwise cracking) when a sensitive material is exposed to a H2S containing gas. Cracks will start anywhere in the microstructure on dislocations and/or MnS inclusions because of hydrogen collecting at such location and building op internal pressure by which the crack will find his way through the microstructure by following the path with the lowest resistance.
>
> The typical cracks due to Hydrogen during welding mostly called cold cracking.
>
> Hope this information is useful.
>
> Best Regards,
>
> Herman Pieper
>
> On 26 mrt, 14:56, jhenn...@deltak.com wrote:
> > Your AI is correct. You may not use the 24C preheat recorded for the
> > SMAW process as a basis for FCAW. Each process is qualified
> > independently as far as essential variables is concerned. QW-200.2(b)
> > states, in part: "The completed PQR shall document all essential
> > variables of QW-250 through QW-280 for each welding process used
> > during the welding of the test coupon". QW-406.1 is an essential
> > variable for the FCAW process and therefore the minimum temperature
> > used during the welding of the FCAW pass(es) becomes the "preheat"
> > value used for the WPS. Of course, the WPS may have a preheat 100F or
> > 55C less than the lowest recorded temperature for the process per
> > QW-406.1.
>
> > Thickness of base metals, thickness of the deposited weld metal have
> > no bearing on this - you have two welding processes and the essential
> > variables shall be recorded for each seperate process and the WPS
> > shall be based on those values.
>
> > In your case, the WPS for the FCAW process (PQR minimum preheat
> > recorded 87C) may have a minimum preheat of 32C.
>
> > If you think about it this makes sense. Assume you were to use GTAW
> > for the root pass and FCAW for the fill and the material was carbon
> > steel with a carbon equivalent of, say, 0.55. This material would
> > have some sensitivity to hydrogen induced cracking. GTAW has very low
> > hydrogen potential, <H4, and would probably require no preheat above
> > ambient in order to prevent HIC. However, standard FCAW with a
> > hydrogen content of >H8 may require preheat to prevent HIC. Would you
> > want to assign no preheat to a WPS of FCAW only?
>
> > We will see what the other pundits have to say. Cheers.
>
> > J
>
> > On Mar 26, 5:49 am, Muhammed Ibrahim <ibrat...@gmail.com> wrote:
>
> > > Hi friends,
>
> > > I would like to clarify one important thing about preheat. My PQR is
> > > qualified with SMAW+FCAW. When I start the welding I preheated to 24
> > > degree C. After root and hot pass compleated with SMAW before
> > > starting FCAW the plate temperature is 87 degreeC. If I split this
> > > PQR to make a WPS only for FCAW, Is it required to preheat the plate
> > > to 87 degree C or can I give 24 degree celcius as preheated in the
> > > starting of PQR test coupon?. Your response will be highly appreciated.
>
> > > The AI insisting to put preheat of 87 degree shall be considered
> > > when we split the PQR for FCAW because when FCAW process starts the
> > > temperature of the plate to be considered as the preheat temperature (87 degreeC).
>
> > > Thanks & Regards
> > > Muhammed Ibrahim PK- Tekst uit oorspronkelijk bericht niet weergeven
> > > -
>
> > - Tekst uit oorspronkelijk bericht weergeven -
>
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