Agree 100% with John . I was involved in the fabrication of second
largest Vacuum Distillation Column / VD Column here in India (as of
2003 ).
regarding the PWHT {correct me if I am wrong }it is not SR on the
whole vessel. PWHT is to be done on the BAND of 300 mm near all the
HIGH THK Weld areas that are classified as PWHT required , and that
is completely ok . {our designer was GOOD , he envisaged the problem
during design & provided all the cleats inside the Vessel either above
/ below the Weld Seams by 200-350 mm either side that made our job
Easy to PWHT the Weld } but make sure that all the bolting is loose at
one end as there is a good thermal gradient in CS & SS that works
opposite to all our interests (will not be in the cladded area as such
it is with the outer shell Vs SS Internals - Thermal Expansion ) .
outer shell starts to bulge as CS and the loose end may not rest if
the cleat tolerances are on the smaller side. I have seen a 150 mm
extension on this particular bottom shell on all directions {this is
during initial fabrication so no problem of falling internals } during
PWHT & retract most of the length after cooling
If internals are packings then try to remove the peripheral packings .
If the internals are trays then
if chimney trays are already welded you may consider a CUT in between
& reweld if the whole equipment is being HT (as SS internals {assumed
SS internals} will not expand with reference to the "bulge" during the
HT of the CS equipment ).
Bolting bar can be left loose at one End . & all other trays are just
bolted to the Trays so I guess your contractor has to do a double work
as internals may be in a heap in the bottom of the shell after HT.
it will be the same for Crude Distillation Column too , for that
matter any Column / Vessel with internals.
L grade esp 316L is chosen as there is no enough carbon left for
sensitisation (formation of chromium carbides) ...
hope this helps
regards
Mayank
On 11/3/10, John Henning <jhenning@deltak.com> wrote:
> Note: this response is directly to the question and applies only to
> referenced L-grade material
>
>
>
> The effect on corrosion properties will be nil to minimal.
>
>
>
> The use of the Low carbon (L) material minimizes any tendency towards
> chrome-carbide (M23C6). For the L-grade austenitic stainless steels
> (3XXL) the carbon is kept below the solubility limit for carbon in solid
> solution. Therefore there is little driving force to cause the
> formation of Cr-carbides that result in stress corrosion issues due to
> Cr depletion associated with elevated temperature exposures. This is
> not to say that no Cr carbide may form along the grain boundaries,
> however if it does it will be discontinuous and will not cause corrosion
> problems. Please refer to the attached generalized phase diagrams.
>
>
>
> The effect on other material properties will be negligible. The
> temperature is insignificant with respect to inducing structural
> changes.
>
>
>
> There may be significant problems however with distortion of the members
> due to the difference in thermal expansion of austenitic alloys vs.
> carbon steel. However, bear in mind that the size of the vessel and
> length of the members play a major role in how much stress will be
> developed due to differential thermal expansion. This may be
> compensated for by leaving critical connections loose enough to allow
> expansion and then retightening after PWHT.
>
>
>
> Other than concerns with the latter point, there is no practical reason
> why the heat treatment could not be performed.
>
>
>
> John
>
>
>
> From: materials-welding@googlegroups.com
> [mailto:materials-welding@googlegroups.com] On Behalf Of asad azmi
> Sent: Wednesday, November 03, 2010 1:44 AM
> To: materials-welding@googlegroups.com
> Subject: [MW:8034] effect of PWHT Temperature at 600 DEG CELSISIUS ON SS
> 316L
>
>
>
> Dear All,
>
>
>
> Greetings.
>
>
>
> Contractor is doing PWHT of the Carbon Steel Vessel along with SS316L
> INTERNALS ( BEAMS, CHANNELS) Bolted with the Vessel. PWHT temperature
> is 600 DEG CELSIUS.
>
>
>
> Pls suggest me whether it is allowed. what will be the effect on
> mechanical proerties/corrosion ressiatnce proerties of SS 316L becuase
> of the PWHT Temperature.
>
>
>
> Thanks in advance for your valuable suggestions.
>
>
>
> Regrads
>
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