PWHT Requirements in Pressure Vessel Inspections

by Murali

What are PWHT requirements in pressure vessel inspections?

Bob's Reply: I prefer to answer to this question in two separate questions as below:

1. When and where PWHT is mandatory by code requirement?

2. How PWHT must be done when it is mandatory?

Pressure Vessel Inspections-Answer to question 1:

If we focus only in ASME Code Section 8 Div 1, there are lots of conditions which cause the PWHT to be mandatory. Some of them are:

a) Based the requirement of UW-2(a) when our service is lethal and our material is carbon steel or low alloy steel.

b) Based the requirement of UW-2(c) when our pressure vessel is unfired steam boiler and our material is carbon steel or low alloy steel.

c) Based the requirement of UW-2(d) when our pressure vessel is subjected to direct firing and our material is carbon steel or low alloy steel.

d) Based the requirement of UCS-56 when our nominal thickness exceeds a specific value, for instance 38 mm for P No.1 material or 0 mm for P No.3 material.

UCS-56 also gives min. PWHT temperature and min. holding time (soaking period) based on P-Nos. and thickness respectively.

e) Similarly to the UCS-56, the same apply for other martial groups i.e. UNF-56, UHA-32, UHT-56 and etc.

f) Based the requirement of UCS-68(b) when our MDMT (Minimum Design Metal Temperature) is below -55 Degree F.

Pressure Vessel Inspections-Answer to question 2:

When one of above requirement applies to us, then we must do PHWT based the requirement of UW-40 which detail to us the PHWT procedure.

Some important requirements in this clause are:

a) For Furnace PWHT, Loading Temperature shall not exceed 800°F, heating rate 400 deg F/hr/inch of thickness, cooling rate 500°F /hr/inch of thickness.

Still air cooling permitted below 800°F. During soaking period, temp difference between hottest and coldest part shall not exceed 150°F.

b) Minimum overlap for PWHT in multiple heats is 5 ft.

Comments for PWHT Requirements in Pressure Vessel Inspections

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Apr 25, 2016
ss NEW
by: Anonymous

you will change the microstructure

Aug 28, 2015
PWHT due to UCS-79(d) NEW
by: Anonymous

Is PWHT mandatory following the welding of a head to a vessel when the extreme fiber elongation of the head is more than 5% and the thickness is more than 5/8 in.?

Feb 03, 2014
PWHT for Lethal Service NEW
by: Essam Hassan

In case, PWHT is mandatory because of the lethal service.
What is the min holding time?
is it based on the thickness and then get the value from table UCS-56?
or it is 15 minutes for all cases?

Jan 08, 2014
by: Rafik


It's true that For the vessels that will undergo PWHT, The nozzle connection shall be as per Fig. UW-16.1 (f-1),f-2), f-3) & (f-4)

Thank you.


Oct 22, 2013
CTOD in-lieu to PWHT NEW
by: Anonymous

Can PWHT replace by fracture toughness test i.e CTOD?

Oct 05, 2013
pwht NEW
by: Anonymous

very good

Apr 27, 2013
hardness test after PWHT NEW
by: vubadien

Dear Sirs,
could anyone help you to explain below concern???

I've a concern about hardness test requirement after PWHT.
I am now inspect the vessel fabrication by ASME VIII div.2 with material SA-516 Gr70 thickness 60mm so after PWHT it is whether hardness test after PWHT or not and is it a scope of A.I?

Jan 31, 2013
PWHT temperature exceedance NEW
by: Welder1

Is there accepted by the ASME Code to use a higher PWHT temperature on the product compare with what you have qualified your QPR according to - example you WPQ is qualified with 580 deg.C, and the product is PWHT with 620 deg.C.
Is that accepted according to ASME Code?

Sep 26, 2012
PWHT holding time effect NEW
by: Anonymous

IF PWHT holding time increases the Tensile value will reduce,

To confirm the tensile applicability at the extended holding time, the check test performed,
Example The vessels which need in service PWHT apart from fabrication PWHT, check test (Simulation test) will conduct for mechanical properties with a Fabrication PWHT holding time plus service PWHT holding time

Aug 26, 2012


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