p91 Pwht. Aws
p91 Pwht. Aws
p91 Pwht. Aws
Heat Treatment of
P91 Steels
These advanced steels are quite different from and require
significantly more attention than traditional chrome-moly alloys
WILLIAM F. NEWELL IR. (wfnewell@pobox.comj is cofounder and vice president of engineering, Euroweld, Ltd., and president ofW.F.
Newell & Associates, Inc., which are both in Mooresville, N C. He is also vice chair oftheAWS DIO Committee on Piping and Tubing, and
a member of the ASME BPVIX and IIIIX, Subgroup on Strength of Weldments.
Based on a presentation made during the Chrome-Moly Steels Conference held Nov. 17 during the 2009 FABTECH International &
AWS Welding Show, Chicago, III.
WELDING JOURNAL
I I 16" 1.87" Thick
16" x 1.87"
. -^-
•
2-1/2 Hour Soak
D- Top
Btai IMmm
Fig. 1 — Temperature gradient in heavy-wall pipe PWHT without in- Fig. 2 — Temperature gradient in heavy-wall pipe PWHT with in-
creased heated band. Minimum ID temperature required to be 1350°F creased heated band. Entire ID is greater than I350°F (730°C) min-
(730°C) (Ref. 5). imum (Ref. 5).
APRIL 2010
range of 1350° to 1470oF (730° to 800oC).
The actual code of construction must be
consulted for specific rules (Refs. 1, 2).
Thermal Gradients
Recently, it has become apparent that
due to the thermal conductivity, especially
P(T)91 and 92, the heated and soak bands
need to be greater than those used (Ret 4)
on the traditional chrome-molybdenum
steels and as outlined in AWS D10.10. This
is especially true on heavy sections where
the thermal gradient between the outside
and inside of a component may be such that
the inside may not be seeing the required
PWHT temperatures. On a 2-in. (50-mm)
or thicker section, thermal gradients of 70°
to 200oF (21° to 930C) are not unusual.
Therefore, verifying procedures with
mock-ups or extensive monitoring of the
weldment during PWHT becomes criti-
cal. Just because the outside or surface
where the heaters are placed reaches tem-
perature, it cannot be assumed that the Fig. 3—Arrangement of preheating electrical resistance pads on 30- x 1-in. (760- x25-mm)
inside or surface opposite the heaters has pipe (Ref. 6).
reached temperature. Normally on heavy
or complex sections, the heated band must
be increased and the maximum possible high as 2.4 wt-%. Weld metal with higher conducted properly, but would exceed the
PWHT temperature used. This is illus- nickel content is used to satisfy some off- lower critical transformation temperature
trated in Fig. 1. The outside reached and shore toughness requirements. Such val- of the adjacent high Ni+Mn weld metal,
maintained the 1400oF (760oC) target ues will result in a lower critical transfor- thus potentially degrading its properties.
temperature, but the inside varied from mation temperature as low as 13350F It is being proposed that during manu-
1340° to 1360oF (727° to 7380C). The min- (7240F), which would be less than that re- facture of components (pipe, fittings, etc.),
imum permitted temperature was 1350oF quired by code or that necessary to tem- material specifications will limit the nickel
(730oC). When an extended heated and per "-B9" weld metal. The problem arises plus manganese to 1.0 wt-% maximum to
soak band is to be utilized, the required when the installer or repair organization avoid this issue. Similar consideration, ex-
temperatures can be achieved — Fig. 2. makes a weld near or repair of this weld cept a 1.5 wt-% maximum, is being given
metal and uses domestic (ASME) code to code rule revisions. In the interim, many
Soft Spots criteria for PWHT. The PWHT could be domestic owner-users are specifying a 1.0
[i fVmn
lustrate one arrangement for preheating 1200
and the resulting effect. These findings fur-
ther substantiate the need for additional 1300
monitoring thermocouples, not just the
minimum number recommended in AWS 1400
D10.10 or other sources.
1500
Weld Metal Composition —
New Issue, Fabrication vs.
Installation 1700
WELDING JOURNAL
wt-% maximum Ni+Mn content for weld it would be impossible to cover all situa- cial attention, or the low bidder got the
metal used on their components and dur- tions. Safety is the primary basis for our job. Sometimes problems arise from just
ing installation (Refs. 1-3). codes and standards. Attempting to dis- plain ignorance of the rules and technical
place blame for a problem or issue be- attributes required to successfully fabri-
ASME and AWS cause "it's not in the code" is technically cate and install CSEF steels.
indefensible. Contractually or legally it
Due to the increased attention CSEF may be true, but from an engineering Summary
steels require, the AWS D10 Committee standpoint, such a defense is unaccept-
removed P(T)91 materials from its exist- able. The problems and challenges sur- Base material development and code
ing guideline publication on welding rounding the CSEF steels cannot be re- acceptance have preceded effort and re-
chromium-molybdenum piping and tubing solved with code rules alone. There is no search in the areas of weldment proper-
(D10.8) and are preparing a new document substitute for sound engineering judg- ties and welding consumables for the
(D10.21, which is still pending) for ad- ment, mock-ups, and experience. This also CSEF steels. Although the base metals
vanced chrome-molys and CSEF steels. extends to material manufacturing and can offer superior properties, these bene-
Revision of D10.10 is also being consid- shop and project supervision for all as- fits can only be realized when the proper-
ered to address thermal gradient issues pects of the work. The best specification ties are maintained and not degraded dur-
being found between existing recommen- or procedure is relatively worthless unless ing manufacture, fabrication, or installa-
dations and CSEF issues. These documents it is followed. tion. From a welder's standpoint, the abil-
are pending consensus with efforts under- These facts should be obvious, but ity to weld the CSEF steels is rather
way to revise the appropriate sections and given the frequency and nature of contin- straightforward. For the CSEF steels,
rules of the ASME code. Efforts at both ued problems, the CSEF steels are not proper preheat, PWHT, and monitoring
ASME and AWS are an evolving process being given the respect and attention re- are not optional, they are mandatory.
and will continue to be so as more is learned quired. The CSEF steels are not like tra- Lessons learned with P(T)91 weld-
about these materials (Refs. 1^1). ditional chrome-molybdenum steels that ments have truly demonstrated that these
are very forgiving. Too often, the root advanced chromium-molybdenum (Cr-
cause of observed technical problems with Mo) steels are quite different and require
Supervision and CSEF steels has its genesis from a com- significantly more attention than the
Experience mercial issue, such as lack of training (no P(T)22 and more traditional chromium-
budget), inexperienced engineering or su- molybdenum alloys. Greater attention to
Codes are rarely prescriptive and only pervision (no budget), overall budget con- weld metal selection, preheat, and demon-
provide what is required, not how to do straints, unrealistic schedule, ignorance strated postweld heat treatment schedules
it. This is intentional and appropriate — of the fact that CSEF steels require spe- are some of the reasons that the CSEF al-
loys must be treated differently.
Competent, experienced engineering
and supervision are required to successfully
obtain the mechanical properties that can
References
APRIL 2010