20889/05-3/12
January 2012
Review of Current Practice for Welding
of Grade 92
For: Valid
Contents
1 Introduction
1 2 Welding procedure
1 2.1 Welding processes
1 2.2 Filler metals
2 2.3 Gas purging
2 3 Heating Cycle
4 3.1 Introduction
4 3.2 Preheat/interpass
4 3.3 Interruption of the heating cycle
5 3.4 Postheating (hydrogen bake)
5 3.5 Cooling after welding
6 3.6 3.6.1 3.6.2 PWHT
PWHT performance and parameters
Localised PWHT
6 6 6 3.7 Heating methods
7 4 Acknowledgements
7 Table 1 Commercially available filler wires for Grade 92 (Note 1) ................................................... 3 Table 3 Welding parameters for Grade 92, from various industrial applications(4) ........................ 9 Figure 1 Typical heating cycle for P(T)92 welds ................................................................................ 4 20889/05-3/12
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1
Introduction
The American designation T/P92 used for tubes and pipe, respectively, indicates a creep
enhanced ferritic steel (CSEF) that was originally developed by Nippon Steel under the
name NF616 as an alternative to Grade 91. This grade is also referred to as ‘Grade 92’,
and is available under the European designation X10CrWMoVNb9-2 (include in EN 102162) as well as American designations such as ASTM A213 T92, A335 P92, A387 Gr92, A182
F92 and A369 FP92.
In general, welding of P92 and similar CSEF grades is considered rather straightforward,
provided that the correct welding procedures, filler metal and heating cycles are applied by
welder with suitable skills.
A review of published data on requirements to be considered when developing welding
procedures for Grade 92 is presented in Sections 2 and 3 and summarised in Table 2. With
regard to filler metals, commercially available matching grades are tabulated in Table 1. In
addition, welding parameter ranges, extracted from various welding procedure
specifications (WPSs) applied in industry for Grade 92 and similar, are summarised in
Table 3.
2
Welding procedure
2.1
Welding processes
The most common welding processes for P92 fabrication are GTAW (TIG) and its narrowgap and hot wire variations, SMAW (MMA), FCAW and SAW. Note: the American
acronyms will be used throughout this document when referring to welding processes.
GTAW typically exhibits higher toughness than weld metal deposited using flux and slag
systems. EPRI1 recommend that the rod diameter is restricted to 3.2mm (1/8in) for manual
GTAW. The FCAW deposition rate is higher than all other arc welding processes (except
SAW), particularly for welding in position. The FCAW process also has considerable
advantages in terms of productivity; in some applications the time saving can be as much
as 40% compared to SMAW2. To achieve these benefits, it has been reported3 that a rutilebased flux system is necessary, which combines excellent operability with the all-positional
capability necessary for fixed pipework. FCAW is expected to replace SMAW, whereas
GTAW will still be required for pipe root runs or small diameter/thin tubes and SAW will be
the preferred option for thick-walled components. Narrow-gap (NG) GTAW is also finding
increased use for thick sections when quality and productivity are paramount.
With regard to the welding technique, toughness can be increased by depositing thinner
layers due to the tempering effect of subsequent layers.
The abovementioned welding processes are generally applied as follows4:
1. Original fabrication:




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GTAW only: mostly for “thin” wall small bore tubes eg 4in diameter, 1/4in thickness
(manual or automatic)
GTAW and SMAW: for root runs and fill+capping runs in ‘thick section’ pipe joints,
respectively
GTAW and FCAW: for root runs and fill+capping runs in ‘thick section’ pipe joints,
respectively
GTAW+SAW or GTAW+SMAW+SAW for ‘thick’ components (1.5in and greater) welded
in workshops
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2. Weld repairs:


SMAW or GTAW depending on component, excavation size and original WPS,
following excavations of the welding defects
SMAW and FCAW, following excavation of the welding defects, where SMAW is used
to deposit buttering runs to temper the heat affected zone (HAZ) and FCAW is used for
filling and capping runs
Note: The list is based on a survey of industrial applications, however, it cannot be
considered exhaustive.
2.2
Filler metals
The increase in steam temperature and pressure with the use of relatively new steels such
as P92 also means higher risk of weldment failure. Therefore proof is required of sufficient
creep rupture properties in the filler metals. Furthermore, sufficient toughness properties at
room temperature are also required to reduce heating costs during pressure tests5. The
presence of alloying elements such as C, Nb, N and W, which are necessary to guarantee
the creep rupture strength but have an adverse effect on toughness, is partially
compensated for by the presence of Ni.
However, similarly to P91-matching filler metals, to avoid transformation of the weld metal
during PWHT, the combined Ni+Mn limit is limited to 1.5%, as both elements decrease the
lower transformation point (Ac1). This reduces the risk of forming austenite at the PWHT
temperature that subsequently transforms to untempered martensite on cooling.‘6.
Matching P92 consumables have been developed by a number of vendors. In many
instances the composition has been based on that of the parent metal, with modifications to
improve toughness3,7 (Table 1).
When procuring Grade 92 filler metals, EPRI1 recommendations are endorsed: weld metal
with low residual elements (X factor < 15), using a -15 or -16 coating instead of a -18
(ASME B&PV II-C classifications) and a Mn/S ratio greater than 50 are recommended. The
maximum diffusible hydrogen content corresponding to classification ‘H4’ for SMAW
electrodes and ‘H5’ for FCAW and SAW (flux) is required to prevent cold cracking.
Chemical and mechanical testing on a ‘per lot per size’ basis for SMAW and FCAW
electrodes and wire is also recommended.
2.3
Gas purging
With regard to back purging, AWS D10.88 identifies the following conditions for Cr-Mo
creep-resistant grades (see also footnote 1 at the bottom of this page):



Cr <4%: no back-purging.
Cr 4-6%: back purging may be required according to the service conditions.
Cr >6%: back purging is always required.
Therefore, P92 requires back purging to prevent oxidation of the root pass. Back purging is
normally carried out with commercially pure Argon and maintained until at least three
passes have been deposited. Purging equipment able to withstand temperatures up to the
required preheat level (Table 2) are commercially available9.
1
AWS D10.8: the members of the American Welding Society’s D10 Committee on Piping and Tubing decided to remove
P(T)91 materials from their existing guideline publication on welding CrMo piping and tubing (AWS D10.8) and prepare a new
document for it (AWS D10.21, ‘Guideline for Welding Advanced Chromium-Molybdenum Steel Piping and Tubing’) and the
other advanced Cr-Mo grades such as P(T)92, P911, P92, P122, T23, etc. A draft version of this document was undergoing
approval at the time of writing.
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Table 1 Commercially available filler wires for Grade 92 (Note 1)
Welding process
SMAW
Manufacturer
Oerlikon (A-L)
Trade name
CROMOCORD 92
Metrode
Kobelco
Chromet 92
CR-12S
Specification
AWS A5.5: E 9018-G
EN 3580-A: E Z CrMoWVNb 9 0.5 2 B 4 2 H5
None
None
EN ISO 3580-A E ZCrMoWVNb 9 0.5 2 B 4 2
H5
EN 1599 Class E ZCrMoWVNb 9 0.5 2 B 4 2 H5
AWS A5.5 Class E9015-B9 (mod.);E9015-G
EN ISO 3580-A E ZCrMoWVNb 9 0.5 2 B 4 2
H5
AWS A5.5 Class E9015-B9 (mod.);E9015-G
None
Not available
Bohler
P92-1% Co
P92-0.5% Ni
Fox P 92
Nippon Steel
Welding
Nittetsu N-616
Not available
Metrode
Bohler
9CrWV
P92-IG
Thermanit MTS 616
A-L
Bohler
Carborod WF92
Thermanit MTS 616
FCAW
Metrode
Bohler
Supercore F92
P92 Ti-FD
None
EN 21952-A WZ CrMoWVNb 9 0.5 1.5
EN ISO 21952-A WZ CrMoWVNb 9 0.5 1.5
AWS A5.28 ER90S-B9 (mod.); ER90S-G
EN 21952-A WZ CrMoWVNb 9 0.5 1.5
EN 21952-A GZCrMoWVNb 9 0.5 1.5
AWS A5.28 ER90S-B9 (mod.); ER90S-G
None
E 91 T1-GM
SAW+Flux
Oerlikon (A-L)
OE-Cromo SF92 +
OP F500
9CrWV + LA491
Bohler P 92-UP+BB910
Thermanit MTS 616
GTAW
GMAW
Metrode
Bohler
Kobelco
Nippon Steel
Welding
Thermanit MTS 616 /
Marathon 543
PF-200S/US-12CRS
PF-200SD/US-12CRSD
Nittetsu Y-616 +
Nittetsu NB-616
None
None
EN 24598A S ZCrMoWVNb9 0.5 1.5/SA FB 2
AWS A5.23 Class EB9 (mod.)
EN 10270 S ZCrMoWVNb 9 0.5 1.5
AWS A5.23 Class EG [B9 (mod.)]
Not available
Note:
(1)
Data collected from various sources10,11,12,13,14,15. The list is not to be considered exhaustive.
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3
Heating Cycle
3.1
Introduction
Figure 1 shows a schematic representation of a typical heating cycle. Note: postheating
may be required after cooling if PWHT cannot be immediately applied, see Section 3.4.
Details on the individual components of the heating cycle are provided in the following
sections.
Preheat
PWHT
760°C
Interpass
Heating rate
100-150°C/h
200°C
min
Cooling rate
150°C/h
300°C
max
Below 400°C cool in
still air
Cool to 80-100°C
RT
Figure 1 Typical heating cycle for P(T)92 welds16
3.2
Preheat/interpass
The available literature and the WPS’s analysed for this review generally agree on a
minimum preheat temperature of 200˚C and a maximum interpass temperature of 300˚C
(some sources recommend 250°C and others 350°, for preheat and interpass,
respectively). EPRI1 also reported that fabricators will go as low as 121°C (250°F) for root
and hot pass layers in thin walled components or when GTAW (TIG) is used. This is
confirmed by Metrode17, who justify the lower preheat temperature with the low level
hydrogen input associated with this welding process. Note that preheat temperatures as low
as 50°C have been used successfully when welding GTAW root passes in P/T91 steels and
it may be possible, as more experience is gained, to reduce the preheat temperature
employed when welding the P/T92 steels.
The reason for indicating a preheat temperature of 200°C is to avoid the completion of
austenite/martensite reaction (the ‘martensite finish’ temperature is approximately 120°C for
P921) hence preventing hydrogen cracks. By limiting the interpass temperature to 300°C,
each pass provides at least a partial martensitic transformation and such martensitic
fraction can be tempered by the subsequent passes.
Therefore, a preheat-interpass range of 200-300˚C represents a balance between allowing
some transformation to martensite and maintaining an acceptable resistance to hydrogen
cracking by the weldment containing some untransformed austenite.
Preheating is best performed using electrical resistance heating elements since the
temperature is easily and accurately controlled by attaching thermocouples to the
component or by the use of optical pyrometers. Radiant gas fired heaters may also be used
with motorised valves to control the gas flow. Direct flame impingement should not be
permitted.
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3.3
Interruption of the heating cycle
Interruption of the welding cycle should be avoided if at all possible, particularly when
welding thick components where the weld area may be subjected to high residual stresses.
If the welding is terminated when the joint is only partially complete there may be
insufficient cross sectional area available to carry any stresses from shrinkage, self weight
etc. Therefore, care should be taken to avoid mechanical and thermal shock until
components have been subjected to PWHT.
In addition the contour of the partially completed weld may be such that there are some
severe stress raisers present. Cracking is therefore a significant risk. If interruption is
unavoidable, at least one fourth of the wall thickness should be deposited and preheat must
be maintained until the groove is completed or a postheating (Section 3.4) implemented.
Hot grinding of the weld may also be used to remove any sharp changes in section and to
provide a smooth contour within the weld preparation. It is also recommended to perform
visual inspection and if possible MPI at the point of interruption and prior to start of further
welding. Cracks or visible defects should be removed prior to further welding.
The AWS welding handbook18 recommends that when components are thicker than
25.4mm (1inch) or the chromium content is greater than 4%, the preheat temperature shall
be maintained throughout the welding operation, even when welding is interrupted. Cooling
to room temperature shall be avoided before the weld is completed. Where a loss of power
means that the preheat cannot be maintained then the joint should be lagged and the joint
cooled as slowly as possible. To prevent a total loss of preheat on very thick partially
completed components a back-up system may be kept on stand-by – for example gas
heaters should there be an interruption to the electricity supply.
Non-destructive examination, MPI as a minimum, may be necessary before restarting
welding to ensure that the component has not developed any cracks. If power is available
then an intermediate PWHT could be considered for the more highly alloyed steels such as
9Cr and 12Cr, volumetric NDE and MPI would be advisable prior to recommencing welding
if this is done. A full PWHT to comply with the specification requirements should be carried
out immediately after completion of the weld (Section 3.6). The length of time within the
specified PWHT temperature range will need to be compared with that applied during
welding procedure qualification.
Stress corrosion cracking (SCC) of as-welded P91 (hence also P92) components has been
encountered when the item has been left in the workshop for a period of time in the aswelded condition, particularly in moist atmosphere. Specific data on times of exposure and
environments are not available in literature, however, components are sometimes left in the
as-welded conditions for weeks, months or even up to a year19. Such delay may occur
when there are a number of welds to complete on a component or when the fabricator is
waiting for a convenient number of components to be welded, before carrying out the
PWHT. Should there be a significant delay between weld completion and PWHT, it would
be advisable to keep the components sufficiently hot to prevent condensation, although this
would prevent X-ray inspection before PWHT. Alternatively, by ensuring a dry environment,
one can still mitigate the risk of SCC whilst allowing X-ray inspection before PWHT.
3.4
Postheating (hydrogen bake)
As shown in Figure 1, PWHT should be carried out directly after cooling to below 100°C. If
PWHT cannot be immediately carried out, postheating is recommended. By maintaining the
preheat temperature for some 4 hours prior to reducing it to ambient temperature, hydrogen
is allowed to diffuse out, decreasing the risk of hydrogen-cracking.
However, compared to the earlier higher carbon alloys, some authors do not considered
postheating to be necessary with P92 (and P91) and claim that welds less than 50mm
(2inch) thick can be cooled slowly to ambient temperature without problems10. In addition, if
the preheat temperature is above the martensite finish temperature, see 3.4 below, then
there will be austenite present and this will reduce the benefits of a hydrogen release
treatment.
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Note: when postheating is carried out, it will mitigate but not eliminate the risk of stress
corrosion cracking, therefore, PWHT shall be carried out as soon as possible and careful
handling is required to keep the components dry and to avoid excessive loading (see 3.3).
3.5
Cooling after welding
To achieve postweld tempering of the martensitic structure, it is important that the weld joint
cools below martensitic temperature before heat treatment. The martensitic finish (Mf)
temperature of P92 is approximately 120°C1. Therefore, it is recommended to cool below
100˚C to allow complete transformation of the weld metal and heat affected zone.
3.6
PWHT
3.6.1
PWHT performance and parameters
An average temperature of 760°C (750-770˚C range) represents a good compromise
between improving toughness and creep strength and ensures a limited holding time (for
lower temperature ranges, the holding time necessary to achieve the required toughness
would be too long). However, it is paramount that the actual composition of the weld metal
and parent metal is known, to ensure that the lower critical transformation temperature is
not exceeded.
The holding time depends on the selected temperature range, however, it may also be
subject to commercial pressure, requiring the shortest possible holding time (hence a
higher temperature would be required). When developing consumables for P92, Metrode10
obtained satisfactory toughness levels at room temperature by stress-relieving at 760°C for
2 hours (GTAW), 2-4 hours (SMAW) and 4 hours (FCAW) irrespective of thickness.
3.6.2
Localised PWHT
Local PWHT is performed when it is not practical to heat treat a complete welded structure
(eg final closure weld on a piping system) or when weld repairs are carried out (particularly
in service, during outages etc).
When performing local PWHT it is very important to determine an adequate soak band
(SB), heated band (HB) and gradient control band (GCB). These will typically encompass a
360° area around cylindrical components. This band should also include any attachments,
nozzles, trunnions etc.

The soak band is the region that will be heated uniformly to the required post-weld heat
treated temperature.

The heated band consists of the soak band, plus a length of adjacent base metal
necessary to control the temperature and limit induced stresses due to temperature
gradients.

The gradient control band consists of the surface area over which insulation and/or
supplementary heat sources are applied.
The size of these areas will be determined by the applicable code or standard. Examples of
codes and standards that deal with local PWHT:

ASME VIII for Pressure vessels fabrication20.

AWS D10.10/D10.10M Recommended Practices for Local Heating of Welds in Piping
and Tubing21.

National Board of Inspectors (NBIC) code for Pressure vessels in-service22.
It has recently emerged23 that for Grades 91 and 92, the heated and soak band required by
standards such as AWS D10.10 may not be adequate, particularly on heavy sections.
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Therefore, it is recommended that local PWHT procedures are verified with mock-ups or
extensive monitoring of the weldment during PWHT, by means of thermocouples.
3.7
Heating methods
The use of inappropriate heating methods, hence the incorrect application of the heating
cycle, particularly of the PWHT, is the major source of problems with welds between
creep-resistant grades. In addition, potential inconsistencies between the heating methods
used in qualifications and those applied in production, in the field more often than in
workshops, must be taken into account.
EPRI1 have reviewed the most common methods used for preheating, post heating and
PWHT in light of their application to high Cr grades, with the following conclusions:
Flame heating: Non-suitable, the use of flame preheating should not be permitted, due to
the risk of localised overheating.
Furnace heating: Suitable, provided thermal gradients within the furnace are controlled.
Resistance heating: Suitable, typically applied for localised preheating or PWHT and for
field operations. Cares should be taken when establishing the extension of the heated
bands (Section 3.6.2) and when placing thermocouples.
Induction heating: Non-suitable, this technology has limitations due to the Curie point on
heavy-wall (beyond 2in) CSEFs.
4
Acknowledgements
ʹVerified Approaches to Life Management & Improved Design of High Temperature Steels
for Advanced Steam Plants – VALIDʹ is a collaboration between the following organisations:
TWI Ltd, Air Liquide UK Ltd, Centrica Energy plc, Doosan Power Systems Ltd, E.ON New
Build & Technology Ltd, Metrode Products Ltd, SSE plc and Polysoude SAS. The Project is
managed by TWI Ltd and is partly funded by the TSB under the Technology Programme
ref: ʹ100816ʹ. The author would like to thank all partners for their contribution to this review.
20889/05-3/12
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Table 2 Summary of common welding practice for P92
Variable
Welding process
Preheat T
Interpass T
Post heating
Cooling before
PWHT
Filler metals
PWHT temperature
PWHT duration
Gas purging
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Commonly applied variant/range
GTAW (including narrow gap and hot
wire variations), SMAW, FCAW, SAW
and combinations
Min 100-150˚C (GTAW)
Min 200˚C (other processes)
Max 300-350˚C
200˚C for 4 hours
Reference section
2.1
3.1
3.4
Not required for:
 Thin sections (under 50mm).
 Thick sections if ‘H4’ or ‘H5’
consumables are used and the weld
is cooled to cool no lower than 80°C
before PWHT.
80-100˚C
See Table 1
750-770˚C
3.6.1
As per code requirements or:
 2 hours (GTAW).
 2-4 hours (SMAW).
 4 hours (FCAW).
Required (Argon)
2.4
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Table 3 Welding parameters for Grade 92, from various industrial applications(4)
Procedure No
Process(es)
1
GTAW
Type of weld
Pipe OD
Pipe WT, mm
Base material
Bevel angle, ˚
Pipe butt
168.3mm
21.97
P91
Root 37.5
Main 8/10
6G
Welding position
(ISO or ASME)
Preheat, min ˚C
Interpass, max ˚C
Wire
diameter,
mm
Travel
speed,
(1)
mm/min
2
FCAW
2.4
250
350
1.2
2.4
215-385
50-100
Current, A(1)
78
160-180
60-100
Voltage, V(1)
12
27
10-13
0.68-1.22
-
Gas shield, l/min
and mix
Gas purge, l/min
Postheating
PWHT,
Temperature, ˚C
/ Time, h
SMAW
Pipe butt
All
4.8-60mm
P91
Root 37.5
Main 10
All, progression
uphill
200(2)
56
Heat input, kJ/mm
3
GTAW
9
18
Ar
75Ar/25CO2
8-10
Until hot pass
760˚C / 5h
10-14
Ar
30-35
350
2.5/3.2/4
60-120/
70-150/
70-150
70-100/
100-145/
140-190
20-22/
20-23/
24-28
-
-
4
GTAW
SMAW
Pipe butt
512.5mm
53mm
P91
Root 30
Main 10
5G, uphill
200(2)
320
GTAW
5
SMAW
Pipe butt
>20
P92
Root 35-40
Main 10
All except vertical
down
100200
150
300
1.6/2.4 2.5/3.2/4
Pipe butt
All
4.7-64
P91
-
Pipe butt
4.8-200
P91
30 included
Pipe butt
All
4-200
P91
30 included
Pipe butt
34in
P92
40 included
9
Narrow –gap
GTAW
Hot Wire
Pipe butt
406.4mm
33
P92
Main 2
5G
3G, uphill
1G
PF
1G
200
200
150
200
200
200
200
250
3.2/4/5
2.4
300
3.25/4
2.4
2.4
-
-
-
400-475
Min
120
90-130/140180/140-180
100125
110-130/
140-160
370-410
110130
90-120 /
110-140
20-24
-
-
27-29
14-16
21-22
-
-
-
-
1
1.1/1.2
GTAW
2.4
3.2/4
80-150
60-150
-
-
30-42
90-150
90-130/
110-180
-
-
104121
10-15
22-28
-
-
13
-
-
-
-
2.33.4
8-15
Ar
6-10
-
-
-
-
Yes (no details)
130˚C x 2h
760±15˚C /
2.5 min/mm
(min 1h)
750±15˚C /
min 2h
Cool to 80-100˚C
before PWHT
760-775˚C /
2.5 min/mm
(min 3h)
2.4
6
SMAW
SMAW
300
2.5/3.2/4
85-90/
60-120 /
55-100
68-70 /
90-124 /
123-175
21-23 /
24-25 /
25-26
1-1.1 /
1-3 /
2-3.6
-
10-30
Ar
8-12
-
6-12
Ar
2-4
-
750˚C / 1.5h
7
SMAW
GTAW
-
8
SAW
GTAW
SMAW
220-250
-
Cool to room
2h
740±10˚C /
16h
(qualified
range)
745-755˚C / 2h
300
1.0
4
12-16
Ar
18-20
Maintain for 3 runs
250-300˚C x 1h
750-760˚C / 3h
Root 90
Hot 115
Fill 110-115
Root 170/220(3)
Hot 190-280(3)
(3)
Fill 260-400
Root 9.3/8.3(3)
(3)
Hot 10.)/9.1
(3)
Fill 10.7/9.5
-
30
Ar
20
Ar
750-770˚C / 4h
Notes:
(1) Collated range of current and voltage applied, considering all weld runs.
(2) Some GTAW+SMAW procedure require 100˚C min preheat temperature for the GTAW root run.
(3) Peak/background (pulsed current).
(4) WPSs for Grade 91 were included as similar welding parameters can be used for both P91 and P92.
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1 K Coleman, “Guideline for Welding Creep Strength-Enhanced Ferritic Alloys”. EPRI, Palo Alto, CA:
2007. 1012748.
2 Arndt Jetal, “The T23/T24 book,
Vallourec&Mannesmann Tubes, 1998.
new
grades
for
waterwalls
and
superheaters”,
3 Marshall A W, Zhang Z, Holloway G B, “Welding consumables for P92 and T23 creep resisting
steels”. In Welding and repair technology for Power Plants. Proceedings, Fifth International EPRI
Conference, Point Clear, AL, 26-28 June 2002. P91 session. Paper P6.
4 Information provided by Alstom Power.
5 Heuser et al, “Properties of matching filler metals for [arc welding] E911 (P911) and P92 [Cr-Mo
steels]”. In Welding and repair technology for Power Plants. Proceedings, Fifth International EPRI
Conference, Point Clear, AL, 26-28 June 2002. P91 session. Paper P3.
6 Bruhl, F, “Verhalten des 9 5-Chromstahles X 10CrMoVNb 91 und seiner Schweissverbindungen im
Kurz- un langzeitversuch”; Dissertation, Graz 1989.
7 Bertoni A et al, “L’acciaio Grado 92: sviluppo dei materiali di consumo e procedure di saldatura”.
Rivista Italiana della Saldatura, N 5, 2005.
8 ANSI/AWS D10.8-96, “Recommended Practices for Welding of Chromium-Molybdenum Steel
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