TWI Current Practice for Welding P92.pdf

(1)

Review of Current Practice for Welding

of Grade 92

For: Valid

20889/05-3/12

January 2012

(2)

(3)

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 10216-2) 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 narrow-gap 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 rutile-based 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:

 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

(5)

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.

(6)

Table 1 Commercially available filler wires for Grade 92 (Note 1)

Welding process Manufacturer Trade name Specification

SMAW Oerlikon (A-L) CROMOCORD 92 AWS A5.5: E 9018-G

EN 3580-A: E Z CrMoWVNb 9 0.5 2 B 4 2 H5

P92-1% Co None

P92-0.5% Ni None

Bohler Fox P 92 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 Thermanit MTS 616 EN ISO 3580-A E ZCrMoWVNb 9 0.5 2 B 4 2

H5

AWS A5.5 Class E9015-B9 (mod.);E9015-G

Metrode Chromet 92 None

Kobelco CR-12S Not available

Nippon Steel Welding

Nittetsu N-616 Not available

GTAW Metrode 9CrWV None

Bohler P92-IG EN 21952-A WZ CrMoWVNb 9 0.5 1.5

Thermanit MTS 616 EN ISO 21952-A WZ CrMoWVNb 9 0.5 1.5 AWS A5.28 ER90S-B9 (mod.); ER90S-G

A-L Carborod WF92 EN 21952-A WZ CrMoWVNb 9 0.5 1.5

GMAW Bohler Thermanit MTS 616 EN 21952-A GZCrMoWVNb 9 0.5 1.5

AWS A5.28 ER90S-B9 (mod.); ER90S-G

FCAW Metrode Supercore F92 None

Bohler P92 Ti-FD E 91 T1-GM

SAW+Flux Oerlikon (A-L) OE-Cromo SF92 + OP F500

None

Metrode 9CrWV + LA491 None

Bohler Bohler P 92-UP+BB910 EN 24598A S ZCrMoWVNb9 0.5 1.5/SA FB 2 AWS A5.23 Class EB9 (mod.)

Thermanit MTS 616 / Marathon 543

EN 10270 S ZCrMoWVNb 9 0.5 1.5 AWS A5.23 Class EG [B9 (mod.)] Kobelco PF-200S/US-12CRS PF-200SD/US-12CRSD Not available Nippon Steel Welding Nittetsu Y-616 + Nittetsu NB-616 Note:

(7)

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.

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. 200°C min 300°C max Cool to 80-100°C PWHT 760°C RT Preheat Interpass Heating rate 100-150°C/h Cooling rate 150°C/h Below 400°C cool in still air

(8)

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 as-welded 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.

(9)

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}

(10)

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.

(11)

Table 2 Summary of common welding practice for P92

Variable Commonly applied variant/range Reference section Welding process GTAW (including narrow gap and hot

wire variations), SMAW, FCAW, SAW and combinations

2.1

Preheat T Min 100-150˚C (GTAW) Min 200˚C (other processes)

3.1 Interpass T Max 300-350˚C

Post heating 200˚C for 4 hours 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.

3.4

Cooling before PWHT

80-100˚C Filler metals See Table 1

PWHT temperature 750-770˚C 3.6.1

PWHT duration As per code requirements or:  2 hours (GTAW).

 2-4 hours (SMAW).  4 hours (FCAW).

(12)

Table 3 Welding parameters for Grade 92, from various industrial applications(4)

Procedure No 1 2 3 4 5 6 7 8 9

Process(es) GTAW FCAW GTAW SMAW GTAW SMAW GTAW SMAW GTAW SMAW SMAW GTAW SMAW SAW GTAW SMAW Narrow –gap

GTAW Hot Wire

Type of weld Pipe butt Pipe butt Pipe butt Pipe butt Pipe butt Pipe butt Pipe butt Pipe butt Pipe butt

Pipe OD 168.3mm All 512.5mm - All - All 34in 406.4mm

Pipe WT, mm 21.97 4.8-60mm 53mm >20 4.7-64 4.8-200 4-200 - 33

Base material P91 P91 P91 P92 P91 P91 P91 P92 P92

Bevel angle, ˚ Root 37.5

Main 8/10 Root 37.5 Main 10 Root 30 Main 10 Root 35-40 Main 10

- 30 included 30 included 40 included Main 2

Welding position (ISO or ASME)

6G All, progression

uphill

5G, uphill All except vertical down 5G 3G, uphill 1G PF 1G Preheat, min ˚C 250 200(2)₂₀₀(2) 100-150 200 200 200 150 200 200 200 200 Interpass, max ˚C 350 350 320 300 300 250 300 220-250 300 Wire diameter, mm 2.4 1.2 2.4 2.5/3.2/4 2.4 3.2/4 1.6/2.4 2.5/3.2/4 2.4 2.5/3.2/4 3.2/4/5 2.4 3.25/4 2.4 2.4 4 1.0 Travel speed, mm/min(1) 56 215-385 50-100 60-120/ 70-150/ 70-150 80-150 60-150 - - 30-42 85-90/ 60-120 / 55-100 - - - 400-475 Min 120 Root 90 Hot 115 Fill 110-115 Current, A(1) 78 160-180 60-100 70-100/ 100-145/ 140-190 90-150 90-130/ 110-180 - - 104-121 68-70 / 90-124 / 123-175 90-130/140-180/140-180 100-125 110-130/ 140-160 370-410 110-130 90-120 / 110-140 Root 170/220(3) Hot 190-280(3) Fill 260-400(3) Voltage, V(1)_{12 27}_{10-13 20-22/} 20-23/ 24-28 10-15 22-28 - - 13 21-23 / 24-25 / 25-26 20-24 - - 27-29 14-16 21-22 Root 9.3/8.3(3) Hot 10.)/9.1(3) Fill 10.7/9.5(3) Heat input, kJ/mm 0.68-1.22 - - - 2.3-3.4 1-1.1 / 1-3 / 2-3.6 - - - - 1 1.1/1.2 -

Gas shield, l/min and mix 9 Ar 18 75Ar/25CO2 10-14 Ar - 8-15 Ar - - - 10-30 Ar - - 6-12 Ar - - 12-16 Ar - 30 Ar

Gas purge, l/min 8-10

Until hot pass

30-35 6-10 - 8-12 - 2-4 18-20

Maintain for 3 runs 20 Ar

Postheating - Yes (no details) 130˚C x 2h Cool to 80-100˚C

before PWHT Cool to room 2h 250-300˚C x 1h PWHT, Temperature, ˚C / Time, h 760˚C / 5h 760±15˚C / 2.5 min/mm (min 1h) 750±15˚C / min 2h 760-775˚C / 2.5 min/mm (min 3h) 750˚C / 1.5h 740±10˚C / 16h (qualified range) 745-755˚C / 2h 750-760˚C / 3h 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).

(13)

1 K Coleman, “Guideline for Welding Creep Strength-Enhanced Ferritic Alloys”. EPRI, Palo Alto, CA: 2007. 1012748.

2 Arndt Jetal, “The T23/T24 book, new grades for waterwalls and superheaters”, Vallourec&Mannesmann Tubes, 1998.

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 Piping and Tubing”, American Welding Society, 1996.

9 http://www.huntingdonfusion.com/en/products/pipe-purging-systems/heat-resistant-for-6-to-46q.html 10 Metrode Ltd, “P92 welding consumables for the power generation industry”, Issue 4, September 2009.

11 http://www.oerlikon-welding.com/

12 Oerlikon Competence, no.5. Nov.2010. pp.7-13, 15-24. Saint-Ouen l”Aumone, France; ALW, 2010. www.airliquide.com; www.oerlikon-welding.com

13 Bohler Welding Guide, Edition 09/2010, published by BOHLER WELDING.

14 Welding Filler Metals, Phoenix-Union-Thermanit, published by BohlerSchweisstechnik Deutschland GmbH, Edition September 2009.

15 Posch et al, “GMA-welding of creep resistant steels with flux cored wires (FCAW): perspectives and limitations”. Welding in the World, vol 53, Special issue 2009. Welding consum,ables. Paper wc-1, pp 619-614.

16 Richardot D, Vaillant JC et al, “The T92/P92 book”, Vallourec&Mannesmann Tubes, 1998.

17 Holloway G B, Zhang Z, Marshall A W, “Properties of T/P92 CrMo weld metals for Ultra Super Critical (UCS) power plant”, 2008, Great Britain

18 AWS Welding Handbook. Vol. 4, “ Materials and Applications - Part 2”, AWS, 1998.

19 Henry J F, “Growing Experience with P91/T91 Forcing Essential Code Changes”, Combined Cycle Journal, First quarter 2005.

20 “ASME Boiler and Pressure Vessel Code, Section VIII, 2011a Edition: “Rules for Construction of Pressure Vessels”, American Society of Mechanical Engineers, 01 July 2011.

21 AWS D10.10: “Recommended practices for local heating of welds in piping and tubing”.

22 Doty W D: “History and need behind the new NBIC rules on weld repair without PWHT”, Welding Research Council Bulletin 412, June 1996 3-8.

23 Newell W F Jr, 2010: “Welding and Postweld Heat treatment of P91 steels”, AWS Welding Journal, US, April 2010. Based on a presentation made during the Chrome-Moly Steels Conference held during the 2009 FABTECH International & AWS Welding Show, Chicago, Ill.