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NEW GENERATION 21/4CR STEELS T/P 23 AND T/P 24 WELDABILITY AND HIGH TEMPERATURE PROPERTIES 75

1 INTRODUCTION

Creep resistant steels for high temperature service area vital part in the construction of power stations. Thepossibilities to increase steam parameters of new boil-ers are restricted if pressure and temperature in themembrane water-walls cannot be simultaneouslyincreased.

The increase of steam parameters requires the develop-ment of new high strength and creep resistant materials.

Conventional ferritic-bainitic steels such as ASTMA 213 T12 (13CrMo4-4) and ASTM A 213 T22 (10CrMo9-10) do not have enough creep strength for the use asmembrane water-walls of USCB (Ultra Super Critical

Boilers). Moreover, the maximum hardness in the HAZof steel T22 (10CrMo9-10) is too high in the as weldedcondition and a post-weld heat treatment is necessaryto reduce the hardness below 350 HV.

For some time, two modified versions of the standard21/4Cr-1Mo steel (P22) are on the market.

The new grades T23 and T24 are ideal candidates forwater-wall tubes of USCB. They are also used for super-heater and re-heater components of conventional boil-ers and Heat Recovery Steam Generators (HRSG) [1,2]. Moreover, thick walled pipes can nowadays be pro-

duced for piping applications in headers and main steam

pipes, in both conventional and HRSG plants.This paper gives data on weldability, mechanical prop-erties, toughness and high temperature behaviour(creep) of these new materials T/P 24 (7CrMoVTiB10-10) which have been developed by Vallourec &Mannesmann Tubes [1, 2], and T/P 23 (7CrWVMoNb9-6). It is the outcome of a research project of the BelgianWelding Institute in collaboration with Laborelec andwith industrial partners (Carnoy Industrial Piping,Cockerill Mechanical Industries, Fabricom, Stork Mec,SAF Oerlicon, Thyssen Welding Germany, Vallourec &Mannesmann Tubes, AIB Vinotte, WTCM).

Both base metals and weldments in tubes and thick

walled pipes have been investigated.

2 BASE MATERIALS

Chemical compositions of steel grades T/P23 and T/P24are compared per ASTM A 213/A335 (ASTM A 335 forP24 in preparation) in Table 1 to other standard steelgrades (T/P22, T/P91) used for high temperature appli-cation in power plants.

The investigated materials in this study were both tubesand pipes. The T23 and T24 tubes have an outsidediameter of 51 mm and a wall thickness of 7.1 mm. The

P23 pipes have an outside diameter of 219 mm and awall thickness of 30 mm. The P24 pipes have an outsidediameter of 370 mm and a wall thickness of 25 mm.

NEW GENERATION 21/4CR STEELS T/P 23 ANDT/P 24 WELDABILITY AND HIGH TEMPERATURE

PROPERTIESA. Dhooge1, J. Vekeman2

1 Belgian Welding Institute/University Ghent, e-mail: [email protected],2 Belgian Welding Institute, e-mail: [email protected]

(Belgium)

ABSTRACT

Creep resistant steels for high temperature service are a vital part in the construction of power stations. The possi-bilities to increase steam parameters of new boilers are restricted if pressure and temperature in the membrane water-

walls cannot be simultaneously increased. The increase of steam parameters requires the development of new highstrength and creep resistant materials. This paper gives data on weldability, mechanical properties, toughness andhigh temperature behaviour (creep) of new materials T/P 24 (7CrMoVTiB10-10) which have been developed byVallourec & Mannesmann Tubes, and T/P 23 (7CrWVMoNb9-6). It is the outcome of a research project of the BelgianWelding Institute in collaboration with Laborelec and with industrial partners. Both base metals and weldments intubes and thick walled pipes have been investigated.

IIW-Thesaurus keywords: Boilers; Tubes and pipes; Creep resisting materials; Weldability; Toughness; Mechanical

properties; Creep strength; Strength; Hardness; Coarse grained heat affected zone; Heat affected zone; Weld zone;

Simulating; Post weld heat treatment; Heat treatment; Reheat cracking; Cracking; Defects; GTA welding; Arc weld-

ing; Gas shielded arc welding; MMA welding; Submerged arc welding; Practical investigations.

Welding in the World, Vol. 49, n 9/10, 2005

Doc. IIW-1693-05 (ex-doc. XI-810-04) recommended forpublication by Commission XI Pressure vessels, boilersand pipelines.

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76 NEW GENERATION 21/4CR STEELS T/P 23 AND T/P 24 WELDABILITY AND HIGH TEMPERATURE PROPERTIES

The chemical composition according to the certificatesis given in Table 2.

The required and actual measured mechanical proper-ties are shown in Table 3.

Conclusion

The delivered base metals show acceptable hardness,strength and toughness and fulfil the requirements ofthe standards.

3 BASIC WELDABILITY STUDY WELD SIMULATION

Weld simulation tests were performed to study the weld-ability of the 21/4Cr steels and to be used as input data formaking the welded joints under field conditions. An exten-sive simulation program was carried out to determine theinfluence of the welding parameters and a Post WeldHeat Treatment (PWHT) on the microstructure, hardnessand the toughness of the Heat-Affected Zone (HAZ).

Table 1 Chemical composition of ferritic creep resistant base materials and of martensitic T/P 91

T/P 22 C Si Mn Cr Mo Ni V W Al B N Nb Ti

21/4Cr-1Mo Min - .25 .30 1.9 .87 - - - - - - - -

ASTM A 213/A335 Max .15 1.00 .60 2.6 1.13 - - - - - - - -

T/P23 C Si Mn Cr Mo Ni V W Al B N Nb Ti

7CrWVMoNb9-6 Min .04 - .10 1.9 .05 - .20 1.45 - .0005 - .02 -

ASTM A 213/A335 Max .10 .50 .60 2.6 .30 - .30 1.75 .030 .0060 .030 .08 -

T/P24 C Si Mn Cr Mo Ni V W Al B N Nb Ti

7CrMoVTiB10-10 Min .05 .15 .30 2.2 .90 - .20 - - .0015 - - .05

ASTM A 213 Max .10 .45 .70 2.6 1.10 - .30 - .020 .0070 .012 - .10

T/P 91 C Si Mn Cr Mo Ni V W Al B N Nb Ti

X10CrMoVNb 91 Min .08 .20 .30 8.0 .85 - .18 - - - .030 .06 -

ASTM A 213/A335 Max .12 .50 .60 9.5 1.05 .40 .25 - .040 - .070 .10 -

Rp0.2 Rm A5 Hardness CVN

(MPa) (MPa) (%) (HV10) (J)

ASTM A 213 > 450 > 585 > 20 < 263 HV10 > 41 (T)> 68 (L)

T24 > 528 > 639 > 20 > 200 245*

> 521 > 640 > 20

P24 > 560 > 640 > 24 > 212 260

> 569 > 654 > 24

* Subsized specimen, value in J/cm2.

Table 2 Chemical composition of investigated tubes and pipes

C Si P S Mn Cr Mo V W Al B N Nb Ti

T23 .060 .220 .014 .002 .470 2.05 .08 .230 1.510 .017 .0019 .009 .050 -

P23 .070 .280 .008 .004 .540 2.08 .08 .220 1.650 .018 .0020 .011 .030 -

T24 .063 .220 .006 .003 .510 2.45 .97 .255 - .003 .0043 .0075 - .051

P24 .060 .290 .014 .003 .410 2.31 .95 .228 - .010 .0046 .0080 - .079

Table 3 Mechanical properties of investigated tubes and pipes

Rp0.2 Rm A5 Hardness CVN

(MPa) (MPa) (%) (HV10) (J)

ASTM A 213/A335 > 400 > 510 > 20 < 230 -T23 > 557 > 644 > 25 > 201 232*

> 537 > 628 > 26

P23 > 552 > 633 > 22 > 203 219

> 553 > 634 > 23

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3.1 Experimental procedure

In weld simulation, impact test specimens are subjectedto weld thermal cycles by computer controlled dynamicresistance heating and simultaneous cooling via watercooled jaws. A programmed heating and cooling thermalcycle is thereby applied to the central working zone ofthe specimen. The microstructures obtained in the cen-tral zone correspond to the microstructures of the HAZobtained in welded joints under field conditions.

Single weld cycles were simulated with different peaktemperatures (Tp1 = 1 350 oC, 1 200 oC and 1 000 oC).The desired cooling time t8/5 between 800 and 500 oCdepends on the welding process one wants to simulate.Cooling times t8/5 between 800 and 500 oC of 10, 30 and60 seconds for the tubes and 10, 20, 40 and 60 sec-onds for the pipes were chosen.

When welding the thin walled tubes (wall thickness7.1 mm) without preheat, the cooling time t8/5 is about 10,35 and 60 seconds for a heat input of respectively 5, 10and 15 kJ/cm. Applying a higher heat input will result inlower cooling rates.

When welding the thick walled pipes (wall thickness 25 30 mm) with a preheat of 150 oC, the cooling time t8/5is much faster compared to the thin walled tubes.Cooling times of 10, 20 and 60 seconds correspond toa heat input of respectively 15, 18 and 26 kJ/cm.Applying a higher heat input will also here result in lowercooling rates.

Besides the single weld thermal cycles, also double

cycles were applied with Tp1 =1350o

C and a secondpeak temperature of either 750 oC or 1 000 oC.

After simulation of four specimens at the same condition,one specimen was used to measure the VickersHardness HV10 and to perform micro structural inves-tigations. The other three specimens were used for

Charpy V impact tests at room temperature to deter-mine the impact toughness. The impact tests on the full-size specimen, with a notch of 2 mm, were performedconform to EN 10045-1 and the tests on the sub-sizespecimen, with a notch of 1 mm, conform to ASTM E23.

The need for a PWHT was evaluated using the follow-ing criteria:

maximum hardness 350HV10 (power generationapplication);

minimum impact toughness at room temperature: 27 J[Pressure Equipment Directive (PED)].

The PWHT was performed in a furnace. The specimenswere heated to the desired temperature at a rate of125 oC/hour and after the holding time was expired, theywere air cooled.

3.2 Test results and discussion

The test results are summarised in Figures 1 to 6.

Some different behaviours were found between tubesand pipes, especially concerning the toughness afterdouble thermal cycles and after PWHT.

In the as welded condition, the lowest toughness andhighest hardness can be expected in the coarse grained

HAZ of both tubes and pipes. The highest hardness isassociated with the fastest cooling rate although the

Figure 1 Weld simulation test results on T23

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hardness variation with different cooling rates is not verypronounced.

For the tubes, fulfilling the hardness criteria (< 350 HV)seems to be easily achievable even without preheat andPWHT. Indeed, in nearly all simulated conditions, thehardness is below 350 HV10. Applying a PWHT reducesthe hardness considerably (see Figures 1 and 2).

Obtaining sufficient toughness in the as welded condi-tion requires an adequate heat input. The lowest tough-ness is found with the fastest cooling rate (t8/5 = 10 sec-onds), but in practice cooling rates will always be lowerin these thin walled tubes, if normal heat input for TIGwelding is used, giving rise to adequate toughness.

Applying a PWHT has a positive effect on the tough-ness in the coarse grained HAZ of T24 but for the T23steel, a slight drop in toughness is found (see Figures 1and 2).

Applying a double weld cycle with Tp2 = 1 000 oCimproves the coarse grained HAZ toughness consider-ably in T23 but Tp2 = 750 oC has a negative effect (seeFigure 1).

Also for the thick walled pipes, the hardness staysmostly below 350 HV, even in the as welded conditionand not much variation is found with different coolingrates. Applying a PWHT reduces the hardness consid-erably.

Toughness in the as welded condition is low in thecoarse grained HAZ of both steels (see Figures 3 and5). For the P23 steel, this can be improved by applying

a PWHT at 740 oC for 1.5 hours (see Figure 4) althoughno improvement was found for T23. For the P24 steel,the improvement by such a PWHT is quite low (seeFigure 6). Such a heat treatment was beneficial for theT24.

Both steels do not show any improvement in toughnessby applying a second weld thermal cycle with a peaktemperature of 750 oC or 1 000 oC as was the case forthe T23 steel with a second peak temperature of1 000 oC. Applying a PWHT at 740 oC for 2 hours onthese microstructures has a positive effect especiallywhen a high second peak temperature is applied(Figures 4 and 6). Indeed, much higher toughness wasfound for a second peak temperature of 1 030 oC com-

pared to 1 000 oC indicating that a second peak tem-perature of 1 000 oC is too low for toughness improve-ment. One can conclude that the overall HAZ toughnesswill improve and be adequate after PWHT.

3.3 Conclusions

The weld simulation tests indicate that adequate hard-ness and toughness can be obtained in tube welds ofboth T23 and T24 materials without preheat and with-out PWHT.

Also for the thick walled pipes, the hardness stays below

350 HV, even in the as welded condition. However, fortoughness reasons, a PWHT at a temperature of about740 oC is necessary.

Figure 2 Weld simulation test results on T24

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Figure 3 Weld simulation test results on P23


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4 REHEAT CRACKING

Reheat cracking, also called stress relief cracking, is apossible cause of weld failures in creep-resistant, pre-

cipitation-strengthened alloys such as ferritic alloy steels.Reheat cracking is defined as an intergranular crackingphenomena occurring in the heat-affected zone (HAZ)and occasionally in the weld metal of a welded joint,being initiated during a post-weld heat treatment (PWHT)or during high temperature service.

A susceptible microstructure is a coarse prior austenitegrain size with strong grain interiors that resist plasticdeformation and weak grain boundaries. Therefore thecoarse-grained heat-affected zone (CGHAZ) is the mostsusceptible region of a weldment.

4.1 Experimental procedureAt the Research Centre of the Belgian Welding Institute,a lot of experience has been gathered with the isother-mal slow strain rate tensile test. In this test, cylindricalspecimens are given a weld simulation cycle with peaktemperature Tp1 =1350 oC to simulate the CGHAZ. Aftercooling to room temperature (t8/5 = 20 seconds), thespecimen is heated to and held at the test temperaturebetween 550 oC and 750 oC. As soon as this tempera-ture is obtained, the specimen is slowly strained to frac-ture at a tensile velocity of 0,5 mm/min. After fracturingthe specimen, its reduction in area is measured toassess the ductility.

Steels are classified as follows:

Susceptibility to reheat-cracking % Reduction in area Extremely susceptible < 5 % Highly susceptible 5 - 10 % Slightly susceptible 10 - 20 %

Not susceptible > 20 %The tests were performed using a Gleeble 1500/20thermo mechanical simulator.

Cylindrical test samples, 110 mm long and with a diam-eter of 6 mm (tubes), respectively 12 mm (pipes), withthreaded ends were used. The samples were taken inaxial direction.

After simulation, a zone of 10 mm in the centre of thespecimen was reduced to 4 mm (tubes), respectively6 mm (pipes) so that specimen fracture during tensiletesting occurred in the simulated zone.

The longitudinal cross-section of the specimen with thelowest reduction of area was investigated using lightoptical micrographs. A second specimen was testedunder the same conditions, but with inert gas (argon)protection to prevent oxidation of the fracture surface, toexamine the fracture surface using scanning electronmicroscopy (SEM). The influence of double weld cycles,second peak temperature Tp2 = 750 or 1 000 oC, hasbeen compared to single weld cycles.

4.2 Test results and discussion

Figure 7 summarizes all test results and shows thereduction in area as a function of the peak temperature.

Both T24 and P24 materials have a reduction in area of

more than 20 % for single cycled specimens in the whole

Figure 7 Reheat cracking test results

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test temperature range, indicating that there is no sus-

ceptibility to reheat cracking in this particular simulatedCGHAZ. Also the double cycled specimens havemicrostructures which are not susceptible to reheatcracking.

The situation is different for the T23 and P23 materials.Here, T23 shows some susceptibility to reheat crackingin the particular simulated coarse grained HAZ at PWHTtemperatures above 675 oC. This means that when T23is used without PWHT, no reheat cracking problems areexpected during service (temperature limited to 585 oC).

The P23 simulated coarse grained HAZ shows a highsusceptibility to reheat cracking in a broad test temper-ature range above 575 oC. Important to note is that

applying a double weld cycle with a second peak tem-perature of 1 000 oC increases the reduction in area toa value above 20 %, indicating that multi-pass weldinggives rise to fine grained microstructures that are notsusceptible to reheat cracking.

Typical SEM pictures for P23 and P24 are shown inFigure 8. One can recognize a fully intergranular fracturealong the prior austenite grain boundaries for the P23coarse grained HAZ and a mixed ductile/intergranularfracture with ductile characteristics on the grain bound-aries for the P24 steel.

The reheat cracking results for T23 and P23 agree withtest results by J.G. Nawrocki [3]. In this study, the reheat

cracking susceptibility was based on the ductility andthe resultant microstructures. HCM2S (Japanese des-ignation of T/P23) was found to be extremely to highlysusceptible to reheat cracking at each energy input andpost weld heat treatment. HCM2S experienced brittleintergranular failure along prior austenite grain bound-aries for single-pass simulations. For double-pass sim-ulations, the ductility increased and the samples failedalong grain boundaries (prior austenite or packet) nor-mal to the tensile axis and exhibited extensive plasticdeformation, indicating that reheat cracking was avoidedwith the use of multi-pass simulations.

It is clear that some caution has to be taken in weldingthe T23 and P23 steel due to their possible susceptibil-

ity to reheat cracking. This is particularly the case forhighly stressed joints for which the following recom-mendations can be formulated:

Avoid too coarse grained HAZ microstructures by:

limiting the heat input (below 2.5 kJ/mm), using multiple layer welding (renormalizing effect), use of tempering beads. Eliminate stress raisers at coarse grained HAZ areasby using: an adequate welding technique, smooth transitions (grinding).

4.3 Conclusions

Both investigated T24 and P24 materials show no sus-ceptibility to reheat cracking in this particular simulatedCGHAZ. Also the fine grained microstructures (double

cycled specimens) are not susceptible to reheat crack-ing.

T23 shows some susceptibility to reheat cracking in theparticular simulated coarse grained HAZ at PWHT tem-peratures above 675 oC. The P23 simulated coarsegrained HAZ shows a high susceptibility to reheat crack-ing in a broad test temperature range above 575 oC.However, the simulated fine grained microstructures arenot susceptible to reheat cracking indicating that reheatcracking can be avoided by using an appropriate weld-ing technique (avoiding coarse grained microstructuresin highly stressed areas).

5 PROPERTIES OF TUBE WELDS

5.1 Welding data

The 7.1 mm thick T23 and T24 steel tubes were weldedwith the GTAW process. Welding was performed withand without preheating, but in all cases without PWHT.Both homogeneous and heterogeneous welds (to T91and AISI 316L tubes) were investigated.

The T23 homogeneous welds and the heterogeneouswelds to T91 were welded with filler metal Union1Cr2WV (diameter 2 mm) from Thyssen Welding. Theheterogeneous welds to AISI 316L were welded with

SAF filler metal Inconel 82 (diameter 2 mm).The chemical composition of the wires (according to thefabricator) and of the all-weld metal is given in Table 4.

P23 P24

Figure 8 SEM pictures or fracture surface (reheat cracking test)

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The welded joints were tested according to EN 288-3:Specification and approval of welding procedures formetallic materials Part 3: Welding procedure tests forthe arc welding of steel.

The destructive tests on the welds included: transverse tensile test according to EN 895; bend tests according to EN 910; impact tests according to EN 875; macroscopic and microscopic examination accordingto EN 1321; hardness test according to EN 1043-1.

5.2 Mechanical test results

The mechanical test results are summarized in Tables 5to 8.

For the homogeneous welds, all welded joints fulfilled thestrength requirements. Also root and face bending couldbe performed without cracking. The impact toughness(measured on subsize specimens) and the hardnessare closely related to the applied welding procedure andespecially to the applied heat input. Lower toughness ismostly correlated with a high heat input (Welds no. 1

and 2 in Table 5 were welded with only 3 weld passesand a heat input up to 35 kJ/cm). Hardness in excessof 350 HV can be encountered when low heat input isapplied (e.g. in weld no. T24-3, welded with a heat inputof 12 - 22 kJ/cm).

The heterogeneous girth welds T23 or T24 to T91,welded with T23 or T24 filler metals, with a preheat of150 oC and with a PWHT (740 oC 2 hours) showed allacceptable strength, hardness and toughness in spite ofthe high heat input used (up to 35 kJ/cm).

Table 4 Chemical composition of welding wires and all-weld metal (homogeneous welds)

C Si P S Mn Cr Mo V W Al B N Nb Ti

T23

Union 1Cr2WV - wire .07 .3 - - .5 2.2 .22 1.7 .002 .01 .05 -

T23/All-weld metal .069 .384 .007 .004 .528 2.15 .029 .22 1.32 .011 .002 - .026 .004

T24

Union 1CrMoVTiB .05 .3 - - .5 2.2 1.0 .22 - .003 .010 .005

T24/All-weld metal .064 .285 .004 .004 .488 2.37 .95 .251 .013 .009 .003 - .008 .051

Table 5 Test results on T23 homogeneous tube welds

Test results on T23 tubes (OD 50.8 7.1 mm)

Weld no. T23-1 Weld no. T23-2 Weld no. T23-3Manual GTAW 3 layers Orbital GTAW 3 layers Manual GTAW 7 layersFiller: Union 1Cr2WV 2 mm Filler: Union 1Cr2WV 2 mm Filler: Union 1Cr2WV 2 mmNo preheat, no PWHT Heat input: 35 kJ/cm Heat input: 10-20 kJ/cm

No preheat, no PWHT Preheat: 150 oC, no PWHT

Transverse tensile testing (EN 895)

Required: greater than specified minimum BM (EN 288-3): min. 510 MPa specified in ASTM A213

Tensile strength BM (MPa) 644-628

Tensile strength girth welds 645 / outside weld 617 / outside weld 645 / outside weld(MPa) / fracture location in base metal in base metal in base metal

635 / outside weld 637 / outside weld 639 / outside weldin base metal in base metal in base metal

Side bending (EN 910)

Required: 120o without cracks of 3 mm in each direction (EN 288-3)Root bends 180o - no cracks 180o - no cracks 180o - no cracksFace bends 180o - no cracks 180o - no cracks 180o - no cracks

Charpy-V impact testing (EN 875)

Required: 27 J (34 J/cm2) in WM at room temperature (PED)

Impact base metal (J/cm2) 232 240 225 / 232

Impact HAZ (J/cm2) 177 216 265 / 219 226 30 27 / 94 179 211 255 / 215

Impact weld metal (J/cm2) 25 25 27 / 26 123 182 123 / 143 228 226 108 / 187

Hardness testing (EN 1043-1)

Required: max. 350 HV10 (power plant applications)

HV10 base metal 201

HAZ max. HV10 face 340 314 334

HAZ max. HV10 root 298 349 287

Weld metal max. HV10 face 343 308 360

Weld metal max. HV10 root 283 325 297

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Table 6 Test results on T24 homogeneous tube welds

Test results on T24 tubes (OD 50.8 7.1 mm)

Weld no. T24-1 Weld no. T24-2 Weld no. T24-3

Manual GTAW 6 layers Orbital GTAW 3 layers Manual GTAW 7 layersFiller: Union 1CrMoVTib 2 mm Filler: Union 1CrMoVTib 2 mm Filler: Union 1CrMoVTib 2 mmNo preheat, no PWHT Heat input: 26.4 kJ/cm Heat input: 12-22 kJ/cm

No preheat, no PWHT Preheat: 150 oC, no PWHT



Tensile strength BM (MPa) 639-640

Tensile strength girth welds 638 / outside weld 631 / outside weld 635 / outside weld(MPa) / fracture location in base metal in base metal in base metal

643 / outside weld 623 / outside weld 632 / outside weldin base metal in base metal in base metal


Required: 120o without cracks of 3 mm in each direction (EN 288-3)

Root bends 180o - no cracks 180o - no cracks 180o - no cracks

Face bends 180o - no cracks 180o - no cracks 180o - no cracksCharpy-V impact testing (EN 875)



Impact HAZ (J/cm2) 233 253 307 / 264 272 91 388 / 250 98 226 211 / 178

Impact weld metal (J/cm2) 91 108 105 / 101 39 7 17 / 21 255 253 255 / 254



HV10 base metal 200

HAZ max. HV10 face 340 339 363

HAZ max. HV10 root 343 360 313

Weld metal max. HV10 face 331 339 366

Weld metal max. HV10 root 336 343 298

Table 7 Test results on heterogeneous tube welds T23 and T24 to T91

Test results on T23 and T24 tubes to T91 (OD 50.8 7.1 mm)

Weld no. T23-4 Weld no. T24-4Manual GTAW 3 layers Manual GTAW 3 layersFiller: Union 1Cr2WV 2 mm Filler: Union 1CrMoVTib 2 mmHeat input: 30 - 35 kJ/cm Heat input: 26 - 35 kJ/cmPreheat: 150 oC, PWHT: 740 C 2 hours Preheat: 150 oC, PWHT: 740 C 2 hours


Required: greater than specified minimum BM (EN 288-3): min. 510 MPa for T23 specified in ASTM A213min. 585 MPa for T24 specified in ASTM A213min. 585 MPa for T91 specified in ASTM A213

Tensile strength girth welds 564 / outside weld in T23 599 / outside weld in T24(MPa) / fracture location 578 / outside weld in T23 588 / outside weld in T24



Root bends 180o - no cracks 180o - no cracksFace bends 180o - no cracks 180o - no cracks



Impact HAZ T23/T24 (J/cm2) 258 265 258 / 260 253 250 260 / 254

Impact HAZ T91 (J/cm2) 194 189 182 / 188 204 228 196 / 209

Impact weld metal (J/cm2) 179 196 191 / 189 216 187 238 / 214



Max. HV10 face 275 308

HAZ T91 root 257 275

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Also the heterogeneous girth welds T23 or T24 toAISI 316L, welded with Inconel 82 filler metal, withoutpreheat and PWHT but with lower heat input (maximum15 kJ/cm) showed all acceptable strength and tough-ness. The hardness in the 21/4 Cr steel HAZ can be inexcess of 350 HV when too low heat input is applied(e.g. in weld no. T24-5, welded with a heat input of 6 13 kJ/cm).

5.3 Conclusions

Both T23 and T24 tubes can be welded without preheatand without PWHT, providing an appropriate heat inputand multiple layer welding is applied. The recommendedheat input is between 15 and 25 kJ/cm.

The heterogeneous girth welds T23 or T24 to T91,welded with T23 or T24 filler metals, with a preheat of150 oC and with a PWHT (740 oC 2 hours) and theheterogeneous girth welds T23 or T24 to AISI 316L,welded with Inconel 82 filler metal, without preheat andPWHT showed all acceptable strength and toughness.The applied heat input should not be below 15 kJ/cm inorder to avoid hardness in excess of 350 HV.

6 PROPERTIES OF PIPE WELDS

6.1 Welding data

P23 and P24 steel pipes were welded with the SMAWand SAW welding process. Both homogeneous and het-

erogeneous welds (to P22 and P91 pipes) were inves-tigated.

In order to define the optimum PWHT conditions, SMAWgirth welded pipes were subjected to different PWHTcycles. The impact and hardness test results are sum-marized in Figures 9 and 10. From these data, it wasdecided to apply a PWHT at 740 oC for two hours.

The chemical composition of the filler metals (accordingto the fabricator) and of the all-weld metal (measuredvalues) is given in Table 9.

The heterogeneous SMAW welds P23 and P24 to P22were welded with filler metal Thyssen SH Chromo 2 KS.The SMAW welds P23 and P24 to P91 were weldedwith Thyssen CrMo9V electrodes [4].

The welded joints were tested according to EN 288-3:Specification and approval of welding procedures formetallic materials Part 3: Welding procedure tests forthe arc welding of steel.

The destructive tests on the welds included: transverse tensile test according to EN 895; bend tests according to EN 910; impact tests according to EN 875; macroscopic and microscopic examination accordingto EN 1321; hardness test according to EN 1043-1.

6.2 Mechanical test results

The mechanical test results are summarized inTables 10 to 15.

Table 8 Test results on heterogeneous tube welds T23 and T24 to 316L

Test results on T23 and T24 tubes to 316L (OD 50.8 7.1 mm)

Weld no. T23-5 Weld no. T24-5

Manual GTAW 5 layers Manual GTAW 4 layersFiller: Inconel 82 2 mm Filler: Inconel 82 2 mmHeat input: 8 - 15 kJ/cm Heat input: 5 - 11 kJ/cmNo Preheat, no PWHT No Preheat, no PWHT


Required: greater than specified minimum BM (EN 288-3): min. 510 MPa for T23 specified in ASTM A231min. 585 MPa for T24 specified in ASTM A213min. 585 MPa for 316L

Tensile strength girth welds 599 / outside weld in T23 621 / outside weld in 316L(MPa) / fracture location 593 / outside weld in 316L 621 / outside weld in 316L



Root bends 180o - no cracks 180o - no cracksFace bends 180o - no cracks 180o - no cracks

Charpy-V impact testing (EN 875)Required: 27 J (34 J/cm2) in WM at room temperature (PED)

Impact HAZ T23/T24 (J/cm2) 238 226 235 / 233 277 307 / 292

Impact HAZ 316L (J/cm2) 307 289 289 / 295 238 245 245 / 243





in HAZ T23/24 root 333 380

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For the homogeneous P23 welds, both SMAW and SAWwelds fulfilled the strength, hardness and toughnessrequirements. In all cases, a preheat of at least 150 oChas been applied and all welds received a PWHT at740 oC for two hours. The heat input seems to play aminor role here. Indeed, the applied heat input variedbetween 8 and 27 kJ/cm.

Also for the homogeneous P24 welds, welded with thesame conditions as the P23 welds, both SMAW andSAW welds fulfilled the strength, hardness and tough-ness requirements. Only SMAW weld P24-4 showed

some cracking in the side bend test due to unacceptableporosities in the weld metal. Also here, the applied heatinput varied largely between 8 and 37 kJ/cm.

The heterogeneous girth welds P23 to P22, welded withP22 filler metals, with a preheat of 150 oC and with aPWHT (740 oC 2 hours) showed all acceptablestrength, hardness and toughness.

Also the heterogeneous girth welds P23 or P24 to P91,welded with P91 filler metal, with a preheat of 150 200 oC and with a PWHT (740 oC for two hours) andheat input ranging from 18 to 25 kJ/cm showed allacceptable strength and hardness. Toughness is noproblem in the HAZ but the weld metal shows low impacttoughness and cracking in side bend testing.

Low impact toughness at RT is not unusual for dissim-ilar welds with low-alloy ferritic steel. Due to the differ-

Figure 9 Influence of PWHT on hardness and impact toughness of P23 SMAW welds

Figure 10 Influence of PWHT on hardness and impact toughness of P24 SMAW welds

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ences in chromium contents between the materialsinvolved, carbon diffuses during PWHT from the low-chromium material into the neighbouring high-chromiumsteel or weld metal. As a result, a carbon-depleted zoneevolves in the low-chromium and a carbon-enriched

zone in the high-chromium material. The extensions ofthese zones depend on tempering time and temperature.These microstructural changes have an effect on roomtemperature toughness. However, the creep rupturestrength of such a dissimilar weld usually is not affected.The fine-grained intercritical HAZ of the low-alloy ferriticsteel still remains the weak zone after long-time expo-sure.

6.3 Conclusions

The 30 and 25 mm thick P23 and P24 pipes, weldedwith matching filler metals, a preheat of minimum 150 oC,

a heat input between 10 and 35 kJ/cm and a PWHT at740 oC for two hours show acceptable strength, hard-ness and toughness.

Heterogeneous girth welds between P23 and P24 toP22, welded with P22 filler metals, with a preheat of150 oC and with a PWHT (740 oC 2 hours) showed allacceptable strength, hardness and toughness.

Heterogeneous girth welds P23 or P24 to P91, weldedwith P91 filler metal, with a preheat of 150 200 oC andwith a PWHT (740 oC for two hours) and heat input rang-ing from 18 to 25 kJ/cm showed all acceptable strengthand hardness but low weld metal toughness (low impactvalues and cracking in side bend testing).

7 CREEP TEST RESULTS ON PIPE BASEMETAL AND WELDMENTS

To determine the creep strength, uniaxial creep tests at575 oC with stresses between 170 MPa and 215 MPa(isothermal creep tests) were performed by LABOR-

ELEC and the Belgian Welding Institute. The time torupture was measured and the fracture location wasdetermined by metallographic examination.

Table 9 Chemical composition of welding wires and all-weld metal (homogeneous welds)

C Si P S Mn Cr Mo V W Al B N Nb Ti Cu

P23 - SMAW .06 .22 .017 .007 .46 2.28 .067 .28 1.72 - .002 .017 .043 .001 -Thyssen Cr 2 WV*

P23 - 3 / SMAW .06 .29 .015 .007 .53 2.33 .073 .25 1.23 .001 .002 - .028 .005 -All-weld metal

P23 - SMAW .05 .38 .011 .007 .55 2.12 .08 .23 1.38 .002 .0009 .015 .006 -SAF AL CROMO E223*

P23 - SAW .075 .22 .017 .001 .47 2.19 .09 .23 1.88 .022 .001 .009 .063 .048 -Thyssen Union S1 Cr 2 WV

Flux: UV 430 TTR-W*

P23 - 5 / SAW .068 .44 .010 .006 1.32 2.09 .03 .19 1.32 .013 .001 - .015 .002 -All-weld metal

P23 - SAW .055 .27 .011 .004 .56 1.93 .08 .23 1.25 < .017SAF AL CROMO SF223* .0001

P23 - 6 / SAW .052 .37 .010 .005 .58 2.18 .058 .22 1.23 .024 .001 - .007 .004 -All-weld metal

P24 - SMAW .06 .44 .008 .005 .54 2.37 .96 .23 - - .002 .034 .032Thyssen CrMoVTiB*

P24 - 3 / SMAW .06 .45 .011 .007 .51 2.30 .91 .26 .021 .006 .003 - .010 .023All weld metal

P24 - SMAW .075 .51 .004 .003 .46 2.47 .97 .22 < .002 .0024 .016 .063SAF AL CROMO E224* .005

P24 - 4 / SAW .08 .41 .008 .005 .41 2.41 .99 .22 .016 .007 .002 - .014 .055All-weld metal

P24 - SAW .06 .24 .004 .002 .53 2.39 1.01 .24 - - .0037 - .073 .14Thyssen Union

S1 CrMoVTiB*

Flux: UV430 TTR-W

P24- 5 / SAW .05 .37 .007 .004 1.17 2.23 .92 .21 .025 .017 .001 - .007 .009 .19All weld metal

P24 - SAW

SAF AL CROMO SF224*

P24 - 6 / SAW .07 .52 .012 .006 .54 2.72 1.06 .24 .020 .038 .003 - .017 .032 .05All-weld metal

* Data from fabricators.

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Table 10 Test results on homogeneous P23 pipe welds

SMAW girth weld in P23 pipes (OD 219 30 mm)

Weld no. P23-3

Filler: Union Cr2WVHeat input: 8 - 27 kJ/cmPreheat 150 oC, PWHT 740 oC - 2 hours



Tensile strength BM (MPa) 633 634

Tensile strength girth welds 581 / outside weld in base metal(MPa) / fracture location 578 / outside weld in base metal



Side bends no cracks > 3 mm



Impact base metal (J/cm2

) 207 219 233 / 219Impact HAZ (J/cm2) 208 150 218 / 192

Impact weld metal (J/cm2) 98 77 35 / 70


Required: max. 350 HV10 (power plant applications) max. 320 HV10 (EN 288-3)

HV10 base metal 203

HAZ max. HV10 face 238

HAZ max. HV10 root 233

Weld metal max. HV10 face 231

Weld metal max. HV10 root 241


SAW girth welds in P23 pipes (OD 219 30 mm)

Weld no. P23-5 Weld no. P23-6Filler: Union S1 Cr2WV Filler: AL CROMO SF 223Heat input: 9 kJ/cm Heat input: 10 - 16 kJ/cmPreheat 180 oC, PWHT 740 oC 2 hours Preheat 150 oC, PWHT 740 oC 2 hours




Tensile strength girth welds 577 / outside weld in base metal 563 / outside weld in base metal(MPa) / fracture location 566 / outside weld in base metal 578 / outside weld in base metal

Side bending (EN 910)Required: 120o without cracks of 3 mm in each direction (EN 288-3)

Side bends no cracks > 3 mm no cracks > 3 mm




Impact HAZ (J/cm2) 215 209 220 / 215 228 217 221 / 222




HV10 base metal 203

HAZ max. HV10 face 256 227

HAZ max. HV10 root 231 221Weld metal max. HV10 face 236 207

Weld metal max. HV10 root 222 227

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SMAW girth welds in P24 pipes (OD 370 25 mm)

Weld no. P24-3 Weld no. P24-4

Filler: Union CrMoVTiB Filler: AL CROMO E 224Heat input: 8 - 27 kJ/cm Heat input: 25 - 37 kJ/cmPreheat 150 oC, PWHT 740 oC 2 hours Preheat 200 oC, PWHT 740 oC 2 hours


Required: greater than specified minimum BM (EN 288-3): min. 585 MPa for T24 specified in ASTM A213





Side bends cracks due to porosities no cracks > 3 mm



Impact base metal (J/cm2

) 260 246 273 / 260Impact HAZ (J/cm2) 115 228 129 / 157 216 114 240 / 190




HV10 base metal 212


HAZ max. HV10 root 247 256

Weld metal max. HV10 face 228 231



SAW girth welds in P24 pipes (OD 370 25 mm)

Weld no. P24-5 Weld no. P24-6Filler: Union S1 CrMoVTiB Filler: AL CROMO SF 224Heat input: 9 kJ/cm Heat input: 10 - 20 kJ/cmPreheat 180 oC, PWHT 740 oC 2 hours Preheat 150 oC, PWHT 740 oC 2 hours


Required: greater than specified minimum BM (EN 288-3): min. 585 MPa for T24 specified in ASTM A213



Side bending (EN 910)Required: 120o without cracks of 3 mm in each direction (EN 288-3)

Side bends no cracks > 3 mm no cracks > 3 mm




Impact HAZ (J/cm2) 277 255 109 / 214 235 242 254 / 244




HV10 base metal 212


HAZ max. HV10 root 260 254Weld metal max. HV10 face 247 239


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Creep rupture tests were performed on the deliveredP23/P24 base materials and the homogeneous SMAW

and SAW welded joints (preheat 150 oC; PWHT740 oC 2 hours). For the welded joints, creep speci-mens were taken transversely from the weld.


Table 14 Test results on heterogeneous pipe welds P23 and P24 to P22

SMAW girth welds P23 and P24 pipes to P22

Weld no. P23-7 Weld no. P24-7

P22 Filler 2.5 and 3.25 mm P22 Filler 2.5 and 3.25 mmHeat input: not available Heat input: not availablePreheat 150 oC, PWHT: 740 oC 2 hours Preheat 150 oC, PWHT 740 oC 2 hours


Required: greater than specified minimum BM (EN 288-3): min. 510 MPa for P23 specified in ASTM A335min. 585 MPa for T24 specified in ASTM A213min. 415 MPa for P22 specified in ASTM A335

Tensile strength girth welds 489 / outside weld in P22 514 / outside weld in P22(MPa) / fracture location 496 / outside weld in P22 487 / outside weld in P22



Root bends no cracks > 3 mm no cracks > 3 mm



Impact HAZ P23/P24 (J/cm2) 182 193 203 / 193 91 75 91 / 86

Impact HAZ P22 (J/cm2) 265 257 212 / 245 294 294 294 / 294





in HAZ T91 root 233 253

Table 15 Test results on heterogeneous pipe welds P23 and P24 to P91

SMAW girth welds P23 and P24 tubes to P91

Weld no. P23-8 Weld no. P24-824 layers 21 layersP91 Filler 2.25 and 3.25 mm P91 Filler 2.25 and 3.25 mmHeat input: 18 - 25 kJ/cm Heat input: 18-23 kJ/cmPreheat 200 oC, PWHT: 740 oC 2 hours Preheat 200 oC, PWHT 740 oC 2 hours


Required: greater than specified minimum BM (EN 288-3): min. 510 MPa for P23 specified in ASTM A335min. 585 MPa for T24 specified in ASTM A213min. 585 MPa for P91 specified in ASTM A335

Tensile strength girth welds 623 / outside weld in P23 623 / outside weld in P24(MPa) / fracture location 623 / outside weld in P23 617 / outside weld in P24



Side bends 2 cracks > 3 mm at 180 cracks > 3 mm at 180

Charpy-V impact testing (EN 875)Required: 27 J (34 J/cm2) in WM at room temperature (PED)

Impact HAZ P23/P24 (J/cm2) 187 143 89 / 140 160 125 135 / 140

Impact HAZ P91 (J/cm2) 145 211 145 / 167 179 194 216 / 196





in weld metal root 263 271

Stress levels were based on isothermal creep data at575 oC from Sumitomo data sheet for HCM2S (P23) and

from Vallourec & Mannesmann own analysis for P24. Ascatter band of 20 % is assumed on the average stressvalues.

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7.1 Creep test results and discussion

on P23(-joints)

The isothermal (575 oC) creep test results are shown in

Figures 11 and 12.The P23 base material from Vallourec & Mannesmannhas a higher creep rupture strength at 575 oC thanSumitomos HCM2S and the test results are above themean rupture curve in the Larson-Miller diagram.

The welded joints have lower creep strength than theP23 base material, but fall within the 20 % scatter bandon the average stress values for the HCM2S base mate-rial. Only the SAW joint (Weld no. P23 - 5) shows a neg-ative trend.

The specimens from the SMAW joint (Weld no. P23 - 3)ruptured in the Sub Critical HAZ. The SAW joints

The results are also plotted in the Larson-Miller diagramwith a constant C in the Larson Miller parameter takenequal to 20. In the Larson-Miller diagram the creepresults are compared with the mean and minimum creep

rupture curves for the base material according to thestandards.

For P23, the mean and minimum creep rupture curvesare calculated on basis of the maximum allowablestresses in the ASME Code Case 2199. According toASME Boiler & Pressure Vessel Code the maximumallowable stress must be smaller than 67 % of the meancreep curve for rupture in 105 hours or 80 % of the min-imum creep curve for rupture in 105 hours.

For P24, the mean creep rupture values for a 105 hoursrupture time are found in the VdTV Data Sheet 533.The minimum stress is around 20 % below the specifiedmean stress.

Figure 12 Creep test results


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showed ruptures in the weld metal or Sub Critical/InterCritical HAZ. Ruptures in the weld metal can be theresult of the relative high stresses used. Also the factthat for some welds, a low Nb content was found in the

weld metal could have contributed:Nb content P23-6 SAW: 0.007 %

Nb content P23-3 and P23-5 SMAW: 0.028 %SAW: 0.015 %

Creep data on welded joints are all above the minimumrequired in the ASME Code Case 2199 as can be seenin the Larson-Miller plot (see Figure 12).

7.2 Creep test results and discussion on P24

(-joints)

The isothermal (575 oC) creep results are shown inFigures 13 and 14.

The creep rupture strength of the delivered P24 basematerial confirms the results obtained by Vallourec &Mannesmanns own analysis and is between the mini-mum and mean creep rupture curve in the Larson-Millerdiagram.

The SMAW and SAW welds no. P24-4 and P24-6, havea lower creep strength than the base material with rup-



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tures in the Sub Critical- HAZ, but fall within the 20 %scatter band of the P24 base material and show a pos-itive trend in creep strength. For the SMAW joint P24-4, the specimen tested at 180 MPa broke with a low

reduction of area in the weld metal, due to a weld defect.When plotting the results of the welded joints in theLarson-Miller diagram, one can see that the creep rup-ture strengths fall outside the minimum creep rupturecurve for P24 base material defined in VdTV datasheet 533 for the higher stresses used. At the loweststress-levels the creep rupture strength is near the min-imum curve for P24 base material.

The SMAW and SAW welds no. P24-3 and P24-5, havea creep strength which falls out the 20 % scatter bandof the P24 base material. A large scatter in rupture timesoccurred.

This is probably due to the Ti which is not always homo-

geneously distributed in the weld metal at different loca-tions in the welded joint, resulting in rupture in the weldmetal with a scatter in rupture times.

The joints P24-4 and P24-6 showed a higher Ti contentin the weld metal than welds no. P24-3 and P24-5,resulting in a better creep strength.

Ti-content P24-4 and P24-6:SMAW: 0.055 % SAW: 0.032 %

Ti-content P24-3 and P24-5:SMAW: 0.023 % SAW: 0.009 %

7.3 Fracture location

For lower service related stresses (and thus longer rup-ture times) the fracture location of the creep specimenwill probably move to the intercritical HAZ (IC-HAZ). Aswith all welded joints in ferritic/martensitic CrMo steels,a lower hardness of the microstructure and lower creepstrength is expected due to the thermal weld cycle. Inthe IC-HAZ, between the Fine Grained HAZ and theuninfluenced base material, of CrMo welds there is onlya partial austenitic transformation due to the heat-inputduring welding. As a result, not all carbides go in solu-tion and during the temperature treatment after weldingoccurs over-ageing: this means that there is a carbidecoarsening and the carbides will coagulate. The resultis a softer microstructure with lower creep properties.

7.4 Conclusions

Lower creep rupture strengths are obtained in the P23SMAW and SAW welded joints than the delivered P23base material, but with creep rupture strengths abovethe minimum required for P23 base material in theASME Code Case 2199. Ruptures occurred either in theweld metal or SC-HAZ.

Lower creep rupture strengths are obtained in P24SMAW and SAW welded joints than the delivered P24base material, but within the 20 % scatterband of thebase material. For lower stresses, the creep rupture

strengths move towards the minimum requirements forP24 base material in the VdTV data sheet 533 withruptures in the SC-HAZ.

Creep results within the 20 % scatterband of the P24base material can be obtained in P24 SMAW and SAWwelded joints, if the Ti in the weld metal can be kept atan acceptable level (above 0.030 %), otherwise the joint

has not enough creep strength with rupture in the weldmetal and with a large scatter in rupture times.

For the relatively high stresses used, ruptures occurredin the weld metal. This is an indication that the weld metalhas a lower creep strength than the base materials.

For lower, service related stresses (and thus longer rup-ture times) the fracture location of the creep specimenwill probably move to the intercritical HAZ.

8 OVERALL CONCLUSIONS

Both investigated new 21/4Cr-Steel tubes and pipes are

very promising high temperature materials for use in powerplants. The base metals fulfil the standard requirementsand appropriate welding consumables are available.

Care must be taken when welding T23 and P23 due tothe susceptibility to reheat cracking but this type ofembrittlement can easily be avoided by using an appro-priate welding technique.

Weld simulation tests and tests on real weldments haveshown that adequate hardness and toughness can beobtained in tube welds of both T23 and T24 materialwithout preheat and without PWHT. Also heterogeneoustube welds (T23 and T24 to T91 with T23 or T24 fillermetal, or to 316L with Inconel 82 filler) showed all

acceptable strength and toughness.Also for the thick walled pipes, the hardness stays below350 HV, even in the as welded condition. However, fortoughness reasons, a PWHT at a temperature of about740 oC is necessary. Both homogeneous and hetero-geneous welds (P23 or P24 to P22), welded with P22filler metal, showed acceptable strength, hardness andimpact toughness. Heterogeneous welds (P23 and P24to P91), welded with P91 filler metal, showed acceptablestrength and hardness, but low weld metal toughness.

Adequate creep strength can be obtained in the homo-geneous pipe welds, providing the Nb and Ti level iskept sufficiently high in the deposited weld metal.

REFERENCES

[1] Arndt J., Haarman K., Kottmann G., Vaillant J.C.,Bendick W., Kubla G., Arbab A., Deshayes F.: The T23/T24Book. New grades for waterwalls and superheaters,Information book from Vallourec & Mannesmann Tubes,1998.

[2] Gabrel J.: Development of T and P23 steel grade,Vallourec and Mannesmann Tubes, Cost 522 SteamPower Plant Progress Report No. 04, 2003.

[3] Nawrocki J.G.: Stress relief cracking of a ferritic alloysteel, IIW Document IX-2001-01.

[4] Heuser H., Jochum C.: Characterization of matchingfiller metals for new ferritic-bainitic steels like T/P 23 andT/P 24, Thyssen Welding Germany.

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