CRACKING OF RECOVERY BOILER COMPOSITETUBES – THERMAL FATIGUE OR STRESS

CORROSION CRACKING?

Hannu Hänninen1 *), Pekka Pohjanne2), Pekka Saarinen1) and Timo Kiesi1)

1)Helsinki University of TechnologyLaboratory of Engineering Materials

P.O. Box 4200FIN-02015 HUT

2)VTT Manufacturing TechnologyP.O. Box 1704

FIN-02044 VTTFinland

ABSTRACT

In the composite tubes of black liquor recovery boilers cracking and corrosion attack hasbeen observed, especially in the floor tubes. Cracks typically initiate in the stainless steelcladding and penetrate through the cladding to the stainless steel/low alloy steel interface,where the cracks normally propagate along the interface and do not penetrate to the lowalloy steel. Cracking occurs also on lower walls, smelt spout openings and air ports andsometimes penetration to low alloy steel base metal has also occurred. The crackingmechanism has been suggested to be either stress corrosion cracking or thermal fatigue. Aspecial case of cracking has been the situation where cracks are present on the inner surfaceof the tubes, when cracking is also starting from the water side. The environmental,thermal, mechanical and metallurgical effects on these cracking phenomena are evaluatedbased on these two different mechanisms of cracking. Special attention is paid to thepossible methods for mitigation of the recovery boiler cracking failures, e.g. by operatingprocedures or materials selection.

*On leave at Institute for Advanced Materials, Joint Research Centre PettenSite, P.O. Box 2 - 1755 ZG Petten, The Netherlands

1. INTRODUCTION

Black Liquor Recovery Boilers (BLRB) are an essential and critical component in pulpmills. Because of the aggressive boiler atmosphere, composite tubing of stainless steel oncarbon steel are used in the critical areas of lower boiler furnace. The composite tubes aretypically made by hot co-extrusion followed by cold rolling, annealing and straightening.Recently, the corrosion resistant overlay is also applied to the carbon steel tube by welding.In boiler manufacturing the tubes are welded to panels, which are adjusted and welded toparts of the boilers. The boilers have built-in residual stresses from manufacturing.Additional stresses arise from in-service loading due to high and often fluctuating operatingtemperature and internal tube pressure. Cracking and corrosion of AISI 304L austeniticstainless steel cladding of composite floor tubes including the wall-tube bends underneaththe smelt bed have been a problem for more than 10 years, which has increased themaintenance costs and lowered the BLRB availability [1-10]. This kind of corrosionproblem exists also in air ports, smelt spout openings and recently in wall tubes betweenfloor and primary air port level being of great concern to pulp mills because of potentialtube failure in the lower furnace that may result in a serious smelt-water explosion.

Over 50% of the Finnish recovery boilers having composite furnace floor tubes haveexperienced cracks in the furnace floor area to some extent. Typical for the floor tubecracking is that cracks can be found over the whole floor surface and it is not possible tofind any special areas where they are concentrated. Cracks have been observed in old aswell as in new BLRBs and no correlation has been observed between differentmanufacturers. 25% of the boilers have experienced pitting and wastage corrosion in thesame areas [4]. Recent findings from North America suggest that in the sloped-floor boilersthe most common areas for cracking are adjacent to the spout wall and in the smelt runareas adjacent to the side walls. Whereas in decanting-floor boilers cracking is more oftenrandomly distributed over the floor [8].

This paper summarises the main ideas and approaches concerning cracking and corrosionmechanisms in BLRBs by characterising the cracking of composite boiler tubes,determining the environments and residual as well as thermal stresses and evaluates thesuitability of some new alternate materials for use in the lower furnace of the BLRBs. Thesecond aim is to provide practical information about durability of materials in differentoperating conditions and situations that occur in BLRBs. Furthermore, purpose is to givematerial and manufacturing solutions as well as recommendations for safe operatingprocedures to improve the maintenance of BLRBs.

2. CRACKING MECHANISMS

Floor tube cracks appear typically in the crown of the tube, on the sides of the tube andclose to or along the toe of the tube-to-fin welds, Figs 1a, b. Cracks initiate in the stainlesssteel cladding and penetrate transgranularly the cladding with little branching to thestainless steel/low alloy steel interface, where the cracks normally end with a corrosion pit(contains mainly S) or grow along the interface, but do not penetrate into the low alloysteel, Fig. 1c,d. Cracks propagating along the interface impede heat transfer and produce acrevice where corrosion can be enhanced. In the worst case delamination and spalling ofstainless steel layer can occur resulting in exposure of the carbon steel to the boileratmosphere. In some cases (e.g., transverse cracking at primary air ports) cracks maycontinue past the stainless steel/carbon steel interface into the carbon steel (Fig. 1e, [11])and then tubes typically show severe overheating and in some cases cracking exists also onthe inner water-side surfaces of the tubes, Fig. 1f. In addition, typically transverse crackingis observed in the AISI 304 type steel fins penetrating the solid fins. These commoncracking patterns and orientations reflect the stress states at the surface of the tubes at thetime cracks form (perpendicular to the tensile main stresses).

In part of the cases cracking clearly relates to thermal fatigue (clear indications ofoverheating and in the worst case cracking on the inner water-side surface of the tubes [9]).But in many of the cases cracking seems to be environment assisted cracking (EAC) andhas features typical of transgranular stress corrosion cracking (TGSCC) of austeniticstainless steels [1,3,4,6]. Since no clear consensus has been achieved on the crackingmechanism, laboratory studies have been performed to identify the conditions under whichSCC and thermal fatigue occur on composite tubes.

2.1 Thermal fatigue

Thermal fatigue is a damage mechanism, which occurs at stresses over the yield strength ofa material. These stresses are developed when the free expansion or contraction by cyclicheating or cooling is restricted. Most often it is the surrounding material that restricts theexpansion or contraction. Thermal fatigue resistance is noticed to be better when the elasticmodulus and the coefficient of thermal expansion (CTE) are small, and when the yieldstrength, toughness and thermal conductivity are high. High strength and toughness aredesired material properties, but they very rarely exist at the same time. Normally carbonsteel and stainless steel, which have different CTEs, are joined together in a compositetube. When the temperature of the composite tube increases, for example, due to steam

blanketing inside the tube, the tube materials tend to expand according to their CTEs. Whenthe materials are joined together, the free expansion is restricted, and thus, thermal stressesare developed.

Thermal fatigue has been considered as one reason for cracking of recovery boilercomposite tubes. The conditions where composite tubes operate are not constant but canvary very much. The solidified layer of smelt against the recovery boiler floor tubes cantemporarily melt for chemical reasons or crack mechanically, e.g. due to fall ing salt cakesfrom the convective section of the boiler, causing high temperature liquid smelt to touch thefloor tubes or steam blanketing to occur inside the tubes. Temperature excursions producethermal gradients, which create thermal stresses in the composite tubes. When thetemperature spikes are high enough and are repeated many times, cracks may finally formin the cladding of a composite tube in the aggressive environment of BLRBs. Temperaturemeasurements of a number of boiler floors indicate that short (<3 min) temperature spikessignificantly (100…300 °C) above the normal operating temperature (300 °C), especiallynear the crown of the floor tubes, really take place. This has not been considered to be thesole cause of cracking, because on the basis of the in-situ temperature measurements nodirect relationship between floor tube cracking locations and thermal spike frequency hasbeen obtained. An insufficient number of high enough thermal cycles for thermal fatiguecracking to occur on the floor tubes takes place as compared to appropriate ASME SectionII I design curve for isothermal fatigue at 427 °C [12]. The existing thermal fatigue dataseems to fall within the S-N curve regardless of the peak temperature of the test cycle(<650 °C max). There is, however, no real thermal fatigue data obtained in BLRBenvironmental conditions. Often also TEM analyses of the dislocation structures are madeto evaluate the fatigue damage based on the cellular dislocation structure [5,6]. However,recent studies showed that the thermal fatigue laboratory test samples and cracked tubesfrom boilers have similar dislocation structures (moderate dislocation density with evendistribution showing tangled structure because of high amount of dislocation loops andsmall lattice defects pinning the dislocations) [9,13]. Cellular dislocation structures can befound only in very severe cases of cracking including marked overheating and possiblecracking also on the inner surface of the tubes. Thus, this microstructural feature can not beconsidered to be a reliable indication of the presence of thermal fatigue.

2.2 Stress corrosion cracking

Stress corrosion cracking takes place when certain aggressive environment exists above acritical temperature at the same time with sufficiently high tensile stress in the material.Stresses causing TGSCC failure of the stainless steel cladding can be residual stresses from

fabrication or stresses having their origin in the difference of CTE of austenitic stainlesssteel cladding and carbon steel core. It has been postulated that SCC during shut-down start-up cycle combined with water washing is one possible mechanism, because aggressivesolutions form by reactions of water and the solidified smelt [1,3,4,6]. AISI 304L typeaustenitic cladding has high tensile stresses after the shut-down, because of different CTEs ofthe carbon steel and the austenitic stainless steel. Stress analyses show that, when compositefloor tubes are cooling from the normal operation temperatures (about 300 oC) tensilestresses in AISI 304L type austenitic stainless steel cladding exceed the yield stress of thematerial at about 220 oC and remain during the whole outage period [8,14].

It is nowadays well known that AISI 304 type steels can crack very rapidly at a low stresslevel (above 1/3 of yield stress) when exposed to a solution containing sodium sulphide(Na2Sx9H2O) or to a concentrated aqueous solution of smelt at temperatures of150…200 °C [8,10,15]. Pure anhydrous Na2S is hygroscopic and in solution it is highlyalkaline. Cracking is able to occur in the solutions of Na2S alone (essential chemicalcompound for cracking) or combined with sodium hydroxide (NaOH) and/or sodiumcarbonate (Na2CO3), i.e., in chemicals of which the smelt consists (NaOH can form byoxidation of Na2S during water wash in smelt solution, by hydrolysis of Na2S or dissolutionof Na2CO3 in water [15]). These smelt solutions have high dry-out temperature (180 °C),and solubil ity of Na2S in water increases with temperature. The dry-out temperature is stillincreased with NaOH (hygroscopic and prevents dehydratation of Na2S) up to 250 °C. In ahighly concentrated and saturated solution of Na2S and NaOH SCC has been demonstratedat temperatures as low as 60 °C in SSRT tests [8,10]. Therefore, if the smelt bed is notcompletely removed during the water wash, the smelt solution may become concentratedand further enrich in Na2S as the temperature increases in boiler start-up. The hydratedsmelt is able to cause cracking in the temperature range below 200 °C (cracking may not beexpected below 150 °C).

In addition, stress analyses and on-line material temperature measurements of floor tubesshow that high tensile stresses may be locally present in the cladding also during operation[4,10,14]. A short-term material temperature transient with a peak temperature of 400 oC isenough to cause tensile stresses that equal the yield stress of AISI 304L type austeniticstainless steel cladding at a typical floor tube operating temperature of 300 oC [14].Therefore, SCC may also be a possible cracking mechanism during operation, provided thatsome kind of liquid and/or gas phase, specific to TGSCC of austenitic stainless steels, isstable and adjacent to the floor tubes during operation.

Chemical enrichment of K, S and Cl close to the tubes may lead to a molten smelt andcreate conditions for SCC in boiler operating conditions, since these elements lower themelting temperature of the smelt [16, 17]. This makes the frozen layer thinner and easierfor the smelt to penetrate. These elements decrease also the viscosity of the smeltenhancing its flow. Therefore, conditions for SCC to occur may exist also during normaloperation conditions of the boiler. However, there is some experience available that nocracking has been observed in composite tube floors, which have not been exposed to waterwashing during outages.

4. THERMAL FATIGUE STUDIES

Thermal fatigue tests for composite tubes have been carried out with a testing device basedon high frequency (150-200 kHz) induction heating, which heats and cools the tuberepeatedly, without dwell times, in order to produce temperature gradients to the compositetubes. The test piece of 120 mm long composite tube was located inside the heating coilduring the heating. The temperature of the composite tube during the test was monitoredwith thermocouples to simulate the thermal cycles measured from real boiler floors. Whenthe surface of the tube reached the desired temperature it was immediately automaticallytransferred to water spray cooling. After cooling to a certain temperature, the tube wastransferred back to the heating position. Because the induction heating affected the claddingas well as the carbon steel, the inside surface of the carbon steel tube was continuouslycooled by pressurized air. Without the air cooling, the temperature of the inner surfacewould have increased to a high value. This would have resulted in internal cracking of lowalloy steel of the sample, which is sometimes also observed in severe cracking cases in realboiler floor tubes [13]. The temperature in the inner surface changed in the same phase withthe temperature in the outer surface, but the temperature range in the inner surface wasmuch smaller in the tests. In the real boilers steam blanketing can result in smaller thermalgradients across the tube wall and cracking in the inner water-side surface of the tubes canalso take place. The purpose of the tests was to heat and cool the cladding surface of thetube in the temperature range of 300…700 °C. The stresses/strains in the short compositetube samples after the tests at room temperature were, however, different from the residualstresses in the real BLRB composite floor tubes after the operation of a boiler, which arehighly tensile.

AISI 309 weld overlay contained rather large cracks after thermal fatigue test similar toAISI 304L (Fig. 2a). Alloy 825 weld overlay exhibited rather small cracks after the test(2000 cycles, 300…700 °C). Alloy 625 weld overlay contained the smallest cracks among

the welded as-fabricated overlays. In annealed alloy 625 weld overlay cladding, the crackswere the smallest of all of the tested materials. However, the number of small cracks in theannealed material was higher than in the as-fabricated welded alloy 625. Co-extruded 3R12and Sanicro 38 materials contained cracks which proceeded straight to the half thickness ofthe cladding. Sanicro 63 material exhibited the smallest cracks among all as-fabricated co-extruded composite tubes. The cracks were slightly smaller than in the as-fabricated weldoverlay 625 material, but the number of cracks was clearly higher in Sanicro 63. Sanicro 65material, on the other hand, contained large cracks, which proceeded mainly transgranularlystraight from the surface to the half thickness of the cladding. Composite tube 3RE28exhibited the largest cracks among all materials. The largest crack proceededtransgranularly through the whole cladding to the interface and continued to the carbonsteel (Fig. 2b). The chromized tube contained quite large cracks, which proceeded from thesurface of the tube to the base material. The cracks in carbon steel (St 45.8 III) proceededfrom the surface of the tube towards the inner surface, but they were not so large as inmany composite tubes. The crack depths of all materials were compared to each other. Thenumbers of cracks of certain size after thermal cycling (300…700 °C) of 2000 cycles areshown in Fig. 3 exhibiting the distribution of cracks in each material.

5. STRESS CORROSION CRACKING

SCC of composite tube materials in shut-down conditions, i.e., when the boiler is waterwashed, or especially during the start-up, when the floor is not completely cleaned fromsolidified hydrated smelt after washing, was investigated with SCC tests in different typesof hydrated Na2S – Na2CO3 salts. Hydrated salt mixtures were chosen, because recentstudies have shown that an aggressive alkaline sulphide containing environment, capable tocause SCC in stainless steel at elevated temperatures, can form on boiler floor duringwashing, when the washing water reacts with the solidified smelt [8,15].

The test results show that SCC risk during start-up cycle should not be under-estimated ifhydrated solidified smelt or smelt chunks and washing water are left on the floor afterwashing. The start-up was simulated by heating the hydrated salt mixtures from roomtemperature up to 150...250 oC. During tests the crystal water was allowed to be releasedand the salt mixtures to dry.

AISI 304 type stainless steel was susceptible to SCC in various sodium sulphide(Na2Sx9H2O), sodium carbonate (Na2CO3), sodium hydroxide (NaOH) and sodiumsulphate (Na2SO4) salt mixtures, Tables 1 and 2 [18]. Cracking did not occur when

sulphide content was low or the carbonate-based salt contained no sulphide. Tests indicatedalso that the nickel-base Alloy 625 has the best resistance against SCC in hot moist sodiumsulphide containing salts of the new alternative composite tube materials. The second bestcomposite tube material was Alloy 825. It was susceptible to SCC in pure sulphide and insulphide salt mixtures containing sodium hydroxide, but it was resistant in Na2S-Na2CO3

salt mixture, that is closer to the real boiler environment, e.g. than the pure sulphide. Theplain carbon steel (pressure vessel steel TStE 355, DIN 17102) was resistant to SCC inNa2S and Na2S-Na2CO3 salt mixtures, where both AISI 304 and Alloy 825 cracked. This isa beneficial result considering the composite tube structure, i.e., if the corrosion resistantcladding fails by SCC it does not necessarily mean that the cracks proceed into the pressurecarrying C-steel. In boilers this may be an indication of the mechanism of cracking, i.e.SCC cracks stop at or grow along the interface, but thermal fatigue cracks are able tocontinue past the interface.

Table 1. Summary of the SCC tests performed in various hydrated salt mixtures insimulated start-up conditions. All water originates from hydrated Na2S x9H2O. (U-bendspecimens, test duration 20...25 h, temperature from 25 to 200oC)

Salt mixture - percentage from dry solids Results

Na2S

[%]

Na2CO3

[%]

NaOH

[%]

Na2SO4

[%]

Conc.

[g/lH2O]

AISI

304

Alloy

825

Alloy

625

C-steel

100 - - - 480 SCC SCC*) No SCC No SCC

20 80 - - 2400 SCC No SCC --- ---

33.3 33.3 33.3 - 1445 SCC SCC*) --- ---

12 53 12 23 4090 SCC SCC**) --- ---*) Small cracks and **) Incipient cracks

Table 2. Effect of sodium sulphide content and moisture to SCC behaviour in simulatedstart-up conditions. (U-bend specimens, test duration 5...6 h, temperature from 25 to230...270oC)

Salt mixture -

Percentage from dry solids

Results

Na2CO3

[%]

Na2S

[%]

Conc.

[g/lH2O]

AISI 304 Alloy

825

Alloy

625

C-steel

100 0 590A) No SCC No SCC No SCC No SCC

90 10 4827B) No SCC --- --- ---

80 20 2407B) SCC --- --- ---A) Na2CO3 x10H2O and B) Na2S x9H2O

SCC of composite floor tubes may occur during operation after thermal cycling (tensilestress state present at operating conditions) due to some aggressive sulphur species presenton the floor tubes. Slow strain rate tests (SSRT) performed with plate materials in somealkali-polysulphide-alkali chloride melts at 300 oC showed that AISI 304L type austeniticstainless steel was slightly susceptible to SCC when exposed to alkali polysulphide-alkalichloride mixtures under argon atmosphere with or without chlorides. In the sameenvironments Alloy 825 was resistant to SCC. [17,18]

7. SUMMARY

There is plenty of evidence that floor tube cracks can initiate and form very rapidly bystress corrosion cracking mechanism in hydrated smelt or concentrated water washsolutions of Na2S and NaOH or Na2CO3. In boiler operation cracking occurs when tensilestresses are present and boiler floor tubes are sufficiently hot (>150 °C). This susceptibilitywindow for cracking should be considered very carefully in the boiler operating proceduresfor water washing, dry-out fires or start-up when the floor is heated above 150 °C while incontact with hydrated smelt or concentrated washing water. For avoiding SCC of floortubes in water wash solutions under the frozen smelt, i.e. saturated hydrated Na2S solutions,or in contact with moist smelt, the critical temperature range (150…200 °C) should beavoided and water wash should be started below 150 °C tube temperature, before start-upfloor should be clean of moist hydrated smelt and drying should be made below 150 °C. Atpresent, the critical temperature of 150 °C for AISI 304 type steel floor tubes is still notwell defined and confirmed.

Thermal cycling of floor tubes causes tensile stresses at operating temperature, but at themoment there is only preliminary evidence of SCC in the operating environments, i.e. inthe smelts possibly enriched of S, K or Cl. If enough thermal transients of more than100 °C exist in the same area of the floor in the aggressive floor environmental conditions,thermal fatigue is certainly possible and its presence is especially evident from the thermaldegradation of the carbon steel tube parts and in some cases from cracks on the inner water-side surface of the tubes.

A number of new alternative composite tube materials, having higher yield strength thanthat of AISI 304 type steel and a CTE value more closely matched to that of carbon steelbase metal, are more resistant (but not immune) both to SCC and thermal fatigue crackingthan the conventional AISI 304L type composite tubes. Alloy 825 can be a good choice forfloor tubes, although it is not completely resistant to SCC in simulated boiler shut-downstart-up tests its resistance is much better than that of AISI 304 steel. And the filedexperience shows no incidence of cracking of Sanicro 38/Alloy 825 at the moment. Ingeneral, weld overlay Alloy 625 and co-extruded Sanicro 63 have turned out to be the mostresistant materials. Thus, they may be considered in places where Alloy 825 is notapplicable.

ACKNOWLEDGEMENT

TEKES, Finnish Recovery Boiler Committee, Andritz-Ahlstrom Oy, Kvaerner Pulping Oy,Savcor Oy and VTT Manufacturing Technology, which is gratefully acknowledged, havefunded this study. The authors want also to thank to the steering committee of SOMAproject for useful discussions and Sandvik AB and Welding Services Inc. for supplying testmaterials.

REFERENCES

1. Pohjanne, P., Hänninen, H., Mäkipää, M. & Ehrnsten, U., TAPPI 7th Int. Symposium onCorrosion in the Pulp & Paper Industry, Orlando, FL, November 16-20, 1992, pp. 259-265.

2. Ingevald, S. & Kiessling, L., ibid., pp. 285-297.3. Hänninen, H., 30 Years Recovery Boiler Co-operation in Finland, Finnish Recovery

Boiler Committee, Helsinki, Finland, 1994, pp. 121-132.

4. Hänninen, H., Pohjanne, P. & Nieminen, P., Proceedings of International Symposiumon Plant Ageing and Life Predictions of Corrodible Structures. Sapporo, Japan, May15-18, 1995, pp. 529-539.

5. Mäkipää, M., Salonen, J., Nenonen, P. & Hakkarainen, T., TAPPI 8th Int. Symposiumon Corrosion in the Pulp & Paper Industry, Stockholm, Sweden, 1995, pp. 189-197.

6. Keiser, J., Taljat, B., Wang, X-L., Maziasz, P.J., Singbeil , D.L. & Prescott, R.,Proceedings of the TAPPI 1996 Engineering Conference, TAPPI Press, Atlanta, GA,U.S.A., 1996, pp. 693-705.

7. Singbeil, D., Prescott, R., Keiser, J. & Swindeman, B., Proceedings of the 1997 TAPPIEngineering Conference, TAPPI Press, Atlanta, GA, U.S.A., 1997, pp. 1001-1023.

8. Keiser, J.R., Sarma, G.B., Wang X.-L., Hubbard, C.R., Swindeman, R.W., Singbeil ,D.L. & Singh, P.M., Proceedings of the 2000 TAPPI Engineering Conference, TAPPIPress, Atlanta, GA, U.S.A., 2000, 11 p.

9. Saarinen, P., Hänninen, H., Tuiremo, J. & Salmi, K., Proceedings of the 2000 TAPPIEngineering Conference, TAPPI Press, Atlanta, GA, U.S.A., 2000, 13 p.

10. Keiser, J.R., Taljat, B., Wang X.-L., Swindeman, R.W., Maziasz, P.J., Meyers, L.E.,Thomas, R.L., Elliott, S.T., Singbeil, D.L., Prescott, R. & Gorog, J.P., Pulp & PaperCanada, Vol. 101, No. 4, 2000, pp. 20-26.

11. Oldestam, T. 1987 Kraft Recovery Operations Seminar, Orlando, FL, U.S.A., 1987,12 p.

12. Swindeman, R.W., Keiser, J.R., Maziasz, P.J. & Singbeil, D.L., 9th Int. Symposium onCorrosion in the Pulp & Paper Industry, Ottawa, Canada, 1998, pp. 209-212.

13. Saarinen, P., Hänninen, H. & Nenonen, P., Proceedings of the 2000 TAPPI EngineeringConference, TAPPI Press, Atlanta, GA, U.S.A., 2000, 17 p.

14. Taljat, B., Zacharia, T., Keiser, J. R. and Wang, W-L., Tappi J., Vol. 82. No. 12, 1999,pp. 99-108.

15. Prescott, R., Eng, P. & Singbeil, D.L. 9th Int. Symposium on Corrosion in the Pulp &Paper Industry, Ottawa, Canada, 1998, pp. 185-188.

16. Klarin, A., Tappi J., Vol. 76, No. 12, 1993, pp. 183-188.17. Mäkipää, M., Oksa, M. & Pohjanne, P. Proceedings of the 2000 TAPPI Engineering

Conference, TAPPI Press, Atlanta, GA, U.S.A., 2000, 11 p.18. Pohjanne, P., Saarinen, P., Mäkipää, M., Oksa, M. & Hänninen, H., VTT Symposium

BALTICA V, 6-8 June, 2001, 12 p.

a) b)

c) d)

e)f)

Figure 1. Cracking in AISI 304L composite floor tubes. Transverse (a) and craze(mosaic) (b) cracking patterns in austenitic AISI 304L, transgranular cracks stop ator grow along the interface of cladding (c, d) as well as can continue past theinterface to low alloy steel (e). (f) Cracking on the inner water-side surface of thetube in the region where extensive cracking is also present in the cladding (signs ofoverheating were also present based on widening of the decarburised zone).

a) b)Figure 2. Cracks in the cladding of a) AISI 309 weld overlay and b) co-extrudedaustenitic stainless steel (3RE28) after thermal fatigue testing (300-700 oC, 2000cycles).

-20

20-4

0

40-6

0

60-8

0

80-1

00

100-

200

200-

300

300-

500

500-

1000

1000

-200

0

2000

-

625/2 (weld overlay, annealed)

625/1 (weld overlay)

St45,8 III

San38

3R12

3RE28

0

20

40

60

80

100

120

140

160

180

Num

ber

of c

rack

s

Crack depth (µm)

Depth distribution o f cracks (300-700 ° C, 2000 cyc les)

625/2 (weld overlay, annealed)San63/1625/1 (weld overlay)825/1 (weld overlay)St45,8 III309/1 (weld overlay)San38San65/13R12Cr-tube3RE28

Figure 3. The summary of crack depths in the composite tube materials examinedafter thermal fatigue cycling.

a) b)

Figure 4. Cracking of AISI 304 U-Bend specimens after SCC tests in hydrated Na2S.(Heating from 20 oC to 220 oC, test duration 25 hours of which 19 hours above 180oC)