Materials and Design 32 (2011) 2763–2770

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Materials and Design

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Microstructure and mechanical properties of dissimilar materials jointsbetween T92 martensitic and S304H austenitic steels

Jian Cao a, Yi Gong a, Kai Zhu a, Zhen-Guo Yang a,⇑, Xiao-Ming Luo b, Fu-Ming Gu b

a Department of Materials Science, Fudan University, Shanghai 200433, PR Chinab Shanghai Institute of Special Equipment Inspection & Technical Research, Shanghai 200062, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 8 September 2010Accepted 4 January 2011Available online 8 January 2011

Keywords:MicrostructureMechanical propertiesDissimilar materials joints

0261-3069/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.matdes.2011.01.008

⇑ Corresponding author. Tel.: +86 21 65642523; faxE-mail address: zgyang@fudan.edu.cn (Z.-G. Yang)

In this paper, T92 martensitic steel and S304H austenitic steel were welded by gas tungsten arc welding(GTAW) process. Microstructural features and mechanical properties of T92 and S304H dissimilar mate-rials joints were investigated. The results showed that the part of the joints with relatively weak tensilestrength was T92 coarse-grained heat affected zone (CGHAZ), while the part of the joints which revealedrelatively weak toughness was weld metal. The decrease of tensile strength in T92 CGHAZ was due to itscoarse tempered martensite structure. Weak toughness of the joints was resulted from the coarsedendritic austenite of the weld metal. However, the weld metal in transverse direction of the jointswas provided higher tensile strength by the orientation distribution of grains compared with T92 CGHAZ.

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1. Introduction

Increased heat efficiency and improved environmental protec-tion are always the innovative driving forces in the developmentof ultra supercritical (USC) boilers for fossil power plants, whosesteam temperature is up to 600 �C and pressure exceeds 27 MPa.Under this USC condition, the heat efficiency can rise to around45%, compared with the value as 41% of supercritical (SC) boilers.However, with the increase of steam parameters, requirementsfor the materials applied in the USC boilers components arebecoming higher. Thus, many new generation steels have beendeveloped in recent years, including T92 (9Cr0.5Mo2WVNb)martensitic steel and S304H (18Cr9Ni3CuNbN) austenitic steel.

T92 steel was developed by the Nippon Steel Corporation ofJapan [1] in the late 1990s by modifying chemical compositionsupon T91 (9Cr1MoVNb) for even more preferable mechanicalproperties at high temperatures. This steel has the manufacturer’sdesignation as NF616 (ASTM Stands A213) and contains 0.5% Mo,1.8% W, as well as small additions of Nb, V and B. Creep strengthof T92 at 600 �C increases about 10–20% compared with that ofT91 at 600 �C [2–13]. S304H steel was developed by Sumitomo Me-tal Industries Ltd on the base of TP304H (0Cr19Ni10). As a newtype of austenitic steel, S304H possesses not only excellent resis-tance to high-temperature corrosion and steam oxidation mainlydue to high Cr content, but also superior creep strength than mar-

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tensitic steels [14–17]. Thus, it is widely used for superheaters andreheaters, which have the abominable service environment in USCboilers. Normally, T92 steel can be used as pipes linking superheat-ers and reheaters. In this case, the welding between T92 and S304Hsteels will be necessary.

Until now, many researches have mainly focused on the proper-ties of T92 and S304H steels. As for T92 and S304H dissimilarmaterials joints, however, there almost hasn’t any report aboutit. Since T92/S304H dissimilar materials joints is obtained by usingmelted filler material to join two steels, the melted filler materialwill re-crystal to form the weld metal part of the joints after weld-ing. In addition, due to the effect of welding thermal cycles, notonly the microstructure of T92 adjacent weld metal but also themicrostructure of S304H adjacent weld metal will both changeduring the welding process. Considering that the mechanical prop-erties of the joints are closely linked with its microstructure. Thus,an in-depth insight into the structure–property relationships ofT92/S304H dissimilar materials joints may have great significancesfor both the dissimilar steels welding process between new gener-ation martensitic and austenitic steels, and the safety of the USCboilers. In our work, on the one hand, mechanical properties ofT92/S304H dissimilar materials joints were carried out throughtensile and impact tests. On the other hand, the microstructuresacross the entire joints were also investigated. What’s more, thedetailed mechanism governing the microstructural evolution ofthe joints during welding process was analyzed by means of theelectron back-scattered diffraction (EBSD) technique, which wasfirstly used to study the process of grain structure developmentof dissimilar materials joints.

Fig. 2. Schematic diagram of welding process of T92/S304H dissimilar materialsjoints.

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2. Experimental

2.1. Materials and welding procedure

Scales of the two base materials T92 and S304H used in thepresent investigation were 57.15OD � 5.08 mm and 57OD �4.5 mm in thickness respectively. Heat treatment conditions ofthem were described below: 1) T92: austenitization was carriedout for 20 min at 1050 �C and then tempering for 60 min at760 �C. 2) S304H: solution treatment at 1100 �C, followed by watercooling until room temperature. Fig. 1 was schematic diagram ofheat treatment processes of T92 and S304H steels. AWS ERNiCr-3(corresponding to INCONEL 82/182) was chosen as the filler mate-rial. The chemical compositions of all three materials are given inTable 1. The T92/S304H dissimilar materials joints were weldedby means of gas tungsten arc welding (GTAW) with pure argongas (Ar) as the shielding gas. The arc voltage and the arc currentused in welding process were 14 V and 230A (type: balance)respectively. The argon purity used for GTAW in this experimentis 99.99%. Fig. 2 was the schematic diagram of the welding processand the part marked with dotted line in Fig. 2 represented thewelding region. After welding, post welding heat treatment(PWHT) was carried out for 1 h at 730–760 �C to eliminate thewelded residual stress. Fig. 3 was the schematic diagram of thePWHT process of T92/S304H dissimilar materials joints and Fig. 4was the schematic diagram of T92/S304H dissimilar materialsjoints.

Fig. 3. Schematic diagram of post welding heat treatment process of T92/S304Hdissimilar materials joints.

2.2. Test methods

Tensile and impact tests of T92/S304H dissimilar materialsjoints were done respectively at room temperature according tothe ASTM E8-04 [18] and E23-02a [19] standards. Optical micros-copy (OM) was used to observe the metallographic microstructuresacross the joints. Three etching solutions were used: (1) T92: picricacid (2, 4, 6-trinitrophenol) 2.5 g, ethanol 20 ml and H2O 20 ml; (2)weld metal: FeCl3 5 g, HCl 20 ml and H2O 20 ml; (3) S304H: CuSO4

4 g, HCl 20 ml and ethanol 20 ml. PHILIPS XL30FEG scanning elec-

Fig. 1. Schematic diagram of heat treatment processes of T92 and S304H steels.

Table 1Chemical compositions of the base materials and filler material samples (wt.%).

Material C Cr Mo V Nb Ni Mn

T92 sample 0.11 8.76 0.36 0.21 0.059 0.25 0.46S304H sample 0.09 18.38 / 0.040 0.49 8.91 0.82ERNiCr-3 filler 0.030 20.0 / / 2.40 72.5 2.90

Fig. 4. Schematic diagram of T92/S304H dissimilar materials joints.

tron microscopy (SEM) was employed to analyze the fractographsof impact specimens. The specimens used for EBSD investigationwere obtained from the joints in location A and B (Fig. 5), and asuitable surface finished for EBSD was obtained by applying themechanical polishing followed by jet electro-polishing in 6% solu-tion of perchloric acid in methanol. The schematic diagram of thespecimen in EBSD system was displayed in Fig. 6, in which ND,RD and TD stood for normal, rolling and transverse directionrespectively. EBSD micrographs were taken on the TD-RD planeso that the RD is horizontal and the ND is vertical.

P S Si N Al W B Cu

0.016 0.002 0.39 0.044 0.01 1.63 0.0033 /0.033 0 0.025 0.11 0.009 / 0.004 2.96/ 0.001 0.04 / / / / 0.01

Fig. 6. The location of specimen in EBSD system.

Fig. 5. The locations of specimens for EBSD.

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3. Results and discussion

3.1. Microstructures

The microstructure of T92 base material is shown in Fig. 7a. Itconsists of fully tempered martensite where carbide particles(M23C6, MC) precipitated from the lath martensites and the prioraustenite grain boundaries during the tempering process [20].The microstructure of S304H base material shown in Fig. 7b con-sists of equiaxed austenitic grains with average grain size of�12 lm. What’s more, M23C6, NbCrN and NbX precipitates are dis-tributed within the austenite grains, which can assure excellentstrength for S304H steel at room temperature [21]. The INCONEL82/182 weld metal exhibits a fully austenitic microstructure andthe shape of grains is dendritic (Fig. 7c). T92 heat affected zone(HAZ) consists of tempered martensite. Furthermore, the regionin the T92 heat affected zone (HAZ) near the weld metal iscoarse-grained heat affected zone (CGHAZ) with an average grainsize of’ �25 lm (Fig. 8a). Beyond this region, the fine-grained heataffected zone (FGHAZ) with an average grain size of �9 lm(Fig. 7d) is seen adjacent to the unaffected T92 base metal. In addi-tion, a lot of carbide particles (M23C6, MC) precipitate and distrib-ute within the grains after PWHT, which improves the strength andtoughness of T92 HAZ. The S304H HAZ has a large equiaxedaustenitic structure with a grain size of �22 lm (Fig. 10). Besidethese, the two interfaces including T92 HAZ/weld metal andS304H HAZ/weld metal are both bonded well and the fusing lineis very clear, seen in Fig. 7e and f. Therefore, the microstructuralcharacteristics of the whole joints can be obviously reflected byFig. 7.

Fig. 8 shows the grain oriented distribution maps of T92CGHAZand the weld metal near T92 CGHAZ side. The color of grain inFig. 8 stands for its orientation. The observation in Fig. 8a demon-strates that T92 CGHAZ is made up of much equiaxed grains whosecolors are randomly distributed. Moreover, combined with the in-verse pole figure map of T92 CGHAZ (Fig. 9a) it is well known thatthe maximum oriented intensity value of T92 CGHAZ near the weldmetal is about only 3. This means that the grains in T92 CGHAZ do

not present the obvious oriented feature and most of the grains inT92 CGHAZ are not conducive to epitaxial growth.

However, while in the weld metal zone, most of coarse colum-nar grains take on green and the direction where these grains growup is from the interface between T92HAZ and weld metal to thecentre part of weld metal zone (Fig. 8b). Furthermore, it can beseen from Fig. 9b that the maximum oriented intensity value ofweld metal is about 16, which indicates that the weld metal pre-sents the oriented feature, i.e. grains in weld metal zone grow upalong the transverse direction of the joints after welding.

Fig. 10 shows the grain oriented distribution map of weld metalnear S304H HAZ side. The black line in Fig. 10 illustrates the inter-face between S304H HAZ and weld metal. Similarly, the color ofgrain in Fig. 10 stands for its orientation. On the left side of theinterface, we can estimate that the columnar grains in weld metalzone have similar colors. Furthermore, the trend that grains growup is from the interface between S304H HAZ and weld metal tothe centre part of weld metal zone. In addition, it can be seen fromthe inverse pole figure map in Fig. 11a that the maximum orientedintensity value of weld metal is about 16 indicating weld metalpresents the oriented feature. That is to say, the weld metal nearS304H HAZ grow up along the transverse direction of the joint afterwelding, which is according with the orientation test results ofweld metal near the T92 HAZ side (Fig. 9b). Therefore, weld metalpart takes on oriented distribution along transverse direction of thejoints.

On the right side of the interface, S304H HAZ, however, is incontrast with weld metal. The colors of equiaxed grains are ran-domly distributed. In addition, it can be seen from Fig. 11b thatthe maximum oriented intensity value of S304H HAZ near the weldmetal is about only 4.5, which shows grains in S304H HAZ do notdisplay oriented feature during the welding process and most ofthe grains in S304H HAZ are not conducive to epitaxial growth.

3.2. Mechanical properties

3.2.1. Tensile propertyResults of the transverse tensile test are listed in Table 2, from

which the tensile strength values of two group specimens arerespectively 689 and 677 MPa, both are higher than that of theASME T92 and S304H standards. So the tensile strength of thejoints can ensure the USC boilers safe service. In addition, the frac-ture of the joints is located in T92 CGHAZ, which indicates tensilestrength of T92 CGHAZ part is relatively weak.

As can be seen from the Fig. 7, although the microstructures ofT92 base material and its HAZ are both tempered martenste, aver-age grain sizes of them are different. Average grain size of T92CGHAZ is larger than that of T92 base material and any part inT92 CGHAZ with the same volume as T92 base material has lessquantity of interface between grains than that in T92 base mate-rial. It is well known that interface between grains can effectivelyobstruct the movement of dislocation and improve the strength ofmaterial [22–24]. Thus, the strength in T92 CGHAZ is weaker thanthat in T92 base material, inducing the strength in CGHAZ de-creased. While in T92 FGHAZ, with the decreasing of average grainsize, any part in T92 FGHAZ with the same volume as T92 basematerial has more quantity of interface between grains than thatin T92 base material. Hence the strength in T92 FGHAZ is strongerthan that in T92 base material and, correspondingly, the strengthin T92 FGHAZ is improved. Similarly, compared with the S304Hbase material, average grain size of S304H HAZ (Fig. 10) is largerthan that of S304H base material and any part in S304H HAZ withthe same volume as S304H base material has less quantity of inter-face between grains than that in S304H base material, which re-sults in a consequent decrease of strength in S304H HAZ. Inaddition, although S304H base material has almost the same grain

Fig. 8. EBSD grain orientation map of (a) T92 CGHAZ, and (b) weld metal near T92 CGHAZ side.

Fig. 7. Metallographic structures of T92/S304H dissimilar materials joints, exhibiting (a) T92 base material, (b) S304H base material, (c) weld metal, (d) T92 fine grained HAZ(FGHAZ), (e) interface between T92 HAZ and weld metal, and (f) interface between S304H HAZ and weld metal.

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size as T92 base material, the strengths of them are different. Dueto high alloy elements content in S304H austenitic steel (Table.1),

solid solution strengthening and dispersion strengthening effectscaused by these alloy elements are stronger in S304H base material

Fig. 10. EBSD grain orientation map of weld metal near S304H HAZ side.

Fig. 9. Inverse pole figure maps of (a) T92 CGHAZ, and (b) weld metal.

Table 2Tensile test results of T92/S304H dissimilar materials joints.

Sample no. Tensile strength (rs, MPa) Rupture position

1 689 T92 CGHAZ2 677 T92 CGHAZASME T92 P620 /ASME S304H P655 /

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than that in T92 base material, indicating that S304H base materialhas higher strength than T92 base material [21]. Likewise, as theaverage grain size of S304H HAZ is similar to that of T92 CGHAZand the strengthening effect caused by alloy elements for S304HHAZ is stronger, in this case, S304H HAZ has higher strength thanT92 CGHAZ. With regard to weld metal, although the strengthen-

Fig. 11. Inverse pole figure maps of (a

ing effect caused by grain boundary is weakest for its coarse den-dritic austenitic grain, orientation distribution of grains in weldmetal zone, giving rise to higher strength than T92 CGHAZ in trans-verse direction of the joints (Figs. 8 and 10). As a result, the fracturelocation is taken place at the T92CGHAZ finally.

3.2.2. Impact toughnessThe impact test results of the joints are given in Fig. 12. It is

clear that the average impact strength value of S304H base mate-rial is the highest. In contrast, the average impact strength valueof weld metal is the lowest. Furthermore, the average impactstrength value of weld metal is 57 J which is higher than ASMEstandard value (>41 J), so the impact strength of the joints is qual-ified. Meanwhile, the average impact strength values of T92 CGHAZ

) weld metal, and (b) S304H HAZ.

Fig. 12. Impact test results of T92/S304H dissimilar materials joints.

Fig. 13. SEM fractographs of (a) T92 base material, (b) T92 FGHAZ, (c) S30

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and S304H HAZ are lower than that of their base materials respec-tively, while average impact strength value of T92 FGHAZ is higherthan that of T92 base material.

Fig. 13 shows the impact fractograph of the joints. The fracturesurfaces of T92 base material and T92 FGHAZ, as well as S304Hbase material exhibit ductile fracture characteristic and consist offiber zone and radiation zone, especially the fiber zone occupys abig proportion. Beside these, there are a large number of small-sized dimples with different sizes and depths on their fracture sur-faces, which displays excellent plasticity of them. As for the S304HHAZ and T92 CGHAZ, their fracture modes both belong to quasi-cleavage fracture. Compared with the fracture surfaces of two basematerials, a larger brittle area is seen on their fracture surfaces.Thus, it can be inferred that the toughness of two base materialsis better than that of two HAZs. Furthermore, the amount of tearridge around dimple in Fig. 13d is much more than that inFig. 13e, indicating that the toughness of T92 CGHAZ is weakerthan that of S304H HAZ. In addition, it can be seen from the frac-ture surfaces of two base materials and their HAZs that there aresome precipitates in the dimples. For T92 base material and it’s

4H base material, (d) S304H HAZ, (e) T92 CGHAZ, and (f) weld metal.

J. Cao et al. / Materials and Design 32 (2011) 2763–2770 2769

HAZ, EDX results show the precipitates on the fracture surfaces ofthem are composed of Cr, Fe, W and C elements (Fig. 14a). So it canbe inferred that the type of precipitates is a M23C6 particle [1]. ForS304H base material and it’s HAZ, EDX results show the precipi-tates on the fracture surfaces of them consist of two types particles.One is composed of Cr, Fe, Ni and C elements (Fig. 14b), and theother is composed of Nb, Cr and N elements (Fig. 14c). Thus, itcan be inferred that the two types precipitates are M23C6 andNbCrN respectively [21]. Since the precipitates on the fracture sur-faces of two base materials are the same with that of their ownHAZs, it also implies that there is no any new type precipitateappearing in the two HAZs after welding. The fracture surface con-sists of many dissociation surfaces at weld metal part, has beensuggested to be the typical brittle fracture feature (Fig. 13f). Hence,the toughness of weld metal part is the weakest, which is accor-dance with impact test results (Fig. 12).

The changes of toughness at different parts of the joints can bewell explained by means of Cottrell–Petch theory [25]. Accordingto the classic Cottrell–Petch theory, when the temperature keepsconstant, two factors, grain size and average orientation factor,are responsible for the toughness of metals and alloys. Moreover,with the increase of grain size or average orientation factor, thetoughness of metals and alloys will be weakened, and vice versa.Based on previously microstructural analyses, during the welding

Fig. 14. EDX spectrums of (a) precipitates on the fracture surfaces of T92 base material aand it’s HAZ.

process, the grain size in the HAZs of T92 and S304H have bothchanged, but orientation factor has not increased since no orienta-tion was formed. Thus, the toughness of them is decided by grainsize. For T92 CGHAZ and S304H HAZ, due to larger grain size thantheir respective base materials, correspondingly, the toughness ofthem decreases. While in T92 FGHAZ, the decrease of grain sizeleads to improvement of its toughness. As a result, the toughnessof T92 FGHAZ is better than that of T92 base material. However,as for weld metal, during the welding process, not only are thecoarse austenitic grains produced, but also the orientation isformed in the transverse direction of the joints. In this case, underthe impact force which is perpendicular to the transverse directionof the joints, crack is formed and easily expands along the grainboundaries of coarse austenitic grains causing brittle fracture. Con-sequently, weld metal part of the joints has the lowest toughnessvalue.

3.3. Microstructural evolution mechanism

Based on former analyses, it is clear that GTAW has taken effecton the microstructures of HAZs and weld metal during the weldingprocess. At the initial stage of welding, the filler material under-goes melting. When the welding process is finished, melted fillermaterial begins to crystallize to form weld metal zone. According

nd it’s HAZ; (b) and (c) precipitates on the fracture surfaces of S304H base material

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to non-spontaneous nucleation theory of welding crystallography[26–28], firstly, melted filler material crystallizes on the base ofadjacent base material, and then produces epitaxial growth to formcoarse columnar crystals under the effect of temperature field.Thus, the direction where the grains in weld metal zone grow upis decided by variation of temperature field. Taking into accountthat the direction where temperature decreases is from the inter-face between base material and weld metal towards the e middlepart of the weld metal zone, the melted weld metal crystallizesalong this direction, i.e. the maximum temperature gradient direc-tion. In other words, the grains in weld metal zone grow up fromthe two interface sides towards the middle part of weld metal zonerespectively and result in orientation distribution along the trans-verse direction of the joint, seen in Figs. 8 and 10. Therefore, thetransverse tensile strength of weld metal is improved for its orien-tation in this direction. While in the two HAZs, because of narrowwidth in transverse direction of the joints, there is no obvious tem-perature gradient change. This suggested that orientation distribu-tions of grains in the two HAZs are random.

On the other hand, for T92 HAZ, since the temperature is higherthan its initial austenitic transition temperature (Ac1) during thewelding process, tempered martensite begins transforming intoaustenite. Furthermore, in T92 CGHAZ, the temperature even ex-ceeds its fully austenitic transition temperature (Ac3), whichmakes austenitic grains grow up and results in coarsening. Whilein T92 FGHAZ, as the temperature is between Ac1 and Ac3, the fineaustenitic grains can be obtained. After welding, with the continu-ous cooling, austenitic grains transform into lath-shaped martens-ite. Finally, T92 CGHAZ has martensitic structure with coarse grainsize and T92 FGHAZ has martensitic structure with fine grain size.However, For S304H austenitic steel, there is no martensite–austenite (M–A) transition during welding process. Only the regionadjacent to the weld metal, due to welding heat effect, has coarseaustenite structure. Thus, microstructural evolution leads to thevariation of mechanical properties of the joints.

4. Conclusions

The investigations performed on the T92/S304H dissimilarmaterials joints have led to the following conclusions.

1. Tensile strength and impact toughness of T92/S304H dissimilarmaterials joints obtained by GTAW process can meet the USCboiler’s requirements. Furthermore, the part of the joints withrelatively weak tensile strength was T92 CGHAZ, while the partof the joints which revealed relatively weak toughness wasweld metal.

2. The coarse tempered martensitic structure of T92 CGHAZ makesthe strengthening effect caused by grain boundary decreasedand results in relatively weak tensile strength in T92 CGHAZ.

3. The weak toughness of weld metal is attributed to its coarsedendritic austenitic structure. Furthermore, the impact fracto-graph of weld metal takes on typical brittle fracturecharacteristics.

4. Grains in weld metal zone grow up respectively from the twointerface sides towards the middle part of weld metal resultingin its orientation distribution in transverse direction of thejoints. Weld metal, due to its orientation distribution, hashigher tensile strength than T92 CGHAZ.

Acknowledgements

The work was supported by both National Natural ScienceFoundation of China (Grant 50871076) and Shanghai Leading Aca-demic Discipline Project (Project Number: B113).

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