复旦大学：《材料失效分析 Materials Failure Analysis》课程教学资源（教学案例）22. Microstructure and mechanical properties of dissimilar materials joints between T92 martensitic and S304H austenitic steels.pdf

Materials and Design 32(2011)2763-2770 Contents lists available at ScienceDirect Materials and design ELSEVIER journalhomepagewww.elsevier.com/locate/matdes Microstructure and mechanical properties of dissimilar materials joints between t92 martensitic and s304h austenitic steels Jian Cao, Yi Gong, Kai Zhu, Zhen-Guo Yang Xiao-Ming Luo, Fu-Ming Gu of Special Equipment Inspection Technical Research, Shanghai 200062, PR China ARTICLE O A BSTRACT Article history: In this paper, 192 martensitic steel and S304H austenitic steel were welded by gas tungsten arc welding (GTAW)process. Microstructural features and mechanical properties of T92 and S304H dis rials joints were investigated. The results showed that the part of the joints with relativel Available online 8 January 201 strength was T92 coarse-grained heat affected zone(CGHAZ), while the part of the joints which revealed elatively we hess was weld metal. The decrease of t strength in 192 CGHAZ parse ten martensite structure. Weak toughness of the joints was resulted from the coarse of the weld metal. however. the weld in transverse direction of the joints milar materials joints was provided higher tensile strength by the orientation distribution of grains compared with T92 CGHAZ. e 2011 Elsevier Ltd. All rights reserved. 1 Introduction tensitic steels[14-17. Thus, it is widely used for superheaters and reheaters which have the abominable service environment in us Increased heat efficiency and improved environmental protec- boilers. Normally, t92 steel can be used as pipes linking superheat- tion are always the innovative driving forces in the development ers and reheaters. In this case, the welding between T92 and S304H of ultra supercritical(USC) boilers for fossil power plants, whose steels will be necessary. steam temperature is up to 600C and pressure exceeds 27 MPa Until now, many researches have mainly focused on the proper Under this USC condition, the heat efficiency can rise to around ties of T92 and S304H steels. As for T92 and S304H dissimilar 45%, compared with the value as 41% of supercritical (SC)boilers. materials joints, however, there almost hasn't any report about However, with the increase of steam parameters, requirements it. Since T92/S304H dissimilar materials joints is obtained by using for the materials applied in the USc boilers components are melted filler material to join two steels, the melted filler material becoming higher. Thus, many new generation steels have been will re-crystal to form the weld metal part of the joints after weld- developed in recent years, including T92(9Cr05Mo2WVNb) ing. In addition, due to the effect of welding thermal cycles, not martensitic steel and S304H (18Cr9Ni3CuNbN)austenitic steel only the microstructure of T92 adjacent weld metal but also the T92 steel was developed by the Nippon Steel Corporation of microstructure of S304H adjacent weld metal will both change Japan 1 in the late 1990s by modifying chemical compositions during the welding process. Considering that the mechanical prop- pon T91(9Cr1MoVNb) for even more preferable mechanical erties of the joints are closely linked with its microstructure. Thus, properties at high temperatures. This steel has the manufacturers an in-depth insight into the structure-property relationships of designation as NF616(ASTM Stands A213)and contains 0.5% Mo, T92/S304H dissimilar materials joints may have great significan 88 W, as well as small additions of Nb, V and B Creep strength for both the dissimilar steels welding process between new gener f T92 at 600C increases about 10-20% compared with that of ation martensitic and austenitic steels, and the safety of the Usc T91 at 600C[2-13 S304H steel was developed by Sumitomo Me- boilers In our work, on the one hand, mechanical properties of tal Industries Ltd on the base of TP304H(OCr19Ni10. As a new T92/S304H dissimilar materials joints were carried out through type of austenitic steel, S304H possesses not only excellent resis- tensile and impact tests. On the other hand the microstructures tance to high-temperature corrosion and steam oxidation mainly across the entire joints were also investigated. What's more, the due to high Cr content, but also superior creep strength than mar- detailed mechanism governing the microstructural evolution of the joints during welding process was analyzed by means of the electron back-scattered diffraction(EBSD) technique, which was Corresponding author. Tel. +86 21 65642523: fax: +86 21 65103056 firstly used to study the process of grain structure development of dissimilar materials joints 0261-3069/s- see front matter o 2011 Elsevier Ltd. All rights reserved doi:10.1016 mates.201101.008

Microstructure and mechanical properties of dissimilar materials joints between 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 aDepartment of Materials Science, Fudan University, Shanghai 200433, PR China b Shanghai Institute of Special Equipment Inspection & Technical Research, Shanghai 200062, PR China article info Article history: Received 8 September 2010 Accepted 4 January 2011 Available online 8 January 2011 Keywords: Microstructure Mechanical properties Dissimilar materials joints abstract 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 materials joints were investigated. The results showed that the part of the joints with relatively weak tensile strength was T92 coarse-grained heat affected zone (CGHAZ), while the part of the joints which revealed relatively weak toughness was weld metal. The decrease of tensile strength in T92 CGHAZ was due to its coarse tempered martensite structure. Weak toughness of the joints was resulted from the coarse dendritic austenite of the weld metal. However, the weld metal in transverse direction of the joints was provided higher tensile strength by the orientation distribution of grains compared with T92 CGHAZ. 2011 Elsevier Ltd. All rights reserved. 1. Introduction Increased heat efficiency and improved environmental protection are always the innovative driving forces in the development of ultra supercritical (USC) boilers for fossil power plants, whose steam temperature is up to 600 C and pressure exceeds 27 MPa. Under this USC condition, the heat efficiency can rise to around 45%, compared with the value as 41% of supercritical (SC) boilers. However, with the increase of steam parameters, requirements for the materials applied in the USC boilers components are becoming higher. Thus, many new generation steels have been developed in recent years, including T92 (9Cr0.5Mo2WVNb) martensitic steel and S304H (18Cr9Ni3CuNbN) austenitic steel. T92 steel was developed by the Nippon Steel Corporation of Japan [1] in the late 1990s by modifying chemical compositions upon T91 (9Cr1MoVNb) for even more preferable mechanical properties at high temperatures. This steel has the manufacturer’s designation as NF616 (ASTM Stands A213) and contains 0.5% Mo, 1.8% W, as well as small additions of Nb, V and B. Creep strength of T92 at 600 C increases about 10–20% compared with that of T91 at 600 C [2–13]. S304H steel was developed by Sumitomo Metal Industries Ltd on the base of TP304H (0Cr19Ni10). As a new type of austenitic steel, S304H possesses not only excellent resistance to high-temperature corrosion and steam oxidation mainly due to high Cr content, but also superior creep strength than martensitic steels [14–17]. Thus, it is widely used for superheaters and reheaters, which have the abominable service environment in USC boilers. Normally, T92 steel can be used as pipes linking superheaters and reheaters. In this case, the welding between T92 and S304H steels will be necessary. Until now, many researches have mainly focused on the properties of T92 and S304H steels. As for T92 and S304H dissimilar materials joints, however, there almost hasn’t any report about it. Since T92/S304H dissimilar materials joints is obtained by using melted filler material to join two steels, the melted filler material will re-crystal to form the weld metal part of the joints after welding. In addition, due to the effect of welding thermal cycles, not only the microstructure of T92 adjacent weld metal but also the microstructure of S304H adjacent weld metal will both change during the welding process. Considering that the mechanical properties of the joints are closely linked with its microstructure. Thus, an in-depth insight into the structure–property relationships of T92/S304H dissimilar materials joints may have great significances for both the dissimilar steels welding process between new generation martensitic and austenitic steels, and the safety of the USC boilers. In our work, on the one hand, mechanical properties of T92/S304H dissimilar materials joints were carried out through tensile and impact tests. On the other hand, the microstructures across the entire joints were also investigated. What’s more, the detailed mechanism governing the microstructural evolution of the joints during welding process was analyzed by means of the electron back-scattered diffraction (EBSD) technique, which was firstly used to study the process of grain structure development of dissimilar materials joints. 0261-3069/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2011.01.008 ⇑ Corresponding author. Tel.: +86 21 65642523; fax: +86 21 65103056. E-mail address: zgyang@fudan.edu.cn (Z.-G. Yang). Materials and Design 32 (2011) 2763–2770 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes

2764 J. Cao et al Materials and Design 32(2011)2763-2770 2.1. Materials and welding procedure Scales of the two base materials t92 and s304h used in the S304H present investigation were 57 150D x 5.08 mm and 570D x 4.5 mm in thickness respectively. Heat treatment conditions of them were described below: 1)T92: austenitization was carried Unit: mm out for 20 min at 1050C and then tempering for 60 min at 60C. 2)S304H: solution treatment at 1100C, followed by water cooling until room temperature. Fig. 1 was schematic diagram of ig2 Schematic diagram of welding process of T92/S304H dissimilar materials corresponding to INCONEL 82/182)was chosen as the filler mate- rial. The chemical col tions of all three materials are given in Table 1. The t92/S304H dissimilar materials joints were weld by means of gas tungsten arc welding (GTAw) with pure argon gas(Ar)as the shielding gas. The arc voltage and the arc current PWH used in welding process were 14 V and 230A(type: balance) spectively. The argon purity used for GTAW in this experiment is 99.99%. Fig 2 was the schematic diagram of the welding process part marked with dotted line in Fig. 2 represented the 730-760℃ egion. After welding, post welding heat treatment was carried out for 1 h at 730-760 C to eliminate the welded residual stress. Fig 3 was the schematic diagram of the Heating rat ≤220℃h PWhT process of T92/S304H dissimilar materials joints and Fig. 4 vas the schem of T92/S304H dissimilar materia 2. 2. Test methods Tensile and impact tests of t92/S304H dissimilar materials Time(min) joints were done respectively at temperature according to Fig 3. Schematic diagram of post welding heat treatment process of T92/S304H the ASTM E8-04 [18 and E23-02a [19]standards. Optical micros- dissimilar materials joints. copy(oM)was used to observe the metallographic microstructures across the joints. Three etching solutions were used: (1)T92: picric acid (2, 4, 6-trinitrophenol)2.5 g, ethanol 20 ml and H20 20 ml; (2) weld metal: FeCl3 5 g, HCI 20 ml and H20 20 ml; (3)S304H: CuSOA T92 ERNICr-3 S304H 4 g, HCI 20 ml and ethanol 20 ml PHILIPS XL30FEG scanning elec B M WM 1100°C Fig. 4. Schematic diagram of T92/S304H dissimilar materials joints. D20min S304H tron microscopy(SEM)was employed to analyze the fractograph 1050°C T92 760°C of impact specimens. The specimens used for EBSD investigation were obtained from the joints in location A and b( Fig. 5),and 60min suitable surface finished for EBSD was obtained by applying the Heating rate mechanical polishing followed by jet electro-polishing in 6% solu- ≤300℃/h tion of perchloric acid in methanol. The schematic diagram of the specimen in EBSD system was displayed in Fig. 6, in which ND RD and tD stood for normal, rolling and transverse direction espectively. EBSD micrographs were taken on the TD-Rd plane Fig. 1 Schematic diagram of heat treatment processes of 192 and S304H steels so that the rd is horizontal and the nd is vertical Table 1 Chemical compositions of the base materials and filler material samples(wt%). 8.76 0.4600160.002 0044001 1.6300033 S304H sample 0.09 18.38 004004 8910.8200330 0.1 ERNiCr-3 flle 0030 20.0 25290 004

2. Experimental 2.1. Materials and welding procedure Scales of the two base materials T92 and S304H used in the present investigation were 57.15OD 5.08 mm and 57OD 4.5 mm in thickness respectively. Heat treatment conditions of them were described below: 1) T92: austenitization was carried out for 20 min at 1050 C and then tempering for 60 min at 760 C. 2) S304H: solution treatment at 1100 C, followed by water cooling until room temperature. Fig. 1 was schematic diagram of heat treatment processes of T92 and S304H steels. AWS ERNiCr-3 (corresponding to INCONEL 82/182) was chosen as the filler material. The chemical compositions of all three materials are given in Table 1. The T92/S304H dissimilar materials joints were welded by means of gas tungsten arc welding (GTAW) with pure argon gas (Ar) as the shielding gas. The arc voltage and the arc current used in welding process were 14 V and 230A (type: balance) respectively. The argon purity used for GTAW in this experiment is 99.99%. Fig. 2 was the schematic diagram of the welding process and the part marked with dotted line in Fig. 2 represented the welding region. After welding, post welding heat treatment (PWHT) was carried out for 1 h at 730–760 C to eliminate the welded residual stress. Fig. 3 was the schematic diagram of the PWHT process of T92/S304H dissimilar materials joints and Fig. 4 was the schematic diagram of T92/S304H dissimilar materials joints. 2.2. Test methods Tensile and impact tests of T92/S304H dissimilar materials joints were done respectively at room temperature according to the ASTM E8-04 [18] and E23-02a [19] standards. Optical microscopy (OM) was used to observe the metallographic microstructures across the joints. Three etching solutions were used: (1) T92: picric acid (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 electron microscopy (SEM) was employed to analyze the fractographs of impact specimens. The specimens used for EBSD investigation were obtained from the joints in location A and B (Fig. 5), and a suitable surface finished for EBSD was obtained by applying the mechanical polishing followed by jet electro-polishing in 6% solution of perchloric acid in methanol. The schematic diagram of the specimen in EBSD system was displayed in Fig. 6, in which ND, RD and TD stood for normal, rolling and transverse direction respectively. EBSD micrographs were taken on the TD-RD plane so that the RD is horizontal and the ND is vertical. Fig. 1. Schematic diagram of heat treatment processes of T92 and S304H steels. Table 1 Chemical compositions of the base materials and filler material samples (wt.%). Material C Cr Mo V Nb Ni Mn P S Si N Al W B Cu T92 sample 0.11 8.76 0.36 0.21 0.059 0.25 0.46 0.016 0.002 0.39 0.044 0.01 1.63 0.0033 / S304H sample 0.09 18.38 / 0.040 0.49 8.91 0.82 0.033 0 0.025 0.11 0.009 / 0.004 2.96 ERNiCr-3 filler 0.030 20.0 / / 2.40 72.5 2.90 / 0.001 0.04 / / / / 0.01 Fig. 2. Schematic diagram of welding process of T92/S304H dissimilar materials joints. Fig. 3. Schematic diagram of post welding heat treatment process of T92/S304H dissimilar materials joints. Fig. 4. Schematic diagram of T92/S304H dissimilar materials joints. 2764 J. Cao et al. / Materials and Design 32 (2011) 2763–2770

J. Cao et aL/ Materials and Design 32 (2011)2763-2770 not present the obvious oriented feature and most of the grains in T92 WM S304H T92 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 grow up is from the interface between T92HAZ and weld metal to the centre part of weld metal zone( Fig. 8b). Furthermore, it can be A 边B seen from Fig 9b that the maximum oriented intensity value of weld metal is about 16, which indicates that the weld metal pre- sents the oriented feature, i. e grains in weld metal zone grow along the transverse direction of the joints after welding. Fig 10 shows the grain oriented distribution map of weld metal near S304H HAZ side. The black line in Fig 10 illustrates the inter Fig. 5. The locations of specimens for EBSD ce between S304H HAZ and weld metal. Similarly, the color of grain in Fig. 10 stands for its orientation. On the left side of the interface, we can estimate that the columnar grains in weld metal zone have similar colors. Furthermore, the trend that grains grow up is from the interface between S304H HAZ and weld metal to the centre part of weld metal zone. In addition, it can be seen from Tested surface RD intensity value of weld metal is about 16 indicating weld metal presents the oriented feature. That is to say, the weld metal near S304H HAZ grow up along the transverse direction of the joint after welding, which is according with the orientation test results of TD weld metal near the t92 HAZ side(fig. 9b). Therefore, weld metal part takes on oriented distribution along transverse direction of the Fig. 6. The location of specimen in EBSD system. On the right side of the interface, S304H HAZ, however, is in contrast with weld metal. The colors of equiaxed grains are ran- 3. Results and discussion domly distributed. In addition, it can be seen from Fig. 11b that the maximum oriented intensity value of S304H HAZ near the weld 3. 1. Microstructure metal is about only 4.5, which shows grains in S304H HAZ do not display oriented feature during the welding process and most of The microstructure of T92 base material is shown in Fig. 7a. It the grains in S304H HAZ are not conducive to epitaxial growth. consists of fully tempered martensite where carbide particles M23C6, MC)precipitated from the lath martensites and the prior 3. 2. Mechanical properties austenite grain boundaries during the tempering process[20] The microstructure of S304H base material shown in Fig. 7b con- 3. 1. Tensile property sts of equiaxed austenitic grains with average grain size of Results of the transverse tensile test are listed in table 2. from 12 Hm What's more, M23C6, NbCrN and NbX precipitates are dis- which the tensile strength values of two group specimens are tributed within the austenite grains, which can assure excellent respectively 689 and 677 MPa, both are higher than that of the strength for S304H steel at room temperature 21. The INCONEL ASME T92 and $304H standards. So the tensile strength of the 82/182 weld metal exhibits a fully austenitic microstructure and joints can ensure the USC boilers safe service. In addition, the frac- the shape of grains is dendritic(Fig. 7c). T92 heat affected zone ture of the joints is located in T92 CGHAZ, which indicates tensile (HAZ) consists of tempered martensite. Furthermore, the region strength of T92 CGHAZ part is relatively weak. in the t92 heat affected zone(HAz) near the weld metal is As can be seen from the Fig. 7, although the microstructures of coarse-grained heat affected zone(cghaz) with an average grain T92 base material and its haz are both tempered martenste, aver size of 25 um(Fig. Sa). Beyond this region, the fine-grained heat age grain sizes of them are different. Average grain size of T92 affected zone (FGHAz) with an average grain size of 9 um CGHAZ is larger than that of t92 base material and any part in ( Fig. 7d)is seen adjacent to the unaffected T92 base metal. In addi- T92 CGHAZ with the same volume as t92 base material has less tion, a lot of carbide particles(M23C6, MC)precipitate and distrib- quantity of interface between grains than that in t92 base mate- ute within the grains after PWHT, which improves the strength and rial. It is well known that interface between grains can effectively toughness of T92 HAz. The S304H HAZ has a large equiaxed obstruct the movement of dislocation and improve the strength of stenitic structure with a grain size of 22 Hm(Fig. 10). Beside material [22-24]. Thus, the strength in T92 CGHAZ is weaker than S304H HAZ/weld metal are both bonded well and the fusing line creased. While in T92 FGHAZ, with the decreasing al ae c haz de- these, the two interfaces including t92 HAZ/weld metal and that in T92 base material, inducing the strength in CGhaz de- is very clear, seen in Fig. 7e and f. Therefore the microstructural size, any part in t92 FGHAZ with the same volume as T92 base characteristics of the whole joints can be obviously reflected by material has more quantity of interface between grains than that in T92 base material. Hence the strength in T92 FGHAZ is stronge Fig 8 shows the grain oriented distribution maps of T92CGHAz than that in t92 base material and, correspondingly, the strength and the weld metal near T92 CGHAZ side. the color of grain inin T92 FGHAZ is improved. Similarly, compared with the $304H Fig 8 stands for its orientation. The observation in Fig 8a demon- base material, average grain size of S304H HAZ(Fig. 10)is larger strates that t92 CGHAZ is made up of much equiaxed grains whose than that of S304H base material and any part in S304H HAZ with colors are randomly distributed. Moreover, combined with the in- the same volume as S304H base material has less quantity of inter verse pole figure map of T92 CGHAZ (Fig. 9a)it is well known that face between grains than that in $304H base material, which re- the maximum oriented intensity value of T92 CGHAZ near the weld sults in a consequent decrease of strength in S304H HAZ. In metal is about only 3. This means that the grains in t92 CGHAZ do addition, although S304H base material has almost the same grain

3. Results and discussion 3.1. Microstructures The microstructure of T92 base material is shown in Fig. 7a. It consists of fully tempered martensite where carbide particles (M23C6, MC) precipitated from the lath martensites and the prior austenite grain boundaries during the tempering process [20]. The microstructure of S304H base material shown in Fig. 7b consists of equiaxed austenitic grains with average grain size of 12 lm. What’s more, M23C6, NbCrN and NbX precipitates are distributed within the austenite grains, which can assure excellent strength for S304H steel at room temperature [21]. The INCONEL 82/182 weld metal exhibits a fully austenitic microstructure and the shape of grains is dendritic (Fig. 7c). T92 heat affected zone (HAZ) consists of tempered martensite. Furthermore, the region in the T92 heat affected zone (HAZ) near the weld metal is coarse-grained heat affected zone (CGHAZ) with an average grain size of’ 25 lm (Fig. 8a). Beyond this region, the fine-grained heat affected zone (FGHAZ) with an average grain size of 9 lm (Fig. 7d) is seen adjacent to the unaffected T92 base metal. In addition, a lot of carbide particles (M23C6, MC) precipitate and distribute within the grains after PWHT, which improves the strength and toughness of T92 HAZ. The S304H HAZ has a large equiaxed austenitic structure with a grain size of 22 lm (Fig. 10). Beside these, the two interfaces including T92 HAZ/weld metal and S304H HAZ/weld metal are both bonded well and the fusing line is very clear, seen in Fig. 7e and f. Therefore, the microstructural characteristics of the whole joints can be obviously reflected by Fig. 7. Fig. 8 shows the grain oriented distribution maps of T92CGHAZ and the weld metal near T92 CGHAZ side. The color of grain in Fig. 8 stands for its orientation. The observation in Fig. 8a demonstrates that T92 CGHAZ is made up of much equiaxed grains whose colors are randomly distributed. Moreover, combined with the inverse pole figure map of T92 CGHAZ (Fig. 9a) it is well known that the maximum oriented intensity value of T92 CGHAZ near the weld metal is about only 3. This means that the grains in T92 CGHAZ do not present the obvious oriented feature and most of the grains in T92 CGHAZ are not conducive to epitaxial growth. However, while in the weld metal zone, most of coarse columnar grains take on green and the direction where these grains grow up is from the interface between T92HAZ and weld metal to the centre part of weld metal zone (Fig. 8b). Furthermore, it can be seen from Fig. 9b that the maximum oriented intensity value of weld metal is about 16, which indicates that the weld metal presents the oriented feature, i.e. grains in weld metal zone grow up along the transverse direction of the joints after welding. Fig. 10 shows the grain oriented distribution map of weld metal near S304H HAZ side. The black line in Fig. 10 illustrates the interface between S304H HAZ and weld metal. Similarly, the color of grain in Fig. 10 stands for its orientation. On the left side of the interface, we can estimate that the columnar grains in weld metal zone have similar colors. Furthermore, the trend that grains grow up is from the interface between S304H HAZ and weld metal to the centre part of weld metal zone. In addition, it can be seen from the inverse pole figure map in Fig. 11a that the maximum oriented intensity value of weld metal is about 16 indicating weld metal presents the oriented feature. That is to say, the weld metal near S304H HAZ grow up along the transverse direction of the joint after welding, which is according with the orientation test results of weld metal near the T92 HAZ side (Fig. 9b). Therefore, weld metal part takes on oriented distribution along transverse direction of the joints. On the right side of the interface, S304H HAZ, however, is in contrast with weld metal. The colors of equiaxed grains are randomly distributed. In addition, it can be seen from Fig. 11b that the maximum oriented intensity value of S304H HAZ near the weld metal is about only 4.5, which shows grains in S304H HAZ do not display oriented feature during the welding process and most of the grains in S304H HAZ are not conducive to epitaxial growth. 3.2. Mechanical properties 3.2.1. Tensile property Results of the transverse tensile test are listed in Table 2, from which the tensile strength values of two group specimens are respectively 689 and 677 MPa, both are higher than that of the ASME T92 and S304H standards. So the tensile strength of the joints can ensure the USC boilers safe service. In addition, the fracture of the joints is located in T92 CGHAZ, which indicates tensile strength of T92 CGHAZ part is relatively weak. As can be seen from the Fig. 7, although the microstructures of T92 base material and its HAZ are both tempered martenste, average grain sizes of them are different. Average grain size of T92 CGHAZ is larger than that of T92 base material and any part in T92 CGHAZ with the same volume as T92 base material has less quantity of interface between grains than that in T92 base material. It is well known that interface between grains can effectively obstruct the movement of dislocation and improve the strength of material [22–24]. Thus, the strength in T92 CGHAZ is weaker than that in T92 base material, inducing the strength in CGHAZ decreased. While in T92 FGHAZ, with the decreasing of average grain size, any part in T92 FGHAZ with the same volume as T92 base material has more quantity of interface between grains than that in T92 base material. Hence the strength in T92 FGHAZ is stronger than that in T92 base material and, correspondingly, the strength in T92 FGHAZ is improved. Similarly, compared with the S304H base material, average grain size of S304H HAZ (Fig. 10) is larger than that of S304H base material and any part in S304H HAZ with the same volume as S304H base material has less quantity of interface between grains than that in S304H base material, which results in a consequent decrease of strength in S304H HAZ. In addition, although S304H base material has almost the same grain Fig. 6. The location of specimen in EBSD system. Fig. 5. The locations of specimens for EBSD. J. Cao et al. / Materials and Design 32 (2011) 2763–2770 2765