J. Cao et aL/ Materials and Design 32(2011)2763-2770 IAZ, EDX results show the precipitates on the fracture surfaces of process, the grain size in the HAzs of 192 and S304H have both them are composed of Cr, Fe, W and c elements( Fig. 14a). So it can changed, but orientation factor has not increased since no orienta- inferred that the type of precipitates is a M23C6 particle[1]. For tion was formed. Thus, the toughness of them is decided by grain S304H base material and it's HAZ, EDX results show the precipi- size. For T92 CGHAZ and S304H Haz, due to larger grain size than tates on the fracture surfaces of them consist of two types particles. their respective base materials, correspondingly, the toughness of One is composed of Cr, Fe, Ni and C elements(Fig. 14b), and the them decreases. while in t92 FGHAZ, the decrease of grain size other is composed of Nb, Cr and n elements( Fig. 14c). Thus, it leads to improvement of its toughness. As a result, the toughness can be inferred that the two types precipitates are M23 C6 and of T92 FGHAZ is better than that of T92 base material. However. NbCrN respectively [21]. Since the precipitates on the fracture sur- as for weld metal, during the welding process, not only are the faces of two base materials are the same with that of their own coarse austenitic grains produced, but also the orientation is HAZs, it also implies that there is no any new type precipita formed in the transverse direction of the joints. In this case, under ppearing in the two HAZs after welding. The fracture surface con- the impact force which is perpendicular to the transverse direction sists of many dissociation surfaces at weld metal part, has been of the joints, crack is formed and easily expands along the grain ggested to be the typical brittle fracture feature( Fig. 13f). Hence, boundaries of coarse austenitic grains causing brittle fracture. Con the toughness of weld metal part is the weakest, which is accor- sequently, weld metal part of the joints has the lowest toughness dance with impact test results(Fig. 12). The changes of toughness at different parts of the joints can be ell explained by means of Cottrell-Petch theory [25 According 3.3. Microstructural evolution mechanism to the classic Cottrell-Petch theory, when the temperature keeps constant, two factors, grain size and average orientation factor, Based on former analyses, it is clear that GTAW has taken effect are responsible for the toughness of metals and alloys. Moreover, on the microstructures of HAZs and weld metal during the welding with the increase of grain size or average orientation factor, the process. At the initial stage of welding, the filler material under toughness of metals and alloys will be weakened, and vice versa. goes melting. When the welding process is finished, melted filler Based on previously microstructural analyses, during the welding material begins to crystallize to form weld metal zone. According Mo 0.0 2000.002.00400 00100012.001400 2000.002.004.006.0080010001200140 Energy-Kev Energy-Kev 2000.002004006.00800100012.00140016001800 Kev Fig. 14. EDX spectrums of (a) precipitates on the fracture surfaces of 192 base material and it's HAZ; ( b)and (c) precipitates on the fracture surfaces of S304H base materialHAZ, EDX results show the precipitates on the fracture surfaces of them are composed of Cr, Fe, W and C elements (Fig. 14a). So it can be inferred that the type of precipitates is a M23C6 particle [1]. For S304H 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 the other is composed of Nb, Cr and N elements (Fig. 14c). Thus, it can be inferred that the two types precipitates are M23C6 and NbCrN respectively [21]. Since the precipitates on the fracture sur￾faces of two base materials are the same with that of their own HAZs, it also implies that there is no any new type precipitate appearing in the two HAZs after welding. The fracture surface con￾sists of many dissociation surfaces at weld metal part, has been suggested 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 be well explained by means of Cottrell–Petch theory [25]. According to the classic Cottrell–Petch theory, when the temperature keeps constant, 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, the toughness of metals and alloys will be weakened, and vice versa. Based on previously microstructural analyses, during the welding process, the grain size in the HAZs of T92 and S304H have both changed, but orientation factor has not increased since no orienta￾tion was formed. Thus, the toughness of them is decided by grain size. For T92 CGHAZ and S304H HAZ, due to larger grain size than their respective base materials, correspondingly, the toughness of them decreases. While in T92 FGHAZ, the decrease of grain size leads to improvement of its toughness. As a result, the toughness of T92 FGHAZ is better than that of T92 base material. However, as for weld metal, during the welding process, not only are the coarse austenitic grains produced, but also the orientation is formed in the transverse direction of the joints. In this case, under the impact force which is perpendicular to the transverse direction of the joints, crack is formed and easily expands along the grain boundaries of coarse austenitic grains causing brittle fracture. Con￾sequently, weld metal part of the joints has the lowest toughness value. 3.3. Microstructural evolution mechanism Based on former analyses, it is clear that GTAW has taken effect on the microstructures of HAZs and weld metal during the welding process. At the initial stage of welding, the filler material under￾goes melting. When the welding process is finished, melted filler material begins to crystallize to form weld metal zone. According Fig. 14. EDX spectrums of (a) precipitates on the fracture surfaces of T92 base material and it’s HAZ; (b) and (c) precipitates on the fracture surfaces of S304H base material and it’s HAZ. J. Cao et al. / Materials and Design 32 (2011) 2763–2770 2769