Effect of Full Annealing PWHT on a Long -term Creep Strength of 2.25Cr-1Mo Steel Welded Joint

Transcription

1 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug Effect of Full Annealing PWHT on a Long -term Creep Strength of 2.25Cr-1Mo Steel Welded Joint Kazuhiro Kimura, Takashi Watanabe, Hiromichi Hongo, Masayoshi Yamazaki, Jun-ichi Kinugawa and Hirosada Irie National Institute for Materials Science, Nakameguro, Meguro-ku, Tokyo , Japan. Abstract The effect of a full annealing post weld heat treatment, in the austenitic single phase temperature region, on the long-term creep strength of has been investigated and compared with that of a normal PWHT, at a temperature below AC1. The creep rupture strength of the welded joint subjected to a normal PWHT is lower, in the short-term, than those of the base metal and weld metal. These differences in creep strength disappear after long-term exposure for about 50,000h at 550 o C. On the other hand, the creep strength of the welded joint subjected to a full annealing PWHT is higher than those of the other steels in the long-term condition. The full annealing PWHT is also expected to reduce the probability of type IV cracking.. Full annealing in the austenite single phase temperature region has been proposed as a new PWHT condition to attain high creep strength and to improve the reliability of welded joints. 1. Introduction In order to maintain good performance and adequate reliability in high temperature structural components, it is important to obtain high creep strength and to control mechanical damage in the weldment and heat affected zone, such as type IV cracking [1-3]. Since the occurrence of mechanical damage is promoted by an inhomogeneous microstructure in the weldment and HAZ, such inhomogeneity in the microstructure should be eliminated. Recently, it has been reported that the creep strength of the 0.5Cr-0.5Mo steel with a fully annealed ferrite and pearlite microstructure is higher than that of steels with microstructures of martensite, tempered martensite and bainite, at very long times [4]. Full annealing, in the austenit ic single phase temperature region, should dissolve any inhomogeneity of microstructur formed during welding and, therefore, may be effective not only for improving long-term creep strength but also for the suppression of mechanical damage in the weldment and HAZ. In the present study, the effect of a full annealing PWHT, in the austenitic single phase region, on the long-term creep strength has been investigated on a welded joint at 550 o C, and the possibility of improving the mechanical properties of the weldment, by a full annealing PWHT, has been investigated, by comparison with normal PWHT conditions.

2 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug Experimental procedure A (JIS SCMV 4NT) plate with a thickness of 50mm was used in this study. The weldment was produced by narrow gap sub-merged arc welding, and the precise welding conditions have been reported in previous papers [1, 3]. The chemical compositions of the base metal and the weld metal are shown in Table 1. There is no significant difference in chemical composition between base metal and weld.. Two types of post weld heat treatment condition, normal condition at a temperature below A C1 (690 o C PWHT), and full annealing condition in the austenitic single phase temperature region (930 o C PWHT), as shown in Table 2, were employed. Full annealing PWHT was conducted at the same temperature as that used for normalizing the base metal. Table 1 Chemical compositions (mass%) of the base metal and the weld metal studied C Si Mn P S Cu Ni Cr Mo Al Base metal Weld metal Table 2 Heat treatment conditions of the base metal and two types of post weld heat treatment Base metal Normalizing Tempering 930 o C x 100min. / Fun cooling 730 o C x 105min. / Air cooling PWHT Normal condition Full annealing condition 690 o C x 28h / Air cooling 930 o C x 30min. / Furnace cooling 3. Results and discussion 3.1 Effect of initial microstructure Creep rupture strengths at 550 o C of s with different initial microstructures, of annealed ferrite and pearlite (JIS STBA 24 [5]), bainite (JIS SCMV 4NT [6]) and tempered martensite (ASTM A542 [7]), are shown in Figure 1. The creep rupture strength of the steel with the microstructure of tempered martensite is higher than that of the other steels in the short-term and large differences in creep rupture strength are observed. These differences in creep rupture strength, however, decrease with decreasing applied stress. In contrast, in the long-term, over about 50,000h, the steel with the annealed ferrite and pearlite microstructure has a tendency to show a slightly higher creep rupture strength than the other steels [8]. From

3 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug these results, it has been concluded that creep the strength of at 550 o C is governed by the inherent creep strength in the long-term, over about 50,000h [9] Bainite Tempered martensite Stress / MPa Ferrite + Pearlite Time to rupture / h 550 o C Figure 1 Stress vs. time to rupture curves of the three types s at 550 o C 3.2 Effect of welding The creep rupture strengths at 550 o C, of the base metal, the weld metal and the welded joint, subjected to a 690 o C PWHT, are shown in Figure 2. In the short-term, the creep rupture strengths of the weld metal and the welded joint, subjected to a 690 o C PWHT, are lower than that of the base metal, and the creep rupture lives of the weld metal and the welded joint, subjected to a 690 o C PWHT, are about 1/3 and 1/5 of that of the base metal, respectively. However, such differences in creep rupture strength decrease with decreasing applied stress. No significant difference in the creep rupture strength is observed for these three materials in the long-term, over about 50,000h. This disappearance of the differences in creep rupture strength in the long-term corresponds to a decrease in creep strength to the level of the inherent creep strength [9], as mentioned above. Bright field TEM images of the weld metals (a) in the as 690 o C PWHT condition, (b) creep ruptured after 77,845h at 550 o C-78MPa and (c) the base metal creep ruptured after 54,581h at 550 o C-78Mpa, are shown in Figure 3. A rapidly solidified and quenched fine microstructure with a high dislocation density was observed in the weld metal even after a PWHT at 690 o C (Fig.3 a). However, significant progress in recovery, such as coarsening of carbide particles and subgrains and decrease in dislocation density, is observed in the weld metal creep ruptured after 77,845h at 550 o C-78MPa (Fig.3 b). The significantly recovered microstructure of this creep ruptured weld metal (Fig.3 b) is similar to that of the base metal creep ruptured after 54,581h at 550 o C-78MPa (Fig.3 c), and there is no obvious difference between these two microstructures.

4 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug Stress / MPa Base metal Weld metal o C PWHT Time to rupture / h JIS SCMV4NT 550 o C Figure 2 Stress vs. time to rupture curves at 550 o C of a base metal, a weld metal and the welded joint subjected to 690 o C PWHT Although the short-term creep strength of the weld metal and the welded joint, subjected to a 690 o C PWHT, are lower than that of the base metal, due to the rapidly solidified and quenched fine microstructure of the weld metal and heat affected zone, such effects of initial microstructure on the creep strength disappear after long-term creep exposure, for about 50,000h at 550 o C. Therefore, it has been considered that the almost identical long-term creep strengths for these materials are governed by the inherent creep strength of the 2.25Cr-1Mo steel [9]. a) Weld metal in the as 690 o C b) Weld metal creep ruptured c) Base metal creep ruptured PWHT condition. after 77,845h at 550 o C after 54,581h at 550 o C. Figure 3 Bright field TEM images of the weld metals (a) in the as 690 o C PWHT condition, (b) creep ruptured after 77,845h at 550 o C-78MPa and (c) the base metal creep ruptured after 54,581h at 550 o C-78MPa.

5 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug Effect of full annealing post weld heat treatment Creep curves at 550 o C-78Mpa, for the base metal, the weld metal and the welded joints subjected to 690 o C PWHT and 930 o C PWHT, are shown in Figure 4. The creep test duration of the welded joint subjected to 930 o C PWHT is about 25,000h, and this creep test is still in progress. The creep rupture strength under the creep test conditions of 550 o C-78MPa is considered to be governed by its inherent creep strength, as mentioned above. The creep strain of about 0.012, at about 25,000h, for the welded joint subjected to 930 o C PWHT, is smaller than those of the other materials, and is about 1/3 of that of the welded joint subjected to a 690 o C PWHT and less than 1/2 of the creep strains of the base metal and the weld metal. The creep strain of the welded joint is clearly reduced by the full annealing post weld heat treatment at 930 o C in the austenit ic single phase region, in comparison with the normal condition of PWHT at the temperature of below A C1. It should be noted that the creep strain of the welded joint subjected to a 930 o C PWHT is smaller than that of the base metal. Consequently, the creep strength of the welded joint subjected to the full annealing post weld heat treatment is higher than that of the base metal in the normalized and tempered condition. Strain JIS SCMV4NT 550 o C - 78MPa 690 o C PWHT Base metal Weld metal o C PWHT Time / h Figure 4 Creep curves at 550 o C-78MPa of the base metal, the weld metal and the welded joints subjected to 690 o C PWHT and 930 o C PWHT Creep rate vs. time curves at 550 o C-69MPa of the base metal, the weld metal and the welded joints subjected to 690 o C PWHT and 930 o C PWHT are shown in Figure 5. The creep test duration of the welded joint subjected to 930 o C PWHT is about 23,000h, and this creep test is still in progress. Very similar creep deformation behaviour, and almost identical creep rupture life, are observed for all the materials, except for the welded joint subjected to 930 o C PWHT. A significant decrease in creep rate, resulting from the full annealing post weld heat treatment at 930 o C, can be clearly observed. The creep rate vs. time curve of the welded joint subjected to 930 o C PWHT shows a humpat about 1,000h, and this minimum creep rate is almost the same as those of the other materials at 1,000h. However, the creep rate of the welded joint subjected to a 930 o C PWHT is smaller than those of the other materials before this hump, and decreases significantly with increasing creep test duration, after the hump, up to about 10,000h. The creep rate at 10,000h is about 1/5 of those of the other materials.

6 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug Creep rate / h Base metal JIS SCMV4NT 550 o C - 69MPa 690 o C PWHT Time / h Weld metal 930 o C PWHT Figure 5 Creep rate vs. time curves at 550 o C-69MPa of the base metal, the weld metal and the welded joints subjected to 690 o C PWHT and 930 o C PWHT. Stress vs. minimum creep rate curves at 550 o C, of the base metal, the weld metal and the welded joints subjected to 690 o C PWHT and 930 o C PWHT, are shown in Figure 6. The minimum creep rates of the weld metal and the welded joint subjected to 690 o C PWHT are larger than that of the base metal in the higher stress region. However, such differences decrease with decreasing stress, and the minimum creep rate at stresses lower than 80Mpa, are almost the same for all the materials, except for the welded joint subjected to 930 o C PWHT. The welded joint subjected to a 930 o C PWHT shows a lower minimum cree p rate than the other materials, even in the low stress region. As the strengthening effect of the full annealing is useful in the long-term where creep strength is governed by its inherent creep strength [4], it may be expected that this improvement in the creep strength by full annealing will be more effective under the low stress service conditions typical of high temperature structural components.

7 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug JIS SCMV4NT 550 o C Minimum creep rate / h Weld metal 690 o C PWHT Stress / MPa 10 Base metal 930 o C PWHT Figure 6 Stress vs. minimum creep rate curves of the base metal, the weld metal and the welded joints subjected to 690 o C PWHT and 930 o C PWHT Moreover, the development of mechanical damage in the weldment, such as type IV cracking, may be suppressed. The chemical compositions of the base metal and the weld meta l are almost the same. In addition, the rapidly solidified and quenched fine microstructure with a high dislocation density found in the weld metal, and the inhomogeneous microstructure found in the heat affected zone, have been completely eliminated during the full annealing post weld heat treatment at 930 o C in the austenitic single phase region. Consequently, it has been concluded that an improvement of the long-term creep strength, and the suppression of mechanical damage, is attained for low alloy Cr-Mo steel weldments by a full annealing post weld heat treatment in the austenitic single phase region. 4. Conclusions Investigation of the effects of a full annealing post weld heat treatment on the creep strength of a welded has shown that: (1) Differences in the short-term creep strength of, caused by varying initial microstructures, decrease with an increase in creep exposure time, and disappear after long-term creep exposure for about 50,000h at 550 o C. (2) Even though the short-term creep strength of the weld metal and the welded joint subjected to 690 o C PWHT are lower than that of the base metal, the creep strength of these materials are almost the same in the long-term, after about 50,000h at 550 o C. (3) The long-term creep strength of the welded joint, subjected to 930 o C PWHT, is higher

8 Effect of Full Annealing PWHT OMMI (Vol. 2, Issue 2) Aug than those of the other materials, and it may be expected that the strengthening effect of the full annealing PWHT on the creep strength of welded joints is more effective under the low stress service conditions typical of high temperature structural components. (4) The development of mechanical damage, such as type IV cracking, may be suppressed by a full annealing post weld heat treatment in the austenitic single phase region, as the microstructural defects in the weldment and heat affected zone have been eliminated. (5) It has been concluded that the improvement of long-term creep strength and the suppression of mechanical damage is attained for low alloy Cr-Mo steel weldment by a full annealing post weld heat treatment in the austenitic single phase temperature region. References 1. T. Watanabe, M. Yamazaki, H. Hongo, J. Kinugawa and Y. Monma, J. Soc. Mater. Sci. Jpn., 45(1996), p T. Ogata and M.Yaguchi, J. Soc. Mater. Sci. Jpn., 47(1998), p T. Watanabe, M. Yamazaki, H. Hongo, J. Kinugawa, T. Tanabe and Y. Monma, J. Soc. Mater. Sci. Jpn., 48(1999), p K. Kimura, H. Kushima, E. Baba, T. Shimizu, Y. Asai, F. Abe and K. Yagi, Proc. 5th Inter. Charles Parsons Turbine Conf., Advanced Materials for 21st Century Turbines and Power Plant, The Institute of Materials, Cambridge, UK, July 2000, p NRIM Creep Data Sheet, Data Sheets on the Elevated-Temperature Properties of 2.25Cr-1Mo Steel for Boiler and Heat Exchanger Seamless Tubes (STBA24), 3B(1986). 6. NRIM Creep Data Sheet, Data Sheets on the Elevated-Temperature Properties of Normalized and Tempered 2.25Cr-1Mo Steel Plates for Boilers and pressure Vessels (SCMV 4 NT), 11B(1997). 7. NRIM Creep Data Sheet, Data Sheets on the Elevated-Temperature Properties of Quenched and Tempered 2.25Cr-1Mo Steel Plates for Pressure Vessels (ASTM A542), 36A(1991). 8. H. Kushima, K. Kimura, F. Abe, K. Yagi, H. Irie and K. Maruyama, Tetsu-to-Hagané, 85(1999), p K. Kimura, H. Kushima, F. Abe and K. Yagi, Mater. Sci. Eng., A (1997), p.1079.