Investigation of Bowing of Steam Turbine Rotor during. Long Term Service*

Transcription

1 Investigation of Bowing of Steam Turbine Rotor during Long Term Service* By Seishin KIRIHARA,** Masao SHIGA,** Mitsuo KURIYAMA,** Ryoichi SASAKI** and Katsukuni HISANO*** Synopsis Rotor bowing is a problem encountered in high pressure (HP) and intermediate pressure (IP) turbine rotors during a long term service. The problem occurred in the solid block of HP and IP turbine rotors where the main steam temperature was 566 C. In this paper, it is indicated that such rotor bowing is caused by the metallurgical heterogeneity of materials in rotor, and that the rotor materials should be manufactured under a precisely controlled heat treatment. The experimental results are summarized as follows: (1) Bowing of the rotor shaft during a long term service is caused by the scatter of creep strain rate in rotor material. (2) Difference of creep strain rate in the rotor section is caused by the non uniform temperature distribution during heat treatment. (3) The asymmetric distribution of sulphur segregation in a rotor section has no effect on the ratio of creep strain rate. (4) Temperature difference during heat treatment should be controlled within 6 C. In this case, the ratio of the maximum to the minimum creep strain rate in rotor shafts may be less than 1.20, and this prevents the rotor shafts from bowing during a long term service. (5) Rotating heat treatment is effective for the prevention of rotor bowing. I. Introduction Rotor bowing problem is experienced in high pressure (HP) and intermediate pressure (IP) turbine rotors during a long term service. The problem occurred in the solid block of HP and IP turbine rotors where the main steam temperature was 566 C and reheat temperature 566 C in the double flow-type of an I P rotor. According to the recent investigation, the rotor bowing occurred in some rotors which were manufactured more than 10 years ago. So, the rotors were adjusted their weight balance after the bowing. However, the bowing occurred again. Generally, it is said that rotor bowing may be caused by ununiform temperature during heat treatment and scatter of creep strain rate in the material used. In this paper, the rotor which showed bowing in service was investigated to clarify the causes of bowing. The contents of study are 1) investigation of the relationship between creep characteristics and metallurgical structure, and 2) development of prevention method of rotor bowing. II. Experimental Method 1. Specimen Table 1 shows the chemical composition. Table 2 shows the mechanical properties before service and the heat treatment condition. This material was sampled from the rotor in which the bowing occurred during 2 years operation (B-1). Figure 1 shows the relation between deflection and service time. Curve B-1 is for the B-1 rotor shaft. Curve B-2 is for the rotor used in another power plant. Figure 1 indicates Table 1. Chemical compositions of used material. (wt%) Table 2. Mechanical properties of used material. * ** *** Received April 10, Hitachi Research Laboratory, Hitachi Works, Hitachi, Ltd., 1984 ISIJ Hitachi, Ltd., Kujimachi, Saiwai-cho, Hitachi 317. Hitachi Research Article (107)

2 (108 ) Transactions ISIJ, Vol. 24, 1984 that the critical deflection 0.3 mm is attained in 2 to 4 years service. Figure 2 shows the sampling position of the creep test specimens. Figure 3 shows the configuration of creep test specimen. 2. Experiments Creep tests were conducted with specimens of 100 mm gauge length and 10 mm diameter at 550 C under the applied stress of 25 kg/mm2. The creep elongation is measured with a dial indicator (accuracy: 10-2 mm) and a differential transformer (accuracy: 10_c mm). Difference of temperature on the parallel portion of the specimen was adjusted within ± 1 C during the test. Brinell hardness test is conducted by using kg load. print. However, sulphur segregation has no effect on creep characteristics (see Fig. 7). 2. Microstructure and Hardness Photograph 1 shows the microstructure of the rotor material (X 100). There is little difference in the III. Experimental Results 1. Chemical Analysis Table 3 and Fig. 4 show the chemical compositions and their distribution, respectively. There is a peak value of sulphur content at the point mm apart from the center axis of the rotor on the concave surface side. On the convex surface side, the peak value of sulphur content is at mm apart from the center axis. Each location corresponds to the center line of the sulphur segregation band revealed by sulphur Fig. 2. Samplin g position of th e creep test specimens. Fig. 1. Relation between deflection and service time. Fig. 3. Configuration of creep test specimen (mm). Table 3. Chemical composition of used material. (wt%) Research Article

3 Transactions ISIJ, Vol. 24, 1984 (109) Fig. 4. Chemical material. composition of used Photo. 1. Microstructures. grain size at the rotor core vicinities on both the concave and the convex surface sides, as shown in the micrographs, C9 and 19 (austenite grain size G.S 4.0), On the other hand, the grain size on the concave surface side is remarkably smaller than that on the convex surface side, as shown in the micrographs C l (G.S 6.0), C3 (G.S 5.5), Ii (G.S 4.5) and 13 (G.S 4.0). This difference in the grain size is supposed to Research Article

4 (110 ) Transactions ISIJ, Vol. 24, 1984 depend on the uneven temperature distribution during austenizing heat treatment. Figure 5 shows the Brinell hardness distribution in the rotor material. Hardness is a little higher on the convex surface side than concave surface side by the uneven temperature distribution during heat treatment. Especially, the hardness around the convex surface is remarkably high. Table 4 shows the creep test result. Figure 6 shows the creep curve at 550 C under 25 kg/mm2 (245 MPa). The creep strain at Cl and C3 on the concave surface side is about twice as large as those at I1 and 13 on the convex surface at creep test time 500 h in spite of the distances from the center axis, mm for C3 and 13, 296 mm for C 1 and Ii. The mean value of the steady state creep rate shown in Table 4 on the concave surface (5.35 X 10_3 %/h) is 1.35 times as large as that on the convex surface (3.98 X 10_3 %/h). Difference of the creep rate on the concave side and convex side is caused by rotor bowing during service. Fig. 5. Hardness distribution section (HB). of rotor Fig. 6. Relation between and testing time. creep strain Table 4. Creep test results.

5 Transactions ISIJ, Vol. 24, 1984 (111) Fig. 7. Relation between and testing time treatment. creep after strain reheat Table 5. Temperature distribution in Sample A-1. Fig. 8. Relation between creep strain and testing time after rotating heat treatment. 685 C for 70 h and furnace cooled. Each curve is almost similar to each other, irrespective of the sampling location of each specimen whether it is on sulphur segregation band or not, and whether it is on the convex surface side or on the concave surface side. Therefore, it appears that sulphur segregation band does not affect the creep strain rate. Rotor bowing is caused by non-uniform temperature distribution during heat treatment. 3. Effect of Sulphur Segregation Band on the Creep Strain Rate Figure 7 shows the effect of sulphur segregation band on creep strain rate. Specimens were sampled from portions in segregation band and no segregation region in the rotor shaft section symmetrically. These specimens were reheated to relieve the effects of creep damage during service and grain size difference by the heat treatment for manufacturing. They were heated at 970 C for 3 h and quenched at a cooling rate of 100 C/h. They were tempered at 4. Improved Creep Strain Rate Distribution on Rotor Shaft by Precisely Controlled Heat Treatment Asymmetric creep strain rate is the result of the ununiform heat treatment. Therefore, temperature distribution should be precisely controlled. Table 5 shows the temperature distribution during heat treatment. Maximum temperature difference was less than 6 C in each case of quenching and tempering. Figure 8 shows creep curves of the rotor shaft heat treated by the rotating heat treatment method. The maximum value of steady state creep strain rate is 2.72x103 - %/h. The minimum value is 2.26x 10-3 %/h. So, the ratio of the maximum to the minimum is In case of the B-1 rotor after 2

6 (112 ) Transactions ISIJ, Vol. 24, 1984 years service, the maximum and minimum steady state strain rates are 4.75x 10_3 %/h and 3.20x 10_3 %/h, respectively. In this case, the ratio is It is suggested that the problem of bowing during a long term service in the rotor shaft is caused by the asymmetric temperature distribution during heat treatment. Iv. Conclusions The experimental results are summarized as follows : (1) Bowing of the rotor shaft during a long term service is caused by the scatter of creep strain rate in rotor material. (2) Difference of creep strain rate in the rotor section is caused by the ununiform temperature distribution during heat treatment. (3) The asymmetric distribution at sulphur segregation in a rotor section has no effect on the ratio of creep strain rate. (4) Temperature difference during heat treatment should be controlled within 6 C. In this case, the ratio of the maximum to the minimum of creep strain rate in rotor shafts may be less than 1.20, and this prevents the rotor shafts bowing during a long term service. (5) Rotating heat treatment is effective for the prevention of rotor bowing. REFERENCES 1) A. Chitty, M. R. Graham and M. C. Marphy: Iron Steel, (1971), April, 95. 2) T. Nakadaira, E. Kanazawa, Y. Kanou, K. Amano and M. Kawai: Preprint, The 8th International Forgemaster Meeting, Steel Casting and Forgings Assoc. Japan, Tokyo, (1977). Research Article