Materials Science Research International, Vol.3, No,1 pp. 31-36 (1997)

General paper

REMAINING LIFE PREDICTION BY THE ISO-STRESS

METHOD OF BOILER TUBES AFTER PROLONGED SERVICE

Osamu KANEMARU, Masaru SHIMIZU, Toshio OHBA, Hideko MIYAZAKI,

Fujio ABE and Koichi MAGIEnvironmental Performance Division, National Research Institute for Metals,

2-2-54, Nakameguro, Meguro-ku, Tokyo 153, Japan

Abstract: The prediction of remaining creep life by the ISO-stress method was investigated for 153500h service-

exposed boiler tubes of 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels. At first, the internal pressure creep

rupture strength of the service-exposed tube specimens of the steels was shown to be the same as the tensile creep

rupture strength of the sheet specimens taken from the tubes. Then the temperature-accelerated creep rupture tests

under iso-stress were carried out by the tensile creep rupture tests, because of simplicity of the tests. The ISO-stress

method was verified for the virgin specimens of the steels by comparing the predicted long-term rupture times as long

as 105h with the data in the NRIM Creep Data Sheets. The predicted rupture times agreed with the observed data

within a factor of 2 or 3. The temperature-accelerated tests of the sheet specimens taken from the service-exposed tubes

showed that the linear extrapolation of the rupture times to the service temperature gave the remaining creep rupture

life of about 8•~105h for the 1.25Cr-0.5Mo-Si steel and about 1.6•~106h for the 2.25Cr-1Mo steel. However, the

ISO-stress method could not be applied for the 9Cr-1Mo steel, because the log tr versus T curve was not linear, where

tr is the creep rupture time and T is the testing temperature. It is concluded that the ISO-stress method is useful when the

significant microstructural change does not take place during accelerated creep rupture tests at high temperatures.

Key words: Remaining creep life prediction, ISO-stress method, Boiler tubes, Prolonged service, Ferritic steels,

Microstructural change

1. INTRODUCTION

For assessment of remaining creep life, creep rupture tests are carried out under stress and/or temperature accelerated conditions, and extrapolations to service stress and temperature are performed using appropriate parametric equations. The Iso-stress method is based on temperature-accelerated creep rupture tests under Iso-stress condition over a range of temperatures above the service temperature and on linear extrapolation of rupture times to the service temperature [1, 2]. Under iso-stress conditions the Manson-Haferd parameter reduces to a linear relationship between log tr and T, where tr is the creep rupture time and T is the testing temperature. The Iso-stress method has successfully been applied to boiler tube materials of low alloy steels such as 0.5Cr-0.5Mo-0.25V [3] and 1Cr-0.5Mo steels [2]. These steels exhibited a linear relationship between log tr and T. However, the applicability of the Iso-stress method has not

yet been systematically investigated for highly Cr contained Ferritic steels.

The purpose of the present research is to investigate the

prediction of remaining creep rupture life by the Iso-stress method for service-exposed boiler tubes of 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels. At first, the correlation between internal-pressure and tensile-creep rupture strength was confirmed for the serviced-exposed steels. Then the

temperature-accelerated tests were carried out by the tensile creep rupture tests. Next, the Iso-stress method was verified for the virgin specimens of the steels by comparing the

predicted long-term rupture times as long as 105 h with the data in the Creep Data Sheets by National Research Institute for Metals (NRIM). Finally, the remaining creep life of the service-exposed boiler tubes of the steels was examined by the Iso-stress method. The applicability of the Iso-stress method was discussed by taking account of the change in microstructural evolution with temperature.

2. MATERIALS AND TESTING PROCEDURES

The chemical compositions of the service-exposed tubes examined are given in Table 1, which were within the requirement by the Japanese Industrial Standards (JIS) for 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels. A part of tube component was removed from service-exposed

Table 1. Chemical composition of materials tested (mass%).

Received August 28, 1996

31

Osamu KANEMARU, Masaru SHIMIZU, Toshio OHBA, Hideko MIYAZAKI, Fujio ABE and Koichi YAGI

reheater which had been exposed for 153500h in a power boiler. The operating conditions and sizes of the boiler tubes are listed in Table 2. The tube specimens, the dimensions of which are shown in Fig. 1, were then subjected to internal

pressure creep rupture tests at 823 and 873 K for up to about 3000h. In order to clarify the effect of fuel ash deposit and oxide layer formed on the outer and inner surfaces of the tubes on the internal pressure creep rupture strength, two types of the tube specimens were prepared. One of them was the as-exposed tubes containing the surface-affected zones. Another was the machined tubes of which outer and inner surfaces were machined to remove the surface-affected zones. These are called in the following the as-exposed and surface-machined tube specimens. The machined thickness was about 0.8mm from the outer surface and about 0.5mm from the inner surface for the 1.25Cr-0.5Mo-Si steel tube, about 0.85mm from the outer surface and about 0.5mm from the inner surface for the 2.25Cr-1Mo steel tube and about 1.5mm from the outer surface and about 0.5mm from the inner surface for the 9Cr-1Mo steel tube. The hoop stress in the internal pressure creep rupture tests was evaluated using mean diameter formula expressed as

(1)

where P is the internal pressure force (MPa), D is the outside diameter (mm) and t is the wall thickness (mm). The details of the internal pressure creep rupture test procedures were described elsewhere [1]. The tensile creep rupture tests were also carried out for the service-exposed materials using sheet specimens taken from the tubes. Figure 2 shows the location of the sheet specimens sampled from the tubes.

The temperature-accelerated creep rupture tests were carried out by the tensile creep rupture tests for both the service-exposed and virgin materials, under iso-stress condition. The sheet specimens shown in Fig. 2 were used for the service-exposed materials. The round bar specimens, the same as those being used in the NRIM Creep Data Sheet Project, were used for the virgin materials. The testing temperatures were selected so that the rupture times were less than about 1000h.

The microstructural observations of the tube specimens were made in the vicinity of the outer surface, the middle

portion and the inner surface of the tubes by an optical microscope, and in the middle portion by a transmission electron microscope (TEM). The identification of precipitates was carried out by X-ray diffraction on electrolytically extracted residues.

3. RESULT AND DISCUSSION

3.1. Correlation Between Internal-pressure and Tensile-creep Rupture Strength

The stress versus time to rupture curves by the internal

pressure creep rupture tests for the service-exposed tubes of the 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels are

shown in Figs. 3, 4 and 5, respectively. The results for the as-exposed and surface-machined tube specimens are shown by the open symbols. The creep rupture strength of the steels was the same for the as-exposed and surface-machined tube specimens. This indicates that the fuel ash deposit and oxide layer formed on the tube surfaces scarcely affect the creep rupture strength of the tubes, although the microstructure in the vicinity of outer and inner surfaces were damaged during service exposure as shown in Figs. 6, 7 and 8.

In Figs. 3, 4 and 5, the results by the tensile creep rupture tests using the sheet specimens taken from the service-exposed tubes (the solid symbols) and the data bands for the

Table 2. Operation conditions of boiler.

Fig. 1. Profile and size of specimens for internal pressure

creep rupture test.

Fig. 2. Location of specimens sampled for the tensile creep rupture test, and profile and size of specimens.

32

REMAINING LIFE PREDICTION OF BOILER TUBES

virgin materials are also shown for comparison. The data in

the NRIM Creep Data Sheets, STBA 23 (1.25Cr0.5MoSi)

[4], STBA 24 (2.25CrMo) [5] and STBA 26 (9CrMo)

[6], are shown as the data bands for the virgin materials.

At higher stresses the creep rupture strength of the service-

exposed materials is lower than that of the virgin materials,

but at lower stresses the creep rupture strength of the present

materials nearly agrees with that of the virgin materials for

the 1.25Cr0.5MoSi and 2.25CrMo steels. The creep

rupture strength of the serviceexposed material of the 9Cr

Mo steel is lower than that of the virgin material. The

stressaccelerated creep rupture tests were unsuitable for the

assessment of remaining life. The temperatureaccelerated

tests using the Isostress method were verified. The creep

rupture strength by the internal pressure creep rupture tests

was the same as those by the tensile creep rupture tests for the

same service-exposed materials. The present results indicate

that the simple tensile creep rupture tests using the sheet

specimens are available to the prediction of the creep rupture

strength of the tubes instead of the complicated internal

pressure creep rupture tests. Then, we adopted the tensile

creep rupture tests for the temperature-accelerated creep

rupture tests under iso-stress which will be described in

the next section.

3.2. Applicability of the Iso-stress Method to Long

term Creep Life Prediction

The results of temperature-accelerated creep rupture tests

under iso-stress for the virgin materials of the 1.25Cr0.5Mo-

Si, 2.25Cr-Mo and 9CrMo steels are shown by the open

symbols in Fig. 9 as a function of testing temperature.

The straight lines also shown in this figure are linear

extrapolations of the short-term creep rupture times to lower

temperatures to predict long-term creep rupture times at low

temperatures. The solid symbols are the results of long-term

creep rupture times as long as 105h which were published in

the NRIM Creep Data Sheets, STBA 23 (1.25Cr-0.5Mo-Si)

[4], STBA 24 (2.25Cr-1Mo) [5] and STBA 26 (9Cr-1Mo)

[6]. The deviation of the predicted rupture times (shown by

the arrows) from the observed ones was estimated by a

parameter ogtr-logtr where the tr and tr are the

predicted and observed rupture times, respectively. It appears

that the ƒÂ depends on materials and stress levels. The

predicted rupture times agree very well with the observed

ones for the 1.25Cr-0.5Mo-Si steel at 118 MPa and for the

2.25Cr-1Mo steel at 7 8 MPa, where the absolute value of

is about 0.13 for both cases. In other cases, the agreement is

not good enough but the predicted rupture times are about 2

or 3 times smaller than the observed ones, =-0.31 to -0.42.

It is concluded that the Iso-stress method can predict the long

term creep rupture life as long as 105h within a factor of 2 or

3 for the steels.

3.3. Remaining Life Prediction of Service-exposed

Materials by the Iso-stress Method

Figure 10 shows the results of long-term creep life

prediction by the Iso-stress method for the service-exposed

Fig. 3. Internal pressure creep rupture test results of

1.25Cr-0.5Mo-Si steel tube exposed for 153500h.

Fig. 4. Internal pressure creep rupture test results of 2.25Cr-Mo steel tube exposed for 153500h.

Fig. 5. Internal pressure creep rupture test results of

9Cr-1Mo steel tube exposed for 153500h.

33

Osamu KANEMARU, Masaru SHIMIZU, Toshio OHBA, Hideko MIYAZAKI, Fujio ABE and Koichi YAGI

Fig. 6. Optical and TEM micrographs for 1.25Cr-0.5Mo-Si steel tube exposed for 153500h.

Fig. 7. Optical and TEM micrographs for 2.25Cr-IMo steel tube exposed for 153500h.

Fig. 8. Optical and TEM micrographs for 9Cr-1Mo steel tube exposed for 153500h.

materials of the three steels. The stress levels were near the

service stress conditions. The temperature-accelerated test results shown by the open symbols exhibit a linear

relationship between log tr and T for the 1.25Cr-0.5Mo-Si and 2.25Cr-1Mo steels but not for the 9Cr-1Mo steel. The

linear extrapolations of the temperature-accelerated test data to the service temperatures predict the remaining creep life

to be about 8•~105h for the 1.25Cr-0.5Mo-Si steel and

about 1.6•~106h for the 2.25Cr-1Mo steel as shown by the

arrows. The bars are also the extrapolated creep rupture

times at the same stress and temperature conditions, by

using the most popular Larson-Miller method based on the

data in the NRIM Creep Data Sheets. The predicted creep

rupture time of 1.25Cr-0.5Mo-Si steel by the Iso-stress

34

REMAINING LIFE PREDICTION OF BOILER TUBES

Fig. 9. Temperature-accelerated test results shown by the open symbols and linear extrapolations of these data to lower temperatures for the steels. The solid symbols are long-term creep rupture times in the NRIM Creep Data Sheets [4-6].

method (shown by the arrow) is shorter than the data band for the STBA 23 steel. The predicted value of 2.25Cr-1Mo steel is a little longer than the data band for the STBA 24 steel. However, the prediction of remaining creep rupture times by the Iso-stress method was not made for the 9Cr-1Mo steel, because of a non-linearity of the log tr versus T curve.

3.4. Microstructural Effects on Temperature-accelerated Test Results

The U-shape of the log tr versus T curve for the 9Cr-1Mo steel shown in Fig. 10 suggests that strengthening effects are operating in the service-exposed specimens at high temperatures above about 1050 K. The temperature-accelerated tests under the iso-stress of 11 MPa were also carried out for the virgin materials of the 9Cr-1Mo steel. This is shown in Fig. 11. The linearity of the log tr versus T curve was observed for the virgin specimens. The difference of the behavior between the service-exposed and virgin materials might be due to the difference of microstructurel change between both materials during the temperature-accelerated tests. Figure 12 shows the TEM microstructures of the service-exposed and virgin specimens of the 9Cr-1Mo steel before and after aging for 24h at 1023 and 1098 K. The aging was carried out to produce the microstructure of the specimens just before the temperature-accelerated creep rupture tests. In this work, the specimens were held for 24h at the testing temperature before the tests. The precipitates in the service-exposed specimens before aging were identified as M23C6, M7C3 and Cr2N. The major components were M23C6. Of the precipitates, M7C3 disappeared after aging at 1023 and 1098 K, indicating that M7C3 dissolved in the matrix. It should be noted that the aging temperatures, 1023 and 1098 K, were much higher than the service temperature, 900 K. The microstructure after aging at 1098 K was quite different from that before aging. A large number of fine needles of Cr2N were distributed in the matrix, while the amount of M23C6 decreased. The fine

Fig. 10. Remaining life prediction of the service-exposed

tubes of the steels by the Iso-stress method.

Fig. 11. Log tr versus T curves by temperature-accelerated tests for virgin (solid symbols) and service-exposed (open symbols) 9Cr-1Mo steel.

35

Osamu KANEMARU, Masaru SHIMIZU, Toshio OHBA, Hideko MIYAZAKI, Fujio ABE and Koichi YAGI

Fig. 12. TEM micrographs of service-exposed and virgin 9Cr-1Mo steel before and after aging.

needles were identified as Cr2N from the shape of precipitates

[7] and X-ray diffraction patterns on electrolytically extracted residues. A similar microstructure was also observed after creep rupture testing at 1098 K. No nitride was observed in the virgin specimens, although the carbides grew in size during aging. Therefore, it appears that the strengthening effects operating in the service-exposed specimens at 1098 K results from the fine precipitates of Cr2N formed at the testing temperature. The formation of Cr2N at 1098 K may have a correlation with the decrease in M23C6 precipitates, although detailed mechanisms are not clear at present. It is concluded that the linear relationship between log tr and T is obtained when the significant change in microstructure with temperature does not take place in the range of temperature-accelerated tests. Furthermore, for a reliable remaining life

prediction, the microstructure evolution during temperature-accelerated creep rupture tests should be similar to that at the service temperature

4. CONCLUSIONS

The remaining creep life prediction of boiler tubes after

prolonged service was investigated for the 1.25Cr-0.5Mo-Si, 2.25Cr-1Mo and 9Cr-1Mo steels by the Iso-stress method. The tubes had been exposed for 153500h in a power plant.

(1) The internal pressure creep rupture strength of the as-exposed tube specimens having fuel ash deposit and oxide layer agreed with that of the surface-machined tube specimens, and with the tensile creep rupture strength using sheet specimens.

(2) The applicability of the Iso-stress method was examined for the virgin specimens of the steels. The long-term creep rupture times as long as 105h predicted by the Iso-stress method agreed with the long-term data in the NRIM Creep

Data Sheets within a factor of 2 or 3.

(3) The remaining creep rupture life of the service-exposed

specimens of the steels was predicted by the Iso-stress

method. The linear extrapolation of the temperature-

accelerated tests to the service temperature gave the

remaining creep rupture life of about 8•~105h for the 1.25Cr-

0.5Mo-Si steel and about 1.6•~106h for the 2.25Cr-1Mo

steel. However, the Iso-stress method could not be applied

for the 9Cr-1Mo steel, because of a non-linearity of the log tr

versus T curve.

(4) The microstructural observations of the service-exposed specimens of the 9Cr-1Mo steel showed the formation of a large number of fine needles of Cr2N in the matrix at 1098 K, while M23C6 carbides were the major components in as-exposed condition. It appears that the fine precipitates of Cr2N operated as a strengthener at 1098 K, causing a longer rupture time at 1098 K and a non-linearity of the log tr versus T curve. It is concluded that for a reliable remaining creep life prediction by the Iso-stress method, the microstructural evolution during temperature-accelerated creep rupture tests should be similar to that at the service temperature.

REFERENCES1. O. Kanemaru, M. Shimizu, T. Ohba, K. Yagi, Y. Kato and

K. Hattori, Int. J. Pres. Ves. & Piping, 48 (1991) 167.2. R.V. Hart, Metals Technology, 3 (1976) 1.3. B.J. Cane and K.R. Williams, Proc. Int. Conf. on Mechanical

Behaviour of Materials (ICM-3), (ed. K.J. Miller and R.F. Smith), Pergamon Press, (1979) 265.

4. NRIM Creep Data Sheet, No.2A (1976).5. NRIM Creep Data Sheet, No.3B (1986).6. NRIM Creep Data Sheet, No.19A (1981).7. M. Kikuchi, T. Sekita, S. Wakita and R. Tanaka, Tetsu-To-

Hagane, 62 (1976) 981 (in Japanese).

36