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Measurement of the tensile and yield strength of boiler steels by small punch and tensile test methods/Aurukatlateraste tombetugevuse ja voolepiiri mootmine kuuliga surumise ning tombekatse meetodil.

1. INTRODUCTION

The increase of durability and operating reliability of power plant components is of vital importance in thermal engineering. For safe and reliable operation of the equipment it is highly significant to be able to determine the life consumption and to estimate the remaining life of the component or the time till the next mandatory inspection as accurately as possible. It allows to optimize the amount and cost of the inspections and to avoid unscheduled outages and unreasonable replacement of the components, which is expensive.

The components of a power plant suffer material deterioration in terms of strength decrease due to the change in the microstructure of the metal, caused by long-term operation at high temperatures under creep and cyclic fatigue. The influence of the long-term operation on tensile properties of boiler steels has been investigated in [1-8]. It has been shown that long-term exploitation of steel at elevated temperatures besides other changes leads also to the decrease of the tensile strength, measured at room temperature. Tensile strength, measured at the operating temperature decreases considerably more [3]. It has been shown [9] that in some cases the yield strength plays the key role in determining the 3R (run, repair, replacement) decision for a particular component.

In order to get reliable integrity assessment and remaining life estimation, the determination of material properties for in-service components in their current state of damage is very important. One possibility to estimate mechanical properties is the extraction of metal samples from the actual components. Two metal sampling machines, one for sampling from external surfaces of power plant components (Fig. 1) and the other one for sampling from the internal surface of the turbine rotor bore (Fig. 2) have been designed and developed. A detailed review of the metal sampling machines as well as metal sampling experience can be found in [10]. Acceptability of metal sampling from the in-service components has been analysed in [10], where it has been shown that metal sampling by using the mentioned sampling machines does not lead to the initiation of inadmissible stress concentrations. Components could stay in further operation without any loss of safety and the sampled areas did not need repair.

Since the metal samples should be small enough in order to allow continued safe operation of the components, the problem of determining the tensile properties on small test pieces arises. Over the past two decades a number of testing techniques, applicable to miniaturized specimens, have been developed. One of such techniques is small punch (SP) testing, proposed by Manahan et al. [11]. This technique has attracted attention and has been investigated by many researchers [12-19].

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

The SP testing technique is based on the determination of the force-displacement curve for a small disk-shaped specimen under a central force. A typical SP force--displacement curve is presented in Fig. 3. This curve can be divided into four distinct parts: I--elastic bending, II--plastic bending, III--membrane stretching, IV--plastic instability. These four deformation mechanisms have been studied in [20]. On the basis of SP test results, the force [F.sub.y] that corresponds to the limit of the elastic regime and initialization of plastic deformations and the maximum force [F.sub.max] can be determined. The relationships between these SP test parameters and tensile properties of the material have been obtained and analysed in [12,14,16].

Despite a number of investigations and reported FE studies of SP testing, this method is not yet standardized. Standardization procedures for determining mechanical properties, based on SP testing, are being developed through cooperation of several laboratories (members of EPERC--European Pressure Equipment Research Council) using both experiments and mathematical models. The results of the EPERC round robin provided foundation of the code of practice, which has been recently published as a Workshop Agreement [21].

The aim of the present work is to analyse the possibility to determine the tensile properties of the power plant component materials by means of the SP testing technique with main attention on the yield strength determination.

2. EXPERIMENTAL

In order to analyse the applicability of the SP testing technique for the determination of tensile strength and yield strength of some boiler steels, SP tests and conventional tensile tests were performed at room temperature. In total, 10 metal samples were extracted from power plant components--steam pipes, superheater tubes (20, 12Ch1MF, 12Ch11V2MF) and drum (16GNM). The chemical composition of these steels is presented in Table 1. Afterwards 32 specimens for SP testing and 24 specimens for tensile tests were prepared from these samples.

Manufacturing of standard cylindrical specimens (d = 3 mm, [L.sub.0] = 5 mm) and tensile tests have been carried out in accordance with EN 10002-1:2001. The miniature disk-shaped specimens for SP tests were prepared with dimensions of 8 mm in diameter and 0.5 mm in thickness by punching and machining with subsequent grinding on abrasive paper. The tests were conducted on a testing machine Instron 8516 under a constant displacement rate of 2.5 mm/min. The SP test fixture is shown schematically in Fig. 4. The specimen holder consists of upper and lower dies, supporting the specimen. A thread on the dies (not shown in Fig. 4) is used to apply a clamping force. Thus the disk specimen is clamped between two dies along the perimeter to the distance of 1 mm at the periphery by a force of 7 kN. The specimen is subjected to a central load, which is transmitted from the punch to the specimen by means of a hardened steel ball 4.8 mm in diameter.

[FIGURE 4 OMITTED]

3. RESULTS AND DISCUSSION

The results of the SP and conventional tensile tests are partially presented in Table 2. The high repeatability (0-2%) of the results of tensile strength, determined from tensile tests, are compared with significant lower repeatability (0-5%) of the maximum force measurements from SP tests. The results show that the maximum punch force is linearly related to the tensile strength (Fig. 5). In order to obtain the relationship between SP maximum force ([F.sub.max]) and tensile strength ([R.sub.m]), a linear least squares regression of experimental data was performed and the following regression equation was obtained (Fig. 5):

Rm = 0.184 Fmax [+ or -] 53, N/[mm.sub.2],

where [+ or -] 53 is standard error of the regression equation in N/[mm.sub.2].

[FIGURE 5 OMITTED]

Let us consider yield strength determination from SP tests in greater detail. There are two different approaches. The relationship between the yield strength and maximum punch force has been investigated in [14] and shown to follow a linear law for an elastic-perfectly plastic material. However, if both characteristics, the yield strength and the tensile strength, are linearly related to the maximum punch force, then the ratio of yield to tensile strength should be constant. On the basis of experience with common boiler steels it was found [22] that there is no functional relationship between yield strength and tensile strength (Fig. 6a) and the ratio of yield strength to tensile strength varies in a wide range (Fig. 6b). This may be explained by the influence of such factors as creep embrittlement, corrosion embrittlement, etc.

According to another approach for yield strength evaluation on the basis of the SP test proposed in [12,16], the yield strength can be determined as a function of the force, corresponding to the initialization of the plastic deformation. However, it has not appeared possible to determine the transition point from the elastic to the plastic region from the SP force-displacement curves of the investigated steels (Fig. 7). This means that the material yield strength could not be determined with sufficient accuracy using SP tests, neither on the basis of maximum punch force nor on the basis of the force, corresponding to the initiation of plastic deformation.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

4. CONCLUSIONS

Small punch tests of miniature disk-shaped specimens and conventional tensile tests of standard cylindrical specimens have been performed at room temperature with some commonly used boiler steels. The comparison of the results has shown that tensile strength of the metal can be reliably obtained as a linear function of the maximum punch force, determined in the small punch test. On the basis of conducted experiments it has been found that yield strength of the materials could not be determined with sufficient accuracy by means of small punch tests, neither on the basis of maximum punch force nor on the basis of the force, corresponding to the initiation of the plastic deformation.

Received 17 September 2008, in revised form 6 February 2009

REFERENCES

[1.] Krutasova, A. Reliability of Power Equipment Metal. Energoatomizdat, Moscow, 1981 (in Russian).

[2.] Berezina, T., Bugay, N. and Trunin, I. Diagnosis and Prognosis of Power Equipment Metal Life. Oekhnika, Kiev, 1991 (in Russian).

[3.] Antikain, P. Metals and Stress Calculation of Boilers and Piping. Energiya, Moscow, 1990 (in Russian).

[4.] Berezina, T. Key to Steels and Alloys in Thermal Engineering. Handbook. Chelyabinsk, 2004 (in Russian).

[5.] Ray, A. K., Tiwari, Y. N., Sinha, R. K., Chaudhuri, S. and Singh, R. Residual life prediction of service exposed main steam pipe of boilers in a thermal power plant. Eng. Failure Anal., 2000, 7, 359-376.

[6.] Nakoneczny, G. J. and Schultz, C. C. Life Assessment of High Temperature Headers. Babcock & Wilcox, 1995.

[7.] Cheruvu, N. S. Degradation of mechanical properties of Cr-Mo-V and 2.25 Cr-1Mo steel components after long-term service at elevated temperatures. Metall. Trans., 1989, V 20A(1), 87-97.

[8.] Smirnova, A., Balashov, Y., Tikhonova, T. and Ivanova, L. Degradation of structure and mechanical properties of steel 20 during long-term operation at temperatures higher than 450[degrees]C. Teploenergetika, 1993, 11, 11-13 (in Russian).

[9.] RD 10-577-03 Instruction of Metal Control and Life Extension of Basic Components of Boilers, Turbines and Piping at Power Plants, 2003 (in Russian); http://www.tehdoc.ru/ files.2045.html (26.03.09).

[10.] Dedov, A., Klevtsov, I., Lausmaa, T. and Neshumayev, D. Method of small samples for assessment of properties of power plant components: sampling devices and stress concentration in dimples. In Proc. Conference Plant Maintenance for Managing Life & Performance BALTICA VII, 2007, vol. 2, 180-192.

[11.] Manahan, M., Argon, A. and Harling, O. The development of a miniaturised disk bend test for the determination of post-irradiation mechanical properties. J. Nucl. Mater., 1981, 103-104, 1545-1550.

[12.] Fleury, E. and Ha, J. S. Small punch tests to estimate the mechanical properties of steels for steam power plant. I. Mechanical strength. II. Fracture toughness. Int. J. Pressure Vessels Piping, 1998, 75, 699-713.

[13.] Parker, J., Stratford, G., Shaw, N., Spink, G. and Metcalfe, H. The application of miniature disc testing for the assessment of creep damage in CrMoV rotor steel. In Proc. Conference Plant Maintenance for Managing Life & Performance BALTICA IV, 1998, vol. 2, 477-488.

[14.] Brookfield, D., Li, W., Rodgers, B., Mottershead, J., Hellen, T., Jarvis, J., Lohr, R., Howard-Hildige, R., Carlton, A. and Whelan, M. Material properties from small specimen using the punch and bulge test. J. Strain Anal., 1999, 34, 423-435.

[15.] Husain, A., Sehgal, D. and Pandey, R. Design of a simple, versatile, small-specimen punch test setup for determination of the mechanical behavior of materials. Exp. Techn., 2002, 33-38.

[16.] Ruan, Y., Spatig, P. and Victoria, M. Assessment of mechanical properties of the martensitic steel EUROFER97 by means of punch tests. J. Nucl. Mater., 2002, 307-311, 236-239.

[17.] Campitelli, E., Spatig, P., Bonade, R., Hoffelner, W. and Victoria, M. Assessment of the constitutive properties from small ball punch test: experiment and modelling. J. Nucl. Mater., 2004, 335, 366-378.

[18.] Milieka, K. and Dobes, F. Small punch testing of P91 steel. Int. J. Pressure Vessels Piping, 2006, 83, 625-634.

[19.] Egan, P., Whelan, M., Lakestani, F. and Connelly, M. Small punch test: An approach to solve the inverse problem by deformation shape and finite element optimization. Comput. Mater. Sci., 2006, 40, 33-39.

[20.] Manahan, M., Browning, A., Argon, A. and Harling, O. The use of small-scale specimens for testing irradiated material. ASTM-STP 888, American Society for Testing and Materials, Philadelphia, PA, 1986.

[21.] CEN Workshop Agreement, CWA 15627:2006 E, "Small Punch Test Method for Metallic Materials". CEN, Brussels, Belgium, December 2007.

[22.] Klevtsov, I., Dedov, A., Bogolyubova, E. and Boyarinova, T. Direct measurement of mechanical properties of metal to be used in manufacture of power plant equipment. Thermal Eng., 2008, 55, 431-434.

Ivan Klevtsov, Andrei Dedov and Artjom Molodtsov

Department of Thermal Engineering, Tallinn University of Technology, Kopli 116, 11712 Tallinn, Estonia; {klevtsov, dedov}@staff.ttu.ee, Artjom.Molodtsov@nj.energia.ee
Table 1 Chemical composition of investigated steels

Steel grade  Content, wt%

               C      Si      Mn      Cr

20           0.17-   0.17-   0.35-   max
             0.24    0.37    0.65    0.25
12Ch1MF      0.08-   0.17-   0.4-    0.9-
             0.15    0.37    0.7     1.2

16GNM        0.12-   0.17-   0.8-    max
             0.18    0.37    1.1     0.3
12Ch11V2MF   0.10-   max     0.5-    11-
             0.17    0.5     0.8     13

              Ni      Mo       V       W

20           max
             0.3
12Ch1MF      max     0.25-   0.15-   max
             0.3     0.35    0.3     0.2

16GNM        1.0-    0.4-
             1.3     0.55
12Ch11V2MF   max     0.6-    0.15-   1.7-
             0.6     0.9     0.3     2.2

              Cu       S       P

20           max     max     max
             0.3     0.04    0.035
12Ch1MF      max     max     max
             0.2     0.025   0.03

16GNM        0.15-   max     max
             0.25    0.04    0.035
12Ch11V2MF   max     max     max
             0.3     0.025   0.03

Steels are designated in accordance with [22].

Table 2. Results of small punch and tensile tests

Steel grade            Small punch test

                [F.sub.max],        Number of
                   mean,            specimens
                     N

12Ch1MF             3132                4
12Ch1MF             2479                2
12Ch1MF             2764                3
12Ch1MF             2423                3
12Ch1MF             2667                4
16GNM               2863                3
16GNM               2915                3
16GNM               3224                3
12Ch11V2MF          3846                5
20                  2807                2

Steel grade     Small punch        Tensile test
                    test

              Repeatability *,      [R.sub.m],
                     %                mean,
                                   N/[mm.sub.2]

12Ch1MF              3                 534
12Ch1MF              2                 480
12Ch1MF              0                 474
12Ch1MF              1                 455
12Ch1MF              0                 443
16GNM                4                 555
16GNM                5                 553
16GNM                3                 541
12Ch11V2MF           4                 823
20                   3                 468

Steel grade               Tensile test

                   Number        Repeatability *,
                     of                 %
                 specimens

12Ch1MF              3                  1
12Ch1MF              2                  0
12Ch1MF              2                  1
12Ch1MF              2                  1
12Ch1MF              3                  2
16GNM                2                  1
16GNM                2                  0
16GNM                2                  0
12Ch11V2MF           3                  1
20                   3                  0

* Repeatability is calculated by dividing the
standard deviation by the mean value.
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Author:Klevtsov, Ivan; Dedov, Andrei; Molodtsov, Artjom
Publication:Estonian Journal of Engineering
Article Type:Report
Geographic Code:4EXES
Date:Jun 1, 2009
Words:2420
Previous Article:Synergy-based approach to quality assurance/Sunergiapohine lahenemine kvaliteedikindlustusele.
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