Failure Analysis of A Bolt in High Pressure Steam Turbine [PDF]

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Failure Analysis of a Bolt in High-Pressure Steam Turbine Article in Journal of Failure Analysis and Prevention · August 2016 DOI: 10.1007/s11668-016-0160-8

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Failure Analysis of a Bolt in High-Pressure Steam Turbine

Xue-qin Kang

Journal of Failure Analysis and Prevention ISSN 1547-7029 J Fail. Anal. and Preven. DOI 10.1007/s11668-016-0160-8

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Author's personal copy J Fail. Anal. and Preven. DOI 10.1007/s11668-016-0160-8

TECHNICAL ARTICLE—PEER-REVIEWED

Failure Analysis of a Bolt in High-Pressure Steam Turbine Xue-qin Kang

Submitted: 25 December 2015 / in revised form: 30 July 2016  ASM International 2016

Abstract In this research, a 25Cr2MoVA steel bolt used to assemble a high-pressure steam turbine cylinder in a power generation plant fractured after approximately 23 years of operation. Macrographic and micrographic analysis, scanning electron microscopy techniques, chemical analysis, tensile, impact and hardness testing were used to fully characterize the component and material properties. Based on thorough investigations, it has been identified that mechanical properties of connecting bolt of the cylinder were weakened by bulk ferrite during heat treatment and coarse grain boundaries during long-term use and these induce the bolt fracture under an impact force during the equipment startup process. It is recommended that all bolts in the same situation should be changed, or that high-pressure steam turbines should be carefully operated in order to avoid great impact during maintenance operations, especially during the equipment startup and shut-down process. This research clearly provides guidance for standard-setting committees and contributes to the prevention of similar types of accidents. Keywords Failure analysis
Mechanical properties
Metallurgical structure
Cleavage

Introduction Modern engineering systems exhibit outstanding performance through the technologies developed in recent decades. Because of these technological advances, the X. Kang (&) China University of Mining and Technology, Xuzhou 221116, Jiangsu, People’s Republic of China e-mail: [email protected]

analysis of most equipment failure is complicated. As part of the analysis, it is critical to identify the reason for the accident and to provide the useful information to prevent similar accidents. Unfortunately, most of the products’ failures are not easily accessible in academic literature since manufactures try not to publicize the drawbacks of their products. However, it is critical to share the previous experience or case studies in order to prevent similar accidents and to improve the current designs [1]. In the recent situation a large portion of electricity is being produced in aging thermal power plants [2], to increase generating efficiency of steam turbine is essential and unavoidable, so the working temperature of the steam turbine should be improved. Although excellent high-quality materials such as CrMoV steel and 12% Cr steel are used in thermal power plant equipments, varying forms of metallurgical degradation caused by creep or fatigue as well as complex working conditions, can affect the components during long-term operations at high temperatures [3]. The steam turbine cylinder bolt is among the most critical and highly stressed components in a modern power plant, and its failure often occurs in early and late operations. The consequences of bolt failure are severe in terms of both safety and economic impact. The economic loss can reach 3 million yuan per day in a 1000 MW power unit under full load operation. For this reason, electric power utilities and manufacturers quantify strict standards to improve the quality of the bolts. However, the deregulation of the electricity market, which has existed for approximately 20 years, has led energy companies away from continuous operations and toward more flexible operating schedules that allow them to maintain profitability in a competitive commercial environment [4]. The principal consequence of deregulation is an increase in annual startup cycles, which exacerbates the failure rate of the bolts. Many researchers have conducted studies about fracture in bolts

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used in steam turbines or properties of CrMoV steel. Ma et al. [5] reported that bolt fracture occurs because there are areas of microsegregation and creep cavities in the verge of the bolt used in the high-pressure main stop valve of the turbine. Liu et al. [6] proposed that fracture in the 30CrMnSiA bolt is caused by hydrogen embrittlement that is, in turn, caused by the high C content and coarse microstructure of martensite. Dong [7] reported that fracture in bolts used in the high-pressure main stop valve of the steam turbine is caused by improper heat treatment. Milan [8] pointed out that several factors, such as surface defects, thread root radius being smaller than the nominal value, low toughness, and stress concentration, have all contributed to crack nucleation. It is important to note that the bolt fracture studied in this literature occurred in the installation and initial stages of use. Callaghan et al. [9] studied the evaluation of low cycle fatigue behavior of 2.25Cr-1Mo steel at elevated temperature and established fatigue toughness to crack propagation. Failure analysis of a bolt used in a highpressure steam turbine over a long period of time and the sudden rupture have not been extensively studied. A better understanding of why a bolt that has been in use for a long time, fractures would not only enhance the safety of operations, but also guide the emendation of standards. In this research, we investigate the exact causes of a steam turbine cylinder breakdown. The fracture of the connecting bolt in the cylinder is fully investigated through various methods; for instance, fractography to check the bolt’s broken topography, tensile, impact and hardness tests to evaluate the material properties, and the energy spectrum analysis to determine the chemical characteristics. Based upon the observations and the identified causes of the failure we present, it is possible to develop a more appropriate standard to prevent similar accidents in future. The following sections describe the details of the systematic process of identifying the cause of the bolt failure.

1085 mm, and its diameter was 100 mm. Threads were ground using a milling machine after heat treatment, and the bolt was machined to a root radius of 0.412 mm and an angle of 60. The nut material was 35CrMoA. The bolts used in the steam turbine cylinder in question were installed in 1987 and maintained every 4 years. Hardness detection, metallurgical structure, crack detection, disassembling, and rigging were carried out in the course of the maintenance operation. The bolt ruptured when the steam turbine restarted after maintenance operations and reached a speed of up to 1000 rpm. The fractured bolt was removed from the cylinder with a round nut. To determine the cause of the bolt failure, the following procedures were abided. Firstly, the ruptured surface of the failed bolt was examined visually, and a full image of the failed bolt and typical macromorphology were taken using a Canon 60D camera. The failed bolt was cut in order to observe and analyze the fracture surface. A sawing machine was used to cut the bolt. Finally, the failed bolt was rendered into various specimens that were used for related experiments. Secondly, the chemical composition of the failed bolt was analyzed using a direct reading spectrometer (SPECTROMAXx) on the basis of GB/T4336-2002. The inclusions and machining quality of the threads were measured using optical metallographic microscopy (PMG3-2-613U) based on the user standard and GB/T 10561-2005, respectively. Scanning electron microscopy (SEM: Hitachi-S3000N) was utilized to investigate the micromorphology of the failed bolt. Finally, tensile, impact and hardness tests were carried out using an electronic universal testing machine (CSS44300), impact tester (JBN-300), and Brinell hardness tester (HB-3000) based on GB/T228-2002, GB/T229-2007, and GB/T2314-2009, respectively. The microstructure was analyzed according to GB/T 13299-1991.

Background and Investigation Procedure

Observation Results

There was an accident due to the failure of a connecting bolt of a steam turbine cylinder. Because of the bolt failure, the whole generating facilities stopped running. The failure bolt was made of 25Cr2MoVA steel. The bolt length was

Visual Observation

Fig. 1 Full image of the failed bolt with the nut

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Figure 1 shows the full image of the failed bolt with the nut. The fracture surface of the bolt is located where nut

Author's personal copy J Fail. Anal. and Preven.

contacted with the cylinder. The head of the bolt broke down from the failed bolt with the nut. The fracture surface of the failed bolt is shown in Fig. 2. The fracture occurred at the root of the thread near the head of the bolt, and the entire transverse section fractured. Visual observation shows that the fracture was not caused by plastic deformation; rather, that the bolt rupture was caused by brittle

fracture. The bolt has a splintery fracture, but the fracture surface is coarse. The source of the crack (the a zone shown in Fig. 2) begins at the root of the thread and then extends to the center of the bolt; this is the b zone shown in Fig. 2. The c zone marked in Fig. 2 is the final fracture zone. Figure 3 shows the details of the crack nucleation at the thread root. Scanning Electron Microscope (SEM) Observations Figure 4 shows the micromorphology of the b zone marked in Fig. 2. It is apparent that the fracture surface shows cleavage fracture characteristics and belongs to intracrystalline crack. There is no evidence of the fatigue failure mode, such as regular striation traces, on the fracture surface. In addition, a river pattern (marked in Fig. 4a) and secondary cracks (marked in Fig. 4b) appear on the fracture surface. These results indicate that the high-pressure steam turbine cylinder bolt was expected to rupture rapidly after the routine maintenance operations, because there were no fatigue failure features on the fracture surface. Machining Quality and Inclusions Inspection of Thread

Fig. 2 Fractured surface of the failed bolt

The machining quality of the thread is shown in Fig. 5. The machining quality of the dedendum (Fig. 5a) and addendum (Fig. 5b) matches the standard agreement. As Fig. 5 shows, the inclusions are A0.5, B0.5, C0.5, and D0.5, according to GB 10561-2005, and these meet the requirements of the matrix material. Chemical Composition

Fig. 3 Detail of the crack nucleation

The chemical composition of the failed bolt was determined by direct reading spectrometer, which is shown in Table 1. It can be seen that the composition of the failed

Fig. 4 Micromorphology of failed bolt fracture surface (a) cleavage feature and river pattern and (b) secondary cracks

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Author's personal copy J Fail. Anal. and Preven. Fig. 5 Optical micrograph of (a) dedendum and (b) addendum of failed bolt thread

Table 1 Chemical composition of the failed bolt (wt.%) Elements

C

Si

Mn

Cr

Mo

V

Measured value

0.25

0.28

0.56

1.67

0.31

0.28

GB/T 3077-2008

0.22–0.29

0.17–0.37

0.40–0.70

1.50–1.80

0.25–0.35

0.15–0.30

Table 2 Tensile properties of failed bolt Performance

Rp0.2 (MPa)

Rm (MPa)

A (%)

Z (%)

Measured value

810.6

945.1

16.2

57.8

DL/T 439-2006

C588

C735

C16

C50

Fig. 6 Necking phenomenon of the typical tensile specimen

bolt is within the specified range according to standard GB/ T 3077-2008.

Mechanical Characterization

Fig. 7 Macromorphology of the tensile specimen

Uniaxial Tensile Test Bolts used in high-pressure cylinders are subjected to tensile stress in the normal working process; therefore, the tensile performance of these materials should be tested. The specimen for the tensile test was cut from the failed bolt along the axial direction. The tensile specimen was fabricated with 100 mm of the gauge length and 10 mm of diameter. The tensile test was performed with a loading rate of 2 mm/min at room temperature, and the test results are shown in Table 2. As a result, the yield stress (Rp0.2),

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tensile strength (Rm), elongation percentage (A), and percentage reduction in area (Z) coincide with DL/T 4392006, as listed in Table 2, but the percentage reduction in area is close to the lower limit. Figure 6 shows the figure of the sample after a single tensile test. The necking phenomenon of the tensile specimen and longitudinal crack is observed. Figure 7 shows the macromorphology of the tensile specimen fracture surface. There is a significant amount of plastic deformation on the fracture surface of the tensile specimen. The

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fracture surface includes a coarse radiation area and longitudinal secondary cracks. Figure 8 shows the micromorphology of the tensile specimen fracture surface using SEM. The fracture surface was mixed with secondary cracks (marked in Fig. 8a) and dimples (marked in Fig. 8b). The character of this macro- and micromorphology is different from the fracture surface of the failed bolt, and this indicates that the fracture of the bolt is not caused by static tensile stress. Impact and Hardness Test Bolts used in high-pressure cylinders that are subjected to impact force when the pressure changes in the cylinder should be tested for their impact properties. The specimen for the impact test was cut from the failed bolt along the axial direction. The impact test was performed at room temperature and 20 C. The test result is shown in Table 3. The impact absorption energy of the ‘‘U’’-shaped notch specimen shows a higher value when compared with the material specifications of DL/T 439-2006, as listed in Table 3. Hardness can reflect the comprehensive performance of materials. A hardness test is commonly used to test material performance because it is a simple operation and can be conducted in a production site without damaging the materials being tested; thus, some standards require its use to measure material hardness. It is necessary to verify

whether excessive heating occurred in the process of running. Therefore, the hardness of the failed bolt is measured at 750 kg of loading and with 10 s of loading time. The hardness result is shown in Table 3. It is apparent that the Brinell hardness value exceeds the range of the material standard DL/T 439-2006, as listed in Table 3. Figure 9 shows the macromorphology of the impact specimen’s fracture surface. The fracture does not exhibit plastic deformation and rather was likely caused by brittle fracture. The fracture surface includes the crack source (a zone), crack radiation area (b zone), and transient fracture area (c zone), which correspond with the a, b, and c zones marked in Fig. 2. Figure 10 shows the micromorphology of the b zone marked in Fig. 9 using SEM. The fracture surface was mixed with cleavage fractures and secondary

Fig. 9 Macromorphology of the impact specimen fracture surface

Fig. 8 Micromorphology of tensile specimen fracture surface (a) dimple fracture and (b) high magnification Table 3 Impact properties and hardness of the failed bolt 20 C

Room temperature Performance

‘‘U’’ shaped

‘‘V’’ shaped

‘‘U’’ shaped

‘‘V’’ shaped

HBS

DL/T 439-2006 (J/cm2)

C59

…

…

…

241–277

Measured value (J/cm2)

69

22.7

31.7

12

280

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Fig. 10 Micromorphology of the impact specimen fracture surface (a) cleavage fracture and secondary cracks and (b) river pattern

Fig. 11 Metallurgical structure of (a) tempering sorbite and ferrite and (b) grain boundary at high magnification

cracks (marked in Fig. 10b). A river pattern shown in Fig. 10a is the typical characteristic of cleavage, which is evidence of brittle fracture [10–12]. When compared with the failed bolt fracture surface (Fig. 4), the macro- and micromorphology agree. This proves that the failed bolt suffered from impact force and then fractured in the process of running. Optical Micrographs Figure 11 shows the metallurgical structure of the failed bolt. The metallurgical structure is tempered sorbite and bulk ferrite. There is some bulk ferrite marked on Fig. 11a on the tempered sorbite matrix of the failed bolt. The grain boundary marked on Fig. 11b is clearly at high magnification, and this image indicates the carbide or impurities gathered at the location shown. According to DL/T 4392006, we know that this phenomenon can change the properties of materials; for example, hot brittleness, although these properties still meet the requirements of the standard under room temperature.

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Discussion Based upon the test and analysis results described in previous sections, the main causes of the steam turbine cylinder accident are the following. There exists bulk ferrite because quenching temperature is low and/or the heat preservation time is short during heat treatment and coarse grain boundaries during long-term use. The bulk ferrite and coarse grain boundaries induce the brittleness of failed bolt. Eventually, the bolt was cracked under impact force during the equipment startup process. It is indicated that the uniform tempered sorbite is the optimum microstructure. However, the mechanical properties can be reduced by bulk ferrite. Coarse grain boundary is mainly due to the accumulation of impurities and this also weakens the mechanical properties of the bolt. In the given accident, it was identified that the first crack occurred in bolt thread roots. Thus, the relatively large stress was concentrated on thread bolt and cracks were gradually propagated under impact force during the equipment startup process.

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Conclusion A 25Cr2MoVA steel bolt used to connect a high-pressure steam turbine cylinder in a power generation plant failed due to the material brittleness and impact force that occurred during the equipment startup process. It is recommended that all bolts in the same situation be changed, or that high-pressure steam turbines should be carefully operated in order to avoid great impact during maintenance operations, especially during the equipment startup and shut-down process. Exacting maintenance standards should be formulated to control the quality of new and in-use parts. Acknowledgments The study reported in this article was supported by ‘‘the Fundamental Research Funds for the Central Universities (2015XKZD01)’’ and ‘‘the Priority Academic program Development of Jiangsu Higher Education Institutions.’’

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