A development of swept-frequency eddy current for aging characterization of heat resistant steel

Abstract

This study presents an application of the swept-frequency eddy current (SFEC) technique for characterization of aging conditions of 5Cr-0.5Mo heat resistant steel by a U-shaped, eddy current probe. An optimal eddy current probe was established and employed to investigate the specimens under different simulated service conditions. Sweep frequencies from 500 to 10,000 Hz were generated to measure the impedance changes using a precision LCR meter. As results, the normalized impedances showed the highest differences in each aging condition at the frequency of 3,200 Hz. Microstructural observations were utilized to give details of the differences in microstructures among aged conditions, which related to the carbide precipitation characteristics of the specimens. In addition, a micro-hardness was tested on the carbide colonies in complement the aged conditions. The results showed that the hardness correlated well to the normalized impedances obtained from the swept-frequency eddy current testing.

Keywords

Eddy current heat-resistant steel swept-frequency carbide precipitates

1. Introduction

Heat resistant steels have been broadly applied in steam turbine power plants due to its outstanding creep and oxidation resistance at high temperature and pressure services. The creep resistance depends on the carbide formation, aging conditions, and coalescent of different types of carbides [1]. Types of carbide will depend on the time and temperature of the parts in services. Chemical composition of these Cr-Mo steels has been designed to withstand in such demanding service conditions which will occur the carbide precipitation after some period of services. Therefore, the occurrence and dissolution of carbide precipitates during service are the aspect of interest in order to determine the age of in-serviced components.

An eddy current testing is generally used in a variety of industries to detect flaws, identify materials as well as the physical properties of materials. Up till now, this technique has been developed to characterize the microstructure of materials by using a principle of measuring the changes in the electrical conductivity and magnetic permeability of materials [2, 3, 4]. Evaluation of material hardness, heat treatment condition, and residual stress were found in various researches [5, 6, 7, 8]. From those results, the conductivity is the greatest key to the eddy current evaluation of non-ferromagnetic materials which directly affected by the material microstructure changes. Unfortunately, as a complex system of high permeability variation, it is difficult to apply the normal eddy current technique to evaluate the microstructure of ferromagnetic materials. The results are also affected by the robustness of measurement. There were some researches played the specific eddy current techniques with surface carbon content on carburized steel [9, 10, 11]. The consequences of their works related with the observing microstructures of carbon content on the specimen surfaces. Additionally, correlation results with the material properties in microstructures confirmed the promising eddy current techniques for quantitative characterization without resorting to destructive testing [12, 13]. Due to the lack of microstructural data of carbide properties and distribution as related to eddy current responses in ferromagnetic materials, a promising eddy system based on the swept frequencies was studied.

This study attempted to use the swept-frequency eddy current to measure the impedance responses as a function of microstructure changes in heat resistant steels. The different service conditions of the specimen (ASTM 335, 5Cr-0.5Mo) simulated by an isothermal aging process were characterized by a specific the U-shape eddy current probe using the Swept-frequency Technique.

2. Materials and methods

2.1 Material and heat treatment

The material for this study was 5Cr-0.5Mo steel provided by the Electricity Generating Authority of Thailand (EGAT) in the shape of a 5-mm thick tube with 40-mm diameter. The chemical composition of the as received tube was given in Table 1.

Table 1
Chemical composition of 5Cr-0.5Mo steel by Optical Emission Spectroscopy

Chemical composition (Wt. %)
C	Cr	Mo	Si	S	P	Mn	Ni	Cu	Al	Sn	Fe
0.14	4.8	0.5	0.42	0.004	0.015	0.44	0.05	0.01	0.004	0.034	Bal.

Temperature ( $T$ ) and time ( $t$ ) of heat treatment were used to simulate the age of services of the boiler tubes in terms of thermal and time experiences developed from the Larson-Miller equation expressed in the Eqs (1) and (2) [13]. Typically, the Larson-Miller equation could be used to correlate with the mechanical properties, the terms of $Q$ /2.3 $R$ ( $Q$ is the activation energy and $R$ is the gas constant) known as the Holloman-Jaffe parameter, and it is very effective for determining hardness changes in Cr-Mo steels. Table 2 showed the heat-treatment parameters for simulating the desired service ages at the service temperature of 560 ${}^{\circ}$ C.

$\displaystyle\textit{LM parameter}=T\left({20+\log_{10}t}\right)=\frac{Q}{2.3R}$ (1) $\displaystyle t_{\textit{heat treatment}}=10^{\frac{\left[{T_{\textit{service}% }\left({20+\log t_{\textit{service}}}\right)}\right]}{T_{\textit{heat % treatment}}}-20}$ (2)

Table 2

Heat-treatment parameters of as received, 100,000, 200,000, 300,000, and 400,000 h specimens

Sample/Aging	Cooling rate	Service temperature	Simulating temperature	Simulating time	LM
(h)	( ${}^{\circ}$ C)	( ${}^{\circ}$ C)	( ${}^{\circ}$ C)	(min)	parameter
(A) As received	–	–	–	–	–
(B) 100,000	100	560	750	170	20.92
(C) 200,000	100	560	750	302	21.17
(D) 300,000	100	560	750	420	21.32
(E) 400,000	100	560	750	530	21.43

The heat-resistant steel coupons (ASTM 335, 5Cr-0.5Mo) with dimensions 25 $\times$ 65 $\times$ 4 mm were subjected to the isothermal aging to simulate the service times of 100,000 h, 200,000 h, 300,000 h, and 400,000 h while the as-received specimen (35 $\times$ 65 $\times$ 4 mm) was also used as a parent material for references. All of the samples were cut, pressed, ground and polished to yield a smooth surface as seen as Fig. 1.

Figure 1.

Aging specimens at simulated service times of: as-received, 100,000, 200,000, 300,000, and 400,000 h (left to right).

2.2 U-shape eddy current probe

In order to increase sensitivity for detection of microstructural changes in heat resistant steel, each eddy current probe was designed and developed using the high-permeability ferromagnetic core in a U-shape wrapped by a copper wire as the schematic shown in Fig. 2. This probe in the previous version was used to assess the microstructures of grade T22 steel in different heat treatments [12]. The advantage was the uniform magnetic flux produced between poles of the U-shape probe parallel to the surface of the specimen, which could be sensitive to the variations of microstructures in the material than the normal pancake eddy current probe.

Figure 2.

Schematic of U-shaped eddy current probe.

The updated version of the U-shape eddy current probe consisted of a ferrite core with its relative initial permeability of 10,000 $\pm$ 25% and frequency bandwidth to 200 kHz at 3 dB from JFE Co. Ltd. The dimension was 16 $\times$ 6 $\times$ 10 mm (W $\times$ T $\times$ H). A copper wire of AWG 40 was turned on the ferrite core to get the optimal output and frequency response. Three ranges of frequency responses, at 300 Hz, 2,000 Hz and 10,000 kHz, were chosen to determine the appropriate number of coil turns by the Eq. (3). As calculated, the ferrite-core probes were coiled to approximately 500 turns for probe (A), 100 turns for probe (B), and 20 turns for probe (C).

$\displaystyle e_{1}=-N_{1}\frac{d\phi}{dt}$ (3)

2.3 Swept-frequency technique and impedance analysis

To determine the optimal sensitivity of the three probes, they were tested and monitored by the swept-frequency technique. A G-tech model 2002 oscillator generated sine wave amplitude at 10 Vp-p to the probes. The probe voltage outputs, which were placed on the surface of the ferrite specimen, were measured as an ideal magnetic circuit. The voltage outputs of the air coil were then used as a reference for examining the voltage output difference. A digital storage Oscilloscope (OSC) (Agilent model 54624A) was employed to measure and collect data while sweeping the frequencies.

Figure 3.

Experiment setup of swept-frequency eddy current technique.

To evaluate the specimen aged conditions, the experimental setup was set as illustrated in Fig. 3. The impedance measurements were performed using an Agilent 4980 precision LCR meter with frequency scanning in the range of 200–10,000 Hz at a voltage of 10 Vp-p inducing the primary eddy magnetic field in the core coil probe and the secondary field on the specimen surface. Five Eq. (5) replicated impedance measurements were made on each specimen from each 5 different specific positions. In this study, the responded impedances were used to determine the effect of microstructural changes of aging conditions. The optimal frequency was selected at the peak of the frequency scanning. The normalized impedances ( $Z$ ) were determined from the obtained data of the LCR precision meter ( $Z_{\textit{measure}}$ ) divided by the impedance of the probe in the air ( $Z_{\textit{air}}$ ). The equation of normalized impedance was expressed as an Eq. (4). It was applied to analyze the effects of different intrinsic losses, peak values, bandwidths and characteristic frequencies.

$\displaystyle\textit{Normalized Z}=\frac{Z_{\textit{measure}}-Z_{\textit{air}}% }{Z_{\textit{air}}}$ (4)

3. Results and discussion

3.1 Results of optimal eddy current probe

The experimental results of probe A, B and C were shown in Fig. 4a–c. The maximum voltage outputs from the steel specimen were 9.9 V for probe A (100 Hz–200 Hz), 8.88 V for probe B (1 kHz–3 kHz), and 3 V for probe C (30–900 kHz). Increasing the test frequency also raised the air-coil voltage output from 6 V to 8 V for probe A, 2.5 V to 6.5 V for probe B and 0.5 V to 2.5 V for probe C. Obviously, probe B offered the maximum output voltage difference, and its frequency response was in the range of 700 Hz–6 kHz ( $-$ 3 dB) as shown in Fig. 4d. Therefore, the possible probe was the U-shaped ferrite core having an initial permeability of 10,000 $\pm$ 25% winding with 100 turns of AWG 40 copper wire (probe B). This design was selected for characterizing the aged behaviors of heat-resistant steel caused by microstructural changes.

Figure 4.

Voltage outputs and frequency response results of U-shaped eddy current probes.

3.2 Results of aging condition measurements

The results of impedance normalization obtained from probe B were presented in Fig. 5. The normalized inductive reactance ( $X_{L}$ ) as a function of normalized resistance ( $R$ ) represented the different aged conditions in Fig. 5a. The results in this plot from left to right were as follows: as-received 100,000 h, 300,000 h and 400,000 h specimens, with the 200,000 h specimen line between the 300,000 and 400,000 h specimens. The significant differences of maximum normalized R from as-received, 100,000 h, 200,000 h, 300,000 h and 400,000 h specimens were 1.05, 1.14, 1.20, 1.18 and 1.21, respectively. This enhancement trended to be only from the resistance changes of the specimens. To understand the frequency response, normalized impedances as a function of frequency were plotted. The frequency response gave the maximum normalized impedance at 3.2 kHz as seen in Fig. 5b corresponding to the frequency bandwidth of the designed probe B. The resistance as the real part in the impedance diagram showed that there was a good separation in the data as a function of different thermal history.

Referred to the Matthiesson’s rule, the impedance of Cr-Mo steel specimens depending on types, volume fraction, and resistivity of carbides could be described as an Eq. (5) [12, 14]. $Z$ is the electrical impedance of ferromagnetic materials and $\rho$ is an individual resistivity of different type carbides.

$\displaystyle Z_{\textit{Total}}=Z_{\textit{lattice}}+X_{1}\rho\left({Fe_{3}C}% \right)+X_{2}\rho\left({Mo_{2}C}\right)+X_{3}\rho\left({M_{7}C_{3}}\right)+X_{% 4}\rho\left({M_{23}C_{6}}\right)+X_{5}\rho\left({M_{6}C}\right)$ (5)

This equation shows various resistivities depending on physical and electrical properties of new carbides. $X$ is the volume fraction of each type of carbides. However, the inductive reactance in the imaginary part depended on the test frequency. It was difficult to determine among the aged specimens from the inductive reactance measurements.

Figure 5.

Experimental results of (a) normalized inductive reactance ( $X_{L}$ ) vs normalized resistance ( $R$ ), (b) normalized impedance ( $Z$ ) vs frequency.

3.3 Results of microstructure analysis

Micrographs of ferrite and carbide precipitates for as-received of 5Cr-0.5Mo steel specimens were shown in Fig. 6a. Carbide evolution at different aging times indicated differences in carbide size and quantity. The remaining carbide particles of the as-received specimen were a result from 5Cr-0.5Mo steel tube manufacturing with the ferrite grain size approximately 20–25 $\mu$ m. The 100,000 h simulated aged specimens showed carbide depletion near the grain boundaries as shown in Fig. 6b. Newly formed small carbides dispersed within the ferrite grains while the existing carbide colonies dissolved in which the grain boundaries were the preferred locations for larger carbides. Figure 6c showed the changes in carbide distribution of the 200,000 h aged specimen. Spheroidal carbides were seen with the uniform ferrite grain size of 20–25 $\mu$ m. The larger carbide particles and colonies in the ferrite matrix and along grain boundaries were observed at this state. At 300,000 h, the microstructure showed the amount of larger carbide colonies, and the carbides dispersed extensively into ferrite grains as seen in Fig. 6d. In Fig. 6e, the microstructure of the 400,000 h specimen showed the carbide growth along the grain boundaries.

3.4 Results of carbide particle size and spatial distribution

Carbide size distributions in the as-received and aged specimens were analyzed by color imaging software as shown in Fig. 7. The carbide distribution of the as-received specimen showed three, dominant, precipitate particle sizes, namely 0.35, 0.45 and 0.65 $\mu$ m as in Fig. 7a. In the 100,000 h specimen, small carbide particles were dispersed inside ferrite grains, but the remaining carbide colonies were absent, and larger precipitates were preferentially located along grain boundaries. Two dominant peaks presented at 0.35 and 0.55 $\mu$ m, which could be seen in Fig. 7b. In Fig. 7c at 200,000 h, the amount of small carbides decreased, but the number of larger carbides (1.65–1.95 $\mu$ m) were more than that of the 100,000 h specimen indicating the carbide growth affected by service periods. The 300,000 h specimen, the amounts of both small and large carbides increased, and two dominant peaks were observed at 0.35 and 0.55 $\mu$ m as shown in Fig. 7d. Carbide particles were coarsening as the particle size increased from 0.35 to 0.55 $\mu$ m. Finally, in the specimen at 400,000 h aged, the carbide size distribution was similar to that of the as-received specimen as a result of carbides dissolution and forming new small carbide particles. Both carbide sizes and quantities were illustrated in Fig. 7e.

Figure 6.

Microstructure of specimens isothermally aged at 1,000X magnification (a) as-received, (b) 100,000 h, (c) 200,000 h, (d) 300,000 h, (e) 400,000 h.

Figure 7.

Size distribution of carbides precipitates (a) as received, (b) 100,000, (c) 200,000, (d) 300,000, (e) 400,000 h.

3.5 Comparison between eddy current and micro-hardness results

Vicker micro-hardness measurement was a traditional semi-destructive testing, which was widely used for microstructural assessment of aging. The average of 3 time measurements with a small load (50 grams and dwell time 15 s) on microstructures were collected to correlate with the maximum normalized impedance data in Fig. 8. A good agreement between the hardness values and the normalized impedances of 5Cr-0.5Mo steel specimens were performed to determine the efficiency of eddy current testing using the swept frequency technique. Both trends in hardness and maximum normalized impedance increased with aging from as-received to 200,000 h. However, there were a drop in both hardness and impedance at the 300,000 h specimen. This was believed to be due to the phase transformation of the specimen after long period time in aging. The normalized impedance trend maintained good correspondence with the hardness trend until reaching the aging time of 400,000 h. This phenomenon illustrated that the eddy probe had a good response for microstructure assessment.

Figure 8.

Correlation between Vickers hardness values and maximum normalized impedance of eddy current testing.

4. Conclusion

Swept-frequency eddy current measurement was developed in this work for characterizing the different aging times of heat-resistant 5Cr-0.5Mo steel. The results could be summarized as the following.

The specific probe in a U-shaped design using 100 turns with the AWG 40 wire gave the highest sensitivity over the response frequency range of 700–6,000 Hz.

At 3,200 Hz, the results of experimental normalized impedance could be used to classify the differences in aged steel specimens.

Carbide precipitates and growth were the main factors that change the electrical resistance at each aging stage.

A good correlation of hardness and normalized impedance at maximum frequency response was observed when using the swept frequency technique.

Footnotes

Acknowledgments

This work was financially supported by the National Metal and Materials Technology Center (MTEC), Thailand (MT-B-55-MET-83-247-G) and the Electrical Generating Authority of Thailand for 5Cr-0.5Mo steel.

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