Magnetic properties evaluation of ageing behaviour in

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authors (J N Mohapatra) acknowledges CSIR for providing the financial support for his research work. References. [1] SikkaV K, Ward G T and Thomas K C 1982 ...

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JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 42 (2009) 095006 (6pp)

doi:10.1088/0022-3727/42/9/095006

Magnetic properties evaluation of ageing behaviour in water-quenched 5Cr–0.5Mo steel J N Mohapatra, A K Panda and A Mitra1 National Metallurgical Laboratory, Council of Scientific and Industrial Research, Jamshedpur-831007, India E-mail: [email protected]

Received 30 October 2008, in final form 17 March 2009 Published 17 April 2009 Online at stacks.iop.org/JPhysD/42/095006 Abstract Magnetic Barkhausen emissions and magnetic hysteresis measurements were carried out on water-quenched 5Cr–0.5Mo steel subjected to ageing at 600 ◦ C up to 5000 h. During initial ageing, this steel exhibited magnetic softening, which was attributed to relaxation of quenching stress in the material as well as decrease in dislocation density and migration of interstitial carbon atoms towards the grain boundary. Further ageing resulted in magnetic hardening owing to the restricted movement of the domain wall by the precipitation of carbides such as M3 C2 , M2 C, M7 C3 where M stands for Fe, Cr or a combination of them. At longer ageing periods, magnetic behaviour was affected by a change in the composition and morphology of the carbides. Massive M23 C6 types of carbides were formed during longer periods of ageing. The coarsening of carbides decreased the pinning density for the domain wall motion and affected the magnetic properties of the steel. The effect of demagnetizing field from voids and non-magnetic massive carbides also affected the magnetic behaviour. Magnetic behaviour and Vickers hardness measurements during ageing have been effectively supported by microstructural evaluations suggesting the capability of the magnetic techniques for assessment of damage during ageing in high temperature 5Cr–0.5Mo steel components.

several micro-mechanisms that create damages in materials. For example, the brittle nature of massive carbides produced during extended periods of service generates micro cracks at the grain boundary and grain boundary triple points. Such micro cracks eventually lead to macroscopic failure of the components. The failure occurs with no significant prior change in the macroscopic appearance of the materials and therefore appears to be spontaneous [4]. At present, the replication technique is used to get information on carbide precipitates. However, it gives information at a localized region and also the technique is labour intensive and time consuming. Attempts are being made to find the influence of carbide precipitates on the physical properties using other techniques such as eddy current, ultrasonic, x-rays, thermal conductivity [5]. In view of the fact that Cr–Mo steels are ferromagnetic, magnetic techniques are gaining importance for the assessment of such steel components. In this work, attempts have been made to investigate the structural change and its effect

1. Introduction Microstructure plays an important role in the mechanical properties of steel. Mechanical properties deteriorate with the degradation of microstructure as is observed during thermal exposition. Care has always been taken to obtain steels of suitable microstructure so that during operation at high temperature structural degradation does not take place. Different versions of Cr–Mo steels have been developed for structural applications having relatively stable microstructures at high temperatures. These steels are extensively used as high temperature components in petrochemical, thermal and nuclear power plants [1–3]. However, exposure of these materials in aggressive environments for an extended period of service generates microstructural change in the form of carbide precipitation, coarsening of carbides, migration of carbides and carbide segregation at the grain boundaries. There are 1

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J. Phys. D: Appl. Phys. 42 (2009) 095006

J N Mohapatra et al 1.1

Cr

Mo

Mn

Si

Fe

0.11

5.47

0.44

0.36

0.38

Bal.

1.0

1.0

(II)

(I)

6

0.9

4

0.8

2

0.7 1

10

100

7

6

0 1000

5

Ageing Time (H)

0.9

4

Normalized RMS voltage

Normalized coercivity

5Cr–0.5Mo

C

Normalized coercivity

Material

8

1.1

Normalized RMSvoltage

Table 1. Chemical composition of 5Cr–0.5Mo steel.

3 0.8

(II)

(III)

(IV) 2

100

500

1000

5000

Ageing Time (H)

Figure 2. Variation of RMS voltage of the MBE and coercivity of water-quenched 5Cr–0.5Mo steel with ageing time at 600 ◦ C. Figure 1. SEM micrograph of water-quenched 5Cr–0.5Mo steel after 1 h of ageing.

time starting from 100 h. The inset of figure 2 shows the ageing behaviour for lower ageing periods. The values have been normalized with respect to 1 h of ageing time. The variations of the magnetic properties were not monotonic functions of ageing. Depending on the variation of properties the entire studied range of ageing time can be divided into four regimes. The initial two regimes are shown in the inset of figure 2, where magnetic softening is observed up to 200 h of ageing (regime-I), as indicated by the increase in the MBE voltage and the decrease in the coercivity. Beyond 200 h of ageing MBE voltage decreased and coercivity increased, indicating magnetic hardness of the materials. This process continued up to 900 h of ageing (regime-II) at 600 ◦ C, beyond which magnetic softening occurred again up to 3300 h of ageing (regime-III), indicated by the increase in the MBE voltage and the decrease in the coercivity. In regime-IV, i.e. beyond 3300 h, the MBE signal again decreased and the coercivity increased up to 5000 h. Figure 3 shows the variation of hardness of the material up to 5000 h of ageing time. The effect of ageing on hardness value was divided into four regimes as in the case of magnetic properties. At the beginning of regime-I, the mechanical hardness of the material decreased rapidly. The rate of change of hardness with ageing time became slower at the end of regime-I and at the beginning of regime-II. A rapid decrease in hardness was observed in regime-III. In regime-IV, the hardness value became almost constant up to the measured ageing period. Figure 4(a) shows the SEM micrograph of waterquenched 5Cr–0.5Mo steel aged at 200 h which revealed precipitation of carbides. The EDS analysis of the carbides at the grain boundary and at the matrix is shown in figures 4(b) and (c), respectively. The carbides were mostly Cr and Morich. As the ageing period increased, more of M2 C, M3 C2 and M7 C3 carbides were precipitated in the matrix and growth of carbides was observed, as shown in figure 5(a) at 700 h of ageing [7–9]. The morphology of the carbides also changed with the ageing period. The magnified form of the carbides

on magnetic properties of water-quenched 5Cr–0.5Mo steels subjected to ageing.

2. Experimental An unexposed 5Cr–0.5Mo steel tube was obtained from an Indian oil refinery unit having chemical composition as shown in table 1. Flat specimens were cut out from the tube which was water quenched after austenizing at 900 ◦ C for 30 min. The microstructure of the water-quenched material is shown in figure 1. The water-quenched material was subjected to thermal exposure at 600 ◦ C for different lengths of time up to 5000 h. The magnetic Barkhausen emissions (MBE) and magnetic hysteresis loop (MHL) measurements were carried out at room temperature by interrupting the exposure for various periods using a surface probe. MHL measurement was carried out to get magnetic hysteresis properties from where coercivity was evaluated. MBE measurement gave the RMS voltage of the Barkhausen emissions. The details of the magnetic measurement were described elsewhere [6]. The magnetizing frequency for MBE measurement was 40 Hz, whereas the hysteresis loop was measured at quasi-dc (50 mHz) field. Microstructural investigation and Vickers hardness of the aged samples were carried out to correlate the mechanical and structural change with the magnetic properties. Scanning electron microscopy, complemented by energy dispersive x-ray (SEM-EDX, Model: JEOL JSM 840A) microanalysis was carried out to study the change in the composition of the carbides with thermal exposure.

3. Results and discussion Figure 2 shows the variation of the RMS voltage of the magnetic Barkhausen emissions and coercivity with ageing 2

J. Phys. D: Appl. Phys. 42 (2009) 095006

J N Mohapatra et al

after 5000 h of ageing, as observed through EDS analysis and are shown in figures 7(c) and (d), respectively. The initial magnetic softening in regime-I was due to the relaxation of quenching in stress, decrease in dislocation density after ageing at high temperature and diffusion of interstitial carbon atoms towards the grain boundary [10, 11]. Such structural variation also affected the mechanical hardness of the materials and the hardness value rapidly decreased to 215 Hv after 50 h of ageing from 308 Hv obtained after 1 h of ageing. Thereafter, the observed slow change in hardness was due to the competing effect of precipitation hardening and the structural relaxation responsible for the rapid decrease in hardness value at the initial stage. A slow variation in the magnetic properties (coercivity and MBE voltage) was also observed near the end of regime-I (inset of figure 2). The precipitation of carbides as observed after 200 h of ageing restricted the magnetic domain wall movements. The obstacle of the domain wall increased the coercivity of the materials. As the domain movement was restricted due to precipitation, the MBE signal consisted of a large number of small amplitude pulses resulting in a decrease in the MBE voltage as observed in regime-II. This process continued until the precipitation of carbides took place. It is clearly observed that more carbide precipitated after 700 h of ageing as compared with 200 h of ageing even though coarsening of some carbide took place. The effect of the precipitation of carbides on the MBE signal can clearly be demonstrated by the pulse height distribution of Barkhausen emissions as shown in figure 8. Increase in carbide density in regime-II resulted in increased pinning density for the domain wall movement, which, in turn, produced a lower amplitude of Barkhausen pulses with increasing ageing period, e.g. 200, 400 and 900 h as shown in figure 8(a).

at such ageing periods in figure 5(b) shows the shape of the carbides. There were mainly two types of carbide morphology observed (a) globular and (b) elongated forms. The EDS analysis showed that the globular carbides were mostly Cr-rich whereas the elongated ones were mostly Mo-rich as shown in figures 5(c) and (d), respectively. As the ageing period increased to 2000 h (figures 6(a) and (b)) carbide size increased and they became massive when the ageing time became 5000 h (figures 7(a) and (b)), which was the maximum ageing period studied in this work. The Cr and Mo compositions of the globular and the elongated carbides increased considerably 325

(I)

300

(III)

(II)

(IV)

Hardness (Hv 30)

275 250 225 200 175 150 125 100 1

10

100

1000

5000

Ageing Time (hours)

Figure 3. Variation of Vickers hardness of water quenched 5Cr–0.5Mo steel with ageing time.

(a)

Element Fe Cr Mo

700

600

Wt% 80.83 16.60 2.57

FeK

At% 80.70 17.80 1.50

Element Fe Cr Mo

(b) 500

400

500

Grain boundary Carbides

Wt% 64.88 31.67 3.45

At% 64.30 33.71 1.99

(c) CrK α

Carbides at matrix

400

300

300

CrK α

200

200

FeL β FeL α CrL β CrL α

100

0 0

100

MoL α MoL β

FeLβ FeL α CrLβ CrL α

FeK β CrK β keV 8.410

0 0

MoL α CrK β MoL β

FeK β keV 8.410

Figure 4. (a) SEM micrograph of water-quenched 5Cr–0.5Mo steel at 200 h of exposure showing number of fine carbide precipitations, (b) EDS of grain boundary carbide and (c) EDS of matrix carbide.

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J. Phys. D: Appl. Phys. 42 (2009) 095006

J N Mohapatra et al

(b)

(a)

1300

Element Fe Cr Mo

1200 1100 1000 900

Wt% 78.95 18.19 2.86

At% 78.83 19.51 FeK α 1.66

1300

(c)

Element Fe Cr Mo

1200 1100 1000 900

800

800

700

700

600

600

Wt% 89.53 5.71 4.76

At% 90.96 6.23FeK α 2.81

(d)

500

500

CrK α

400

400 300

300

200

200 100

FeLβ FeL α

0

MoL β MoL α

0

CrK β 5

FeK β

FeL β FeLα CrL β CrL α

100 keV

0 0

10

MoLα

CrKα

MoLβ

FeK β

CrKβ 5

keV 10

Figure 5. (a) SEM micrograph of water-quenched 5Cr–0.5Mo steel at 700 h of exposure showing precipitations of large number of carbides, (b) micrograph showing the existence of carbides with globular and elongated morphology, (c) EDS of globular carbide and (d) EDS of elongated carbide.

(a)

(b)

Figure 6. (a) SEM micrograph of water-quenched 5Cr–0.5Mo steel at 2000 h of exposure showing coarsened carbides and (b) morphology of the carbides.

With a further increase in the ageing period (regime-III), carbides coarsening became predominant as compared with the formation of new carbides. As a result, the pinning densities effectively decreased and inter carbide distances increased. Hence domain wall movement took place without much hindrance and the material became magnetically softer, resulting in a decrease in coercivity and an increase in the RMS voltage of the MBE signal. Such coarsening of carbides was clearly observed when the SEM micrographs of the 2000 h aged sample (figure 6(b)) were compared with the 700 h aged sample (figure 5(b)). The variation of carbide size with ageing period is shown in table 2. As the effective pinning density decreased, the domain wall could move a longer distance without hindrance resulting in the reduction of low amplitude Barkhausen pulses and the enhancement of the number of high

amplitude pulses as observed in figure 8(b). The decrease in hardness, as found in regime-III, was due to the lowering of Cr and Mo concentrations in the matrix [12, 13]. With a further increase in the ageing period, there was not much of change in the hardness value. When the ageing period was further enhanced, i.e. in regime-IV, carbides became massive as shown in figure 7(b) for the 5000 h aged sample. The Cr content in the massive globular carbides became high and Mo concentration in the elongated carbides also enhanced drastically, as observed in the EDS spectrum analysis shown in figures 7(c) and (d), respectively. Such an increase in Cr and Mo carbides resulted in the depletion of those elements in the matrix. The depletion of Cr and Mo in the matrix reduced the corrosion resistance and the solid solution strengthening of the material, respectively 4

J. Phys. D: Appl. Phys. 42 (2009) 095006

J N Mohapatra et al

(a)

(b)

1000

1300

Element Fe Cr Mo

1200 1100 1000 900

Wt% 53.26 40.85 5.89

At% 53.96 43.63 3.41

(c)

900 800 700

800

Element Fe Cr Mo

Wt% 80.59 9.84 9.57

At% 83.32 10.93 5.76

FeK α

(d)

600

CrK α

700

500

600

FeK α

400

500

Mo L α

400

300

300

CrK α

200

200

FeLβ FeL α CrL β CrL α

100 0 0

MoL α

100

CrK β FeK β

MoL β

keV 5

0

FeLβ FeL α MoL β CrL β SiK α CrL α

FeK β CrKβ

keV

5

10

10

Figure 7. (a) SEM micrograph of water-quenched 5Cr–0.5Mo steel at 5000 h of exposure showing large number of massive carbides, (b) morphology of the carbides, (c) EDS of globular carbide and (d) EDS of elongated carbide. 3000

3000

900 h 400 h 200 h

2000

2000 h

2000

1500

1000

500

(b)

900 h

2500

No of pulses

No of pulses

2500

(a)

3300 h 1500 1000 500

0 0.0

0.1

0.2

0.3

0 0.0

0.1

Pulse height (Arb.Unit)

0.2

0.3

Pulse height (Arb.Unit)

3000

(c)

No of pulses

2500

2000

3300 h

1500

1000

4000 h 500

0 0.0

0.1

0.2

5000 h

0.3

Pulse height (Arb.Unit)

Figure 8. Pulse height distribution of the magnetic Barkhausen emissions at (a) 200 h, 400 h and 900 h, (b) 900 h, 2000 h and 3300 h and (c) 3300 h, 4000 h and 5000 h of ageing.

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J N Mohapatra et al

carbides towards the grain boundary. This microstructural change in the form of coarsening of carbides and their migration towards the grain boundary deteriorated the strength of the material. Magnetic softening took place due to the increase in the inter particle distance and the decrease in the number of carbides. When the carbides became massive, the demagnetizing field offered by the massive carbides hindered the domain wall motion, thereby increasing the magnetic hardening. The present correlations of magnetic properties with microstructural changes will be helpful for damage assessment of aged 5Cr–0.5Mo steels by magnetic non-destructive evaluation techniques.

Table 2. Carbide size with ageing period. Ageing period (h)

Shape of carbide

Type of carbide

Maximum size of observed carbides (µm)

700

Spherical Elongated

Cr-rich Mo-rich

2000

Spherical Elongated

Cr-rich Mo-rich

5000

Spherical Elongated

Cr-rich Mo-rich

Diameter = 0.39 Length = 0.65 Thickness = 0.35 Diameter = 0.6 Length = 1.26 Thickness = 0.39 Diameter = 0.69 Length = 1.91 Thickness = 0.74

Acknowledgments

[14, 15]. The massive carbides became very brittle and created voids at the grain boundary triple point when the material was under stress. The massive carbides were either nonmagnetic or weakly magnetic. The non-magnetic carbides and/or voids created a demagnetizing field, which, in turn, reduced the applied magnetic field. Due to this demagnetizing field, the material in regime-IV experienced less magnetic field compared to other regimes. Hence the RMS voltage became lower. The voids and the demagnetizing force also restricted the domain wall movement and hence an increase in coercivity was observed in regime-IV as compared with regime-III. In this regime, the pulse height distributions and their effect on Barkhausen emissions voltage was quite complicated. A drastic change in the pulse height distribution was observed here (figure 8(c)). In this regime the number of small amplitude pulses decreased considerably and, at the same time, a few very high amplitude pulses were observed. In spite of such an increase in large amplitude pulses, the RMS voltage of the Barkhausen emissions decreased with the ageing period in regime-IV as the total number of pulses was less.

The authors are grateful to the Director, NML Jamshedpur, for kindly permitting the publication of this work. One of the authors (J N Mohapatra) acknowledges CSIR for providing the financial support for his research work.

References [1] SikkaV K, Ward G T and Thomas K C 1982 Ferritic Steel for High Temperature Applications (American Society for Metals) p 65 [2] Tsai M C and Yang J R 2002 Mater. Sci. Eng. A 00 1 [3] Moorthy V, Vaidyanathan S, Laha K, Jaykumar T, Rao K B and Raj B 1997 Mater. Sci. Eng. A 231 98 [4] Govindaraju M R, Kaminski D A, Devine M K, Biner S B and Jiles D C 1997 NDT & E Int. 30 11 [5] Othani T, Ogi H and Hirao M 2006 Acta Mater. 54 2705 [6] Mitra A, Mohapatra J N, Swaminathan J, Ghosh M, Panda A K and Ghosh R N 2007 Scr. Mater. 57 813 [7] Das S K, Joarder A and Mitra A 2004 NDT & E Int. 37 243 [8] Moorthy V, Vaidyanathan S, Jaykumar T and Baldev R 1998 Phil. Mag. A 77 1449 [9] Michaud P, Delagnes D, Lamesle P, Mathon M H and Levaillant C 2007 Acta Mater. 55 4877 [10] Jiles D C 1988 J. Phys. D: Appl. Phys. 21 1196 [11] Mohapatra J N, Panda A K, Gunjan M K, Bandyopadhyay N R, Mitra A and Ghosh R N 2007 NDT & E Int. 40 173 [12] Homolova V, Janovec J, Zahumensky P and Vyrostkova A 2003 Mater. Sci. Eng. A 349 306 [13] Koning K 1993 Proc. 16 Vortragsveranstaltung Langzeitverhalten Warmfester Stable and Hochtempeaturwerkstoffe (VDEh Duesseldorf, Germany, November 1993) p 45 [14] Rajan T V, Sharma V P and Sharma A 2006 Heat Treatment (Prentice-Hall) p 283 [15] Ryu K S, Nahm S H, Kim Y B, Yu K M and Son D 2000 J. Magn. Magn. Mater. 222 128

4. Conclusion Water-quenched 5Cr–1Mo steel showed significant microstructural variations with ageing. Initial ageing up to 200 h indicated magnetic softening (MBE RMS increase, coercivity drop), which was due to the stress relaxation and diffusion of interstitial carbon atoms towards the grain boundary. Intermediate ageing led to precipitation of carbides, which resulted in magnetic hardening owing to pinning of the domain wall motion. During long-term ageing a magnetic softening was observed, which is attributed to coarsening of carbides at the expense of smaller carbides and migration of

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