Journal of Metallurgy and Materials Science, Vol. 48, No. 3, July-September 2006, pp. 169-173Printed in India, © NML, ISSN 0972-4257

Evaluation of ageing behavior in modified 9Cr-1Mosteel by magnetic barkhausen emission technique

J. N. MOHAPATRA, A. K. PANDA, M. K.GUNJAN and A. MITRA*National Metallurgical Laboratory, Jamshedpur-831007

Abstract : The paper addresses the effect of ageing on the magnetic, mechanical andstructural behavior of modified 9Cr-1Mo steels. Magnetic Barkhausen Emission (MBE)technique has been used to evaluate the magnetic properties. Initial magnetic softeningdue to stress relaxation effects was indicated by an increase in MBE rms voltage. Thesubsequent precipitation of fine carbides above 50hrs of ageing was indicated by magnetichardening (rms decrease). However, such precipitation hardening was indicated bymonotonic increase in mechanical hardness upto 500hrs of ageing. Scanning electronmicroscopy (SEM) indicated the formation of such fine carbides. Ageing beyond 1000hrsled to coarsening of carbides and also compositional changes in the matrix as well as thegrain boundary as observed from Energy dispersive Analysis of X-ray (EDAX). EDAXrevealed the alteration in Cr and Mo concentration in the matrix and grain boundarycarbides with aging.

Key Words : 9Cr-1Mo Steels, Magnetic Barkhausen Emission, Microstructure andMechanical Hardness.

INTRODUCTION

The 9Cr-1Mo steels find applications in high thermal environments typically pressure vessels,piping systems and petrochemical industries [1-3]. The modified version of these steels bearinteresting characteristic properties making them potential replacements of the conventionalCr-Mo as well as different groups of stainless steels [4]. During prolonged high temperatureexposure, the steels undergo microstructural changes that ultimately determine the servicelife of the component [5-8]. In order to access the extent of damage in these high temperaturecomponents, the use of different Non-destructive Evaluation (NDE) methodologies are gainingimportance. NDE by Magnetic Barkhausen Emission (MBE) technique is found to be effectivein condition monitoring of such components as the magnetic properties change sensitivelywith microstructural changes [9-10].

The present work is an attempt to put forth a correlation between Magnetic BarkhausenEmission characteristics with those of microstructural and mechanical changes in modified9Cr-1Mo steel subjected to high temperature exposure.

EXPERIMENTAL PROCEDURES

The material is modified 9Cr-1Mo steel with an additional elements V and Nb which improvethe mechanical properties. The composition of the material is given in the Table-1.

Table-1 : Composition of modified 9Cr-1 Mo Steel

C Mn Si Cr Mo V Nb

0.09 0.4 0.38 8.27 0.94 0.19 0.08

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* Corresponding author E-mail : amitra@nmlinida.org

The as-received material in the form of a tube was cut into several specimens to carry outaging at 600OC for different durations. Magnetic Barkhausen Emission (MBE) studies werecarried out for aged samples at a frequency of 40Hz. The signal was analyzed after filtering ata bandwidth of 30 -300kHz. Microstructural evaluation was carried out using Scanning electronmicroscope (SEM, JEOL-400). The echant used for the material was picral. The Mechanicalhardness was measured using 3kg load in a hardness tester machine (System AFFERY).

RESULTS AND DISCUSSION

Magnetic Barkhausen Emission (MBE) waveforms were obtained for samples aged for differentduration and shown in Fig. 1. Similar types of waveforms were obtained for a range of agingtemperatures from which rms voltage was calculated. The variation of rms voltage with agingtemperature is shown in Fig. 2. The rms voltage initially increased upto 50 hours of aging.Such magnetic softening at the initial stage is attributed to stress relaxation effects. Beyond 50

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170 J. MET. MATER SC., Vol. 48 No. 3, 2006

Aged for 1hr

Am

plit

ud

e (A

.U)

Time (mSec)

Aged for 500hr

5mSec

Aged for 2200hr

Fig. 1 : Variation of amplitude of the MBEwaveform with time of exposure

0 500 1000 1500 2000 2500

2

4

6

RMS Voltage(A.U)

Vickers Hardness(HV30)

Exposure Time(Hours)

RM

S V

olt

age

(A.U

)

214

216

218

220

222

224

226

228

Vickers H

ardn

ess(HV

30)

Fig. 2 : Variation of RMS voltage and VickersHardness with time of Exposure

hours of aging, the material became magnetically softer indicated with an increase in rmsvoltage. However, mechanical hardness increased monotonically upto 500 hours of aging.The cause of such magnetic and mechanical behavior after aging was examined using SEMand the micrographs are shown in Fig. 3. As compared to 1hr of exposure (Fig-3a), aging upto500 hours shows the precipitation of fine carbides of MX (M= V, Nb, X=C, CN) type(Fig. 3b). The increase in the number density of such carbides led to precipitation strengthening

Fig. 3 : Micrographs of 9Cr-1Mo samples aged at (a) 1hrs, (b) 500hrs and (c) 2200 hrs

(a) (b) (c )

of the material. This was also shown from MBE pulse height distribution (Fig. 4a) and powerspectrum (Fig. 4b) analysis which gives good explanation of the microstructural change duringaging. In comparison to 1hour of aging, the material aged for 500 hrs showed large number oflow amplitude pulses while the number of higher amplitude pulses decreased. The powerspectrum also indicated a decrease in amplitude after 500hrs of ageing. This suggested thatthe precipitation of fine carbides impeded domain wall motion leading to magnetic hardening.

EVALUATION OF AGEING BEHAVIOR IN MODIFIED 9Cr-1Mo STEEL

J. MET. MATER SC., Vol. 48 No. 3, 2006 171

0.0 0.1 0.2 0.3

0

500

1000

1500

2000

2500

3000

No

of

Pu

lses

Pulse Height

1 hour 500 hour 2200 hour

(a)

50 100 150 200 250 3000.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.10

2200hours

500hours

1hour

Am

plit

ud

e (A

.U)

Frequency (kHz)

(b)

Fig. 4 : Effect of ageing on (a) Pulse height distribution and (b) Power spectrum of theMBE signal in 9Cr-1Mo steel

As a consequence, the material becomes magnetically harder with a reduced rms voltage. Theprecipitated carbides also obstructed the dislocation movement to give good creep strength.To analyze the compositional variation in the microstructure, Energy dispersive analysis of x-rays (EDAX) was carried out for the aged samples and shown in Fig. 5. EDAX studies revealedthat the concentration of Chromium and Molybdenum in the matrix carbides decreased as theageing period increased from 1hour (Fig. 5a) to 500 hours (Fig. 5b). This reduction inconcentration of Cr and Mo led to the formation of finely disperse carbides in the matrix.There was a corresponding increase in the concentration of these elements in the grain boundarycarbides [Fig. 5 (d) & (e)] which impeded dislocation motions at the grain boundary leading toincrease in mechanical hardness.

Further ageing upto 1000hrs of exposure did not reveal any change in soft magnetic properties.However, on ageing beyond 1000hrs the soft magnetic properties of the material startedimproving, exhibited by a significant increase in rms voltage. To study the effect of this extendedageing, SEM micrograph of the material exposed for 2200hours was obtained and shown inFig. 3c. The micrograph shows coarsened carbides of Chromium and Molybdenum richaccumulated at the grain boundaries. The EDAX analysis shows that after 2200hours of agingthe concentration of Chromium increased significantly in the matrix carbides (Fig. 5c) whileit decreased from the grain boundary carbides (Fig. 5f). The coarsening of Chromium carbidesnot only facilitated easy domain wall movement but also enhanced their ferromagnetic couplingthat was revealed by significant increase in MBE rms voltage. The growth of carbides after2200hrs of ageing was also indicated by reduction of low amplitude pulses and a correspondinghigh amplitude pulses compared to 500hrs aged material as shown in Fig. 4a. Frequencyspectrum (Fig. 4b) also revealed a drastic increase in the amplitude. It is also in view that thesecoarsened carbides do not facilitate any further strengthening effect and hence no significantchange was observed in the mechanical hardness. Though the Mo content also increased inthe matrix, prominent change was observed in the grain boundary (Fig. 5f). This drastic increasein Mo concentration at the grain boundary carbide led to the formation of Mo-rich Laves

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172 J. MET. MATER SC., Vol. 48 No. 3, 2006

phase. The formations of such Mo-rich phases, which are brittle in nature, are reported to bethe cause of material degradation in 9Cr-1Mo steels.

V K αV K β

V L αV L β

CrK α

CrK βCrL αCrL β

FeKα

FeKβ

FeL αFeL β

MoLαMoLβ keV0

100

200

300

400

500

600

700

800

900

1000

1100

1200

1300

0 5 10

(a)

V K αV K β

V L αV L β

CrKα

CrKβCrLαCrLβ

FeKα

FeKβFeLαFeLβ

MoLαMoLβ ke

V0

500

1000

1500

0 5 1

(d) 1 hr 1 hr

V K αV K β

V L αV L β

CrK α

CrK βCrL αCrL β

F e Kα

F e KβFeL αFeL β

M o LαM o Lβ

keV0

500

1000

1500

2000

2500

3000

0 5 10

(b)

V K αV K β

V L αV L β

CrKα

CrKβCrL αCrL β FeKβFeLαFeLβ

M o Lα

M o LβkeV0

500

1000

1500

0 5 10

(e) 500 hrs 500 hrs

SiK α V K αV K β

V L αV L β

CrK α

CrK βCrL αCrL β

FeKα

FeKβ

FeL αFeL β

MoLαMoLβ

keV0

500

1000

1500

2000

0 5 10

(c )

SiK α

SiK βV L αV L β

CrK α

CrK β

CrL αCrL β

FeKα

FeKβ

FeL αFeL β

MoLα

MoLβ

keV0

500

1000

1500

0 5 10

(f) 2200 hrs 2200 hrs

Fig. 5 : EDAX analysis data (a), (b) and (c) of matrix carbides while (d), (e) & (f) refers tograin boundary carbide for 1hr, 500hrs and 2200 respectively aged samples

CONCLUSION

The effect of ageing on the magnetic, mechanical and microstructural development in modified9Cr-1Mo steels was investigated. Magnetic Barkhausen emission technique was found to givegood correlation between the magnetic properties and structural variation on ageing. Initial

EVALUATION OF AGEING BEHAVIOR IN MODIFIED 9Cr-1Mo STEEL

J. MET. MATER SC., Vol. 48 No. 3, 2006 173

ageing upto 50hrs revealed magnetic softening which was attributed to stress relaxation. Thiswas subsequently followed by magnetic hardening upto 500 hrs of ageing which was causeddue to impediment to domain wall motion by initially precipitated fine carbides. Mechanicalhardness increased monotonically indicating precipitation hardening effects. The coarseningof carbides beyond 1000hrs of ageing showed significant magnetic softening. This pronouncedsoftening behavior was indicated by high amplitude pulses for the material exposed to 2200hrs of ageing. At such extended ageing, there was compositional change in the matrix as wellas the grain boundary carbides. The Mo-rich Laves phases formed at the grain boundaries thatare responsible for damage to modified 9Cr-1Mo steels.

ACKNOWLEDGEMENT

The authors thank Director, NML for kindly giving permission to publish this work.

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