Creep behaviour study of virgin and service exposed 5Cr–0.5Mo steel using magnetic Barkhausen emissions technique

doi:10.1016/j.jmmm.2008.04.152

Journal of Magnetism and Magnetic Materials

Volume 320, Issue 18, September 2008, Pages 2284-2290

https://doi.org/10.1016/j.jmmm.2008.04.152 Get rights and content

Abstract

Creep damage behaviour of water quenched 5Cr–0.5Mo steel has been studied using magnetic Barkhausen emissions (MBE) technique. The results were compared with the materials having same composition but used in service for 15 years to demonstrate the potentiality of the magnetic technique for in-situ evaluation of extent of creep damage of components. The rms voltage of magnetic Barkhausen signal for the virgin sample decreased at the initial stage of the expended creep life where new carbides are formed. As soon as the growth of the carbides took place at the expense of the smaller ones, MBE voltage started increasing due to the decrease of pinning density. However, in case of 15 years of service exposed sample, growth of carbides already took place and hence MBE voltage increased even during the initial stage of laboratory creep testing. As soon as the void started forming in the samples (both for virgin and service exposed one), the rate of increase of MBE voltage started decreasing. The formations of such cavities were observed through SEM micrograph analysis.

Introduction

The components of the thermal power plant and petrochemical industries are made up of Cr–Mo steels that experiences high-temperature and stresses [1], [2], [3]. The high temperature exposure for prolonged time under stress causes microstructural changes [4] i.e., carbide precipitates and migrates towards the grain boundary; grain and sub-grain grow as well as micro voids form. Such microstructural change affects the mechanical properties of the material [5]. The estimation of material degradation to get the remaining life of the components is a challenge for modern society [6], [7]. If such estimation can be done non-destructively, it will not only prevent the catastrophic failure of components but also save material loss and make the use of the components economical. It has been observed that magnetic Barkhausen emissions are quite sensitive to the microstructural changes in steel [8]. The technique was successfully used to study the creep behaviour of Cr–Mo steel [9]. Attempts have made in this work to study the effect of Barkhausen emissions on the structural changes during creep in virgin 5Cr–0.5Mo steel and compare the result with service-exposed one so that the technique could be useful for evaluation of in-service components.

Section snippets

Experimental

Service-exposed 5Cr–0.5Mo steel tube with ferrite–martensite microstructure obtained from petrochemical industry has been used for the present study. The material was used in service for 15 years at an average temperature of 400 °C and pressure 28.11 MPa. Flat specimens were prepared from the tube for creep testing. Creep test at the laboratory was performed at 600 °C and 60 MPa stress. Creep test was also carried out on specimens cut out from the virgin tube obtained from the industry. The

MBE signal and microstructure for water quenched 5Cr–0.5Mo steel

RMS voltage of magnetic Barkhausen emissions signals for water-quenched virgin sample after different creep exposure is shown in Fig. 3 along with the creep strain data. RMS voltage initially decreased with expended creep life (25%), which was close to the primary creep region according to the creep strain curve. It started increasing after 35% of expended creep life, which was in the secondary creep stage. This increase in rms voltage continued till failure of the sample although the rate of

Discussion

The creep behaviour of virgin materials and 15-year service-exposed 5Cr–0.5Mo steel has been studied in the present work. The purpose of the study is to see how the magnetic Barkhausen emissions technique is useful for on-site experimentation. The initial martensitic microstructure was transformed to bainitic after 15 years of service exposure. Substantial increase of Cr and Mo percentage of the grain boundary carbides of the service-exposed materials as compared to virgin sample indicated that

Conclusion

Magnetic Barkhausen emissions technique was used to investigate water quenched virgin and service exposed 5Cr–0.5 Mo steels subjected to creep at 600 °C and 60 MPa tensile stress. In the virgin material three distinct regions were found correlated with the three stages of creep. RMS voltage decreased in the primary region due to the increase in pinning density for the formation of newer carbides in the materials. In the secondary region where the coarsening of carbides and their migration towards

Acknowledgement

The authors are thankful to the Director, NML for his permission to carryout the work and publishing the paper.

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