NDT Technology

NDT Technology

Nondestructive Evaluation of Reformer Tubes in Direct Reduction of Iron Unit by Eddy Current Test Method

Document Type : Original Article

Authors
Karim Yosefpour Shirsavar 1
Majid Abbasi 2
Seyed Jamal Hosseinipour 3
1 MSC Student, Faculty of Materials Engineering, Babol Noshirvani ‎University of Technology, Mazandaran, Iran.‎
2 Associate Professor, Materials Engineering Department, Babol Noshirvani University of Technology
3 ‎Associate Professor, Faculty of Materials Engineering, Babol Noshirvani ‎University of Technology, Mazandaran, Iran.‎
10.30494/jndt.2021.277416.1065
Abstract
The eddy current nondestructive evaluation was investigated to ‎estimate the service life of cast ‎austenitic stainless steel tubes in the reformer unit of the direct ‎reduction iron ore plan. This ‎evaluation was performed at 60 kHz based on the ‎impedance plane and as a point inspection ‎method. Reference tubes with specified microstructure ‎and service life were used for ‎calibration. The optical and SEM microscopes were used‎‏ ‏‎for ‎microstructural studies. The ‎microscopic examination showed that the as-cast microstructure has ‎austenitic matrix with ‎eutectic cells ‎containing chromium and ‎niobium carbides. It was also ‎observed that with ‎increasing operating time and increasing the intensity of aging, the ‎microstructural changes occur ‎more severely. These changes include oxidation and carburization of ‎surface, creating of ‎destructive phases, and depletion of chromium ‎in the austenite matrix, variation ‎of eutectic cells ‎morphology and formation of the creep microcracks. The results of eddy current ‎evaluation ‎showed that microstructural changes due to increasing the service life have a significant ‎effect ‎on the electromagnetic response of the tube. As the life of the tubes increases, the self-‎‎inductance resistance index increases sharply and the resistivity index decreases. The interaction ‎of these two index is ‎indicated as a point on the ‎impedance plane of for each tube. Comparison ‎of the position of these points in the impedance plane indicates the ‎existence of linear changes ‎between electromagnetic and aging behaviors. Therefore, the service ‎life of the reformer tubes ‎can be estimated from the electromagnetic responses in the eddy current ‎method.‎
Keywords
Reformer
Direct Reduction of Iron
Life Assessment
Cast Stainless Steel
Eddy Current Testing
Impedance &lrm
Plane

Subjects

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  • Receive Date 15 March 2021
  • Revise Date 29 April 2021
  • Accept Date 07 May 2021