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Magnetic Characterisation of In-service Process Heater TubeS.K.Das, B.Ravi Kumar, S. Palit Sagar, R.N. Ghosh, D.K. Bhattacharya, A. Mitra
National Metallurgical Laboratory, Jamshedpur, India
|Fig 1: Optical micrograph showing the microstructures from outer and inner surfaces of the service exposed tube|
In virgin tube, grain boundary carbide network started from the edges. Figures-2 (a) & (b) shows the microstructures of the virgin and service exposed samples. SEM-EDAX analysis reveals that the grain boundary Cr-rich carbides (Cr3C7) of the virgin sample transformed to coarser carbides (Cr23C6) after service exposure. Chromium depleted regions also detected by EDAX at inner and outer edges of the service exposed tube.
|| Fig 2: SEM Micrographs showing grain boundary carbides (Cr7C3) network in virgin tube (a) transformed to coarse carbides (Cr23C7) (b).|
To identify the carbides and the phases of the carbide free zones, X-ray diffractograms were taken and the phases are summarized in Fig-3 (a) and (b) for the virgin and service exposed samples respectively. The virgin sample showed Cr7C3 and NbC type carbides together with solid solution of Cr and Ni in g -Fe. Cr7C3 type carbides transformed to Cr23C6 type. This type of transformation was observed earlier [3-5]. Cr-, NiCr-, FeCr- oxides were formed at the outer most layer of the in service sample. The next thin layer of the sample becomes Cr2O3 and a very thin layer of MnFe- and MnCr- oxide was found at the inner edge of the tube.
Fig 3: The phases at different locations of the tubes detected by XRD - (a) virgin tube (b) service exposed tube. Layer thickness is not drawn to the actual scale.|
In service exposed tube, the oxides at the outer surface was formed due to the exposure of the tube in air at high temperature (>1100 °C). The formation of Cr oxides decreased the Cr content at the outer layer of the tube and dissolution of carbides took place, which led to the grain boundary carbides free region near the outer surface.
At the inner surface the diffused carbon from the naphtha cracking interact with the chromium and formed carbides and grew with the grain boundary carbides. After reaching a critical size the carbides came out from the surface as a metal dusting. The existence of dusting was observed in SEM at the inner edge of the tube (figure- 4). The metal dusting and depletion of Cr led to the dissolution of near by carbides at the inner surface of the tube, which resulted to grain boundary carbide free region. Thus the oxidation at the outer surface and carburisation at the inner surface led the Cr depletion region near both the surface. The variation of Cr concentration in the matrix from outer to the inner edge of the service expose tube is shown in figure-5. It was found that Cr depletion was more at the outer surface.
|Fig 4: Microstructure of the service exposed tube from inner edge showing the sign of metal dusting.||Fig 5: Chromium concentration profile in the matrix from outer to inner edge of the service exposed tube analyzed by EDAX.|
The virgin tube was non-magnetic. Hence, the magnetic properties were evaluated only on in-service tube. A hysteresis loop was plotted from the sample and is shown in figure 6(a). Hysteresis loop was also plotted [figure 6(b)] after removing 1-mm surface layer and ferromagnetic property was almost vanished.
|Fig 6: (a) Hysteresis loop obtained from the inner surface of the service exposed tube and (b) after removing 1-mm surface layer from inner surface.|
To understand the origin of magnetism, it is necessary to look the Fe-Ni-Cr phase diagram, which is shown in figure 7.
|Fig 7: Fe-Ni-Cr phase diagram. indicate carburised and oxide layers|
The oxidation or carburisation process decreases the carbon content within the matrix and increases Fe-Ni content relatively. Due to this compositional variation, the initially non-ferromagnetic steel become ferromagnetic. The appearance of ferromagnetism depends upon the depletion of Cr as well as Cr/Ni ratio in the matrix. As the Cr depletion due to oxidation was more for the present tube, only the outer surface was magnetic. To make inner layer ferromagnetic, more carburisation and hence longer period of service exposure is needed.
A probe was developed to measure magnetization by non-destructive way. Using this technique the magnetization was measured at different locations of the tube at two different frequencies to get different depth of penetration (figure-7). Difference in output signal at two different frequencies indicated the feasibility of the technique to measure the magnetic property due to carburisation of the tube.
|Fig 8: Magnetic induction at two different frequencies at different locations of service exposed tube by using magnetic NDT technique.|
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