Magnetic evaluation of the external surface in cast heat-resistant steel tubes with different aging states

•The oxide scale exerts a strong influence on the magnetic response.•Samples with a slight oxide formation exhibit a non-ferromagnetic behavior.•The oxide scale thickness is directly proportional to the magnetic response intensity.•Samples exposed to higher temperatures present a thicker oxide scale...

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Bibliographic Details
Published in:Journal of magnetism and magnetic materials 2018-06, Vol.456, p.346-352
Main Authors: Arenas, Mónica P., Silveira, Rosa M., Pacheco, Clara J., Bruno, Antonio C., Araujo, Jefferson F.D.F., Eckstein, Carlos B., Nogueira, Laudemiro, de Almeida, Luiz H., Rebello, João M.A., Pereira, Gabriela R.
Format: Article
Language:English
Subjects:
Aging
Aging (metallurgy)
Approximation
Austenitic stainless steels
Chromium
Creep (materials)
Destructive testing
Eddy current testing
Eddy currents
External surface
External walls
Heat resistant steels
HP steel
Magnetic fields
Magnetic force microscopy
Mechanical properties
Micrographics
Nondestructive testing
Oxidation
Oxide scale
Oxidizing atmospheres
Scale (corrosion)
Scanning electron microscopy
Sensors
Service life
Steel
Steel tubes
Structural damage
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Summary:•The oxide scale exerts a strong influence on the magnetic response.•Samples with a slight oxide formation exhibit a non-ferromagnetic behavior.•The oxide scale thickness is directly proportional to the magnetic response intensity.•Samples exposed to higher temperatures present a thicker oxide scale. Heat-resistant austenitic stainless steels have become the principal alloys for use in steam reformer tubes in the petrochemical industry due to its mechanical properties. These tubes are typically exposed to severe operational conditions leading to microstructural transformations such as the aging phenomenon. The combination of high temperatures and moderate stresses causes creep damages, being necessary to monitor its structural condition by non-destructive techniques. The tube external wall is also subjected to oxidizing atmospheres, favoring the formation of an external surface, composed by an oxide scale and a chromium depleted zone. This external surface is usually not taken into account in the tube evaluation, which can lead to erroneous estimations of the service life of these components. In order to observe the magnetic influence of this layer, two samples, exposed to different operational temperatures, were characterized by non-destructive eddy current testing (ECT), scanning DC-susceptometer and magnetic force microscopy (MFM). It was found that the external surface thickness influences directly in the magnetic response of the samples.
ISSN:0304-8853
1873-4766
DOI:10.1016/j.jmmm.2018.02.051