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Research Papers: Materials and Fabrication

Failure Analysis of a 9–12%Cr Steel Superheater Tube

[+] Author and Article Information
E. Pour saeidi1

Department of Mechanical Engineering, Zanjan University, Zanjan 31585-381 Iran; IPPR Research Laboratory, Iran Power Plant Repair Co., Karaj, Iranepsaeidi@mail.znu.ac.ir

M. Aieneravaie, M. R. Mohammadi Arhani

IPPR Research Laboratory, Iran Power Plant Repair Co., Karaj, Iran

1

Corresponding author.

J. Pressure Vessel Technol 131(6), 061401 (Sep 23, 2009) (9 pages) doi:10.1115/1.3147982 History: Received May 03, 2008; Revised May 06, 2009; Published September 23, 2009

The failure analysis of 9–12% chromium steel tubes, containing about 2.5% molybdenum, is discussed in the present study. The component is used in a steam power plant boiler as a high-temperature superheater tube and has been in service for about 100,000 h. The failure occurred without appreciable wall thinning. Specimens were taken from the region beneath the fracture surface and investigated by optical and electron microscopes. The microstructure was composed of ferrite and grain boundary particulate carbides. The results indicated that the fracture was initiated because of the bending of the tube near the anchor and propagation of the crack through the interfaces between massive carbides and matrix (sensitized zone). Final fracture has occurred as a result of an overload due to the decreasing of load carrying section produced by crack propagation.

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Copyright © 2009 by American Society of Mechanical Engineers
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Figures

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Figure 1

(a) General aspect showing the location of cracking and whole fracture surface; (b) multiple cracking in one-half of the fracture surface, 7×; (c) the flat surface with parallel lines and a little plastic deformation, 7×; (d) deposits formed on the outer surface of the tube, 7×; and (e) parallel lines due to cracking of the oxide scale on the inner surface of the tube at the initiation site of the fracture, the arrow indicating the origin of the fracture, 20×

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Figure 2

(a) SEM micrograph of the tube from the region that is referred to as sensitized zone, (b) EDX point chemical analysis from the center of a grain, (c) EDX chemical analysis of the bulk material, and (d) EDX point chemical analysis from a grain boundary carbide

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Figure 3

Microstructures of the HTSH 9–12%Cr steel tube: (a) optical micrograph of the transverse section of the tube at the outer surface showing the etch attack, (b) SEM micrograph from the outer surface section, (c) optical micrograph of the axial section of the tube near the outer surface showing the continuous network of carbides at grain boundaries, and (d) optical micrograph of the crack path through the grain boundaries

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Figure 4

(a) Multiple cracking at the sensitized zone of the outer surface (white arrows), 40× and (b) secondary cracks, 64×

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Figure 11

Schematic representation of the temperature gradient in the heat transfer from hot flue gas to steam (5)

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Figure 10

The bending stress due to tube buckling on the outer surface as a function of temperature difference

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Figure 9

(a) Schematic view of a bended beam under the influence of an axial force P, and (b) the free-body diagram for the left-hand part of the beam

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Figure 8

Schematic view of a straight cap ended high-pressure vessel

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Figure 7

Schematic view of the section of a tube and its dimensions

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Figure 6

The view of HTSH bundle and the position of failed tube

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Figure 5

Initiation sites of the final fracture from the inner surface (white arrows), 40X

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