Engineering Failure Analysis 13 (2006) 75–79

www.elsevier.com/locate/engfailanal

Analysis of a boiler pipe rupture

Zhang Baoyou *, Lai Zhonghong, Cui Yuexian, Fan Xigang

Analysis and Measurement Center, Harbin Institute of Technology, Harbin 150001, PR China

Received 28 October 2004; accepted 29 October 2004

Available online 31 March 2005

Abstract

The rupture of a boiler pipe was studied by using chemical analysis, scanning electron microscope and energy dis-

persive spectroscopy. The results show that excess temperature caused by obstruction of stream flow was associated

with the bubble clusters on the bore surface, which resulted in the creep deformation responsible for elongated grains

and wall thinning. The pipe burst because the high temperature strength was below the designed standard.

� 2005 Elsevier Ltd. All rights reserved.

Keywords: Burst; Boiler pipe; Creep deformation

1. Introduction

A water cooling boiler pipe made of 20 steel burst after five months usage. The outside diameter and

thickness of wall are 38 and 4 mm, respectively. The steam temperature is 450 �C, and the pressure is

382 MPa. A segment 165 mm in length containing the rupture was cut from the pipe.

2. Experimental procedure and results

2.1. Macroscale analysis

A large approximately elliptical gap, with long axis of 53 mm and minor axis of 34 mm, can be observed

from the pipe, as shown in Fig. 1. The sharp fracture edge about 0.5 mm in thickness is thinner than in the

other regions. The bore close to the fracture surface is clean, corresponding to the location of a bubble

1350-6307/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2004.10.015

* Corresponding author. Tel.: +86 451 8641 4867; fax: +86 451 8641 4234.

E-mail address: xigangfan@163.com (Z. Baoyou).

Fig. 1. The morphology of gap.

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cluster 180 mm long. The other regions are covered with thick oxide film. The wall thickness at one end isuniform and at the other is non-uniform as seen from the section, with the minimum thickness of 2.8 mm

close to the gap (Fig. 2).

2.2. Analysis of material inclusions

The inclusions are contain sulfides, aluminum and spherical oxides.

2.3. Optical microstructures

Specimens were cut from the gap and the end of the analyzed segment. The microstructure was measured

using scanning electron microscope (SEM). The microstructures of the segment ends are mixtures of ferrite,

pearlite and cementite. Spherical or blocky cementite are located in the ferrite grain boundaries. The major-

ity of pearlite is spheroidized (Fig. 3).

As shown in Fig. 4, the region close to the gap comprises ferrite, pearlite and cementite, and the pearlite

is also spheroidized. The ferrite grains in the regions next to the gap were elongated along the deformation

direction of the pipe.

2.4. Analysis of oxide or bore

From Fig. 5, a layer of rust on the bore surface can be observed. The EDS result (Fig. 6) indicates that it

is Fe-bearing oxide. The ductile fracture is shown in Fig. 7. The composition of the surface involves the

elements Al, Si and Ca (Fig. 8).

Fig. 2. The section of pipe.

Fig. 3. The microstructure of the segment end.

Fig. 4. The microstructure of the gap.

Z. Baoyou et al. / Engineering Failure Analysis 13 (2006) 75–79 77

Fig. 5. The oxide morphology of the surface of inwall.

Fig. 6. EDS result for Fig. 5.

78 Z. Baoyou et al. / Engineering Failure Analysis 13 (2006) 75–79

3. Discussion

The microstructures of the pipe involve ferrite, pearlite and cementite. The majority of pearlite is sphero-

idized. The evolution of microstructure indicates that the pipe was used under high temperature, but the

highest temperature was not above the AC1 temperature of steel (735 �C). The evolution of microstructure

resulted from excess temperature which reduced the creep strength and resulted in the pipe expanding underthe steam pressure.

The greatly elongated grains at the edge of the gap show that the pipe failed under fast deformation.

Fig. 7. The cracked surface morphology.

Fig. 8. EDS result for Fig. 7.

Z. Baoyou et al. / Engineering Failure Analysis 13 (2006) 75–79 79

The excess temperature is related to bubble clusters. The rough surface obstructed the water flow, caus-

ing the overheating and excess temperature. The encrustation of the bore, one of the reasons of overheating,

is probably related to water quality.

4. Conclusions

The excess temperature, on account of the obstruction of stream flow associated with bubble clusters on

the surface of local regions, resulted in the creep deformation leading to wall thinning and rupture.