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TECHNICAL ARTICLE—PEER-REVIEWED

Corrosion Fatigue Failure Analysis of a Supporting Pipe Weld in Deaerization Plant Qiaoling Chu . Min Zhang

Submitted: 2 July 2015 / in revised form: 28 July 2015  ASM International 2015

Abstract The aim of this work is to reveal the failure mechanism of a supporting pipe weld after 20 years of service in a deaerization plant. The characterization methods included inductively coupled plasma, optical microscopy, scanning electron microscope, X-ray energy dispersive spectrometer, mechanical tests, and finite element analysis (FEA). The results showed that the supporting pipe weld failed by corrosion fatigue cracking. Cracks initiated from the surface pits which were due to corrosion. FEA analysis revealed that stress concentration occurred in the failed weld. This paper brings out the details of the investigation and suggests remedial measures to improve performance of such pipe welds in deaerization plants. Keywords Weld

Corrosion fatigue
Failure analysis


Introduction In steam production plants, the most serious problems perhaps involve water side corrosion which results from the presence of high oxygen levels in the system. In order to solve these problems, deaerators are applied to reduce dissolved oxygen in boiler feedwaters. Generally, the deaerator system can be considered as the critical plant component in the low pressure feedwater system [1]. In this work, a supporting pipe applied in such a deaerization plant exhibited cracks in the welds after 20 years of service. Figure 1a shows the schematic Q. Chu (&)
M. Zhang College of Materials Science and Engineering, Xi’an University of Technology, Xi’an 710048, China e-mail: [email protected]

drawing of the supporting pipes. The supporting pipes and steam pipes were made of ASTM A312 Type 321 austenitic stainless steel. These pipes were joined by shielded metal arc welding (SMAW). AWS ER321 electrode was employed as the filler material. The supporting pipes had an outer diameter of 26.7 mm with wall thickness of 3 mm. The steam pipes had an outer diameter of 88.9 mm with wall thickness of 7.62 mm. The smaller pipes were installed to support the steam pipes. Cracks occurred in the weld metal and extended into the matrix material as exhibited in Fig. 1b. This paper analyzes the failure mechanism of this failed weld.

Experimental Set-Up The pipes displayed in Fig. 1a were made of ASTM A312 Type 321 stainless steel. Chemical analysis by inductively coupled plasma (ICP) was applied to identify the matrix and weld materials of this failed pipe. A cross section of the welded joint was prepared to determine the microstructure characteristics. The metallographic samples were prepared by grinding and successive wet grinding emery papers and fine polishing with diamond paste. The etching solution was a mixture of 15 ml HNO3 and 45 ml HCl (Aquaregia solution). The fracture surfaces were observed with a stereo microscope and a scanning electron microscope (SEM) equipped with an X-ray energy dispersive spectrometer (EDS). Tens