Failure Analysis of AISI-304 Stainless Steel Styrene Storage Tank
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TECHNICAL ARTICLE—PEER-REVIEWED
Failure Analysis of AISI-304 Stainless Steel Styrene Storage Tank Muhammad Sajid Ali Asghar • Fawad Tariq Ashraf Ali
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Submitted: 6 March 2010 / in revised form: 1 April 2010 / Published online: 30 April 2010 Ó ASM International 2010
Abstract This paper presents the failure analysis of AISI304 stainless steel tank that was fabricated by welding and used for the storage of styrene monomers. After about 13 years of satisfactory operation, significant cracking was observed adjacent to the weld joints and in base plate near tank foundation. Weld repair was by shielded gas arc welding using AISI 308 stainless steel filler wire. The failed base plate was replaced with the new AISI 304 base plate of same thickness. After a short period of time, seepage was observed along the weld bead. Upon nondestructive testing cracks were found in the heat-affected zone and in the base plate. The failure investigation was carried out on welded and base plate samples using spectroscopy, optical and scanning electron microscopy, fractography, SEM–EDS analysis, microhardness measurements, tensile and impact testing. The results revealed transgranular cracks in the HAZ and base plate, and the failure was attributed due to stress corrosion cracking. Cracks initiated as a result of combined action of stresses developed during welding and the presence of a chloride containing environment due to seawater. It was further observed that improper welding parameters were employed for weld repair which resulted in sensitization of the structure and postweld heat treatment to remove weld sensitization and minimize the residual stresses was not done. Keywords Stress corrosion cracking Styrene AISI-304 Fractography Transgranular cracks Carbides Dye penetrant SEM
M. S. A. Asghar (&) F. Tariq A. Ali Department of Materials Engineering, NED University of Engineering and Technology, Karachi 75270, Pakistan e-mail: [email protected]
Introduction and Background The selection of stainless steels for chemical storage tanks and vessels is based on mechanical properties, corrosion resistance, fabrication characteristics (such as weldability), working environment, service temperature, and cost. However, corrosion resistance and mechanical properties are generally the most important factors whenever selecting a grade of stainless steel for a given application [1]. Among the many 300 series austenitic stainless steel grades, AISI-304 SS is widely used for the construction of large storage tanks for styrene monomers, because of its corrosion resistance in many environments and its relatively low cost as compared to other 300 series steels. Moreover, another important factor for choosing AISI 304 SS is that it is chemically inactive to the styrene monomer. However, this corrosion resistance is not found in all environments; especially in marine environments which are well known to induce serious stress corrosion cracking (SCC) problems due to the presence of chloride ions [2–4]. Stress corrosion cracking is a fo
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