ABSTRACT

Despite of excellent merits in mechanical and chemical properties of duplex class stainless steels, stringent requirements in welding procedure qualification and fabrication stage have regulated not only the application of various high-efficiency welding processes but also applicable welding parameters. This study is focused on hardness requirement which is known as one of the most stringent factor in welding of duplex class material and this study aims to evaluate the effects of machining and welding methodology on hardness value of duplex and superduplex stainless steel weldments.

INTRODUCTION

Currently, duplex and superduplex stainless steels are widely used for piping system in offshore unit because of its excellent combination of strength and corrosion resistance properties. Despite of excellent merits in mechanical and chemical properties of duplex class stainless steels, stringent requirements in welding procedure qualification and fabrication stage have regulated not only the application of various high-efficiency welding processes but also applicable welding parameters. First of all, this research aims to investigate the main factor to cause irregular hardness increase on the surface of specimen by machining methodology using various experiments and proposes an optimum surface preparation for hardness testing. Hardness limits for 22%Cr duplex stainless steel (UNS 31803) and 25%Cr superduplex stainless steel(UNS 32760) are often taken from NACE MR0175 and most of the oil major companies used to regulate hardness limit on basis of their own specifications. Because Rockwell C hardness measurement just written in NACE standard is somewhat coarse and not ideal for accurate measurement of discrete regions, such as small weld zone and heat affected zone (HAZ) and there is no standardized conversion table or function for duplex materials between Rockwell and Vickers system in ASTM E140, many efforts have been proceeded to produce new hardness correlation function for duplex materials until now.

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