Abstract

Twisting and U-bending of heat exchanger tubing are common practices employed to enhance operating conditions and plant design. Both methods result in cold plastic deformation which can have an impact on the mechanical and corrosion properties of the material. This paper is focused on the effect of the cold plastic deformation on austenitic stainless steel (UNS N08935) and the necessity of subsequent solution annealing. The material qualification was evaluated by mechanical and corrosion properties using results of hardness, pitting corrosion resistance (ASTM G48 Method C), and stress corrosion cracking in sour conditions (NACE TM0177). The results will be able to demonstrate if the cold work deformation on alloy 35Mo will affect the use of this alloy in heat exchangers applications in the refinery industry in the cold-worked condition without the need of re-solution annealing.

Introduction

UNS N08935 is a new versatile super austenitic alloy with extreme pitting resistance as indicated by its pitting resistance equivalent number (PREN) of 52. It can be used in a broader temperature range than superduplex and hyperduplex stainless steels, offers good weldability and is more cost-effective than Nickel-based materials which make the grade a good candidate for O&G applications, refineries, and chemical industries.1,2

In heat exchangers, twisted tube technology and U-bundle design are used to achieve better heat transfer efficiency. Shargay et al. showed that heat treatment of UNS S32100 austenitic stainless steel twisted tubes is required to have equivalent chloride stress corrosion cracking resistance to non-twisted tubes.3 The tensile residual stress introduced by cold deformation will usually reduce the pitting corrosion resistance.4 As twisting and U-bending is a common practice to enhance operating condition and plant design, especially in the heat exchanger, this investigation is evaluating the effect of cold-working or plastic deformation on pitting corrosion resistance and the stress corrosion cracking resistance in sour conditions. For this reason, the aim of this paper is to evaluate the effect of cold deformation on the mechanical and corrosion properties of UNS 08935 alloy and to demonstrate if this alloy is still suitable to be used in heat exchangers applications in the refinery industry in the cold-worked condition without the need of re-solution annealing. The susceptibility of pitting corrosion, chloride stress corrosion cracking, and sulfide stress corrosion cracking will be performed in the twisted tubes and-bends specimens of UNS 08935. In addition, welded tubes of UNS 08935 will be evaluated.

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