ABSTRACT

The majority of directional, measure-while- and logging-while drilling tools as well as non-magnetic drill collars (NMCD) are manufactured using austenitic CrMn-stainless steels. These proprietary grades meet the demanding requirements in terms of mechanical, magnetic and corrosion resistance properties necessary for these applications. However, these grades might become susceptible to pitting and environmentally assisted cracking (EAC) when subjected to high chloride-containing water-based drilling fluids at elevated temperatures. Consequently, Ni-stabilized austenitic stainless steels with higher contents of chromium and molybdenum are preferred in these environments. In a similar manner as for CrMn-stainless steels, thermo-mechanical processing including warm or hot working as well as welding can lead to sensitization of these grades. Sensitization in CrMn-stainless steels was indeed the driving force behind the introduction of quality assurance testing per ASTM A262 in API specification 7-1. Recent studies have shown that ASTM A262 Practice A has limitations when used in high Ni-containing grades used in drilling technology, though. In this study, ASTM G48 Method A was evaluated as a potential standardized test method for assessing sensitization and pitting corrosion resistance of CrNi-stainless steels used in drilling equipment. Results from ASTM G48 were also compared with results from more traditional quality assurance methods like Charpy V-notch impact energy and ASTM A262 Practice A obtained on several commercially available grades.

INTRODUCTION

Pitting and environmentally assisted cracking (EAC) are a major concern when austenitic CrMn-stainless steels, which are commonly used for non-magnetic drill collars (NMCD)1, are exposed to high halide-containing drilling fluids. Even though the fast propagation of environmentally assisted cracks can partially be controlled by the introduction of compressive residual stresses2, surface treatments are not intended to address localized corrosion resistance. In fact, it has been demonstrated that CrMn-stainless steels become prone to pitting corrosion when exposed to high Cl streams at elevated temperature1,3-5, which ultimately can also lead to stress corrosion cracking (SCC) when sufficient stresses are present.6-9 Therefore, corrosion resistance becomes a crucial property for operational reliability and integrity of drilling equipment. To tackle this challenge, nickel (Ni)-stabilized austenitic stainless steels with higher contents of chromium and molybdenum are preferred in these harsh environments. In fact, the number of grades that became commercially available as well as the utilization rate of non-magnetic CrNi-stainless steels in drilling technology has significantly increased during the past decade.

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