Mechanical and Corrosion Properties of Fe-Cr-Mn-C-N Austenitic Stainless Steels for Drill Collars
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In recent years, petroleum drilling operations have been conducted under severe conditions, such as those present in sour gas wells. In order to load the drill bit, the drill collar is attached on the upper side of the drill bit, which enhances the overall efficiency of drilling. The collar is subjected to severe abrasion in sour environment. In these environments, electrochemical corrosion such as pitting corrosion will be predominant, and hydrogen sulfide and carbon dioxide are particularly problematic, as these gases are highly corrosive. Corrosion may manifest itself gradually, but it causes catastrophic failure of many materials. For example, hydrogen sulfide encourages environmental-assisted cracking, such as stress corrosion cracking and hydrogen embrittlement. This can cause premature failure through an interconnected network of destructive cracks that
EUNKYUNG LEE, Research Associate, and BRAJENDRA MISHRA, Professor, are with the Department of Mechanical Engineering, Worcester Polytechnic Institute, Worcester, MA. Contact e-mail: [email protected] JISEUNG RYU and SEOL JEON, Ph.D. Candidates, are with the School of Materials Science and Engineering, Pusan National University, Busan, South Korea. BRUCE R. PALMER, Professor, is with the Texas A&M University at Qatar, Doha, Qatar. Manuscript submitted September 16, 2015. Article published online April 1, 2016 2550—VOLUME 47A, JUNE 2016
rapidly spread through otherwise unaffected metal. Metals which undergo such an attack can fail much more rapidly than metals that are subjected to more common, uniform corrosion. Therefore, pitting corrosion resistance[1] is required for this application along with superior general corrosion resistance and excellent mechanical properties of high strength and ductility. Furthermore, drill collar is contacted with electronic sensing devices, and non-magnetic property is required. For these reasons, Fe-Cr-Ni austenitic alloys have previously been used in down-hole applications, but these alloys have shown inadequate pitting corrosion resistance and limited mechanical performance.[2] Fe-Cr-Mn-based stainless steels with interstitially alloyed nitrogen (instead of nickel) have been of interest, mostly because of the enhancement of the noticeable mechanical and corrosion resistance properties of these nitrogen steels.[3,4] According to previous research on alloying nitrogen atoms in austenite, nitrogen atoms increase the concentration of free electrons, which enhances the metallic interaction. These studies showed that nitrogen increases the thermodynamic stability of the austenite phase and contributes to short-range atomic ordering in a wide range of Fe-Cr-Ni alloys. Short-range ordering is well known to contribute to the distribution of alloying elements (such as chromium), and improves corrosion properties, including the sensitization of steels.[5] Despite the promising properties of nitrogen stainless steels, nitrogen has a low solubility in iron and high pressure processes are needed to fabricate the alloys,[3,4] in addition to the i
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