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APPLICATION OF MARTENSITIC, MODIFIED MARTENSITIC AND DUPLEX STAINLESS BAR STOCK FOR COMPLETION EQUIPMENT
Rashmi B. Bhavsar CAMCO Products & Services 703 0 Ardmore Houston, Texas 77054 U.S. A. Raimondo Montani Foroni, S.p.A. 21055 Gorla Minore (VA) VIA A. Colombo, 285 Italy
ABSTRACT Martensitic and duplex stainless steel tubing are commonly used for oil and gas applications containing COz. Completion equipment manufacturing requires use of solid round bar or heavy wall hollows. Material properties for this stock are not identical in all cases. Material properties as well as corrosion characteristics are discussed for 13Cr, 13Cr - 5Ni - 2Mo and 25Cr alloys. Corrosion testing of modified or Enhanced 13Cr solid bar stock, UNS S41425 and other compositions in HzS - Cl- and pH is Corrosion testing of various super duplex bar stock at reported in coupled and uncoupled condition. Impact value requirements, various H$ - chlorides and temperature in CO* environment is reported. welding issues and special consideration required for these alloys for completion equipment is discussed. Modified 13Cr and Super Duplex Oil Country Tubular Goods (OCTG) are readily available, however, availability of completion equipment raw material compatible with these OCTG is limited. Keywords: Martensitic, 410 Stainless Steel, 420 Modified Stainless Steel, 13Cr, Super 13Cr, Modified 13Cr, Super Duplex, Stress Corrosion Cracking, Hydrogen Sulfide, Carbon Dioxide, Chlorides, PH.
Copyright01998 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be made in writing to NACE International, Conferences Division, P.O. Box 218340, Houston, Texas 77218-8340. The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A.
Stainless steel tub&us are widely used for oil and gas wells in environments containing COZ and small amounts of H#. Downhole completion equipment such as subsurface safety valves, landing nipples, packers, gas lii mandrels, and other tools commonly use solid round bar or heavy wall hollow as raw material, Bar stock provides flexibility of manufacturing various diameter parts out of one bar. Thin wall, cold worked tubular materials are not suitable as a starting raw material for completion equipment since Downhole equipment requires polished bores, dynamic seal distortion would occur upon machining. surfaces or require a tight drift and the distortion can not be tolerated for these components. Cold worked tubing and casing as raw stock is not suitable for these close tolerance components because of the distortion and are available with a limited wall thickness. The wall thickness required for completion equipment may exceed 1.5 to 2 in. (3.8 to 5 cm), and coupling stocks available in cold worked corrosion resistant alloys (CBA) are usually less than 1 in. (2.54 cm) thick. It is very common to use bar stock from heat treatable alloys that provides larger flexibility plus allows welding and forging to be used as fabrication techniques. API XT is not applicable to most completion equipment because the mill practices are different for solid bar and heavy wall hollows from oil country tubular goods (OCTG). Heat treatable raw stock is required for forged or welded products such as gas lifl mandrels. Alloy used for manufacturing completion equipment may not be the same as the tubing alloy and requirements applicable to OCTG may be similar, but would not be exactly the same for solid bars, which must be considered when ordering completion equipment. Many planned projects may utilize special modified 13Cr or duplex stainless steel OCTG at higher strength levels for economical reasons; however, tubing accessories may not be available in the same alloy with the same strength levels and corrosion resistance, which may have availability or cost implications.
API 5CT 13Cr OCTG are made from AISI 420 modified stainless steel. Bar stock and heavy wall hollows, used for completion equipment, are available in 410 or 420 modified stainless steel. Since 13Cr stainless steel is a fully martensitic alloy and is relatively easier to manufacture, it is commonly used for tubulars. 410 stainless steel has been used as forgings, castings (CA15) or as bars. Prior to 1994, 13Cr was not listed in the NACE Standard MRO175. When customers specified compliance to the MB0175 for equipment to be run with 13Cr tubulars, 410 stainless steel was utilized. It was grandfathered in MB01 75 from earlier days based on usage in the wellhead industry. As the North Sea area activities grew, it became necessary to supply 410 stainless steel with improved, consistent performance, and there was also a need for weldable 13Cr for gas lift mandrels. 420 modified stainless steel, due to higher carbon content, was not preferred for welding; as a result, 410 stainless steel was utilized for welded products. A specification for 410 stainless steel (410 - 13Cr) with higher chromium levels, to a 12.5% minimum, lower sulfur and phosphorus content with a minimum toughness requirement was developed. Samples of newly specified 410 stainless steel, with controlled chemistry and 420 modified stainless steel, from various suppliers, were tested to meet minimum corrosion and cracking resistance. Testing was carried out in 0.145 psi (0.010 Bar) H2S and 72.5 psi (5.0 Bar) CO* with a notched c-ring at 167F (75C) for 1,000 hours. The test criteria identified poor quality materials for this condition. Performance of this new 410 stainless steel, called 410 13Cr stainless steel, and 420 modified stainless steel was found to be similar in these types of environments. Table 1 lists the three grades, commercially available: AISI 410 stainless steel, 410 - 13Cr stainless steel and 420 modified stainless steel, and points out the differences and similarities in these alloys.
The major difference between 13Cr stainless steel OCTG from heavy wall and bar stock is the toughness values. As shown in Figures 1 and 2, as the bar diameter increases, the toughness value decreases. 13Cr stainless OCTG exhibit impact values meeting an average 40 Joules minimum at -10C as required by the North Sea operators (this is due to thinner wall and higher amount of working in OCTG). Large diameter 12/13Cr bar stock can not meet these values consistently. Current specification for 13Cr bar stock, including large diameter, has charpy impact values of 20 Joules (15 R.lbs.) minimum average at 10C in the longitudinal direction. (A higher probability of rejection would occur for the requirements of 40 Joules at -1OC.) The North Sea area operators and the Norsok Standard have accepted the 20 Joules average charpy values at - 10C and these alloys have performed well over the last 10 years. 420 modied stainless steel can be welded, however, due to high carbon content, is susceptible to cracking. To maintain a hardness below 22 HRc maximum in the heat affected zone, the required stress relieve temperature would be higher than the tempering temperature where the base metal would not be able to meet the required minimum design yield strength. 410 stainless steel, either 12 or 13Cr, due to lower carbon than 420 modified stainless steel, is successfully used for welding. Gas lift mandrels are fabricated by either quench and temper or stress relieve after welding to 22 HRc maximum hardness. (Filler metals are available that would provide similar toughness in the weld metal to the base metal.) 12 and 13Cr (410 13Cr) stainless steel has been used successfully for gas litI mandrels without stress corrosion cracking in the North Sea area as well as in many other oil and gas wells in various fields throughout the world. These steels are used as 80 ksi (551 MPa) minimum yield strength for completion equipment to be compatible with API L80 - 13Cr tubing. Use of these alloys at 95 ksi (615 MPa) minimum yield strength, and higher, is not recommended since tempering temperature required to achieve high yield strength can lower toughness as well as corrosion resistance. Generally the tempering temperature for these grades is limited to above 1,150F (620C) and preferably above 1,200F (650C). 21 The NACE Standard MB0175 lists 420 Modified stainless steel in the quench and temper condition to 22 HRc maximum hardness up to a maximum H2S partial pressure of 1.5 psi (0.1 Bar) in production environments with a produced water pH 2 3.5. Literature cites the H2S limit to be anywhere from 0.15 psi (0.01 Bar) at 3.5 pH to 15 psi (1.0 Bar) H2S at 4.5 PH.~ However, 12/13Cr stainless steel downhole equipment has been successfully used at higher levels of H2S without sulfide stress cracking (SSC).
Modified 13Cr, or the so called Super 13Cr, stainless steel tubulars with low carbon, 11.5 to 13Cr, 4 to 6% Ni, 1 to 2.5% MO, were introduced with improved weldability for flow lines. They were also offered as OCTG at 95 ksi (615 MPa) and 110 ksi (758 MPa) minimum yield strength. Because of nickel and molybdenum in these alloys, they offered higher toughness and higher general corrosion resistance compared to standard 13Cr stainless steel. Pitting and weight loss corrosion resistance in CO2 environment Several tubing suppliers offered their version of were higher and were suitable up to 302F (150C) modified martensitic alloys including 15Cr stainless steel. Though they were claimed to have higher resistance to sulfide stress cracking, these alloys were found to be susceptible to low levels of H2S. Performance of these alloys varied, depending on chemical composition. Susceptibility of sulfide stress corrosion cracking depended on pH and chloride levels in the test fluids. Literature review indicated