Paper No. 96 APPLICATION OF MARTENSITIC, MODIFIED MARTENSITIC AND DUPLEX STAINLESS STEEL 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 reported in coupled and uncoupled condition. Corrosion testing of various super duplex bar stock at various H$ - chlorides and temperature in CO* environment is reported. Impact value requirements, 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. Copyright 01998 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.
Transcript
Paper No.
96
APPLICATION OF MARTENSITIC, MODIFIED MARTENSITIC AND DUPLEX STAINLESS STEEL
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 reported in coupled and uncoupled condition. Corrosion testing of various super duplex bar stock at various H$ - chlorides and temperature in CO* environment is reported. Impact value requirements,
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.
Copyright 01998 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 distortion would occur upon machining. Downhole equipment requires polished bores, dynamic seal 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.
12/13Cr STAINLESS STEELS
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 167°F (75°C) 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.
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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 -10°C 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 - 10°C in the longitudinal direction. (A higher probability of rejection would occur for the requirements of 40 Joules at -1O’C.) The North Sea area operators and the Norsok Standard have accepted the 20 Joules average charpy values at - 10°C 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 1,150”F (620°C) and preferably above 1,200”F (650°C). 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 STAINLESS STEELS
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 were higher and were suitable up to 302°F (150°C)” Several tubing suppliers offered their version of 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 that at higher levels, that is at 100,000 ppm chlorides and at pH below 4, tolerance to H$ concentration was very low, and cracking occurred at room temperature below 1 psi (0.07 Bar) of H$.4,51
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OCTG suppliers of modified Super 13Cr stainless steel did not supply solid bar or heavy wall hollows as they were set up to mass-produce tubulars only and it was commercially unattractive for them to pursue the tubing accessory market. Due to the proprietary nature of these alloys, chemical composition of the bar materials varied from different suppliers and their resistance to SSC needed to be evaluated. Table 2 lists the typical chemical composition for the alloys evaluated.
Initial testing was performed on duplicate samples of three heats of alloy UNS S41425 using NACE TM0177 Method A (tensile test) for SSC and Method C (c-ring) for high temperature testing (Table 3). There were no failures in tensile test of any of the samples in 1.5 psi (0.1 Bar) H2S, 5% NaCl at 3.5 pH. The samples were shiny at the end of 30 days testing and no cracks were found. Similarly, no failure of c-ring occurred in 0.45 psi (0.03 Bar) H$ and 15% NaCl at 194°F (90°C) or in 1.5 psi (0.1 Bar) H2S and 5% NaCl at 348°F (175°C). Slow strain rate testing was also performed on duplicate specimens of two heats in CO2 - Cl environments at higher temperatures to assess stress corrosion cracking in the absence of H2S. UNS S41425 solid bars had ratios, for percentage reduction of area and time to failure, higher than 0.9 indicating no stress corrosion cracking at high temperature (150°C) in the absence of H2S.
As shown in Table 4, UNS S41425 and modified 13Cr bar stock from a second supplier (Supplier B), was tested per TM0177 Method A. UNS S41425 performed better than the other sample but did not perform as good as the tubing sample in 1 psi (0.07 Bar) H#, 90,000 ppm Cl- and 4.2 pH. Three out of four samples of UNS S41425 passed but one specimen had small pits. One of the pits had a crack at the bottom of the pit when examined at 100x under a microscope and was considered a failed sample. The failure in this test seemed to be related to a higher concentration of chlorides. other bar stock materials failed in a short period of time under the same test environment. One of the tubing samples tested had a pit but no crack was detected at the bottom of the pit. 110 ksi (758 MPa) minimum yield strength material was evaluated at a lower level of H2S since some of the literature cited higher susceptibility to SSC for higher strength tubing materials. UNS S41425 showed susceptibility in 0.15 psi (0.1 Bar) H$ and 30,000 ppm Cl at 3.5 pH but did not fail a pH of 4.5 in the same concentration of H2S or at lower levels of H$i (i.e. at 0.05 psi or 0.003 Bar) at 3.5 pH. Susceptibility of modified 13Cr stainless steel depends upon concentration of H2S, chlorides and the pH of the test fluids and susceptibility seems to be higher for higher strength materials.
When coupled to steel, both tubing and bar stock materials experienced SSC failure in 1.5 psi (0.1 Bar) H2S and 4.5 pH in a very short time due to rapid hydrogen charging which makes these alloys very susceptible to SSC. However, when the temperature was raised to 150°F or the pH raised to 4.5, failure did not occur under the same environmental condition (Table 5). The absence of cracking at higher temperature is helpful for downhole tools as temperature is generally above 75°F (24°C). Standard 13Cr (420 Modified)
stainless steel did not experience such a short time failure in similar test condition. Application should be reviewed using a modified 13Cr when coupled to a carbon steel or low alloy steel tubulars and caution should be applied accordingly.
Modified 13Cr or Enhanced 13Cr stainless steels offer higher strength as well as higher toughness. Charpy v-notch values for solid large diameter bars are in excess of 90 Joules and typically in excess of 135 Joules at -10°C significantly higher than 410 - 13Cr or 420 Modified stainless steel. Higher strength makes these alloys attractive as it can offer equipment with larger bore with a given OD or with a reduced OD for the same ID. Improved corrosion resistance and higher strength make it more attractive for larger usage, however SSC resistance has to be evaluated for each grade of modified 13Cr stainless steel for the specific application. Bar stock material availability is restricted and performance depends on the chemical composition and manufacturing practices. When using modified 13Cr tubing it is important to keep this in
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mind and equipment manufacturers should be consulted for availability of equipment in modified 13Cr for the given environment.
Modified 13Cr pipes are welded for flow lines using duplex stainless steel filler metal to avoid post weld heat treatment. Welding development for downhole gas lift mandrel applications was carried out using bar stock material and matching filler metal. Welding filler metal was developed and procedure qualification was performed per ASME Section IX. For side pocket mandrel applications, post weld quench and temper was performed to achieve uniform properties across weld and heat affected zones. Side pocket mandrels are usually installed at the lower part of the well as chemical injection mandrels or as gas lift mandrels. Gas lift mandrels are spaced out up and down the tubing string, normally from 1,000 ft. to 8,000 ft. (300 to 2,400 meters). Since they are generally exposed to higher temperatures, corrosion testing using the NACE tensile test was performed at 300°F (149°C). The welded section was in the middle of the gage length and failure did not occur in 1 psi (0.07 Bar) H# and 90,000 ppm chlorides at 300°F (149°C). SSC performance at low to moderate temperatures is yet to be evaluated.
DUPLEX STAINLESS STEELS
Earlier duplex stainless steels with 22% chromium offered around 65 to 70 ksi (448 to 482 MPa) minimum yield strength in the annealed condition. At this lower strength, they are not useful for completion equipment, since most tubing is 80 ksi (55 1 MPa) minimum yield strength or higher. They are cold worked to 110 ksi (788 MPa) minimum yield strength or higher as OCTG but not as heavy wall hollows or bar stock and they are not considered for completion equipment.
The new family of “Super Duplex” stainless steels with 25% chromium and pitting resistance equivalent (PRE) over 40 were introduced and offered 80 ksi (551 MPa) minimum yield strength in the annealed condition. These alloys containing chromium-nickel-molybdenum-nitrogen, with or without tungsten and copper, offer higher corrosion resistance and higher strength and are likely candidates for completion equipment. Ahoy UNS S39277 has a tungsten content of 0.75 to 1.5%, which improves the resistance to localized corrosion similar to molybdenum. However, tungsten does not accelerate the
precipitation of sigma phase and shows improved resistance in chloride containing environments.61 Since most duplex stainless steel tubing is used with 110 ksi (788 MPa) minimum yield strength, these duplex stainless bars in the annealed condition with 80 ksi (551 MPa) minimum yield strength are not compatible with the tubing mounted components and are not commonly used for downhole tools.
Sulfide stress corrosion cracking evaluation was carried out at room temperature on an 8 in. (203 cm) diameter solid round bar of UNS S39277 using tensile test and solution per NACE TM0177. No
cracking occurred at 90 or 100% of yield strength in the coupled to steel condition in saturated H2S with 5% NaCl and 0.5% acetic acid. Since completion equipment is exposed to elevated temperatures and duplex stainless steel shows higher susceptibility at around 175°F to 195°F (80 to 9O”C), corrosion testing was carried out at 176” F (80°C) and 375°F (190°C). 25Cr super duplex stainless steels with similar chemical composition, as shown in Table 6, were also evaluated. C-ring test specimens were removed from
solid round bar stock [up to 8 in. (203 cm) diameter] and heavy wall raw material in the annealed condition. Nominally, 0.250 in. (0.63 cm) was removed from the OD of each material for clean up.
The required outer fiber stress levels were obtained utilizing the mathematical formula of NACE TMO177-90, Method C for OD type stressing. A modified version was utilized for ID type stressed rings. All c-rings were stressed to 100% at test temperature yield strength utilizing the modified method of
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paragraph 8.5.1.2. (1990 Edition). This modification essentially doubled the required deflection deformation. The gas phase was initially introduced into the environmental autoclave at room temperature, then readjusted at temperature to the required partial pressure. At approximately 10 day intervals 75-100 psi (5 to 7 Bars) was bled from the autoclave and replenished with the correct premixed composition. 75% of the autoclave’s volume contained the liquid phase; 25% was allocated to the gas phase. To minimize the adverse effects of liquid phase oxygen contamination in these sour gas environmental evaluations, controlled laboratory procedures and techniques were practiced.
Environmental parameters and the mathematical methodology of c-ring outer fiber stress versus deflection calculations are critical variables in the performance of super duplex stainless steels exposed to sour gas surroundings in laboratory testing. At 375°F (190°C) with 25% NaCl and 5 or 10 psi (0.34 or 0.7 Bar) H2S, no environmental cracking or preferential attack of the ferrite phase (PAFP) was apparent. Lowering the temperature to 176°F (80°C) with the same quantities of H2S and NaCi produced severe cracking. Reducing the NaCl concentration to 20% with HzS concentrations of 3 or 5 psi (0.20 or 0.34 Bar) still produced cracking and preferential attack of ferrite phase at 176°F (Table 7). This is significant as subsurface safety valves and some gas lift mandrels are used at shallow depths and at these low temperatures, caution should be used in specifying these materials in production environments.
When the NaCl concentration was lowered to 15% with 3 or 10 psi (0.20 or 0.7 Bar) HzS (HCO3 buffered), alloys UNS S39275 and UNS S32760 produced cracking; ahoy UNS S39277 produced variable performance from its different product forms, i.e. 30-40% apparently cracked. Lowering the H2S concentration to 0.5 psi (0.03 Bar) with 15% NaCl produced no cracking or preferential attack.
As well established by earlier studies, the temperature regime around 176°F (80°C) can be considered SSC critical for the super duplex stainless steel; the higher 375°F (190°C) temperature evaluation did not produce evidence of SCC.
The stress condition of the test specimen is important. The plastically deformed 100% applied stress material seems to enhance the tendency to produce SCC versus lower degrees of deflection (stress). Highly stressed environmentally wetted surfaces seem to have a higher probability to come down with PAPP. A precursor to SCC of these type materials seems to involve the preferential attack of the materials’ ferrite phase. Within the interior of such zones, enhancement of localized solution chemistry aggressiveness occurs. Subsequent interaction with stress may produce SCC.
The center of the large diameter bar will cool very slowly during annealing of large diameter bars. It is likely that secondary phases, such as sigma and laves phases, would be present below certain depths, lowering the performance of these large diameter materials. It is recommended that microstructures be evaluated to detect secondary phases. Alternately, impact testing at -50°F (-46°C) or lower temperature in transverse direction helps to detect detrimental effect of these phases. Significantly lower charpy v-notch values will indicate presence of undesirable phases. A good practice for large diameter bars [above 6 in. (152 cm) diameter] would be to bore a hole through prior to annealing to reduce cross sectional areas and provide uniform cooling.
Duplex stainless steels are not commonly used for downhole tools in production environments due to the high rate of corrosion under acidiing conditions. Additionally, they are more susceptible to cracking in the temperature range typical of downhole environments. However, because of their high pitting resistance, they are preferred for completion equipment in treated sea water injection wells.
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In higher level of H,S and for higher strength requirements nickel based alloys, such as Incoloy 925 or Inconel 718, are recommended. Due to higher cost of raw material and diiculty of machining these alloys, higher cost of the equipment should be expected. Higher cost would be justified in more severe environments.
CONCLUSION
Mill practices are different for solid round bars from OCTG and alloys utilized for completion equipment may not have identical properties as the tubing ahoy. When specifying completion equipment, requirements for these alloys should be considered. 410 - 13Cr stainless steel is available with similar corrosion resistance as 13Cr stainless steel OCTG and can be welded successllly.
Modified 13Cr stainless steel offers higher strength and toughness and excellent corrosion resistance in CO2 environments containing chlorides compared to API L80 - 13Cr. However, it is susceptible to sulfide stress corrosion cracking in H2S and chlorides at room temperature in low pH fluids. The susceptibility increases with lower pH, higher HzS or higher chlorides. UNS S41425 modified 13Cr with 95 ksi minimum yield strength performed better than other modified 13Cr stainless steel bar stock. Modified 13Cr stainless steel bar or tubing with Ni - MO were more resistant to general corrosion in all test conditions, but were more susceptible to SSC when coupled to steel. Availability of new alloys in bar stock with similar performance to OCTG can be an issue and equipment manufacturers should be consulted prior to specifying higher strength alloys.
Duplex stainless steel showed most susceptibility at 176°F (80°C) compared to 375°F (190°C). UNS S39277 performed better than UNS S32760 or UNS S32750. Procurement of completion equipment made from stainless steel requires addressing proper specifications and careful evaluation of these alloys for the given environment, plus availability of proper raw material to produce a tubing accessory.
REFERENCES
1. NACE Standard MR0175 - “Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment”, 1997 Edition.
2. “Acid Corrosion in Wells (COz, H2S): Metallurgical Aspects”, Jean Louis Crolet; Elf Acquitaine,
SPE 1983.
3. Proposed ballot for NACE Standard MRO175 dated September 5, 1997.
4. “Corrosion Resistance of 13Cr, 5 Ni - 2 MO Martensitic Stainless Steel in CO2 Environment Containing Small Amount of H2s”, M. Ueda, T. Kushida, K. Kondo, T. Kudo; Sumitomo Metals Industries, Ltd.; Corrosion 92, Paper No. 55.
5. “Sulfide Stress Cracking Resistance of Supermartensitic Stainless Steel for OCTG”, L. Scoppio, M. Barteri - Centor Svilsppo Material: S.p.A., G. Cumino - Dahnine Tubi Industriali: S.r.1.; Corrosion 97, Paper No. 23.
6. “The Effect of Tungsten and Molybdenum on the Performance of Super Duplex Stainless Steels”,
