Development of weldable martensitic stainless steel line pipe by HF-ERW process

Transcription

1 Stainless Steel World 99 Conference Development of weldable martensitic stainless steel line pipe by HF-ERW process SSW PAGE 331 Authors: Naoshi Ayukawa (Pipe & Tube Sales Dept, Nippon Steel Corporation, Japan), Hideyuki Nakamura (Nagoya R&D Laboratories, Nippon Steel Corporation, Japan), Motofumi Koyuba (Pipe Dept, Nagoya Works, Nippon Steel Corporation, Japan), Akihiro Miyasaka (Nagoya R&D Laboratories, Nippon Steel Corporation, Japan), Manabu Ueda (Pipe Dept, Nagoya Works, Nippon Steel Corporation, Japan), Hitoshi Asahi (Steel Research Laboratories, Nippon Steel Corporation, Japan), Koichi Nose (Steel Research Laboratories, Nippon Steel Corporation, Japan), Abstract A new type of martensitic stainless steel line pipe having excellent weldability and high corrosion resistance in wet CO 2 environments manufactured by the highfrequency electric-resistance welding (HF-ERW) process was developed on a commercial scale. The newly developed pipe is characterised by greater productivity compared with the conventional martensitic stainless steel seamless pipes, higher dimensional accuracy, in respect of thickness and outside diameter and capability of manufacturing thinner-wall pipes. 1 Introduction Recently, oil wells and natural gas wells containing considerable amounts of carbon dioxide (CO 2 ) have been increasingly going into service, and martensitic stainless steel oil country tubular goods (OCTG) (ex. AISI 420) are used for such environments. As for line pipe use, however, martensitic stainless steel pipes have not been popular because of its poor weldability. Although some new types of seamless martensitic stainless steel pipe with improved weldability have been developed (refs 1, 2) and going into service since a few years ago, no HF-ERW martensitic stainless steel pipe has been fully developed. In this study, we have developed, on commercial scale, a new HF-ERW martensitic stainless steel line pipe with excellent field weldability and corrosion resistance in wet CO 2 environment. The newly developed steel contains 11 13% Cr. It also contains Ni and a little amount of Mo. Amounts of C and N are extremely low. The well-controlled chemical composition makes it possible to obtain high corrosion resistance and good weldability at the same time. As for manufacturing process, we have developed a new method for inert gas shielding in seam welding process. This prevents Cr oxide formation during seam welding, and sound weld seam has been obtained. The newly developed HF-ERW martensitic stainless steel line pipe has higher productivity than seamless martensitic stainless line pipes. In addition, it has higher dimensional accuracy, and thinner wall thickness is available. In the following, the design concept for material, the manufacturing process, and the dimensional and mechanical properties of the trial products are described. The mechanical properties and corrosion resistance of the HF-ERW portion and the welded joint are also described. 2 Concept of materials design 2.1 Target properties Figure 1 shows the service environment of martensitic stainless steel line pipes. The target of the development was to obtain a martensitic stainless steel that has sufficient corrosion resistance in the wet CO 2 environment shown in figure 1. Table 1 shows the target properties including mechanical ones. The target low-temperature toughness in the base metal, the ERW portion and the welded joint was set at 40J or above in terms of the absorbed energy at 20 C in the Charpy V-notch impact test. Girth welding was assumed to be carried out without pre-heating and post weld heat treatment (PWHT). Target strength grade was set at American Petroleum Institute (API) X80 considering the matching with weld materials. 2.2 Basic concept Figure 2 shows the basic concept to obtain weldable martensitic stainless steel line pipes having tough welded joint, excellent weldability and high corrosion resistance. To obtain good CO 2 corrosion resistance,

2 SSW PAGE 332 Development of weldable martensitic stainless steel line pipe by HF-ERW process Cr content must be 11% or above. Lower C and N contents are required to keep the hardness in the heat-affected zone (HAZ) from increasing. To prevent the formation of coarse δ-ferrite that impairs the toughness of the HAZ, Ni, which is a γ-phase stabilising element other than C and N, is added to produce a fully martensitic structure in the HAZ. Mo is added to increase the sulphide stress cracking (SSC) resistance in the wet CO 2 environments containing trace amounts of H 2 S (0.001 bar or below). 3 Trial products of HF-ERW pipe 3.1 Chemical composition and manufacturing process Table 2 shows the chemical composition of the ultra-low C, ultra-low N, 11% Cr, 3.5% Ni and 1% Mo steel developed. Figure 3 shows the manufacturing process. Air cooling after hot rolling forms a homogeneous martensitic structure in the base metal. The obtained strip was annealed below the Ac 1 temperature to reduce the strength to desired level. This strip was formed into a pipe by HF-ERW process with a diameter of 10 3 / 4 in. in a most advanced ERW pipe mill at Nagoya Works. Welding was done in an inert-gas atmosphere so as to prevent the formation of chromium oxide and in-line seam heat treatment was applied to reduce the ERW portion hardness. 3.2 Mechanical properties and microstructure Strength Table 3 shows the results of tensile tests for the base metal and HF-ERW portion. Because of the proper heat treatment applied to the hot-rolled strip, strength of the X80 grade was stably obtained. The tensile test specimen containing the HF- ERW portion ruptured in the base metal after tensile test. This showed that good welding was done. Strength at elevated temperatures are illustrated in figure 4. A decline in yield strength with increasing temperature was small. The yield strengths at temperatures from 100 to 150 C, which are expected service temperatures, were as high as those at a room temperature Toughness Figure 5 shows the results of the Charpy impact test on the base metal and the HF- ERW portion. Table 4 shows the results of a V-notched impact test conducted on the HAZ of the seam heat treatment with various positions of the notch. Figure 6 schematically shows the HAZ of the seam heat treatment. Both the base metal and the HF- ERW portion proved to have enough absorbed energy at 20 C Crack tip opening displacement (CTOD) Table 6 shows the results of the CTOD test. The target CTOD value of 0.15 (δ = 0.15mm) was satisfied in all areas including the HF- ERW portion Hardness Table 5 shows the Vickers hardness of the base metal and the HF-ERW portion. The actual Vickers hardness was under 300 (Hv < 300), which meets the target hardness of not higher than Microstructure Figure 7 shows the microstructure of the base metal and the HF-ERW portion. Both the base metal and the HF-ERW portion proved to have homogeneous tempered martensite structures. 3.3 Pipe dimensions Table 7 shows the roundness and the eccentricity of the trial pipes. The roundness and the eccentricity averaged 0.15% and 0.9%, respectively. The dimensional accuracy was much better than that of ordinary seamless pipes and comparable to that of ordinary HF-ERW pipes. 3.4 Welded joints Table 8 shows the gas tungsten-arc welding (GTAW) conditions. Figure 8 shows the hardness profile and macrostructure in the welded joint. Table 9 shows the results of the tensile tests on the welded joints. All tensile specimens ruptured in the base metal. As is evident from table 10, the welded joint have enough absorbed energy. Table 11 shows the results of the CTOD test. The welded joint, like the base metal, has sufficient property. 4 Corrosion properties 4.1 General corrosion properties Corrosion tests at 100 C in 25 mass% NaCl aqueous solution were conducted for base

3 metal, HF-ERW portion and girth weld portion. The test conditions are listed in table 12. The results are tabulated in table 13. In any conditions, the corrosion rates were lower than 0.01mm/y and no crevice corrosion was observed. 4.2 Sulphide stress cracking (SSC) properties An SSC test were carried out under conditions which are listed in table 14. The results are listed in table 15. The critical H 2 S partial pressure for this line pipe is estimated to be bar in 5% NaCl solution. 4.3 Cathodic protection properties Resistance to hydrogen embrittlement under cathodic protection was tested under conditions listed in table 16. Even at 1194mV vs SCE, which corresponds to an over cathodic protection, no crack was observed. The steel was immune from hydrogen embrittlement even under cathodic protection. Stainless Steel World 99 Conference 5 Conclusions 1 A new type of weldable martensitic stainless steel line pipe, with thin wall thickness and high dimensional accuracy, has been developed by applying improved inert-gas shielding technology in HF- ERW process. 2 The developed pipes were weldable without pre-heating and PWHT. All investigated properties, including those of HF- ERW portion and welded joint, proved to satisfy the target mechanical properties and corrosion resistance requirements. References 1 H. Asahi, K. Nose, H. Inoue, H. Tamehiro, T. Terasawa, M. Koyuba and N. Ayukawa: Corrosion and Mechanical Properties of Weldable Martensitic Stainless Line pipes, Super martensitic Stainless Steels 99, May K. Nose, H. Asahi, H. Tamehiro and H. Inoue: The Material Design and Properties of Martensitic Stainless Steel for Flowline Line Pipes, 16th International Conference on Offshore Mechanics and Arctic Engineering, The Offshore Mechanics and Arctic Engineering Division of the American Society of Mechanical Engineers, ISBN , April SSW PAGE 333

4 SSW PAGE 334 Development of weldable martensitic stainless steel line pipe by HF-ERW process Figure 1. Application image of development. Figure 2. Basic concept of alloy design for new weldable martensitic stainless line pipe. Figure 3. Manufacturing process.

5 Stainless Steel World 99 Conference SSW PAGE 335 Figure 4. Strength at elevated temperatures. Figure 5. Charpy impact test results of base metal and ERW portion (transverse direction, 1/2 sub-size)

6 SSW PAGE 336 Development of weldable martensitic stainless steel line pipe by HF-ERW process Figure 6. Schematic diagram of ERW portion and seam heat treatment zone. Figure 7. Microstructure of ERW portion and base metal µm Figure 8. Macrograph and hardness profile of weld cross-section.

7 Stainless Steel World 99 Conference SSW PAGE 337 Process HF-ERW Grade X80 equivalent (YS 550 MPa, TS 620 MPa) CO 2 corrosion Corrosion rate < 0.1 mm/y (100 C, 10 bar, CO 2, sweet) Toughness* ve J (full size, transverse) Hardness* Hv 350 Weldability no cold crack (without pre-heating and PWHT) Table 1. Target performance of development. * including weld joint HAZ C Si Mn P S Cr Ni Mo Ti N Aiming < < 0.03 < < Actual Table 2. Chemical composition of tested steel (wt.%). Location Orientation YS (MPa) TS (MPa) El. (%) Base metal Transverse ERW portion Transverse Aiming Table 3. Tensile test results. Notch position Temp ( C) Energy (J), average Shear area (%), average ERW portion (weld centre) WC + 1mm WC + 2mm WC + 5mm WC + 12mm* Table 4. Charpy impact test results of ERW portion and seam heat treatment zone (transverse direction 1/2 sub-size). * WC + 12mm: boundary of the visible seam heat treatment zone Average Max. Base metal ERW portion Seam heat treatment zone Table 5. Hardness of base metal and HF-ERW HAZ. Location Spec. Orientation/size Temp. ( C) CTOD (δ/mm), average Base metal BS7448 Part 1 Transverse/B 2B (B=5mm) ERW portion (weld centre) BS7448 Part 1 Transverse/B 2B (B=5mm) WC + 5mm BS7448 Part 1 Transverse/B 2B (B=5mm) Criteria Table 6. CTOD test results.

8 SSW PAGE 338 Development of weldable martensitic stainless steel line pipe by HF-ERW process Pipe no. Roundness Eccentricity % 1.03% % 0.90% % 0.90% % 0.64% % 1.03% Average 0.15% 0.15% Table 7. Roundness and eccentricity of pipes. Roundness = (max. OD min. OD) / nominal OD 100% Eccentricity = (max. WT min. WT) / nominal WT 100% OD = outer diameter WT = wall thickness Welding method Filler Diameter Groove Preheat Shield gas Heat input 1 5 pass GTAW superduplex (25% Cr) 2.4mm V not applied Ar kj/mm cap. 1.7 kj/mm Interpass max. 150 C PWHT not applied Table 8. Girth welding condition. Orientation TS (MPa) Failure position Transverse 770 Base metal 781 Base metal 776 Base metal Table 9. Results of weld tensile tests. Notch position Temp. ( C) Energy (J), average Shear area (%), average Weld metal FL (fusion line) FL + 2mm FL + 5mm Boundary of the visible HAZ Table 10. Charpy impact test results of girth-welded joints (transverse orientation, 1/2 sub-size). Location Spec. Orientation/size Temp. ( C) CTOD ((/mm), average Weld metal BS7448 Part 1 transverse/b 2B (B=5mm) Fusion line BS7448 Part 1 transverse/b 2B (B=5mm) Criteria Table 11. CTOD test results of girth-welded joints. Sweet Sweet (H 2 S) Temperature 100 C 100 C H 2 S partial pressure bar CO 2 partial pressure 10 bar 10 bar NaCl contents 25 mass% 25 mass% Test duration 91 days 91 days Table 12. Conditions of general corrosion tests.

9 Stainless Steel World 99 Conference SSW PAGE 339 As-welded + shot-blasted Machined Base + ERW Girth Base + ERW Girth Sweet < 0.01 mm/y < 0.01 mm/y < 0.01 mm/y < 0.01 mm/y Sweet (H 2 S) < 0.01 mm/y < 0.01 mm/y < 0.01 mm/y < 0.01 mm/y Table 13. Results of general corrosion tests. Temperature 25 C H 2 S partial pressure bar CO 2 partial pressure 1 bar NaCl contents 5 mass% ph 4 Stress 100% YS Test duration 30 days Table 14. Conditions of SSC test (4-point bend beam test). NaCl (%) Test conditions Results H 2 S (bar) Girth, machined Girth, as-welded + shot-blasted ERW, machined NC, NC NC, NC NC, NC Table 15. Results of SSC test. NC = no crack Specimen location/orientation Girt weld cap/transverse weld ERW weld cap/seam weld (not flattened) Specimen surface finish Girth weld cap surface was: As-welded specimen: left intact Machined specimen: machined flat + finished (320 grid) ERW seam weld was left intact Stress 100% YS Test solutions 3.5% NaCl ph 5.4 (measured before test) 5.8 (for 1194mV) ~ 6.3 (for 894mV (measured after test) Cathodic potential 1194mV vs SCE ( 1150mV vs Ag/AgCl) 894mV vs SCE ( 850mV vs Ag/AgCl) Duration 90 days Temperature 25 C Gas environment Open air Table 16. Conditions for cathodic protection tests (4-point bend beam test). Cathodic potential Girth weld ERW seam weld As-welded specimens Machined specimens As-pipe specimens 1194mV vs SCE No crack No crack No crack 894mV vs SCE No crack No crack No crack Table 17. Cathodic protection test results.

10 SSW PAGE 340 Development of weldable martensitic stainless steel line pipe by HF-ERW process