IPC2012-90271

Proceedings of the 2012 9th International Pipeline Conference IPC2012 September 24-28, 2012, Calgary, Alberta, CanadaIPC2012-90271 MATERIAL DEVELOPMENT OF X80 FOR STRAIN-BASED DESIGN APPLICATIONS Christoph RIVINIUS Aktien-Gesellschaft der Dillinger Hüttenwerke Dillingen, Germany Andreas LIESSEM Europipe GmbH Mülheim, Germany Volker SCHWINN Aktien-Gesellschaft der Dillinger Hüttenwerke Dillingen, Germany Martin PANT Europipe GmbH Mülheim, Germany Jens SCHRÖDER Europipe GmbH Mülheim, Germany ABSTRACT Due to the further increasing demand for natural gas, the construction of long-distance pipelines traversing through seismically active regions or arctic regions with ground movement caused by permafrost phenomena will become more and more necessary. To guarantee the safe operation of those pipelines, the pipe material has to fulfill strain-based design requirements in the coated condition. Hence in longitudinal direction low yield-to-tensile ratios, high uniform elongation values and a roundhouse shape of the stress-strain curve combined with sufficient strength values in transverse direction are essential. The basis for appropriate pipe properties is an adequate design of the plate material. To achieve these objectives the microstructure has become a central element. Nevertheless, it has to be taken into account that the cold deformation during the pipe manufacturing process and the heat treatment of the pipe during the subsequent coating have a major influence on the final line pipe behavior. The current paper describes recent development steps and approaches. The mechanical properties of the different concepts will be compared and the advantages and disadvantages will be highlighted. INTRODUCTION The increasing demand for natural gas requires the construction of pipelines passing through seismically active regions or regions with discontinuous permafrost. Strain-based design takes account of the particular challenges of these special environmental conditions. To provide a sufficient plastic reserve, the material has to feature a low yield-to-tensile ratio (Y/T), high uniform elongation in longitudinal direction and a specific shape of the stress strain curve. The development of high strength line pipe material for strain-based design applications has been driven forward successfully in the recent years [1-4]. Also the design methods and prediction tools have improved continuously [5]. For the plate material development the understanding and the control of the microstructure are the key to success. Besides the chemical composition, the cooling process has the most significant influence on the formation of microstructure [6-7]. Minor differences in cooling conditions can strongly affect the mechanical properties such as uniform elongation and Y/T [8]. Since cold deformation during the pipe forming process such as UOE and the heat treatment during the pipe coating process considerably alter the mechanical properties [9-11], it is necessary to ensure that good plate properties can be transferred to the final product. Only if plate material development and pipe production are well coordinated, can the best compromise of material properties be tailor-made for any pipeline project. PLATE & PIPE PRODUCTION The Basis for the plate production of the described grade X80 steel for strain-based design applications is an excellent slab quality. The highly optimized steelmaking process at Dillinger Hüttenwerke (DH) leads to lowest contents of phosphorus, sulphur and total oxygen [12]. A fully vertical type caster with bending and straightening after complete solidification avoids the accumulation of inclusions and allows the production of heavy plates with very homogeneous mechanical and technological properties [13]. 1 Copyright © 2012 by ASME Y/T, longitudinal, % 90 85 80 75 70 65 60 Y/T UEL 11,0 10,5 10,0 9,5 9,0 8,5 8,0 The pipes with a 48 inch diameter were manufactured by the U-O-E process at the 18 m production line of EUROPIPEs large diameter pipe mill in Mülheim. After plate input, edge crimping and U-forming, the plates were formed to so-called “slit pipes” in the 60 kt O-ing press. The subsequent longitudinal welding process comprises the submerged arc welding with four-wire technology for the inside and five-wire technology for the outside weld seam, respectively. After welding the pipes were expanded to adjust their final shape. A non-destructive inspection of weld seam and pipe ends according to customer’s requirements was performed prior to the final inspection of pipe surface and geometry. In order to simulate the coating process some pipes were heated at a temperature range of 190 to 200 °C with a total holding time of app. four minutes using the induction spool at Mülheim Pipe Coating GmbH (MPC), a subsidiary of EUROPIPE, in the vicinity of the large diameter pipe mill. A similar temperature cycle was applied on test coupons for small scale testing in order to simulate the as-coated condition. PLATE CONCEPTS The chemical composition that is used for the current development of grade X80 for strain-based design applications is listed in table 1. The low carbon equivalents (Pcm according to API 5L [15] and CE according to CSA-Z234.1-07 [16]) ensure a good weldability and the added micro alloying elements lead to improved strength, toughness and strain aging resistance. Due to competing demands like high strength and high uniform elongation in combination with a very low yield-totensile ratio, a conventional bainitic plate concept will not be able to fulfill all requirements after pipe forming and coating. Hence, a first approach was the development of a ferritic martensitic dual phase (DP) steel (“concept A”). Figure 1: Comparison of yield to tensile ratio (Y/T) and uniform elongation (UEL) of plate material of conventional X80 and of concept A plate material for strain-based design (longitudinal direction, mean values) Figure 1 presents the test results for the yield-to-tensile ratio and the uniform elongation (UEL) in longitudinal direction of conventional X80 plate material and of the concept A plate material. The strain-based design concept shows a Y/T that is considerably reduced compared to the conventional concept and an UEL that is significantly increased. The microstructure of the concept A material, presented in figure 2, consists of a matrix of bimodally distributed polygonal ferrite grains and a self tempered martensitic second phase. The latter is, on one hand, responsible for sufficient strength of the overall material and influences, on the other hand, the dislocation structure of the ferrite grains and thus, the strain hardening behavior. Although the concept A plate material showed a good combination of strength and plastic deformation capacity, the DP steel concept had room for improvement: The relatively large mean grain size of the ferrite matrix caused reduced Charpy V-notch (CVN) upper shelf energies and the elongated grain shape in combination with a linear-shaped martensite arrangement lead to a pronounced material anisotropy. Significant differences between strength values in longitudinal and transverse direction are the consequence. Table 1: Carbon content, alloying and carbon equivalent of grade X80 steel C (wt.%) 0,05 pcm Mn (wt.%) 1,9 Alloying SiCuNiMo Micro Alloying NbTi Pcm 0,18 CE 0,26 = C+ Si Mn + Cu + Cr Ni Mo V Si Mn Cu Ni Cr + Mo + Nb + V + + + + + 5 B , CE = C + F ⋅ ( + + + + + 5B ) 30 20 60 15 10 30 6 15 20 5 2 Copyright © 2012 by ASME UEL, longitudinal, % After reheating the slabs to temperatures that assure a proper dissolution of micro-alloying elements but impede disadvantageous austenite grain coarsening, thermo mechanical (TM) rolling was performed at two four-high-reversing stands to a final thickness of 23.7 mm. The subsequent cooling was carried out using a MULPIC (Multi Purpose Interrupted Cooling) device that allows a precise control of the cooling conditions such as cooling rate and final cooling temperature [14]. 100 95 Conventional X80 Concept A 12,0 11,5 (a) M F M F 10 µm 1 µm (b) Figure 3: Second phase microstructure with martensite (M) and M/A-constituents (marked with arrows) within the ferrite matrix phase (F) of concept B plate material 1 µm Figure 2: Dual phase microstructure of concept A, consisting of (a) polygonal ferrite (F) with self tempered martensitic second phase (M) (a and b) This material shows a more complex microstructure of granular bainite [17] with a strongly reduced ferrite grain size and a complex second phase consisting of martensite and M/A (see figure 3). The concept B adds excellent CVN toughness to the good compromise of the overall properties of the concept A material. Figure 4 compares the upper shelf of the CVN transition curves of the plate material for concept A and concept B. The latter shows impact energies in the region of 500 J down to testing temperatures of -50 °C whereas the concept A material merely offers mean values of 220 J at -40 °C. Furthermore, comparing the BDWTT transition curves of plate material for the concepts A and B, concept B material shows slightly improved test results (see figure 5). The adjustments of cooling parameters like cooling start temperature, cooling finish temperature and cooling rate resulted in a new plate concept, further on called “concept B”. CVN transverse, J 500 400 300 200 100 0 -60 -50 -40 -30 -20 -10 Concept B Concept A BDWTT transverse, %SA 600 100 75 50 25 0 -60 -50 -40 -30 -20 -10 0 Concept B Concept A test temperature, °C Figure 4: Upper shelf energies of the CVN transition curves for plate material of concept A and concept B (transverse direction, mean values) test temperature, °C Figure 5: BDWTT transition curves for plate material of concept A and concept B (transverse direction, mean values) 3 Copyright © 2012 by ASME Even at temperatures of -50 °C its mean values stay above 75 % shear area, whereas the concept A test results already start drifting into the transition region at these temperatures. Nevertheless, at test temperatures of -20 °C, both materials show good BDWTT results with mean values between 85 and 90 % shear area. Another difference between the concepts A and B was the slight reduction of the anisotropy between longitudinal and transverse direction. This results in higher values for the yield strength in longitudinal direction with still sufficient tensile strength. Values of Y/T remained very low in combination with an excellent uniform elongation. respectively. The longitudinal direction was tested in the aswelded condition and after coating simulation. Table 2 summarizes the target results for the base metal tensile tests. The mean values of the tensile tests for both concepts are given in table 3 and 4 for the as-welded and as-coated condition, both longitudinal and transverse. While the longitudinal specimens are rectangular, transversal specimens are round bar. The strength properties in longitudinal direction in the as-welded condition are for information only whereas the properties after coating simulation were fixed. The yield strength is reduced to 510 MPa compared to the requirements in hoop direction. However, the minimum tensile strength requirement is 625 MPa in both directions. Furthermore a low yield-to-tensile ratio and a minimum value for uniform elongation in excess of six percent in longitudinal direction are defined. A gain in yield strength owing to the coating simulation is evident (see tables 3 and 4). This effect is mainly based on the thermal activated diffusion of carbon that accumulates at the stress fields of the dislocations during aging. The tensile strength is not as affected by the simulated coating thermal cycle. Concept C provides a higher strength level than concept B. The pipes made from plates of concept C reveal approximately 30 MPa higher strength values whereas Y/T-ratio and uniform elongation are only slightly influenced by the different rolling concept. Generally, the influence of a coating temperature of approximately 200°C on the stress-strain curve of longitudinal tensile tests is presented in figure 7. The pipe material shows a stress-strain diagram with round-house shape before and after coating: yielding occurs continuously and no yield platform or Lueders elongation can be observed, though the slope of the curve is increased on the coated sample [18]. The BDWT test results tested on pipe specimens with full wall thickness show that concept C provides a significantly lower BDWTT transition temperature, making this concept eventually suitable for low temperature projects (see figure 8). In addition to the strength and BDWTT properties of the pipe body the fracture toughness in the base metal and heat-affected zone of the longitudinal seams was investigated. F M 1 µm Figure 6: Second phase microstructure with martensite (M) and M/A-constituents (marked with arrows) within the ferrite matrix phase (F) of concept C plate material Further development of concept B lead to a slightly modified microstructure with a higher content of M/A (see figure 6). The differences between concept B and this new “concept C” will be explained more precisely in the following section. PIPE TEST RESULTS Plates from concept B and from modified concept C were formed, welded and expanded to pipes on the 18 m production mill at Mülheim. The pipes were sampled and tensile tests have been performed in longitudinal and transverse direction, Table 2: Targeted values for the tensile test of X80 base metal direction transverse longitudinal longitudinal condition as welded as welded as coated YS 0.5 (MPa) 555 - 690 info 510 – 630 TS (MPa) 625 – 825 info 625 – 775 Y/T (%) ≤ 93 info ≤ 90 UEL (%) not required info ≥6 4 Copyright © 2012 by ASME Table 3: Tensile test results for pipe material (concept B, mean values) direction transverse longitudinal longitudinal condition as welded as welded as coated YS 0.5 (MPa) 578 508 532 TS (MPa) 678 650 658 Y/T (%) 85 78 81 UEL (%) 8.9 7.9 Table 4: Tensile test results for pipe material (concept C, mean values) direction transverse longitudinal longitudinal condition as welded as welded as coated YS 0.5 (MPa) 603 540 557 TS (MPa) 718 650 679 Y/T (%) 84 80 82 UEL (%) 8.3 8.0 For this purpose CVN samples with the notch in the HAZ and the base metal were taken. Either concept delivered adequate CVN properties. In summary, higher CVN properties in the HAZ were observed in concept B with 250 J average compared to concept C with 190 J average. In the base metal 410 J average value was measured in concept B and 350 J in concept C at -15°C testing temperature. CONCLUSION In order to fulfill the challenging demands of a grade X80 for strain-based design, a dual phase microstructure approach was pursued as a first concept A. The material of this concept showed a good combination of strength and plastic deformation capacity but the CVN test results revealed relatively low upper shelf energies. Concepts B and C with a more complex microstructures of granular bainite showed advanced CVN impact energies and improved BDWT test results. Both concepts met the supplement requirements of grade X80 for strain-based design. However concept C with a higher amount of M/A constituent in the microstructure is favoured for its slightly increased strength and BDWTT properties. All plate concepts show noticeable differences between the strength level in transverse and longitudinal direction. If these differences are too large it is hard to find a suitable balance between yield strength in transverse direction and tensile strength in longitudinal direction. On the other hand, during heat treatment for coating yield strength and yield-to-tensile 700 600 as coated as welded 100 BDWTT tranverse, %SA 75 50 Concept C B stress, MPa 500 400 300 200 100 0 0 1 Concept A B 25 0 -100 2 3 4 5 strain, % Figure 7: Effect of coating process on the shape of stress strain diagram (concept B, pipe material) -80 -60 -40 -20 test temperature, °C 0 20 Figure 8: BDWTT transition curves for pipe material of concept B and concept C (transverse direction, single values) 5 Copyright © 2012 by ASME ratio are elevated while uniform elongation is reduced. Therefore a certain level of anisotropy is considered necessary to guarantee sufficient high elongation values in longitudinal direction in combination with high strength values in transverse direction even after the pipe coating process. 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