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7KHHIIHFWVRIVXO¿GHVWUHVVFUDFNLQJRQWKH mechanical properties and intergranular cracking of P110 casing steel in sour environments
Hou Duo, Zeng Dezhi , Shi Taihe, Zhang Zhi and Deng Wenliang
6WDWH.H\/DERUDWRU\RI2LODQG*DV5HVHUYRLU*HRORJ\DQG([SORLWDWLRQ6RXWKZHVW3HWUROHXP8QLYHUVLW\&KHQJGX Sichuan 610500, China
© China University of Petroleum (Beijing) and Springer-Verlag Berlin Heidelberg 2013
Abstract: 9DULDWLRQDQGGHJUDGDWLRQRI3FDVLQJVWHHOPHFKDQLFDOSURSHUWLHVGXHWRVXO¿GHVWUHVV cracking (SSC) in sour environments, was investigated using tensile and impact tests. These tests ZHUHFDUULHGRXWRQVSHFLPHQVZKLFKZHUHSUHWUHDWHGXQGHUWKHIROORZLQJFRQGLWLRQVIRUKRXUV
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SUHORDGVWUHVVRIWKH\LHOGVWUHQJWK ıs PHGLXPVLPXODWHGIRUPDWLRQZDWHU7KHUHGXFWLRQLQ WHQVLOHDQGLPSDFWVWUHQJWKVIRU3FDVLQJVSHFLPHQVLQFRUURVLYHHQYLURQPHQWVZHUHDQG 54%, respectively. The surface morphology analysis indicated that surface damage and uniform plastic GHIRUPDWLRQRFFXUUHGDVDUHVXOWRIVWUDLQDJLQJ,PSDFWWRXJKQHVVRIWKHFDVLQJGHFUHDVHGVLJQL¿FDQWO\
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Key words: Acidic solutions, high-temperature corrosion, hydrogen embrittlement, intergranular FRUURVLRQVXO¿GHVWUHVVFUDFNLQJ
1 Introduction corrosion cracking (SCC) which is an anodic cracking mechanism. 6WHHOVUHDFWZLWKK\GURJHQVXO¿GHIRUPLQJPHWDOVXO¿GHV Specifically, testing methods using the bend specimen and atomic hydrogen as corrosion byproducts. The atomic JHRPHWU\GHVFULEHGLQWKH$670VWDQGDUGV $670 hydrogen can diffuse into the metal matrix, creating hydrogen ,QWHUQDWLRQDO DQGWKHGRXEOHFDQWLOHYHUEHDP '&% HPEULWWOHPHQWDQGVXEVHTXHQWFUDFNLQJ7KLVLVWHUPHGVXO¿GH WHVWGHVFULEHGLQWKH1$&(VWDQGDUGV 1$&(,QWHUQDWLRQDO stress cracking (SSC) and is more severe at low pH values. 2005) are used in design/fitness for service evaluating and 6XO¿GHVWUHVVFUDFNLQJLVEHFRPLQJDNH\LVVXHGXHWRKLJK for determining material qualifications and specifications levels of H2S and CO2 in new natural gas reservoirs (Carneiro +D\ &UDYHURHWDO *HOGHUHWDO HWDO .HUPDQLHWDO investigated the susceptibility of stainless steel to SSC in sour Furthermore, severe working conditions, to which environments containing H2S, CO2 and chloride using slow drilling equipment is exposed in reservoirs, require high VWUDLQUDWHWHVWLQJ 6657 $OEDUUDQHWDO VWXGLHGWKH grade steels capable of resisting SSC and withstanding high effects of heat-treatments on pipeline steel resistant to SSC, mechanical loads under conditions that encourage hydrogen XVLQJOLQHDUHODVWLFIUDFWXUHPHFKDQLFV /()0 RQVWDQGDUG HPEULWWOHPHQW +( =KDRDQG
*Corresponding author. email: [email protected] to aqueous H2S/CO2 environments with high H2S partial Received December 17, 2012 3HW6FL
SUHVVXUHV6HYHUDOUHVHDUFKHUV 6ULQLYDVDQDQG.DQH reported that the susceptibility of pipeline steels to HIC in
6ULQLYDVDQDQG7HEEDO 1HVLFHWDO 6PLWKDQG NaCl solutions saturated with high pressure of H2S depended Joosten, 2006) discussed that there are currently no generally RQWKHK\GURJHQSHUPHDELOLW\RIVWHHO0DQ\VWXGLHV 6KLPDGD accepted prediction algorithms for any form of H2S corrosion. HWDO 2WHJXLHWDO 6WROODQG:LONLQVRQ There are still many uncertainties about the corrosive have also investigated the conditions that make metals reactions that lead to pitting, which is the most common mode more prone to intergranular cracking and have assessed of sour service equipment failure. Zhang et al (2011) found the deformation mechanisms of hydrogen embrittlement that the hydrogen sulfide content in some wells exceeded induced by temperature susceptibility. In addition, other 10vol.% in the Sichuan sour gas fields in China. In these VFLHQWLVWV 9HQHJDVHWDO 3DOHWDO KDYHUHSRUWHG wells, the partial pressure of H26UHDFKHGDERXW03DDQG the mechanisms associated with intergranular corrosion and WKHWHPSHUDWXUHZDV &6LQFHQRSUDFWLFDOH[SHULHQFH VXO¿GHFUDFNLQJEXWWKHUHLVQRWPXFKLQIRUPDWLRQDERXW66& had been available for the corrosion of carbon steels under susceptibility in V-notched specimens and loss in mechanical these extreme conditions, it is difficult to design effective properties caused by hydrogen damage in sour environments. evaluation methods for the corrosion-resistance of materials. In this study, we used P-110 casing steel specimens to Zhang et al (2011) studied the ability of sulfur to accelerate examine loss in tensile and impact strengths under sour the corrosion of carbon steel in wet H2S environments, conditions. Using a high temperature high pressure (HTHP) showing that sulfur severely accelerated SSC. However, DXWRFODYH KRPHPDGH03D& DORDGLQJ¿[WXUHIRU the methods discussed above cannot be used to evaluate the WHQVLOHVSHFLPHQVDQGDORDGLQJ¿[WXUHIRUEHQGVSHFLPHQV variations and degradations of mechanical properties of steel we measured the tensile and impact properties of P-110 equipment and casings in sour wells, and cannot suggest casing specimens before and after they were exposed to the the strength criteria as a standard. For these reasons, it is VRXUHQYLURQPHQWV WHPSHUDWXUHDW& WRWDOSUHVVXUH important to learn about hydrogen embrittlement, study the 03D +26SDUWLDOSUHVVXUH03DDQG&22 partial pressure 2 hydrogen concentration and evaluate the effects of material 03D SUHORDGVWUHVVRIWKH\LHOGVWUHQJWK ıs IRU properties in order to develop new testing methods and hours. identify proper strength criteria for well casings in sour environments. ([SHULPHQWDO To date, most research on hydrogen embrittlement has investigated absorption of hydrogen into line pipe steels and 2.1 Materials and corrosive medium susceptibility of these steels to hydrogen induced cracking Standard tensile test specimens with an outside diameter (HIC) in solutions saturated with high pressure H2S and CO2. (OLD]HWDO UHSRUWHGWKDWVWHHO¶VVXVFHSWLELOLW\WR+,& of 6.35 mm were machined from P-110 casing (an outside and its absorbed hydrogen content increased as the partial GLDPHWHURIPPDZDOOWKLFNQHVVRIPPDQG D\LHOGVWUHQJWK ı RINVL 03D DFFRUGLQJWRWKH pressures of H2S and CO2 increased. Several researchers s 0LFKOHUDQG1DXPDQQ :RRGWOLDQG.LHVHOEDFK 1$&(70PHWKRGDVVKRZQLQ)LJ
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G (mm) 25.4
R (mm) L (mm) 101±1 F -0.02 (mm) ij10 -0.07
Fig. 1 Dimensions of standard tensile test specimens
Sub-size specimens for the Charpy impact test were size specimens were 10 mm × 5 mm × 55 mm, containing a 2 PDFKLQHGIURP3FDVLQJDFFRUGLQJWRWKH$670( mm-deep V notch. The chemical composition of P-110 casing DQG$670$VWDQGDUGV7KHGLPHQVLRQVRIWKHVHVXE is listed in Table 1. 3HW6FL
Simulated water was prepared to match as closely as 700 possible the analysis of formation water obtained from ı=ıs×80%=606 MPa 600 the Puguang gas-field in Sichuan Province, China and its ȟ=0.28% properties are listed in Table 2. The corrosive medium was 500 saturated with H26 SXUH DQG&22 SXUH DQG N and CH is 2 SXUH ZDVXVHGIRUSUHVVXUL]DWLRQ+2 4 400 not used in this test. 300
Table 1 Chemical composition of P-110 casing Stress, MPa 200 Chemical composition, wt% Steel grade 100 CSi0Q PSCr0R Others 0 P110 0.32 0.34 0.013 0.006 2.42 Ni 0.00 0.08 0.16 0.24 0.32 Strain, % Table 2 Properties of simulated formation water Fig. 3 Stress-strain curve of the tensile specimen during preloading Composition, g/L Density pH Salinity 3 + + 2+ 2+ ņ ņ ņ g/cm value g/L Na K Ca 0J Cl SO4 HCO3
10.0 11.4 2.43 126.5 0.71 0.71 UHPRYHGIURPWKHORDGLQJ¿[WXUHDQGWKHQWKHWHQVLOHWHVWLQJ was conducted on these specimens. All evaluations were repeated three times and averaged. 2.2 Tensile testing This autoclave was designed and constructed to allow the The tensile specimen was mounted onto a specialized introduction of multiphase flow. The maximum operating ORDGLQJ¿[WXUHIRUWHQVLOHVSHFLPHQVƗ, shown in Fig. 2. With SUHVVXUHDQGWHPSHUDWXUHZHUH03DDQG&7KH DQ076WHQVLOHWHVWLQJPDFKLQH 076&RUSRUDWLRQ loading fixture and the autoclave were made of Hastelloy 0LQQHDSROLV86$ DSUHORDGZDVDSSOLHGRQWKHWHQVLOH C-276. The difference in the thermal expansion coefficient specimen until the stress and strain of the specimen reached EHWZHHQ+DVWHOOR\&DQGFDUERQVWHHOZDVRQO\ . 03D ıs DQGUHVSHFWLYHO\7KHVWUHVVVWUDLQ ȝP P & DW& DQGWKHVWUDLQRIWKH3FDVLQJ curve of a tensile specimen is shown in Fig. 3. Then, the VSHFLPHQZDV PP 7KHUHIRUHWKHIL[WXUHZDV nut number was screwed down to maintain a constant load able to maintain a constant load on the specimen during
ıs) on the specimen so that the stress and strain did not H[SRVXUHWRDVRXUHQYLURQPHQWIRUKRXUV change during the corrosion test. The assembly of the loading For comparison, four groups of tensile specimens ¿[WXUH Ɨ with the tensile specimen were removed from the pretreated at different conditions were prepared and used testing machine and then immediately placed in a 6-liter to conduct following examinations. Tensile specimens autoclave, shown in Fig. 4. In these stress corrosion tests, the without any pretreatment were designated Specimens A specimens were immersed in the simulated formation water and Specimens B were obtained after the tensile specimens
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CO2SDUWLDOSUHVVXUHVZHUHNHSWDWDQG03DUHVSHFWLYHO\ ıs. The specimens which had been exposed to the sour and the water flow rate was controlled at 2 m/s. Following HQYLURQPHQWIRUKRXUVZLWKDSUHORDGRIıs, were exposure to the sour environment, the specimens were named Specimens C (the pH value of the formation water was DQG6SHFLPHQV' WKHS+YDOXHRIWKHIRUPDWLRQZDWHU Screw-down the nut number to keep a constant load was adjusted to 3 using CH3COOH). The true stress-strain curves of each group of specimens were drawn to analyze the results of the tensile test. The KDUGHQLQJLQGH[RIHDFKVSHFLPHQZDVFDOFXODWHGIURP(T Loading fixture 7KHVWUDLQHQHUJ\GHQVLW\ZDVFDOFXODWHGIURP(T ZKLFK represents the total mechanical energy of the P-110 casing specimen consumed until failure, and numerically equals the area under the stress-strain curve from zero to fracture point. K Fracture toughness ( IC ZDVFDOFXODWHGIURP(T Tensile specimen Displacement gange NNN Nxyxii i y i n ¦¦¦iii 111 NN2 (1) 2 Nxii x ¦¦ii 11
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Stirring apparatus Loading fixture for tensile specimens Loading fixture for bend specimens Valve Gas out Temperature pressure monitor Pressure meter
H2S+CO2
Corrosion medium
N2 H2SCO2 CH4
Insulation Heater
Fig. 4 Schematic of the autoclave for H2S and CO2 corrosion tests
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ysln i d a dd ddd d aa21ªº aaa 523 d a F F () (1)«» 1 () () 0.421() d dd ddd F FFʌ ¬¼28 FFF (a) (b) where nLVWKHKDUGHQLQJLQGH[ N is the total number of PHDVXUHPHQWSRLQWV s is the true stress, s=ı*(1+İ e is the Fig. 5 The necked section (a) and the top view of true plastic strain, e=ln(1+İ İLVWKHVWUDLQGLPHQVLRQOHVV the fracture plane (b) of the tensile specimen U* is the strain energy density, J/m3 V is the specimen volume, m3 P is the critical load of crack propagation, F WKHFUDFNWLSUHDFKHGıs when the displacement of the GHWHUPLQHGE\WKHVWDQGDUGPHWKRGRXWOLQHGLQ$670( V-notch was 0.12 mm, which can be seen from the stress- A 2 L N1 0 is the cross-sectional area of the specimen, m is the displacement curve in Fig. 7. d d HORQJDWLRQRIWKHVSHFLPHQP ıLVWKHVWUHVV03D a and F The three-point bend testing was also conducted at 60 are the diameter of the fibrous zone and the cross-sectional &DQG03DLQWKHDXWRFODYHIRUKRXUVDQGWKHVWUHVV diameter of the fractured neck, respectively, as shown in Fig. LPSRVHGRQWKHVSHFLPHQZDVPDLQWDLQHGDWıs. In this P is the critical load, determined by the standard method F autoclave, the H2S and CO2 partial pressures were kept at 1 RXWOLQHGLQ$670( DQG03DUHVSHFWLYHO\ WKHZDWHUÀRZUDWHZDVFRQWUROOHG DWPV DQGWKHS+YDOXHRIWKHFRUURVLYHPHGLXPZDV 7KUHHSRLQWEHQGWHVWLQJDQGLPSDFWWHVWLQJ $IWHUKRXUH[SRVXUHWKHVXEVL]HVSHFLPHQQDPHG The procedure for installing the Charpy impact specimen DV6XEVL]HVSHFLPHQV(ZDVUHPRYHGIURPWKHWKUHH RQWKHWKUHHSRLQWEHQG¿[WXUHƘ is shown in Fig. 6. The load point bend fixture and the impact testing was immediately imposed on the specimen was adjusted by the displacement of performed at room temperature to obtain crack propagation F the V-notch, which was controlled by the nut number. A dial force QVKRZQLQ)LJ)RUFRPSDULVRQRWKHUVXEVL]H indicator was mounted to measure the specimen displacement specimens without any pretreatment, named as Sub-size and then the stress applied at the crack tip of the specimen VSHFLPHQV)ZDVDOVRWHVWHGDW&$OOHYDOXDWLRQVZHUH FRXOGEHFDOFXODWHGE\¿QLWHHOHPHQWDQDO\VLV7KHVWUHVVDW repeated for three times and averaged. 3HW6FL a Screw down the nut number f () W to keep a constant load aa a a 1.99 ( )(1 )ªº 2.15 3.93 2.7( )2 a WW«» W W 3( ) ¬¼ W aa3 2(1 2 )(1 ) 2 WW
P B where QLVWKHFUDFNH[WHQVLRQIRUFH is the specimen 0.25mm B WKLFNQHVV N is the specimen thickness between the roots of WKHVLGHJURRYHV WLVWKHVSHFLPHQZLGWK S is the span of the specimen, S=4W a is the crack size. K K The value of Q was equal to the fracture toughness ( IC) 2 §·KQ when the (,BBN , Wt a ) 2.5 . Otherwise, the test is ¨¸ Impact specimen ©¹V y K not a valid IC test.
Dial indicator 8 mm rod
30°±2°
Striking edge 0.25 mm rod 4 mm C D Fig. 6 Assembly of three-point bend test for the sub-size specimens Specimen
1000
Stress: 606 MPa Displacement: 0.12 mm AB 800 Center of strike
80°±2° 600 Loading point 1 mm rod Stress, MPa
400 Fig. 8 The impact test for measurement of the crack extension F force ( Q RIVXEVL]HVSHFLPHQV(DQG)
200 0.05 0.10 0.15 0.20 3 Results and discussion Displacement, mm 3.1 Loss of mechanical properties and hydrogen Fig. 7 Stress-displacement curve and preloading point damage 0HFKDQLFDOSURSHUWLHVRIWHQVLOHVSHFLPHQVH[SRVHGWR different conditions are listed in Table 3. The P-110 casing steel impact specimens were exposed The results of Specimens B indicate that the elastic to sour environments and simultaneously underwent three- deformation of the casing specimen caused by stress was SRLQWEHQGLQJVWUHVV$IWHUKRXUVWKHVSHFLPHQVZHUH recoverable when the stress was removed, and its tensile taken from the autoclave and immediately impact tests strength and yield strength actually increased due to K were conducted at room temperature. The value of Q was strain hardening. The results of tensile tests performed on FDOFXODWHGIURP(T Specimens C and D were stable and reliable. From these tests, we found that for the specimens exposed to sour PS a HQYLURQPHQWVDW&DQG03DZLWKDSUHORDGRIı , Kf Q () s Q 3 W (4) the deformation of the casing specimens cannot be recovered, BB W 2 N resulting in significant losses in mechanical properties. Furthermore, these results show that the fracture toughness with GUDVWLFDOO\UHGXFHGWR03DāP1/2, this is mainly due to 3HW6FL
Table 3 Tensile properties of tensile specimens exposed to different environments
Tensile Reduction A 23.6 67.2 215 B 1030 62.2 220 203 C 66.5 50.1 D 22.0 66.5 50.0 corrosion by H2S and CO2. The tensile specimens examined 1200 in this study represented two different types of strain aging: ordinary strain aging occurred in Specimens B, which could 1100 EHUHFRYHUHG VSHFLDOVWUDLQDJLQJFDXVHGE\FRUURVLRQGXH to H2S and CO2, which occurred in Specimens C and D and 1000 could not be recovered. This special strain aging due to H2S 900 and CO2 corrosion also led to decays in the strength, ductility, and toughness of the casing specimens. The reduction in Specimens A 800 tensile strength, yield strength and fracture toughness of the Specimens B casing steel specimens can be attributed to the degradation of stress, MPa True 300 Specimens C, D the steel mechanical properties caused by combined effects 200 of the sour environment and the stress state. This shows that 100 the casing mechanical properties will significantly reduce 0 in wellhead and bottom conditions due to the combined 024 6810 corrosive effect. Compared with Specimens A, the true stress- True strain, % strain curves of Specimens C and D have moved down, as Fig. 9 True stress-strain curves of specimens A, B, C and D V VKRZQLQ)LJ ,WLVIRXQGWKDWWKHVWUDLQHQHUJ\GHQVLW\ İ) RI6SHFLPHQV%FDOFXODWHGE\LQWHJUDWLQJLQFUHDVHGE\- m3, while those of Specimens C and D decreased by 15.0 J/ fracture of Specimens C, which had a smaller cross-sectional m36LPLODUO\)LJ VKRZVWKDWWKHVWUDLQHQHUJ\SHUXQLW area than Specimens A and a large number of cavity-like volume of the casing specimens changed after the casing structures. The differences between Specimens A and C are was placed in service in sour well conditions. The specimens as follows: the cross-sectional area of Specimens C decreased exposed to air (Specimens B) show an increase in strength and while the crystalline area increased. Specimens A showed and a decrease in plasticity. Because of strain hardening greater plastic deformation after yielding, and then necking induced by preloading, local plastic deformation (dislocation occurred followed by fracturing. Specimens C showed creep movement) occurred, the grain size increased, and the behavior and dislocation gliding during the stress corrosion accumulation of dislocations increased the local strength test, causing no significant yield platform or deformation. (the local strength increased due to the cumulative effect of The total elongation of Specimens C was less than that of grain growth and dislocation accumulation). For Specimens Specimens A, which was caused by hydrogen permeation C and D, the strength and plasticity decreased due to H2S, and hydrogen gathering at local defects in Specimens C. An CO2 contained in the corrosive medium. The strain energy increase in space between crystals observed in Specimens loss would occur in sour environments and the loss energy is C resulted in plastic deformation that occurred at inclusions equivalent to hydrogen surface damage induced by SSC. and grain boundaries. Slippage then occurred along the close- The surfaces of specimens C and D were investigated packed (111) direction in the body-centered cubic (BCC) with a scanning electron microscope (Quanta 450, American crystal structure. In the process of tensile test, shear stresses )(, DQGDODUJHQXPEHURIQDUURZFUDFNVDQGRWKHUVLJQVRI acted in the directions of planes (111) and ( ), which caused corrosion were observed to have formed due to the combined necking and then a reduction in dislocation glide. Hydrogen effects associated with the H2S and CO2 corrosion medium. permeation led to surface damage, which allowed hydrogen The corrosion was accelerated at local surfaces because of atoms to permeate even further. The cavity-like structures combined actions of stress concentration and pitting. The continued to expand since the stress kept increasing during specimens showed plastic deformation, increased lengths, WKHWHQVLOHWHVWLQJ(YHQWXDOO\ODUJHFDYLW\OLNHVWUXFWXUHV and reduced outer diameters, as shown in Fig. 10. The narrow were formed, resulting in decreases in strength, ductility, and cracks formed on the specimen surface had a width of 100 toughness of the P-110 casing steel specimens. ȝPDQGDGHSWKRIȝP The experimental results show that the combined action The tensile fracture of Specimens A, shown in Fig. 11(a) of sour environment and applied stress caused losses in the DQG)LJ E UHSUHVHQWHGD¿EURXVIUDFWXUHWKDWKDGPDQ\ mechanical properties of P-110 casing steel specimens. The secondary cracks. Fig. 11(c) and Fig. 11(d) show the tensile mechanical properties of casing specimens, resisting external Pet.Sci.(2013)10:385-394 391 forces and corrosion, were reduced by H2S and CO2, which increased, local plastic deformation occurred when the made the casing specimens more susceptible to brittle fracture hydrogen pressure equaled the yield strength. When the and failure. At first, hydrogen diffusion and aggregation hydrogen pressure equaled the atomic force, micro-cracks only occurred at defects. As the hydrogen pressure gradually initiated. Hydrogen was adsorbed onto the metal surface 1.1 1.2 1.3 1.4 (c) Local surface inregularities induced by H2S, CO2 corrosion (a) Macro-morphology of specimens from H2S, CO2 corrosion 119.6ȝm 102.5 85.43 68.34 51.26 34.17 17.09 CD B ࣕ ࣕ’ 0.0 A (b) Plastic deformation of specimen induced by H2S, CO2 corrosion (d) The size and morphology of surface narrow cracks Fig. 10 Macro-morphology, plastic deformation and surface damage of Specimens C Standard tensile fracture 2mm 100ȝm (a) Tensile fracture morphology of Specimens A (b) Fiber area fracture morphology of Specimens A SSC tensile fracture 2mm 50ȝm (c) Tensile fracture morphology of Specimens C (d) Fiber area fracture morphology of Specimens C Fig. 11'LIIHUHQFHLQIUDFWXUHPRUSKRORJ\RI¿EURXV]RQHRIVSHFLPHQV$DQG& 392 Pet.Sci.(2013)10:385-394 and further concentrated at defects. This changed the local (4) were consistent, indicating that both of these formulas potential and decreased the surface energy of the metal, are accurate and reliable. The results also indicate that strain resulting in crack initiation and propagation. Eventually, aging phenomena occur in bend specimens, and the notch hydrogen permeation enhanced dislocation formation, VHQVLWLYLW\LQFUHDVHGDIWHUH[SRVXUHWRVRXUHQYLURQPHQWVDW multiplication and motion, which caused stress concentration DORDGRIıs. The impact performance and resistance to and plastic deformation to reach a critical state. H[WHUQDOLPSDFWIRUFHVRI3FDVLQJVSHFLPHQVGHFUHDVHG due to bending in sour environments (H S and CO ) at high 3.2 Reduction of impact toughness and intergranular 2 2 temperature and pressure. These results are consistent with fracture tensile tests performed on Specimens C and D, which were The impact tests were performed at room temperature on DOVRH[SRVHGWRVRXUHQYLURQPHQWVDW&DQG03D7KH WKHVXEVL]HVSHFLPHQVEHIRUHDQGDIWHUH[SRVLQJWKHPWRWKH P-110 casing specimens showed strain aging as well as loss sour environments at 60 °C and 10 MPa and test results are in mechanical properties in both the tensile and bend stress listed in Table 4. corrosion tests. The aging phenomenon was induced by the The impact properties of P-110 casing specimens corrosive effects of the sour environments containing H2S and decreased after three-point bend testing conducted in sour CO2. The stress, hydrogen damage and microstructural change environments. Their reductions in impact energy and fracture caused the formation and accumulation of dislocations, and K toughness ( IC) were 28% and 54%, respectively. The then micro-crack nucleation, and ultimately degradations of values of fracture toughness calculated from Eqs. (3) and the mechanical properties of the specimens. Table 4,PSDFWSHUIRUPDQFHRIWKHVXEVL]HLPSDFWVSHFLPHQVEHIRUHDQGDIWHUH[SRVXUHWRVRXUHQYLURQPHQWV 6XEVL]H Impact energy Yield strength Displacement corresponding Crack initiation Crack propagation Fracture toughness W F S W W K 1/2 specimen t, J gy, kN to yield strength gy, mm energy m, J energy a, J IC, MPa.m F 104.5 11.62 1.21 28.8 75.7 80.3 E 74.4 10.09 1.08 19.9 54.5 36.7 The characteristics of the impact properties of P-110 steel 16 120 specimens after corrosion tests primarily highlight differences between the load-displacement and energy-displacement- FXUYHV&RPSDUHGWRWKHFXUYHVRI6XEVL]HVSHFLPHQV 12 90 F without any pretreatment, curves of specimens that underwent SSC testing were offset, resulting in a decrease in 8 60 the total displacement, as shown in Fig. 12. In addition, the Load-displacement curve Energy-displacement curve Load, kN energy consumed by crack formation and crack propagation Energy, J Regular line: Standard VLJQLILFDQWO\GHFOLQHGIRU66&WHVWLQJVSHFLPHQV 6XEVL]H Bold line: HTHP bend SCC VSHFLPHQV( 7KHVHUHVXOWVDUHH[SODLQHGE\WHQVLOHDQGEHQG 4 30 stress corrosion tests, 1) cracks were formed more easily, 2) VSHFLPHQVKDGUHGXFHGUHVLVWDQFHWRH[WHUQDOLPSDFWIRUFHV 0 0 3) specimens had reduced fracture toughness, and 4) fracture 0 7 14 21 and failure occurred more easily. Stress concentration and Displacement, mm micro-cracks occurred at the bottom of the V-notch in impact WHVWVSHFLPHQVGXHWRWKHVWUHVVRISUHORDGXSWRıs Fig. 12'LVSODFHPHQWORDGDQGGLVSODFHPHQWHQHUJ\FXUYHVRIVXEVL]H and the corrosive test conditions at 60 °C, 10 MPa, where VSHFLPHQVEHIRUHDQGDIWHUH[SRVXUHWRWKHVRXUHQYLURQPHQWZLWKDEHQGLQJ for 168 hours specimens were submerged in simulated formation water for VWUHVVRIıs 168 hours. The notch sensitivity of P-110 casing specimens is shown in Fig. 13. 7KHGLVORFDWLRQIUHH]RQHDWWKHFUDFNWLSKDGDKLJK LQFUHDVHGXSWRıs in the three-point bending test while degree of strain and nonlinear elasticity. In general, when other conditions were not changed, the bend specimen P a force ( Q) greater than the force of atomic bonding was showed intergranular cracking at the bottom of the V-notch, applied, micro-cracks would nucleate at the dislocation-free as shown in Fig.14. The crack initiated at the root of the ]RQHRUDWWKHERWWRPRIWKH9QRWFK)RUGXFWLOHPDWHULDOV 9QRWFKDQGWKHQH[SHULHQFHGQXFOHDWLRQDQGSURSDJDWLRQ PLFURFUDFNQXFOHDWLRQDQGGLVORFDWLRQVRXUFHVH[WHQGHGLQ along the V-notch due to notch sensitivity. Crack growth was WKHSODVWLF]RQHXQGHUFRQVWDQWVWUHVVDQGGLVSODFHPHQW,QWKH accompanied by corrosion effects, and crack propagation H[SHULPHQWWKLVOHGWRPLFURFUDFNVEHFRPLQJPLFURYRLGV and bifurcation occurred at local sites of inhomogeneity in which changed the properties of the P-110 casing specimens the material. Re-nucleation and further propagation were from strong and tough to weak and fragile during the tensile again prompted by the corrosion effects of H2S and CO2. DQGEHQGVWUHVVFRUURVLRQH[SHULPHQWVLQVRXUHQYLURQPHQWV %HFDXVHRIWKHFUDFNJURZWKWKHVL]HVRIWKHJUDLQVORFDWHG $IWHUWKHORDGLPSRVHGRQWKHVXEVL]HVSHFLPHQV near cracks were changed. The crack propagated along the Pet.Sci.(2013)10:385-394 393 direction of structural defects, especially at the crack tip. concentration, it changed the metallurgical structure and grain Thus, the constant stress led to corrosion and continuous VL]HVDQGDOVRIRUPHGVWUHVVFRQFHQWUDWLRQHIIHFWVDWWKHFUDFN crack nucleation, which led cracks to continue propagating tip, hydrogen trapping, and hydrogen damage. The crack K and developing. Eventually, intergranular cracks were propagated until the stress at the crack tip reduced to IC, then formed due to the sensitivity of the V-notch caused by stress crack propagation terminated. Pitting corrosion SCC a Crack Depth of penetration, initiation K ım ISCC Temperature a a, Crack Pressure n growth Corrosive medium K IC Boundary Fast K = ı ¥Cʌa I m fracture Grain Fig. 130HFKDQLVPDQGLQÀXHQFLQJIDFWRUVRILQWHUJUDQXODUFUDFNLQJFDXVHGE\K\GURJHQGDPDJH and hydrogen embrittlement Impact fracture Notch (a) Notch sensitivity crack initiation (b) Crack growth and intergranular cracking HTHP bend SCC intergranular cracking (c) Crack growth and microstructural change (d) Crack tip and microstructural change Fig. 14 1RWFKVHQVLWLYLW\DQGLQWHUJUDQXODUFUDFNLQJFDXVHGE\VXO¿GHVWUHVVFUDFNLQJLQVRXUHQYLURQPHQWVDW&03D EHQGLQJVWUHVVRI ıs, 1 MPa H2S partial pressure, 2 MPa CO2 partial pressure, 168 hours) 4 Conclusions RIıs in a sour environment, which was shown by tensile and impact testing. 1) P-110 casing steel specimens underwent strain aging 2) Surface damage and loss of mechanical properties and were prone to fracture and even failure under a preload ZHUHIRXQGLQ3FDVLQJVSHFLPHQVDIWHUH[SRVXUHWRD 394 Pet.Sci.(2013)10:385-394 sour environment. Dislocation formation, multiplication and Mic hler T and Naumann J. Hydrogen environment embrittlement of motion were promoted by hydrogen permeation, resulting austenitic stainless steels at low temperatures. Hydrogen Energy. in the P-110 steel specimens being more prone to crack 2008. 33(8): 2111-2122 nucleation and propagation. Nat ional Association of Corrosion Engineers. NACE TM0177- 3) In sour wells, defects and stress concentration on 2005. Laboratory testing of metals for resistance to sulfide stress cracking and stress corrosion cracking in H S environments. NACE P-110 steel may lead to the acceleration of corrosion and 2 International. 2005. Houston TX WKHH[WHQVLRQRIPLFURFUDFNVLQWRPLFURYRLGVWKHUHE\ 1HVLF6&DL-<DQG/HH./$PXOWLSKDVHÀRZDQGLQWHUQDOFRUURVLRQ changing the casing properties from strong and tough to weak prediction model for mild steel pipelines. Corrosion, 3-7 April 2005, and fragile, making them more susceptible to intergranular Houston TX fracture. 2WHJXL-/)D]]LQL3*0DVVRQH-HWDO,QWHUJUDQXODUVXVFHSWLELOLW\LQ 7KHH[SHULPHQWDOPHWKRGVRIWHQVLOHDQGLPSDFW failures of high pressure tubes. Pressure Vessels and Piping. 2009. 86 testing can be used to quantitatively evaluate the mechanical (2): 211-220 performance of casings in sour environments. Selecting more Pal S, Ibrahim R N and Raman R K-S. Studying the effect of appropriate materials for casing used in sour environments VHQVLWL]DWLRQRQWKHWKUHVKROGVWUHVVLQWHQVLW\DQGFUDFNJURZWK may help reduce the susceptibility of hydrogen embrittlement for chloride stress corrosion cracking of austenitic stainless steel on future casing materials. using circumferential notch tensile technique. Engineering Fracture Mechanics. 2012. 82(1): 158-171 Ritchie R O, Cedeno M H-C, Zackay V F, et al. 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