COMPARISON OF TWO TESTING METHODS FOR DETERMINATION OF RESISTANCE OF GRADE X65 STEEL TO HYDROGEN INDUCED CRACKING

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1 COMPARISON OF TWO TESTING METHODS FOR DETERMINATION OF RESISTANCE OF GRADE X65 STEEL TO HYDROGEN INDUCED CRACKING Petr JONŠTA a, Karel MATOCHA b a VSB-TU Ostrava, Faculty of Metallurgy and Materials Engineering, 17. Listopadu 15/2172, Ostrava Poruba, Czech Republic, petr.jonsta@vsb.cz b RESEARCH & DEVELOPMENT, Ltd., Pohraniční 693/31, Ostrava - Vítkovice, Czech Republic, karel.matocha@mmvyzkum.cz Abstract Pipelines that are used in the oil industry and that are in contact with crude oil or natural gas may succumb to severe degradation due to hydrogen sulfide. One of the well known degradation processes is called hydrogen induced cracking (HIC) and it is a form of internal hydrogen damage cause by the development of small cracks oriented parallel to the surfaces of the steel. These cracks tend to link up with other cracks due to a build up of internal pressure in the hydrogen damage zones in the steel and the resultant stress fields around the zones. This link up of the cracks tends to produce the characteristic stepwise crack appearance. The evaluation of material for resistance to HIC is performed by NACE TM standard [1]. The test method is describe as exposing unstressed test specimens to standard test solutions saturated with hydrogen sulfide at ambient temperature and pressure. After a specific time (usually 96 hours) the test specimens shall be removed and evaluated. Each test specimen shall be sectioned in accordance with NACE TM standard and polished metallographically for determination of test parameters Crack Sensitivity Ratio (CSR), Crack Length Ratio (CLR) and Crack Thickness Ratio (CTR). These parameters can be also determinate by ultrasonic immersion method using the AGFA NDT Krautkramer device. The paper is focused on the comparison of parameters CSR, CLR and CTR of grade X65 steel in accordance with API 5L determination [2] by two different methods, ultrasonic immersion method and metallographically in accordance with NACE TM Both methods showed similar results. Keywords: hydrogen induced cracking (HIC), metals, testing methods 1. INTRODUCTION Hydrogen induced cracking (HIC) is a problem encountered in carbon and microalloyed steels operating in aggressive hydrogen sulfide containing environment, especially in steels used for pipelines and pressure vessels in petrochemical industry. It is generally recognized that the resistance of steels to HIC depends on some microstructural features, e.g. chemical composition, micropurity, character of microstructure, strength level [3-5]. The evaluation of pipeline and pressure vessel steels for resistance to HIC is standardly performed to NACE TM standard. Resistance of steel has been evaluated on the basis of key parameters value CLR, CTR, CSR. These parameters can be also determined by ultrasonic immersion method using the AGFA NDT Krautkramer device. The work is devoted to the comparison of the two testing methods. 2. EXPERIMENTAL MATERIAL Plate 15 mm in thickness manufactured of grade X65 steel according to API 5L has been used for experiments. The chemical composition of the steel is given in Table 1. Yield strength and ultimate strength of steel were determined as 513 MPa and 540 MPa.

2 Table 1 Chemical composition of the steel (wt. %) C Mn Si P S Cu Mo Nb V Ti N Al Ca Microstructural analysis of steel was conducted after the etching in nital solution using a light microscope ZEISS NEOPHOT 32. The microstructure of the studied steel was ferritic-perlitic and is shown in Figs. 1 a, b. a) b) Fig. 1 Microstructure of grade X65 steel 3. EVALUATION OF RESISTANCE TO HIC Three specimens of studied steel about dimensions 100 mm x 20 mm x 15 mm were prepared and exposed for 96 hours in corrosion solution in accordance with the NACE TM 0284 standard. Experiment was carried out in the accredited chemical laboratory of MATERIAL & METALLURGICAL RESEARCH, Ltd. 3.1 immersion method evaluation The ultrasonic immersion method was applied using AGFA NDT Krautkramer device, see Figs. 2, 3. Test specimens are arranged in immersion tank and ultrasonic s scan process was in accordance to the rules defined in advance [6]. The evaluation of the crack parameters Crack Length Ratio (CLR), Crack Thickness Ratio (CTR) and Crack Sensitivity Ratio (CSR) is done automatically. An example of results is shown in Fig. 4. Fig. 2 General view on AGFA NDT Krautkramer device Fig. 3 Detail of arranged specimens

3 cracks detected specimen Fig. 4 An example of the ultrasonic immersion test results 3.2 evaluation The sampling method and metallographic evaluation was in fully acorrdance with the NACE TM standard, see Fig. 5, 6. Examples of the found cracks are on the Fig. 7. a -crack length(mm), b -crack thickness(mm), W -specimen width(mm), T -specimen thickness(mm) Fig. 5 Sampling method Fig. 6 Schema of crack measurement Formulas used for calculating CLR, CTR and CSR parameters are marked 3.2.1, and

4 a) b) Fig. 7 Examples of found cracks 4. RESULTS The Comparison of measured CLR, CTR and CSR parameters is documented in Fig. 8. The obtained results have shown that differences in parameters determined by both used methods are very low. CLR (%) 16 15, , ,5 13 Immersion CTR (%) 2,00 1,50 1,00 0,50 0,00 Immersion a) Evaluation b) Evaluation 0,2 CSR (%) 0,15 0,1 0,05 0 c) Immersion Evaluation Fig. 8 Results for the evaluation of CLR, CTR and CSR parameters 5. CONCLUSION The paper was focused on the comparison of the two testing methods for determination of CLR, CTR and CSR parameters describing the resistance of pipeline steels to HIC. Experiment was carried out on the plate, 15 mm in thickness of grade X65 steel. Parameters CLR, CTR and CSR were determined by ultrasonic immersion method with use the AGFA NDT Krautkramer device and metallographically with fully accordance to NACE TM In all cases we obtained very good agreement of all measured experimental results with those of comparative ones. It is possibly to say that ultrasonic immersion method was very good and faster alternative method to the metallographic evaluation in our occurrence.

5 ACKNOWLEDGEMENTS This work was realized under the support of research projects MSM (Ministry of Education of Czech Republic), GAČR 106/08/1243 (Grant Agency of Czech Republic). REFERENCES [1] NACE Standard TM , Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen Induced Cracking. Item No , ISBN , 2005 NACE international. [2] API Specification 5L, Specification for Line Pipe, Forty-second edition, January Effective date: July 1, [3] SOZAŃSKA et al. Examination of Hydrogen Interaction in Carbon Steel by Means of Quantitave Microstructural and Fractural Descriptions. Materials Characterization 46, 2001, pp [4] ELBOUJDAINI, M., REVIE, R., W. Metallurgical Factors in Stress Corrosion Cracking (SSC) and Hydrogen-Induced Cracking (HIC). J Solid State Electrochem (2009). Published online: 25. March 2009 [5] GYU, T., P. et al. Effect of Microstructure on the Hydrogen Trapping Efficiency and Hydrogen Induced Cracking of Line pipe steel. Corrosion Science 50 (2008), pp [6] AV CORROSION TESTING: Hydrogen Induced Cracking Sensitivity Tests According to NACE TM 0284 and Related Specifications. QS-Manual, Institut für Materialprüfung und Werkstofftechnik, Clausthal - Zellerfeld, Deutschland, 2002, pp. 12