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51317--9715-Problems Related to the Slow Strain Rate Test Performance and the Specimen Characterization in Austenitic Corrosion Resistant Alloy Tubes

The specimen surface, effect of filling rate were examined. Characterization of the primary fracture surface and the gauge section of the tested specimens was done by scanning electron microscopy and subsequent classification according to the NACE TM0198 standard.

Product Number: 51317--9715-SG
ISBN: 9715 2017 CP
Author: Pilar Esteban
Publication Date: 2017
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The slow strain rate test (SSRT) is a method of screening corrosion resistant alloys (CRAs) for resistance to stress corrosion cracking at elevated temperatures in sour oilfield production environments. Moreover it is used as a quality control test included as a part of the material specification requirements because of the short time to obtain results.However there are some issues related to the performance of the testing and the characterization of the tested specimens which have a direct impact on the final result of the test. The purpose of this paper is to highlight some of these difficulties encountered during the testing of different CRAs grades such as N08028 N08535 and N08825. One of these issues is the influence of the final grinding of the specimen (grit paper number longitudinal or transversal grinding lubrication…) to have a proper surface finish which does not affect the performance of the material during the testing. A second problem is the method of pressurizing the autoclave to reach a repetitive value of the gas total pressure at elevated temperatures. In this sense some testing procedures establish the value of H2S pressure at room temperature but do not specify what should be the condition in the autoclave in testing condition. Taking account of that it appears to be a big challenge to test the material exactly at the same conditions when autoclaves of different sizes and shapes are implied in the testing. Once the test is finished other problems arise related to the tested specimen characterization. One of them is the accuracy in the measurement of the reduction of area which is a critical parameter to classify the result of the test. In addition regarding the visual examination of the test specimen NACE TM0198 proposes four different classes depending on the features observed on the primary fracture surface and the type of secondary cracks in the gauge section that may be not easily attributes. Another problem appears when the ductility ratios are above 0.8 where a further analysis on cross sections is required by scanning electron microscopy (SEM) in order to distinguish fissures from SCC secondary cracking focussed in the necked region. To complete the tested specimen characterization a deep evaluation of the primary fracture by SEM has been performed with slight loss of ductility (<20%) for the three grades. Special features have been observed; some of them clearly related with SCC and some others of more difficult interpretation.Slow strain rate testing is a rapid method with a strong potential for ranking alloys for material selection purposes. However it is a test method subjected to different issues that may affect the results and lead to inconsistent conclusions.

Key words: slow strain rate test, corrosion-resistant alloys, specimen preparation, reduction in area measurement, fracture surface characterization, filling rate, testing pressure

The slow strain rate test (SSRT) is a method of screening corrosion resistant alloys (CRAs) for resistance to stress corrosion cracking at elevated temperatures in sour oilfield production environments. Moreover it is used as a quality control test included as a part of the material specification requirements because of the short time to obtain results.However there are some issues related to the performance of the testing and the characterization of the tested specimens which have a direct impact on the final result of the test. The purpose of this paper is to highlight some of these difficulties encountered during the testing of different CRAs grades such as N08028 N08535 and N08825. One of these issues is the influence of the final grinding of the specimen (grit paper number longitudinal or transversal grinding lubrication…) to have a proper surface finish which does not affect the performance of the material during the testing. A second problem is the method of pressurizing the autoclave to reach a repetitive value of the gas total pressure at elevated temperatures. In this sense some testing procedures establish the value of H2S pressure at room temperature but do not specify what should be the condition in the autoclave in testing condition. Taking account of that it appears to be a big challenge to test the material exactly at the same conditions when autoclaves of different sizes and shapes are implied in the testing. Once the test is finished other problems arise related to the tested specimen characterization. One of them is the accuracy in the measurement of the reduction of area which is a critical parameter to classify the result of the test. In addition regarding the visual examination of the test specimen NACE TM0198 proposes four different classes depending on the features observed on the primary fracture surface and the type of secondary cracks in the gauge section that may be not easily attributes. Another problem appears when the ductility ratios are above 0.8 where a further analysis on cross sections is required by scanning electron microscopy (SEM) in order to distinguish fissures from SCC secondary cracking focussed in the necked region. To complete the tested specimen characterization a deep evaluation of the primary fracture by SEM has been performed with slight loss of ductility (<20%) for the three grades. Special features have been observed; some of them clearly related with SCC and some others of more difficult interpretation.Slow strain rate testing is a rapid method with a strong potential for ranking alloys for material selection purposes. However it is a test method subjected to different issues that may affect the results and lead to inconsistent conclusions.

Key words: slow strain rate test, corrosion-resistant alloys, specimen preparation, reduction in area measurement, fracture surface characterization, filling rate, testing pressure

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51316-7861-Slow Strain Rate and Rising Step Load Hydrogen Embrittlement Testing of UNS N07718 - STG 32

Product Number: 51316-7861-SG
ISBN: 7861 2016 CP
Author: Brian Kagay
Publication Date: 2016
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