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Susceptibility of Plasma Nitrided 17-4 PH to Sulphide Stress Cracking in H2S-Containing Environments

The stainless steel 17-4 PH (H1150D) is a cost-effective alloy for oilfield components due to its combination of high strength, toughness, and good corrosion resistance. Plasma nitriding improves the surface strength of the 17-4 PH and this work suggests that the nitrided can the resistance to SSC.

 

Product Number: 51317--9342-SG
ISBN: 9342 2017 CP
Author: Mario Coseglio
Publication Date: 2017
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Context: The oil and gas industry is facing a major challenge as the processes are migrating to environmentally aggressive reserves found in ultra-deep water operations where significant amounts of hydrogen sulphide (H2S) carbon dioxide (CO2) and elemental sulphur can lead to premature failures. The selection of materials for oilfield components is one of the main concerns due to its complex interactions with the sour environment. Field failures have been frequently attributed to Sulphide Stress Cracking (SSC). It can occur due to the applied stress in the presence of hydrogen sulphide and its associated with the entry of atomic hydrogen into the metal substrate although its mechanisms still unclear.Gap: The martensitic precipitation-hardening stainless steel 17-4 PH (or UNS S17400 - ASTM A564 Type 630 or DIN 14542) at the double-aged condition (H1150D) is one of the grades recommended by NACE MR0175 / ISO 15156 for wellhead christmas tree valves and other subsurface oilfield components. Despite its favourable combination of properties such as high strength relatively high toughness good corrosion resistance and a reasonable cost recent reports of field failures in the oil and gas sector at stress levels below the safe limits defined by the industry standards poses a challenge for the definitions of its limits of applications in sour environments. Recently a technical circular restricted the use of this material to applications where the sustained stress do not exceed 50% of the specified minimum yield strength and safe limits of chloride content and temperature have not been defined.Purpose: Some surface engineering technologies such as nitriding carburizing and nitrocarburizing have been used to improve the surface strength of the 17-4 PH. Temperature is a critical parameter for nitriding of precipitation-hardening stainless steels as it cannot exceed its ageing temperature. However plasma-based surface modification at low temperature (320-420°C) has been successfully carried out with particular interest in enhancing the tribological aspects of the 17-4PH while not compromising its corrosion performance. The purpose of this work is to determine the susceptibility of plasma-based surface-modified 17-4 PH (H1150D) to SSC in H2S-containing environments. This will contribute to the better understanding of the mechanisms of failure in order to help defining the limits of application so that this material can be safely and economically selected for oilfield components submitted to sour environments.Methodology: The SSC resistance of the plasma nitrided 17-4 PH in simulated produced water and other parameters defined by the intended service will be evaluated in accordance with the NACE TM0177 – Method A (Standard Tensile) and Standard Bent-Beam with four-point bend configuration.Results and Conclusion: Preliminary results indicate that the plasma nitrided case can be effective in reducing the SSC susceptibility with further analysis being carried out to confirm the corrosion resistance and to understand the interactions of the plasma nitrided layer and the H2S-containing environment.

Key words: sulfide stress cracking, SSC, precipitation-hardening stainless steel, 17-4 PH, UNS S17400, hydrogen sulfide, plasma nitriding

Context: The oil and gas industry is facing a major challenge as the processes are migrating to environmentally aggressive reserves found in ultra-deep water operations where significant amounts of hydrogen sulphide (H2S) carbon dioxide (CO2) and elemental sulphur can lead to premature failures. The selection of materials for oilfield components is one of the main concerns due to its complex interactions with the sour environment. Field failures have been frequently attributed to Sulphide Stress Cracking (SSC). It can occur due to the applied stress in the presence of hydrogen sulphide and its associated with the entry of atomic hydrogen into the metal substrate although its mechanisms still unclear.Gap: The martensitic precipitation-hardening stainless steel 17-4 PH (or UNS S17400 - ASTM A564 Type 630 or DIN 14542) at the double-aged condition (H1150D) is one of the grades recommended by NACE MR0175 / ISO 15156 for wellhead christmas tree valves and other subsurface oilfield components. Despite its favourable combination of properties such as high strength relatively high toughness good corrosion resistance and a reasonable cost recent reports of field failures in the oil and gas sector at stress levels below the safe limits defined by the industry standards poses a challenge for the definitions of its limits of applications in sour environments. Recently a technical circular restricted the use of this material to applications where the sustained stress do not exceed 50% of the specified minimum yield strength and safe limits of chloride content and temperature have not been defined.Purpose: Some surface engineering technologies such as nitriding carburizing and nitrocarburizing have been used to improve the surface strength of the 17-4 PH. Temperature is a critical parameter for nitriding of precipitation-hardening stainless steels as it cannot exceed its ageing temperature. However plasma-based surface modification at low temperature (320-420°C) has been successfully carried out with particular interest in enhancing the tribological aspects of the 17-4PH while not compromising its corrosion performance. The purpose of this work is to determine the susceptibility of plasma-based surface-modified 17-4 PH (H1150D) to SSC in H2S-containing environments. This will contribute to the better understanding of the mechanisms of failure in order to help defining the limits of application so that this material can be safely and economically selected for oilfield components submitted to sour environments.Methodology: The SSC resistance of the plasma nitrided 17-4 PH in simulated produced water and other parameters defined by the intended service will be evaluated in accordance with the NACE TM0177 – Method A (Standard Tensile) and Standard Bent-Beam with four-point bend configuration.Results and Conclusion: Preliminary results indicate that the plasma nitrided case can be effective in reducing the SSC susceptibility with further analysis being carried out to confirm the corrosion resistance and to understand the interactions of the plasma nitrided layer and the H2S-containing environment.

Key words: sulfide stress cracking, SSC, precipitation-hardening stainless steel, 17-4 PH, UNS S17400, hydrogen sulfide, plasma nitriding

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