Abstract

The precipitation hardened nickel alloys are designed for Oil & Gas applications requiring high mechanical strength and toughness combined with high corrosion resistance in sour environments. Over recent years there has been increasing industry demand to improve quality control and categorise the various PH Nickel alloy grades resistance to Hydrogen Stress Cracking (HSC) for critical High Pressure-High Temperature environments. This is a complex corrosion mechanism with many factors including composition, strength, microstructure and grain boundary cleanliness1,2. Evaluation efforts have used multiple techniques to measure the effects of HSC resistance with this paper concentrating on the Slow Strain Rate Test (SSRT) according to TM0198 Appendix C and the quality control of API6ACRA.

The purpose of the paper is to present results using the TM0198 slow strain rate test method in a hydrogen charging environment and show the Hydrogen Stress Cracking resistance of the UNS grades N09945, N09946, N07718 and N07725 in relation to mechanical properties and microstructural characterization of mill heats. Process control of the composition and processing has shown the HSC resistance of N09946 can be optimized with the plastic elongation ratio values in the range of 0.7 to 0.4 dependent on the grade and heat treatment. The processing of N07725 manufactured according to API6ACRA has shown the materials elongation ratio as a measure of resistance to HSC can be significantly improved from values of approximately 0.16 to ratio's above 0.45. The SSRT results are compared with mechanical properties determined according to API6ACRA and detailed microstructural analysis.

Introduction

The Precipitation Hardenable (PH) Nickel alloys N09925, N07718, N09945, N09946 and N07725 are widely used for critical downhole oil field applications such as high strength tubing hangers and completion equipment. The materials are particularly useful in High Pressure/High Temperature wells where high strength and corrosion resistance are required in H2S containing production fluids. Over the last 20 years a limited number of field failure investigations in PH Nickel alloys have been related to the presence of sufficient amounts of intergranular precipitates promoting hydrogen embrittlement, which results in brittle cracking of UNS alloys N077183,4 and alloy N077255 and N077166. The investigations on these failures of Nickel PH alloy 718 have studied the potential for grain boundary precipitates to impair the materials resistance to hydrogen stress cracking by causing brittle fracture by decohesion at the particle/ matrix interface. Cassagne et al4 suggested that hydrogen embrittlement is promoted by any inter-granular second phase precipitate irrespective of chemical composition. Liu et al in a separate study on alloy UNS N07718 also showed local transgranular cleavages were initiated from the delta phase/ matrix interfaces in the presence of pre-charged hydrogen6. Liu et al7 in a separate study on alloy 718 also showed local transgranular cleavages were initiated from the delta phase/ matrix interfaces in the presence of pre-charged hydrogen(6) Mannan et al8 has also shown that the presence of significant amounts of any second phase lowers time –to- failure, % elongation and reduction of area ratios for high temperature sour environment SSRT tests. In this investigation Slow Strain Rate Testing (SSRT) on plain specimens at ambient temperatures in a Hydrogen charging environment have been used to show the effects of hydrogen on a range of high strength PH Nickel alloys mechanical properties.

This content is only available via PDF.
You can access this article if you purchase or spend a download.