ABSTRACT

Precipitation Hardened (PH) nickel alloys have been used extensively and successfully in the Oil & Gas Industry for many years. These materials offer high strength and outstanding corrosion resistance in many aggressive environments and they are a common selection for high-strength equipment for downhole, wellhead, subsea and Christmas tree applications. However, several high-profile failures have occurred, including tubing hangers, cross-overs and subsea bolts with alloys such as UNS N07718, UNS N07716 and UNS N07725. In all these cases, the mechanism identified was Hydrogen Assisted Cracking (HAC) as a result of embrittlement from atomic hydrogen absorbed by the alloy in specific operating environments.

PH nickel alloys exhibit complex microstructures with multiple potential secondary and tertiary phases. They require carefully controlled thermomechanical processing and heat treatment to deliver performance that is adequate for the intended applications If processed improperly (particularly during hot working and heat treatment), the resultant microstructure may adversely affect the material properties and suitability for the intended service. Despite the number of scientific and technical contributions produced over the last years, the interaction between these complex microstructural features, the service environment and atomic hydrogen is still not well established. This is further complicated by variations in testing approach used to study and simulate hydrogen charging conditions. The present paper provides insights on the HAC failure mechanism for API 6ACRA PH nickel alloys comparing findings from numerous studies. In addition, implications for currently adopted standards and emerging specifications are also presented and discussed.

INTRODUCTION

UNSa N07718 has been employed by the aerospace industry for hot turbine sections since the 1960’s. This material was adopted by the oil and gas industry in the 60’s and 70’s to deliver a high strength material option for increasingly hot and high pressure wells. The adoption while largely successful, did have early failures between the late 90’s and the early 00’s due to Hydrogen Assisted Cracking (HAC). Lessons learned from such failures, which will be reviewed in the following sections, resulted in manufacturers and operators coming together to create APIb Specification 6A7185 which better ensured HAC-resistance by close control of microstructure and all influencing process steps. For example, chemistry is restricted to make the most likely deleterious phase (d phase) less likely, and metallographic assessment assures that the phases present (including d phase) are not distributed deleteriously (by avoiding continuous, or semi-continuous grain boundary networks and avoiding the most damaging form of delta phase - acicular platelets).

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