ABSTRACT
The study of hydrogen stress cracking of various Nickel based alloys has recently been reported by others and is a subject of increasing interest. This paper describes results for different test methods, including Slow Strain Rate Testing (SSRT), Incremental Step Loading (ISL), and Constant Load Verification (CLV).
A new Ni-Cr-Mo-Fe alloy UNS N08830 alloy was recently evaluated for resistance to Hydrogen Induced Stress Cracking (HISC), simulating conditions arising during cathodic protection in a subsea environment.
The unique set of test methods and conditions included SSRT and ISL, all using pre-charged specimens with ongoing continuous charging during testing. Test specimens utilized different geometries, including smoothed and notched, along with round and rectangular cross sections.
The paper draws key conclusions based on comparisons of test methods, and also compares UNS N08830 alloy results to other high strength CRA’s used in Oil and Gas subsea production equipment.
INTRODUCTION
Cases of Hydrogen Embrittlement affecting subsea equipment and components have been well documented for some time.1,2,3 The source of embrittlement investigated in this study is theorized to emanate from the evolution of hydrogen gas generated by cathodic protection systems in seawater, with several factors possibly contributing to a material’s susceptibility.4
A joint industrial project (JIP) was formed in 2005 out of Norway to investigate the susceptibility to Hydrogen Induced Stress Cracking (HISC) of duplex and martensitic stainless steels, which resulted in a recommended practice issued in 2008.5,6 More recently, Ni based alloys have been investigated after some field experiences demonstrated susceptibility to HISC.7 NTNU(1) in Norway performed HISC testing in 2014-16 on various austenitic alloys for comparative purposes. To this end, Incremental Step Load (ISL) testing was performed on precipitation hardened nickel alloys along with strain hardened austenitic alloys. One of the alloys included in the study was UNS N08830, a recently introduced new grade suitable for drilling tools for severe oil & gas downhole environments. The nominal chemical composition of the alloy is listed in Table 1.
The new alloy can achieve high yield strength approaching 1170 MPa (170 ksi) through strain hardening, making it a candidate material for various components in tools used for drilling, completions and subsea applications. The alloy retains significant strength at elevated temperatures, giving it advantages over leaner austenitic alloys and super-duplex stainless steels, while offering economic advantages as compared to higher cost Precipitation Hardened (PH)nickel alloys.8
