Abstract

UNS N08935 is a new versatile austenitic alloy with the composition 35Ni-30Fe-27Cr-6.5Mo-0.28N and a PREN of 52. It has an extreme pitting resistance which makes the grade a good candidate for O&G applications. The alloy is available in several strength levels with good ductility: seamless tubes with 62 ksi, 140 ksi and 180 ksi. It is an austenitic grade that is worked to strength which results in an alloy with very high resistance to hydrogen effects. According to NACE MR0175 / ISO 15156 the grade will be a Type 4c material.

For the 62 ksi material the resistance against Hydrogen Induced Stress Cracking (HISC) has been demonstrated by constant load testing of specimens polarized to -1050 mVSCE. For the cold worked material Slow Strain Rate Testing (SSRT) at Test Level VI in NACE MR0175 / ISO 15156, a condition within Type 4d is discussed.

With its high mechanical strength and pitting resistance as well as being not prone for HISC, UNS N8935 is an interesting candidate for subsea applications. Further, UNS N08935 is interesting as tubing for HPHT-wells due to its very high strength.

Introduction

UNS N08935 is a newly developed 6% Mo grade that has a high Pitting Resistance Equivalent Number (PREN) of 52. Earlier data has been presented for heat exchanger tubing where extreme pitting resistance was shown. In addition general corrosion results in Hydrochloric Acid and Sulfuric Acid have been reported.1 For solution annealed tubes from the same heats as used in this work, a Critical Pitting Temperature, CPT, at 100.8°C and 102.5°C in modified ASTM(1) G1501 with 3M MgCl2, Magnesium Chloride, was reported.2 Those investigations were made on annealed material. A mean CPT in ASTM G48E3 of 70°C for butt welded tubes with Alloy 59 as filler metal was reported.4

UNS N08935 is a structurally stable austenitic alloy where high pitting resistance and mechanical strength have been combined. This has been achieved by alloying with nitrogen while keeping the nickel content high. With such high nickel content as 35% the material is very structure stable and not prone to form intermetallic phases. Another aspect is that the material is fully austenitic which indicates that the alloy is not prone to hydrogen embrittlement. Some failures have occurred on precipitation hardened Ni-base alloys due to hydrogen embrittlement.5 It should be noted that UNS N08935 is cold worked to strength and not precipitation hardened. The nitrogen content around 0.28% can be dissolved in the austenite and hence no chromium nitrides are formed. The high nickel content together with 6.5 % molybdenum and 26-27% chromium indicates that UNS N08935 should have good resistance against Stress Corrosion Cracking (SCC) in H2S environments.

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