Hydrogen-Assisted Cracking Behavior of Ni Alloy 718: Microstructure, H Testing Protocol, and Fractography
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INTRODUCTION
PRECIPITATION-HARDENED Ni-base super-alloy UNS07718 is used in critical components of oil-field structures for their inherently high fracture toughness and corrosion resistance.[1–8] However, the high fracture toughness of the alloy reduced significantly in H
DHINAKARAN SAMPATH, GIDEON OBASI, and ROBERT AKID are with the Department of Materials, University of Manchester, Manchester, UK and with the International Centre for Advanced Materials, University of Manchester, Manchester, UK. Contact e-mail: [email protected], dhinakaran.sampath@ manchester.ac.uk ROBERTO MORANA is with the International Centre for Advanced Materials, University of Manchester and with the BP Exploration Operating Company Limited, Sunbury on Thames, UK. Manuscript submitted October 27, 2019; accepted September 27, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
evolving environments indicating its susceptibility to hydrogen-assisted cracking (HAC).[9] UNS07718 (hereafter referred to as alloy 718) is an aerospace material consisting of a variety of elements forming metallurgically different types of phases viz. c¢, c¢¢, d, laves, TiN, and NbC in an FCC c matrix. The aging treatment of alloy 718 was modified to meet the HAC resistance required for oil and gas components, and the heat treatment procedure has been standardized in API standard 6ACRA-718 (2013) for oil and gas applications.[10] The over-aged variants are preferred over the under-aged variants to achieve the desired mechanical properties due to significant variability in the mechanical properties of under-aged variants.[11] Kagay et al.[12] established that the under-aged variant of alloy 718 performed better than the peak-aged and over-aged variants based on SSRTs under in-situ H charging in deaerated 0.5 M H2SO4 solution at 21 C. However, the
under-aged variant was reported to be difficult to manufacture.[11] The current API standard has stipulated the required minimum yield strength and acceptable microstructural features of over-aged variants such as ‘‘discrete’’ and ‘‘isolated’’ presence of precipitates at grain boundaries (GBs) to increase resistance to H-induced intergranular (IG) cracking.[10] The approach is to employ an overaged variant with isolated d phase at the GBs for better HAC resistance. However, a series of costly and catastrophic HAC failures [1,2,5,13] culminated in a need to re-examine the microstructural requirements and heat treatment schedule of alloy 718 recommended by the API standard.[10] There has been a limited number of research studies that have investigated the HAC performance of microstructural variants suggested by the API standard. Therefore, a systematic understanding of HAC performance of the API variants of alloy 718 linked to microstructural features and fracture modes is essential in the development of new alloy variants. The complex microstructural effects on HAC have been previously observed in Ni alloys–alloy 718,[14–16] Monel K500,[17,18] alloy C276,[19] and Fe-Ni-Cr alloy.[20,21] To illustrate the extremes of manifestatio
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