Sulfide stress cracking of nickel-containing low-alloy steels

Abstract

Low-alloy steels (LAS) are extensively used in oil and gas (O&G) production due to their good mechanical properties and low cost. Even though nickel improves mechanical properties and hardenability with low penalty on weldability, which is critical for large subsea components, nickel content cannot exceed 1-wt% when used in sour service applications. The ISO 15156-2 standard limits the nickel content in LAS on the assumption that nickel concentrations above 1-wt% negatively impact sulfide stress cracking (SSC) resistance. This restriction excludes a significant number of high-strength and high-toughness alloys, such as Ni-Cr-Mo (e.g., UNS G43200 and G43400), Ni-Mo (e.g., UNS G46200), and Ni-Cr-Mo-V grades, from sour service applications and can be used only if successfully qualified. However, the standard is based on controversial research conducted more than 40 years ago. Since then, researchers have suggested that it is the microstructure that determines SSC resistance, regardless of Ni content. This review summarizes the advantages and disadvantages of nickel-containing LAS in terms of strength, weldability, hardenability, potential weight savings, and cost reduction. Likewise, the state of knowledge on the effect of nickel on hydrogen absorption as well as SSC initiation and propagation kinetics is critically reviewed.

Effect of Nickel on the Hydrogen Stress Cracking Resistance of Ferritic/Pearlitic Low Alloy Steels

“…2 Likewise, an increasing number of reservoirs can be in the arctic region and exposed to extreme temperature gradients. 1,3,4 Harsh environments invoke strict material requirements. In this regard, researchers argue that, in the next 10 to 15 years, there will be an increased demand for low alloy steels (LAS) with a unique combination of mechanical, corrosion, and technological properties.…”

“…For example, LAS with (i) homogeneous through-thickness yield strength (YS) levels above 690 MPa, (ii) ductile to brittle transition temperatures (DBTT) below -60°C, and (iii) adequate sour service resistance could become the norm in high-pressure, high-temperature (HPHT) and arctic equipment. 4,5 LAS are widely used in oil and gas production due to their excellent mechanical properties and low cost. 4 Mechanical properties are improved by alloying.…”

“…4,5 LAS are widely used in oil and gas production due to their excellent mechanical properties and low cost. 4 Mechanical properties are improved by alloying. LAS often contain manganese, chromium, molybdenum, and nickel.…”

“…Ni, an element that does not form carbides and has a solubility in the bcc ferrite structure of about 7 wt%, 4,6 increases strength and toughness simultaneously, while lowering DBTT, increasing hardenability, and imparting the lowest penalty on weldability. 4 The use of LAS in environments containing H2S is governed by Part 2 of the ISO 15156 Standard, 7 which restricts the Ni content to a maximum of 1 wt% for use in any H2S condition due to concerns regarding sulfide stress cracking (SSC) resistance, as stated in Annex A of the standard. At present, carbon and low alloy steels exceeding the 1 wt% Ni restriction require complicated and expensive qualification testing, which often implies a de facto ban in allowable Ni content.…”

“…In this regard, exceeding the critical transformation temperature causes the formation of untempered martensite upon cooling, which is known to be detrimental to SSC resistance. 4 Overlapping effects from varying chemical compositions and changing microstructures have also obscured the intrinsic effect of Ni in several subsequent works as discussed by Kappes et al 4 The effect of Ni on SSC resistance was extensively investigated in the 1980's. 9,[11][12][13][14][15][16][17][18][19] Although most researchers suggested that Ni did not play a direct role in sour service performance, the engineering community has yet to reach consensus as to whether the cap on nickel was scientifically justified.…”