Low alloy steels are one of the most commonly used material systems in oil and gas fields as they can be heat treated to appropriate strength levels including higher strengths such as 758 MPa (110 ksi) and 862 MPa (125 ksi) minimum yield strength while providing economical solutions for various oilfield conditions. Higher hardenability of low alloy steels is an important factor to ensure proper heat treatment to higher strength levels and this is typically achieved by addition of elements such as Chromium (Cr) Molybdenum (Mo) Nickel (Ni) etc. in the alloy chemistry. It is also essential to ensure adequate toughness in these high strength steels to reduce risk of brittle fracture. Increasing Ni content in the chemistry of low alloy steel can provide increased hardenability while maintaining good toughness when heat treated to high strengths. However the guidelines of NACE MR0175/ISO 15156-2 currently restrict the maximum Ni content to 1% mass fraction and in general recommend use of Cr-Mo type low alloy steels such as 41XX series in sour (H2S) service. This has generally led to exclusion of low alloy steels containing higher Ni such as 43XX series in sour service. In this paper an effort is made to evaluate the sulfide stress cracking (SSC) resistance of common grades of Cr-Mo and Ni-Cr-Mo steels heat treated to high strength using NACE TM0177 Method A testing. This would also assist when comparing the cracking resistance of high strength low alloy steels with greater than 1% mass fraction Ni content to those which are within this limit.Keywords: high strength low alloy steel Cr-Mo Ni-Cr-Mo sulfide stress cracking (SSC) 1% Nickel content

With increasing oil & gas demand and depletion of sweet reserves, oil & gas companies in the regional
economies are focusing towards the exploitation of sour resources. This necessitates the use of pipelines
and down-hole tubing made from special steels with significant resistance to hydrogen-induced cracking
(HIC). These steels are produced through specific technologies for enhanced chemical composition control
and microstructural engineering to incorporate the required strength, weld ability and improved HIC
resistance. It is well established that the HIC initiates at sites with microstructural heterogeneities whether
due to presence of gross nonmetallic inclusions or the micro-structural constituents. The presence of central
segregation further aggravates the conditions particularly when the final pipe sizes require the longitudinal
slitting of the coils. Presence of non-metallic inclusions in the steel makes it vulnerable to hydrogen-induced
cracking under wet H2S environment. The mechanism of HIC begins with the generation of hydrogen atoms
by corrosion reaction of H2S and Fe in the presence of free water. The hydrogen atoms then diffuse into
steel and accumulate around the inclusions. The higher number of inclusions equates to the more sites
available for hydrogen adsorption. Recombination of hydrogen atoms to H2 molecules builds up a heavy
gas pressure in the interface between matrix and inclusions. Cracking initiates because of the tensile stress
field caused by hydrogen gas pressure and crack propagates in the surrounding steel matrix. The
longitudinal slitting exposes the internal microstructural abnormalities to the skelp edges which are then
incorporated in the thermally stressed weld zone. While the post-weld heat treatment (PWHT) mostly
homogenizes the weld zone microstructure, the presence of excessive central line features cannot be
completely removed thereby making this zone more prone to HIC attack

Stainless steel UNS S17400 (17-4PH) has been successfully used in oilfield services outside the traditional NACE MR0175/ISO 15156 limits for permanent equipment. The exact operational envelops of 17-4PH (HH1150), including the tensile threshold stress, sour gas partial pressure, temperature, and exposure time that enable the crack-free usage of 17-4PH (HH1150) are not well established. For service equipment, NACE MR0175/ISO15156 currently provides exemptions from the tight environmental restrictions of permanent equipment, but instead limits the maximum applied stress to a debatable 60% of the specified minimum yield strength (SMYS). In this investigation, the sulfide stress-corrosion cracking of 17-4PH is revisited through 51 new NACE TM0177 Methods A tests conducted over 240 hours minimum (480hrs in certain cases). Under unrestricted sour gas partial pressures, the threshold tensile stress below which cracking does not occur is between 45% and 60% of the SMYS at ambient temperature. Alloy 17-4PH is also less susceptible to sulfide stress cracking as temperature increases from 70°F (21°C) to 350°F (177°C). Risk of sulfide stress cracking is also greatly mitigated when delta ferrite is controlled. With reduced delta ferrite, as provided by two out of three tested heats, and reverted austenite promoted by both chemical composition and longer aging treatments, no cracking is seen at 60% stress level up to 45psi H2S (0.31MPa); at 45% stress level, this value is increased to 120psi (0.83MPa) based on newly-collected test data.