Sulfide Stress Cracking Resistance Of Nitrogen-Strengthened Stainless Steels

Abstract Although the bulk of the materials used in oil field operations are carbon steel or alloy steel, stainless alloys are employed in critical areas where salt water, C02, and H2S may be present. Relatively recently, a number of nitrogen-strengthened austenitic stainless steels have become commercially available. They have substantially higher strength than the 300 series of stainless steels, plus other attractive features. This report describes sulfide stress cracking tests performed on a number of these alloys. Most were found to be somewhat susceptible to cracking, depending on the stress level. It was determined that this was due to their high manganese content. The mechanism responsible for cracking was not firmly established. One commercial nitrogen-strengthened stainless steel, XM19, was highly resistant to sulfide stress cracking despite a manganese content of 5%. This difference is attributed to the superior corrosion resistance of the alloy. Introduction Although the bulk of the materials used in oil field operations are carbon steel or alloy steel, stain less alloys are employed in critical areas such as pumps, valves, and down-hole instrumentation housings. In many such operations, significant amounts of salt water, C02, and H2S are present, creating severe problems of general corrosion, pitting, and sulfide stress cracking. Corrosion-resistant materials also are required in other supporting areas, such as work and supply boats for off-shore platforms. Among the alloys that have been used in this area are Type 410 and S17400 stainless steels, and nickel alloys N05500, and N07750. While all of these alloys have adequate strength, each has certain deficiencies. Both Type 410 and S17400 stainless steels are subject to sulfide stress cracking, pitting, and crevice corrosion. The nickel alloys, although highly resistant to the environment, are considerably more expensive than the stainless steels and less readily available. Most austenitic stainless steels have adequate corrosion resistance for oil field operations, and are immune to sulfide stress cracking as long as they are free fromo('martensite resulting from cold work. However, they do have rather low strength, with only 30 to 35 ksi 0.2% yield strength in the annealed condition. Relatively recently, a number of nitrogenstrengthened austenitic stainless steels have become commercially available. These alloys have nearly twiCE the 0.2% yield strength of the 300 series of stainless steels, giving them a definite design advantage over the older alloys. Several also have other attractive features, such as excellent resistance to brine, seawater, and other corrosive environments, and one alloy (S21800) is much less subject to galling and seizing than other corrosion-resistant alloys. All are completely non-magnetic, even when severely cold worked. This is important for certain instrument applications, where magnetism can seriously affect their operation. This report describes the propensity of a number of nitrogen-strengthened stainless steel alloys to stress crack in H2S environments.