Sulfide Stress-Cracking Resistance of Nitrogen-Strengthened Stainless Steels
Authors: Gaugh Robert R. 
Published: December 2007

Although most materials used in oilfield operations are carbon steel or alloy steel, stainless alloys are used in critical areas where salt water, CO2, and H2S may be present. Relatively recently, a number of present. Relatively recently, a number of nitrogen-strengthened austenitic stainless steels have become commercially available. This paper 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 carbon steel and alloy steel are used most often in oilfield operations, stainless alloys are used in critical areas such as pumps, valves, and downhole instrumentation housings. In many such operations, significant amounts of salt water, CO2, 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 offshore platforms.Among the alloys that have been used in this area are Types 410 and S17400 stainless steels, and nickel alloy Types NO5500 and NO7750. While all these alloys have adequate strength, each has certain deficiencies. Types 410 and S17400 stainless steels both 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 are less readily available.Most austenitic stainless steels have adequate corrosion resistance for oilfield operations, and are immune to sulfide stress cracking as long as they are free from 1 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 nitrogen-strengthened 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; one alloy (Type S21800) is much less subject to galling and seizing than other corrosion-resistant alloys. All are completely nonmagnetic, even when severely cold worked. This is important for certain instrument applications, where magnetism can affect their operation seriously.This paper describes the propensity of a number of nitrogen-strengthened stainless-steel alloys to stress crack in H2S environments. The investigation was performed in two parts. The first was to determine the performed in two parts. The first was to determine the resistance of various commercial nitrogen-strengthened stainless steels to sulfide stress cracking as a function of applied stress. The second was to perform additional work on experimental alloys to determine why most of these steels were found to be somewhat susceptible to cracking.

Experimental Work

The test specimens used for the first phase of this work were made from commercially produced bar stock of the following stainless steels: Grades XM11, XM17, XM19, XM28, XM29, and Types S21800 and 304N. Specimens of Types 304 and 316 were included for controls.

JPT

P. 482