ABSTRACT
A study of the corrosion resistance of multiple high N, high Mn austenitic stainless steels is presented, focusing on a newly developed alloy with increased pitting resistance. The alloys vary principally by the amount of Mo, Ni, Mn, Cr and N. High Mn and Cr content increase the solubility of N, and the N acts to stabilize the austenitic matrix, improve pitting resistance, and increase strength. The alloys were tested in the work hardened condition, with yield strengths ranging from 850-1020 MPa. Resistance to pitting was measured by multiple methods. Electrochemical testing was used to determine the pitting potential and critical pitting temperature (ASTM standard G150) in an 8% chloride solution. Ferric chloride pitting tests (ASTM standard G48) were run to compare pitting mass loss and to confirm the critical pitting temperature. Results generally followed expected trends with the new alloy demonstrating improved pitting resistance as predicted by the PREN. Susceptibility to intergranular attack and stress corrosion cracking were also tested, with less definitive results. These alloys are typically used as components in non-magnetic drilling equipment and should be considered for applications where a combination of corrosion resistance and high strength are required.
INTRODUCTION
High nitrogen alloys are well suited for a number of applications requiring corrosion resistance and high strength. Nitrogen is a well known alloying addition to Fe-base austenitic alloys used to increase the stability of the non-magnetic austenitic phase. Nickel can also produce the same effect, but the element is expensive and can lead to issues with stress corrosion cracking over ranges typically seen in the 300 series stainless steels. Nitrogen has the added benefit of increasing the pitting resistance in conjunction with Mo and improving both the annealed and cold worked strength. When designing alloys for high N content, Mn is often added to increase N solubility. Cr also serves to increase the N solubility and corrosion resistance, but must be balanced in regards to ferrite formation. High N grades are often defined by the pitting resistance equivalent number (PREN), which takes into account the beneficial effects of Cr, Mo, and N on resistance to chloride pitting,
