Fabrication of ultrahigh nitrogen austenitic steels by nitrogen gas absorption into solid solution
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9/27/03
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Fabrication of Ultrahigh Nitrogen Austenitic Steels by Nitrogen Gas Absorption into Solid Solution T. TSUCHIYAMA, H. ITO, K. KATAOKA, and S. TAKAKI For the purpose of fabricating ultrahigh nitrogen austenitic steels (1 mass pct N), the phenomenon of nitrogen absorption into solid solution was thermodynamically analyzed and applied to Fe-Cr-Mn system ternary alloy. During the annealing of the steel in a nitrogen gas atmosphere of 0.1 MPa at 1473 K (nitrogen absorption treatment), the nitrogen content of the steel was increased with the absorption of nitrogen gas from the material surface and then saturated when the system reached a state of equilibrium. Effect of the steel composition on an equilibrium nitrogen content was formulated taking account of interactions among Cr, Mn, and N atoms, and the condition for fabrication of ultrahigh nitrogen austenitic steels was clarified. The nitrogen addition to ultrahigh content markedly increased proof stress and tensile stress of the austenitic steels without losing moderate ductility. For example, Fe-24Cr-10Mn-1.43N (mass pct) alloy has 830 MPa in 0.2 pct proof stress, 2.2 GPa in true tensile stress, and 75 pct in total elongation. As a result of tensile tests for various nitrogen-bearing austenitic steels, it was found that the proof stress is increased in proportion to (atomic fraction of nitrogen)2/3.
I. INTRODUCTION
NITROGEN addition to austenitic stainless steels has many advantages including that (1) the strength of the steels is greatly increased without lowering ductility too much;[1,2] (2) nickel content can be reduced because nitrogen is a strong austenite stabilizer, which markedly lowers the Ms temperature;[3,4] and (3) corrosion resistance, especially pitting corrosion resistance, is also improved.[1,4,5] Therefore, the processing and properties of high nitrogen stainless steels have being widely studied for the purpose of complying with the increased demands on stainless steels for structures in chemical and energy industries,[6,7] marine and offshore environments,[7,8] surgical and dental applications,9 and so on. However, the nitrogen content of the industrially produced materials is 0.4 pct at most, that is to say, we do not make the most of the advantages of nitrogen addition. Development of the technique to add more nitrogen could improve the properties of austenitic stainless steels further. In general, high nitrogen stainless steels are usually produced by melting in a high-pressurized nitrogen gas atmosphere. There is a report that a stainless steel containing solute nitrogen of 1.1 pct had been made using 4 MPa nitrogen gas.[10] However, the pressurized melting process requires special equipment, and, besides, it sometimes causes the problem of formation of blowholes on solidification.[11] This prevents the increase of nitrogen to the ultrahigh concentration region. (In this article, steels containing over 1 pct N are referred to
T. TSUCHIYAMA, Lecturer, and S. TAKAKI, Professor, are with the Department of Materials
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