Abstract
Nitrogen is one of the most effective elements for improving not only the strength but also the corrosion resistance of stainless steels. In this study, three low carbon and high nitrogen martensitic stainless steels, which contain less than 0.1 mass% C and more than 0.45 mass% N, have been produced through the pressurized induction melting process, in which nitrogen is introduced from the pressurized N2 atmosphere. The hardness and corrosion resistance of these steels were mainly investigated in various heat treatment conditions. The hardness of these steels after spheroidal annealing treatment was about 95 HRB and the cold workability was superior to that of AISI440C, which was a high carbon martensitic stainless steel. The hardness of these steels after hardening and sub-zero treatment was about 53 to 56 HRC, which was lower than that of high carbon martensitic stainless steel. In tempering process, however, high nitrogen steels showed the secondary hardening by about 4 point in HRC compared with the quenched hardness after sub-zero treatment and had the maximum tempered hardness of 56 to 60 HRC at around 723 K. The corrosion resistance of hardened and tempered materials under 723 K exhibited better than that of AISI304 evaluated by pitting potential in 3.5 % NaCl aqueous solution. Both remained Cr2N in hardening and precipitated Cr2N in tempering were found to degrade the corrosion resistance of high nitrogen martensitic stainless steels. The best balanced developed steel had the hardness of 60 HRC and better corrosion resistance than AISI304 at the same time under the optimized heat treatment condition.
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