Abstract
EVANS1 has recently described improvements in mechanical properties of 20 Cr : 25 Ni steels containing 0.5 and 1.5 wt. % Ti by nitriding to give a dispersion of TiN precipitates. 0.025 inch thick strip nitrided in nitrogen at 1,050° C showed increased hardness and a marked improvement in creep resistance compared with a conventional Nb-stabilized 20 Cr : 25 Ni steel. Our own work, recently confirmed by Evans (private communication), shows that nitriding under those conditions produces large quantities of chromium nitrides (CrN and Cr2N) in addition to titanium nitride and so cause a catastrophic decrease in the corrosion and oxidation resistance. This behaviour is consistent with the thermodynamic stabilities of the nitrides2,3.
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