Thermodynamic Study on the Transformation of Austenite into Martensite in Iron-High Nitrogen and Iron-Carbon Binary System

An investigation on the transformation of austenite into martensite based on the statistical thermodynamics was carried out by using the iron-nitrogen binary alloys containing about 1.8 to 2.8 weight per cent nitrogen. The results obtained were compared with those of the iron-carbon binary alloys. The M_s point lowered with the increase in nitrogen content, reaching room temperature at about 2.2 weight per cent nitrogen showing a complete austenitic structure. M_s points of the iron-nitrogen and of iron-carbon alloys containing the same atomic per cent of respective solutes agreed well with each other. T_0^ temperature (〓F^=0) of the iron-nitrogen alloy fell nearly parallel to the change in M_s point with the increase in nitrogen content. On the other hand, the driving force necessary to initiate martensite transformation increased with the increase in nitrogen content. The effect of carbon on the thermodynamic properties of martensite transformation was similar to that of nitrogen. Stabilities of martensites and supersaturated ferrites of the iron-nitrogen and iron-carbon alloys were also discussed