PHASE-TRANSFORMATIONS DURING TEMPERING OF THE FE-15CR-1MO MARTENSITESCONTAINING NITROGEN OR CARBON

Hardness measurements, dilatometry, internal friction measurements, Mo ssbauer spectroscopy and transmission electron microscopy are utilized in order to study the effect of tempering on the microstructure of a stainless martensitic steel containing 15% Cr, 1% Mo and 0.6% N. A sim ilar carbon steel containing 15% Cr, 1% Mo and 0.6% C is used for comp arison. Tempering of alloy Fe-15Cr-1Mo-0.6N in the low temperature ran ge of 353-473 K leads to formation of hexagonal epsilon-nitride (Fe,Cr )2N, which is followed by precipitation of the orthorombic xi-nitride (Fe,Cr)2N at temperatures of 573-773 K. The hexagonal nitride Cr2N is precipitated at 923 K and preferably formed at grain boundaries. The a lloy Fe-15Cr-1Mo-0.6C shows the expected tempering behaviour. epsilon- carbide (Fe,Cr)2C and cementite (Fe,Cr)3C are precipitated during low temperature ageing, followed by the formation of Cr7C3 carbides after the temperature has risen to 873 K. With a similar interstitial conten t the amount of retained austenite in the nitrogen martensite is nearl y twice as high as in the carbon one. Furthermore, the thermal stabili ty of the retained austenite of the nitrogen alloy is substantially hi gher than that of the carbon steel.