Calorimetric and Mössbauer spectroscopy examinations of ADI

The study presents a new approach to the mechanism of isothermal transformation in ADI. The discussion was based on the results of calorimetric analysis of the thermal effects that take place in the specimen when heated after austempering and on the results of analysis of the phase composition of cast iron examined by Mossbauer spectroscopy. The examinations were carried out on specimens taken from casting of 60 mm diameter subjected to austempering at 280, 330 and 380 o C. Upon heating, on the DSC diagram, a strong exothermic effect of about 20 o C range was observed to take place (it was not observed in the common cast iron), and its beginning, depending on the austempering temperature, fell to the range of 469 to 490 o C. The heat of this effect was proportional to the austenite content in matrix. The Mossbauer examination of phase composition in a specimen „frozen” from 650 o C revealed the decay of austenite and the appearance of products of its decomposition, i.e. α3 ferrite and Fe2,4C type carbides. So, it has been concluded that the exothermic effect present upon heating of cast iron during austempering (and due to the decomposition of austenite) should correspond to the endothermic effect due to the energy absorption necessary for carbon to be „retained” in γst austenite during stage I of the transformation (γo→ γst + α1).