The influence of the heating rate on the point АС3 and on austenite grain growth

In the work the influence of the heating rate, temperature and soaking time in γ-area upon austenite grain growth in Сt6 steel of standard composition was investigated in this work. The experiments were carried out at the installation for complex investigations of alloys over a wide range of electric heating rate for the specimens of 10mm in diameter and 50mm long. Changes in the specimens’ temperature and length were registered. The critical point АС3 was determined by the inflection of the dilatometer curve and then it was defined more accurately metallographically using optical and electronic microscopes. АС3 point, determined by the dilatometer curve proved not to show austenite forming completion precisely. The metallographic method made it possible to establish that there still remained 1,5-2,0% of ferrite, which dissolves in austenite completely at subsequent temperature raise by 10-15ºC at increased and high heating rates. Original austenite grain, which is formed after the temperature has reached АС3 +10-15ºC depends substantially upon the heating rate within 0,1-5ºC interval. Subsequent acceleration of the cooling rate up to 10-30ºC/sec has practically no influence upon the original grain, which remains at 10 mcm level. With temperature growth from АС3 point to 1000ºC the original austenite grain grows mostly at low heating rate (0,1ºC/sec). An increase of the heating rate up to 5-30ºC/sec) results in substantial deceleration of grain growth. Isothermal soaking enlarges austenite grain the more, the more is the soaking temperature. At 1000ºC and 20 minutes’ soaking time the grain size is twice as big as that at 950ºC. Austenite grain grows even less when the soaking time is 900ºC.