Abstract
In order to obtain fundamental information on the characteristics in heat-treatment of graphitized structural low alloy steels, the effects of graphite size, austenitization temperature and additions of Ni and Cu on the behavior of austenitization of the graphitized steels were studied. The degree of austenitization was measured over a temperature range of 800°∼1000°C by means of the hardness test after water quenching.
The main results obtained were as follows:
(1) Time-temperature-austenitization diagrams of the graphitized steels shifted to the longer time side than those of normalized steels. For example, at 950°C, the graphitized steels required 2∼5 sec to start austenitization and 20∼2000 sec to finish, whereas the normalized steel 3∼5 sec to start and 7∼10 sec to finish.
(2) At a lower temperature (800°C or 850°C), the graphitized steels, especially one containing coarse graphite particles, showed a different austenitization behavior that proceeds in two stages.
(3) The steels of fine graphite particles were austenitized more quickly than those of coarse graphite.
(4) The graphite size affects the behavior of austenite nucleation, so that in the steels of coarse graphite particles, the austenite nucleates on the ferrite-graphite interface and on ferritegrain boundaries in the steels of fine graphite particles.
(5) Although the effects of additional elements were rather obscure, a small amount of Cu and Ni seemed to change the effects of the graphite size.
