ARAKR İTİK SICAKLIKLARDAN ÖSTEMPERLENMİŞ FERRİTİK KÜRESEL GRAFİTLİ DÖKME DEMİRDE YENİ FERRİT OLUŞUMU.

In the present study, during austempering, microstructure formations in the ferritic ductile irons austempered from intercritical temperature ranges have been investigated. For this purpose a ferritic ductile iron was intercritically austenitised (partially austenitised) at intercritical temperature ranges and then austempered in a salt bath held at austempering temperature of 365°C for various times. Following austempering from various intercritical annealing temperatures, microstructure variation graphics were created to illustrate the transformation of products quantitatively as a function of austempering time. Metallographic measurements showed that during austempering, the total volume fraction of the transformed phases from the original austenite formed at intercritical austenitization temperature is approximately equal to the volume fraction of original austenite for all austempering times. Within each of the austempered specimen from intercritical temperature ranges, in addition to proeutectoid and bainitic ferrite, the new ferrite (epitaxial ferrite) introduced into the structure. The new ferrite volume fraction increased with increasing intercritical austenitizing temperature but transforming percentage of new ferrite from the original austenite present at intercritical austenitizing temperature increased with decreasing intercritical austenitizing temperature. Increasing the austempering time causes new ferrite, bainitic ferrite and high C austenite to displace martensite. Some specimens were also conventionally austempered from 900°C for comparison. The new ferrite was absent in the conventionally austempered samples. [ABSTRACT FROM AUTHOR]