اثر دما و زمان آستمپرينگ بر مورفولوژي بينين و ريزسختي چدنهاي گرافيت فشرده آستمپر شده

Effect of Austempering Temperature and Time on Bainite Morphology and Microhardness of Austempered Compacted Graphite Cast Irons

In this research the effect of austempering temperature and time on the bainite morphology and microhardness of the compacted graphite cast irons were investigated. For this purpose, unalloyed compacted graphite cast iron with a carbon equivalent of 4% was cast. The as-cast specimens were austenitised at 900oC for l00min. and austempered at 300 and 400oC for 2, 30, 60, and 90 minutes. All of the heat treated and also the as-cast samples underwent metallographic studies and their infrastructures were observed by optical microscope. The amount of retained austenite and bainitic ferrite were studied by X-Ray Diffractometry. To study the microstructure and distribution of the phases in the matrix microhardness evaluation tests with 50gr and l00gr loads were carried out. The results revealed that those samples austempered for 2min. showed martensite in the microstructure. Prolonging the austempering time removes the martensite and produces ferritic bainite and retained austenite. Nucleation of bainitic ferrite was observed within a very short time after starting the austempering. After 30min. the bainitic ferrite and retained austenite were formed and the second stage of austempering process was started. By increasing the austempering time the retained austenite starts to decompose and its amount decreases. XRD observations revealed the greatest amount of the retained austenite for austempering process at the time of 30min. Lower bainite and upper bainite were observed at the austempering temperatures of 300oC and 400oC respectively. Microhardness measurements showed minimum hardness at 30min.