Effect of Austempering Treatment on Microstructure and Mechanical Properties of D6AC Steel

Document Type : Original Articles

Authors

khodamorad Abbaszadeh

hasan Saghafian

shahram Kheirandish

10.22067/ma.v22i2.10907

Abstract

In this research, the influence of austempering time on the microstructure, tensile and impact properties of the ultrahigh strength D6AC steel was studied. For this purpose, samples were first austenitized at 910° C for 40 minutes, and then austempering treatment at 330° C for 3.5, 5, 7.5, 10, 12.5, 15, 20 and 1440 minutes with the subsequent oil-quenching stage was conducted in order to obtain different steel microstructures. Steel microstructures were examined using the optical and scanning electron microscopes. Mechanical properties of steel samples were determined by performing the tensile and Charpy impact tests. The results showed that the yield and tensile strengths of the samples with lower bainite-martensite microstructure first increased and then decreased with increasing the austempering time when compared with the martensitic steel. However, its impact energy and ductility always increases with increasing the austempering time. Microstructural examinations showed that the onset of a peak in the strength-austempering time curve is due to the partitioning of the prior austenite grains by lower bainite and the plastic constraint which is induced on the lower bainite by the surrounding hard martensite. The fracture surface of impact samples with lower bainite-martensite microstructure showed a mixed mode including the quasi-cleavage and the ductile fracture represented by dimples. However, the fracture surface of the impact sample with martensitic microstructure exhibited the brittle intergranular fracture phenomenon.

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