Influence of Austempering Heat Treatment on Microstructure and Mechanical Properties of AISI 9255 High Silicon Steel

Abstract

The present work is mainly centered on austempering of high silicon steel at different austempering conditions and its influence on microstructure and mechanical properties of AISI 9255 high silicon steel. The tensile specimens machined as per the ASTM standard were initially austenitized at three levels of temperatures namely 850^oC, 900^oC and 950^oC for 30 minutes.Austempering was done in a molten salt bath at different levels of temperatures namely 280^oC, 320^oC, 360^oC and 400^oC for 90 minutes.

X-ray diffractometer, scanning electron microscope and optical microscope were used to characterize all heat-treated specimens. At lower austempering temperatures namely 280°C and 320°C the samples exhibit lower bainitic microstructure while the higher temperatures such as 360°C and 400°C exhibited upper bainitic microstructure. The specimen austempered at 280^oC for 90 minutes after austenitizing at 850^oC for 30 minutes showed highest UTS of 1680 MPa with a percentage elongation of 8.7 percent.