Mechanical Properties Optimization of Structural Steels for Structural Applications by Austempering

Abstract

Combination of mechanical properties like strength, hardness and toughness are normally required for structural applications. These mechanical properties can be obtained by altering the microstructures of alloys. Heat treatment is the most common method to alter the microstructures of alloys to obtained desired mechanical properties. In this research work, the effect of different austempering temperatures and times on the mechanical properties of AISI 4340, AISI 4140 and 30CrMnSiA are investigated. Scanning Electron Microscope (SEM) and X- rays Diffraction were used for structural characterization of samples after each treatment. It was observed that most of microstructure consist of bainite along with different amounts of martensite and retained austenite and fraction of each phase in microstructure depends on the austempering temperature and time. Micro hardness test and impact tests were carried out in order to study the effect of microstructure evolution on the mechanical properties of each alloy after austempering. The results show that hardness of samples decreases as function of time up to first 60 minutes then it start increasing slightly. The effect of austempering conditions on impact strength show that impact strength of samples increases as function of increasing time up to 60 minutes and then it start decreasing. Both hardness and impact strength of the samples increases as a function of decreasing temperature. Mechanical characterization of samples indicate that development of dual microstructure of bainite-martensite impart better combination of mechanical properties. AISI 4340 especially austempered at lowest temperature provides best combination of strength and toughness among studied alloys.