COMPARATIVE STUDIES ON MECHANICAL PROPERTIES OF AISI 4340 HIGH-STRENGTH ALLOY STEEL UNDER TIME- QUENCHED AND AUSTEMPERED CONDITIONS

Abstract

In the present work, the mechanical properties and microstructures of AISI 4340 high strength alloy steel under different conditions are investigated. Generally time quenching is substituted for surface hardening wherein martensite structure is formed in the case of the softer products towards core but austempering gives always uniform grained bainitic structure without any residual stresses and retained austenite. Austempering is given to hot working dies and springs to obtain maximum elastic limit. To assure uniform finer grains in the specimen normalizing treatment is given prior to other treatments. Samples were prepared and all of them were normalized at 9000C for 2 hrs and then air cooled to room temperature. After normalizing specimens are separated into 2 groups, timed quenching is performed on one set and austempering heat treatment is performed on another set of specimens. During timed quenching, the specimens are heated to 9000C. After reaching 900 0C, the specimens are soaked at that temperature for two hours. The specimens are then taken out of the furnace and quenched in engine oil for less than 4 seconds followed by air cooling. Hardness, microstructure and impact tests were performed on the timed quenched specimens to analyze the hardness value, grain size distribution and the energy absorbed respectively. Another set of the normalized specimens were taken and austempering was carried out on them. During austempering the specimens were heated at 9000C for 2 hrs in one furnace followed by transferring the specimens to the second furnace immediately within 10 seconds, which was maintained at 3200C and the specimens were soaked in the second furnace for 2 hrs and 45 min and then furnace cooling was done Hardness, microstructure and impact tests were again performed and obtained values were compared with timed quenched specimens. Difference in the mechanical properties of AISI 4340 steel at timed quenched and austempered conditions was observed. It is found that grain size of the austempered specimen is higher than the time quenched specimen. It is also observed that the austempered specimen has maximum strength and very good impact energy as compared to that of the timed quenched specimen. As a result mechanical properties like tensile strength, impact strength are improved in the austempered specimens than the timed quenched specimens.