Isothermal Treatment at Low Temperatures Applied to AISI 4340 Steel: Effects on Hardness, Toughness and Microstructure

Abstract

The effect of the parameters of isothermal treatment at low temperatures, also called Austempering, on the impact toughness, hardness and microstructure of AISI 4340 steel was investigated. For this purpose, specimens for impact toughness tests were machined according to ASTM E 23-93a. For hardness tests, prismatic bars of 10mmx10mmx30 mm were manufactured. Austempering was performed in a salt bath furnace using 55% sodium nitrate and 45% potassium nitrate. Three temperatures: 300, 325 and 350 °C, and four soaking times: 1 min, 10 min, 1 hour and 10 hours, were used. Hardness tests were performed on Rockwell durometer and three indentations per sample on HRc scale were made. The impact toughness test was done with the Charpy method. The specimens, previously machined, were subjected to austempering, and then subjected to the respective tests. The maximum hardness value 56.75 HRC was found at 300 °C with 10 min of soaking time, and the maximum impact toughness value was 27.67 J, found under the same conditions. It was observed that the hardness and impact resistance are not always inversely related during Austempering. In all microstructure tests, a biphasic structure composed of lower bainite with lath martensite in different proportions was observed, showing that the bainitic reaction is never complete. it is concluded that when alloyed steels are subjected to the Austempering treatment, a decrease in hardness with the soaking time or variation in temperatura, is not always accompanied by an increase in impact toughness, since undissolved alloyed carbides can produce embrittlement at any time and temperature interval.