LOW-TEMPERATURE IMPROVEMENT OF MECHANICAL-PROPERTIES OF AISI-4340 STEEL THROUGH HIGH-TEMPERATURE THERMOMECHANICAL TREATMENT

The low-temperature mechanical properties of AISI 4340 ultrahigh-strength steel can be dramatically improved by high-temperature thermomechanical treatment (HTMT). A comparison was made with the mechanical properties developed by the conventional heat-treatment (CHT). When the steel was hot forged by 50 pct reduction at 1177 K followed by direct oil quenching and subsequent tempering at 423 K, the slow-bending fracture energy of fatigue-precracked steels was significantly improved, with notably increased strength, owing to a moderate increase in ductility over the temperature range of 123 to 293 K, and the ductile-to-brittle transition temperature (DBTT) in the Charpy impact test was remarkably lowered. The marked development was achieved in the slow-bending fracture energy, with moderate improvement in strength, owing to significantly increased ductility over the temperature range of 123 to 293 K, when the steel was deformed by 50 pct reduction at 1473 K followed by direct oil quenching and subsequent tempering at 453 K. The treatment also improved the shelf energy and DBTT in the Charpy impact test. The improved mechanical properties are attributed to the subcell structure introduced in austenite by processing HTMT and inherited by martensite. The present work shows that an HTMT steel is attractive for low-temperature ultrahigh-strength applications when suitable combinations of deformation temperatures with tempering conditions are applied to the steels.