Improved lower temperature fracture toughness of ultrahigh strength 4340 steel through modified heat treatment

Abstract

A modified heat treatment has been suggested whereby lower temperature plane-strain fracture toughness (KIC) of 4340 ultrahigh strength steel is dramatically improved in developed strength and Charpy impact energy levels. The modified heat-treated 4340 steel (MHT-4340 steel) consists of a mixed structure of martensite and about 25 vol pct lower bainite which appears in acicular form and partitions prior austenite grains. This is produced through isothermal transformation at 593 K for a short time followed by an oil quench (after austenitizing at 1133 K and subsequent interrupted quenching in a lead bath at 823 K). The mechanical properties obtained at room temperature (293 K) and 193 K have been compared with those achieved using various heat treatments. Significant conclusions are as follows: the MHT-4340 steel compared to the 1133 K directly oil-quenched 4340 steel increased theKIC values by 15 to 20 MPa • m1/2 at increased strength and Charpy impact energy levels regardless of the test temperature examined. At 193 K,KIC values of the MHT-4340 steel were not less than those of the 1473 K directly oil-quenched 4340 steel, in whichKIC values are significantly enhanced at markedly increased strength, ductility, and Charpy impact energy levels. The MHT-4340 steels compared to austempered 4340 steels at 593 K, which have excellent Charpy impact properties, showed superiorKIC values at significant increased strength levels irrespective of test temperatures. The lower temperature improvement inKIC can be attributed to not only the crack-arrest effect by acicular lower bainite but also to the stress-relief effect by the lower bainite just ahead of the current crack.