Effect of preform heat treatment on the flow formability and mechanical properties of AISI4340 steel

The present study reports the effects of variation in preform heat treatment on the flow formability and mechanical properties of flow formed tubes of AISI4340 steel. Preforms are subjected to three different types of heat treatments namely, annealing (ANN), spheroidizing (SPH) and hardening and tempering (HT). Sound quality seamless tubes have been produced by a two-pass backward flow forming method. Tensile properties in terms of yield strength, ultimate tensile strength (UTS), and percentage of elongation have been evaluated for both the preforms and subsequently flow formed tubes. The present study demonstrates that a combination of very high strength (UTS = 1320 MPa) with reasonable ductility could be achieved for flow formed HT tubes, whereas a combination of moderate strength (UTS > 1050 MPa) with good ductility has been obtained for flow formed ANN and SPH tubes in the as-flow formed condition. Evolution of tensile properties and flow formability of the differently heat-treated preforms are correlated to the microstructural features. Effects of microstructural features and mechanical properties of the preforms on the flow formability have further been evaluated by monitoring spindle load, spring-back, and ovality of the formed tube after each pass. It has been shown that microstructural features such as size and shape of secondary phases and strain hardening characteristics of the preform play a critical role in obtaining good flow formability in the present material.