Effects of Intercritical Heat Treatment on Phase Transformation, Microstructures and Mechanical Properties of Hypoeutectoid Steel
Xu Zuoren;Wang Zifang
A study has been conducted of the intercritical heat-treated AISI 4340 steel withprior martensitic structure. Actually, the phase transformation from martensite toaustenite during the intercritical treatment may lead to nonequilibrium situations. De-termination of compositional changes and quantitative changes due to partitioning can-not be accurately obtained from the equilibrium phase diagram. Under these circums-tances, martensite undergoes isothermal reaustenitization at an appropriate temperaturein the (α+γ) two phase region of the equilibrium phase diagram (730℃--760℃). Thepercentage of transformation may increase with increasing time until all the acicularferrites are annihilated. The rate of reaction increases with increasing temperature.These phenomena can be explained through thermodynamic analysis. Acicular and glo-bular austenite morphologies are thus formed, depending on the austenitilizing tempera-ture and the heating rate. In nature, they are dependent on the thermodynamic drivingforce as well as on the diffusion rate and distance of the alloying elements. The mechanical properties and the fracture behavior of the two phase alloy withboth martensitic and acicular ferritic structures have also been ascertained and studied.Experiments showed that the dual phase structure would lead to an increase in tough-ness while the strength remained essentially unchanged. The nature of the fracturesurfaces at room temperature indicates that a critical strain model is more applicable.The positive effect of acicular ferrite on toughness can be explained by the arrest ofcracks originated in martensite in the ductile ferrite therm of the normal strain distri-bution in the yielded zone. Grain refinement by intercritical treatment may also bebeneficial for enhancing toughness.