Modeling of flow stress of bainitic and martensitic functionally graded steels under hot compression

In the present paper, hot deformation behaviour of functionally graded steels (FGSs) is investigated by means of hot compression tests at different temperatures and strain rates. Functionally graded steels (FGSs) consist of ferrite (α), austenite (γ), bainite (β) and martensite (M) co-existing in different configurations and produced via electroslag remelting (ESR). Zener–Hollomon parameter (Z–H equation) is evaluated to describe the mean flow stress of FGSs (with αβγ and γMγ structures) as a function of temperature and strain rate. The values of stress power (n) and activation energy (Q) are also determined. A new analytical model is developed to determine the flow stress under hot deformation behaviour. The model considers a phase mixture law and boundary layers properties which are obtained by experimental investigation on single phase materials. Finally, the values assessed by using the new analytical frames are compared with those obtained from new experimental tests and a sound agreement is found.

► Hot deformation behaviour of αβγ and γMγ composites is investigated.
► The MFS is fitted by the Z–H equation based on the hyperbolic sine function.
► A theoretical model is proposed to describe the MFS of FGSs under hot compression.
► The model just requires the knowledge of the properties of the boundary layers.
► The error between the theoretical values and the experimental data is very limited.