Abstract
The requirements placed on closed-die-forged parts of advanced steels have been increasing recently. Such forgings demand an innovative approach to both design and heat treatment. It is important to obtain high strength and sufficient ductility in closed-die forgings. High strength, mostly associated with martensitic microstructure, is often to the detriment of ductility. Ductility can be improved by incorporating a certain volume fraction of retained austenite in the resulting microstructure. Among heat treatment processes capable of producing martensite and retained austenite, there is the Q&P process (Quenching and Partitioning). This process is characterized by rapid cooling from the soaking temperature to the quenching temperature, which is between Ms and Mf, and subsequent reheating and holding at the partitioning temperature. Thus, strength levels of more than 2000 MPa combined with more than 10% elongation can be obtained. This experimental programme involved steels with 2.5% manganese. Forgings of these steels were heat treated using an innovative process in order to obtain an ultimate strength of more than 2000 MPa combined with sufficient elongation. Thanks to a higher manganese level, the Mf was depressed as low as 78°C, and therefore quenching was carried out not only in air but also in boiling water. Holding at the partitioning temperature of 180°C, when carbon migrates from super-saturated martensite to retained austenite, took place in a furnace. The effects of heat treatment parameters on the resulting mechanical properties and microstructure evolution in various locations of the forging were studied.
Highlights
Closed-die forgings made by hot forming a steel stock are shaped with the use of plastic deformation [1]
quenching and partitioning (Q&P) processing leads to predominantly martensitic microstructures which typically contain 10 or more percent retained austenite [15,16,17,18,19,20]
3 Discussion of results Experimental treatment which included the Q&P process led to mostly fine martensitic microstructure with retained austenite and a small fraction of bainite in all parts of the forging (Figs. 4 – 8)
Summary
Closed-die forgings made by hot forming a steel stock are shaped with the use of plastic deformation [1]. The process routes which are currently used with classical forging steels have been optimized over the years to a point where there is no more space for substantial enhancement and improvement of mechanical properties This applies in particular to strength, service life and safety where the last-named aspect is provided mainly by the fracture behaviour of the particular steel. This article introduces one of the available processes for achieving high strength and sufficient ductility: quenching and partitioning (Q&P) This advanced technique has a major weakness in that the quenching temperature must be controlled and kept above the Mf. In terms of practical implementation, Q&P processing is relatively straightforward with steel workpieces of constant cross-section where temperature field control and behaviour is substantially simpler than in intricate parts. Highstrength steels exhibit good ductility, with elongation values around 10% which is a relatively high value for martensitic materials
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