Understanding the low temperature end of the hot ductility trough in steels

The low temperature end of the hot ductility trough has been examined for steels which have been solution treated at similar to 1300 degrees C before tensile testing in the temperature range of 1000-600 degrees C. Failure in the trough in this region is intergranular ductile and occurs by strain intensification in the thin film of ferrite surrounding the prior austenite grain. The strain causes voiding to occur at the inclusions situated at the boundaries, the cavities gradually linking up to give failure. In steels which are solution treated before tensile testing, the depth of the trough is shown to be controlled by the volume fraction of the second phase particles, their size and the separation between the particles. Recovery in ductility on the low temperature side of the trough is solely dependent on being able to produce a sufficiently large quantity of ferrite to prevent strain concentration (similar to 40%). Often this has to await the test temperature falling below the AR(3) in which case wide troughs are formed. However, if conditions are right, very narrow troughs can be produced in which the ferrite that is formed is deformation induced. The width of the trough at the low temperature end of the trough is shown to decrease with increase in strain rate, refinement of the austenite grain size, increase in cooling rate from the solution treatment temperature, decrease in the volume fraction of sulphides situated at the austenite grain boundaries and reduction in the Mn and C contents. The depth of the trough decreases in a similar manner with all these variables except for C and Mn, where for the former there is no effect and for the latter, increasing the Mn level reduces the depth. Narrow troughs on this side of the trough are dependent on being able to form deformation induced ferrite in sufficiently large amounts so as to improve the ductility at temperatures above the AR(3). A model is proposed to account for most of these observations.