· Tratamientos térmicos ·
A A Nofal, H Nasr El-din and M M Ibrahim
Central Metallurgical Research and Development Institute, (CMRDI), Cairo, Egypt. 67th WFC
This work aims at studying the influence of thermomechanical
treatment, either by ausforming just after quenching and before the onset of
austempering reaction or either by cold rolling after austempering.
In the first part of this work, ausforming of ADI up to 25% reduction in height
during a rolling operation was found to add a mechanical processing
component to the conventional ADI heat treatment, thus increasing the rate of
ausferrite formation and leading to a much finer and more homogeneous
ausferrite product. The kinetics of ausferrite formation was studied using both
metallographic as well as XRD-techniques. The effect of ausforming on the
strength values was quite dramatic (up to 70 and 50% increase in the yield and
ultimate strength respectively). A mechanism involving both a refined
microstructural scale and an elevated dislocation density was suggested.
Nickel is added to ADI to increase hardenability of thick section castings, and
ausforming to higher degrees of deformation is necessary to alleviate the
deleterious effect of alloy segregation on ductility.
In the second part of this work, the influence of cold rolling (CR) on the
mechanical properties and structural characteristics of ADI was investigated.
The variation in properties was related to the amount of retained austenite (γr)
and its mechanically induced transformation. In the course of tensile
deformation of ADI, transformation induced plasticity (TRIP) takes place,
indicated by the increase of the instantaneous value of strain-hardening
exponent with tensile strain. The amount of retained austenite was found to
decrease due to partial transformation of γr to martensite under the CR strain.
Such strain-induced transformation resulted in higher amounts of mechanically
generated martensite. The strength and hardness properties were therefore
increased, while ductility and impact toughness decreased with increasing CR
reduction.
