Thermomechanical treatment of austempered ductile iron

摘要

The production of lightweight ferrous castings with increased strength properties became unavoidable facing the serious challenge of lighter aluminum and magnesium castings.The relatively new ferrous casting alloy ADI offers promising strength prospects,and the thermo-mechanical treatment of ductile iron may suggest a new route for production of thin-wall products.This work aims at studying the influence of thermomechanical treatment,either by ausforming just after quenching and before the onset of austempering reaction or 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 compared 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 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,while 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.