Equal Channel Angular Pressing Process Of Martensitic Steel Structure Evolution And Mechanical Properties Study

Equal Channel Angular Channel Pressing(ECAP)is one of the important methods to obtain bulk nano-metals.The refinement mechanical of grains can be fulfilled by three mechanisms during ECAP:The first one is through the severe plastic deformation causing dislocation and dislocation boundary to segment large grains,which caused the grain refinement—dislocation segment mechanism.The second is by severe deformation forming the deformation twins,then twins boundary segmenting large grains,which causing the grain refinement—twins refinement mechanism.The third is with severe deformation inducing martensitic phase transformation to producing the new grains,which leading to the grain refinement—phase refinement mechanism.In this paper,martensite heat-resistant steel was selected to carry out ECAP test at room temperature.Microstructural evolution and the changing of second-phase particles' appearance and distribution were researched in detail;Mechanical behaviors,effect of annealing methods on both structures and properties of ECAPed samples,the mechanisms of making materials tenacious by ECAP+annealing treatment were also researched.After 1 pass ECAP of martensite heat-resistant steel,in the effects of shearing force, martensite laths had been distort and fragment,and formed a large number of ultrafine sized dislocation cell.In the effects of severe shearing force,the re-dissolution of second-phase particles were happened.The room temperature tensile properties showed that the strength of yield and tensile had a obviously increase(the yield strength had increased～52%).But uniform plastic deformation ability was dramatically reduced and almost completely lost their ability of work hardening.The deformed samples had been annealed at different temperature.The results showed that with the increasing of annealing temperature,the phenomenon of tissues reversion were more and more prominent.In the condition of 680℃and 720℃annealing,a small amount of recrystallization was formed.The second-phase particles precipitated in the matrix and displayed diffuse uniform distribution.The second-phase particles were smaller than initial samples.With the increasing of annealing temperature,the room temperature tensile properties(yield and tensile strength)of annealing samples was declined rapidly.Meanwhile,the ability of uniform plasticity deformation restored in different degree.In the condition of 680℃annealing treatment,the uniform plasticity deformation ability was similar to the level of initial state,and the strength was higher than initial state.720℃for uniform plastic deformation ability and strength was higher than the initial state.After 4 passes ECAP,grains changed into equiaxed shape completely.The average size of grains was between 200nm and 300nm.Organization was composed of a large number of ultra-fine grains and sub-grains(or dislocation cell).Room temperature tensile properties showed that with the increase of pressing passes,the strength was increased gradually.But the speed of strength increase was slow.4 passes samples attained the highest strength(σ_b=1638MPa,σ_0.2=1515MPa).All of the sample's work hardening ability were lower。After 680℃2h and 6h annealing treatment of each passes,reversion and recrystallization were happened in varying degrees.And with the increase of extrusion passes,the phenomenon of reversion and recrystallization is more obvious.Grains internal had a lower dislocation density.The tensile properties showed that the strength in higher passes annealing treatment samples is lower than the former one's.The uniform elongation was similar,but the fracture elongation was increased.After 1 Pass ECAP,the increasing strength increase of samples is the results of dislocation strengthen and fine-grians strengthen.After annealing treatment of samples,the effects of dislocation strengthen were weakened.The second-phase particles re-precipitated, and in diffuse uniformly distribution.Meanwhile annealing treatment acquired stable ultra-fine grains.The second-phase particles played the role of strengthening by precipitation strengthen(Orowan mechanism)and hindrance grains growth.