Research On Microstructures And Mechanical Properties Of Medium Manganese Content Fe-Cr-B Alloy/42CrMo Steel Composite Roll

Roll is an important component in metal forming process.It is difficult to meet both requirements in strength and toughness at same time,which is made by a single metallic material.However,the composite roll which is fabricated by two different kinds of metals demonstrates excellent performances due to the combination of high hardness and wear resistance in its cladding layer with high toughness in its roll core.Thus,it has been the trend of the future roll.Fe-Cr-B alloy is a new type of material with ultra-high hardness,strength and excellent wear resistance,but poor impact toughness.The author tried to increase manganese content to enhance the toughness of the iron-based alloy.So,the experimental composite rolls that selected middle-manganese Fe-Cr-B alloys with different manganese-content as the cladding layer and 42CrMo low-alloyed steel as the core were fabricated.Research work mainly focused on cladding layers and composite interface.The microstructures,phase composition and distribution of alloying elements were analyzed by optical microscopy?OM?,scanning electron microscopy?SEM?,energy dispersive spectroscopy?EDS?and X-ray diffraction?XRD?.The macro-hardness of the cladding layer was tested by Rockwell hardness tester and impact toughness measured using corresponding samples.Based on these observation and analysis,the mechanism of the variation of microstructure and mechanical properties of the cladding layer and composite interface was explored.The research results were obtained as follow:The as-cast microstructure of the cladding layer is consists of matrix,elongated pre-eutectic borides,coarse eutectic crystallographic and honeycomb-like structure,which are composed of?-Fe,M₂B,M₃?C,B?,M₂₃?C,B?₆ and?-Fe.Moreover,the microstructure becomes more and more coarse,and the continuity and integrity of the eutectic boride network worse because of the difference of cooling rates from the outer to inner cladding layer.Correspondingly,the hardness decreases gradually and starts to rise slightly when closes to the interface.After quenching?1050?/30min,air cooling?and tempering?550?/45min?,the type of phases in the cladding layer is still unchanged while their proportions and morphology are changed,showing the coarse eutectic boride network partly broken and the matrix structure transformed into tempered martensite in which a large number of fine and dispersed particles precipitated.Correspondingly,the hardness of cladding layer increases and gets steady,but the impact toughness does not improve,to certain degree,comparing with the relevant as-cast samples.The interface microstructure of the as-cast composite roll is divided into three parts,namely,the cladding sub-zone on the left which is composed of the matrix and the reticular eutectic boride,the roll core sub-zone on the right which contains massive ferrite grains and pearlite groups,and the interface zone which is filled by fine acicular martensite.Due to the compositional difference between the middle manganese Fe-Cr-B alloy and 42CrMo low-alloy steel,the concentration of alloying elements decrease continuously from the cladding layer to the roll core.After quenching?1050°C/30min,air cooling?,the reticular eutectic boride in the cladding breaked,and the matrix transforms into lath martensite and trace amount of restrained austenite,the roll core sub-region into acicular martensite and a small amount of hard precipitated phases,while the interface microstructure into a dense acicular martensite structure.After tempering?550°C/45min?,the morphology of reticular eutectic boride does not change,but the acicular martensite in the matrix transforms into tempered sorbite,and the microstructure of the core sub-region turns into a tempered sorbite structure and a certain amount of massive precipitated phases,while the composite interface structure turns into dense tempered sorbite.Comparing with the corresponding as-cast samples,the impact toughness gets slightly improved after tempering.In addition,the increase of manganese content in the middle-manganese Fe-Cr-B alloy can fine the microstructure,and enhance the hardness of cladding layer,but has no significant effect on the microstructure and mechanical properties of composite interface.