Microstructure And Properties Of Cr5 Steel By Laser Surface Modification

The whole iron and steel industry meets some difficulties and has to transform product mix and trim capacity.For steel companies,it is imperative to reduce their production and capital costs,which have caused great pressure to steel companies by roll consumption and spares of backup rolls.Strengthening and remanufacturing of backup rolls by laser surface technologies both improve service life and implement rapid repair of backup rolls with different hardness,which reduce the roll consumption and spares of backup rolls and have great realistic meaning.As one critical part,backup rolls need high properties and stability,so it is extremely difficult to strengthen and remanufacture backup rolls.At present,some relatively mature technologies are used to strengthen and remanufacture backup rolls,for example bushing technology and surface welding.In this thesis,laser alloying technology and laser cladding technology are used to strengthen and remanufacture one specific backup roll.Composition and microstructure design are researched to solve the problems of preventing cracks,controlling of hardness and enhancing of wear resistance.Optical Microscope(OM),Scanning Electron Microscope(SEM),Electron Probe Microanalysis Analysis(EPMA),Transmission Electron Microscope(TEM),hardness tester and friction and wear tester are used to analyze microstructure and properties of Cr5 steel modified by different processes with different materials.Strengthening and wear mechanisms are also discussed.Laser alloying was implemented on Cr5 steel using Cr-B4C alloy powder.The in situ reaction between Cr and B4C provided Cr,B,C for laser alloying layers.Fe-based layer with hypoeutectic was obtained.Dendrites contained martensite and retained austenite.Inter dendrites consisted of lamellar ferrite and lamellar compounds.The thickness of eutectic lamellas ranged from 80nm to 150nm.With the increase of scanning speed,the fraction of eutectic lamellas increased due to the increase of alloy content in the molten pool.Then the heighten of precipitation strength enhanced hardness,wear resistance and crack sensitivity.The average hardness of the laser alloying layer prepared with the power of 2000W and the scanning speed of 8mm/s was 889.9HV,which was 1.8 times as much as that of the the traditional quenched layer(approximately 480HV).In the friction and wear test,the major abrasion type of the laser alloying layer and the traditional quenched layer was adhesive wear.The volume loss of the laser alloying specimen was only 29.5 percent of that of the traditional quenched specimen due to the thin eutectic lamellas.Fe-based alloy layers with excellent wear resistance can be obtained by laser alloying using Cr-B4C alloy powder.The hardness of laser alloying layers are higher than the requirement of the specific roll.The problem of crack has not been solved by changing scanning speed.To reduce crack sensitivity and adjust the hardness of laser alloying layers,Cr-B4C-Ni alloy powder was used to prepare laser alloying layers.The austenite amount increased and the fraction of eutectic lamellas decreased with the increase of Ni content in the alloy powder.Then lower precipitation strength and solution strength decreased hardness and crack sensitivity of layers.When the Ni content was 75wt.%,the average hardness of the layer was about 443.1HV,which fulfilling the requirement of the specific roll.In the friction and wear test,The volume loss of the laser alloying specimen was only 81.7 percent of that of the traditional quenched specimen.Laser alloying layers with suitable hardness and excellent wear resistance can be obtained using Cr-B4C-Ni alloy powder.Due to the addition of Ni,the crack sensitivity of laser alloying layers decreases.When F313 alloy powder was cladded on Cr5 steel by FL-DLS21-DLight-3000 type semiconductor laser,the optimized process parameters were 1600W,10mm/s,11.5g/min and 30%.Hardness of F313 cladding layers ranged from 600HV to 796HV,which was obviously higher than that of the specific roll.To adjust the hardness of laser cladding layers,pure nickel powder and iron powder were mixed with F313 alloy powder in various proprotions,respectively.Then the mixed powders were cladded on Cr5 steel.The austenite amount increased with the increase of Ni content in the alloy powders.The hardness of layers increased due to the lower solution strength.When the Ni content was 10wt.%,the average hardness of the layer was 505.1HV,which fulfilling the requirement of the specific roll.In the friction and wear test,the volume loss of the cladding specimen was 10 percent less than that of the traditional quenched specimen.Adopting 10wt.%nickel powder mixed with F313 alloy powder,wear resistant cladding layers with the hardness fulfilling the requirement of the specific roll can be obtained.The hardness of cladding layers decreased with the increase of Fe content in the alloy powder due to lower solution strength.When the additive amount of iron powder was 50wt.%,the average hardness of the cladding layer was 487.8HV,which fulfilling the requirement of the specific roll.In the friction and wear test,the volume loss of the cladding specimen was 15 percent less than that of the traditional quenched specimen.Adopting 50wt.%iron powder mixed with F313 alloy powder,wear resistant cladding layers with the hardness fulfilling the requirement of the specific roll can be obtained.By properly boosting the laser energy density,adopting the 316L alloy powder as the first layer on the neck of backup roll and conducting the preheating and heat preservation of cladding,a large area of DF105 cladding layer without apparent defect was obtained.The surface hardness of DF105 cladding layer was tested by a HL-hardness tester after turning process.The average hardness of DF105 cladding layers was 58.55HRC,which was higher than that of the specific roll.Microstructure of the DF105 cladding layer contained equiaxed dendrites and columnar dendrites.The hardness of columnar dendrites was a little higher than that of equiaxed dendrites.In the friction and wear test,the volume loss of the DF105 cladding specimen was 27.4 percent less than that of the traditional quenched specimen.A large area of wear resistant cladding layer without apparent defect can be fabricated using customized DF105 alloy powder.Its hardness is higher than that of the specific roll.