Study On Process And Microstructure Of Laser Cladding High Speed Steel On Metallurgical Roll Surface

Metallurgical roll materials have high loss,easy failure characteristics and practical requirements for remanufacturing.Laser cladding technology provides a new way for surface strengthening,remanufacturing and life extension,and it is an important technical means to save materials,protect the environment,improve production efficiency and reduce production costs.Based on the laser cladding for metallurgical roll surface strengthening and remanufacturing technology,a tough,wear-resistant and corrosion-resistant metallurgical bonding working layer can be prepared on the surface of the inexpensive roll core material,which significantly improves the overall performance of the roll surface.The purpose of this study is to use a multi-pass cladding and multi-layer gradient cladding to prepare a metallurgical bonded clads with excellent structure and performance on the surface of common metallurgical rolls,and to repair and remanufacture metallurgical rolls.It would provides theoretical basis for surface cladding and remanufacturing and repairing metallurgical roll materials by laser cladding technology,and provides technical support and data for industrial applications.In this paper,45 steel and Nodular Cast Iron(NCI)metallurgical roll core materials are used as the base material to explore the feasibility of laser cladding different types of high speed steel for surface strengthening and repair.By optimizing the laser cladding process parameters,an abrasion resistant cladding layer with good metallurgical bonding is prepared.Optical microscopy(OM),scanning electron microscope(SEM)with energy dispersive spectroscopy(EDAX),X-ray diffractometer(XRD),Vickers microhardness tester,PARSTAT4000 electrochemical workstation,CST550 electrochemical noise/galvanic corrosion tester and analysis instrument such as high temperature friction and wear tester are used to study the microstructure,composition,phase,hardness,corrosion resistance,galvanic corrosion and high temperature wear performance of the prepared laser cladding layer,and analyze the wear and corrosion mechanism of the cladding layer and the electrochemical compatibility under the service conditionsIn this paper,a metallographic method based on material science is proposed.The metallographic method is used to observe the shape of the cladding layer by metallographic observation,and determine the shape,size and integrity of the cladding layer and the bonding state with the matrix.As while,the microhardness distribution on the cross section of the cladding layer is determined and the wear resistance of the cladding layer is inferred.Under suitable process conditions,the cladding layer has regular morphology and metallurgical bonding,high hardness and uniform distribution.Metallographic method is simple,efficient and easy to implement,suitable for a variety of laser cladding process optimization.With Nd:YAG pulsed laser,the suitable process parameters for single-pass cladding T15 high-speed steel on the surface of nodular cast iron are current 240A,scanning speed 2.5 mm/s,defocusing amount 13mm,preset powder thickness 0.2mm.The addition of 50%Ni60 self-fluxing alloy can effectively control the cracking of the cladding.The suitable process parameters for cladding M2 high speed steel are current 210A,scanning speed 2.5 mm/s,defocusing amount 13mm,preset powder thickness 0.2mm.The suitable process parameters for cladding 10V high speed steel are current 180A,scanning speed 2.5 mm/s,defocusing amount 14mm,preset powder thickness 0.2mm.Preheating the matrix can reduce cladding cracking,remelting can avoid 10V cladding crackingUsing high-power CO2 gas laser,the suitable process parameters for single-pass cladding T15 and YT12 on the surface of nodular cast iron are laser power 1800W,scanning speed 6mm/s,spot diameter 4mm,preset powder thickness 1mm.Ni60 transition layer can effectively avoid cladding cracking.The suitable process parameters for single-pass cladding DL2501 and DL155 on 45 steel surface are laser power 4000W,spot diameter 4mm,lateral feed rate 11.5 g/min,and scanning speeds of 800 mm/min and 400 mm/min,respectively.In the case of the high-power direct output semiconductor laser,the suitable process parameters for single-pass cladding 10V on the surface of ductile cast iron are laser power 2300W,scanning speed 10 mm/s,spot size 11.5x2.5mm,preset powder thickness 1mm.On the basis of the laser cladding process optimization experiment,under the appropriate process conditions,the multi-pass laser cladding samples with smooth surface,continuous overlapping and regular shape were obtained,and The effects of laser cladding process on cladding process,microstructure and properties of cladding layer were studied.Cladding cracking occurs when the T15 high speed steel is clad on the surface of nodular cast iron by pulsed laser.The phase composition of the laser cladding layer contains ?-Fe,WC1-x and V4C3,etc.The average hardness of the cladding layer is 600HV,and the highest value is 682HV,which is about 2.2 times that of the nodular cast iron matrix.The use of pulsed laser to clad M2 high speed steel on the surface of nodular cast iron does not cause cracking.The laser cladding layer is densely structured,metallurgically combined with the substrate,and has no defects such as crack and pores.The morphology of the M2 high-speed steel cladding layer is characterized by a small amount of cellular crystals and a large number of dendrites.The W,Mo,Cr and V elements in the cladding layer are enriched in the dendrites,and the phase composition contains ?-Fe and ?-Fe,WC1-x,etc.,the average hardness of the cladding layer is 750 HV,which is about 2.4 times that of the ductile iron matrix.When the cladding layer is worn at a high temperature of 500 ? for 30 minutes,the weight loss of the clad is about 56%of that of the nodular cast iron matrix,the friction coefficient is significantly reduced and stable,and the main wear mode of the cladding layer is abrasive wear.High temperature wear resistance is greatly improved compared with nodular cast iron base.Cladding cracking occurs when a 10V high-speed steel is cladded on the surface of nodular cast iron by pulsed laser.After remelting the laser clad layer,the obtained remelted layer is densely organized,metallurgically bonded to the substrate,and has no defects such as crack and pores.The remelted 10V high speed steel cladding layer contains a large amount of VC-rich spherical particles;the average hardness of the remelted layer is 650HV,which is 2.1 times the microhardness of the matrix.The wear loss of the remelted layer at a high temperature of 500? for 30 minutes is about 50%of that of the nodular cast iron matrix.The main wear mode of the remelted layer is abrasive wear.Using high-power continuous CO2 laser,NTT and NYY multi-pass and multi-layer laser cladding samples with smooth surface,continuous overlapping,regular shape and Ni60 transition can be prepared on the surface of nodular cast iron.The thickness of the cladding layer prepared is 1400?m?1600?m,no macroscopic and microscopic defects such as cracks and pores,and metallurgical bonding with the matrix.The Ni60 transition layer is an effective way to avoid cladding cracking.The microstructure of the NTT cladding layer is mainly composed of a Ni-rich phase and a dispersed phase of a relatively high content of alloying elements such as V,W,and the content of precipitated phase increases slightly from the substrate to the surface of the coating.The NTT cladding layer is composed of austenite phase,V4C3,WC1-x,CoCx and other multi-alloy phase coexisting structures,in which V4C3,WC1-x,CoCx and other hard phases play the role of dispersion strengthening.The maximum hardness of NTT cladding layer It is 716HV,which is 2.2 times that of the substrate.The weight loss rate of 500? high temperature wear for 30 minutes is about 36%of the nodular cast iron matrix,which has excellent wear resistance.The NYY cladding layer microstructure consists mainly of Cr-depleted dendrites and a second phase of Cr-rich,C-rich elements formed between dendrites,with a dendritic wall of up to 500 ?m.The NYY cladding layer is ?-Fe,(Fe,Ni),Cr7C3 and other phases.The strengthening Cr7C3 phase is mainly distributed between the grain boundaries.The maximum hardness value of the NYY cladding layer is 739HV,which is about 2.3 times that of the matrix.The weight loss rate of 500? high temperature wear for 30 minutes is about 85%of the nodular cast iron matrix,which has better wear resistance.A gradient cladding layer can be prepared on the surface of 45 steel using a high power continuous CO2 laser.The clad has continuous layer without defect such as crack and pores and metallurgical bonding with the 45 steel matrix.Replacing the base material and adjusting the cladding material composition has effect to reduce the difference in thermal properties and is an effective way to avoid cracking.The microstructure of the cladding layer is characterized by rapid solidification of various morphologies such as dendrites and equiaxed crystals,which are composed of ?-Fe,CrNiFe-C and Cr7C3.The composition of the alloying elements in the cladding layer is uniformly distributed macroscopically,and Mo and Cr are enriched in the dendrites in the microdomains.The microhardness is a gradient distribution with a surface hardness of 748HV and a transition layer hardness of 552HV,which is 3.74 and 2.76 times the hardness of the matrix,respectively.Under the optimized process conditions,the laser power is 4000W and the scanning speed is 400 mm/min.The wear of the cladding layer is 25.78%of the wear of the substrate under the same conditions.The wear mechanism of the cladding layer is abrasive wear,and the wear of the matrix is abrasive wear and adhesive wear,the cladding layer has good wear resistance.A 10V high speed steel cladding layer can be prepared by using a semiconductor high power direct output semiconductor laser.The obtained clad has continuous layer and continuous flatness and is free of cracks and pores,and is metallurgically bonded with the nodular cast iron substrate.Replacing laser type from Nd:YAG to SC laser to change the interaction between the laser and the material is an effective way to avoid the phenomenon of pulsed laser cladding cracking.The microstructure of the cladding layer is characterized by lamellar,massive,spherical particles and other rapid solidification structures,composed of ?-Fe,VC,y-Fe,Cr23C6 and FeV.The composition of the alloying elements in the cladding layer is uniformly distributed macroscopically,and the microdomains are segregated by V.The cladding layer has a microhardness of 700HV,which is 2.2 times the matrix hardness of 310HV.Under the optimized process conditions,the wear of the cladding layer is 43%of the wear of the substrate at 30 min wear.The wear mechanism of the cladding layer is abrasive wear,friction and wear and 750? thermal shock test results show that the cladding layer has good wear resistance and thermal shock resistance.The polarization curve and EIS test results show that 45 steel along the substrate direction to the cladding working layer,the corrosion difficulty sequence is DL155 working layer>DL2501 substrate layer>45 steel matrix,in a gradient state.Compared with the substrate,the corrosion current density and corrosion rate of the matrix are one order of magnitude lower,which indicates that the corrosion resistance of the 45 steel roll surface after laser cladding treatment is significantly improved.It can be seen that the high Cr,Ni element and fine grain strengthening effect in the cladding layer can enhance the corrosion resistance.The M2 high-speed steel clad is about 400mV higher than the corrosion potential of the substrate,and it is not easy to electrochemically corrode,but its self-corrosion current and corrosion rate are not obvious,indicating that the corrosion resistance of the M2 high-speed steel coating to the NCI matrix is different,improved but not significant.The equivalent circuit of the laser cladding layer of 45 steel and NCI substrate in 3.5 wt%NaCl corrosion medium is R(QR),and the corrosion resistance mechanism is surface physical mechanical barrier.45 steel/DL2501,45 steel/DL155 and NCI/M2 galvanic couples in 3.5wt%NaCl solution galvanic corrosion occurred and the SEM morphology showed obvious pitting corrosion.According to HB 5374-87,the galvanic pair is E-grade corrosion,the galvanic corrosion is poor,and the electrochemical is incompatible.The galvanic corrosion kinetics are all anodic activation polarization,and the even geometry of the large cathode and small anode accelerates anodic dissolution.However,under the actual working conditions,the defect-free roll which is locally strengthened and repaired by laser cladding technology,exhibit the geometry of the large anode and small cathode in the repaired area and the substrate will inhibit the galvanic corrosion and behave as electrochemical compatibility in the service environment.