| Laminar cooling roller is one of the key equipments in laminar flow cooling system, which is in the laminar cooling phase between finishing export and reeling machine. It is a transmission equipment of strip steel.Laminar cooling rolls contact the hot strip steel directly, so the loss caused by wearing is amazing. And the surface quality and serving time of the laminar cooling rolls have important effects on the efficiency of production, product quality and manufacturing costs. Thus, how to improve and repair the wearing surface and elongate the service life of the laminar cooling rolls a lot in industry is important. Thermal spraying, spray welding and induction heat quenching are widely used to improve the surface properties of laminar cooling rolls. But there are some drawbacks in the processing craft and properties. As a new surface treatment technology, laser cladding can obtain excellent properties coatings with a metallurgical bonding between the coating and the low-cost substrate. The aim of the present work is using laser cladding techniques to fabricate coatings with wear and corrosion resistance and firm bond, improving the serving life and finding a new way to strengthen and repair laminar cooling roller.In this paper, powder side-delivered laser cladding is used to obtain substrate and working coatings with metallurgical bond on the 45# steel substrate with optimal laser processing parameters. OM, SEM, XRD, micro-hardness, electrochemical workstation and wear tester are used to analyze the microstructure, composition, phase constituents, micro-hardness, wear and corrosion resistance properties at high temperature of the coatings.For the powder of DL2501, the optimal processing parameters of single-track laser cladding substrate layer are spot diameter 3 mm, defocusing amount 340 mm, Powder feeding speed 11.5 g/min, laser power 4000 W, scanning speed 800mm/min. At the processing parameters, single-track laser cladding substrate layer has clean and good forming quality surface with metallurgical bond. There is no defect like hole and flaw in the coating. Its hardness is higher (552.3 HV) and more stable. he cladding zone displays typical laser rapid solidification morphology, which includes plane crystal, columnar crystal and dendrite. The microstructure of the heat-affected zone is quenched martensite. The substrate microstructure is still pearlite and ferrite.Multi-track laser cladding layer without defect like hole and flaw is obtained on matrix at 0.5 overlapping ratio based on the optimal laser processing parameters. From the matrix to the surface of cladding layer, the contents of alloy element Cr increase obviously, Ni, Si and Mo increase slightly. While elements Fe and C decreases. Due to the effect of dendritic segregation, content of Fe is higher in grain, while intergranular content of Cr, Ni and Mo is higher. XRD analysis shows that the cladding substrate layer consists of a-Fe, Cr-Ni-Fe-C,Cr7C3 and some carbide of Mo. Element Si distributed evenly between each phase in the form of solid solution.For the powder of DL155, the optimal processing parameters of single-track laser cladding substrate layer are laser power 4000 W, scanning speed 400 mm/min. At the processing parameters, single-track laser cladding substrate layer has clean and good forming quality surface with metallurgical bond. There is no defect like hole and flaw in the coating. Its hardness is higher (748.3 HV) and more stable.Multi-track laser cladding layer without defect like hole and flaw is obtained on substrate layer at 0.5 overlapping ratio based on the optimal laser processing parameters. From the substrate layer to the surface of working layer, the contents of alloy elements Cr and Ni decrease slightly, While element Mo increases. Content of other elements remain the same. The analysis result of the working layer is basically identical with the substrate layer.Laser working cladding has higher corrosion potential and impedance, lower corrosion current and corrosion ratethan the metal matrix. The working layer has not only excellent corrosion resistance but thermal fatigue resistance. After the thermal fatigue test, there is not any flaw and drop on the surface. And the metallurgical bond of laser cladding is very firmly and stable. Furthermore the working layer has the outstanding wear resistance of high temperature. The main wear form of working layer is abrasive wear. After wear for 30 min, the weight loss (0.0239 g) of the laser cladding layer is 25.78% of that of the 45# steel (0.0927 g). |
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