Laser Surface Alloying And Direct Deposition Of Roll Materials On Metallurgical Hot Rolls

Rolls are the main consumptive parts which can deform the metals and determine the efficiency of the rolling mills and the quality of the mill bars. The quality and lives of them have an important effect on the efficiency of rolling production, product quality and manufacturing costs. How to improve the wear resistance and elongate the service life of rolls matters a lot in reducing the consumption of rolls.In recent years, in addition to the improvement in the manufacture technique and materials of rolls, induction heat quenching, puddle-welding and thermal spraying are widely used to improve the wear resistance of rolls. But there are some drawbacks in the obtained coatings. Laser surface alloying (LSA) and laser-aided direct metal deposition (LADMD) can fabricate dense coatings with a metallurgical bonding with substrates, thus improving the wear resistance of the substrates. The aim of the present work is to use laser techniques to fabricate dense, wear resistant coatings which have a metallurgical bonding with the substrates. It can offer theoretical instructions on repair and strengthening of rolls by LSA, fabrication of new work layers by LADMD and technical support for their further application.The substrates for LSA were high-Ni-Cr infinite chilled cast iron rolls, nodular cast iron rolls and 70MnV cast steel rolls. The 1018 mild steel was used as the substrates for the LADMD experiments. High power laser was used to carry the experiment. Coatings with good metallurgical bonding with substrates were fabricated with optimal processing parameters. Optical microscopy (OM), scanning electron microscopy (SEM) attached with energy dispersive x-ray spectrometry (EDX), X-ray diffractometer (XRD), profilometer, microhardness tester and wear tester were used to analyze the microstructure, composition, the distribution and propagation of cracks, phase and wear behavior of the coatings.The laser alloying technique was applied to the high-Ni-Cr infinite chilled cast iron roll coated with C-B-W-Cr nano-carbide ceramics. The results revealed that the alloyed layers combined metallurgically with the substrate with a lot of cracks and pores found in part. With conditions that the laser power, spot diameter and overlap ratio remained unchanged, the thickness of the coating changed slightly with varying scanning speed, but the ratio of cracks increased and the hardness of the alloyed layer increased then decreased with the increase of scanning speed. The optimal scanning speed was 11 m/min, when the laser power, beam diameter and overlapping ratio were 7.2kW, 0.8-3mm and 33.3%, respectively. In this case, the average thickness of alloyed layer was 0.29mm with an average microhardness up to 1001HV_0.05, ie, about 1.53 times as high as that of the substrate of 656HV.C-B-W-Cr powders were used as the alloying powders on nodular cast iron rolls. The fabricated layers had metallurgical bonding with the substrates. The thickness of layers was 0.33-0.37mm. The number of pores and cracks went done and the hardness of the layer increased with the increase of the laser specific energy. The optimal processing parameters were: laser power 4kW, laser spot diameter 1.5mm and overlap ratio 33.3%. The thickness of this coating was 0.37mm and the average microhardness was 1201 HV, which was 1.4 times higher than that of the substrate. The layer was composed of proeutectic carbides and ledeburite eutectic which contained martensite, residual austenite and cementite. Wear test results at 500℃in ambient air showed that the wear resistance of the coating was 0.6 times higher than that of the nodular cast iron substrate. The wear mechanism of the layer was the mixture of adhesive wear, abrasive wear and oxidation wear.Metallurgical bonding was achieved between the 70MnV cast steel roll substrate and the layer with NiCr-Cr₃C₂ powders by LSA. The layer was dense, pore and crack free. As the scanning speed increased, the thickness of the layer and the HAZ decreased, the content of the retained austenite increased and the hardness and wear resistance of the layer increased. The phases were influenced by the scanning speed with only a little variation in the content. When the laser power is 4kW, scanning speed is 2.2m/min, spot diameter is 1.5mm and overlap ratio is 33.3%, the obtained layer had the highest hardness and wear resistance. In this case, the thickness and hardness of the layer is 0.48mm and 858HV_0.1, respectively. Wear test at 500℃in ambient air showed that the wear resistance of the layer was 8.8 times of that of the substrate after sliding for 904.32m. The improvement of wear resistance was contributed to the co-effect of the grain refinement, solution strengthening, the toughγ-Fe matrix,Cr₇C₃, Fe₃C and martensite hard phases and the good bonding between them.LADMD was used to fabricate Co-285, Co-285+30wt% and (Co-285+30wt%WC) +0.8wt%Y coatings on 1018 mild steel substrates.The optimal parameters for the Co-285 coating were laser power 0.8kW, spot diameter 0.5mm, powder flow rate 8.6g/min and the scanning, overlapping ratio 50% and scanning speed 0.375m/min. The coating was pore and crack free with a microhardness of 420HV_0.5 with the optimal parameters. The coating was composed ofα-Co solid solution, Cr₂₃C₆ and Co₃W. Volume loss of the coating after wear against the Al₂O₃ ball in the ambient air in the room temperature for 60min was 1.4mm3. Wear mechanism of the coating was a mixture of adhesive wear, plastic deformation, abrasive wear and oxidation wear.The optimal parameters for the Co-285+30wt% and Co-285+30wt%WC+Y coatings were: laser power 1kW, spot diameter 0.5mm, powder flow rate 8.5g/min, overlapping ratio 50% and scanning speed 0.3m/min. The average hardness of the coating without the undissolved WC was 751HV_0.5, which was 1.83 times of that of the Co-285 coating (420HV_0.5). Co-285+30wt%WC+Y coating was pore and crack free while the Co-285+30wt%WC coating had some micro-cracks and pores. XRD results indicated that the Co-285+30wt%WC coating was composed of WC, NiCoCr solid solution, W₂C, CoC_x, Cr₃C₂ and Cr₂₃C₆, while Co-285 +30wt%WC+Y coating was composed of WC, NiCoCr solid solution, W₂C, CoC_x, Cr₇C₃, Cr₂₃C₆ and Co₆W₆C. Volume loss of the Co-285+WC and Co-285 +WC+Y coatings after wear against the Al₂O₃ ball in the ambient air in the room temperature for 60min was 0.18mm³ and 0.17mm³, respectively. The wear volume loss of the Co-285 coating was 7.8 times of that of the Co-285+WC coating and 8.2 times of that of the Co-285+WC+Y coating. Wear mechanism of the two coatings was also a mixture of adhesive wear, plastic deformation, abrasive wear and oxidation wear, but the adhesive wear and abrasive wear of them were greatly suppressed. The Co-285+WC coating was remelted by laser to investigate the remelting effect on the microstructure and hardness of the coating. Results indicated that the microstructure in the remelting layer was uniform without pores or cracks and the WC had a large dissolution. Furthermore, the microhardness was improved from the 770HV_0.5 in the Co-285+WC coating to 963HV_0.5 in the remelting layer. Laser remelting is good to improve the quality of the deposited coatings. The addition of 0.8wt% Y reduced the residual stress, crack or pore ratio and hardness in the Co-285+WC coating, whilst improved the toughness and wear resistance a little without changing the wear mechanism.Laser surface alloying improved the wear resistance of the nodular cast iron and cast steel rolls. The effect is especially obvious when the roll material is of low carbon and alloying elements content. The deposited Co-285 coating is dense and of good properties, but the hardness and wear resistance of it need to be improved. The addition of WC can improve the wear resistance of the Co-285 coating and laser surface re-melting improved the distribution of microstructure and the microhardness of the Co-285+WC coatings. The addition of Y decreased the crack rate and the microhardness of the Co-285+WC coatings while improved the wear resistance of it at the same time.