Study On Surface Strengthening Technology Of Cooling Roll Main Shaft

Cooling roll,as an important part of metallurgical equipment,which works between precision rolling and coiling in the hot rolling process to transport and cool the finished strip.The cooling roll main shaft is required to have high hardness,corrosion resistance and wear resistance due to cold and thermal shock,alternating load and sliding friction of high-temperature steel plate when working.Entrusted by the local metallurgical equipment company,this research explores the surface strengthening of the key parts of the cooling roller main shaft-journal and key groove by laser quenching instead of the original nitriding process of the cooling roller main shaft.Owning to the actual production conditions of the industry,QPQ salt bath nitriding and laser quenching were performed on 42Cr Mo steel substrate to obtain the relevant performance data of the two processes.The optimized laser quenching process parameters were obtained by orthogonal test,and the key groove of the cooling roller main shaft was tested to verify the feasibility of process replacement.(1)After QPQ salt bath nitridation with different process parameters,42Cr Mo steel can obtain dense surface nitridation layer,whose surface hardness is higher than the matrix.When the nitridation temperature is560?and the nitridation time is 120min,the nitridation effect is the best.The depth of nitridation layer is 200?m,the surface hardness is657.1HV_0.2,which is about 2.5 times that of the substrate.The friction coefficient is 0.418 that is 29.9%lower than the substrate,and the wear amount is 0.7mg that is only 29.2%of the substrate.The main wear mechanism is abrasive wear.The corrosion voltage of the sample is-0.76928V,and the self-corrosion current density is 4.2759×10^-6A/cm².The corrosion resistance is improved.(2)Based on the original process parameters of the enterprise,the spot size is fixed at 13mm×2mm.The orthogonal experiment was designed,taking laser power,scanning speed and overlap amount as three factors,each factor taking three levels,making L₉(3⁴)orthogonal test table,and obtaining the optimized process parameters:laser power2000W,scanning speed 8mm/s and overlap amount 2mm.The surface structure of the sample is dense.The cross-sectional structure consists of transformation hardening zone,transition zone and matrix.The depth of hardened layer is 800?m and the surface hardness is 771.0HV_0.2that is about 2.9 times of matrix's.The friction coefficient is 0.394 that is 33.9%lower than the substrate's and the wear amount is 0.4mg that is only 16%of the substrate.The main wear mechanism is abrasive wear.The corrosion voltage of the sample is-0.76037V,and the self-corrosion current density is 2.1613×10^-6A/cm²,and the corrosion resistance is greatly improved.The optimized technological parameters were used to laser quench the key groove of the cooling roller main shaft.The macroscopic appearance of the quenching zone on the inner surface of the key groove is good.The hardened layer depth is 600?m,the surface hardness value is 679.5HV_0.2,the friction factor is 0.408,the wear amount is 0.6mg,the corrosion voltage is-0.669095V,and the self-corrosion current density is 4.6604×10^-6A/cm².Since the laser beam has a certain angle with the inner surface of the key groove during processing,the energy density decreases due to the incomplete vertical scanning,which leads to a slight decrease in surface hardness,wear resistance and corrosion resistance compared with the vertical scanning sample,but all meet the performance requirements of the key groove.(3)From the performance parameters of laser quenching surface hardness,hardening layer depth,wear resistance and corrosion resistance,it is feasible to replace the nitriding process of the cooling roller main shaft by surface modification of the key parts of the journal and key groove of the cooling roller main shaft by laser quenching process,which can effectively reduce the production cost and produce positive economic benefits.The optimized laser quenching process parameters in this research have been incorporated into the enterprise process documents for guiding production.