| The sharpness of domestic knives and scissors is poorer than that of foreign knives and scissors as carbide in high-carbon martensitic stainless steel used as knives and shears is large in size and quantity and unevenly distributed.In this study,8Cr13MoV steel was selected as the experimental steel.Based on the realization of carbide dispersion control,the formation and control technology of eutectic carbide,the critical control process of secondary carbide and nano-scale carbide were studied,which provided scientific basis for optimizing electroslag remelting process,heat working and heat treatment process and improved the sharpness of knives.Industrial scaled 8Cr13MoV electroslag remelting ingot with 300 kg in weight was dissected and analyzed.The types,distribution and growth behaviors of eutectic carbide in the ingot were ascertained,which laid the foundation of controlling eutectic carbide.The eutectic M7C3 carbides in 8Cr13MoV ESR ingot were composed of large-size bulky carbide,fibrous carbide and small-size spherulitic carbide with agglomeration behavior.The bulky carbide precipitated first,and then the fibrous and spherulitic carbide precipitated.Microsegregation and high cooling rate led to the rapid increase of supersaturation of alloy elements in the residual liquid phase,and promoted the transformation of eutectic carbide morphology from bulky to fibrous and spherulitic.The difference of cooling conditions at the top and bottom of electroslag ingot resulted in the inhomogeneity of size and distribution of eutectic carbide along the axis of electroslag ingot.The axial uniformity of the size and distribution of eutectic carbide was improved by gradually reducing the electroslag remelting current.Compared with constant current electroslag remelting,the eutectic carbide in the electroslag remelting ingot with decreasing current obviously gained better control.The volume fractions of eutectic carbide were 0.9%at the top center and 0.2%at the top edge of the modified electroslag remelting ingot while they were 1.8%and 1.5%in the traditional electroslag remelting ingot,respectively.The size and volume fraction of eutectic carbide could be significantly reduced by increasing homogenization annealing temperature and prolonging annealing time.The volume fraction of eutectic carbide in hot rolled plate decreased from 1.43%to 0.05%when the temperature and time increased from 1100℃-0.5 h to 1200℃-2 h,and eutectic carbide basically dissolved.The secondary carbide in 8Cr13MoV knives was large in quantity and unevenly distributed,which provided a strength increment less than 10 MPa and decreased the performance of knives.The quantity and distribution of large secondary carbide were effectively controlled and the hardness and sharpness of the knives were improved by solution treatment for annealed sheet.The results indicated that compared with the conventional austenitizing heat preservation treatment at 1050℃ for 5 min,after solution treatment at 1100℃ for 30 min,the number density of secondary carbides decreased from 0.19/μm2 to 0.03/μm2,the initial cutting performance was increased by 101.3%and the cutting edge retention was increased by 55.9%.It is difficult for 8Cr13MoV knives to obtain a large number of dispersed nano-scale carbides for secondary hardening during conventional quenching and tempering.Roll forging and delayed low temperature tempering were put forward.The results indicated that after roll forging at 600℃,the number of the carbides increased,the average size of carbides decreased,the grains were refined and the hardness and sharpness of the cutting edge were effectively improved.A large number of nano-scale carbides precipitated in 8Cr13MoV knives during delayed low-temperature tempering at 220℃,which resulted in significant secondary hardening effect and improved the sharpness of the knive.Compared with tempering at 220℃ for 6 h,the initial sharpness of the scissors was increased by 98.7%,and the sharpness durability was increased by 56.0%. |
PDF Full Text Request