Reseach And Development Of New Martensitic Phase Chang Wear-Resisitant Materials

Paper pulp grinding discs are the core and wearing parts of the pulp section of the paper pulp system,consuming a huge amount in the pulp production line and also a key component in determining the quality of the pulp.As we all know,China is the birthplace of papermaking technology.However,due to the late start and development of wear-resistant materials for domestic pulp grinding discs,immature application,and insufficient investment,the low quality of domestic pulp grinding discs cannot meet the high quality development requirements of the domestic pulp industry,seriously hindering the development of high-end domestic pulp equipment.This study summarizes and analyses the problems that exist in the mill production site by following up on the pulp mill manufacturing plant in the field and recording its actual production situation.Secondly,relevant information was reviewed to understand the composition,microstructure and preparation process of foreign advanced grinding discs,and to rationalize and improve the production process and disc composition according to the actual production situation.Furthermore,the heat treatment process of the grinding discs is investigated,and the microstructure evolution mechanism of martensitic stainless steel for pulp grinding discs is investigated researched,with the characterization and testing methods of OM,SEM,EDS and TEM.The new grinding discs are used in the pulping line to compare the service life and stability of the discs with those of advanced foreign grinding discs.In this course,the failed discs are conducted in the failure analysis and data from the analysis is fed back into production.This work is carried out in a closed-loop feedback process to promote the step-by-step development of high-quality pulping discs,and the quality of the discs has been improved in stages up to now.(1)Improved preparation process: The complex working environment of the abrasive discs involves a combination of slurry impact,abrasive wear,and corrosion wear.The new wear-resistant material has been redesigned to control the precipitation and composition of each phase,and the internal microstructure of the material.The pouring process has been changed to a teapot-type pouring package(ladle)for the pouring operation,and zirconia type filters are used in the clad sand type.Quantification of the non-metallic inclusions was performed with metallurgical microscopy.The results showed that the casting molding rate of the abrasive flakes increased by 5% after teapot ladles and filters have changed.The number and size of non-metallic inclusions were significantly reduced.This work provides a detailed and reliable practical basis for the exploration and improvement of the molding process for pulp making mill flakes.(2)Heat treatment process: The heat treatment process is divided into three steps,namely: annealing → quenching → tempering.High temperature annealing can relieve the internal stress concentration of the grinding disc and promote the microstructure homogeneity.By doing so,a better austenite phase was attained in high-temperature,which prepared the disc for the next quenching process.This study shows that the ferrite content of the grinding discs increases with the extension of the annealing holding time at 1050°C.After grinding discs were heated and oil-quenched with a box-type muffle furnace,their microstructures showed good hardness properties.However,the surface of the grinding teeth and air contact generated an oxidation layer,which even led to surface decarburization,affecting the surface performance of the grinding discs.In view of these problems,the vacuum oil quenching heat treatment furnace was used to explore the grinding disc process,which effectively avoided the peeling and decarburization layer on the surface of the grinding disc and achieved good results.The final tempering treatment relieved the stress concentration generated by quenching,formed tempered martensite,and promoted the role of the second phase precipitation.This work provides an effective and reliable practical and theoretical basis for the exploration of heat treatment processes for pulping mill blades.(3)The developed grinding discs were used in the pulping line,and meanwhile their service life and circumstances were tracked.The results showed that the service life of the grinding discs has increased by 50% compared to the previous improvement,and no abnormal failures such as broken teeth have occurred,indicating they are in good condition.Failure analysis shows that internal inclusions are one of the main causes of wear and aggravation of wear.The spherical carbides in the precipitated carbides have the effect of pinning the grain boundaries of the grinding disc matrix and also have the tendency to inhibit wear.Tensile fracture analysis confirms that the cast grinding discs have a high number of defects and low toughness.This work clarifies the forms of failure of pulp grinding discs and provides valid and reliable practical data for the next stage of pulp grinding disc quality.