Microstructure Refinement And Stengthening And Toughening Mechanisms Of High Chromium Hot Work Die Steels By Trace Nanoparticles

The service life of hot work die steels is directly determined by its comprehensive performance,such as impact toughness,high-temperature tensile property,thermal fatigue resistance,high-temperature wear resistance,etc.The main factors affecting the comprehensive service performance are the high-temperature microstructure stability,high-temperature softening resistance,and oxidation resistance of the hot work die steels.High strength and toughness at room and high temperatures are important indicators,however,the bottleneck of the traditional enhancement and toughening methods for hot work die steels has been reached at present.In order to break through the bottleneck of strengthening and toughening,starting from refining the microstructure,this work proposes a new way to improve the antioxidant property by increasing the Cr content of steel,synchronized with the homogeneous dispersion of nanoparticles in the melt,to achieve refining the microstructure of the hot work die steels,strengthening the room temperature toughness of the hot work die steels,significantly enhancing the high temperature microstructure stability and antioxidant resistance,and finally achieving the purpose of improving the service performance.In this paper,single-phase TiC and dual-phase TiC+TiB₂ nanoparticles reinforced high chromium steels were prepared for the first time by using the master alloy method to successfully incorporate nanoparticles into high chromium steels and achieve they uniform dispersion.The effects of single-phase TiC and dual-phase TiC+TiB₂nanoparticles on the room temperature and high temperature mechanical properties of high chromium hot work die steels were investigated,and the mechanism of nanoparticles manipulating microstructure,the strengthening and toughening of high chromium hot work die steel was analyzed.The cold-heat fatigue resistance of high chromium steels enhanced by single-phase TiC and dual-phase TiC+TiB₂ nanoparticles was studied,and the strengthening mechanism of the nanoparticles to enhance the cold-heat alternating fatigue resistance of high chromium steels was revealed.The following three main innovations were achieved:1.Based on the multi-scale in-depth comparative analysis of the microstructure evolution of single-phase TiC and dual-phase TiC+TiB₂ nanoparticles enhanced high chromium steels,the manipulation mechanisms of nanoparticles on the microstructure evolution of high chromium steel hot work die steel were revealed.In liquid-solid phase transformation,nanoparticles promote the nucleation of austenitic dendrites,hinder the growth of austenitic dendrites,and inhibit the segregation of solute atoms.During forging,nanoparticles inhibit dynamic recovery and recrystallization and hinder the growth of recrystallized grains.In quenching and tempering,grain refinement provides more grain boundaries to promote martensite nucleation and make the slats smaller and more uniform.The higher dislocation density,chromium content,and more uniform solid solution of elements provide nucleation sites and nucleation energy for precipitates,which promotes finer and more dispersed nanocarbide phases.2.Based on the comparative analysis of room and high temperature mechanical properties and friction and wear behaviors of high chromium hot work die steels reinforced by single-phase TiC and dual-phase TiC+TiB₂ nanoparticles,the strengthening and toughening mechanisms of high chromium hot work die steels reinforced by nanoparticles at room temperature and the high temperature was revealed.The yield strength,tensile strength,and impact toughness of the high chromium hot work die steel at room temperature were 1810 MPa,1556 MPa,and 480 J/cm² with the addition of 0.02 wt.%TiC+TiB₂ nanoparticles,which were increased by 9.9%,23.6%,and 33.0%,respectively.At room temperature,200℃and 400℃,the volumetric wear with the addition of 0.02 wt.%of TiC+TiB₂nanoparticles was categorized as 1.88×10⁸μm³,4.22×10⁸μm³,and 3.22×10⁸μm³,which increased by 72.6%,54.4%,and 73.8%,respectively.Room temperature mechanical properties strengthening mechanism:fine grain strengthening,Orowan strengthening,and thermal mismatch strengthening,among which Orowan strengthening is the most dominant reinforcement mechanism.Room temperature toughness enhancement mechanism:nanoparticle-manipulated high chromium steels grain refinement is conducive to intergranular coordination deformation so that the crack extension path increases with the deflection of cracks and delays fracture.High-temperature strengthening and toughening mechanism:more dispersed carbides at the grain boundary prevent the dissolution of tempered martensite and improve its high-temperature stability;more diffuse nano-carbides at grain boundaries pinned grain boundaries,hindering grain boundary slip.Room temperature wear mechanism:fine grains and fine precipitates prevent the initiation and expansion of cracks in the surfaces and subsurfaces of the contact surfaces,reduce material stripping and spalling,and lower the degree of abrasive wear.High-temperature wear resistance mechanism:nano-strengthened high chromium steels is strong enough to withstand higher temperatures and stresses,reducing plastic deformation and softening,decreasing adhesive wear,and reducing oxidative wear.Moreover,the high-strength matrix prevents the intrusion of abrasive particles and reduces abrasive wear.3.Based on the comparative analysis of the cold-heat fatigue resistance behavior and oxidation resistance of single-phase TiC and dual-phase TiC+TiB₂ nanoparticles enhanced high chromium steels,the mechanisms of oxidation resistance and cold-heat alternating fatigue resistance of nanoparticles enhanced high chromium hot work die steels is revealed.At 1000,2000,and 3000 thermal fatigue cycles,the crack extension lengths of the high chromium steels with 0.02 wt.%TiC+TiB₂nanoparticles were 92.18μm,396.32μm,and 908.29μm,respectively,and the thermal fatigue crack lengths were reduced by 48.3%,49.4%,and 33.4%,respectively.Nanoparticles enhance the finer and more uniform microstructure of high chromium steels so as to inhibit crack initiation and hinder crack extension during cold-heat fatigue cycling,and improve the ability of high chromium steels to resist cold-heat fatigue crack initiation and extension.The carbide is finer and uniformly dispersed,and the uniform fine microstructure shows good coordination in the process of cold-hot fatigue,which promotes stress dispersion,prevents stress concentration,and inhibits crack initiation;the finer the grain,the more grain boundaries,the stronger the hindering effect on crack extension,so that the crack growth path becomes longer,to delay crack extension.It is revealed that the addition of nanoparticles to high chromium steels can reduce the thickness of the thermal fatigue oxide layer,improve the structure of the oxide layer,form a thicker and denser Cr_xO_ylayer,inhibit oxidation pitting,improve the oxidation resistance of high chromium steels during the thermal fatigue cycle,and prevent the initiation and expansion of oxidation cracks and thermal fatigue cracks.The coefficient of thermal expansion between the oxide and the matrix of the die steels matrix is different,and less oxidation forms a smaller stress concentration,which slows down the further initiation and extension of cracks.