Thermodynamic Design Of Phase Equilibrium For High Boron HSS And Formation Mechanism Of Boron Carbide Hard Phases

The application o f high-speed steel has gradually expanded fro m traditional too l materials to high-temperature rolls,bearings,mechanical parts and high-end co ld-working mo lds,and the demand is increasing because o f its high hardness,excellent wear resistance and heat resistance.However,the traditional product ion cost of HSS wear-resistant materials is high because it contains a large amount of metal elements such as W,Mo,Cr,V and Co,and the,which seriously restricts the further pro mot ion and applicat ion of the material.In recent years,with the boost ing cos o f r aw metals,the development o f new wear-resistant materials with low production cost as to satisfy the requirements of working condit ions has beco me an important scient ific problem.The prvious reports show that boride hard phase with higher hardness and better thermal stabilit y can be obtained by adding proper amount of boron in tradit ional HSS wear resistant materials,which has a favorable applicat ion prospect.However,the effect o f boron on the precipitat ion behavior of hard phase of boron carbide and t he relat ionship between phase equilibrium thermodynamics and microstructure propert ies during so lidification areneeded to be further illuminated.In this paper,first ly,the so lidificat ion process,precipitat ion phase,microallo ying,heat treatmentparamet ersandstrength-toughness mechanism o f high-boron high-speed steel are analyzed,systemat ically.Moreover,the format ion mechanism and thermodynamic design method o f boron-carbide hard phase are also invest igated Based on the sub-latt ice lattice model,sub-regular solut ion model and densit y funct ional theory,a thermodynamic model of precipitated phase was constructed.As a result,the electronic structure and thermodynamic properties of the precipitates were elucidated.The influence o f allo y content on p recipitated phase was determined.The co mpet it ion mechanism of allo y elements during precipitat ion was revealed.The regulat ion mechanism of allo y elements o n Microstructure and properties was explored.Based on the results o f co mposit ion optimizat ion,a series of high-boron high-speed steel samples were successfully prepared.The microstructure,morpho logy and element distribut ion of as-cast allo y were studied.The mechanism of quenching and tempering on microstructure and the mechanism o f thermodynamic mo del contro lling the propert ies o f as-cast allo y were revealed and verified.Meanwhile,the dynamic process of allo y distribut ion and phase transformat ion in so lidificat ion behavior of high-boron high-speed steel was also studied.The growth behavior of primary austenite,eutectic precipitate,secondary precipitate and the format ion mechanism of precipitate were explored.Consequent ly,this workprovided theoret ical basis for further contro lling and improving the propert ies of boron high-speed steel.The fo llowing conclusions wre drawn:?1?First-principles calculations showed that the hardness o f boron-carbide phase increases with the increase o f B content,and the hardness o f W₂B was the highest?14.56 GPa?.On this basis,a thermodynamicmodelsuitableforthe precipitat ion phase o f Fe-M1-M2-M3-M4-C-B system was constructed,which was in goo d agreement with the results of Thermo-Calc thermodynamic software.In addit ion,the t ype of allo ying elements was an important factor affect ing precipitat ion and precipitation process.What's more,the B and Cr were beneficial to the precipitat ion of M₂?B,C?boron carbide,while The W and Mo could pro mote the precipitation of M ₃?B,C?boron carbide,and the V has the greatest influence on the precipitat ion o f M?B,C?boron carbide.?2?The optimum components o f thermodynamic design?B:1.4¹.6wt.%,C:0.40⁰.50 wt.%,W:0.5¹.0 wt.%,Mo:1.0¹.5 wt.%,Cr:3.0⁴.0wt,V:0.75¹.0 wt.%?were provided.It confirmed that as-cast microstructure consists of pearlite,ferrite,retained austenite,a small amount of martensite and boron carbide,among which boron carbide mainly distributes along grain boundaries.In the as-cast structure,there ws a certain segregat ion in the allo y distribution,and the carbon mainly distributed in the matrix,while boron mainly distributed in the grain boundary.Most allo ying elements such as W,Mo,Cr and V form boron carbides at the grain boundary with C and B,and a small amount of them exist in the matrix.?3?Under the guidance o f heat treatment,the matrix structure o f quenched high boron HSS transforms into martensite with good toughness.Meanwhile,the hardness and impact toughness were improved with the increasing o f quenching temperature.Tempering treat ment could promot e the transformation of retained austenite into tempered martensite.The optimum tempering temperature was 550?,and the highest hardness was64.4 HRC.The tribo logical behavior of high boron HSS mainly incorporated oxidat ion wear,adhesion wear and abrasive wear.When the quenching temperature was 1050?,the friction coefficient?0.425?and relat ive wear rate?79%?were the lowest?4?The characterist ic temperatures o f boron carbide precipitat ion were1350?,1220?,1100?and 1000?,respect ively.The microstructure o f primary austenite includeddendrite and eutect ic product?eutect ic austenite and eutectic boron carbide?which were distributed along grain boundar y after quenching at 1350?.M₂C carbides and M₂?B,C?boron carbides were sparated out as an interdependent mixed gr owth mode during the eutect ic reaction.What's more,The M₃?B,C?boron carbides precipitated at 1000?displayed bulk or granular,which were sparated out along the grain boundary between eutectic boron carbides and primary austenite or between eutectic boron carbides and eutect ic austenite.Consequent ly,the precipitat ion reaction o f M₃?B,C?boron carbide had undergone four stages:diffusion o f allo y elements,aggregat ion of allo y elements at grain boundaries,nucleat ion and growth o f M₃?B,C?,respect ively.