Toughneing Mechanism Of Q&P Heat Treatment On High Boron Iron-Based Alloy And Its Effect On Microstructure And Wear Resistance

High boron iron-based alloys are a sort of wear-resistant materials with boron as the main alloying element.The hardness of boride（mainly Fe₂B）is higher than Cr₇C₃and Fe₃C,which contributes a lot to the wear resistance of the alloys.Moreover,boron with excellent neutron absorption ability also broadens alloys application.However,the net-like or fish-bone like distribution of brittle phase Fe2B results in extremely low toughness of the alloys.In view of this deficiency,the composition design of experimental alloys were carried out in this desertation.The first principle was applied to determine the addition amount of Mn,and the quenching and partitioning（Q&P）heat treatment was used to improve the net-like distribution of boride and the microstructures of matrix.The effects of high temperature austenitizing temperature and Q&P process parameters on the microstructure and mechanical properties of the alloy were investigated,and the mechanisms of toughening and wear resistance improvement were also clarified.The first-priciples calculation results show that the Mn doping is beneficial to improving the toughness of Fe₂B.When the doping concentration of Mn in the Fe₂B unit cell is 5.6 wt.%,the charge density between B-B becomes highest and the weak B-B bond along the[002]crystal direction is enhanced,which improve the intrinsic brittleness of Fe₂B.Based on the results,the addition amount of Mn in the 1.6 wt.%B iron-based alloy was determined to be 4.0 wt.%.Additionally,Al and Si elements are also added to the alloy to suppress the precipitation of carbides during Q&P heat treatment.The solidification microstructure of the alloys re composed of proeutectic austenite dendrites and eutectic structure.The eutectic boride shows net-like or fishbone morphology,while the dendritic structure and the austenite cooling structure in the eutectic are martensite and retained austenite（RA）.The improvement of net-like distribution of boride,caused by modification,increases the i MPact toughness of the alloys by about 6.91%～11.75%,but degrades their wear resistance.High temperature austenitizing promotes the fracture or spheroidization of net-like or fish-bone like boride,which increases the i MPact toughness of the alloys by more than 15.0%,with a maximum value of 5.02 J·cm^-2,but the hardness are decreased slightly.After being Q&P heat-treated,the matrix of alloys are composed of fresh martensite（FM）,secondary martensite（SM）and RA.With the decrease of quenching temperature（Tq）,the volume fraction of RA increases gradually.In addition,the morphology of RA is mostly changed from blocky into film-like,and its distribution also becomes uniform.But less amout of residue austenite after quenching,caused by too low Tq,will result in the decrease in RA volume fraction.With the increase of Tq,the hardness of alloys decrease,while the values of impact toughness increase first and then decrease.When Tq remains unchanged,the volume fractions of RA will gradually increase with the extension of quenching time（tq）.The hardness of the alloys will gradually decrease,and the i MPact toughness will increase first and then decrease.The increase of partitioning temperature（Tp）can promote the increase of carbon partitioning,but too high Tp（440℃）will widen the spacing between the martensite laths and lead to the decomposition of some residue austenite.With the increase of partitioning time（tp）,the volume fractions of RA increase gradually,and the mismatch between RA and martensite increases from 0.1896 to 0.196.When tp is extended to more than 60 s,more carbides will be precipitated in the matrix.The fracture of net-like boride and the increase of film-like RA effectively improve the toughness of the alloys.The maximum impact toughness of the alloys after being Q&P heat-treated are between 7.52～8.21 J·cm^-2,which is more than75.0%higher than that of the as-cast alloys.In terms of wear resistance,the strong TRIP effect induced by film-like RA can improve the hardness of the worn surface.In addition,the volume expansion caused by martensitic transformation can effectively contain boride particles.Therefore,the alloys can maintain good wear resistance even if the hardness are decreased.