| High Cr white iron（HCWI）materials have been widely used in the fields of machinery,mining,and metallurgical industries.The good wear resistance of HCWIs mainly results from the hard eutectic M7C3（M=Fe,Cr）carbides in the matrix,but large and coarse M7C3 carbides are detrimental to the performance of HCWIs.Many strategies such as addition of alloying elements and ceramic particles or rapid cooling have been adopted to modify the microstructure of HCWIs,but have not been very successful.In the present study,Ti3Al C2 was used as a precursor to in situ form Ti Cx in HCWI at high temperatures.In situ formed C-deficient Ti Cx grains distribute around M7C3 particles and inhibit their growth,achieving a fine microstructure.But a part of Al from Ti3Al C2decomposition dissolving in matrix contributes to the improved performance.In situ Ti Cx/HCWI composites have been prepared with pressureless sintering,hot-press sintering and spark plasma sintering technology,respectively.The influencing factors including the particle size of Ti3Al C2 precursor powder,heating rate,sintering temperature,and holding time on the dimension and distribution of Ti Cx and refinement of M7C3 were systematically studied.In addition,the development mechanism of Ti Cx in ferreous matrix was analyzed.Moreover,the Rockwell hardness（HRC）,flexural strength,fracture toughness and wear resistance were measured,and the influence of microstructure and bonding interfaces on the the above properties was investigated.Furthermore,comparative study on the microstructure,mechanical properties,wear resistance and oxidation resistance of two kinds of composites reinforced with directly added Ti C（Ti C/HCWI）and in situ formed Ti Cx（Ti Cx/HCWI）has been conducted.The following creative results have been achieved:（1）The in situ formed Ti Cx grains in the temperature range 1350-1500?C are spherical in shape,and 1-2?m in size.The fine Ti Cx grains distribute around M7C3particles and inhibit their growth,effectively refining the structure of M7C3.The nucleation and initial growth of Ti Cx obeys the classical crystal growth mechanism,but its following growth is controlled by nonclassical crystal gowth mechanism.（2）The particle size of Ti3Al C2 precursor,sintering temperature,heating rate,and holding time have not much impact on the dimension of in situ formed Ti Cx,whereas the sintering temperature and holding time can promote the homogeneous distribution of Ti Cxgrains in matrix.The controllability of key influencing factors lays the foundation for the large-scale preparation of Ti Cx/HCWI composites with a refined and stable microstructure.（3）Two-step sintering method,i.e.,pressureless-sintering at 1500?C for 5 min to guarantee the homogeneous distributing of Ti Cx and then hot-pressing at 1200?C under20 MPa for 1 h to achieve densification,was adopted to prepare dense 4.3～21.5 wt.%Ti Cx/HCWI composites.The mechanical properties and wear resistance were considerably improved in Ti Cx/HCWI composites as compared with unreinforced HCWI.A Rockwell hardness of HRC71.6,a flexural strength of 310 MPa,and a fracture toughness of 10.19 MPa·m1/2 were achieved in a 17.2 wt.%Ti Cx/HCWI composite;in particular,flexural strength and fracture toughness are over 7 times higher than those of unreinforced HCWI.In addition,the 17.2 wt.%Ti Cx/HCWI composite exhibits a good wear resistance of 315 cm-3 under a load of 123 N,which is more than 6 times that of unreinforced HCWI.Refined microstructure and strong bonding interfaces in Ti Cx/HCWI contribute to the improved performance.（4）Comparative study on the two kinds of 17.2 wt.%Ti Cx/HCWI and 17.2 wt.%Ti C/HCWI composites showed that the former has a finer microstructure and better mechanical properties and wear resistance than the latter.The oxidation test also demonstrated that the Ti Cx/HCWI composites have good oxidation resistance as compared with Ti C/HCWI,with a mass gain of 0.23 mg/cm2 after oxidation at 600?C for96 h in air.The oxidation kinetics obeys the parabolic law. |
PDF Full Text Request