| High boron iron based alloys are a sort of new wear resistant materials which contain much cheap boron element instead of expensive alloying elements. There are many hard borides and carbon-borides in the microstructure of the alloys, which mainly contribute for the high wear resistance of the materials. However, the net shape of the borides and carbon-borides greatly deteriorates the toughness and performance of the alloys, so that the alloys couldn’t be applied more widely. In the dissertation, the net shaped borides and carbon-borides in Fe-Cr-B-C system high boron iron based alloys were intensively researched so as to control and optimize their quantity, shape and distribution in the matrix, and to improve the mechanical properties and wear resistance of the alloys.First in this dissertation, the influence of the boron content (1.0%-2.5%) on the shape, quantity of the boride and carbon-boride phases, and the mechanical properties of the materials was researched, by keeping other elements stable, i.e., C 0.45%, Cr 5.0%, W 1.0%, Mo 1.8% and V 0.8%. The results show that, with the boron content increasing, the hard phases also increased in quantity, following an equation y= 15.765 x+ 1.3692, and became thick and coarsening in shape, at the same time the hardness of the materials increased gradually in 51.1-53.4 HRC, but the toughness decreased in 4.84～2.41 J/cm; the optimal property combination, i.e., hardness of 51.8 HRC and toughness of 4.35 J/cm2, could be obtained when boron was 1.5%。Then, the thermodynamics and mechanical properties of the hard borides and carbon-borides in high boron iron based alloys was calculated with the first-principle method. The resulted data show that, M2B type borides are more stable than the M3(B,C) typed carbon-borides in thermodynamics, and the stability and mechanical properties of both M2B and M3(B,C) phases could be improved by resolving Cr and Mo elements in.To control the shape of the hard phases in the high boron iron based alloys, single modificators such as rare earth (RE), magnesium, titanium were applied to treat a high boron iron based alloy with 1.5%B and 0.45%C, respectively, and the influence of the modification element content on the hard phases was researched. In case of RE modification, the net shape of boride and carbon-boride was refined. As for Mg modification, the refinement of the net shape of boride and carbon-boride was obviously, and the austenite was slightly refined. As for Ti modification, the wall of the net shaped hard phases became thinner, also the austenite was slightly refined. The nucleation effects of modification substances such as CeO2, Ce2O2S, MgS and TiC were found and discussed. The optimal content for each single modification element was obtained, i.e.,0.6%RE,0.5%Mg and 0.4%Ti, and as well as the corresponding size of the hard phases 8.75 μm,6.57 μm and 6.78 μm. As a result, the hardness and toughness of the alloy were 53.5 HRC and 5.1 J/cm2 for 0.6%RE modification,53.3 HRC and 4.7 J/cm2 for 0.5%Mg modification,53.5 HRC and 5.6 J/cm2 for 0.4%Ti modification, compared with 51.8 HRC and 4.35 J/cm2 without modification.Based on single modification of the alloy, complex modification with 0.6%RE+0.5%Mg+0.4%Ti was further researched. The results show that, hard phases were almost transformed from net shape to needle and particulate, with an average equivalent diameter of 5.26 μm, increased in number but decreased obviously in volume fraction; however, the alloying element content such as Mo, W and V in the matrix was increased; therefore, the hardness and toughness of the alloy reached 56.0 HRC and 9.0 J/cm2. The modification mechanism was also analyzed. The results show that, RE, Mg could eliminate O and S element in the iron melt; in the solidification, modification substances such as CeO2, Ce2O2S, MgS, TiC could work as heterogeneous cores of austenite and borides (carbon-borides) due to very low mismatch with them; Ce and Mg atoms could be absorbed at the surface of austenite or borides (carbon-borides) to block their coarsening, so hindered the formation of net shape; RE and Ti could also formed big inclusion to serve as heterogeneous cores of austenite and borides (carbon-borides).Based on the complex modification of the alloy, heat treatment was applied to control the net shape of the hard phases. The results show that, with the increasing of quenching temperature in 950℃-1100℃, both the size and the volume fraction of the hard phases increased slightly first, then decreased slightly. The optimal heat treatment was 1000℃ quenching and 500℃ tempering. In that case, the hard phases were completely transformed from net shape to particulate, with an average diameter of 5.9 μm, and increased in number; the mechanical properties of the alloy reached 1584.3 MPa for bending strength,59 HRC for hardness and 11.5 J/cm2 for toughness which was increased by 13.9% and 164.4%, respectively, compared with the alloy without modification.Finally, the high temperature wear resistance at 500℃ of the alloy without modification as cast (MO), the alloy by complex modification as cast (HO) and the alloy by complex modification and 1000℃ quenching + 500℃ tempering heat treatment (H2) were researched, compared with W18Cr4V high speed steel (HSS). The results show that, the wear resistance of M0, H0 and H2 reached 1.0,1.1,1.4 times of that of HSS. The wear mechanism of the alloys were adhesive wear, oxidation wear, abrasive wear and cracking of hard phases. The superior wear resistance of the alloy by complex modification and heat treatment was attributed to the the hard borides and carbon-borides with high hardness, even distribution and particulate shape, and lath like martensite matrix with high toughness and hardness.In sum, the dissertation controlled and optimized the shape, size and distribution of the hard borides and carbon-borides in Fe-Cr-B-C system high boron iron based alloys, by boron content optimization, complex modification, complex modification and heat treatment, obviously improved the mechanical properties and wear resistance of the alloys, analyzed and discussed the modification mechanisms of various modificators on the shape of the hard phases. The dissertation should contribute for the more wide application of high boron iron based alloys, and be helpful to related researches. |
PDF Full Text Request