| Pack boriding treatment is a widely used surface treatment method to improve the service life of H13 hot work die steel. In order to improve the shortcomings of high temperature and long processing time, using surface deformation pretreatment,adding rare earth(RE) element, employing direct current field and some other composite boriding processing technologies have aroused widespread concerns.Assisted by high-energy shot peening pretreatment(HESP) and catalysis effects of rare earth, the low temperature pack boriding at 580℃ was achieved by our team. In this paper, the high temperature friction and wear properties of the high temperature(850℃) pack borided layer composed of Fe2 B single-phase, assisted by HESP pretreatment and assisted by catalysis effects of RE element, and the low temperature(580℃) borided layer were studied. The surface roughness of the samples, as well as the thickness, microstructure, hardness and phase composition of borided layers were researched through optical profiler, scanning electron microscopy(SEM), microhardness tester and in-situ nano hardness testing system,and X ray diffraction(XRD), respectively. The friction coefficient and wear volume of samples were measured by UMT-3 high temperature wear test equipment and optical profiler. The macro and micro morphologies of worn surfaces, the relative content of oxygen of the debris, the phase composition of worn surfaces were analyzed by the SEM, energy spectrum analysis(EDS), XRD and some other test facilities. In addition, the wear mechanisms of borided layers were discussed. The main conclusions are as followings:1. The Fe2 B monophase borided layer was fabricated in annealed H13 steel with the pack boriding treatment at 850℃ after the subsequent vacuum heat treatment.The microhardness of the borided layer is about 1600HV0.1 and its thickness is about37μm. The friction coefficient decreased with the increasing of temperatures and its mutation was due to the layers began to wear out. Conversely, the wear volume increased with the increasing of the wear test temperature, and it climbed up rapidlyat 600℃.2. At the wear test temperature of 400℃, the main wear mechanisms of Fe2 B monophase borided layer obtained from high temperature pack boriding at 850℃ are mild oxidation wear and fatigue wear. At 500℃, the main wear mechanism transforms into oxidation wear. Above 600℃, oxidation wear is the dominant wear mechanism. At 700℃, the wear mechanism is severe oxidative wear and fatigue spalling of the borided layer appeared.3. Compared with the borided sample without HESP, the microhardness of borided layer assisted by HESP increased about 100 HV, and its thickness also increased by about 30%, but the surface roughness increased significantly. The valley morphology induced by HESP can play a role of storing debris and then reduce initial friction coefficient. The wear volumes of borided samples assisted by HESP were all lower than the borided samples without HESP under the same test condition.4. The oxide scales formed on the wear surface of the high temperature(850℃)borided steel assisted by HESP and the bare steel are Fe2O3, and the main wear mechanism was oxidative wear. Owing to the compressive residual stress and the refined grains of borides assisted by ABSP, the fatigue spalling was not found in the borided layer along the edge of wear scar, while the delaminations of borides appeared on the sample without ABSP.5. The thickness, microhardness and densification of the borided layers were improved significantly by adding 2.5% and 5% RE in the boriding agent, while the surface roughness increased slightly. Compared to the borided sample without RE in the boriding agent, the thickness of borided layer assisted by 5% RE in boriding agent increased by about 27%, and the microhardness increased by about 60 HV.When the content of RE is 10%, the borided layer possessed the maximum thickness,but its microhardness and densification decreased obviously, and the surface roughness increased to Ra 3.32.6. Compared to the borided sample without RE in the boriding agent, the worn out time of sample assisted by 5% RE in boriding agent prolonged two times and its wear volume decreased 21%. When compared with the sample without boriding treatment, the wear property of sample assisted by 5% RE in boriding agent increased by nearly 1.6 times. The sample with boriding treatment assisted by 5%RE in boriding agent possesses relatively superior high temperature wear performance.7. After low temperature pack boriding treatment, Fe2 B and Fe B dual phase layer was obtained in H13 steel. The surface roughness of low temperature borided sample assisted by HESP was lower than the that of the polished sample, which is about Ra 1.87 and Ra 2.06, respectively. The hardness and densification of low temperature borided sample assisted by HESP are higher than those of polished sample, and their nano-hardness are 23 Gpa and 18 GPa, respectively.8. When the wear test temperature is 300℃, the main wear mechanism of low temperature borided layer is fatigue wear and slight oxidative wear. At 400℃, the wear mechanism began to change to oxidative wear. Above 500℃, oxidative wear intensified. The wear resistance of low temperature borided layer prepared in the present study decreased significantly at 600℃, which cannot effectively protect the substrate. |
PDF Full Text Request