Study On The Microstructures And Properties Of Laser Cladding On The Surface Of Brake Disc For Myriameter Deep Drilling Rig

Owing to suffering from drastic friction, high thermal load and large brake force, brake disc in drilling rig has three failure forms as following: high temperature oxidation, wear failure due to over-load brake and thermal fatigue crack. Thermal spraying and surfacing are often applied to strengthen or repair the surface of brake disc, which has great limitation.5kW laser with CO2 flow transverse was used for cladding Fe-based alloy and Fe/Cr3C2 powder on 35CrMo steel surface. The microstructures and phase compositions as well as the surface morphologies of sample after high temperature oxidation test, friction and wear test were analyzed by means of optical microscopy, scanning electron microscopy and X-ray diffraction. The content of alloying elements in the bonding zone and cladding zone with the surface oxide were studied by energy spectrum technology. The claddings'hardness together with resistance to high temperature oxidation, thermal fatigue, friction and wear were tested, and whose mechanism were also investigated.The results show that the microstructures of the laser cladding layers were fine and defect-free and good metallurgical bonding between the coating and the substrate was obtained. The main phase of Fe-based cladding is composed ofγ-Fe supersaturated solid solution, M7C3 carbide and (Fe, Cr) ordered phase. The microstructure at the cladding layer bottom was the typical extension growth on plane crystal, the morphology of grains changed from planar and cell to dendrite which was coarser. The microstructure at the top and middle was the regular pine-tree crystal and the microstructure at the top was the small pine-tree crystal, respectively. After adding Cr3C2 into the powder, the claddings contain nascent M7C3 carbide, fine dendrite and lamelliform eutectic structure. Its phase species maintains unchanged, while its quantity increases. The hardness of Fe-based cladding is 1.5 times of that of substrate. The friction and wear properties of the coatings are the synthetical characteristics of the friction pairs. It depends on system strongly and have complicated space-time characteristic. The wear mass loss and the friction coefficient of the coating/45# steel friction pair is more than that of coating/graphite-based powder metallurgy friction disk friction pair at the same condition. And iron-based coating registers better wear resistance in sliding against both 45# steel and graphite-based powder metallurgy friction disk at the same condition than substrate. For 45# steel/coating friction pair, the wear resistance of the coating with 10% and 20% chromium carbide is 3 and 10 times as much as that of the matrix. Friction coefficients of the coatings decreased with the increase of hardness. For coating/graphite-based powder metallurgy friction disk friction, the wear resistance of the coating is twice as much as that of the matrix.High-temperature oxidation tests were researched at 600℃. Oxidation test results at high temperature shows that the high-temperature oxidation resistances of the Fe-based alloy layers and the Fe-based alloy with Cr3C2 layers were better than the matrix. Iron-based alloy cladding possesses good high temperature oxidation, which lies in the formation of FeCr2O4 on the surface of the sample. This spinel oxide is of compact structure, less defect and poor conductivity. The increase of chromium element after adding Cr3C2 to powder makes the formation of continuous and complete chromium oxide, so whose high temperature oxidation resistance is better than that of Fe-based alloy cladding.After 50 times thermal-cooling cycling from 600℃to room temperature, substrate and iron-based alloy cladding possess excellent thermal fatigue resistance with the absence of crack, while crack is present in the coatings with Cr3C2. As the increase of Cr3C2, thermal-fatigue property worsen owing to the decrease of the cycle times of crack initiation, which is caused by the difference of thermal expansion coefficient of hard phase and the matrix.The high temperature oxidation performance, thermal fatigue performance and wear resistance of iron-based alloy claddings with good formability and high hardness are excellent when laser power is 3.5kW and scanning speed fluctuates from 150mm/min to 300mm/min, which can be considered as the surface modification of brake disc.