| 45~# and 35~# steels are most commonly used medium carbon steel with excellent properties in strength, plasticity and tenacity. However, their surface strength and wear resistance are not good enough to meet applications in many fields. Therefore, surface strengthening and modification of these materials are necessary. Surface treatments such as surface thermal processing, surface cladding and alloying of iron based metals have been becoming one of the most important technologies in surface engineering. With the development of high power lasers, much progress has been made in laser surface processing such as the effect of processing parameters on the microstructure and surface properties of materials, optimization of processing parameters and quality control as well as the design of surface materials. The main topics and achievements of this thesis are as follows: 1. The advancement in laser surface processing The advasncement in laser surface processing was summarized. Related material systems, equipments , methods, processing parameters, computer simulation of temperature field and practical applications of this technology are described. The study on the theory of solid-liquid interface of laser cladding is also introduced. 2. Quenching of 45?and 35?Steels with a wide-band CO2 Laser Beam The surface quenching of 45# steel is carried out with a wide-band CO2 laser beam. The microstructure and the micro-hardness distribution within the hardened layer were studied. Single quenching width larger than 10mm is obtained. The micro-hardness is 2?.5 times as high as the unquenched area while the depth of the hardened layer reaches I .9-2.0mm. Both the micro-hardness and the microstructure distribution within the hardened layer indicated that the hardened layer consists of three sub-layers, overheated surface layer, uniform phase transformation layer, and transition layer. With v=4mmls and off-focus length 18mm, the optimal laser power lies between 3kW and 3.3kW. Wide-band laser quenching of 35# steel indicates that a hardened bandwidth of 15mm and depth of 0.53mm were obtained with laser power of 3.0--~3.3kW. The dependence of the micro-hardness on the depth in the hardened layer was measured. It shows that the micro-hardness ranges from 554 to 675Hv, which is about 2.3?.5 times as high as that of the untreated area. The microstructure study indicates that the laser quenched layer can be divided into complete hardened layer, transition layer and high-temperature tempered layer, respectively. 3. Laser cladding of Ni-based alioy/WC Based on the laser cladding experiment of Ni-based alloy/WC, the effect of WC contents on the micro-quality, microstructure and distribution of micro-hardness was analyzed. It shows that the stability of the solid-liquid interface deceases with the increase of the WC particles. With the increase of WC contents, the microstructure changes from planar crystallite to cellular crystallite and dendrite at the interface while it changes from radial to dendrite at the upper part of the layer. The micro-hardness is independent of the laser power whatever WC contents. The micro-cracks become more serious with increasing the WC content. 4. The effect of RE oxides on the crack tendency and microstructure of NI/WC cladding layer The effect of RE oxides on the improvement of the cladding layer quality has been studied. The results indicate that the oxides (MgO, CeO2, Y203) can greatly reduce the crack tendency, improve the micro-quality and refine... |
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