Fig.4 Microstructures of self-protective paste boronizing specimens with different rare earth addition (a) 7wt%, (a) 7wt%, (b) 8wt%, (c) 9wt%, (d) 10wt% It can be seen from Fig.4 that on the surface of the self-protective paste boron samples with rare earth added, a certain boronizing layer occurred. Under the circumstances of 8% rare earth addition and 9wt% rare earth respectively, boronizing layer was regular and closely integrated, which brought good boron effect. However, with the increase of the amount of rare earth added (as shown in Fig.4(d)), boronizing layer expanded further towards the sample, and the structure of which was irregular and its uniformity fell. From the hardness data of the boron samples (as shown in Fig.5), it can be seen that the hardness achieved the maxi value when the rare earth addition was 9wt%. Therefore, it can be drawn that in the current boronizing conditions and in the process of low-temperature boronisation, the optimum amount of rare earth added is 9wt%. 7 8 9 10 22 24 26 28 30 32 34 hardness HB rare earths addition wt% Fig.5 Hardness of self-protective paste boronizing specimens at different rare earth addition 3.4 Discussion In the process of boronisation, the boron agent supplier in the paste had reaction and generated activate B atoms which had been absorbed by the iron and steel sample surface and were segregated and enriched on crystal defects such as grain boundary and the surface phase boundary and space [11], Then had a short-range diffusion in the direction of the internal lattice and matrix. With the continuous migration of B atoms, in the lattice when the conditions of concentration and fluctuation of energy were ready, Fe2B crystal nucleus formed. Then with the crystal grain’s growing up, Fe2B- γ phase boundary propeled toward the inner matrix [12]. The protective paste in which quartz powder, the covering layer of fly ash and coal gangue were as the main ingredients, has the characteristics of high strength, good sealing and easy peeling in boronizing temperature, could achieve good boron effect. 20μm (d) (c) 20μm
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