Boronizing 및 Multi-Boronizing 處理에 의한 鋼材의 表面物性 向上에 관한 硏究
(A) Study On the Improvement of Surface Properties for Steel by Boronizing and Multi-Boronizing Treatments
Boronizing Multi-Boronizing 강재 표면물성;
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This paper has been investigated on the effects of surface properties with various boronizing techniques. Three kinds of boronizing processes were applied, such as the paste thermal diffusion boronizing, plasma aided paste boronizing, and multi-boronizing, and the various substrate alloys were used, such as carbon steels, stainless steels, Mar-M 247 superalloy. The boronizing behaviors and properties of the boride layers were examined in the various substrate alloys by analyzing the thickness of the boride layer, distribution of hardness, wear resistance, corrosion resistance, and high temperature oxidation etc. In the case of paste thermal diffusion boronizing treatment, the thickness of boride layer reduced with increasing the carbon content, and increased with the treatment temperature and time. The XRD results showed that the phase of the boride layer consisted of FeB and Fe_(2)B on carbon steels, and FeB, Fe_(2)B, and CrB on the 12Cr steel. The boronizing treatment improved the wear, erosion resistances and corrosion resistance under acidic solutions. The resistance of high temperature oxidation was also improved by the boronizing treatment. Plasma aided paste boronizing treatment was applied to overcome disadvantages of the thermal diffusion boronizing treatment which resulted in distortion of the sample. This process is useful method because the boronizing mechanism is combined with gas and liquid reaction. The experimental results showed that the boride layer by the plasma aided paste boronizing formed at lower temperatures and shorter treatment time and lower activation energy for boronizing than that by the thermal diffusion process. However, the properties of the boride layer were similar to those of thermal diffusion process. The XRD results showed that the FeB, Ni_(3)B, CrB phases were found in the STS 304 alloy. The corrosion resistance decreased in 1 mole sulpuric acid at 298°(K). The effect of multi-boronizing treatment on the surface properties was examined in the hot working tool steel (STD61) and the Mar-M247 superalloy by analyzing the boronizing behavior and boride layer. The boronizing behavior and properties of boride were similar to the results of the thermal diffusion boronizing and plasma boronizing. Ce was distributed ununiformly in the boride layer and a Cr-rich zone was observed in the FeB boride layer. The hot corrosion resistance of the Mar-M247 superalloy was examined in Na_(2)SO_(4)-NaCl molten salt. The XRD results showed that the Ni_2Al_3 phase formed between the aluminized layer and substrate when the surface modification temperature was below 1273°(K), however, the NiAl phase formed when the temperature was above 1273°(K). The intensity of the XRD peak in the NiAl phase increased after the post heat treatment, where the NiAl phase resulted in enhancing the hot corrosion resistance of the aluminized layer. The suitable mixed ratio of pack materials for aluminizing was 25 wt.% Al : 4 wt.% NH4Cl : 71 wt.% Al_(2)O_(3) powder. The 170μm thickness of aluminide layer was obtained at 1273^vK for 20 hours. The corrosion resistance of boro-aluminized Mar- M247 specimens decreased due to blocking outward diffusion of Cr by boron. It was also found that the cohesion between the oxide scale and aluminized layer decreased during thermal cycling in Na_(2)SO_(4)-NaCl molten salt.