碳工具鋼在鹽浴中的滲鉻研究

Abstract

為了提升碳工具鋼表面的耐磨性，以及改善在高溫環境下的氧化與腐蝕問題，因此本研究嘗試於碳工具鋼表面進行滲鉻處理。實驗分兩部分進行：(1) 在高溫(820℃以上)硼砂浴中滲鉻以被覆鉻碳化物；(2) 在低溫(600~700℃)氯化鹽浴中長時間滲鉻處理。試片在經上述方法處理後，研究其表面所形成的被覆層厚度、硬度分佈、鉻碳化物種類、耐氧化性、耐腐蝕性以及實際的滲鉻深度，所得結果如下： 1. 無論是經高溫硼砂浴滲鉻處理或低溫氯化鹽浴滲鉻處理，於試片表面所形成之被覆層種類大致相同，均為Cr7C3與Cr23C6；被覆層厚度與此兩種化合物之成份比例有關，隨著被覆層的增厚，Cr7C3所佔的百分比增加。 2. 對於上述兩種處理類型，在被覆層厚度相差不大的情況下，經由低溫氯化鹽浴滲鉻處理後試片的滲鉻深度將比經高溫硼砂浴滲鉻處理者來得較深。 3. 經滲鉻處理過後的試片，其硬化能將會有所提升，如此可提高試片表層之硬度值。 4. 試片對高溫環境之氧化抵抗，將隨著被覆層厚度的增加，耐氧化的效果愈好；而在被覆層厚度相近的前提下，兩種處理類型的耐氧化性差異不大，表示試片抵抗高溫氧化的能力只與被覆層厚度有關，而與滲鉻深度無明顯關聯。 5. 經被覆鉻碳化物層後的試片在比室溫略高的環境下(40℃)，對硝酸與硫酸有較佳之抗蝕性，對鹽酸則較差；然而若細看被覆層厚度的影響，則會發現隨著厚度的增加，試片抗鹽酸腐蝕的能力會有所提升；此外，這兩種處理類型的試片在被覆層厚度相近的情形下，腐蝕試驗所得的結果相當一致，顯示耐蝕性也只與被覆層的厚度有關，而與滲鉻深度無直接關聯。

In order to enhance the wear-resistance of carbon tool steel and improve the anti-oxidation and anti-corrosion in high temperature atmosphere, the feasibility of chromizing on the surface of carbon tool steel was investigated. This research was divided into two parts. First, the specimen was chromized in a high temperature(above 820℃) borax bath in order to ascertain the relationship between carbide thickness and treatment parameters. Second, the specimen was chromized in a lower temperature (between 600~700℃) chloride salt bath for long time periods. After the above treatments, the properties of chromized specimen were studied through measuring the thickness of carbide layer and the surface hardness distribution, identifying the type of chromium carbide, comparing the ability of anti-oxidation and anti-corrosion of specimens, and examining the chromizing depth. The results obtained were shown as follows: 1. No matter the specimen was treated in a high temperature borax bath or in a lower temperature chloride salt bath, the forming carbide layer was almost the same type, Cr7C3 and Cr23C6. The thickness of carbide layer was related to the proportion of Cr7C3 and Cr23C6 compounds. The thicker the carbide layer was, the more Cr7C3 compound it contained. 2. Between borax bath chromization and chloride salt bath chromization, the chromizing depth of latter would be deeper than the former for the same thickness of carbide layer. 3. The hardenability of chromized specimen would be better than untreated one, so the surface hardness of specimen would be improved. 4. The anti-oxidation of specimen in high temperature atmosphere was dependent on the thickness of carbide layer. There was no direct relationship between the anti-oxidation and the chromizing depth. When the specimen had a thicker carbide layer, it would be provided with a better anti-oxidation ability. 5. The chromized specimen had better resistance to the corrosion caused by nitric acid and sulfuric acid at 40 degrees centigrade, but it could not sustain the corrosion caused by hydrochloric acid. The ability against corrosion by hydrochloric acid; however, could be improved when the specimen had a thicker carbide layer. As the result of no. 4, the anti-corrosion of specimen was dependent on the thickness of carbide layer but independent on the chromizing depth.