電漿氮化及真空熱處理對熱作模具鋼耐蝕性之影響

Effect of Plasma Nitriding and Vacuum Heat Treatment on the Corrosion Resistance of Hot Work Tool Steels

本研究探討電漿氮化及真空熱處理對SKD61及DH31S熱作模具鋼之微觀結構及耐蝕性之影響。實驗試片以真空爐在1000、1025、1050℃三種溫度持溫，經2 kg/cm2的N2氣淬，並於200 ~ 650℃的溫度區間進行回火。以直流（direct current）電壓於530℃進行10 h電漿滲氮（plasma nitriding）。分析方法包含OM、XRD及尺寸變形觀察，並進行硬度及極化曲線試驗。結果顯示，試片在1025℃持溫20 min後以N2氣淬，硬度可達到53.5 HRC （SKD61）和51.0 HRC （DH31S），並可在550℃回火溫度觀察到二次硬化。由慢速掃描XRD分析，兩種鋼料經氣淬及回火後，主要結構均為Martensite；於530℃經10 h電漿氮化處理後，表面有Fe4N相產生。由極化曲線試驗得知，DH31S之極化曲線可觀察出有鈍化區（passive region）的產生，且腐蝕電位-0.3 V較滲氮前及收料原材之-0.6 V及-0.7 V高，顯示電漿氮化處理對熱作模具鋼之耐蝕性有明顯的幫助。

In this study, two hot work steels have been conducted in the vacuum furnace to furnish the quenching and tempering processes. Some specimens will also be plasma nitrided. SKD61 and DH31S hot work steels were used as the experimental materials. A series of vacuum heat treatment has been conducted at austenitizing temperatures, 1000 ~ 1050 ℃, and tempering temperature ranging from 200 to 650 ℃ to obtain the tempered martensitic microstructure. The electrochemical polarization analysis, microstructure, X-ray diffraction and hardness measurements of the treated specimens were performed. The electrochemical tests were proceeded in a 3.5 wt% NaCl aerated solution. It has been found that the hardness value of SKD61 and DH31S specimens were 53.5 and 51.0 HRC, respectively, by austenitizing at 1025 ℃ for 20 minutes and then N2 gas quenching. The dominant peaks of the γ'-phase (Fe4N) were appeared in the case of the plasma nitrided specimen. Potentiodynamic polarization curves showed that the corrosion potential of nitrided DH31S specimens were higher than those of the non-nitrided and as-received specimens. Besides that, the anodic polarization curve of the plasma nitrided DH31S specimen showed a passive region. It shows that the corrosion resistance of hot work steels can be improved by the plasma nitriding.