鹽浴軟氮化對SKD61熱作模具鋼耐熱熔損及熱疲勞性之影響

Influence of Salt-bath Nitrocarburizing on the Hot Erosion and Thermal Fatigue Resistance of SKD61 Hot-work Steels

本文探討JIS SKD61熱作模具鋼經鹽浴軟氮化處理之微結構、熱熔損及熱疲勞性之變化，選用傳統型（A）及低Si高Mo含量之成分改良型（B）熱作模具鋼為研究材料。實驗試片使用1025℃持溫25分鐘，進行2 kg/cm2氮氣氣淬至常溫，於200 ~ 650℃區間進行不同溫度回火處理，並選用600℃回火試片進行580℃鹽浴軟氮化。研究結果顯示，試片經1025℃沃斯田體化持溫25 min後經氮氣氣淬，硬度可達54.5±0.5 HRC（傳統型A）及55.0±0.2HRC（改良型B），在525℃回火（55.7±0.2 HRC和55.4±0.1 HRC）觀察出有二次硬化現象。傳統型A及改良型B經氣淬及回火後，試片主要結構均為麻田散體相，氮化處理試片表面主要為Fe3N與Fe4N相。熱熔損試驗分析顯示改良型鋼種比傳統型更能抗鋁熔損，且經580℃鹽浴軟氮化80分鐘更有效的降低熔損率，分別降低7.9%（A）及8.4%（B）。改良型鋼種之耐疲勞性優於傳統型SKD61鋼種，且鹽浴軟氮化並不能有效的提升耐熱疲勞性，傳統型A材氮化後試片之熱疲勞裂紋由80μm成長到340μm。

Two hot-work steels, standard A and modified B (low Si and high Mo content), were vacuum heat treated using different quenching and tempering processes, changes in microstructure and corrosion resistance of these steels were studied. Some specimens, austenitized at 1025 ℃ for 25 min, were gas quenched to room temperature and tempered at 200 ~ 650 ℃. Selected specimens were salt bath nitrocarburized at 580 ℃. The microstructures, X-ray diffraction, heat crack and aluminum erosion tests were conducted. The results showed that the hardness values of the as-quenched specimens, austenitized at 1025 ℃ for 25 min, were 54.5 ± 0.5 HRC (standard A) and 55.0 ± 0.2 HRC (modified B), and the phenomenon of secondary hardening was observed at 525 ℃ (55.7 ± 0.2 HRC and 55.4 ± 0.1 HRC). According to X-ray diffraction analyses, martensite phases appeared on the standard A and modified B steels after gas quenching and tempering, and both Fe3N and Fe4N phases were found in the nitrocarburized specimen. Results from aluminum erosion resistance tests showed that the loss rate of the modified specimen B was lower than that of the standard one, and salt-bath nitrocarburizing could effectively reduce the loss rates of the two steels by 7.9% (A) and 8.4% (B). From the results of heat crack tests, it was observed that the heat crack resistance of modified B was better than that of standard A. However, salt-bath nitrocarburizing could not effectively enhance the thermal fatigue resistance, and the fatigue crack depth of the nitrocarburized specimen grew from 80 μm to 340 μm.