Chen Hao, Chen Tai-Cheng, Hsu Hsiao-Hung, Tsay Leu-Wen
Department of Optoelectronics and Materials Technology, National Taiwan Ocean University, Keelung 202301, Taiwan.
Department of Material Research, National Atomic Research Institute, Taoyuan 325207, Taiwan.
Materials (Basel). 2025 Apr 28;18(9):1995. doi: 10.3390/ma18091995.
Quench-polish-quench (QPQ) nitro-carburizing of AISI 4140 steel in a salt bath was performed in this study. Nitro-carburizing in a salt bath enhanced the formation of Fe-nitride on the outer surface layer. Moreover, the oxidizing treatment formed a thin oxide layer decorated on the outermost part of the QPQ-treated sample. The dense compound layer formed after nitro-carburizing in a salt bath consisted of refined granular FeN and transformed to FeN after post-oxidation treatment. Micro-shot peening (MSP) was adopted before QPQ treatment to increase the treated steel's fatigue performance. The results indicated that MSP slightly increased the thickness of the compound layer and harden depth, but it had little effect on improving the fatigue strength/life of the QPQ-treated sample (SP-QPQ) compared to the non-peened one (NP-QPQ). A deep compressive residual stress (CRS) field (about 200 μm) and a hard nitrided layer showed a noticeable improvement in the fatigue performance of the QPQ-treated ones relative to the 4140 substrates tempered at 570 °C. The ease of slipping or deforming on the substrate surface was responsible for its poor resistance to fatigue failure. The cracking and spalling of the brittle surface layer were the causes for the fatigue crack initiation and growth of all of the QPQ-treated samples fatigue-loaded at/above 875 MPa. It was noticed that fatigue crack initiation at the subsurface inclusions was more likely to occur in the SP-QPQ sample fatigue-loading at 850 MPa or slightly above the fatigue limit.
本研究对AISI 4140钢在盐浴中进行了淬火-抛光-淬火（QPQ）氮碳共渗处理。盐浴中的氮碳共渗增强了外表面层Fe氮化物的形成。此外，氧化处理在QPQ处理后的样品最外层形成了一层薄氧化层。盐浴中氮碳共渗后形成的致密化合物层由细化的颗粒状FeN组成，后氧化处理后转变为FeN。在QPQ处理前采用微喷丸（MSP）来提高处理后钢的疲劳性能。结果表明，与未喷丸处理的样品（NP-QPQ）相比，MSP略微增加了化合物层的厚度和硬化深度，但对提高QPQ处理样品（SP-QPQ）的疲劳强度/寿命影响不大。相对于在570℃回火的4140基体，QPQ处理样品中的深压缩残余应力（CRS）场（约200μm）和硬氮化层使其疲劳性能有显著改善。基体表面易于滑动或变形是其抗疲劳失效能力差的原因。脆性表面层的开裂和剥落是所有在875MPa及以上疲劳加载的QPQ处理样品疲劳裂纹萌生和扩展的原因。值得注意的是，在850MPa或略高于疲劳极限的疲劳加载下，SP-QPQ样品更有可能在亚表面夹杂物处萌生疲劳裂纹。
