Yuan Yasha, Lin Yichou, Wang Wenyan, Shi Ruxing, Wu Chuan, Zhang Pei, Yao Lei, Jie Zhaocai, Wang Mengchao, Xie Jingpei
School of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471023, China.
Longmen Laboratory, Luoyang 471000, China.
Materials (Basel). 2025 Jan 13;18(2):334. doi: 10.3390/ma18020334.
In response to the intensifying competition in the mold market and the increasingly stringent specifications of die forgings, the existing 55NiCrMoV7 (MES 1 steel) material can no longer meet the elevated demands of customers. Consequently, this study systematically optimizes the alloy composition of MES 1 steel by precisely adjusting the molybdenum (Mo) and vanadium (V) contents. The primary objective is to significantly enhance the microstructure and thermal-mechanical fatigue performance of the steel, thereby developing a high-performance, long-life hot working die steel designated as MES 2 steel. The thermal-mechanical fatigue (TMF) tests of two test steels were conducted in reverse mechanical strain control at 0.6% and 1.0% strain levels by a TMF servo-hydraulic testing system (MTS). The microstructures of the two steels were characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The results indicate that throughout the entire thermomechanical fatigue cycle, both steels exhibit initial hardening during the low-temperature half-cycle (tension half-cycle) and subsequent continuous softening during the high-temperature half-cycle (compression half-cycle). Furthermore, under the same strain condition, the cumulative cyclic softening damage of MES 1 steel is more pronounced than that of the newly developed MES 2 steel. The number, width, and length of cracks in MES 2 steel are smaller than those in MES 1 steel, and the thermomechanical fatigue life of MES 2 steel is significantly longer than that of MES 1 steel. The microstructures show that the main precipitate phase in MES 1 steel is Cr-dominated rod-shaped carbide. It presents obvious coarsening and is prone to inducing stress concentration, thus facilitating crack initiation and propagation. The precipitate phase in MES 2 steel is mainly MC carbide containing Mo and V. It has a high thermal activation energy and is dispersed in the matrix in the form of particles, pinning dislocations and grain boundaries. This effectively delays the reduction in dislocation density and grain growth, thus contributing positively to the improvement in thermomechanical fatigue performance.
针对模具市场竞争日益激烈以及模锻件规格要求越来越严格的情况，现有的55NiCrMoV7（MES 1钢）材料已无法满足客户日益提高的需求。因此，本研究通过精确调整钼（Mo）和钒（V）的含量，系统地优化了MES 1钢的合金成分。主要目的是显著改善该钢的微观组织和热机械疲劳性能，从而开发出一种高性能、长寿命的热作模具钢，命名为MES 2钢。通过TMF伺服液压测试系统（MTS），在0.6%和1.0%应变水平下，以反向机械应变控制方式对两种试验钢进行了热机械疲劳（TMF）试验。采用扫描电子显微镜（SEM）、电子背散射衍射（EBSD）和透射电子显微镜（TEM）对两种钢的微观组织进行了表征。结果表明，在整个热机械疲劳循环过程中，两种钢在低温半循环（拉伸半循环）期间均表现出初始硬化，随后在高温半循环（压缩半循环）期间持续软化。此外，在相同应变条件下，MES 1钢的累积循环软化损伤比新开发的MES 2钢更为明显。MES 2钢中裂纹的数量、宽度和长度均小于MES 1钢，且MES 2钢的热机械疲劳寿命明显长于MES 1钢。微观组织显示，MES 1钢中的主要析出相是以Cr为主的棒状碳化物。它呈现出明显的粗化，易于引起应力集中，从而促进裂纹萌生和扩展。MES 2钢中的析出相主要是含Mo和V的MC碳化物。它具有较高的热激活能，以颗粒形式弥散分布在基体中，钉扎位错和晶界。这有效地延缓了位错密度的降低和晶粒长大，从而对热机械疲劳性能的改善起到了积极作用。
