【作者】 韩立广；

【导师】 赵宇光；

【作者基本信息】 吉林大学 ， 材料加工工程， 2008， 硕士

【摘要】 本论文以热作模具钢为研究对象,研究了瞬态高能量脉冲电流处理对热作模具钢组织和性能的影响。研究发现脉冲电流处理后在热作模具钢热疲劳试样的切口前端形成热影响区,该区内的组织明显细化且硬度提高。脉冲电流处理提高了热作模具钢的抗拉强度,但也使其延伸率有所降低。冲击实验的结果表明,适当的脉冲电流处理可以提高热作模具钢的冲击韧性。热磨损实验证明,脉冲电流处理提高了热作模具钢的高温抗氧化性、抗热磨损性能。脉冲电流处理延缓了热疲劳裂纹的萌生,降低了热疲劳裂纹的扩展速率,提高了热作模具钢的抗热疲劳性能,并通过细晶强化、位错强化、弥散强化、应力转换强化四种机制解释了其作用机理。在对带有预制宏观裂纹的热作模具钢试样进行脉冲电流处理时,发现脉冲电流处理使预制裂纹尖端曲率半径增大、钝化,起到了裂纹止裂的目的。本文采用有限元分析的方法模拟研究了带有预制切口的热作模具钢试件在脉冲电流处理时的温度场、应力场。模拟计算表明在脉冲电流放电时,预制切口前端升温迅速且温升较大,在切口前端形成了很大的压应力场。模拟计算结果和实际实验有很好的对应关系,这对脉冲电流技术的实际工程应用中的参数优化具有重要的指导意义。更多还原

【Abstract】 important foundation of process equipment was paid more and more attention .As the basement of die industry the die steel was developed raptly in recent years. According to the working conditions the die steel can be divided into cold working die steel and hot working die steel. The hot working die is in service under extreme condition, repeatedly heated and cooled by hot metal and cooling medium. Therefore the hot working die material is requested to be with the properties of high strength, high hot hardness, good hot stability and good resistance of thermal fatigue. The main invalidation form of hot working die is makeup of fracture, distortion, thermal fatigue and hot wear. As to one die, the invalidation form may be only one, also be some of them, it is lied on the working condition and property of die material.For prolonging the life of dies to deduce the production cost, we must to improve the property of hot working die steel when the working condition is immutable. Presently, to improve the property of the material, we can optimize the composition, improve the preparation technics, do surface treatment and pulse current stimulation. The pulse current technology was developed in recent years, as a new technology it was paid more and more attention. The main effect of pulse current stimulation on metal material is that the microstructure is refined, the electroplastic effect, the bulk of metal glass is crystallized, the fatigue of materials is recovered so as to prolonging the life of materials. In this paper, the hot working die steel is the research object. The effects of high-energy instant impulse current stimulation on microstructure and property of the die steel are studied. The main achievements of the study are as follows:1. After pulse electric current stimulation a heat affected zone (HAZ) is formed ahead of specimen notch. The area of HAZ is gradually enlarged with the prolonging of discharging duration and the increase of discharging voltage. However, when the discharging duration is too long (>240ms), part of the surface is melted. The microstructure of the HAZ is refined after pulse current stimulation. The effect is more evidently with the prolonging of the discharging duration, and the increase of the discharging voltage. At the same discharging duration and voltage but different stimulation phases, the effect of the pulse electric current on the microstructure is different. The effect is more evident simulated after 500 cycles of thermal fatigue. The XRD side inclination and fixedΨmethod measurement show that the surface residual stress of the spacemen stimulated by pulse electric current after 500 cycles of thermal fatigue is negative, and the value is increased with the increase of discharging duration and discharging voltage.2. The hardness of HAZ is improved after pulse electric current stimulation, and the amplitude is lager with the prolonging of the discharging duration. The ultra tensile strength of hot work die steel is improved after pulse electric current stimulation. At different stimulation phases the effect of the tensile property of hot work die steel is different. The strengthening effect is much more effective in the specimen stimulated after thermal fatigue. In this experiment, the longer the discharging duration and the higher the discharging voltage is, the more the ultra tensile strength of the specimen is improved. However, the elongation of electro-stimulated specimen is much lower than that of the un-electro-stimulated one, and the higher the ultra tensile strength the lower the elongation it is. The impact toughness of hot working die steel is improved after properly pulse current stimulation, which will be reduced when the discharging duration is too long. In this experiment, 40 cycles is the best discharging duration. The high-temperature wear resistance is improved after pulse current stimulation. And the long the discharging duration is the more the high-temperature oxidation resistance and high-temperature wear resistance is improved.3. The resistance of thermal fatigue the resistance of the initiation and propagation of thermal crack of hot work die steel are improved after pulse electric current stimulation, especially for the steel after thermal fatigue for 500 cycles. In this experiment, the longer the discharging duration and the higher the discharging voltage is, the more the resistance of thermal fatigue. The prefabricate crack is passivated and the curvature of the tip was decreased after properly pulse electric current stimulation. Finally, the crack is prevented. The improvement of thermal fatigue resistance is explained though four mechanism of refining strengthening, dislocation strengthening, dispersion strengthening and stress counteraction strengthening.4. Traditionally, the forecast of the temperature filed and stress filed come from pulse current stimulation is based on the experiment and experience curve or experience formula. To study the temperature filed and stress filed just though experiment is difficult and can not completely forecast the effect of the pulse current stimulation to the whole specimen. The study adopt finite element method simulated temperature field and thermal stress field of the specimen with prefabricate notch stimulated by pulse current. Calculation result shows that, the temperature rise is instantaneous and a thermal compressive stress field formed on the tip of the notch, during electro-pulsing discharging process. The simulation result is corresponding with the practical experiment; it is very important to the application of the pulse current technology in engineering.更多还原