Study on the Microstructure and Mechanical Properties of Vanadis 4 Tool Steel by Adding VC and Cr3C2 Powders through Vacuum Sintering and Heat Treatments

• Main Authors: Kuang-Yao Lee, 李光耀
• Other Authors: Shih-Hsien Chang
• Format: Others
• Language: zh-TW
• Published: 2017
• Online Access: http://ndltd.ncl.edu.tw/handle/jqtt79

碩士 === 國立臺北科技大學 === 材料科學與工程研究所 === 105 === Vanadis 4 steel is a powder metallurgy tool steel which has high levels of vanadium, chromium, and molybdenum elements in the matrix. Generally speaking, Vanadis 4 tool steels manufactured by powder metallurgy have been widely used due to their high strength, wear resistance and ductility. In addition, metal matrix composites (MMCs) is are new materials with different types of ceramics as reinforcement phase dispersed in the metal matrix. Vanadium carbide (VC) and chromium carbides (Cr3C2) have excellent hardness and high melting temperatures, effectively inhibiting grain growth and thermal stability with Fe-based alloys; therefore, they are great options for reinforcing tool steels. According to the above statements, this study utilizes the VC and Cr3C2 carbides to improve the mechanical properties of the Vanadis 4 tool steels. In this study, different ratios of VC (1, 3 and 5 wt%) and Cr3C2 (1, 3 and 5 wt%) powders were added to Vanadis 4 steel powders and then mixed by ball milling in order to find the optimal vacuum sintering parameters of Vanadis 4 tool steels. After compressing and forming, all the specimens were sintered from 1225°C to 1275°C by vacuum sintering for 1 h. Furthermore, a series of heat-treatments (quenching followed by tempering) was performed, by which the optimal samples were heated to 1020°C and maintained at that temperature for 100 min for quenching, with 0.5 MPa of N2 for quenching media, and then subjected to sub-zero treatment (-150°C for 60 min). Meanwhile, the tempering temperature was held at 480°C for 180 min and repeated twice. Finally, various material characterization techniques were used to evaluate the specimens’ properties, including porosity, hardness and transverse rupture strength (TRS). Corrosion tests were also conducted; the microstructures were analyzed using XRD, OM, SEM and EDS techniques. The experimental results showed that good mechanical properties were obtained by the addition of 3 wt% VC powders sintered at 1250°C for 1 h. Meanwhile, the apparent porosity was 0.27%, and TRS and hardness reached 1953.3 MPa and 82.1 HRA, respectively. On the other hand, this study also revealed that the apparent porosity reached to 0.22%, and that a TRS value of 1821.2 MPa and a hardness value of 82.4 HRA were obtained by the addition of 5 wt% Cr3C2 powders, which were sintered at 1250°C for 1 h. Furthermore, when the optimal sintered specimens of Vandis 4 tool steel were obtained by adding VC and Cr3C2 powders (3 wt% VC and 5 wt% Cr3C2 sintered at 1250°C for 1 h, respectively), followed by heat-treatments and sub-zero treatments, the TRS obviously increased to 2066.3 and 2085.0 MPa, respectively. However, there is no significant difference in the hardness value.