Abstract
AISI D2, an alloy tool steel, has been widely used as a cold-work die steel because of its excellent wear resistance, toughness, and machinability. However, when it is used as a mold or tool under a high load, high hardness and wear resistance are required to improve its service life. This study aimed to apply ultrasonic nanocrystal surface modification (UNSM), a local surface hardening technology, to the surface of D2 steel. To maximize the surface improvement effect achievable through UNSM, it is important to select the optimal process conditions according to the characteristics of the base metal. Therefore, the effects of UNSM were compared for different initial hardness values of D2 in this study. To this end, the base metal was subjected to different heat treatment conditions to exhibit different hardness values (approximately 40, 45, 50, and 55 HRc). Changes in the surface of the base metal were observed according to the three main parameters of the UNSM process: the load, inter-pass interval, and feed rate. No significant changes in the surface roughness and hardness were caused by the feed rate. As the inter-pass interval of UNSM decreased and the static load increased, the surface roughness of the base metal increased. However, the specimen with low initial hardness (D2-H40), the roughness improvement decreased as the load increased. On the other hand, the surface hardness improvement increased as the inter-pass interval decreased and the load increased regardless of the initial hardness of the base metal. It also was found that the specimen with the highest initial hardness (D2-H55) exhibited the greatest hardness improvement rate of 9% and smallest hardened layer thickness of 220 μm. In addition, the largest compressive residual stress (− 1,130.6 MPa) was formed on the surface of D2-H55 after UNSM, which occurred because the base metal with high initial hardness had limited plastic flow due to fine grains and low ductility, causing thus the energy repeatedly delivered by UNSM to be concentrated in a limited area. Consequently, reducing the inter-pass interval is effective on improving the surface roughness and hardness regardless of the material hardness. However, the load must be applied at an appropriate level, depending on the material hardness. Therefore, when D2 is heat-treated and used according to the product requirements, it will be effective to consider the changes after UNSM treatment based on hardness after heat treatment that were derived in this study.
Highlights
AISI D2 or D2 is a tool steel that has been widely used as a high-carbon, high-chromium alloy steel
It was found that the surface roughness improvement rate increased as the inter-pass interval decreased regardless of the initial hardness of the base metal
For D2-H40, the specimen with an initial hardness of 40 HRc, the roughness improvement decreased under a load of 50 N, which was excessive
Summary
AISI D2 or D2 is a tool steel that has been widely used as a high-carbon, high-chromium alloy steel. It has been employed in various industries owing to its excellent toughness and wear resistance and has mainly been utilized as a cold-work tool steel, such as in cutting tools, forming rolls, and dies for press. It is necessary to improve the mechanical characteristics and fatigue lives of coldwork tool steels to be used as parts and dies [1]. The mechanical and microstructural characteristics of metallic materials are improved by heat treatment. Heat treatment makes the microstructure finer and increases the wear resistance by generating martensitic structures. The mechanical characteristics can be controlled by varying the heat-treatment conditions according to the product requirements
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