T15 High Speed Steels Produced by High-Temperature Low-Pressure Short-Time Vacuum Hot-Pressing Combined with Subsequent Diffusion-Bonding Treatment

Abstract

Currently, hot isostatic pressing (HIP) is widely used to produce highly alloyed high speed steels (HSSs) in an industrial scale; however, the HIP’s production cost is very high. Another powder consolidation approach with low production cost, namely vacuum hot-pressing (VHP), has hitherto received limited attention. The present work aims to develop an innovative solid-state VHP approach, producing HSSs with large cross-sectional sizes via a VHP facility having low loading capacity, thus further decreasing production cost. In doing so, VHP is performed at a sufficiently high temperature such that the pressure leading to full densification can be significantly reduced to a magnitude as low as several MPa; simultaneously, VHP is completed within a timeframe as short as several seconds to minutes, retaining fine carbide sizes; subsequently, the as-VHP HSS is diffusion-bonding treated (DBT-ed) at a relatively low temperature, achieving full metallurgical bond between powders while minimizing carbide growth. In the present work, T15 HSS was processed using the above VHP approach. The VHP temperature as high as 1200 °C was selected and consequently, the minimal pressure leading to full densification was decreased to ~7 MPa. By controlling displacement of pressing punch to a value corresponding to full densification, the VHP was competed for only 15 min. The almost fully dense as-VHP T15 HSS exhibits submicrometric carbide sizes smaller than those in the as-HIP counterpart, but incomplete metallurgical bond between powders. After diffusion bonding treatment at a relatively low temperature of 1100 °C for 2–4 h, the extent of metallurgical bond between powders is significantly enhanced with insignificant carbide growth. After regular quenching and tempering, the VHP plus DBT-ed T15 HSSs exhibit smaller average primary carbide sizes and similar hardness and three-point bend fracture strength, relative to those in the HIP counterpart after similar quenching and tempering.