EFFECT OF Nb, Ta, AND Ti MICROALLOYING ON THE SECONDARY HARDENING OF Mo-W TOOL STEEL

This work investigates the influence of microalloying elements on the tempering process of a hot-work tool steel. The study examines the austenitisation temperatures and grain growth of a high thermal conductivity hot-work tool steel, with the addition of various microalloying elements: Nb+0.06 w/% Nb, Ta+0.03 w/% Ta, and Ti+0.006 w/% Ti, compared to a reference sample. Thermodynamic calculations are used to analyse the influence of microalloying on the transformation temperatures and carbide formation. Austenitisation temperatures of (1030, 1060, 1080, and 1100) °C are selected, and subsequent tempering is performed at (540, 580, 600, 620, and 640) °C. The investigation focuses on analysing the microstructure, hardness, and grain size. The results show that the microalloying elements have a positive influence on the retention of the grain size during austenitisation and on the enhancement of hardness during tempering. Electron microscopy is utilized to analyse the microstructure, which indicates the prevalence of Mo-W carbides. The carbides exhibit coarsening and morphological changes during high-temperature tempering. The secondary hardening peaks occur at temperatures between (600 and 620) °C, and are more pronounced by microalloying.