THE INFLUENCE OF Nb, Ta AND Ti MODIFICATION ON HOT-WORK TOOL-STEEL GRAIN GROWTH DURING AUSTENITIZATION

In order to explore the influence of niobium, tantalum and titanium modification on grain growth in high-thermal-conductive hot-work tool steel during austenitisation at high temperatures, three types of modified steels (sample HTCS-130 + 0.06 w/% Nb, sample HTCS-130 + 0.03 w/% Ta, sample HTCS-130 + 0.006 w/% Ti) based on reference (sample HTCS-130 – 0) were prepared. The effect of different austenitisation temperatures (1030, 1060, 1080 and 1100) °C on hardness after quenching and grain size were investigated. The results show that there is a positive effect on the mechanical properties and a decreased grain-growth effect in the modified steel samples. The precipitation behaviour of the carbides was also investigated with electron microscopy. The Mo-W carbides were relatively weak at retaining grain size, but their pinning effect was increased with the incorporation of other carbide-forming elements like Nb and Ta. MC-type carbides on the grain boundary were effective at grain-boundary pinning. Nb even further increased the resistance by forming NbC carbides. The addition of Ti, on the other hand, proved to be ineffective due to the intergranular precipitation of the formed carbonitrides.