Quantitative measurements of the eutectic carbide morphology have been conducted for HK40 and HP steels aged at 1100°C up to 3000h. The average area (S), perimeter (L) and volume fraction of eutectic carbides increased with the increase in aging time, and the precipitation density decreased. These morphological parameters of the HK40 steel changed monotonously with the increase in aging time. While, in the HP steel, there were no changes in these parameters up to 1000h, but after 1000h or more, all of these parameters changed abruptly. Using a parameter of L2/S to assess quantitatively the shape of eutectic carbides, it was found that the shape of eutectic carbides in the HP steel turned to complicated ones after long term exposure, whereas that in a HK40 steel turned to rounded ones. In a short time, the number of secondary carbides precipitated on dislocations in the HK40 steel was larger than that in the HP steel. After long term exposure, many secondary carbides were observed in the HP steel, irrespective of no carbides in the HK40 steel. Secondary carbides remaining after long term exposure in the HP steel coalesced with eutectic carbides and the shape of eutectic carbides turned to complicated ones. It is concluded that the changes in the morphology of eutectic carbides with aging time depends on the precipitation morphology of secondary carbides.
