A Study of Hot Hardness of Pure Iron and Pure Tantalum by a Newly Developed Ultra High Temperature Hardness Tester

An ultra high temperature hardness tester which is capable of reading the hot hardness value up to 1700°C was devised recently. The hardness tester has an extra ordinary construction to protect the influence of high light scattering from the ultra highly heated specimen, and the impression image is obtained by reflected electrons from a scanning type electron microscope. The specimen surface image, therefore, is clearly visible over the whole temperature range. The specimen temperature is elevated by electron bombardment up to 2000°C and the temperature is calibrated by observing surface images of various pure metals in the melting and solidifying states. For example, 3.8 mmg of pure platinum (meiting pt. 1770°C) image is clearly observed during the melting procedure. An indenter is located inside of a heating furnace, so the both specimen and indenter tip are heated to the desired temperature. The indenter tip temperature, therefore, is elevated sufficiently before applying a dead weight to a specimen. In a general relation between the hot hardness value and temperature for pure iron and pure tantalum, the inflection points are at lower temperatures rather than those in the previous data. Most of the hot hardness values for pure iron are also slightly lowered in high temperature range over the inflection point. The creep effect on the hot hardness is obviously recognized at the temperatures over the inflection point, but this effect may not extend over the temperatures below 500°C for pure iron. And a higher hot hardness value is always observed under a lower test load except over 800°C. On the other hand, the all-sapphire indenter gives a higher value of hot hardness than the sapphire tipped indenter. This result may also be considered the difference of thermal conductivity from the all-sapphire and sapphire tipped indenters. Hot hardness measurements have long been done with the diamond tipped indenter which gives a much higher value of hardness. Diamond has a higher thermal conductivity than sapphire and is apt to react with various metallic specimens at higher temperature. Moreover, the use of the higher test load is not suitable for an accurate measurement of hot hardness because of the possibility of thermal drift between a specimen and the indenter tip and the promotion of the creep effect. Therefore, it seems more desirable to conduct the hot hardness measurement under a constant test load as low as 50 g either with the all-sapphire indenter.