Abstract
There are some foundamental problems in the structure observation of carbon steels by a high temperature microscope, e.g., the rate of thermal etching and the homogeneity between the surface and the interior structural changes. The authors studied some of these basic problems on the structures of carbon steels which were observed in the temperature range from room temperature up to 1000°C for various holding times (maximum 95 min) in vacuum by a high temperature microscope.The results obtained were as follows:(1) The grain boundaries began to be observed faintly at 600°C and clearly at 700°C by thermal etching. The effects of the heating rate and holding time on the thermal etching were small, but the effects of the test temperatures were large.(2) The surface microstructures of 0.5%C carbon steels differed from the interior ones when cooled at the rate 50°∼70°C/min. This may be caused by the difference of cooling rates between the surface and the interior.
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