抄録
The author observed many hot-tear fractures and metallographic structures around hot-tears in sand-cast carbon-steel castings containing 0.20∼0.25 % carbon and having wall thickness of 10∼50 mm. The results are summarized as follows :
1. Almost all of the observed hot-tears show indications that they have grown between dendrites of primary crystals including a small amount of liquid metal.
2. The fracture surface of interdendritic hot-tear takes microscopically various characteristic shapes, such as (a) mulberry-like shape, (b) thorny shape, and (c) fin-like shape. The author believes that these shapes all come from dendritic structure of the tear surface and are determined by the relation between the velocity of growth of dendrites and, the velocity of relative motion of tear surfaces moving away from one another.
3. Two or three vein-like segregates described before by K. Singer et al are usually found near each hot-tear. They indicate that the tensile strain of a hot-zone of casting in the later stage of solidification is usually concentrated in a few liquid films.
4. Within the scope of the present observation, hot-tearing is not caused by non-metallic inclusions; it occurs before sulphide inclusions are formed.
5. In a few eases, hot-tearing occurs not between dendrites of primary crystals but on boundaries of primary austenite grains after complete solidification. This kind of hot-tear can easily be distinguished from interdendritic one, and there have been found several sets of conditions for the occurrence of the austenitic hot-tear.
