In a certain period during solidification of a metal casting cooling rate near the center is higher than that at the surface; the difference in the rate is particularly large at and immediately after the completion of salidification. This may give rise to a considerable magnitude of tensile stress at the center of the casting, and hence, in some cases, to internal hot tearing. A calculation was performed to estimate the extent of the stress of this type in sphere castings. Major assumptions made were (1) the material of casting was purely plastic without elasticity in the temperature range considered, (2) strain rate in a given material was dependent solely on temperature and three-axial stresses at the given place and moment and (3) cooling rate was linearly related to the distance from the center. Temperature, cooling rate, material constants and other variables were grouped into several non-dimensional numbers and equations were derived from the theory of steady creep under three-axial stresses. The equations were solved numerically by using a computer. Calculation was performed for both solid and hollow spheres, the latter corresponding to a casting with some liquid remaining in the core. The calculated stress and strain rate, expressed in a general form using the non-dimensional numbers, at various cooling conditions and material constants were plotted against the distance from the center. Interpretation of the graphs thus obtained was discussed. A sphere 100cm in diameter cast from 0.2%C steel in sand mold was taken as a practical example. The calculatad stress was relatively small when only the skin was solid, the remainder being still liquid. However, it rose to a value as high as 1 kg/mm² at the center immediately after the end of solidification. It was shown that the tensile stress inside the casting could be minimized by appling external pressure; the pressure necessary for the purpose could be estimated from the result of the present calculation.

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  On observing macrostructures and sulfur print figures around hot tears in many steel ingots it was found that tears were always surrounded by abnormally equiaxal grain structure. By X-ray analysis, it was proved that a hot tear started from inversely segregated sulfides in the outer skin of steel castings. Hot tearing is considered to occur at two stages : primary tearing occurs during formation of a solid skin, the gap being filled with liquid containing higher solute concentration from which sulfur rich liquid is segregated at the last stage of solidification; secondary tearing probably occurs at the sulfur rich liquid film. A weak point in the solid skin was mostly formed by discontinuous rising of molten metal in ingot mold. The tendency of hot tearing decreased with increase of carbon, silicon and manganese content, and with decrease of sulfur content. The influence of copper and phosphor on the hot tearing was not obvious.
  In order to prevent hot tearing, pouring needs to be made at lower temperature and at lower rate. Reducing atmospher in the mold during pouring prevents the primary tearing in solid skin. Mn/S ratio over 30 is also very effective.

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  Surface defects of large castings of plain carbon steel and alloyed steel which being detected by magnaflux can be roughly classified into hot tear, cold crack, sulfide inclusion and creasy surface. The characteristic of each defect is summarized as follows;
  1. Hot tear can be further divided into three types, namely open crack, hair crack and internal crack. All of them have the dense part of sulfur print, sulfide inclusions of type II being found in hot tear. In case of the open crack the fracture is dendritic and oxidized, its microstructure being quite abnormal as compared with the sound part, while in case of the internal crack it looks like microshrinkage.
  2. Cold crack is a stress cracking which being mainly caused by improper cooling or heating and has a linear form. It can not be distinguished by sulfur print.
  3. As cast surface is sensitive to sulfur print and sulfide inclusions of type II exist throughout in the depth of 0.5mm.
  4. Sulfide inclusions do not always accompany with cracking. It forms networks in sulfur print when it presents much.
  5. Creasy surfaces are of 100 to 150mm length and found on drag surface. They are caused due to improper pouring. The depth of surface folding is approximately 1 to 2mm. There exist many globular oxide inclusions around the defect but it has no relation with the density of sulfur print. Sulfide inclusions and creasy surfaces are not harmful and so it need not repair by welding.

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  Various difficulties are experienced in the conventional welding tin-bronze castings. In this work, therefore, a particular interest is returned to a burning-in method, namely the most traditional and simplest way.
  Specified tensile test bars of 88-6-4-2 bronze showed more improved tensile properties after having been burnt-in rather than as-cast ones. Their tensile strength and elogation were 29.5 kg/mm² and 52.2%, while those of as cast ones were 28.3 kg/mm² and 48.6% respectively. Oxide inclusions were of the most troublesome defect. A rapidly cooled structure was formed in the burnt-in area in which columnar crystals grew from the original casting. This fact indicates that the burning-in is available to repair such defects as pressure leakage. Pre- and after-heating were essential to the burning-in treatment. Heating up to 500°C had little effect on the mechanical and morphological properties of 88-6-4-2 bronze, while heating above 500°C decreased the strength slightly and elongation considerably. Heating above 700°C caused coarsening grain size and diffusing lead and copper-rich phases. Hence the pre- and after-heating were made at 400 to 500°C. As compared with this, the coated electrode welding caused oxide inclusions and pinholes, and allowed poor mechanical properties of tensile strength 15.3 kg/mm² and elongation 10.0%. Oxyacetylene welding, caused blow holes and a number of pinholes, and also gave poor properties of tensile strength 22.4 kg/mm² and elongation 16.0%. Both were undesirable for heavy duty repairing.

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