TRANSACTIONS OF SECTION B. 563 



3. The Closing and Welding of Blowholes in Steel Ingots. 

 By Professor Henry M. Howe, LL.D. 



In the solidification of molten or liquid substances, especially those of high 

 melting-point, two classes of cavities are likely to form : gas bubbles called 

 ' blowholes,' and a central contraction cavity called a ' pipe.' 



The blowholes represent (a) the progressive concentration in the molten or 

 liquid mother mass of the gases initially present, a concentration carried on to 

 supersaturation, and to the liberation of part of this gas from the supersaturated 

 layers; and perhaps (b) in some cases, such as that of the solidification of steel 

 ingots, the formation of a gas from chemical reaction brought about by fall of 

 temperature or by passage from the liquid to the solid state. In the case of steel 

 ingots there are indications that carbonic oxide is thus formed during solidification 

 by the union of carbon and oxygen present side by side in the molten metal. 



The formation of the central ' pipe ' is due to the cooling, and hence 

 contraction of the different layers of the mass aeoliotachically, i.e., at different 

 rates inter se. In the first stages of solidification the very outside of the mass, 

 especially if it is cast in a cold iron mould, cools much faster than the still 

 deeper seated layers. The early excess of contraction of the skin, caused by this 

 excess of cooling, is resisted by the lagging interior, with the result that the 

 skin is virtually stretched beyond its normal dimensions, so that when 

 solidification is complete the interior, which in the latter part of the cooling 

 has to cool through a greater range of temperature and hence has to contract more 

 than the skin, no longer suffices to fill that skin completely, and this deficit 

 of volume of the interior is represented by a central cavity overlying the region 

 in which the last of the solidification occurs. This same excess of contraction of 

 the earth's crust in its early stages should later throw that crust into great 

 compression, which may be an important element in volcanic and earthquake 

 phenomena. , 



Blowholes themselves tend in effect to expand the volume of the interior as 

 a whole without changing the volume enclosed by the skin— i.e., the outer 

 dimensions and thus to lessen the deficit or pipe. 



In case of steel ingots this pipe may reach very deep into the axis, and, 

 because it is hard to work up, may compel us to disregard as much as one-third 

 of the ingot in order to get sound unpiped metal. To avoid this some makers 

 of steel of a composition favourable to welding have purposely allowed blowholes 

 to form rather abundantly, so as to prevent the formation of a pipe, and, relying 

 on the ease with which such steel welds, have tried to get flawless metal by 

 welding these blowholes up in the process of rolling the ingot out into its final 

 form, such as that of a boiler plate. 



This procedure is of great economic importance, in that it enables the steel- 

 maker to avoid the serious discarding which would be necessary in case his 

 ingots were free from blowholes and hence deeply piped. But many intelligent 

 metallurgists have condemned this practice on the ground that the closing of 

 blowholes is impossible, because the gas which they contain must remain ever 

 present during the rolling, though of course somewhat compressed. 



In some late investigations I have carried out two lines of inquiry as to 

 whether the gas of the blowholes is qualitatively absorbable and whether the 

 sides of the blowholes themselves are qualitatively weldable under the conditions 

 of actual manufacture. Both lines proceed by comparing the metal in slabs cut 

 from the original ingot without rolling, with metal cut from a boiler plate into 

 which that same ingot was rolled, and cut in such a way as to separate and 

 distinguish those parts of the metal in the plate which had originally been porous 

 when in the ingot from those which had originally been compact. 



The first line showed that the enormous differences in density which existed 

 between the porous and the compact parts of the ingot were practically completely 

 obliterated in rolling the metal down into a boiler plate. In one case the initial 

 difference of 16 per cent, in density was completely removed ; in the other the 

 initial difference of 10 per cent, in density was reduced to one-fiftieth its original 



This tended strongly to confirm the strong antecedent probability that the 

 blowhole gases could be reabsorbed during the rolling process, thanks to its 

 high temperature and pressure. 



