Blowhole formation

Burns D. Beech J., Blowhole formation during solidification of iron alloys. Ironmaking and steelmaking (Quarterly), (1974) 4, 239-250... 

When molding narrow slits, the risk of a blowhole formation is added to the angle or edge weakness. The problem can be bypassed by using steel pieces to be galvanized, which are previously inserted into models. 

Concrete, on the other hand, offers a variety of problems. In the pouring and placement of concrete walls, the formation of blowholes or voids is inevitable. These imperfections vary in size and quantity by the way the concrete is placed and compacted. If the immersion vibrators are kept just below the surface of the concrete during the pour and raised at the same rate as the concrete, the formation of blowholes is kept under reasonable control. In actual practice, this procedure is difficult to control thus causing a variety of patterns and textures to appear from one pour to the next. Figure 49-5 shows a variety of blowhole patterns. 

The most important factor affecting equilibrium gas composition is temperature. Deviations from equilibrium can be explained by the presence of pyrolysis gas, arising during thermal decomposition, by uneven gas and solid distribution, due to channeling, baking, clinkering or formation of blowholes, or by inadequate rates of reaction. 

FIGURE 4.52 Edge weakness and the formation of blowholes when molding narrow grooves (a) model (b) nickel layer (c) edge weakness (d) formation of blowholes ... 

The occurrence of the latter three types of behaviour is determined the growth rate and they are illustrated schematically in Fig. 5.19. Woodruff has shown a good example of the formation of an elongated blowhole in an analogue system (Fig. 5.20). 

Fig. 5.20 Growth of a gas bubble at the interface in solidifying salol (fnucleatton and incorporation into the interface (b) formation oj the elongated blowhole...

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