I talk about your problem with a siderurgical specialist named Jorge Madías (he is in RG), and he said: "It would have to see if this is a porosity by shrinkage, by evolution of gases or by surface reactions. The shrinkage cavity porosity usually is centralized but may be moved if there are any heterogeneity in the heat flow. The porosity outgassing during solidification may be located at a distance from the skin of the ingot, for example 20 mm to 30 mm. Finally, the porosity of the liquid metal reactions with moisture or carbon dust strainer is usually superficial"