9. Defectology

Thermite welding has a number of weaknesses:

• disturbed internal cleanliness (blowholes, hercynites) due to inadequate settling times, combined with reactions that are sometimes too slow;

• As far as blowing is concerned, the maximum reaction temperature should be limited to 2,000°C by adding an adequate quantity of cooling scrap, since the Richardson diagram (figure 1 ) shows that, above this temperature, carbon becomes more reductive than aluminum and favors the appearance of carbon monoxide CO (blowing);

• of this metallographic set, fractures such as the one shown in figure