In this third part of the Basic Metallurgy series, we examine some of the problems that can occur—defects in manufacture—that can lead to problems in both manufacture and later in service with the customer. Typically, the main problems are casting defects, such as pipes and porosity, and cracks that can occur during and after manufacture. Porosity can arise from solidification shrinkage or from gas evolution during solidification. There are many causes of cracking in alloys, with the cracks generally intergranular and all similar in appearance. It is not easy to pinpoint the cause.

The common causes of such defects, some of which can lead to difficulties in manufacturing quality and to cracking and fracture, and the steps that can be taken to minimize their occurrence are reviewed. These include poor quality start materials that lead to contamination and embrittlement; poor melting practice leading to casting defects, segregation, porosity and blisters; poor ingot or material working practice; incorrect annealing practice including quench cracking and fire cracking; and stress corrosion cracking.

Author: Dr. Christopher W. Corti