Figure 1 (left). Failed Sampling – Core Microstructure (SAE 1010); Figure 2 (right). Failed Stamping - Edge Microstructure (SAE 1010)

50X - 5% Nital solution

Question: “I’ve heard the term ‘Stead’s Embrittlement,’ but I know little about it. Can you tell me what this is, and do you have any examples?”

Answer: Stead’s brittleness (embrittlement) is a type of brittle condition resulting from excessive grain growth that often manifests itself in transcrystalline fracture in the coarse-grained structure. The phenomenon was named after J.E. Stead, who explained its cause.

The most common cause is prolonged annealing (crystalline growth of strained ferrite) in low-carbon steel held at a temperature in the range of 500-750°C (930-1380°F), which is below the critical temperature. The result is that the crystals enlarge, and the steel loses part of its strength and ductility. It normally takes a considerable length of time (days) to produce this effect, and it can normally be corrected by reheating the steel above the critical temperature.

This effect is most often associated with cold rolling during finishing operations to obtain smooth surfaces, deep drawing or stamping of low-carbon (<0.20%C) steel. The fracture usually occurs at about 45° or greater to the direction of rolling (see Figures 1 and 2).

The failed part has a carbonitrided case depth of 0.3 mm (.012") and a surface hardness of 62 HRC. The failure occurred at a 90º bend.