A forging die in a 250-ton press producing brass valves began to show signs of fatigue after a few thousand hits. By the time it reached 30,000 hits, the die was badly damaged and was submitted for analysis along with one of the last forgings produced. The investigation included visual and macroscopic inspection, metallographic and chemical analysis, SEM imaging, optical profilometry, mechanical property testing, and EDX analysis. The die was made of chromium hot-work tool steel and the forgings were made of CuZn39Pb3 heated to an initial working temperature 700 deg C. The entire surface of the die was covered with fatigue cracks and many fillets had been plastically deformed. Several other types of damage were also observed, including areas of oxidation, corrosion pits, voids, abrasive wear, die adhesion, and thermal fatigue. Fatigue cracking was the primary cause of failure with significant contributions from the other damage mechanisms.

A cross-recessed die of D5 tool steel fractured in service. The die face was found to be subjected to shear and tensile stresses as a result of the forging pressures from the material being worked. The presence of numerous slag stringers was revealed by microscopic examination of an unetched longitudinal section taken through the die. The pattern was microscopically revealed after etching with 5 % nital to be due to severe chemical segregation or banding. Considerable variation in the hardness, corresponding to the banded and non-banded regions across the face of the specimen was observed. The fracture was found to have originated near the high-stress region of the die face examination of the fracture surface. Failure of the die was concluded to have originated in an area of abnormally high hardness which is prone to microcracking during heat treatment for this grade of tool steel