Fracturing of Gray Iron Door-Closer Cylinder Castings Caused by Lack of Foundry Control Over Chemistry
Door-closer cylinder castings manufactured of class 30 gray iron were breaking during machining. The manufacturing source reported that a random sampling of castings from this lot had hardnesses from 180 to 210 HRB. Based on the color of the components, heat treatment of these castings was suspected. Metallurgical examination on two representative castings supported the conclusions that the cracks in these gray iron door closers that were present either before or during the heat treatment were attributed to a substandard microstructure of the wrong type of graphite combined with excessive ferrite. This anomalous structure is caused by shortcomings in the foundry practice of chemical composition, solidification, and inoculation control. Judging from the microstructure, the strength of the material was lower than desired for class 30 gray iron, and the suspected heat treatment further reduced the strength. Recommendations included that the chemistry and inoculation should be controlled to produce type A graphite structure. The chemistry control should aim for a carbon equivalent close to 4.3% to achieve adequate fluidity for thin sections and to alleviate gas defects.