Brittleness in Bolts and Other Threaded Fasteners

This paper covers an investigation of brittleness of bolts heat treated to have partial decarburized, carbon restored, and cyanided surfaces. Whereas previous studies seemed to show a demarcation at about 40 Rockwell C in hardness for brittle and nonbrittle behavior, this study shows that partial decarburization will combat brittleness up to a hardness level of 44.5 Rockwell C and that cyaniding and carbon restoration can induce brittleness below 40 Rockwell C under some conditions of loading.

In addition to hardness and surface condition, the effect of six threads and near zero threads in the tension zone is shown. The fewer number of threads increased brittle behavior.