Axial loading fatigue tests were carried out on high tension bolts made of boron steel and Cr-Mo steel. The initiation and the subsequent growth of micro fatigue cracks in the bottom of thread were examined through the micro-fractography using SEM.
The results obtained were as followed:
(1) The fatigue strength of boron steel bolts was quite superior to that of Cr-Mo steel bolts. For the stress ratio of the actual stress used in the engine to the mean stress, σa/σm=0.2, the fatigue limit of boron steel bolts was 83MPa, while the other was 54MPa and 65% of the former.
(2) Both boron steel and Cr-Mo steel showed almost the same monotonic and cyclic 0.2% proof stresses.
(3) The fractography using SEM showed a remarkable difference in the initiation and subsequent growth of micro cracks in the bottom of thread between boron steel and Cr-Mo steel bolts. The fatigue fracture surface of boron steel bolt near the bottom of thread consisted of the former austenite grain boundary, while that of Cr-Mo steel bolt occured within the grain and did not correspond to the microstructure.
(4) The former austenite grain size of boron steel was about twice as large as that of Cr-Mo steel. Based on the criterion that fatigue cracks initiate when the size of plastic zone at the bottom of thread exceeds the grain size, it was interpreted that the larger grain size of boron steel caused its higher fatigue limit.
