35CrMo is a widely used medium-carbon alloy steel for high-strength fasteners, but its performance in high-stress applications is highly dependent on manufacturing quality and is subject to notable failure risks. The steel is capable of achieving high strength levels—for instance, 10.9-grade bolts with nominal tensile strengths exceeding 1000 MPa. However, field failures are frequently reported.
Failure analyses of 35CrMo bolts from wind turbines reveal a recurring root cause: surface decarburization during heat treatment. In one case, a 10.9-grade bolt fractured at the thread root; testing showed a surface carbon content of only 0.009% and a decarburization depth of 23.5 μm, exceeding standard limits. This decarburization drastically reduces surface hardness and fatigue resistance, creating a weak zone where stress concentrates and cracks initiate. A separate study of M24 bolts that failed after just one month in service confirmed the same mechanism: excessive and uneven decarburization led to fatigue fracture under design loads. Additionally, research on hydropower applications suggests that 35CrMo bolts may exhibit lower fatigue life compared to lower-strength alternatives like 45 steel in certain loading conditions. Therefore, while 35CrMo is intrinsically capable of high strength, stringent control of heat treatment atmospheres and thorough inspection are mandatory to prevent premature fatigue failure in critical bolting.
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