Abstract: Cracked and uncracked ultra-high strength martensitic steel parts and the corresponding raw materials steels were analyzed due to phenomenon of cracking of the parts in their safe service period. The U-shaped bending beam experiment results in constant strain test show that both the raw materials of cracked and uncracked parts have good resistance to delayed cracking caused by hydrogen. Due to the difference of the production processes, the microstructure of the raw materials is plate martensite for the cracked part and lath martensite for the uncracked part, respectively. When machining into the same kind of parts, due to the structural difference of the martensite, the internal collaborative deformation uniformity is different in machining and forming processes, thus the residual stress in the cracked parts is larger. In addition, the stress concentration is caused by the formation of a large number of dislocation pile-ups in the structure of cracked parts, the local stress reaches the limit first, resulting in cracking.

Key words: ultra-high strength martensitic steel, microstructure, internal stress, dislocation, cracking failure