STP645: Use of Laboratory Failure Simulation Exemplars to Study Intergranular Fracture Modes in 9Ni-4Co-0.20C Steel

    Young, JD
    Supervisor and member of technical staff, Metallurgy Unit of Materials and Producibility, Los Angeles Division, Rockwell International, Los Angeles, Calif.

    Kumar, A
    Supervisor and member of technical staff, Metallurgy Unit of Materials and Producibility, Los Angeles Division, Rockwell International, Los Angeles, Calif.

    Pages: 17    Published: Jan 1978


    Abstract

    During structural spectrum load testing of the aft-fuselage structure of the B-1 aircraft, a 124-mm-long crack was detected in the 9Ni-4Co-0.20C steel primary structure. Review of the fracture location revealed that the crack path was adjunct to a production weld used in the fabrication of the 727-kg (1600-1b) forged and welded horizontal-vertical stabilizer support assembly. Upon removal and examination of the fracture on a scanning electron microscope, two distinct fracture modes were observed: (a) at the origin, a 0.5 to 1-mm intergranular fracture mode was observed, followed by (b) 123 mm of transgranular high-stress low-cycle fatigue crack propagation.

    Fatigue crack growth rate analysis on the scanning electron microscope showed that the fatigue crack started propagation at the onset of testing, strongly suggesting a pre-existing crack which was corroborated by the observation of an intergranular fracture mode at the origin, which exhibited no evidence of cyclic growth.

    A laboratory failure exemplar study program was conducted to establish standards for intergranular fracture mechanisms possible in 9Ni-4Co-0.20C steel, fabricated using the same environmental conditions as present during manufacturing. The salient features of the fractographs of known failure mechanisms were used to establish the cause of service failure intergranular cracking to be hydrogen embrittlement.

    Keywords:

    fractography, intergranular fracture, hydrogen embrittlement, weldments


    Paper ID: STP38084S

    Committee/Subcommittee: E08.01

    DOI: 10.1520/STP38084S


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