STP462

    Effect of Tempering Temperature on Fatigue-Crack Propagation in 4340 Steel

    Published: Jan 1970


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    Abstract

    The fatigue-crack propagation behavior of heat-treated 4340 steel has been studied as a function of tempering temperature from 400 to 800 F and at +80 and -50 F test temperatures. The Griffith-Irwin fracture-mechanics analysis of the data was used for through-thickness cracks in center-notched sheet specimens. Special emphasis was placed on the phenomenon of tempered martensite embrittlement, which occurs in the 500 to 700 F range of tempering temperatures, in order to see whether it can be detected by fatigue testing. The fatigue-crack-growth test results could be represented by an empirical equation of the form d2a/dN = CK)m. The exponent m was found to vary with tempering temperature and test temperature. The fatigue-crack-growth rate at -50 F was higher than at room temperature. No evidence of tempered martensite embrittlement was observed, since the crack-growth rate was actually a minimum in the embrittlement range, the crack-growth rates decreasing as the tempering temperature increases to 600 F, and then increasing again with higher tempering temperatures. The effect of ethyl alcohol on fatigue-crack growth and the relationship between KIc and the fracture-mode transition are discussed.

    Keywords:

    crack propagation, fatigue, impact behavior, low-temperature tests, high-strength steel, tempering temperature, martensite, embrittlement, testing, evaluation


    Author Information:

    Anctil, AA
    Mechanical Engineer and Physical Metallurgist, Metals Laboratory, Army Materials and Mechanics Research Center, Watertown, Mass.

    Kula, EB
    Mechanical Engineer and Physical Metallurgist, Metals Laboratory, Army Materials and Mechanics Research Center, Watertown, Mass.


    Paper ID: STP32049S

    Committee/Subcommittee: E08.06

    DOI: 10.1520/STP32049S


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