Ferritic steels such as the 4140 in a B7 stud lose their toughness as they get colder, and below a certain temperature they can break like glass. The Charpy test measures how much energy the material absorbs as it breaks at the temperature the specification sets, which is usually related to, but not always the same as, the service temperature. Low-temperature bolting grades such as ASTM A320 L7 and L43 must pass it; ordinary B7 does not have to.
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model dataSpecimen, supports and striker drawn to ASTM E23 and ASTM A370. Pendulum proportions are typical of a 300 J (220 ft·lbf) machine; the energy shown is for illustration. Conditioning, timing and the three-specimen rule from ASTM A370; the L7 temperature and energies from ASTM A320 Table 2.
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