STP466

    An Evaluation of the Charpy Impact Test

    Published: Jan 1970


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    Abstract

    For mild steels intended for service temperatures down to -20 to -30 C it is normal to prescribe a Charpy V-notch test with a minimum requirement of 2.8 kgf∙m. Experience has proved that this is a satisfactory criterion for this type of steel. With regard to steels for lower service temperatures and those with higher tensile strength there is a degree of uncertainty. A minimum impact energy of 3.5 to 4.15 kgf∙m at test temperatures immediately below service temperature often is prescribed.

    The investigations described here, cover hot rolled plates from six European steelworks. The materials are from 20 heats (one plate from each) and are intended for welded structures with service temperatures down to -50 C. The steels have an ultimate tensile strength from 47.3 to 67.5 kgf/mm2 and a yield point from 32.1 to 48.9 kgf/mm2.

    The nil-ductility transition (NDT) temperature was determined and for the 20 heats lies between -40 and -75 C. Impact tests were carried out as Charpy V-notch tests taken lengthwise (KVL) and transverse (KVT) to the rolling direction. Charpy pressed notch tests also were carried out as longitudinal tests (KPL).

    The KVL tests show considerable scattering in the whole temperature range from -15 to -80 C. This tendency to scattering varied greatly from heat to heat. There is no unambiguous correlation between the NDT temperatures and the KVL values. In the transition range the transition curves exhibit two kinks giving the curves an approximate double S-form. This may be interpreted as an expression of bimodality in the range around the kinks. The NDT temperature lies in most cases in the range where the kinks are situated.

    The KVT values show considerably less scattering than the KVL values and no tendency to bimodal distribution at the actual test temperatures. An interesting observation is that the KVT values at the respective NDT temperatures of the different steels show very little variation. The average value for all the KVT tests carried out at NDT temperature is 3.21 kgf∙m with a standard deviation of only 0.57 kgf∙m. This seems to indicate that for this type of low temperature steel a criterion based on the KVT value at a constant testing temperature may be applied in order to sort out the steels which have a NDT temperature higher than the testing temperature.

    The KPL values generally show little scattering. There is, however, a very marked bimodal distribution over a small temperature range which begins at 30 to 40 C over the NDT temperature.

    Microfractographic investigations with extraction replicas taken from just below the base of the notch of KVT specimens show thin discontinuous slag film in the fracture surface. The presence of slag films does not appear to affect the fact that the NDT temperature is independent of the rolling direction but seems to affect the level of the NDT temperature.

    Keywords:

    steels, impact tests, low-temperature tests, plates, welded joints, temperature tests, microfractographic investigation, slags, drop-weight test, scattering, transition curve, pressed notch, evaluation, tests


    Author Information:

    Harsem, Ø
    Chief metallurgical research engineer and director of metallurgical research, Det norske Veritas, Oslo,

    Wintermark, H
    Chief metallurgical research engineer and director of metallurgical research, Det norske Veritas, Oslo,


    Paper ID: STP32056S

    Committee/Subcommittee: E28.07

    DOI: 10.1520/STP32056S


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