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Commercially available specimens of pure molybdenum were immersed in a graphite-filled Ta container and heated at a relatively low temperature of 1373 and 1473 K for 1.2 to 18.0 ks. Changes in the yield and maximum strength were evaluated by a three-point bending test at a temperature between room temperature and liquid-nitrogen temperature. Then two parameters, critical stress (apparent intergranular fracture strength) and critical temperature (DBTT), were determined. Finally, the fracture mode was examined using SEM and the apparent transgranular fracture strength was estimated from the relationship between the critical stress and the fracture mode.Results are summarized as follows. (1) Both the critical stress and the critical temperature changed drastically with carbon addition and exhibited a peak value under optimum conditions: 1373 K-3.6 ks and 1473 K-1.2 ks. (2) The fracture mode changed from intergranular to transgranular after carbon addition. (3) The change in the critical stress as a function of the increase in carbon content in this work almost agrees with that in the previous work of Hiraoka et al. (4) The apparent transgranular fracture strength was unchanged (about 990 MPa) after carbon addition of 10 mass ppm or less. This value is almost equal with that before carbon addition. The apparent transgranular fracture strength was lowered after excess carbon addition.

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