Internal friction in a martensitic high-carbon steel

This paper presents a study on defect mobility in a high-carbon martensitic tool steel. Mechanical spectroscopy investigations are performed in the te mperature range 80-700 K. Torsion pendulum (about 1 Hz) and flexural vibrat ing-reed (about 3 kHz) techniques are used and compared in order to obtain thermal activation parameters. Differential scanning calorimetry (DSC) meas urements are also performed in the same temperature range in order to detec t structure transformations. The internal-friction spectra obtained as a fu nction of temperature show four peaks P1, P2, P3 and P4. The physical mecha nisms responsible for the peak formation are analysed on the basis of the p eak activation energies, the influence of cold work and tempering on the in ternal-friction spectra and the comparison with DSC. The conclusions are su pported by present information available on similar materials. It is found that P1, P2 and P4 are all relaxation peaks and are related to dislocation movement in the martensite and to their interaction with solute atoms. P3 s hould be considered as a maximum in the internal-friction spectrum related to a transformation of the material, probably leading to carbide-forming re actions. This transformation coincides with a hardness decrease, which mark s the first step of steel tempering.