Internal friction in martensitic, ferritic and bainitic carbon steel; cold work effects

We present the comparative analysis of the temperature dependent internal friction (IF) spectra for 1.23 wt.% carbon steel with martensitic, bainitic or ferritic structure as well as cold-work effects. Samples that have a martensitic structure at room temperature show a characteristic spectrum consisting of five peaks and an exponential background. Tempering at 800 K transforms the sample structure to a completely ferritic one. All peaks are erased upon tempering, except the peak P5 identified as a Snoek–Köster (S-K) relaxation, the amplitude of which is however drastically reduced. The Snoek–Köster peak is also present in the bainitic structure as well as in initial ferrite, but with an amplitude much lower than in martensitic samples. Cold work performed on tempered samples at room temperature either by bending or roll milling is followed by the formation of a very broad double peak between 200 and 300 K. A similar peak is also found in initial ferrite, which has been subjected to heavy machining. A local minimum in the IF spectrum is found at the temperature of cold work and post-aging. This minimum is the effect of dislocation pinning by carbon precipitates.