Model of the evolution of acoustic emission as the randomization of transient processes in coupled nonlinear oscillators

The behavior of a crack as a resonator radiating acoustic emission (AE) pulses at instants of sudden growth is investigated theoretically and experimentally. This resonance behavior of a growing crack is determined to a large extent by surface waves propagating along its edges. The crack can therefore be regarded as an acoustic resonator excited at the instant of growth of its tip. Transformations in the form of high-frequency harmonic and combination-frequency subharmonic generation are observed in the spectra of the AE signals. The final stage in the evolution of AE is characterized by the transition to a wideband noise spectrum. These facts lead to the hypothesis that bifurcations analogous to those encountered in the onset of dynamic chaos take place in the AE process. This hypothesis forms the basis of a mathematical model of the AE process as a system of coupled nonlinear oscillators, each corresponding to an individual crack. The initial displacement in one of the interacting cracks is adopted as the bifurcation parameter. Spectra calculated by computer simulation exhibit qualitative agreement with the evolution of the spectra obtained in the processing of data from physical experiments

Retained Austenite Transformation during Heat Treatment of a 5 Wt Pct Cr Cold Work Tool Steel

Retained austenite transformation was studied for a 5 wt pct Cr cold work tool steel tempered at 798 K and 873 K (525 degrees C and 600 degrees C) followed by cooling to room temperature. Tempering cycles with variations in holding times were conducted to observe the mechanisms involved. Phase transformations were studied with dilatometry, and the resulting microstructures were characterized with X-ray diffraction and scanning electron microscopy. Tempering treatments at 798 K (525 degrees C) resulted in retained austenite transformation to martensite on cooling. The martensite start (M-s) and martensite finish (M-f) temperatures increased with longer holding times at tempering temperature. At the same time, the lattice parameter of retained austenite decreased. Calculations from the Ms temperatures and lattice parameters suggested that there was a decrease in carbon content of retained austenite as a result of precipitation of carbides prior to transformation. This was in agreement with the resulting microstructure and the contraction of the specimen during tempering, as observed by dilatometry. Tempering at 873 K (600 degrees C) resulted in precipitation of carbides in retained austenite followed by transformation to ferrite and carbides. This was further supported by the initial contraction and later expansion of the dilatometry specimen, the resulting microstructure, and the absence of any phase transformation on cooling from the tempering treatment. It was concluded that there are two mechanisms of retained austenite transformation occurring depending on tempering temperature and time. This was found useful in understanding the standard tempering treatment, and suggestions regarding alternative tempering treatments are discussed. (C) The Author(s) 2017.Funders: Uddeholm AB</p