Hongyun Luo

Effects of strain and strain-induced α'-martensite on passive films in AISI 304 austenitic stainless steel.

In this paper, the effects of strain and heat treatment on strain-induced α-martensite of AISI 304 stainless steel tubes were measured by X-ray diffraction. Moreover, the effects of strain and content of α-martensite on passivated property on the surface of the material in borate buffer solution were evaluated by electrochemical technique. The results showed that the volume fraction of α-martensite increased gradually with the increase of tensile strain for as-received and solid solution samples. However, α-martensite in as-received sample was more than that in the solid solution sample. The electrochemical impedance spectroscopy results showed that the solid solution treatment improved corrosion resistance of the steel, especially for samples with small strain. Moreover, acceptor densities were always higher than donor densities for as-received and solid solution samples. With the increase of strain, the increase tendency of acceptor density was more significant than that of donor density. We also found that the total density of the acceptor and donor almost increased linearly with the increase of α-martensite. The present results indicated that the increased acceptor density might lead to the decreased corrosion resistance of the steel.

Producing Nanostructured 304 Stainless Steel by Rolling at Cryogenic Temperature

Effect of rolling temperature on deformation-induced α′-martensite and reverted austenite was investigated. Rolling experiments were carried out at room temperature (without inter-pass cooling) and at liquid nitrogen temperature, respectively. The phase components, orientations, and microstructures are characterized by X-ray diffraction, electron backscattered scattering detection, and transmission electron microscopy. The (200)α′ martensite was found to be of the preferred orientation in sample rolled at cryogenic temperature. An average grain size of about 300 nm combined with high yield strength (∼1500 MPa) and good elongation (∼35%) was obtained by the cryogenic temperature rolling and 240 s annealing treatment.

Investigations of the fatigue damage in 16Mn steels by wavelet-based acoustic emission technique

16Mn steel is widely used in the large structural machines such as cranes. Fatigue is one of the most common failure modes in this material. Thus, monitoring of the fatigue damage in 16Mn steel is important for the health monitoring of these large engineering structures. In this paper, fatigue crack propagation tests were carried out for hot rolled and low temperature tempered 16Mn steels. Acoustic emission (AE) signals obtained from the fatigue tests were processed by wavelet packet transform (WPT) analysis. The energy distribution in each component of the AE signals was calculated for different specimens. Ratio of energy was extracted as a feature parameter. The results showed that the ratio of energy in each node was different in the hot rolled and low temperature tempered 16Mn steels respectively. The ratio of energy depended on the micro-structure. Based on the SEM fractography observations in this study, these results could be well explained by the different fatigue crack propagation mechanisms.

Acoustic Emission Monitoring of Brittle Fatigue Crack Growth in Railway Steel

Rail networks are gradually getting busier with rolling stock traveling at higher speeds and carrying heavier axle loads than ever before. In order to consider the further application of acoustic emission (AE) for railway damage monitoring, the fatigue and AE characteristics during the fatigue crack growth (FCG) process in a rail steel were investigated in this study. For better recognition and prediction fatigue damage of rail steel, the relationship between the micromechanism and the characteristics of AE signals were studied. The FCG experiments were carried out, and the AE signals emitted from FCG process were studied by AE parameters and Fast Fourier Transformed (FFT) spectra analysis. The AE results were also compared with the fracture surfaces. The results show that the AE technique is capable of acquiring and identifying brittle fracture events during FCG of rail steel.

An Integrated Processing Method for Fatigue Damage Identification in a Steel Structure Based on Acoustic Emission Signals

Abstract This paper presents an integrated processing method that applies principal component analysis (PCA), artificial neural network (ANN), information entropy and information fusion technique to analyze acoustic emission signals for identifying fatigue damage in a steel structure. Firstly, PCA is used to build different data spaces based on the damage patterns. Input information from each sensor is diagnosed locally through ANN in the data space. The output of the ANNs is used for basic probability assignment. Secondly, the first fusion operation adopts Dempster-Shafer (D-S) evidence theory to combine the basic probability assignment value of ANNs in the different data space of a sensor. Finally, the fusion results of each sensor are combined by D-S evidence theory for the second fusion operation. In this paper, information entropy is used to calculate the uncertainty and construct basic probability assignment function. The damage identification method is verified through four-point bending fatigue tests of Q345 steel. Validation results show that the damage identification method can reduce the uncertainty of the system and has a certain extent of fault tolerance. Compared with ANN and ANN combined with information fusion methods, the proposed method shows a higher fatigue damage identification accuracy and is a potential for fatigue damage identification.

Effect of Specimen Thickness on Fatigue Crack Propagation and Acoustic Emission Behaviors in Q345 Steel

The effects of specimen thickness on fatigue crack growth rate (FCGR) and acoustic emission (AE) behaviors of Q345 steel were investigated. The four-point bending fatigue tests were carried out with AE monitoring simultaneously. Based on the thickness effect analysis, fatigue behavior studies, and AE investigations, the effects of specimen thickness on AE signal and AE source mechanisms during fatigue crack propagation were proposed. The results show that as specimen thickness increased, the FCGR was accelerated slightly, while the AE count rate was increased significantly, suggesting that AE signal was more sensitive to the changes in thickness. By analyzing the AE signals at the new plastic yielding area and the crack tip micro-fracture process, AE source mechanisms were explained. These results suggest that the effects of thickness must be considered to obtain a more accurate estimation of fatigue crack propagation through AE technique.

The Effect of Low Temperature Sensitization on Corrosion Resistance of Ultrafine-Grained Type 321 Stainless Steels

The ultrafine grain 321 stainless steel was produced by cold rolling. The microstructure, after 87% cold rolling and annealing at 380 and 450xa0°C, has been characterized using transmission electron microscopy and x-ray diffraction. The degree of sensitization (DOS) was investigated by means of double loop electrochemical potentiokinetic reactivation test. The results showed that the DOS was higher in samples annealed at 380xa0°C than that in samples annealed at 450xa0°C. The strain-induced martensite promoted sensitization at 380xa0°C and the reversion-transformed austenite-restrained sensitization or promoted healing of chromium-depleted zone at 450xa0°C. Although the activation and reactivation current densities in samples sensitized at 450xa0°C were higher than those in samples sensitized at 380xa0°C, higher corrosion resistance of the former was attributed to lower DOS, which was proven further by electrochemical impedance spectroscopy experiment.