In-situ monitoring of ultrasonic attenuation during rotating bending fatigue of carbon steel with electromagnetic acoustic resonance

Using electromagnetic acoustic resonance (EMAR), we studied the evolution of the surface-shear-wave attenuation and phase velocity during rotating bending fatigue of a 0.45% C steel. In the EMAR method, we used a magnetostrictively-coupled electromagnetic acoustic transducer (EMAT) for the contactless measurements of the axial shear wave that is a surface-shear wave, propagating in a cylinder-specimen circumference direction with the axial polarization. The attenuation coefficient always showed sharp peaks around 90% of the fatigue life, independent of the fatigue-stress amplitude. In addition to the ultrasonic measurements, we made crack-growth observations using replicas and measured recovery of attenuation and velocity after stopping the cyclic loading just before and after the peak. From these results, we concluded that the evolution of the ultrasonic properties is caused by a drastic change in dislocation mobility being accompanied by the crack growth.     

Laser–ultrasonic absorption measurements in low carbon steels

We have refined the contactless laser–ultrasound reverberation technique to measure ultrasonic absorption on small metallic samples. In this technique, a sample is supported by a holder which is ultrasonically decoupled from the sample. A pulsed laser is used to generate an acoustic pulse. After the pulse has mode converted and scattered sufficiently to fully insonify the sample, the decrease in the noise-like ultrasonic signal is recorded as a function of time using a laser-interferometer. A joint time–frequency analysis technique is used to extract an absorption spectrum from the signal. In this paper, the technique is demonstrated in a frequency bandwidth ranging from 1 to 7 MHz, and in a dynamic range of 0.003 to 0.3 dB μs⁻¹. Measurements made on samples of three low-carbon steel grades, namely ultra low carbon (ULC), low carbon (LC), and high strength, low-alloy steels (HSLA), clearly show that ultrasonic absorption varies with steel grade. The technique was utilized to study the effect of a magnetic field on the ultrasonic absorption of an annealed ultra low carbon steel sample. It was found that magnetoelastic effects are responsible for a large fraction of the total absorption.     

Effect on marine immersion corrosion of carbon content of low alloy steels

This paper attempts to reconcile information from a number of different sources about the effect of small changes in carbon content on the immersion corrosion of specimens of normal commercial mild and low alloy steels. It does so through interpreting the data reported in the literature in terms of the recently proposed theoretically based phenomenological model for marine immersion corrosion. This model postulates different corrosion phases as corrosion progresses. When the experimental results are interpreted in terms of the model it is found that carbon content has minimal effect on the kinetically controlled corrosion phase. The next phase, when corrosion rate is controlled by oxygen diffusion, is also unaffected, in agreement with theoretical predictions. However, carbon content does affect the two anaerobic phases, with increased corrosion as the carbon content and the water temperature increase. The model allows apparently conflicting observations to be reconciled and shows that carbon content may be influential for longer-term corrosion and for corrosion in tropical waters.     

Influence of laser hardening and resulting microstructure on fatigue properties of carbon steels

Cylindrical specimens of a CSN 12050 carbon steel, equivalent to the UNS G 10420 steel, with two different initial microstructures, normalized and heat treated, were surface processed without melting by a 2.5 kW, CO₂ laser to study the effects of laserbeam hardening and resulting microstructure on fatigue properties and mechanisms. Two configurations of circumferential laser passes were made, resulting in one and three separate surface hardened lines, respectively. Fatigue resistance was studied using alternating bend tests. A detailed metallographic study and x-ray measurements of surface stresses were carried out. It was shown that the laser beam hardening under different conditions either reduced or slightly improved the fatigue life.     

Fatigue damage monitoring of round shape specimen by acoustic microscopy

Acoustic microscope can detect microstructural features with high resolving power compared to other lower frequency acoustic techniques. Many non-destructive evaluations using acoustic microscope, several of them used in industrial fields, were investigated. However, acoustic measurement of curved surface structure was restricted to basic studies as conventional acoustic microscope system was designed for flat specimen. We have developed a modified acoustic microscope for curved surface specimen testing and have measured acoustic image of round bar specimen. In this study, using this system, velocities of round bar specimen during fatigue test are monitored using the V(z) curve method. Measurement errors depending on curved surface measurement were investigated and it was found that the acoustic velocity tends to increase as fatigue damage increases.     

低碳钢回火过程中Cu、Nb析出的相互作用

研究了两种不同成分的低碳钢在600 ℃条件下经不同时间回火处理后的硬度变化,并对600 ℃回火18 h的样品析出物进行组织观察、TEM观测及粒度统计。结果表明：C-Nb钢和C-Cu-Nb钢在600 ℃条件下,短时间回火硬度趋势相反,长时间回火硬度趋势平缓,说明Cu和Nb的析出过程独立存在,析出时间和规律不同;Cu-Nb钢中Nb析出物颗粒尺寸明显大于C-Nb钢,说明Cu的添加促进了Nb析出物的长大。     

Evaluation of fatigue damage in reinforced concrete slab by acoustic emission

The applicability of acoustic emission (AE) technique for evaluation of fatigue damage in reinforced concrete (RC) slabs under cyclic loadings in both laboratory and as a structure in service is studied. The fundamental test performed in laboratory shows that the cracking process can be practically monitored by the measurement of AE signals. Analysis of the relationship between loading phase and AE activity indicates that the final stages of the fracture process can be evaluated by detecting AE signals generated near the minimum loading phase. Comparison of the results from the structure and that from the laboratory specimen demonstrates that AE energy can be an effective parameter for the evaluation of fatigue damage in RC slabs in service.     

Application of the acoustic resonance method to evaluate the grain size of low carbon steels

The velocities and attenuations of ultrasonic waves in low carbon steels were measured precisely and automatically by the acoustic resonance method using a Lorentz-type EMAT. The attenuations were measured from the decay of a signal from the resonant vibration after stopping the external excitation at the predetermined resonant frequency, while the velocities were calculated directly from the resonant frequencies and material thicknesses. It was not possible to predict the yield strength from the velocity measurements. There was a close relationship between grain size and attenuation in the specimens which consist of ferrite and pearlite. The attenuations measured at a frequency of about 5 MHz showed a good correlation with the average grain size and yield strength. The yield strength could be evaluated within the accuracy of ±50 MPa by the acoustic resonance method. The results would be used for on-line evaluation of the grain size and the mechanical strength of steels in industry.     

Elucidating the variables affecting accelerated fatigue crack growth of steels in hydrogen gas with low oxygen concentrations

The objective of this study was to quantify the effects of mechanical and environmental variables on oxygen-modified accelerated fatigue crack growth of steels in hydrogen gas. Experimental results show that in hydrogen gas containing up to 1000 v.p.p.m. oxygen fatigue crack growth rates for X52 line pipe steel are initially coincident with those measured in air or inert gas, but these rates abruptly accelerate above a critical ΔK level that depends on the oxygen concentration. In addition to the bulk gas oxygen concentration, the onset of hydrogen-accelerated crack growth is affected by the load cycle frequency and load ratio R. Hydrogen-accelerated fatigue crack growth is actuated when threshold levels of both the inert environment crack growth rate and K_max are exceeded. The inert environment crack growth rate dictates the creation of new crack tip surface area, which in turn determines the extent of crack tip oxygen coverage and associated hydrogen uptake, while K_max governs the activation of hydrogen-assisted fracture modes through its relationship to the crack tip stress field. The relationship between the inert environment crack growth rate and crack tip hydrogen uptake is established through the development of an analytical model, which is formulated based on the assumption that oxygen coverage can be quantified from the balance between the rates of new crack tip surface creation and diffusion-limited oxygen transport through the crack channel to this surface. Provided K_max exceeds the threshold value for stress-driven hydrogen embrittlement activation, this model shows that stimulation of hydrogen-accelerated crack growth depends on the interplay between the inert environment crack growth increment per cycle, load cycle frequency, R ratio and bulk gas oxygen concentration.     

凝固初期施加超声波对低碳钢含气程度的影响

研究了在凝固初期施加超声波对低碳钢含气程度的影响。结果表明,较低功率的超声波可以更加有效的脱除钢锭内气体;高过热度下的超声波处理有利于降低钢锭含气程度;为了降低钢锭含气程度,最有利的超声处理时间为15 s。并分析了产生上述结果的机理。     

低碳微合金Ti-Nb可焊钢中的N及其第二相粒子

研究了系列低碳微合金Ti-Nb可焊钢中的N含量对模拟焊接热影响区(HAZ)高温奥氏体晶粒尺寸和冲击韧度的影响及其第二相粒子的作用.对试样进行了大热输入焊接热模拟,测定了高温奥氏体晶粒尺寸和焊后试样中的冲击韧度值,并用透射电子显微镜萃取复型法观察了典型试样中第二相粒子(Ti,Nb)N的形态及分布特征.结果表明,钢中Ti、N含量及第二相粒子的尺寸和数目与高温奥氏体晶粒尺寸及冲击韧度值具有很好的对应关系;钢中的N由于生成了细小弥散分布的第二相粒子(Ti,Nb)N而细化了高温奥氏体晶粒,改善了焊后韧性.低碳微合金Ti-Nb钢中适宜的含氮量有一个范围.     

Influence of tensile straining on the permeation of hydrogen in low alloy Cr-Mo steels

The effect of dynamic tensile straining on hydrogen permeation through low alloy Cr-Mo steel samples was studied with the electrochemical permeation technique. The hydrogen steady-state permeation current through large flat tensile specimens mounted between the two compartments of an electrochemical permeation cell undergoes different types of changes, depending on the charging conditions, on the steel’s composition and microstructure and on the strain rate. Dynamic trapping of hydrogen to strain-induced dislocations which leads to a deviation of the permeation current below the initial steady-state value is mostly observed when the external hydrogen activity and the strain rate are large. However, the hydrogen permeation current through Cr-Mo steels with a bainitic microstructure appears to be less sensitive to tensile straining up to large deformation levels than a lower alloyed Cr-Mo steel with a ferrito-pearlitic microstructure. On the other hand, the enhancement of the steady-state hydrogen permeation current observed during tensile straining if specific experimental conditions are met strongly suggests a mechanism of hydrogen transport by mobile dislocations which contributes to hydrogen permeation.     

Effect of silicon and prior deformation of austenite on isothermal transformation in low carbon steels

Isothermal transformation (TTT) behavior of the low carbon steels with two Si con-tents (0.50 wt pct and 1.35 wt pct) was investigated with and without the prior deformation. The results show that Si and the prior deformation of the austenite have significant effects on the transformation of the ferrite and bainite. The addition of Si refines the ferrite grains, accelerates the polygonal ferrite transformation and the formation of M/A constituents, leading to the improvement of the strength. The ferrite grains formed under the prior deformation of the austenite become more ho-mogeneous and refined. However, the influence of deformation on the tensile strength of both steels is dependent on the isothermal temperatures. Thermodynamic calcu-lation indicates that Si and prior deformation reduce the incubation time of both ferrite and bainite transformation, but the effect is weakened by the decrease of the isothermal temperatures.     

Characterization of fatigue damage in unidirectional GFRP composites through acoustic emission signal analysis

Fatigue damage progression in composite materials is governed by different failure mechanisms, each of which contributes to the damage to a different extent. To assess the cumulative damage undergone by the material and to estimate the residual life, it is necessary to discriminate and characterize the failure mechanisms. This necessitates an on-line technique which can be used to monitor and measure the damage progression as it occurs. Acoustic emission (AE), an on-line monitoring tool, is ideally suited for this purpose. To understand fatigue damage in terms of different failure mechanisms using this technique, it becomes necessary to identify and establish their AE characteristics. This paper discusses the experimental investigations carried out on unidirectional glass fibre reinforced plastic (GFRP) composite specimens in which the acquired AE data was analysed utilizing pattern recognition (PR) techniques. The results obtained from the experiments show that three different failure mechanisms which primarily contribute to the damage at any given stage can be discriminated and characterized. Further, an attempt is made to estimate the cumulative damage to predict residual life.     

Effect of boron addition on the microstructures and mechanical properties of thermomechanically processed and tempered low carbon bainitic steels

Thermomechanical process and tempering heat treatment were employed to produce the experimental steel plates. The effect of boron addition on the microstructure and mechanical properties of low carbon bainitic steels was studied in this paper. Microstructure observation and crystallographic features were conducted by using optical microscopy, SEM, TEM and electron back scattering diffraction (EBSD) analysis. The results showed that under the same rolling processes and heat treatment conditions, a substantia...     

Thermographic detection of fatigue damage of reactor pressure vessel (RPV) steels

An IR (IR) thermography technique, as a nondestructive evaluation technique, was applied to investigate the fatigue damage of reactor pressure vessel (RPV) steels during 20 Hz and 1000 Hz fatigue testing. Five stages of temperature profile were observed: an initial increase of the average specimen temperature, a region of temperature decrease, an equilibrium (steady-state) temperature region, an abrupt increase of the temperature, and a drop of temperature following specimen failure. The relationship between the temperature, stress-strain state, and fatigue behavior is discussed. Both thermodynamic and heat-transfer theories are applied to model the observed temperature variation during fatigue. The stress-strain state of the material has been back-calculated from the observed temperature profiles. The predicted and measured temperature evolutions and mechanical behavior during fatigue were found to be in good agreement. Thermography appears to provide a useful method of investigating the stress-strain behavior during fatigue.     

Equilibration of plain carbon and alloy steels with endothermic carburizing atmospheres: Part I. Activity of carbon in plain carbon steels

Experimental data on the activity of carbon in commercial plain carbon steel, AISI type 1010, common shim-stock material, at 871 °C, 927 °C, 1000 °C, and 1038 °C, equilibrated with common endothermic carburizing atmospheres, were found to be generally in agreement at relatively high carbon concentrations with previously published data on pure iron equilibrated with CO-CO₂ mixtures. However, noticeable differences were found at low carbon concentrations. The carbon contents of type 1010 steel were generally higher compared to those of previously published data regarding pure iron except at very high activities of carbon. The carbon contents of type 1010 steel foil specimens by weight gain were generally higher than or equal to those chemically analyzed; however, they corresponded with each other within 0.02 wt.% except at very low carbon potentials.     

Influence of chromium,molybdenum and cobalt on the corrosion behaviour of high carbon steels in dependence of heat treatment

High chromium martensitic steels are designed to provide high corrosion resistance in combination with high strength. Some of these steel grades contain primary carbides for improving the wear resistance, e.g. the steel 440C. The present paper mainly deals with the effect of chemical composition and microstructure on the corrosion properties. Different experimental alloys were produced in the shape of small ingots. The influence of the alloying elements chromium, molybdenum, cobalt, and carbon on the corrosion properties was studied. The results can be summarized as follows: Chromium and molybdenum improve the corrosion resistance, however, only the content of these elements in solid solution in the steel matrix is effective. In case of cobalt the corrosion resistance decreases. The reason is the interaction between cobalt and carbon and its effect on the chromium content in the steel matrix. The calculated pitting resistant equivalent number of high chromium martensitic steels is only limited valid, because there is a major effect of carbide precipitation on the corrosion behaviour. Further investigations were focused on the heat treatment. Especially the effect of the tempering temperature of these steels was studied. The tempering temperature is most relevant for secondary hardening carbide precipitation, which lowers the chromium content of the matrix with detrimental influence on the corrosion properties. The carbide precipitation and chromium distribution was characterized by means of energy filtered transmission electron microscopy (EFTEM).     

应变幅对第二代镍基单晶高温合金DD11在980℃条件下低周疲劳性能的影响

通过对第二代镍基单晶高温合金DD11在980℃条件下的低周疲劳性能测试及表征,研究了在不同应变幅(△ε/2=0.5~1.2%)对循环应力响应行为和断裂模式的影响。建立了显微组织演变和疲劳行为之间的联系。实验结果表明,该合金发生了循环软化行为并且随着应变幅的提高,循环软化程度降低。γ"的粗化以及垂直于加载轴方向的γ通道加宽有利于位错运动的进行,因此造成了循环软化。当应变幅为0.5%时,位错回复也是造成循环软化的原因。随着应变幅增加至0.8%后,γ"的粗化以及垂直于加载轴方向的γ通道加宽程度降低,位错在两相界面上发生了塞积,造成了循环软化程度的降低。疲劳失效模式从扩展区的正断模式转变为了瞬断区的剪切断裂模式。本研究有利于建立单晶高温合金涡轮叶片疲劳失效模式、循环应力响应行为和组织三者的关系,对涡轮叶片的设计使用具有一定的指导意义。     

Probabilistic modeling and sizing of embedded flaws in ultrasonic non-destructive inspections for fatigue damage prognostics and structural integrity assessment

The paper presents a systematic method and procedure for probabilistic fatigue life prediction using non-destructive testing data under uncertainty. The procedure is developed using uncertainty quantification models for detection, sizing, fatigue model parameters and inputs. The probability of detection model is based on a classical log-linear model coupling the actual flaw size with the NDE reported size. Using probabilistic modeling and Bayes theorem, the distribution of the actual flaw size is derived for both NDE data without flaw indications and NDE data with flaw indications. Fatigue damage and structural integrity assessment are suggested based on the developed method and procedure. A turbine rotor example with realistic NDE inspection data is presented to demonstrate the overall methodology. Calculation and interpretation of the results based on risk recommendations for industrial applications are given. The influence of the NDE detection threshold to the assessment results, and error analysis of the assessment results are discussed in detail.