Characterisation of ferritic stainless steel by Barkhausen techniques

Magneto-acoustic emission (MAE) and magnetic Barkhausen noise (MBN) sensing techniques were developed and employed to characterise plastically deformed and heat-treated AISI 430 ferritic stainless steel samples. These results have been compared to the mechanical hardness, coercivity and residual stress of the samples. MAE and MBN were shown to decrease with increasing permanent material deformation. It was found that the inverse of MAE (absolute energy) and MBN (RMS) are linearly proportional to hardness. With increased deformation, the resultant change in dislocation density was found to effect material coercivity. It has been shown that the inverse of MAE absolute energy and MBN have an exponential relationship to the change in material coercivity. The results are explained in terms of the different mechanisms that effect dislocation-domain wall interactions. A new measurement parameter has been developed for microstructural characterisation called MAE absolute energy and has proved to be a useful quantitative method in MAE waveform measurement.     

Magnetic field shielding technique for pulsed remote field eddy current inspection of planar conductors

Pulsed remote field eddy current (PRFEC) can be used for detection of deeply buried flaws in non-magnetic flat plates. However, detection sensitivity for buried defect is low due to the great depth and the weak detecting signal. To improve the defect detection sensitivity, the PRFEC probe is optimized with the magnetic field shielding techniques in this work. On one hand, the exciting coil with shielding structure can focus the indirect magnetic field to penetrate through the plate. On the other hand, the detecting coil and the region between these two coils with shielding material can block and minimize the magnetic field energy diffusion along the direct coupling path. In order to investigate the shielding technique which blocks the direct coupling magnetic field, simulation studies using finite element method (FEM) has been first conducted, where three different PRFEC probes have been simulated and compared. Both detection abilities to defects in different directions and to thickness variation have been analyzed by using the second probe with shielding structure. The conclusions derived from the simulation study have been validated through experimental studies. Both simulation and experimental studies have indicated that the shielding technique can improve the detection sensitivity of subsurface defects.     

Comparison of magnetic nondestructive methods applied for inspection of steel degradation

A series of low carbon steel specimens is investigated in the frame of a chain of magnetic non-destructive measurements on round robin samples, organized by the Universal Network for Magnetic Non-Destructive Evaluation. The samples have been plastically deformed by cold rolling to five consecutive stages of deformation. They were examined by several different nondestructive magnetic methods and the results were compared with each other and with the destructive mechanical measurements of Vickers hardness and ductile-brittle transition temperature. Very well correlated and qualitatively very similar relations between the magnetic and mechanical parameters were found in all cases, and for all the methods. Difference between the results of different measurements was found only in the sensitivity of the measurement with respect to the independent parameter, which is a promising message for the potential practical applications and possible future standardization of magnetic nondestructive methods.     

Inspection of steel degradation by magnetic adaptive testing

A series of low-carbon steel samples is investigated in the frame of a chain of magnetic non-destructive measurements on round robin samples, organized by the Universal Network for Magnetic Non-Destructive Evaluation. The samples have been plastically deformed by cold rolling to five consecutive stages of deformation. They are examined by the method of magnetic adaptive testing, typical by its low required magnetization of the samples. Results of the non-destructive magnetic tests are compared with the destructive mechanical measurements of Vickers hardness and ductile–brittle transition temperature. Linear, sensitive correlation is found between the magnetic parameters and the two mechanical ones.     

Neural network analysis for corrosion of steel in concrete

Corrosion of the steel embedded in concrete plays a vital role in the determination of life and durability of the concrete structures. Several researchers have studied the corrosion behaviour of the embedded steel and the different types of protective measures that are available to control the corrosion. However, little work has been done to recognize, identify the performance and predict the behaviour of the steel over a long term. Hence, this work concentrates on recognizing the behavioural pattern of the embedded steel and predicting its potential characteristics using artificial neural network (since the potential of the embedded steel is used to determine whether the steel is corroding or not as per ASTM C 876-91).A systematic study to develop a suitable method that can accept, analyze and evaluate experimental data that are at random and/or influenced by external, unpredictable variables has been carried out, using the back propagation method. This method is fast and is able to produce an output that has minimum error for this experimental setup. This has resulted in the development of back propagation neural network, that can train and test the system, to calculate the specified parameter for different conditions and recognize the behavioural pattern. Using this methodology, the corrosion of the steel embedded in concrete is analyzed and it is observed that the error encountered is only about 5% for the predictions made from the analysis.     

Magnetic Barkhausen emission technique for detecting the overstressing during bending fatigue in case-carburised En36 steel

The effect of bending fatigue at different maximum stress levels on the magnetic Barkhausen emission (MBE) has been studied in case-carburised En36 steel specimens. The low frequency MBE profile has been measured after unloading the specimen at different number of fatigue cycles. It has been found that, beyond 1000 MPa, the MBE peak height decreases just after few thousand cycles and the percentage reduction in the MBE peak increases with maximum bending stress level. The reduction in MBE peak at lower stresses (<1400 MPa) is attributed mainly to the effect of residual stresses becoming more compressive below the surface due to the application prior tensile stress. At higher stresses (1500 MPa), the variation in the MBE peak also indicates the effect of cyclic hardening and softening with progressive fatigue cycles. The MBE profiles measured after monotonic loading and unloading with different maximum stress levels also show similar reduction in the MBE peak with increase in pre-stress level. However, at higher stresses (>1400 MPa), the cyclic loading shows larger reduction in the MBE peak than the monotonic loading. This is attributed to the effect of cyclic microplasticity induced enhancement of dislocation density in addition to the residual stress modification. This study clearly shows the MBE technique can be used to detect the maximum stress level seen by the specimen beyond 1000 MPa. Any overstressing of this case-carburised steel beyond the fatigue limit of 1150 MPa can be easily detected from the percentage reduction in the MBE peak. Since the crack propagation stage is insignificant in these hard steels, the detection of any overstressing using the MBE technique would be very useful in assessing and preventing the impending catastrophic failure.     

Dissimilar spot welding of dual phase steel/ferritic stainless steel: phase transformations

Phase transformations in dissimilar resistance spot welds of dual phase steel and ferritic stainless steel are analysed. In contrast to a full martensitic microstructure predicted by the Schaeffler and Balmforth diagrams, a ferrite–martensite microstructure was observed in the fusion zone. The formation of ferrite phase in the fusion zone can be attributed to the rapid cooling rate of resistance spot welding, which suppresses the post-solidification ferrite–austenite transformation. The grain growth and martensite formation were main metallurgical features of the heat affected zone of ferritic stainless steel side. Microstructure gradient of heat affected zone in dual phase steel side was dictated by martensitic transformation. The effect of weld thermal cycle on the mechanical performance of the joint is discussed.     

Ultrasonic inspection for circumferential butt joint of austenitic stainless steel with carbon steel

0 IntroductionThe flowleading pipe in pusher of shipping diesel en-gine is constructed by butt welding of 1Cr18Ni9Ti auste-nitic stainless steel circumferential strip with low carbonsteel circumferential strip. It is easy to form stress cracks,slag inclusions, and blowholes etc inside the weldedseam. Thus after turning for the weldment it is necessaryto perform nondestructive test for the welded joint.The weldment is very large in dimension and it is adouble deck construction with vertical a…     

Magnetic Barkhausen measurements for evaluating the formation of Lüders bands in carbon steel

Localized inhomogeneous plastic deformation phenomenon was experimentally investigated in a structural steel by using the magnetic Barkhausen noise measurements. ANSI 1050 steel plates, throughout oriented in the rolling transversal direction, were selected additionally inducing different strains. Scanning of the magnetic Barkhausen energy (MBN_energy) shows the formation of Lüders bands as well as the displacements in the material deformation zone. The MBN_energy angular dependency on the applied stress and on the anisotropy coefficient were determined as well as the presence of Lüders bands in samples oriented according to the rolling direction.     

The sticking behavior of an ultra purified ferritic stainless steel during hot strip rolling

During hot rolling of ferritic stainless steels (FSSs), sticking can cause severe surface defects both on work rolls and strips especially for ultra purified FSSs with the total amount of carbon and nitrogen being less than 150 ppm. In the present paper, high temperature oxidization behavior of the ultra purified FSSs was characterized. A new experimental method was invented to simulate the sticking behavior of ultra purified 21%Cr FSS, by which specially designed specimens were hot compressed with different processing parameters to simulate hot strip rolling process. The specimens were surface conditioned prior to the simulation test, either by mechanically polishing or by pre-oxidization to form oxide scales with certain thickness. Observation on the cross-section of sticky regions indicated that cracks had been formed by cyclic heating and cooling during hot deformation had been acting as nucleation sites for sticking particles. The results indicated that oxide scale could avoid the direct contact between roll surfaces and rolled materials to help reducing the sticking occurrence. The temperature region for easy sticking occurrence in the ultra purified 21%Cr FSS has been measured to be from 900 to 1100 °C, which is enlarged as compared to other plain FSSs. The tendency for sticking occurrence decreased with increasing strain rate and decreasing the hammer or work roll surface roughness.     

Use of capacitive and GPR techniques for the non-destructive evaluation of cover concrete

Two non-destructive techniques, namely ground penetrating radar (GPR) and a more original approach based on capacitive measurement, have been considered in this research project that focuses on the evaluation of cover concrete moisture content. Following a numerical modeling step for the capacitive technique, the two methods were successfully compared during an experimental campaign conducted in the laboratory against several control test slabs.     

Application of eddy currents induced by permanent magnets for pipeline inspection

This paper presents an alternative to the common concentric coil method to induce low-frequency eddy currents in ferromagnetic pipe and tubes. Pairs of permanent magnets rotating around the central axis of these cylinders in proximity of the surface can be used to induce high current densities in the material that is the object of the inspection. Anomalies and wall thickness variations are detected with an array of sensors that measure local changes in the magnetic field produced by the current flowing in the material. This electromagnetic technology is being developed for pipeline inspection platforms that either crawl slowly inside a pipe to maneuver past physical barriers or are pushed by flexible rods. These devices move down the pipeline independent of the product flow, and potentially stop for detailed defect assessment. Fundamental finite element modeling analysis and experimental investigations performed during this development have led to the derivation of a first-order analytical equation for designing rotating exciters and positioning sensors. The rotating permanent magnet system has the potential for broader application because the sensor configurations can be small in physical size, allowing them to pass obstructions that currently prevent inspection using available NDE implementations.     

Full surface inspection methods regarding reinforcement corrosion of concrete structures

For reinforced concrete structures a localisation of all significant critical areas can only be done by a full surface inspection. The economic advantages are obvious: uncritical areas have not to be repaired expensively 1 . The first step of the assessment should always be a visual inspection 2 . The range of deterioration causes can be limited and the degree of deterioration may be estimated roughly. The inspection program can be adjusted to the requirements. By means of a full surface potential mapping areas with a high risk for chloride induced reinforcement corrosion can be localised, although no deteriorations are visually detectable at the concrete surface. In combination with concrete cover depth and resistivity measurements areas with corrosion promoting exposure conditions can be localised even if the reinforcement is not yet depassivated. The following publication gives an overview about the essential full surface investigation methods to localise critical areas regarding corrosion of steel in concrete. The selection of methods is based on the inspection procedure given in 2 .     

Corrosion behavior of ferritic-martensitic steel T91 in supercritical water

The corrosion behavior of a ferritic-martensitic steel T91 exposed to supercritical water at 500 °C and at two different dissolved oxygen concentrations, 25 ppb and 2 ppm, for exposure times up to 505 h was characterized. In this corrosion experiment, all the exposed samples formed a stable oxide layer, but of varying thicknesses. The dissolved oxygen concentration played a significant role in the oxidation behavior of this alloy. For lower oxygen concentration exposure, phase distribution analysis showed a duplex oxide structure, consisting of layered magnetite and spinel phases. Higher oxygen concentration resulted in an additional hematite enriched layer in the outer oxide region. Furthermore, samples exposed to high oxygen content supercritical water exhibited a more porous oxide scale with weaker adhesion to the substrate. Samples of T91 steel whose surface was modified by oxygen ion implantation were tested in the lower oxygen supercritical water environment and were noted to exhibit a lower oxide layer thickness compared to the untreated alloy. The formation of nanometer-sized oxides precipitates at the surface due to oxygen ion implantation appears to influence the nucleation and growth texture of the oxide scale, which may play a role in reducing the oxide layer thickness.     

Modeling the cracking of cover concrete due to non-uniform corrosion of reinforcement

In situations when external chloride penetration is the cause of depassivation, the corrosion process may start from the outer region of a rebar, which might expand non-uniformly. Therefore, the main objective of the present work is to explore the effect of non-uniform corrosion on cracking behavior of cover concrete. The influences of concrete heterogeneities and the porous layer generated at the rebar/concrete interface on the failure patterns and the corrosion level of cover concrete are considered. The random aggregate structures of concrete are built, and the concrete is regarded as a composite composed of three phases, i.e. the aggregate, mortar matrix, and the interfacial transition zones (ITZs). The plasticity damaged model is employed to describe the mechanical properties of the mortar matrix and the ITZs, and it is assumed that the aggregate is elastic. Non-uniform radial displacement with a half ellipse shape is adopted to describe the expansion distribution of the corrosion products. The failure pattern and the corrosion pressure of cover concrete, and the critical corrosion level when the cover concrete cracks due to non-uniform corrosion expansion are studied based on the meso-scale numerical method. The comparison of the simulation results with the available test results on the failure pattern of cover concrete shows fairly good agreement. Moreover, the influence of meso-structural heterogeneities is explored, and the cracking behavior obtained under non-uniform and uniform expansion conditions are compared. Finally, the influences of cover thickness, rebar diameter and the location of rebar (namely side-located rebar and corner-located rebar), on the failure pattern and the corrosion level are examined.     

铁素体不锈钢表面皱折及其评价方法

皱折缺陷是中铬铁素体不锈钢发生较大塑性变形后容易出现的表面缺陷。这种缺陷往往破坏了不锈钢制品的外观,因此需要对制品进行研磨和抛光以消除皱折的影响。然而,额外的加工工序大幅增加了企业的生产成本,同时不可避免地造成了环境污染。那么,正确认识和评价铁素体不锈钢的表面皱折缺陷就显得非常重要。本文主要讨论界定铁素体不锈钢表面皱折缺陷,并简要介绍表面皱折缺陷的形成原因,进而厘清表面皱折缺陷的评价参数,最后针对已有测试方法存在的问题提出可供参考的解决办法。     

Effect of joining temperature on the microstructure and strength of tungsten/ferritic steel joints diffusion bonded with a nickel interlayer

A diffusion bonding process, for joining of tungsten to ferritic steel using nickel as an interlayer, was developed for nuclear component application. The effect of joining temperature on the microstructure and tensile strength of the joint was investigated in this work. Metallographic analysis revealed that a good bonding was obtained at both the tungsten/nickel and nickel/steel interfaces, and the diffusion products were identified in the diffusion zone. Nano-indentation test across the joining interfaces demonstrated the effect of solid solution hardening in the diffusion zone. Tensile test showed that the maximum average tensile strength of ∼200 MPa was obtained for the joint diffusion bonded at 900 °C. The results were discussed in terms of the joining temperature and of the residual stress generated during joining process.     

铁素体不锈钢板坯连铸机技术要点

结合铁素体不锈钢连铸过程高温凝固特性,拟新建的双流铁素体不锈钢板坯连铸机采用直弧型连续弯曲连续矫直的特殊技术,选用全程无氧化浇铸、大容量中间包、结晶器自动液面控制减少钢水中夹杂物;采用摩擦力监测、结晶器漏钢预报保证浇铸的连续性;合理设计动态一冷和二冷配水等提高铸坯表面质量,并设计采用电磁制动、低过热度浇铸、二冷电磁搅拌、二冷动态轻压下技术提高铸坯内部质量。     

Effect of W and Ce additions on the electrochemical corrosion behaviour of 444-type ferritic stainless steel

In order to achieve excellent heat resistance and corrosion resistance, high melting-point and rare earth elements were added to ferritic stainless steel. The effect of W and Ce addition in ferritic stainless steel on the corrosion resistance in 0.1?M NaOH solution was studied. Potentiodynamic polarisation curves indicated that W- and Ce-containing samples had lower corrosion rate and lower passive current density. The electrochemical impedance spectroscopy measurements revealed that W–Ce-containing samples had higher polarisation resistance values and thicker passive films. Capacitance analysis indicated that the semiconducting behaviour and the properties of passive film remain unchanged. The addition of W and Ce led to a decrease in donor and acceptor density, which improved the passive film stability.     

不锈钢盖成形工艺与模具设计

分析了不锈钢盖零件的结构和工艺性,针对零件边缘凸起相对较高,成形过程中容易产生破裂的情况,采用2道工序,第1道工序为拉深锥形和凸起,第2道工序是翻边(浅拉深),用同一副模具成形不同高度的凸起。生产实践证明:不锈钢盖经过这2道工序加工成形,工艺合理,模具结构紧凑,产品质量良好,满足生产要求。