Numerical analysis of multipoint CDW welding process on stainless AISI304 steel bars

Capacitor Discharge Welding is a joining process characterized by high-energy density; it is an electrical resistance welding process, since the heat source is the Joule effect induced by a capacitive discharge at the welding contact zone. The main aims of this work are to introduce achieved technological progresses in the Multipoint CDW applications and to present the developments of a numerical tool to be used for the prediction of the welding condition and the joints final properties.An advanced 3D FEM model has been conceived in order to consider the geometrical complexity of the CDW process applied on circular AISI 304 bars, originated by special multipoint contact profiles; the evolution and influence of the temperature field was studied with accurate simulations of the CDW process.     

Microstructure evolution in AISI 304 stainless steel during near rapid directional solidification

Abstract

The microstructure evolution of near rapidly directionally solidified AISI 304 stainless steel was investigated in the present paper. It is found that the microstructure consists of δ ferrite dendrites with developed sidebranches and interdendritic austenite (γ) under the temperature gradient (G) of 20 K mm^–1 and growth rate (V) of 1·0 mm s^–1. Coupled growth microstructures of thin lamellar ferrite and austenite begin to form at a higher growth rate of 2·0 mm s^–1. The formation mechanism of the coupled microstructures is analysed based on the nucleation and constitutional undercooling criterion that the δ ferrite phase and austenite phase form alternately before the steady state growth of each phase is reached due to larger undercooling. With further increase of the growth rate up to 3·0 mm s^–1, the morphology of the δ ferrite transforms from lathy to cellular.     

Influence of ultrasonic cavitation on surface residual stresses in AISI 304 stainless steel

The effect of ultrasonic cavitation in water on residual stress changes in AlSl 304 stainless steel has been investigated. Studies indicate that high-intensity ultrasonic cavitation introduces a very high compressive residual stress at the surface (due to work-hardening) even for short durations of exposure at ambient temperatures. With increased exposure, the stresses become more compressive; however, they tend to reach a saturation value. Different combinations of temperature, time and cavitation intensity were tried out and the best effects were noticed for a treatment temperature of 5 °C. 304 stainless steel was chosen for the present study on account of its amenability to strong work-hardening. The test specimen was attached to the tip of an ultrasonic vibrator and immersed in the cavitating liquid, i.e. water. However, even in situations where the specimen was kept in a stand-off position close to the vibrator tip (with water in between) similar effects were noticed. The maximum depth of hardening was found to be about 70 m. During this process, there was also a mild roughening of the surface. An incidental observation pertains to the formation of both and martensites at the surface detectable by X-ray diffractometer recordings for specific conditions of cavitation treatment. The required high intensities of vibration in this study were obtained through an in-house built highpower ultrasonic generator working at a frequency around 20 kHz.     

Fracture failure analysis of AISI 304L stainless steel shaft

Fracture failure analysis of an agitator shaft in a large vessel is investigated in the present work. This analysis methodology focused on fracture surface examination and finite element method (FEM) simulation using Abaqus software for stress analysis. The results show that the steel shaft failed due to inadequate fillet radius size and more importantly marking defects originated during machining on the shaft. In addition, after visual investigation of the fracture surface, it is concluded that fracture occurred due to torsional–bending fatigue during operation.     

Fretting wear of nitrogen-implanted AISI 304 stainless steel

Fretting wear resistance of nitrogen-implanted AISI 304 stainless steel was measured and compared to that of unimplanted steel. After 5 × 10³ cycles, unimplanted steel revealed severe damage on non-slip area and adhesive-type wear on microslip region whereas nitrogen-implanted stainless steel was still undamaged. The improved fretting wear resistance is explained to be due to the increased load carrying capacity and decreased adhesion of nitrogen-implanted steel.     

Abnormal grain growth in AISI 304L stainless steel

The microstructural evolution during abnormal grain growth (secondary recrystallization) in 304L stainless steel was studied in a wide range of annealing temperatures and times. At relatively low temperatures, the grain growth mode was identified as normal. However, at homologous temperatures between 0.65 (850 °C) and 0.7 (900 °C), the observed transition in grain growth mode from normal to abnormal, which was also evident from the bimodality in grain size distribution histograms, was detected to be caused by the dissolution/coarsening of carbides. The microstructural features such as dispersed carbides were characterized by optical metallography, X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, and microhardness. Continued annealing to a long time led to the completion of secondary recrystallization and the subsequent reappearance of normal growth mode. Another instance of abnormal grain growth was observed at homologous temperatures higher than 0.8, which may be attributed to the grain boundary faceting/defaceting phenomenon. It was also found that when the size of abnormal grains reached a critical value, their size will not change too much and the grain growth behavior becomes practically stagnant.     

Evolution of dynamic recrystallisation in AISI 304 stainless steel

Abstract

The nucleation and development of dynamic recrystallisation (DRX) has been studied via hot torsion testing of AISI 304 stainless steel. The DRX behaviour was investigated with microstructural analysis and slope changes of flow stress curves. The characteristics of serrated grain boundaries observed by SEM, electron backscattered diffraction and TEM indicated that the nucleated DRX grain size was similar to that of the bulged part of the original grain boundary. The DRX of the alloy was nucleated and developed by strain induced grain boundary migration and by the necklace mechanism. Before the steady state in the flow curve at 1000 ° C and 0.5 s^-1, the dynamically recrystallised grains did not remain a constant size and gradually grew to the size of fully DRX grains at steady state (30 μm). The calculation of the grain size was based on X _DRX (volume fraction of dynamically recrystallisation) under the assumption that the nucleated DRX grains grow to the steady state continuously. It was found that the calculated grain size of the alloy was good agreement with that of the observed grain size. It is expected that a fine grained steel can be obtained by controlling hot deformation conditions on the basis of newly developed equations for predicting DRX behaviour.     

The edge-cracking of AISI 304 stainless steel during hot-rolling

The hot-rolled plates of AISI 304 stainless steel, containing edge cracks of different intensities, were examined. The austenitic matrix of the steel contained small amounts of ferrite inhomogeneously distributed across the width and the thickness of the plate. A correlation was found between ferrite content and edge cracking: the higher the ferrite content the longer the edge cracks. Among the chemical elements present in the steel, the most critical effect on ferrite content was exerted by carbon and nitrogen. The longest edge cracks were observed for plates with the lowest content of carbon and nitrogen. A possible contribution of steel chemistry and heating temperature to changes in the steel phase composition and the probability of edge cracking is discussed.     

The thermal conductivity of AISI 304L stainless steel

A compilation and critical analysis of the thermal conductivity () of AISI 304 stainless steel (SS) between 100 and 1707 K has been given in the literature. The author represented his recommended values of by an inflection in the A versus temperature relationship between 300 and 500 K. Because a physical mechanism had not been identified that would produce such a temperature dependence in of 304 SS, interest was generated in the possible existence of an as yet undiscovered phenomenon that might cause such an inflection. Consequently, experimental verification of the inflection was sought. The present paper presents recent measurements of , the electrical resistivity, and the absolute Seebeck coefficient of 304L SS from 300 to 1000 K and of the thermal diffusivity () from 297 to 423 K. The values computed from the a measurements were within ± 1.6% of the directly measured An inflection was not observed in the temperature dependence of between 300 and 500 K. After careful evaluation and because a physical mechanism still has not been identified which would produce such an inflection, the authors conclude that the inflection in the vs T relationship reported in the literature was caused by the data analysis technique.     

Friction stir welding of AISI 304 austenitic stainless steel

The objective of this work is to demonstrate the feasibility of friction stir welding (FSW) AISI 304 austenitic stainless steels. The tool used was formed of a tungsten‐based alloy. The specimens were welded on an 11 kW vertical milling machine. Defect‐free welds were produced on 2.5 mm plates of hot‐rolled AISI 304 austenitic stainless steels at travel speeds ranging from 40 to 100 mm/min with a constant rotating speed of 1000 rpm. Tensile strengths and hardness values of the weld interface were determined and microstructure features of these samples were investigated.     

Stress induced martensite transformation texture in AISI 304 austenitic stainless steel

Abstract

The phenomenological theory of martensitic transformation was applied to a tension induced martensitic transformation in an AISI 304 austenitic stainless steel in order to estimate the transformation texture. Input data were obtained from the published literature. Calculated pole figures were constructed assuming a variant selection process based on Patel and Cohen’s theory, which emphasises that a mechanical component of free energy is the driving force for martensitic transformation at temperatures above martensite start M_s. The results showed a remarkably good match between the calculated and published measured data.     

Low-cycle fatigue behaviour of notched AISI 304 stainless steel specimens

Low-cyclic fatigue tests were conducted on semi-circle notched and V-notched specimens made of AISI 304 stainless steel. Extensive scanning electron microscopic examination of the fracture surface was also carried out to correlate the microscopic fracture surface features with the macroscopic fatigue loading parameter for this steel. The elastic-plastic fatigue test results indicated a noticeable cyclic hardening phenomenon and also a great influence of the maximum cyclic stress, the mean stress and the notch geometry on both the fatigue life and the fatigue behaviour process. Using careful sensitivity and regression analysis correlations between the macroscopic fatigue parameters on the one hand and the macroscopic and the microscopic fracture surface features on the other, these correlations are presented and clearly documented and discussed for the two notch geometries investigated.     

Economical and ecological cryogenic machining of AISI 304 austenitic stainless steel

 While it is a clean alternative to conventional machining using environmentally polluting cutting oils and emulsions, cryogenic machining using liquid nitrogen has been reported to increase cutting forces and shorten tool life when cutting AISI 304 austenitic stainless steel. This paper presents improved results by using an economical cryogenic cooling approach designed after studying the cryogenic properties of the stainless steel material. By injecting a small amount of liquid nitrogen to the chip–tool interface, but not to the workpiece, this approach yielded a 67% tool-life improvement at 3.82 m/s and a 43% improvement at the medium speed of 3.40 m/s when compared with conventional emulsion cooling. It improved machining productivity and reduced production cost. In this study, different cryogenic machining approaches were compared in the machining test using commercial carbide inserts. The results show the cooling approach is crucial in attaining the benefits of cryogenic machining in cutting stainless steel. Received: 7 February 2000 / Accepted: 30 April 2000     

拉深比对304不锈钢圆筒件残余应力的影响

304不锈钢圆筒拉深件在其口部易发生纵向开裂,外壁残余拉应力过大是造成其纵向开裂的根本原因之一.本文结合有限元法分析不同拉深比所得圆筒拉深件的残余应力,用纳米压痕试验研究了拉深比对304不锈钢圆筒拉深件筒壁残余应力的影响.结果表明:304不锈钢圆筒拉深件外壁的残余拉应力从筒底到口部先增大后减小,最大残余应力出现在筒壁中部约60%筒壁高度处;纳米压痕测得拉深比为1.43、1.54、1.67和1.82拉深圆筒件筒壁的最大残余应力分别为391.87、745.30、793.74和1 013.1 MPa;最大残余应力随拉深比的增大而增大.与其他文献对比分析,此研究结果是正确可靠的.     

Numerical and experimental analysis of microstructure formation during stainless steels solidification

The aim of this work is to examine microstructure formation during the solidification of unidirectional solidified AISI 304 stainless steel. Numerical and experimental results indicate that this numerical model allows a precise analysis of the AISI 304 stainless steel microstructure formation. This model determines temperature profiles, position of liquid and solid isotherms, thermal parameters (thermal gradients, tip rate movement, rate cooling), and finally, the secondary inter dendritic spacing. This model was tested by comparing the experimental values results, and thus a reasonable correlation was found.     

Growth behaviour of small surface fatigue cracks in AISI 304 stainless steel

A series of axial tensile fatigue tests (R = 0.1) was carried out to investigate the initiation and the growth behaviours of very small surface fatigue cracks under two different surface conditions (viz. smooth and pitted surfaces) of AISI 304 stainless steel at room temperature. This paper deals with both of the two approaches regarding the analysis of fatigue: the approach based on the concept of fracture mechanics and low cycle fatigue. In particular, both the initiation and growth of cracks and the coalescence of small cracks by fatigue in the specimen have been investigated by the methods of surface replicas and photomicrographs. Quantitative information such as the initiation period, growth and coalescence behaviours of small cracks, and crack growth properties were systematically obtained. The results show that the accurate determination of these parameters is critical for the application of fracture mechanics to fatigue life assessment.     

Selective dissolution of austenite in AISI 304 stainless steel by bacterial activity

Selective attack in an AISI 304 stainless steel weld metal has been developed after three months in service in well water. Welding zones showed a severe corrosive attack that in some cases led to the steel perforation. Optical and scanning electron microscopy (SEM/EDX) revealed a selective attack. An in-depth analysis showed indications of microbiological activity which could be responsible of the severe attack.