Thick Stainless Steel Research Articles

Hydrogen outgassing and permeation in stainless steel and its reduction for UHV applications

Stainless steel, grade 304L and 316L are widely used structural steel in most of the Ultra High Vacuum (UHV) systems in present-day research. Large size vacuum chambers and beam lines are used in much scientific applications, few examples are CERN accelerators, ITER, LIGO, Neutrino observatory, FAIR, Space research, etc. The selection and operation of the large size vacuum pumping system and the material to be used for UHV application is one of the crucial tasks for the vacuum system engineers as it relates the capital and operational cost linked to it. Any failure in operation has a direct impact on the project execution timeline and the huge refurbishment cost. Hydrogen in structural steel and its reduction for ultra-high vacuum systems is one of the major challenges for the present day. Therefore, prior understanding is required for knowing hydrogen content and its permeation and outgassing behaviour in vacuum system establishment and operation. Based on the available literature, a theoretical model for estimating permeation and outgassing rate is discussed. Relating the permeation and outgassing, the theoretical model contains first and second-order Fick's law and their solutions. Permeation and outgassing rate of 3 mm thick stainless steel is calculated for different thermal bake-out conditions and are discussed. A brief discussion (after going through various references) is mentioned for lowering the outgassing rate for hydrogen in steel.

Underwater laser cutting of thick stainless steel blocks using single and dual nozzles

Laser cutting was performed using a 6-kW fiber laser with two different nozzle configurations to extend the permissible cutting thickness of steel blocks in a water environment. The first nozzle configuration involved a single minimum length nozzle (MLN) operating at an upstream gas gauge pressure of 15 bar; the second configuration consisted of a truncated aerospike nozzle coupled with an MLN. These configurations were designed to form a core air-filled cavity with the submerged jet to have denser and more lengthened characteristics for extending the possible transmission distance of the high-power laser beam, which is vulnerable to water absorption. Before underwater cutting, we performed a Schlieren visualization measurement of the gas jet’s flow behavior as it was discharged from the single and dual nozzles in the ambient air, after which, the stainless steel blocks were laser-cut underwater using both nozzle configurations. The thickness of the selected steel blocks varied from 60 to 80 mm according to the capabilities of the nozzles used to form the air cavity.

Monitoring and analysis of melt-assisted deformation behavior of 304L stainless steel during multipass laser forming process using IR pyrometer and laser-based displacement sensor

In the present study, online monitoring of melt-based laser forming using a 2 kW Yb-fibre laser to understand the effect of partial sheet melting on bending behavior and mechanism to achieve precise deformation is investigated. A 1 mm thick 304L stainless steel sheet was used as substrate. The melt and non-melt-based sheet deformation and temperature signals were monitored using a laser-based non-contact type displacement sensor and an IR pyrometer, respectively for five consecutive scans on the same path having laser power, scan speed and spot diameter as input parameters. Variation in melt depth with process parameters and its effect on counter bending angle, net bending angle and significance of melt percentage with respect to sheet thickness on dictating the bending behavior was investigated. The bending mechanism reported in case of melt-based laser forming was found to vary from established solid metal sheet forming mechanisms and was termed as “melt-based TGM” mechanism. A maximum bending angle of 9.2° was obtained after five scans with 82 μm of melt depth. Microstructural and Phase variations of the melt and non-melt zone were studied for understanding their effects on material properties for melt–based laser forming. Finally, the mechanical behavior like modulus, Nano hardness and micro hardness of the melt and non- melt formed samples were studied to understand the applicability of melt-based laser formed sheets.

Distortion and residual stresses in thick plate weld joint of austenitic stainless steel: Experiments and analysis

Abstract The present work aims to evaluate distortion and residual stresses in thick plate weld joints of austenitic stainless steel. Quantification of residual stresses in thick austenitic stainless steel weld joint is important because it is not desirable to carry out stress relieving after welding owing to the susceptibility to sensitization in the heat-affected zone. Therefore, systematic studies were carried out to predict and measure distortion and residual stresses in the austenitic stainless steel plate weld joint. Variables influencing the generation of residual stresses such as heat input, welding speed, constraints during welding were considered. 20 mm thick plate weld joint of austenitic stainless steel material was prepared using gas tungsten arc welding (GTAW) in unclamped condition and distortions were measured during welding after each pass. Sub-surface and through-thickness residual stresses were measured using blind hole drilling (BHD) and deep hole drilling (DHD) techniques respectively. Thereafter, the weld joint was modeled using the finite element method and analyzed for residual stresses and distortions. Thermal analysis was carried out using actual welding parameters and temperature-dependent material properties to generate temperature profile and compare it with the measured values. The temperature profile along with necessary boundary conditions was used for structural analysis to obtain distortion and residual stresses. Experimental and finite element analysis results for distortion and residual stresses compare well. Details of the experimental and numerical analysis results of distortion and residual stresses for thick section plate weld joints have been discussed in this paper. Also, the effects of differences in yield strengths of weld and base metals on distortion and residual stress results have been brought out.

Crystal plasticity finite element–Marciniak-Kuczynski approach with surface roughening effect in predicting formability of ultra-thin ferritic stainless steel sheets

Forming Limit Diagrams (FLDs) of 0.1 and 1 mm thick ultra-thin ferritic stainless steel (FSS) sheets are predicted using a combination of the crystal plasticity finite element (CPFE) approach and the Marciniak-Kuczynski (M-K) model. In a scale of few microns, especially for thin materials, surface roughness significantly affects sheet formability; therefore, the M-K model is further modified to address the surface roughening effect. The CPFE combined M-K model with the surface roughening effect can reproduce experimental FLDs of two FSS sheets, unlike the method without considering the surface roughening effect. The results highlight the importance of surface roughening, particularly in thinner sheets, where the portion of surface roughness is an order higher than that in the thicker sheets. The evolution of the deformation texture under various loading paths is further investigated and discussed. The findings coincide with reports of previous studies.

Modeling and investigation on multi-wire electrochemical machining (MWECM) assisted with different flushing strategies

With growing technological needs, downscaling and productivity of fabricating micro features has become very crucial. In this context, multi-wire electrochemical machining (MWECM), an advanced form of wire electrochemical machining (WECM) process, is recently being introduced. This anodic dissolution process which is still at its primordial stage produces array micro features in a single run. The research work, portrayed here, is focused on the development of suitable MWECM setup for the fabrication of array micro slits with desired criteria. A mathematical model has been formulated to find out the correlation between process parameters and the width of the micro slit that has been validated later. Depending upon indigenously developed setup, attempts have been made for finding the best flushing parameter settings when axial electrolyte flow, PZT vibration and newly introduced combined approach consisting of those two have been used during experiments. For this, experimentations have been carried out with tungsten wire having a diameter of 50 μm on 100 μm thick stainless steel (SS 304) sheets. Moreover, the experimental results have been analyzed for gaining further insight into the effects of different electrolyte flushing strategies with varying wire feed rate on required output criteria. A comparison has been established between different electrolyte flushing strategies introduced on multi-wire electrodes during experiments for selecting the most suitable technique for cleaning the machining gap. Also, an array of micro slits and complex micro features using the MWECM process considering the best process parameter setting as obtained have been fabricated in order to explore the process efficiency.

Geometry and absorptance of the cutting fronts during laser beam cutting

The geometry of the cutting front and the cutting kerf was measured with an online high-speed x-ray diagnostic system. X-ray videos from fusion cutting of 10 mm thick stainless steel samples were recorded with a frame rate of 1000 Hz. A three-dimensional reconstruction of the time-averaged geometry of the cutting front and cutting kerf out of these images made it possible to apply ray-tracing for calculating the overall absorptance and the distribution of the absorbed irradiance at the cutting front. When increasing the feed rate, it was observed that the local inclination of the lower part of the cutting front with respect to the laser beam increased as well as the locally absorbed irradiance on the cutting front. This also leads to an increase in the overall absorptance with increasing feed rates.

Evaluation of through-thickness residual stresses in conventional and narrow grooved stainless steel welds

Thick AISI 304L stainless steel plates were welded using the gas metal arc welding process, and through-thickness residual stresses were evaluated by finite element simulation and the deep hole drilling technique. 3D moving heat source-based thermo-mechanical models were implemented to evaluate through-thickness residual stresses. The effects of the weld groove geometries and external restraints on the pattern of through-thickness residual stresses were studied. The maximum magnitude of locked-in residual stresses was recorded beneath the top surface, at a depth of around 6 mm. In comparison to conventional weld groove, the narrow weld groove configuration exhibited a 20–40% reduction in peak residual stresses. A significant rise in residual stresses was observed in constrained welds. The effect of the yield strength of the filler material on the evaluation of the through-thickness residual stress distribution in the course of finite element modeling was illustrated. The evolution of through-thickness residual stresses was also assessed concerning each weld pass.

Weld Strength of Laser Welded SS316 and SS321

Nowadays, due to economic considerations, stainless steel is frequently used in dissimilar welds configurations– alloyed steel, carbon steel, copper, titanium. Current research in laser welding aims at joining a large category of structurally dissimilar materials, like copper, stainless steel or aluminum components. The weld quality of 10mm thick stainless steel grades of 316 and 321 laser welded in butt configuration is studied. SS321 is well known for its corrosion resistance property especially dissimilar metal corrosion ie., galvanic corrosion. SS316 provides high strength to weight ratio even at high temperature which makes its as a potential candidate for aerospace application. The joint quality is characterized in terms of hardness, microstructure and inspected using NDT. Scanning Electron Microscope and Optical Microscope were used to study the microstructure of welded specimen. The microhardness test is conducted on the weld joints to quantify the effect of weld parameters in mechanical strength of the joints. Radiography test was conducted to identify the defects in the specimen.

Attenuation of neutron and photon-induced irradiation damage in pressurized water reactor pressure vessels

The present work investigates the attenuation of neutron and photon-induced irradiation damage in a Pressurized Water Reactor (PWR)-like mock-up Reactor Pressure Vessel (RPV) using Tripoli-4® Monte Carlo simulations with a particular emphasis on the presence of a thick stainless steel heavy reflector between the core and RPV. Results show that the photon-induced damage is well described by the exponential law. The neutron-induced damage attenuates quicker than the exponential form near the outer surface of RPV. Nevertheless, an exponential form can represent the attenuation of neutron-induced damage within 5% discrepancy for penetration < 17 cm in a typical 22 cm thick RPV and within 20% in the outermost 3 cm. The exponential form with an additional negative term fits the attenuation of neutron-induced damage in RPV as the negative term considers the “leakage” of neutron near the outer surface. It is observed that the presence of a Gen III-like representative heavy reflector reduces the estimated neutron-induced damage by a factor of 2. On the other hand, this paper verifies that the neutron flux with energies above 0.5 MeV (ϕ>0.5) is more representative of the displacement damage attenuation than the flux above 0, 0.1, and 1 MeV in RPVs. The damage rate is approximatively equal to Kϕ>0.5 with K=9.5×10-22DPA/n∙cm-2 in PWR RPVs.

Investigation on the melt ejection and burr formation during laser fusion cutting of stainless steel

When the melt is ejected from the cut kerf by an assist gas during laser fusion cutting, a fraction of it may adhere to the lower cut edge and form a burr. Adjusting the process parameters to minimize burr formation is a challenging task that becomes more and more difficult as sheet thickness increases. The burr length has significant relevance when the quality of a cut is evaluated, but the underlying mechanisms that control its formation are still not fully understood. In this paper, new results of an experimental investigation focused on identifying characteristic melt ejection regimes are presented. The experiments were conducted on 6 mm thick stainless steel sheets using a disk laser at a power of up to 10 kW. The melt flow exiting the kerf channel and the temporal formation of burr were analyzed using two high-speed cameras. The dependency of melt ejection regimes from process parameters as well as their influence on burr formation is discussed. Furthermore, the authors demonstrate that melt ejection forming a compact and stable threefold outflow is a characteristic property of a burrfree cut.

Study on surface morphology and recast layer microstructure of medium thickness stainless steel sheets using high power laser cutting

With the rapid development of high-power lasers, efficiency of laser cutting and thickness of cutting material have been greatly increased. In this study, a 10 kW fiber laser was used to cut 18 mm thick 316L stainless steel sheets. The surfaces and cross sections of samples were observed by an optical microscope and a scanning electron microscope. It was found that the quality of laser cutting was closely related to its process parameters, and the most important defects during the laser cutting process included striation and dross. On this basis, effects of process parameters such as defocusing amount and cutting speed on the kerf morphology and microstructure of the recast layer were investigated. The cutting surface was clearly divided into four characteristic areas in the vertical direction; the surface morphology and the recast layer microstructure of each area were analyzed. The results of the nanoindentation test showed that microhardness of the recast layer increased significantly comparing with that of the substrate, which was due to the refined microstructure of the recast layer. The influence of the notch effect caused by the dross at the bottom of the cutting surface on the crack source is greater than that caused by the surface striation, so the crack source is more likely to be generated near the dross. The parameters for good cutting quality in this experiment are as follows: 10 bar N2 assist gas pressure, 9 kW of laser power, −15 mm of defocus, and 350 mm/min of cutting speed.

20 kW laser welding applied on the international thermonuclear experimental reactor correction coil case welding

International thermonuclear experimental reactor correction coil cases are made of heavy, thick, high strength, and high toughness austenitic stainless steel 316LN. The BTCC (bottom and top correction coils) case has the dimension of 2.5 × 7 m2 and cross section of 239.8 × 146.7 mm2, side correction coil case has the dimension of 7.2 × 7.6 m2 and cross section of 147.8 × 168 mm2, and they will be closure welded after winding pack insertion. The 20 mm welding depth, dozens of meter of welding length, strict welding requirements, large size, and complex configuration bring a big challenge to this closure welding work. 20 kW high power laser welding is selected as the main welding method because of the advantage of potential welding deformation control and its penetrating ability and cracking resistance. The welding parameter is developed that can cover the assembly gap from 0 to 0.5 mm with a good welding quality. A special test coupon is designed for the welding procedure qualification, and related tests are carried out to qualify the joint properties of bend, tensile, and impact. Finally, a full-scale BTCC case is welded. After welding, ultrasonic testing confirms that almost all welds satisfy the weld seam quality requirement. The recorded temperatures less than 250° indicate that the temperature induced by welding will not harm the internal winding pack. The dimensional deviation of the inner face is less than 4 mm and also satisfies the tolerance requirement of ±2 mm for the BTCC case.

Impact of the thermally induced focus shift on the quality of a laser cutting edge

The phenomenon of thermal focus shift in optical systems, mainly caused by thermal lensing, is well known. There are numerous publications dealing with this effect, but they do not answer the question of how relevant this problem is to the practical application of lasers in laser cutting machines. In this paper, we examine the effect of the thermally induced change of the focus position on the quality of the cut edge when laser cutting 3 mm thick stainless steel with an 8 kW disk laser. After measuring the magnitude of the focus shift, its impact on the quality of the resulting cutting edge was determined for three different scenarios: recommended process parameters, increased speed, and decreased gas pressure. Roughness and perpendicularity tolerance were used to quantify the quality of the cutting edges according to the ISO standard. It becomes clear that it is difficult to explain the effect of the change in the focus position on quality aspects in general, as this also depends on other process parameters. Nevertheless, it can be shown that the resulting cut edge quality decreases in all three scenarios as a result of the thermal focus shift. This problem cannot be avoided by simply changing the process parameter corresponding to the focal position in the opposed direction of the thermal behavior before starting. Even though the reduction of quality might not be severe today, the focus shift should be considered for all applications that require a very high cutting quality. Furthermore, this effect cannot be neglected anymore as laser power will continue to rise in the future.

Assessment of non-uniform residual stress field of the thermal sprayed stainless steel coatings on aluminium substrates by the integral hole drilling method

Thermal spray is one of the most used techniques to produce coatings on structural materials. Such coatings are used as protection against high temperatures, corrosion, erosion and wear. The combined action of high pressures, temperatures and spraying conditions give rise to non-uniform residual stresses. The latter plays an important role in coating design and process parameters optimization. The present work highlights the influence of coatings thickness on the evolution of residual stresses in layered materials. Therefore, thick stainless steel coatings (ASTM 301) of different thicknesses are manufactured by wire arc spraying on aluminium alloy substrates (ASTM 2017A). For a better bond strength, a Ni–Al bond coat is first deposited. Furthermore, a numerically supported hole drilling strain gage method for residual stress field evaluation is proposed. Required calibration coefficients, for the strain–stress transformation formalism based on the integral method, are computed through finite element calculations using Abaqus software. The results indicate that the maximum residual stresses, for all thicknesses, are tensile and range from 140 to 275 MPa. The bond coat does not seem to affect the stress field. Also, it was found that the mean equivalent Von-Mises stress decreases with increasing coating thickness; hence reducing the interfacial adhesion energy of the sprayed materials.

Bipolar plate research using Computational Fluid Dynamics and neutron radiography for proton exchange membrane fuel cells

This work presents the development of liquid-cooled industry-scale bipolar plates for improved water management in PEM Fuel Cells. The methods used for the design development are based on Computational Fluid Dynamics (CFD) modelling and simulation, and Neutron Radiography experiments to analyse liquid water distributions within the cell for different operating conditions.A novel 140 cm2 bipolar plate was designed and manufactured on 0.1 mm thick stainless steel using pre-coated strip steel. CFD modelling carried out for the novel design predicted a significant improvement in terms of cell performance, as well as a more uniform temperature distribution within the membrane. Liquid water distributions were later analysed by neutron radiography experiments, defining a set of different operating conditions (current density, stoichiometry, inlet gases dew point, and cell temperature).Electrochemical and neutron radiography results are presented for all cases and the influence of the operating conditions is discussed. Liquid water distributions within the cell are also analysed and compared against the CFD model results obtained. The influence of the gas flow configuration (reactant gases and cooling water) is clearly observable in the results.

Assessment of the defect stability for large structures submitted to thermal shock loading through the Gfr criterion

This paper proposes a formulation and a numerical protocol to allow the application of the Gfr criterion for thick stainless steel components submitted to thermal shock loading. In a first part of the paper, a short description of the criterion initially defined in the S. Marie's PhD is proposed. The proposition to extend the criterion to thermal loading is then made based on the elastic-plastic stresses determined on the crack free model and the classical influence function formulation.Following this description and proposition of extension, two numerical applications are proposed in order to illustrate the applicability of the modified protocol and its coherence with already existing schemes.In a third part, a validation through the comparison to a large scale test performed on CF8M cast stainless steel material is made. This test was chosen since the Gfr criterion is a candidate for defect stability evaluation in cast components. It is then shown that the parameter can be determined on standard specimen, then used to predict the ductile crack growth in large component made of complex materials such the duplex cast stainless steel.

Development and First Test of the 15 T Nb3Sn Dipole Demonstrator MDPCT1

Fermilab in the framework of the U.S. Magnet Devel-opment Program (MDP) has developed a $Nb_3Sn$ dipole demonstrator for a post-LHC hadron collider. The magnet uses 60-mm aperture 4-layer shell-type graded coils. The cable in the two inner-most layers has 28 strands 1.0 mm in diameter and the cable in the two outermost layers has 40 strands 0.7 mm in diameter. An inno-vative mechanical structure based on aluminum I-clamps and a thick stainless steel skin is used to preload $Nb_3Sn$ coils and support large Lorentz forces. The maximum field for this magnet is limited by 15 T due to mechanical considerations. The first magnet assembly was done with lower coil pre-load to achieve 14 T and minimize the risk of coil damage during assembly. This paper describes the magnet design and the details of its assembly procedure, and re-ports the results of its cold tests.

Formability studies on plasma arc welded duplex stainless steel 2205 sheet

AbstractSheet metal forming is cost‐effective manufacturing process and hold a significant key position in fabrication works. Welding being a popular technique the industries primarily prefer for joining of sheet metal formed parts. The inherent material properties changes at the weld metal and heat affected zone post welding process, hence the impact and changes on mechanical strength aspects need to be studied. The current study focuses on influence of plasma arc welding on formability of 1.6 mm thick duplex stainless steel 2205 sheet using Erichsen cupping test by gauging the height of the cup formed. The performed tests such as uniaxial tensile, microhardness showed better mechanical properties and decrease in formability noted from Erichsen cupping test for weld metal compared to base metal. Finite element analysis of Erichsen cupping test is conducted using ABAQUS software and results are matched with experimental outcomes for validation. The comparison shows that a deviation of less than 5 % is noticed between actual and predicted formability index values. Microstructure examination reveals that, equiaxed grains at the weld center and columnar grains at sides typically formed in the weld metal region. The decrease in formability of weld blank compared to base blank is attributed to an increase in ferrite content. This is supported by amount of ferrite content measured in weld metal is 55 % and base metal is 52 %. The scanning electron micrographs (base and weld blank) reveal the mode of failure is ductile.

Understanding root humping in high-power laser welding of stainless steels: a combination approach

High-power laser welding is an efficient way for joining the thick plates in the industries. However, full penetration laser welding of thick plates is prone to form weld imperfections such as undercuts, root humps, and porosity. In this paper, numerical simulation and laser welding experiments are combinedly carried out to investigate the dynamics of the keyhole and molten pool and understand the formation mechanism of root humping during full penetration fiber laser welding of thick stainless steel plates. Besides, the microstructure of the welded joint is analyzed. Good agreements are obtained between the numerical simulation and experimental results. The results indicate that recoil pressure, surface tension, and gravity are the main driving forces for the formation of root hump. The melt flow driven by the recoil pressure near the bottom of the keyhole is an initial and direct formation cause of the root hump. Welding speed is an important factor for the growth of root hump. Local solidification of the rear weld pool path is a hinder for the continuous growth of root hump and can cause a periodic root humping weld.