Determination of plasticity following deformation and welding of austenitic stainless steel

Intergranular strain has been associated with high-temperature cracking of welded pipework in 316H austenitic stainless steel material used in nuclear power plant heat exchangers. In this study, neutron diffraction has been used to study the development of intergranular strains in plastically-deformed and welded 316H stainless steel. Measurements have been made of the intergranular strain evolution with increasing plastic strain in base material, and correlated with further measurements made in samples extracted from welded pipes, where the pipes were welded following plastic deformation to different levels of plastic strain. Strong tensile strain evolution was seen on the compliant 200 grain family. The results were correlated with various proxy measures of plastic strain, including hardness and diffraction peak width, and excellent agreement was obtained

Effects of plastic strain history on the properties of stainless steel boiler tube welds

The subject of this dissertation is the study of the effects of fabrication history (prestraining, welding and heat treatment) on the mechanical properties of austenitic stainless steel thin wall boiler tubes. These tubes are usually cold bent to shape, and sometimes swaged, prior to interconnection by welding. The bends require solution heat treatment before welding. In addition, subsequent to welding, the residual stresses should be relieved. It is sometimes not practically feasible to follow these constructional practices strictly especially when a whole boiler is constructed as a single unit and becomes too large and complex and contains different tubing materials. As a result of this fabrication history, the mechanical properties of boiler tube materials can be significantly altered. Sample tubes simulating the fabrication steps were supplied by British Energy for this project. The primary aim of the study was to determine spatially resolved room-temperature tensile properties using digital image correlation (DIC) by testing cross-weld specimens machined from the thin wall welded tubes (with plain or prestrained base metal) before and after the heat treatment. The experimental procedure which is used to retrieve the tensile properties from these integrated tests was validated through finite element simulation. Digital image correlation, which is a full-field strain measurement technique, was implemented in order to obtain the local stress-strain curves from regions less than a square millimetre in area and to extract the corresponding local tensile properties such as offset proof strength. The variation of the 0.2% offset proof strength was successfully obtained along these specimens. Evidence of strain hardening due to constraint and weld thermomechanical cycles was found in the plain base metal near the weld pool and evidence of softening was seen in prestrained base metal. On the other hand, after the heat treatment, the effect of prestraining and welding is cleaned out and the strength along the specimen was almost homogenized. However, aswelded cross-weld specimens with prestrained base metal have demonstrated unusual local stress-strain behavior in the weld-affected region. For a better understanding of this behavior, tension test of a cross-weld specimen with a high strength mismatch between the weld metal and base metal was simulated using the finite element method. It was found that the strength mismatch in the specimen, in combination with the experimental procedure, may cause some anomalies in the local stress-strain curves. It was also confirmed that these anomalies are not very detrimental for the determination of the proof strength on the specimens with strength mismatch. Material characterization of the welds and detailed hardness surveys on crossweld specimens were performed. Plastic strain is known to be detrimental for high temperature performance of austenitic stainless steel tubes, therefore, the degree of the plastic deformation should be known before these tubes enter service. DIe, hardness, electron back-scattered diffraction and neutron diffraction (peak width and anisotropy strain) were used to determine the amount of plastic strain in the as-welded tubes. It was observed that there is a good agreement between the predictions of plastic strain in 20% prestrained and welded tube

Effect of prior cold work on the mechanical properties of weldments

Heat exchanger units used in steam raising power plant are often manufactured using many metres of austenitic stainless steel tubes that have been plastically formed (bent and swaged) and welded into complex shapes. The amount of plastic deformation (pre-straining) before welding varies greatly. This has a significant effect on the mechanical properties of the welded tubes and on the final residual stress state after welding. The aim of the present work was to measure and understand the combined effects of pre-straining and welding on the properties and residual stress levels in stainless steel tubing weldments. Effects of plastic deformation were simulated by plastically straining three identical stainless steel tubes to different strain levels (0%, 10% and 20%). Then each tube was cut into two halves and welding back together. The variation in mechanical properties across weldments was measured using digital image correlation (DIC) and a series of strain gauges (SG). Residual stresses were measured on the 0% (undeformed) and 20% prestrained and welded tubes by neutron diffraction. It was found that the welding process had a marked effect on the tensile properties of parent material within 25mm of the weld centre-line. Evidence of cyclic strain hardening was observed in the tube that had not been pre-strained, and evidence of softening seen in the 10% and 20% pre-strained tubes. Macroscopic residual stresses were measured to be near zero at distances greater than 25 mm from the weld centre-line, but measurements in the 20% pre-strained tube revealed the presence of micro residual stresses having a magnitude of up to 50 MPa

Removal of silver (I) from aqueous solutions with clinoptilolite

The aim of the present work was to investigate the ability of natural zeolite, clinoptilolite, to remove silver ions from aqueous solution. Towards this aim, batch adsorption experiments were carried out and the eect of various parameters on this removal process has been investigated. The eects of pH, adsorption time, metal ion concentration and the acidic treatment on the adsorption process were examined. The optimum pH for adsorption was found to be 4.0. It was found that acid treatment has a substantial eect on the metal uptake. In adsorption studies, residual Ag+ concentration reached equilibrium in a short duration of 45 min. Maximum adsorption capacity, 33.23 mg Ag+/g zeolite, showed that this adsorbent was suitable for silver removal from aqueous media. Adsorption phenomena appeared to follow Langmuir and Freundlich isotherms

Feedback regulation in gene networks

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2007.Includes bibliographical references (p. 97-104).Cellular genetic information is encoded in DNA. The passage of this information from DNA to proteins is regulated at multiple levels and each level gives cells the chance to control the structure and function of their components. Transcriptional regulation is an important part of this multi-level process. By using feedback loops as part of transcriptional gene networks, cells can tune the level and stability of gene expression. In the first half of my thesis, I will discuss how isogenic cells can be re-programmed to have varying levels of memory associated with previous growth conditions and how genetic noise limits the stability of this cellular memory. Noise in gene expression, through the phenotypic heterogeneity it promotes, has the potential to be a mechanism implemented by cells to cope with the uncertainties in environmental conditions. By randomly expressing multiple phenotypes, each fit to a certain environment, cells can survive unexpected changes in the extracellular environment. In this way, a population can hedge its bets against environmental uncertainty. Depending on how often the cells choose to display various phenotypes, the population can range from being highly diverse (heterogeneous) to being less diverse (homogeneous). In the second half of my thesis, I will discuss how the degree of phenotypic diversity for an isogenic population of cells can be tuned by re-engineering a gene network. I will present results from experiments which test the effect of noise-induced diversity on population fitness in the presence of fluctuating environments. The results demonstrate that for an optimum population growth in fluctuating environments, cells need to match the rates of inter-phenotypic switching to the frequency of environmental changes.by Murat Acar.Ph.D

Transient computational fluid dynamics modelling of the melting process in thermal bonding of porous fibrous media

A continuum model of the melting process in porous fibrous media is introduced. The fluid flow, heat transfer and phase change within the porous nonwoven web is numerically solved using computational fluid dynamics. Boundary conditions from an experimentally validated whole system model of a typical industrial machine, producing fibrous webs are incorporated. The presented model shows the capability to investigate the phase change during heating of the thermoplastic fibres during nonwoven web formation. Moreover, the fibres' geometrical information and constitutive equations, describing the material behaviour are included. The approach considers the fibre thickness, sheath fraction, and thermophysical properties like melting temperature, latent heat of fusion and the liquid fraction, enabling the assessment of different fibre types and to determine the properties of the fabric. The model results reveal that the web porosity has the most significant effect on the melting process among the considered parameters. Thermal gradients that occur inside the web are due to the combined convection and latent heat of fusion effect, which stores heat to melt the fibres. The model is applicable to a wide variety of systems ranging from textiles, fibrous beds, ceramics, membranes and porous composite materials. © IMechE 2012 Reprints and permissions: sagepub.co.uk/ journalsPermissions.nav

Optimisation of machine components in thermal fusion bonding process of porous fibrous media: Material optimisation for improved product capacity and energy efficiency

A comprehensive parametric study to aid in the material optimisation of the thermal fusion bonding machine components for improved product capacity and energy efficiency is introduced. The effect of conveyer belt and drum cover material type on the thermal bonding performance has been systematically investigated. A previously validated 2D computational fluid dynamics model based on the theory of porous media has been used for the study. The PEEK material is determined to be showing the greatest optimisation potential for higher production rates. The optimum material match for the highest production rates has been determined, considering the effect of material type on the thermal bonding time. Improved product quality associated with a uniform temperature distribution has been achieved by using PEEK material. The conveyer belt component has been determined as the highest energy-absorbing component and could be optimised