Showing papers in "Journal of Materials Engineering and Performance in 2001"

Materials for ultrasupercritical coal power plants—Boiler materials: Part 1

Abstract: The efficiency of conventional boiler/steam turbine fossil power plants is a strong function of the steam temperature and pressure. Research to increase both has been pursued worldwide, since the energy crisis in the 1970s. The need to reduce CO2 emission has recently provided an additional incentive to increase efficiency. Thus, steam temperatures of the most efficient fossil power plants are now in the 600 °C (1112 °F) range, which represents an increase of about 60 °C (108 °F) in 30 years. It is expected that steam temperatures will rise another 50 to 100 °C (90 to 180 °F) in the next 30 years. The main enabling technology is the development of stronger high-temperature materials, capable of operating under high stresses at ever-increasing temperatures. Recently, the EPRI performed a state-of-the-art review of materials technology for advanced boiler/steam turbine power plants (ultrasupercritical power plants). The results of the review show that with respect to boilers, high-strength ferritic 9–12Cr steels for use in thick section components are now commercially available for temperatures up to 620 °C (1150 °F). Initial data on two experimental 12Cr ferritic steels indicate that they may be capable of long-term service up to 650 °C (1112 °F), but more data are required to confirm this. For higher temperatures, austenitic steels and Ni-based alloys are needed. Advanced austenitic stainless steels for use as super and reheater tubing are available for service temperatures up to 650 °C (1112 °F) and possibly 700 °C (1292 °F). Ni-based superalloys would be needed for higher temperatures. None of these steels have been approved by the ASME Boiler Code Group so far. Higher-strength materials are needed for upper water walls of boilers with steam pressure above 24 MPa (3400 psi). A high-strength 2-1/2%Cr steel recently ASME code approved as T-23 is the preferred candidate material for this application. Field trials are in progress. This paper will present the results of the EPRI review in detail, relating to boiler material. Results relating to turbine materials are presented in a companion paper as Part 2.

The adiabatic correction factor for deformation heating during the uniaxial compression test

Abstract: The isothermal uniaxial compression test is a common method to determine the flow stress of metals. For accurate flow stress data at strain rates >10−3 s−1, the data must be corrected for flow softening due to deformation heating. The first step in the correction is to determine the increase in temperature. An adiabatic correction factor, η, is used to determine the temperature between strain rates of 10−3 to 101 s−1. The adiabatic correction factor is the fraction of adiabatic heat retained in the workpiece after heat loss to the dies, η=(ΔT ACTUAL)/(ΔT ADIABATIC), where ΔT ADIABATIC=(0.95 f σdɛ)/(ρC p ). The term η is typically taken to be constant with strain and to vary linearly (0 to 1) with log ( $$\dot \varepsilon $$ ) between 10−3) and 101 s−1. However, using the finite element method (FEM) and a one-dimensional, lumped parameter method, η has been found to vary with strain, die and workpiece thermal conductivities, and the interface heat-transfer coefficient (HTC). Using the lumped parameter method, an analytical expression for η was derived. In this expression, η is a function of the die and workpiece thermal conductivities, the interface heat-transfer coefficient, workpiece heat capacity, strain, and strain rate. The results show that an increase in the HTC or thermal conductivity decreases η.

Thermal interface materials

Abstract: Thermal interface mateials for facilitating heat transfer by conduction across two adjacent surfaces are reviewed. They include thermal fluids and pastes, solders, phase change materials (PCMs), and resilient thermal conductors.

Effects of surface preparation on pitting resistance, residual stress, and stress corrosion cracking in austenitic stainless steels

Abstract: Surface finishing treatments such as shot blasting and wire brushing can be beneficial in improving the integrity of machined surfaces of austenitic stainless steels. These operations optimize in-service properties such as resistance to pitting corrosion and stress corrosion cracking (SCC). In this study, ground steel surfaces were subjected to a series of sand blasting and wire brushing treatments. The surfaces were then characterized by their hardness, surface residual stress state, and resistance to stress corrosion and pitting corrosion. Some samples were selected for depth profiling of residual stress. It is found that surface hardening and the generation of near-surface compressive residual stress are the benefits that can be introduced by sand blasting and brushing operations.

Low cost corrosion damage mitigation and improved fatigue performance of low plasticity burnished 7075-T6

Abstract: Low plasticity burnishing (LPB) has been investigated as a surface enhancement process and corrosion mitigation method for aging aircraft structural applications. Compressive residual stresses reaching the alloy yield strength and extending to a depth of 1.25 mm (0.050 in.) deeper than typical corrosion damage is achievable. Excellent surface finish can be achieved with no detectable metallurgical damage to surface and subsurface material. Salt fog exposures of 100 and 500 h reduced the fatigue strength at 2×106 cycles by 50%. The LPB of the corroded surface, without removal of the corrosion product or pitted material, restored the 2×106 fatigue strength to greater than that of the original machined surface. The fatigue strength of the corroded material in the finite life regime (104 to 106 cycles) after LPB was 140 MPa (20 ksi) higher than the original uncorroded alloy and increased the life by an order of magnitude. Ease of adaptation to computer numerical control (CNC) machine tools allows LPB processing at costs and speeds comparable to machining operations. Low plasticity burnishing offers a promising new technology for mitigation of corrosion damage and improved fatigue life of aircraft structural components with significant cost and time savings over current practices.

Microstructure and Mechanical Properties of Weld Fusion Zones in Modified 9Cr-1Mo Steel

Abstract: Modified 9Cr-1Mo steel finds increasing application in power plant construction because of its excellent high-temperature properties While it has been shown to be weldable and resistant to all types of cracking in the weld metal and heat-affected zone (HAZ), the achievement of optimum weld metal properties has often caused concern The design of appropriate welding consumables is important in this regard In the present work, plates of modified 9Cr-1Mo steel were welded with three different filler materials: standard 9Cr-1Mo steel, modified 9Cr-1Mo, and nickel-base alloy Inconel 182 Post-weld heat treatment (PWHT) was carried out at 730 and 760 °C for periods of 2 and 6 h The joints were characterized in detail by metallography Hardness, tensile properties, and Charpy toughness were evaluated Among the three filler materials used, although Inconel 182 resulted in high weld metal toughness, the strength properties were too low Between modified and standard 9Cr-1Mo, the former led to superior hardness and strength in all conditions However, with modified 9Cr-1Mo, fusion zone toughness was low and an acceptable value could be obtained only after PWHT for 6 h at 760 °C The relatively poor toughness was correlated to the occurrence of local regions of untransformed ferrite in the microstructure

Effects of Cooling Time and Alloying Elements on the Microstructure of the Gleeble-Simulated Heat- Affected Zone of 22% Cr Duplex Stainless Steels

Abstract: The effects of austenite stabilizers, such as nitrogen, nickel, and manganese, and cooling time on the microstructure of the Gleeble simulated heat-affected zone (HAZ) of 22% Cr duplex stainless steels were investigated The submerged are welding was performed for comparison purposes Optical microscopy (OM) and transmission electron microscopy (TEM) were used for microscopic studies The amount of Cr2N precipitates in the simulated HAZ was determined using the potentiostatic electrolysis method The experimental results indicate that an increase in the nitrogen and nickel contents raised the δ to transformation temperature and also markedly increased the amount of austenite in the HAZ The lengthened cooling time promotes the reformation of austenite An increase in the austenite content reduces the supersaturation of nitrogen in ferrite matrix as well as the precipitation tendency of Cr2N The optimum cooling time from 800 to 500 °C (Δt8/5) obtained from the Gleeble simulation is between 30 and 60 s, which ensures the austenite content in HAZ not falling below 25% and superior pitting and stress corrosion cracking resistance for the steels The effect of manganese on the formation of austenite can be negligible

Influence of Service-Induced Microstructural Changes on the Aging Kinetics of Rejuvenated Ni-Based Superalloy Gas Turbine Blades

Abstract: Rejuvenation of Ni-based superalloy gas turbine blades is widely and successfully employed in order to restore the material microstructure and properties after service at high temperature and stresses. Application of hot isostatic pressing (HIP) and re-heat treatment can restore even a severely overaged blade microstructure to practically “as-new” condition. However, certain service-induced microstructural changes might affect an alloy’s behavior after the rejuvenated blades are returned to service. It was found that advanced service-induced decomposition of primary MC carbides, and the consequent changes of the γ-matrix chemical composition during the rejuvenation, can cause a considerable acceleration of the aging process in the next service cycle. The paper will discuss the influence of the previous microstructural deterioration on the aging kinetics of rejuvenated gas turbine blades made from IN-738 and conventionally cast GTD-111 alloys.

Influence of microalloying and heat treatment on the kinetics of bainitic reaction in austempered ductile iron

Abstract: The effect of different contents of alloying additions and austenitizing temperature on the transformation kinetics of austenite in a ductile iron austempered at 300 and 400 °C has been investigated in the present study. X-ray diffraction, optical microscopy, and hardness measurements were used to determine the transformation kinetics of low Ni iron, low Mo iron and low Ni, Mo iron during austempering at 300 and 400 °C for 1 to 480 min after austenitizing at 850 and 930 °C for 120 min. Nickel and molybdenum in used contents are shown to delay the bainitic transformation without the undesirable features. Decreasing the autenitizing temperature is shown to increase the driving force for stage I of reaction but to have only a small effect on stage II kinetics. This shifts the position of the processing window to short periods of time and leads to opening of the processing window, which is closed for higher autenitizing temperatures. A more uniform austempered microstructure can be obtained with a decrease of autenitizing temperature. Decreasing the autenitizing temperature has the disadvantage of reducing the austemperability.

Effects of alloying elements on the mechanical properties and corrosion behaviors of 2205 duplex stainless steels

Abstract: The effects of alloying elements on the microstructure, mechanical properties, and corrosion behaviors of duplex stainless steels (DSSs) have been investigated in this study. Experimental alloys were prepared by varying the concentrations of the constituent elements in DSSs. Hot ductility test, tensile test, charpy impact test, and corrosion test were performed to evaluate the properties of the experimental alloys. The results showed that the extent of edge cracking of DSSs increased with the increasing value of the crack sensitivity index (CSI). The higher the hot ductility index (HDI) was, the better the hot ductility of DSSs achieved. Austenite (γ) stabilizer generally caused a decrease in the strength and an increase in the charpy impact absorbed energy of the stainless steel. On the contrary, ferrite (α) former exerted its beneficial effect on the strength but became detrimental to the toughness of DSSs. The presences of sulfur and boron also caused a decrease in the impact energy, but nitrogen and carbon hardly affected the toughness within the concentration range tested in this study. The value of pitting nucleation potential (E np ) of different nitrogen contents in 3.5 wt.% NaCl solution at room temperature was almost the same, but the value of pitting protection potential (E pp ) among these alloys was increased with increasing the content of nitrogen. The susceptibility to stress corrosion cracking (SCC) of DSSs was high when tested in boiling 45 wt.% MgCl2 solution. On the other hand, the time to failure of the experimental steels in 40 wt.% CaCl2 solution at 100 °C was longer than that in MgCl2 solution. Nitrogen could affect the SCC behavior of DSSs in CaCl2 solution through the combinative effects by varying the pitting resistance and the slip step dissolution. An optimum nitrogen (N) content of 0.15 wt.% was found where the highest SCC resistance could be obtained. Although γ phase exhibited better resistance to SCC, cracks were found to penetrate through α and γ grains or to propagate along the α/γ interface. As a result, a mixed transgranular plus intergranular mode of fracture surface was observed.

Performance evaluation of HSLA steel subjected to underwater explosion

Abstract: Performance evaluation of High Strength Low Alloy (HSLA) steel subjected to underwater explosion is of interest to materials engineers because of its structural applications in ships and submarines. Circular and rectangular plates were investigated for their explosive response because they represent panels of a ships plating. Underwater explosion bulge tests were carried out with increasing shock intensity on 4 mm thick circular plates of 290 mm diameter and rectangular plates of 300×250 mm to study the plastic deformation and the onset of fracture. Empirical models were developed for the prediction of depth of bulge of the plates. A fresh set of tests with various explosive charge quantities and stand offs were carried out which showed good agreement with the models. Failed edges of the plate showed slant fracture suggesting ductile mode of failure. Scanning Electron Microscopic (SEM) fractographic examination showed dimple features suggesting micro void coalescence.

Comparative constitutive constants for hot working of Al-4.4Mg-0.7Mn (AA5083)

Abstract: Torsion tests on as-hot-rolled Al-4.4Mg-0.7Mn alloy were conducted over the strain rate range 0.1 to 10 s−1 and the temperature range 200 to 450 °C. As temperature decreased and strain rate increased, the flow curves exhibited peaks that were lower and broader, followed by lower softening, and closer approach to a steady-state regime partly because fracture occurred at higher strains. In constitutive analysis, the power law was found to be inappropriate, but the exponential law was suitable. The hyperbolic sine function was found to fit more closely with stress multipliers between 0.04 and 0.06 MPa−1. The activation energy was found to be 162 kJ/mol. These results were shown to be in reasonable agreement with previous studies when allowance was made for variations in composition and microstructure.

Kinetics of strain aging in bake hardening ultra low carbon steel—a comparison with low carbon steel

Abstract: The kinetics of the static strain aging process have been analyzed in a vacuum-degassed ultra low carbon bake hardenable (ULC BH) steel with a total carbon content of 20 wt.ppm through measurement of the strength properties. The influence of prestrain and free interstitial carbon content has been studied. The kinetic results were compared with those of a BH low carbon (LC) steel. In the derivation of the time exponent and the activation energy, only the first stage of aging was considered. It was observed that, at all prestrain levels and matrix solute carbon contents, the initial aging process in the ULC steel obeyed the t 2/3 kinetic law and the kinetics were not influenced by the changes in dislocation structure due to prestrain and the dissolved carbon content. In comparison, the aging process and the kinetics in the LC steel were found to be significantly influenced by the amount of prestrain. The presence of carbide particles in LC steels can modify the aging kinetics.

Introduction of Compressive Residual Stresses in Ti-6Al-4V Simulated Airfoils via Laser Shock Processing

Abstract: Ti-6Al-4V (Ti-64) simulated airfoils were laser shock processed with two laser power densities (4 and 9 GW/2) for each of three pulse repetition treatments (1, 3, and 5 shocks/spot). The microstructural effects of laser shock processing (LSP) on the Ti-64 were studied via scanning electron microscopy (SEM). Ultrasonic nondestructive inspection (NDI) was conducted to ensure that the LSP treatments resulted in no internal damage to the simulated airfoils. In-depth residual stress and cold work measurements were made using x-ray diffraction. No substantial changes due to LSP were found in the microstructure, and no internal damage was detected during NDI or metallographic sectioning. It was found that the in-depth residual stress and cold work states induced by LSP were a function of laser power density and pulse repetition. It was possible to induce compressive residual stresses in the direction most critical for the prevention of fatigue-crack growth throughout the thickness of the simulated airfoil leading edge.

Strain-Induced Porosity during Cogging of Extra-Low Interstitial Grade Ti-6Al-4V

Abstract: The phenomenon of strain-induced porosity (SIP) in extra-low interstitial (ELI) grade Ti-6Al-4V with a transformed β starting microstructure is investigated to understand its origin during α-β cogging. For this purpose, the constitutive behavior of the material is coupled with finite-element method (FEM) simulations of the cogging process. Two distinct types of SIP relevant to cogging speeds and temperatures, viz., shear cracking and void nucleation, are identified. While the former occurs at the prior β grain boundaries below 825 °C, the latter occurs at the prior colony boundaries when the deformation temperature is close to the β transus. The FEM simulations have shown that deformation conditions existing in the midregion of the billet are favorable for void nucleation. The mechanism of void growth in the presence of tensile residual stress and temperature during resoaking is modeled using the Cocks-Ashby coupled growth model. Repeated cogging and resoaking steps cause multiplication of void population in large numbers. To avoid both types of defects in any region of the billet, a practical solution has been developed by introducing a differential temperature profile from the surface to the center, and the validity of the proposed scheme is verified with FEM heat-transfer simulations.

Electropolishing of 316L stainless steel for anticorrosion passivation

Abstract: 316L stainless steel is deemed an indispensable material in the semiconductor industry. In many instances, the surface of the production equipment needs to be treated for low-corrosion passivation, good finish, weldability, and cleanliness. The process characteristics of electropolishing meet these requirements well. The current study investigates the effects of the major processing parameters on the anticorrosion performance and the surface roughness. The electrolyte with 10% water content and a ratio between H2SO4 and H3PO4 of 4 and 6 has been proven to be successful, showing no corrosion pitting points on the specimen surface. The electrolyte temperature of 85±10 °C and the electrical current density of 0.5 to 1.0 A/cm2 are found to be optimal. The processing time beyond 3 to 5 min produces no further improvement. The addition of 10% glycerin provides a very fine surface (maximum roughness of 0.05 µm), while the anticorrosion performance is deteriorated. The results obtained are useful for the manufacture of the semiconductor equipment.

On the tensile strength and hardness relation for metals

Abstract: A method for predicting the ultimate tensile strength (S u ) of a material from Brinell-type hardness tests is described for several metals including steel, aluminum, and copper alloys. The prediction of S u is based on a consistent relationship between S u and a material’s hardness coefficient, K d . A simple experimental procedure for determining K d from indentation-hardness test data is presented. The relationship between S u and K d is found to be 1/3 for all cubic metals. Comparisons between predicted and experimentally determined values of S u are made for each of the materials studied, and sources of error between the two S u values are discussed.

Mechanisms of erosion-corrosion on a cobalt-base alloy and stainless-steel UNS S17400 in aggressive slurries

Abstract: Two materials, which are candidates in the construction of drill bits, have been studied in severe liquid-solid impingement conditions. Analysis of the weight loss was carried out on Stellite X40 (Co-base) and UNS S17400 (stainless steel) materials at 30, 60, and 90° angles of impingement under erosion-corrosion conditions at the free-corrosion potential and with applied cathodic protection. A preliminary analysis of the corrosion behavior of the materials in static conditions was the starting point for the electrochemical analysis. A more extensive corrosion analysis under erosion-corrosion conditions was carried out through direct current (DC) potentiodynamic tests with varying angles of impingement. The results were used to identify the contribution of mechanical erosion and electrochemical corrosion for both materials under different angles of impingement and to determine the extent of interaction between corrosion and mechanical-erosion processes.

Distortion Minimization During Gas Quenching Process

Abstract: During gas quenching, independent process parameters include the preheat temperature of the component and the temperature of the circulated gas. One of the most important dependent process parameters is the heat transfer coefficient between the component and the circulated gas. The heat transfer coefficient has significant influence on the quenching results, such as distortion, residual stresses, and hardness distribution. Large distortions after quenching will increase the cost due to the post-quenching processes, such as the grinding and hot rectification. The objective in this research is to minimize the distortion caused by quenching. In this paper, the surface of a component is divided into several regions, and different values of the heat transfer coefficient are imposed on each region. Constraints on the residual stresses and surface hardness distribution are imposed to improve the service properties. The heat transfer coefficients are ideal design variables to optimize the gas quenching process. The commercial finite element software, DEFORM-HT, is used to predict the material response during the quenching process. The response surface method is used to obtain the analytical models of the objective function and constraints in terms of the design variables. Once the closed-form response surface models are obtained, a commercially available design optimization tool, design optimization tool (DOT), is used to search for the optimum design point. This paper summarizes the methodology used to optimize the gas quenching process together with an application of a steel disk example.

Graphitization of steels in elevated-temperature service

Abstract: Prolonged exposure of carbon and low alloy steel components to temperatures exceeding 800 °F (427 °C) can result in several kinds of material microstructural deterioration; for example, creep cavitation, carbide coarsening and/or spheroidization, and, less commonly, graphitization. Graphitization generally results from the decomposition of pearlite (iron + iron carbide) into the equilibrium structure of iron + graphite and can severely embrittle the steel when the graphite particles or nodules form in a planar, continuous manner. Graphitization has resulted in the premature failure of pressure boundary components, including high energy piping and boiler tubes. Failure due to graphitization continues to be of concern in long-term aged carbon and carbon-molybdenum steels, both in weldments and in base metal, where, as recently reported, prior deformation or cold work could accelerate the graphitization process. This paper describes the characteristics and kinetics of graphitization, reviews pertinent laboratory and field experience, and summarizes time-temperature service regimes within which graphitization can be anticipated.

Determination of the toughness of in-service steam turbine disks using small punch testing

Abstract: Knowledge of the material toughness is crucial in assessing the integrity of heavy section steel components. Conventional tests to determine the toughness involve extraction of large blocks of materials and therefore are not practical on in-service components. On the other hand, conservative assumptions regarding toughness without regard to actual data can lead to expensive and premature replacement of the components. Previous EPRI studies have demonstrated the use of a relatively nondestructive technique termed the “small punch test” to estimate the fracture appearance transition temperature (FATT) and fracture toughness (KIc) of high-temperature turbine rotor steels and nuclear reactor pressure vessel steels. This paper summarizes the results of research into the feasibility of extending the small punch test to characterize the toughness of the 3 to 3.5% NiCrMoV (3–3.5NiCrMoV) low alloy steel used for fossil and nuclear power plant low-pressure (LP) steam turbine disks. Results of the present study show that the small punch transition temperature, Tsp, is linearly correlated with FATT, so that measurement of Tsp permits estimation of the standard Charpy FATT through empirical use of the correlation. The statistical confidence prediction uncertainty bands for the correlation were found to be narrow enough to make the small punch- based FATT estimation practical for this alloy. Additionally, independent KIc measurements made by PowerGen, UK, on some of the same test materials were in excellent agreement with measurements made here, indicating that the small punch KIc measurement can be reproducible across laboratories. Limited testing for fracture initiation toughness showed, as has been demonstrated for other materials, that the small punch test-based initiation fracture toughness (KIc) determination was within ±25% of the ASTM standard measurement of KIc, suggesting that the test method can be used for direct determination of fracture initiation toughness.

Hot-roll bonding of Al-Pb bearing alloy strips and hot dip aluminized steel sheets

Abstract: In this paper, the basic bonding mechanism between two materials of practical importance is identified. One of the materials is carbon steel, which has been aluminized on its surface by immersion in molten aluminum. This step produced a Fe-Al intermetallic compound layer. The other material is an Al-Pb alloy (a bearing material). The two materials were hot roll bonded together. It was found that the Fe-Al intermetallic compound broke into discontinuous blocks during the hot rolling operation. The block of intermetallic compound remained bonded to the steel. The overall bond between the Al-Pb strip and the steel strip resulted from two different bonds. The Al-Pb strip and the Fe-Al intermetallic compound (this is called the “block bond” in this paper) and the Al-Pb strip and the bare steel surface in the area where the block separated from the steel substrate (this is called the “blank bond” in this paper).

Corrosion properties of inconel 617 alloy after heat treatment at elevated temperature

Abstract: Inconel alloys find wide application in industry as high-temperature resistance materials. In the present study, refurbishment of the Inconel 617 alloy after 37,000 h of operation in the field is carried out through the heat-treatment process. The electrochemical response of the heat-treated alloy is determined through potentiodynamic testing of the surfaces. The heat-treatment process is carried out at 1175 °C for 1 and 2 h in an air free furnace. The corrosion rate is estimated from TAFEL and polarization measurements. The surface morphology after the electrochemical tests is studied using scanning electron microscopy (SEM), while the material characterization at the surface is carried out using energy disperse spectroscopy (EDS). It is found that the corrosion resistance improves considerably for the workpieces subjected to 1 h heat treatment. The depletion of Cr and Mo at grain boundaries results in excessive pitting in this region. Moreover, enrichment of Cr at the surface after 1 h heat treatment reduces the corrosion current.

Iterative taguchi analysis: optimizing the austenite content and hardness in 52100 steel

Abstract: Three iterations of Taguchi designed experiments and analyses were used to determine optimal thermal treatments for minimizing retained austenite content while maximizing Rockwell hardness (HRC) in AISI 52100 bearing steel. Experimental variables chosen for this study included austenitizing and tempering temperatures, tempering time and cold treatment. After one iteration, tempering temperature and cold treatment were seen to have the greatest effect on austenite content while austenitizing and tempering temperatures had the greatest influence on hardness. After the second and third experimental iterations, two thermal treatments were noted each producing hardness of 58–59 HRC in combination with zero retained austenite as measured by x-ray diffraction.

Factors controlling the magnesium weld morphology in deep penetration welding by a CO2 laser

Abstract: In laser welding with power density beyond 104 W · mm−2, the formation of plasma cavities, commonly referred to as keyholes, leads to deep penetration welds with high aspect ratios. In this paper, the morphologies of keyhole welds produced with a 6 kW CW CO2 laser on two die-cast magnesium alloys, AZ91 and AM50, are compared. It was found that the two magnesium alloys responded differently to laser welding. Though irregular weld cross-section profiles were consistently observed on each materials, bead dimensions often varied with the welding variables in contrasting ways. For both alloys, important characteristics of the weld beads such as depth, width, crown height (hump), and surface ripples were analyzed as a function of the welding parameters, most particularly the heat input. Results show that the use of heat input, a variable grouping two welding parameters into one, was often inadequate in characterizing the bead morphology. Several explanations are given, including base metal vaporization, but the process of bremsstralung absorption explains it well and rationalizes many observed characteristics of laser weld morphology.

Elimination of intergranular corrosion susceptibility of cold-worked and sensitized AlSl 316 SS by laser surface melting

Abstract: Susceptibility to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) due to sensitization is one of the major problems associated with austenitic stainless steels. Thermal exposures encountered during fabrication (welding, hot working, etc.) and elevated temperature service may lead to sensitization of components of austenitic stainless steels. Laser surface melting (LSM) is an in-situ method to increase the life of a sensitized component by modifying the surface microstructure without affecting the bulk properties. In this paper, the results obtained in the attempt to improve IGC resistance of coldworked and sensitized 316 SS by LSM are presented. Type 316 SS specimens cold worked to various degrees ranging from 5 to 25% reduction in thickness and sensitized to different degrees by exposing at 898 K for different durations were laser surface melted using continuous wave (cw) CO2 laser. ASTM standard A262 practice A, optical metallography, and ASTM standard G108 were used to characterize the specimens before and after LSM. Influence of prior deformation on the desensitization behavior was evaluated for the laser melting conditions adopted during the investigation. Complete dissolution of M23C6 due to laser melting and suppression of re-precipitation due to rapid quenching result in a desensitized homogenous microstructure, which is immune to IGC. Under identical laser melting conditions, the extent of desensitization decreases with an increase in the degree of cold work, and hence, higher power levels and an extended interaction time must be adopted to homogenize the sensitized microstructure with prior cold work.

Influences of porosity on mechanical and wear performance of pseudoelastic TiNi-matrix composites

Abstract: Pores usually exist in sintered tribo-composites and may strongly affect the performance of the composites. In this work, the influence of pores on the wear behavior of sintered pseudoelastic TiNi-matrix tribocomposites was investigated. In particular, changes in the density of pores and corresponding variations in wear resistance of the composites were studied. It was demonstrated that mechanical properties and the wear resistance of the composites were strongly affected by voids. The wear resistance was enhanced when the density of pores was reduced by using wax to enhance the compaction during pressing. It was also interesting to observe that pores are sealed during the wear process. This results in improving wear resistance during wear, especially under high loads. Some other contributing mechanisms are also discussed.

Brazeability of the 6061-T6 Aluminum Alloy with Al-Si-20Cu-Based Filler Metals

Abstract: The bond strength of the 6061-T6 aluminum alloy brazed with Al-12Si, Al-9.6Si-20Cu, and Al-7Si-20Cu-2Sn filer metals at a low temperature of 550°C is evaluated. The fractography of these brazements after tensile tests was observed using scanning electron microscopy (SEM). It was found that joints with good integrity can be produced with Al-7Si-20Cu-2Sn filler metal because it can be used in a temperature range of 504 to 526 °C, about 70 °C lower than the traditional Al-12Si filler metal. It was shown that joints of 6061-T6 aluminum alloy as the base metal, when brazed at 550 °C for 60 min using this new filler metal and ward, and after being subjected to a T6 treatment, possessed a high bonding strength of about 121 Mpa.

Creep properties of austenitic stainless-steel weld metals

Abstract: The creep behavior of two austeitic stainless-steel weld metals was investigated. Two AISI 316L stainless-steel base plates were welded together using the submerged arc-welding process. Creep tests were carried out on the welds at constant load, over a stress range of 100 to 400 MPa, and in the temperature range of 600 to 700 °C. The relationships between stress and minimum secondary creep rate at a constant temperature were obtained with Norton’s law. The results showed that AISI 347 weld metal presented a higher creep resistance with lower values of the minimum strain rate, and, consequently, it exhibited a longer life before rupture than AISI 316L weld metal. However, this weld metal showed a lower ductility value than AISI 316L weld metal. The weld-metal microstructure survey, performed before and after the creep testing, has shown different amounts of delta ferrite, which was strongly dependent on time, temperature, and stress level.

Mechanical property and cutting performance of yttrium-reinforced Al 2 O 3 /Ti(C,N) composite ceramic tool material

Abstract: Effects of yttrium on the mechanical property and the cutting performance of Al2O3/Ti(C,N) composite ceramic tool material have been studied in detail. Results show that the addition of yttrium of a certain amount can noticeably improve the mechanical property of Al2O3/Ti(C,N) ceramic material. As a result, the flexural strength and the fracture toughness amount to 1010 MPa and 6.1 MPam1/2, respectively. Cutting experiments indicate that the developed ceramic tool material not only has better wear resistance but also has higher fracture resistance when machining hardened #45 steel. The fracture resistance of the yttrium-reinforced Al2O3/Ti(C,N) ceramic tool material is about 20% higher than that of the corresponding ceramic tool material without any yttrium additives.