Loss Of Toughness Research Articles

Relationship between electrochemical characteristics and SCC of X70 pipeline steel in an acidic soil simulated solution

Stress corrosion cracking (SCC) of X70 pipeline steel in simulated solution of the acidic soil in Yingtan in China was investigated using slow strain rate test (SSRT), SEM and potentiodynamic polarization technique. Experiment results indicate that X70 steel is highly susceptible to SCC as applied potential reduces, which is manifested in loss of toughness and brittle fracture. Constant polarization current can detect the occurrence of SCC. The lower the polarization current is the sooner stress corrosion cracking occurs. The SCC mechanisms are different at varying potentials. When potential is higher than open circuit potential, anodic process controls SCC, whereas when potential is far lower than open circuit potential, cathodic process controls SCC, and between these two potential regions, a combined electrochemical process controls the SCC. Stress or strain has a synergistic effect with electrochemical reactions to accelerate the cathodic hydrogen evolution process, which makes the X70 pipeline steel to be more susceptible to SCC.

Diversity and activity of aquatic fungi under low oxygen conditions

Summary1. The objective was to test whether a decrease in oxygen concentration in streams affects the diversity and activity of aquatic hyphomycetes and consequently leaf litter decomposition.2. Senescent leaves of Alnus glutinosa were immersed for 7 days in a reference stream, for fungal colonization, and then incubated for 18 days in microcosms at five oxygen concentrations (4%, 26%, 54%, 76% and 94% saturation). Leaf decomposition (as loss of leaf toughness), fungal diversity, reproduction (as spore production) and biomass (ergosterol content) were determined.3. Leaf toughness decreased by 70% in leaves exposed to the highest O2 concentration, whereas the decrease was substantially less (from 25% to 45%) in treatments with lower O2. Fungal biomass decreased from 99 to 12 mg fungi g−1 ash‐free dry mass on exposure to 94% and 4% O2 respectively. Sporulation was strongly inhibited by reduction of dissolved O2 in water (3.1 × 104 versus 1.3 × 103 spores per microcosms) for 94% and 4% saturation respectively.4. A total of 20 species of aquatic hyphomycetes were identified on leaves exposed to 94% O2, whereas only 12 species were found in the treatment with 4% O2 saturation. Multidimensional scaling revealed that fungal assemblages exposed to 4% O2 were separated from all the others. Articulospora tetracladia, Cylindrocarpon sp. and Flagellospora curta were the dominant species in microcosms with 4% O2, while Flagellospora curvula and Anguillospora filiformis were dominant at higher O2 concentrations.5. Overall results suggest that the functional role of aquatic hyphomycetes as decomposers of leaf litter is limited when the concentration of dissolved oxygen in streams is low.

Effects of RTM Mold Temperature and Vacuum on the Mechanical Properties of Epoxy/Glass Fiber Composite Plates

The purpose of this study is to investigate the effects of mold temperature, application of vacuum at resin exit ports, and initial resin temperature on the mechanical properties of epoxy matrix woven glass fiber reinforced composite specimens produced by resin transfer molding (RTM). For this purpose, six mold temperatures (25, 40, 60, 80, 100, and 120°C), two initial resin temperatures (15 and 28°C), and vacuum (0.03 bar) and without vacuum (~1 bar) conditions are utilized. Specimens are characterized by ultrasonic C-scan inspection, mechanical tests (tensile, flexural, and impact), thermal analyses (ignition loss and TGA) and scanning electron microscopy. It is observed that mechanical properties of the specimens produced at a mold temperature of 60°C with the application of vacuum and initial resin temperature of 28°C proved to be the highest (e.g., 16, 26, and 43% higher tensile strength, flexural strength, and Charpy impact toughness, respectively, compared to the lowest values attained with mold temperatures other than 60°C while other variables are kept constant). It has been shown that application of vacuum contributes to the final mechanical properties of the produced composites by lowering the percentage of `voids'. In fact, without the application of vacuum, the deteriorations in mechanical properties are as high as 26% loss in Charpy impact toughness and 5% losses in tensile and flexural strength. Additionally, lowering the initial resin temperature is shown to alter mechanical properties (e.g., 14, 12, and 18% losses in tensile strength, flexural strength, and Charpy impact toughness, respectively, when the initial resin temperature is decreased from 28 to 15°C).

Toughness decrease of PLA‐PHBHHx blend films upon surface‐confined photopolymerization

The present research investigates the effect of photoinduced grafting reaction on the bulk properties of melt processed poly(L-lactic acid) (PLA)-poly[(3-hydroxybutyrate)-co-(3-hydroxyhexanoate)] (PHBHHx) blend films. PLA-PHBHHx blend films, comprising 10 wt % PHBHHx showed a remarkable toughness improvement. From dynamic mechanical analysis of melt processed PLA-PHBHHx blend films, the blend appears to be noncompatible. Unfortunately, PLA-PHBHHx blend films underwent rapid physical aging as characterized using differential scanning calorimetry, resulting in a significant toughness loss. Physically aged films regained the original toughness on annealing at 60 degrees C for 30 min. Annealed PLA-PHBHHx blend films also underwent physical aging leading to a significant toughness loss. Hydrophilic monomers like acrylic acid and acrylamide were successfully photopolymerized from the film surface using a sequential, two-step photografting approach. The resultant films were characterized using water contact angle goniometry, ATR-FTIR spectroscopy, and mechanical testing. PLA-PHBHHx blend films lost their toughness significantly on surface modification and this was assigned to UV-assisted solvent induced crystallization as characterized using wide-angle X-ray diffraction analyses.

Influence of Orthogonal Overload on Human Vertebral Trabecular Bone Mechanical Properties

The aim of this study was to investigate the effects of overload in orthogonal directions on longitudinal and transverse mechanical integrity in human vertebral trabecular bone. Results suggest that the trabecular structure has properties that act to minimize the decrease of apparent toughness transverse to the primary loading direction. The maintenance of mechanical integrity and function of trabecular structure after overload remains largely unexplored. Whereas a number of studies have focused on addressing the question by testing the principal anatomical loading direction, the mechanical anisotropy has been overlooked. The aim of this study was to investigate the effects of overload in orthogonal directions on longitudinal and transverse mechanical integrity in human vertebral trabecular bone. T(12)/L(1) vertebral bodies from five cases and L(4)/L(5) vertebral bodies from seven cases were retrieved at autopsy. A cube of trabecular bone was cut from the centrum of each vertebral body and imaged by microCT. Cubes from each T(12)/L(1) and L(4)/L(5) pairs were assigned to either superoinferior (SI) or anteroposterior (AP) mechanical testing groups. All samples were mechanically tested to 10% apparent strain by uniaxial compression according to their SI or AP allocation. To elucidate the extent to which overload in orthogonal directions affects the mechanical integrity of the trabecular structure, samples were retested (after initial uniaxial compression) in their orthogonal direction. After mechanical testing in each direction, apparent ultimate failure stresses (UFS), apparent elastic moduli (E), and apparent toughness moduli (u) were computed. Significant differences in mechanical properties were found between SI and AP directions in both first and second overload tests. Mechanical anisotropy far exceeded differences resulting from overloading the structure in the orthogonal direction. No significant differences were found in mean UFS and mean u for the first or second overload tests. A significant decrease of 35% was identified in mean E for cubes overloaded in the SI direction and then overloaded in the AP direction. Observed differences in the mechanics of trabecular structure after overload suggests that the trabecular structure has properties that act to minimize loss of apparent toughness, perhaps through energy dissipating sacrificial structures transverse to the primary loading direction.

Morphology and mechanical properties of carbon fiber reinforced composites based on semicrystalline polyimides modified by carbon nanofibers

Abstract As previous attempts to recrystallize a thermoplastic polyimide matrix, while preparing carbon fiber reinforced composites, had resulted in embrittlement of the matrix, a new concept was attempted, based on adding a small quantity of acicular nanoinclusions, namely, vapor grown carbon fibers (VGCF). The new concept was based on the expected dual action of VGCF, comprising their power to nucleate the matrix jointly with their ability to reinforce the matrix through an effective stress transfer mechanism. First, regeneration of crystallinity with 1–3 wt.% VGCF was confirmed, showing also that for such low loadings the melt viscosity remained on a practical level. Then by screening a range of mechanical properties it was shown that the reinforcing action of VGCF compensated for the loss of toughness that was associated with high crystallinity.

Beneficial Effects Induced by High Temperature Cycling in Aged Duplex Stainless Steel

The cyclic behavior of type DIN 1.4460 duplex stainless steels in as-received and aged conditions was investigated at room temperature and at 500°C. At room temperature, the aged samples showed embrittlement effects such as loss of toughness and reduction of fatigue life. At 500°C, beneficial effects of the synergy between temperature and strain cycling were observed. It is proposed that at high temperature in the ferrite the strain cycling can decompose the chemical composition fluctuations, promoting a demodulation of the spinodal decomposition formed by aging.

Controlling Morphology of Injection Molded Structural Foams by Mold Design and Processing Parameters

By the invention of foam processing techniques using physical blowing agents, the area of foam injection molding has caught the attention of product developers in industry as well as of researchers. Compared to conventional injection molding, the foaming approach delivers a great potential in the manufacturing as well as in the part itself, offering a monetary benefit, being vantages for example automotive and consumer goods producer (Hyde, L.J., Kishbaugh, L.A. and Katterman, J.A. (2002) [1]. How Microcellular Foam Molding Changes the Cost Structure of Injection Molded Automotive Components: A Review of the Process and Automotive Applications, SAE Technical Paper Series). The pros are facing several cons in the part quality, notably an impaired surface finish, an over-pronounced loss in mechanical strength and toughness or some uncertainties in subsequent processing steps like varnishing and plastic welding. Actual researchers are approaching those challenges in different ways (Dassow, J. (2003) [2]. Foamed Parts with Excellent Surface Quality, Kunststoffe-Plast Europe, 93(9): 65). On the one hand, the choice of the appropriate polymeric materials has a big impact on the morphologies and the properties of the obtained foam injection molded parts. The mold as well as the processing parameters have a big effect on the obtainable foam morphologies and thus the final part properties. This study presents some possibilities to control the morphology of structural foams at high-density reductions (>50%) by an intelligent mold and process design. Parameters affecting the morphology of the foamed part like the foaming temperature, the cavity pressure, and the expansion ratio are varied. Structural properties of the foamed part like surface finish, overall density, and skin layer-thickness as well as mechanical properties are examined.

Effect of displacement dose and irradiation temperature on tensile and fracture toughness properties of titanium alloys

At a low dose level of 0.001 dpa the Ti6Al4V (α + β) alloy showed softening and at higher doses an increase in hardening and lack of work hardening after irradiation at 60 °C. The (α + β) alloy seemed to suffer from plastic instability when irradiated to a dose level of 0.3 dpa at 60 °C. At elevated temperatures a substantial amount of hardening was observed in the (α + β) alloy when tested in the irradiated condition. Earlier studies have shown that the fracture toughness behaviour of the irradiated Ti5Al2.5Sn (α) and (α + β) alloys were quite similar at ambient temperatures. At elevated temperatures, fracture toughness of the irradiated (α + β) alloy decreased more than that of the (α) alloy when compared to the results obtained in the unirradiated condition. The large irradiation hardening and loss of fracture toughness in the (α + β) alloy appears to be related to radiation dose level, temperature and radiation-induced precipitation in the (α + β) alloy.

Effects of Nitrogen on Weld Metal Microstructure and Toughness in Submerged Arc Welding

The effects of nitrogen content on weld metal impact toughness in submerged arc welding were investigated and interpreted in terms of microstructural changes and solid solution hardening. The weld metal impact toughness in as-welded condition decreased with increasing nitrogen content from 110 to 200 ppm. The weld metal microstructure changed with increasing nitrogen content; ferrite with second phase increased at the expense of tough acicular ferrite. In addition to microstructural changes, the microhardness of acicular ferrite increased gradually with the nitrogen content. Therefore, the loss of impact toughness can be attributed to a combination of the effects of microstructural changes and solid solution hardening.

Fabrication and properties of Ti 3AlC 2 particulates reinforced copper composites

The ternary carbide Ti3AlC2 was reactively synthesized from the elemental powder mixtures of Ti, Al and activated carbon. Cu/Ti3AlC2 composites were fabricated by hot-pressing method. Their microstructures and properties were investigated. The results demonstrated that Ti3AlC2 is a promising reinforcement for copper and the Cu/Ti3AlC2 composites exhibited lower electrical conductivity and much superior flexural strength to pure copper matrix without remarkable loss of fracture toughness.

Effect of cooling rate on the microstructure and mechanical properties of Nb-microalloyed steels

We describe here the effect of cooling rate on the microstructure and mechanical properties of Nb-microalloyed steels that were processed as structural beams at three different cooling rates. Nb-microalloyed steels exhibited increase in yield strength with increase in cooling rate during processing. However, the increase in the yield strength was not accompanied by loss in toughness. The microstructure at conventional cooling rate, primarily consisted of polygonal ferrite-pearlite microconstituents, while at intermediate cooling rate besides polygonal ferrite and pearlite contained significant fraction of degenerated pearlite and lath-type ferrite. At higher cooling rate, predominantly, lath-type (acicular) or bainitic ferrite was obtained. The precipitation characteristics were similar at the three cooling rates investigated with precipitation occurring at grain boundaries, on dislocations, and in the ferrite matrix. The fine scale (∼8–12 nm) precipitates in the ferrite matrix were MC type of niobium carbides. The microstructural studies suggest that the increase in toughness of Nb-microalloyed steels with increase in cooling rate is related to the change in the microstructure from predominantly ferrite-pearlite to predominantly bainitic ferrite.

Evaluation of properties and microstructure as a function of tempering time at intercritical temperatures in HY-80 steel castings

Prior work on a failed HY-80 Bridge Access Trunk (BAT) casting indicated the cause of failure to be improper processing techniques. As-received HY-80 casting material showed a non-homogenous microstructure with two distinct microconstituents present: undertempered martensite and a layered martensite–ferrite structure. Heat treatment temperatures from in the intercritical range (721–799 °C) produced microstructures that differ from the desired uniform microstructure of tempered martensite and were similar to those found in the failed casting. In order to further examine the relationship between processing and microstructure, it was decided to vary the time for which the steel was held in the intercritical temperature range. This additional work was warranted by the medium intercritical heat treatment results in the previous study [Holthaus JE, Koul MG, Moran AL. Property and microstructure evaluation as a function of processing parameters: large hy-80 steel casting for a US navy submarine. Eng Fail Anal 2006;13(1):1397–409] and its similarity to the failed casting microstructures. An important finding of this study is that, contrary to normal behavior during tempering, HY-80 steel tempered in the intercritical range demonstrates a severe loss of toughness; which can be exaggerated for longer hold times and higher temperatures. To confirm the hypothesis that the presence of brittle martensite formed by improper heat treatment was the cause of failure, SEM section fractography was employed to directly examine the microstructure underlying the fracture surface and to identify a correlation between microstructure and fracture mode.

Anisotropy of age-related toughness loss in human cortical bone: A finite element study

A mechanistic understanding of the role of bone quality on fracture processes is essential for determining the underlying causes of age-related changes in the mechanical response of the human bone. In this study, a previously developed cohesive finite element model was used to investigate the effects of age-related changes and the orientation of crack growth on the toughening behavior of human cortical bone. The change in the anisotropy of toughening mechanisms with age was also studied. Finite element method (FEM) simulations showed that the initiation toughness decreased by 3% and 8%/decade for transverse and longitudinal crack growth, respectively. In contrast, fracture resistance curve slope for transverse and longitudinal crack growth decreased by 2% and 3%/decade, respectively. Initiation fracture toughness values were higher for the transverse than for the longitudinal for a given age. On the other hand, propagation fracture toughness values were higher for longitudinal than for transverse crack growth for a given age. With respect to age, the toughness ratio for crack initiation decreased by 6%/decade, but that for propagation showed almost no change (less than 1%). In light of these findings, an analytical model evaluating the crack arresting feature of cement lines, is proposed to explain the factors that determine crack penetration into osteons or its deflection by cement lines.

High temperature corrosion of structural materials under gas‐cooled reactor helium

AbstractThe Generation IV International Forum has selected six promising nuclear power systems for further collaborative investigations and development. Among these six concepts, two candidates are Gas Cooled Reactors (GCR), namely the Very High Temperature Reactor (VHTR) and the Gas‐cooled Fast Reactor (GFR). The CEA has launched a R&D program on the metallic materials for application in an innovative GCR. Structural GCR alloys have been extensively studied in the past three decades. Some critical aspects for the steels and nickel base alloys resistance under the service conditions are microstructural stability, creep strength and compatibility with the cooling gas. The coolant, namely helium, proved to contain impurities mainly H2, CO, CH4, N2 and steam in the microbar range that interact with metals at high temperature. Surface scale formation, bulk carburisation and/or decarburisation can occur, depending on the atmosphere characteristics, primarily the effective oxygen partial pressure and carbon activity, on the temperature and on the alloys chemical composition. These structural transformations can notably influence the mechanical properties: carburisation may induce a loss in toughness and ductility whereas decarburisation impedes the creep strength.There is a valuable theoretical as well as practical knowledge on the corrosion of high temperature alloys in the primary circuit of a GCR but this past experience is not sufficient to qualify every component in a future reactor. On the one hand, the material environment could be significantly different from the former GCR's, especially regarding the higher temperature. On the other hand, the materials of interest are partly different. Ni‐Cr‐W alloys, for instance, may offer significant improvement in the maximum operating temperature as far as the mechanical properties are concerned. However, their corrosion resistance toward the GCR atmosphere is still unknown.We describe here our first corrosion tests of Haynes 230, a high strength Ni base alloy containing tungsten, under low oxidising helium at 750 °C or 950 °C. The experiments were carried out in a purposely‐designed device that allows controlling low impurity partial pressure. The test duration was up to 1000 h. The corrosion behaviour was assessed through gravimetry and microscopy. Some specimens were investigated using EDS and DRX. The results under low oxidizing potential are compared to the oxidation kinetics in air. Tentative interpretations are proposed based on published models.

An Understanding of HSLA-65 Plate Steels

HSLA-65 plate steels can be produced using one of five plate manufacturing techniques: normalizing, controlled rolling (CR), controlled rolling followed by accelerated cooling (CR-AC), direct quenching and tempering (DQT), or conventional quenching and tempering (QT (b) eliminate or substantially reduce the need for preheating during welding; (c) resist susceptibility to hydrogen-assisted cracking (HAC) in the weld heat affected zone (HAZ) when fusion (arc) welded using low heat-input conditions; and (d) depending on section thickness, facilitate high heat-input welding (about 2 kJ/mm) without significant loss of strength or toughness in the HAZ. However, application of this plate manufacturing process and of these controls produces significant differences in the metallurgical structure and range of mechanical properties of the HSLA-65 plate steels both among themselves and versus conventional higher strength steel (HSS) plates. For example, among the HSLA-65 plate steels, those produced by Q&T exhibit minimal variability in mechanical properties, especially in thicker plates. Besides variability in mechanical properties depending on plate thickness, the CR and CR-AC plate steels exhibit a relatively higher yield strength to ultimate tensile strength (YS/UTS) ratio than do DQT and Q&T steels. Such differences in processing and properties of HSLA-65 plate steels could potentially affect the selection and control of various secondary fabrication practices, including arc welding. Consequently, fabricators must exercise extreme caution when transferring allowable limits of certified secondary fabrication practices from one type of HSLA-65 plate steel to another, even for the same plate thickness.

Accelerated cyclic oxidation testing protocols for thermal barrier coatings and alumina‐forming alloys and coatings

AbstractThe oxide spallation resistance of oxide scales and ceramic thermal barrier coatings is a key design factor for developing high‐temperature alloy systems.Determination of the lifetimes of such alloy and coating systems is highly desirable. However, as improved systems are developed, lifetimes become so long that the time required to test a system to failure becomes prohibitive. Therefore, reliable protocols for accelerated testing and lifetime prediction are needed. This paper describes two attempts at developing such protocols. The first involves modification of the NASA COSP model to predict cyclic oxidation behavior of alloys and metallic coatings and the incorporation of acoustic emission data into this model. The second involves use of an indentation technique to induce spalling of thermal barrier coatings (TBCs) after short‐term thermal exposures.The first effort involves using the COSP Model, developed at NASA, as the basis for the prediction of oxide spallation. Acoustic emission measurements are used in an attempt to obtain critical parameters in the model from short‐time experiments for a variety of alloys and coatings which rely on alumina scales for oxidation resistance. The model is then used to predict the lives of these alloys and coatings when subjected to cyclic oxidation at 1100°C.A principal concern with ceramic thermal barrier coatings (TBCs) used in gas turbines is their loss of adhesion during service, leading to coating spallation. In this paper, an overview is given of an indentation test for brittle coatings on ductile substrates which is used to quantify decreases in interfacial toughness of TBC systems due to cyclic high‐temperature exposures. The indentation test involves penetration of the TBC and the oxide layer below it, inducing plastic straining in the underlying metal bond coat and superalloy substrate. The indentation strains cause an axisymmetric delamination of the TBC and oxide layers. Measurement of the extent of the delamination, coupled with finite‐element modeling, provides a measure of the adherence of the coating. Test results are presented tracking the loss of interfacial toughness for EBPVD TBC systems cyclically exposed at 1100°C.

Cohesive finite element modeling of age-related toughness loss in human cortical bone

Although the age-related loss of bone quality has been implicated in bone fragility, a mechanistic understanding of the relationship is necessary for developing diagnostic and treatment modalities in the elderly population at risk of fracture. In this study, a finite element based cohesive zone model is developed and applied to human cortical bone in order to capture the experimentally shown rising crack growth behavior and age-related loss of bone toughness. The cohesive model developed here is based on a traction–crack opening displacement relationship representing the fracture processes in the vicinity of a propagating crack. The traction–displacement curve, defining the cohesive model, is composed of ascending and descending branches that incorporate material softening and nonlinearity. The results obtained indicate that, in contrast to initiation toughness, the finite element simulations of crack growth in compact tension (CT) specimens successfully capture the rising R-curve (propagation toughness) behavior and the age-related loss of bone toughness. In close correspondence with the experimentally observed decrease of 14–15% per decade, the finite element simulation results show a decrease of 13% in the R-curve slope per decade. The success of the simulations is a result of the ability of cohesive models to capture and predict the parameters related to bone fracture by representing the physical processes occurring in the vicinity of a propagating crack. These results illustrate that fracture mechanisms in the process zone control bone toughness and any modification to these would cause age-related toughness loss.

EELS study of niobium carbo-nitride nano-precipitates in ferrite

Micro-alloying steels allow higher strength to be achieved, with lower carbon contents, without a loss in toughness, weldability or formability through the generation of a fine ferrite grain size with additional strengthening being provided by the fine scale precipitation of complex carbo-nitride particles. Niobium is reported to be the most efficient micro-alloying element to achieve refinement of the final grain structure. A detailed microscopic investigation is one of the keys for understanding the first stages of the precipitation sequence, thus transmission electron microscopy (TEM) is required. Model Fe–(Nb,C) and Fe–(Nb,C,N) ferritic alloys have been studied after annealing under isothermal conditions. However the nanometre scale dimensions of the particles makes their detection, structural and chemical characterization delicate. Various imaging techniques have then been employed. Conventional TEM (CTEM) and high resolution TEM (HRTEM) were used to characterise the morphology, nature and repartition of precipitates. Volume fractions and a statistical approach to particle size distributions of precipitates have been investigated by energy filtered TEM (EFTEM) and high angle annular dark field (HAADF) imaging. Great attention was paid to the chemical analysis of precipitates; their composition has been quantified by electron energy loss spectroscopy (EELS), on the basis of calibrated ‘jump-ratios’ of C–K and N–K edges over the Nb–M edge, using standards of well-defined compositions. It is shown that a significant addition of nitrogen in the alloy leads to a complex precipitation sequence, with the co-existence of two populations of particles: pure nitrides and homogeneous carbo-nitrides respectively.

Investigation Of Aging Heat Treatment OnMicrostructure And Mechanical Properties Of316L Austenitic Stainless Steel Weld Metal

Austenitic stainless steel alloys are used extensively in heat resistant structural components in power generating and chemical industries. Austenitic stainless steel weld metal exhibits a duplex microstructure consisting of a dispersion of high temperature delta-ferrite in austenite. It has been shown that aging between 500–700°C results in progressive dissolution of the delta-ferrite and precipitation of M23 C6 carbides and intermetallic phases. In this paper, the effects of aging at temperatures between 550–850°C for periods of up to 100 hours on the microstructure and mechanical properties of 316L austenitic stainless steel weld metal have been investigated. Austenitic stainless steel 316L plates of 20 mm thickness were welded using the gas tungsten arc welding technique. Tensile and impact test specimens were machined from the weld sections and subjected to various aging heat treatments. Based on the results of mechnical testing it was found that whereas the strength properties show comparatively small variations due to aging, the tensile ductility and impact toughness are strongly affected by increasing aging temperature and time, such that aging at 850°C for 5 hour results in a 90% reduction in the value of impact energy of the weld metal. The loss of impact toughness and tensile ductility has been discussed based on the dissolution of delta-ferrite and precipitation of the intermetallic sigma phase.