Charpy V-notch Impact Tests Research Articles

The effect of flame straightening on the microstructure and mechanical properties of different strength steels

In many cases, flame straightening is unavoidable after welding for the reduction of deformation. Due to the not very concentrated heat source, the process can cause significant changes in the microstructure, especially in high strength and wear-resistant steels. Due to their different physical properties, the effects vary depending on the flammable gases (acetylene, propane). The situation is complicated by the fact that the manual technology carries a risk of overheating, which can have detrimental effects on the mechanical properties. During our experiments, three steels are investigated (S355J2 + N, XAR400, S960QL). The thermal cycles for the physical simulations were determined by thermocouple measurement during real experimental conditions. Three peak temperatures (1000 °C, 800 °C and 675 °C) and two types of industrial cooling conditions (air and water cooling) were studied. The samples were examined by optical microscopy tests, hardness testing and Charpy V-notch impact tests. During straightening the XAR400 showed high sensitivity to softening even in the lower temperature range, while hardening occurred in the S960QL steel at a higher peak temperature values during water cooling. The inter- and supercritical temperature should be avoided in all steels; however, the subcritical temperature can be beneficial to the toughness properties of the S960QL and XAR400.

Fracture Characterization of Base Metal, Seam Weld, and Girth Weld of Welded Line Pipe Steel at Room and Low Temperatures

For fitness-for-service assessment of pipeline cracks, it is necessary to examine toughness values of pipeline steels and their variations with temperature. In this study, experimental values of toughness (KJC) at room temperature have been obtained for base metal (BM), seam weld (SW), and girth weld (GW) in an API X65 gas transportation pipeline in order to investigate the structural integrity of whole structure. The toughness tests employed side-grooved compact tension specimens, obtained from the original pipe, to characterize the resistance to crack extension curves based on the unloading compliance technique. The values of JIC for tested specimens were found, and then, KJC values are obtained using ASTM E1820 guideline as 302, 262, and 166 MPam1/2 for BM, SW, and GW materials, respectively. Charpy V-notch (CVN) impact tests at room and low temperatures were also conducted to determine low-temperature effects on the toughness, and to examine ductile–brittle transition temperature values in the samples under study. The capability of CVN test for prediction of toughness in the tested specimens was also investigated. The error values in using CVN test results are large enough to justify that the CVN test is not suitable for prediction of toughness accurately.

Impact Toughness Characteristics of SM570-TMC Steel Joint Using Welding Wire Containing 0.4% Nickel at Different Level of Heat Input

The study was conducted to evaluate the impact toughness of flux-cored arc welded of SM570-TMC steel joint under different heat inputs, 0.9 kJ/mm (low heat input) and 1.6 kJ/mm (high heat input). Welding wire containing 0.4%Ni was selected on this experiment. Multi-pass welds were performed on SM570-TMC steel plate of 16 mm in thickness with a single V-groove butt joint on flat position (1G). The evaluation consists of observations on microstructure using an optical microscope and SEM-EDS, and mechanical properties including tensile, microhardness Vickers and Charpy V-notch (CVN) impact test at temperatures of 25, 0 and-20 °C. Results showed that the impact toughness of the base metal (BM) was higher than the weld metal (WM) at all test temperatures. Hardness and impact toughness of WM at low heat input was observed higher than when applied a high heat input. The welded samples at low and high heat inputs had high of tensile strength, and the fracture seemly occurs on the BM. Microstructure observation showed that at a high heat input, larger grains and microsegregation were observed. It might affect on decreasing their impact property.

Fatigue Behavior of Welded API 5L X70 Steel Used in Pipelines

In this study, fatigue failure behavior of welded X70 pipeline steel was investigated by rotating bar bending fatigue tests performed at room temperature. S–N curves of base metal, weld and heat-affected zone (HAZ) were plotted. Tension tests, hardness measurements and Charpy V-notched impact tests were carried out for mechanical characterization. Samples were examined with optical microscope and scanning electron microscope (SEM) microstructurally. Fracture surfaces were examined with SEM. In addition to fractographic analysis, striation counting method was used to estimate the crack growth rate of base metal, weld and HAZ. Stress intensity factor (∆K) ranges were also calculated. Tensile properties and hardness values of base metal and weld were found to be almost the same. Charpy impact test results show that base material has approximately twice impact energy than weld and HAZ at room temperature. However, Charpy impact energies of HAZ and weld decreased noticeably at − 30 °C, while very slight decrease was observed for base metal. The comparison of high-cycle fatigue and crack growth behaviors shows that base metal has better fatigue behavior of all from the point of crack initiation and propagation. Moreover, our findings also support the idea that striation counting can be used as a fractographic approach to estimate the crack growth behavior of materials.

Improvement of low temperature impact toughness through flux modification for submerged arc welded low carbon steel E350 plates

Objectives: To analyse the effects of rutile (TiO2) and Alumina (Al2O3) on impact toughness properties of Submerged arc welded joints for low temperature applications. Methods: Al2O3 and TiO2 have been added to F7AZ/PZ-EL8 Submerged arc welding (SAW) flux in varying quantities and the base material used for joining is low carbon structural steel having E350 grade. To verify the effect of these elements by flux modification, the Charpy V-Notch impact tests, scanning electron microscope (SEM) fractography and visible light microscopy (VLM) have been conducted. Findings: Charpy V-Notch impact testing and analysis have shown that there is considerable improvement in the impact properties of welds with the addition of 20% TiO2 and 20% Al2O3 in the 80% of flux as separate constituents i.e., when these elements have been added in the flux in separate form or two new modified fluxes were prepared, where one is doped with alumina and other is with rutile. Fractography of welds with addition of 20% alumina in 80% F7AZ/PZ-EL8 SAW fluxes are having best impact properties at lower temperature and addition of 20% rutile also as almost effective as alumina in flux. consequently, mode of failure is ductile fracture with dimples as a result of good amount of acicular ferrite microstructure. Novelty: Analysis of previous studies implicated that there is lack of significant studies in the low temperature impact properties of low carbon steel submerged arc welds. Present study shows that the significant improvement in the toughness properties of welds at lower temperature of -40◦C can be achieved through flux modification, it would ensure the application of Low carbon steel structures at lower working temperature. The applications may be pressure vessels, ships or containers. Keywords: Submerged arc welding (SAW); low temperature impact toughness; flux modification; low carbon steels

Superior cryogenic toughness of high-Mn austenitic steel by welding thermal cycles: The role of grain boundary evolution

The high-Mn austenitic steel is expected to improve mechanical properties by grain boundary adjustment. The role of grain boundary on cryogenic toughness of high-Mn austenitic steel subjected to various welding thermal cycle was investigated by the means of instrumented Charpy V-notch impact tests and electron backscatter diffraction (EBSD) analysis. The absorbed energy of samples at −196 °C decreased from 203 J to 163 J with the increase of peak temperature from 600 °C to 900 °C. When the peak temperature increased to 1000 °C, 1100 °C and 1300 °C, the absorbed energy of sample increased to 185 J, 204 J and 196 J, respectively. Phase transformation and abnormal grain growth were absent in the samples. The lowest cryogenic toughness in sample with the peak temperature of 900 °C was thus attributed to the highest proportion of the special grain boundary ∑3. Furthermore, during the cryogenic impact test, the crack formation energy was corresponded to the proportion change of the low angle boundary. While, the crack propagation energy was closely related to the proportion change of special grain boundary of ∑3 rather than the high fraction of ∑3 by welding thermal cycle. It is proposed that the increasing low angle boundary of 2–5° and the activation ∑3 special grain boundary are responsible for the superior cryogenic toughness for high-Mn austenitic steel.

Improvement of strength, toughness and ductility in ultrafine-grained low-carbon steel processed by warm bi-axial rolling

A 0.15%C-0.3%Si-1.5%Mn steel bar with ultrafine elongated grain structures of transverse grain size 1.2 μm was fabricated via multi-pass bi-axial rolling process at a warm working temperature. For comparison, conventionally quenched and tempered 0.29%C steel and 1.03%C steel with a martensitic structure and 0.15% low-carbon steel with a ferrite/pearlite structure were also prepared. The full-size Charpy V-notch impact and tensile tests were conducted at a temperature range of −196 °C to 200 °C, and the relationship between microstructures, yield stress, reduction in area and impact energy was studied. In the developed steel bar, the main orientation in the microstructure changed in the cross-sectional plane, and it was dominated by {001}<100> cube orientation at the center, {111}<110> at the quarter and rolling direction//<110> at the surface. Crack branching started to occur with decreasing temperature in the Charpy test. The fracture surfaces are very complicated at temperatures below −100 °C and the specimen did not separate into two pieces even at a low temperature of −196 °C. The strength–toughness balance of the developed steel was significantly improved compared with conventional steels. This advantage in the developed steel was also seen in the strength–reduction in area balance. As a result, the steel fabricated by warm bi-axial rolling was best balance in correlation between strength and ductility and between strength and toughness at all temperatures.

Design Considerations to Improve Charpy Instrumented Strikers.

Instrumented impact testing allows the applicability of conventional Charpy tests to be extended toward assessing mechanical properties such as dynamic fracture toughness and dynamic tensile properties. In this work, we present design considerations for engineering instrumented strikers for Charpy V-notch impact testing. Specific attention is given to the mechanical and geometric features, as well as the placement of strain gauges and corresponding bridge circuits for instrumentation. These design considerations are intended to make the sensitivity invariant to the location and distribution of impact forces. The concepts presented in this work were applied to an actual instrumented striker, which was then statically calibrated. Data from this calibration indicate that the device has good repeatability, shows a linear response, and is relatively insensitive to impact location.

Enhancement of Impact Toughness in Mild Steel Weldments under Low Temperature Applications

In this research, mild steel weldments are tested by varying nickel content into the weld beads. Three mild steel plates have been joined using three electrodes with similar chemical composition except nickel content 0%, 9-11% and 19-21% respectively, by SMAW process and keeping heat input constant at all. The performance of welded specimens was evaluated by Charpy V-notch impact tests under different temperature conditions (25°C, 0°C, -20°C, -40°C, -60°C). It was found that the weldment with 0% nickel content is suffering rapid transition from ductile to brittle at-60°C, thus toughness reduces to approximately 1/5th of its value at room temperature. Microstructure revealed that at inter-dendritic regions mainly martensite was found. In dendrite core regions of the low carbon weld metals, a mixture of upper bainite, lower bainite and a novel constituent coalesced bainite formed. The fractured surface pattern was also observed using SEM, to reveal the ratio of area underwent ductile or brittle type of failure.

Effect of PTFE coating on enhancing hydrogen embrittlement resistance of stainless steel 304 for liquefied hydrogen storage system application

The phenomenon of hydrogen embrittlement phenomenon is known to be a major obstacle to proposed to overcome this phenomenon. In the present study, polytetrafluoroethylene (PTFE), which is known to be an effective hydrogen adsorption and desorption material, was coated on the surface of stainless steel 304 to improve its hydrogen embrittlement resistance. To make a hydrogen embrittlement environment, electrochemical hydrogen pre-charging was applied to the PTFE-coated stainless steel 304. To investigate the effects of PTFE coating on the hydrogen embrittlement resistance of stainless steel 304, the Charpy V-notch impact (CVN) test was performed under three different temperatures: 25, −83, and −196 °C. Additionally, hydrogen concentration, electron back scatter diffraction (EBSD), and scanning electron microscopy (SEM) evaluations were carried out to verify the results of the CVN impact test. The PTFE coating did not have a significant effect on the quantitative reduction of hydrogen concentration; however, we confirmed its excellent performance in terms of toughness reduction due to the increase in hydrogen loading time at room temperature.

Effect of martensite–austenite constituents on impact toughness of pre-tempered MnNiMo bainitic steel

The martensite-austenite constituents are known to deteriorate the impact toughness of alloyed steel. In order to reduce this adverse effect, the pre-tempering process at low temperatures was introduced before the tempering of intercritical heat treatment process of a MnNiMo bainitic steel. The effects of pre-tempering temperature on the decomposition of martensite-austenite constituents and impact toughness properties of the pre-tempered specimens were investigated. The results of instrumented Charpy V-notch impact testing show that the impact toughness of the materials can be significantly improved by the pre-tempering process, especially for the condition of pre-tempering at 400 °C. After pre-tempering at 400 °C, more martensite-austenite constituents are decomposed into ferrite and small carbides, resulting in the reduction of microcracks nucleation sites. The residual martensite-austenite constituents become mainly the island-like, which can effectively hinder the propagation of secondary cracks. In addition, the pre-tempering at 400 °C increases the fraction of high-angle grain boundaries and refines the size of blocks, which can effectively prevent or deflect the propagation of microcracks. All of those microstructure changes are beneficial to the crack propagation energy. The results suggest that the application of the pre-tempering process is a feasible method to enhance the impact toughness of the studied alloyed steel.

The determination of the weakest zone and the effects of the weakest zone on the impact toughness of the 12Cr2Mo1R welded joint

For the actual welded joints, determination of the weakest zone in the welded and the effects of the weakest zone is very important in characterization of the welded joints. In this paper, microstructure feature and charpy toughness of the 12Cr2Mo1R for pressure vessels steel welded joints were systematically investigated. The standard Charpy V-notch impact test specimens were prepared on the base metal, weld metal and various zones of the heat affected zone. The weakest zone of the 12Cr2Mo1R welded joint and its effects on the impact toughness of the welded joint were studied. It shows that at −30 °C, the weakest zone of the welded joint is the coarse-grained heat affected zone (CGHAZ), which impact toughness is 65 J, and its microstructure is composed of a large number of coarse granular bainite and a few blocks ferrite. With the fraction of CGHAZ on the ligament at the notch tip increases, the impact toughness decreases. The closer the CGHAZ location from the notch is, the less the impact toughness of the sample is. Above two aspects fully reflect the effect of the weakest zone on the toughness of the joint. This work reveals the effect of fraction of weakest zone in front of the notch tip on the impact toughness of the 12Cr2Mo1R welded joints.

Modification of mechanical properties of submerged arc welds by adding Mn2O3– or TiO2–NPs directly to the beveled surface of low-carbon steels

This work was aimed to analyze the changes in the mechanical properties of submerged arc welds due to the growth of the acicular ferrite (AF) phase promoted by addition of Mn2O3– or TiO2-nanoparticles (Mn2O3– or TiO2-NPs). Such Mn2O3– or TiO2-NPs were uniformly deposited along the V-groove, formed from joining two beveled surfaces of low-carbon steel plates to be welding. Mechanical properties of the weld beads with and without NPs were determined through Tensile, Vickers microhardness and Charpy V-Notch (CVN) impact tests; meanwhile their microstructural features changes were followed by Optical and Scanning Electron Microscopies (OM and SEM). Tensile Strength, Yield Strength and Vickers microhardness values were lower for weld beads obtained with addition of Mn2O3– or TiO2-NPs compared to those obtained without NPs additions. Namely, 13.5, 17.5 and 11.4%, respectively, lower by adding Mn2O3–NPs, and 15.8, 24.6 and 12.4%, respectively, lower by adding TiO2-NPs. In contrast, the CVN impact energy values were determined to be about 60 or 30% higher by adding Mn2O3– or TiO2-NPs, respectively. OM and SEM micrographs of weld beads with Mn2O3– or TiO2-NPs show a clear increase in the surface density and length of the AF needles, in comparison to the weld bead without NPs. Namely, it was determined that in average, the length of the AF needles increases 131 and 155% by adding Mn2O3– or TiO2-NPs, respectively. Moreover, the increase in the CVN impact energy and the surface density of AF needles can be correlated with the decrease in the average dimple diameter and with the cleavage sheets width, respectively. Thus, it can be pointed out that the addition of Mn2O3- or TiO2-NPs has a direct influence in the size of the AF needles, which caused the improvement of the corresponding weld bead toughness.

Heat Treatment, Impact Properties, and Fracture Behaviour of Ti-6Al-4V Alloy Produced by Powder Compact Extrusion.

The mechanical properties of titanium and titanium alloys are very sensitive to processing, microstructure, and impurity levels. In this paper, a blended powder mixture of Ti-6Al-4V alloy was consolidated by powder compact extrusion that involved warm compaction, vacuum sintering, and hot extrusion. The as-processed material with an oxygen content of 0.34 wt.% was subjected to various annealing treatments. The impact toughness of heat-treated material was determined using Charpy V-notch impact testing at room temperature. An emphasis was placed on establishing a relationship among fracture behaviour, microstructure, and the resulting properties of tested material. From the results, it is apparent that the highest impact toughness value of 19.3 J was achieved after α/β annealing and is comparable with typical values given in the literature for wrought Ti-6Al-4V. In terms of fracture behaviour, it is quite apparent that the crack propagation behaviour of powder-produced material is rather complex compared with the limited amount of data reported for ingot counterparts.

Effect of Welding Heat Input on Simulated HAZ Areas in S960QL High Strength Steel

When the weldability of high strength steels is analyzed, it is the softening in the heat-affected zone (HAZ) that is mostly investigated, and the reduction of toughness properties is generally less considered. The outstanding toughness properties of quenched and tempered high strength steels cannot be adequately preserved during the welding due to the unfavorable microstructural changes in the HAZ. Relevant technological variants (t8/5 = 2.5–100 s) for arc welding technologies were applied during the HAZ simulation of S960QL steel (EN 10025-6) in a Gleeble 3500 physical simulator, and the effect of cooling time on the critical HAZ areas of single and multipass welded joints was analyzed. Thermal cycles were determined according to the Rykalin 3D model. The properties of the selected coarse-grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse-grained (ICCGHAZ) zones were investigated by scanning electron microscope, macro and micro hardness tests and instrumented Charpy V-notch pendulum impact tests. The examined HAZ subzones indicated higher sensitivity to the welding heat input compared to conventional structural steels. Due to the observed brittle behavior of all subzones in the whole t8/5 range, the possible lowest welding heat input should be applied in order to minimize the volume of HAZ that does not put fulfillment of the allowed maximal (450 HV10) hardness at risk and does not lead to the formation of cold cracks.

Microstructural assessment of AISI 1021 steel under rapid cyclic heat treatment process

Abstract This research was aimed at using image analysis to describe the effects of rapid cyclic heating on mild steel. AISI 1021 steel sample used in this study was subjected to quenching heat treatment followed by 1, 2, 3, and 4-cycles of rapid heat treatment. The as-treated steel samples were characterized by Brinell hardness tests, Charpy V-notch impact tests, optical microscopy, and image analysis using Image J software. The results revealed that the grain size decreased from 1.07 μm in the control sample to 0.79 μm in the third cycle sample and increased to 0.86 μm in the fourth cycle sample. However, the results revealed that two-cycles of rapid heat treatment was enough to produce ultra-fine grains and impact ductility in mild carbon steel.

Charpy impact toughness and transition temperature in ferrite–perlite steel

Las pruebas de impacto son una importante herramienta para la obtención de importantes propiedades, como la tenacidad al impacto y la temperatura de transición ductil-fragil, para la selección de un material en diseño mecánico. Se evaluó la tenacidad al impacto mediante pruebas de impacto Charpy en acero de fase doble (ferrita-perlita) de 10 mm de espesor a temperaturas que oscilan entre 90 ºC y -60 ºC. Las superficies de fractura fueron analizadas por microscopio óptico (MO). Debido a la alta dispersión, mostrada en los resultados, se emplaron numerosas pruebas en las mismas condiciones y el uso de métodos estadísticos para obtener un valor confiable. La temperatura de transición calculada (45 ºC) y la resistencia al impacto de Charpy se analizaron en función de la microestructura y el equivalente de carbono

Effect of Si content on the microstructures and the impact properties in the coarse-grained heat-affected zone (CGHAZ) of typical weathering steel

This study aims to make clear the content limit of silicon (Si), which normally favors weathering steel but probably impairs its impact properties in the coarse-grained heat-affected zone (CGHAZ) induced by the conventional arc welding. Thermal simulation with a Gleeble 3500 simulator was thus carried out to reproduce the CGHAZ of this type of steel with differing silicon (Si) content from 0.12% to 0.48%–0.75%. Charpy V-notch impact tests and microstructure observations using optical microscopy, transmission and scanning electron microscopy (TEM and SEM), and electron backscatter diffraction (EBSD) were performed. The results showed that a microstructure composed of granular bainite (GB), acicular ferrite (AF), lath bainite (LB), and martensite/austenite (M/A) constituents formed in 0.12%Si and 0.48%Si samples, while the LB scarcely appeared in 0.75%Si sample. When the Si content increased from 0.12% to 0.48%–0.75%, the LB decreased and in contrast, the GB increased. In addition, the area fraction (average size) of the M/A constituents increased from 5.8% (0.70 μm) to 9.2% (1.32 μm), while the number fraction (mean size) of effective grains with high-angle grain boundaries (HAGBs) decreased (increased) from 27% (3.77 μm) to 10% (5.71 μm). Accordingly, the Charpy impact energy of the CGHAZ decreased sharply from 208 to 42 J. The increased large M/A constituents significantly enable the microcrack nucleation, while the reduced HAGBs inefficiently impede the microcrack propagation, finally leading to a deteriorated toughness in the CGHAZ of 0.75%Si steel. The silicon content in this steel should not exceed 0.48%.

Dynamic Behaviour of a High-Strength Structural Steel at Low Temperatures

The main objective of this experimental study is to determine the effect of low temperatures on the mechanical behaviour of Strenx 960 Plus high-strength structural steel at different strain rates and stress triaxialities. For this purpose, a comprehensive experimental campaign was designed to characterise the material at a wide range of temperatures and loading rates. The stress triaxiality was varied by testing specimens with different geometry. First, to determine the ductile-to-brittle transition temperature, instrumented Charpy V-notch impact tests were carried out at a range of temperatures from + 20 °C down to − 90 °C. The impact energy dropped gradually with decreasing temperature, but a clear transition temperature could not be identified. A fractography study exhibited a clear dimple structure, revealing predominantly ductile fracture at all temperatures. Then, uniaxial tension tests on smooth and pre-notched axisymmetric specimens under both quasi-static and dynamic loading rates were carried out at room temperature and low temperatures. These tests were conducted to characterise the rate-dependence of the stress–strain behaviour and the failure strain. The results revealed that under quasi-static conditions the flow stress increased with decreasing temperature, while the failure strain was nearly independent of the temperature. Dynamic tensile tests using the same specimen geometries were conducted in a split Hopkinson tension bar at + 20 °C and − 40 °C. The material exhibited a positive strain rate sensitivity at all investigated temperatures. This experimental study reveals that the Strenx 960 Plus steel retains its ductility at temperatures as low as − 40 °C. Brittle failure could not be observed even with combined high strain rate, high stress triaxiality and low temperature.

Charpy Impact Properties of Hydrogen-Exposed 316L Stainless Steel at Ambient and Cryogenic Temperatures

316L stainless steel is a promising material candidate for a hydrogen containment system. However, when in contact with hydrogen, the material could be degraded by hydrogen embrittlement (HE). Moreover, the mechanism and the effect of HE on 316L stainless steel have not been clearly studied. This study investigated the effect of hydrogen exposure on the impact toughness of 316L stainless steel to understand the relation between hydrogen charging time and fracture toughness at ambient and cryogenic temperatures. In this study, 316L stainless steel specimens were exposed to hydrogen in different durations. Charpy V-notch (CVN) impact tests were conducted at ambient and low temperatures to study the effect of HE on the impact properties and fracture toughness of 316L stainless steel under the tested temperatures. Hydrogen analysis and scanning electron microscopy (SEM) were conducted to find the effect of charging time on the hydrogen concentration and surface morphology, respectively. The result indicated that exposure to hydrogen decreased the absorbed energy and ductility of 316L stainless steel at all tested temperatures but not much difference was found among the pre-charging times. Another academic insight is that low temperatures diminished the absorbed energy by lowering the ductility of 316L stainless steel.