Charpy V-notch Research Articles

Microstructure and mechanical properties of high strength steel deposits obtained by Wire-Arc Additive Manufacturing

Wire-arc additive manufacturing has become an alternative way to produce industrial parts. In this work 15 kg walls are built with an effective building rate of 4.85 kg/h using an ER100 wire providing good tensile properties and toughness under welding conditions. The thermal evolution of the walls during manufacturing is measured by thermocouples and an IR camera: it depends on process parameters, deposit strategy and the size of the part. The walls are then characterised as deposit and after heat treatment through hardness, tensile and Charpy-V notch tests. The results show a fine microstructure with unexpected retained austenite and coarse allotriomorphic ferrite in the as deposited walls. The final hardness values vary from about 220 to 280 HV2; the yield stress and tensile strength are 520 and 790 MPa, respectively, and a toughness of about 50 J is obtained at room temperature. The heat treatment transforms the retained austenite, leading to an improvement of the yield stress to 600 MPa.

Optimization of the Sintering Parameters for Materials Manufactured by Powder Injection Molding

The properties of sintered and heat-treated steels produced from the Catamold 8740 material by powder injection molding were examined. Parts made of the Catamold 8740 low-alloy mild carbon steel are used in the military, automotive, and other industries where high reliability and resistance to dynamic loads are required. The Catamold 8740 chemical homogeneity was studied as a function of powder injection molding parameters: heating rate, sintering temperature, holding time, and subsequent heat treatment. Microscopic analysis showed that Catamold 8740 was a mechanical mixture of powders in various sizes (carbonyl iron, nickel, Fe–Mo, Fe–Cr, Fe–Si-Mo, Fe–Si). When the heating rate of the Catamold 8740 powder mixture increased from 2 to 5 °C/min, the microstructural heterogeneity caused by uneven compaction decreased and the alloy formation process activated at 900–1340°C. The impact strength of Charpy V-notch test samples changed from 10.39 to 11.52 J/cm2 with higher sintering temperature and heating rate increasing from 2 °C/min (1270°C) to 5 °C/min (1340°C). When holding time increased from 30 to 90 min at a sintering temperature of 1340°C, the material became denser, the pores rounded, and the ferrite matrix homogeneous. However, the impact strength decreased by a factor of 1.4 because of coarser grain sizes and brittle fracture. Heat treatment processes reduced the impact strength after sintering. The results obtained were used to optimize the powder injection molding parameters. The influence of chemical and dimensional inhomogeneity of the starting Catamold 8740 powders and sintering kinetics on the structure and mechanical properties of parts operating under impact loads was established.

Neutron diffraction bulk texture study with impact property correlation of electron beam welded dissimilar Fe-7%Al alloy to steel joints

Present work focused on the crystallographic bulk texture study of dissimilar Fe-7percentage Al alloy to mild steel joints by electron beam welding using time-of-flight neutron diffraction technique. Textural properties directly correlated with V-notch Charpy impact study. Different processing conditions like the use of beam oscillation and by changing welding speed on texture change and its impact on the performance of the joint was evaluated. Neutron diffraction was performed using general material diffractometer beamline at ISIS neutron radiation source. Texture analysis and their variation obtained in terms of pole-figures, inverse pole figures and orientation distribution functions (ODFs). Beam oscillation shows a uniform and ordered textured grains in both fusion zone (FZ) and heat-affected zone (HAZ) due to more heat mixing that reduced unidirectional solidification gradient. ODF study reveals more gamma fibre components in both FZ and HAZ compared to its non-oscillating counterparts and higher welding speed joints, which predominantly shows more Goss, rotated Goss and cube/alpha fibre components. Charpy impact strength shows approximately 60% improvement in value for the joints, which was prepared by using beam oscillation whereas 20% reduction for higher welding speed condition. Also, X-ray tomography study of Charpy broke sample showed very lesser and shallow cracks and in smaller volume percentage of 0.45 compared to joints produced by the non-oscillating beam showing maximum value of 2.99%.

Sub-sized and miniature CVN specimen conversion methodology

Presently most structural integrity assessment procedures still allow the use of Charpy-V notch impact tests as a measure of fracture toughness. The use is generally made through some more or less reliable correlations between standard Charpy-V notch energy and fracture toughness or tearing resistance. A problem arises if the structure has such a geometry that standard size Charpy-V notch specimens cannot be used. Application standards contain some guides on how to convert sub-sized specimen data to correspond to full size specimens, but these are often inaccurate and limited in their application range. Procedures like ASTM A370, BS7910 and API 579 give some advice on the use of sub-sized Charpy-V specimens but none of them cover the whole Charpy-V transition curve, nor all possible specimen dimensions. Here, a new simple procedure is presented. It is shown that it is applicable over the whole transition curve, thus enabling a point-wise conversion of sub-sized and miniature Charpy-V data to correspond to full size specimens. The new procedure is applicable for steel strengths ranging from 200 MPa to 1400 MPa.

Testing of Welded Joints between Pipes for Pressure Vessels Made of Hot Rolled Fine-Grained Steel P460NL1

This paper contains results of tests performed in order to determine mechanical properties of steel P460NL1, used as filler material during the execution of welded joints. Arc welding of samples from which the specimens were taken was carried out through the application of welding process 111, because it is one of the processes for the execution of pipelines for pressure equipment. Microspecimens with diameter of Ø1,5 mm were tested in order to determine tensile properties of material taken from the heat-affected zone and weld metal, while specimens with diameter of Ø6 mm were tested in order to determine tensile properties of parent material. Standard Charpy V-notch specimens were used in order to determine impact energy. Results of metallographic tests which refer to the structure of a pipe welded joint are also presented.

Additive Manufacturing with Superduplex Stainless Steel Wire by CMT Process

For many years, the oil and gas industry has utilized superduplex stainless steels due to their high strength and excellent corrosion resistance. Wire arc additive manufacturing (WAAM) was used with superduplex filler wire to create walls with different heat input. Due to the multiple heating and cooling cycles during layer deposition, brittle secondary phases may form such as intermetallic sigma (σ) phase. By inspecting deposited walls within wide range of heat inputs (0.40–0.87 kJ/mm), no intermetallic phases formed due to low inter-pass temperatures used, together with the high Ni content in the applied wire. Lower mechanical properties were observed with high heat inputs due to low ferrite volume fraction, precipitation of Cr nitrides and formation of secondary austenite. The walls showed good toughness values based on both Charpy V-notch and CTOD (crack tip opening displacement) testing.

Numerical modelling of dynamic ductile fracture propagation in different lab-scale experiments using GTN damage model

Initiation and propagation of ductile fractures are a major consideration during the design of high-pressure pipelines. Consequences of a pipeline failure can be catastrophic thus structural integrity must be ensured over several decades. Traditional lab-scale experiments such as the Charpy V-Notch (CVN) and Drop Weight Tear Test (DWTT), impact experiments on a notched three-point bending sample, are widely used to measure the fracture toughness of a material. However, with increasing wall thickness and the transition to high-grade steels in the pipeline industry, the size-effect of the specimen and inverse fracture became prominent issues. A new testing methodology called the Dynamic Tensile Tear Test (DT3) is currently investigated as to address the issues presented by the current state of the art. In this study, a numerical investigation is conducted on the CVN, DWTT and DT3 experiments to compare the modelling of dynamic ductile fracture propagation in three different testing scales using the Gurson-Tvergaard-Needleman (GTN) damage model. X70 and X100 pipeline steel grades are used to model material behaviour. For each considered lab-scale experiment, the dynamic ductile fracture behaviour was successfully reproduced using the GTN damage model.

On the effect of long service exposure: changes in microstructure and mechanical properties of Ni-based superalloy

In the present study, the microstructure and the mechanical properties of GTD-111 nickel-based superalloy were investigated. The alloy was in service as the GE-MS9001 gas turbine 1st stage rotating blade for 105,000 hours at a temperature between 950 to 1000 °C. Two sets of samples were extracted from the airfoil and the root of the blade. Then, they were compared for the microstructural and the mechanical properties changes after the high temperature service. Stress-rupture and Charpy V-notch (CVN) tests were conducted on the samples at 871 °C and two temperatures of 25 and 900 °C, respectively. The microstructure and the fracture surface of the samples were analyzed using a scanning electron microscope (SEM). The results showed degradation in the microstructure and the mechanical properties of the airfoil compared to those of the root due to the long-term service at elevated temperatures. The loss in mechanical properties was due to the coarsening ofγ’ precipitates and the formation of brittle phases in the grain boundaries.

Capturing Variability in the Fracture Response of TC128B Steel using Damage Mechanics

A deformation and fracture model was developed for TC128B steel that is valid for temperatures between -80 and 24 °C and for various strain-rates. The Modified-Mohr-Coulomb (MMC) damage mechanics model was used in which the failure strain is dependant on both stress triaxiality and Lode angle. Several mechanical tests, finite element simulations and optimization methods are required to calibrate damage model parameters accurately. Calibration of damage models is often performed at only a single temperature, strain-rate and for a specific grade of steel. However, there are variations in the fracture response due to temperature, strain-rate, and nominal differences in the same grade of TC128B. The current work utilized a recent comprehensive damage model developed by Paredes et al. (2018) for a single grade of TC128 at room temperature as a starting point for model calibration. Damage model parameters were determined based on only a limited number of mechanical test data, mainly tensile and Charpy V-notch tests. It was found that the model could accurately capture the ductile fracture response of the material for a variety of temperatures and strain-rates. Distinct consideration was given to accurately capturing the brittle fracture response at low temperatures.

On the Fracture Behavior of a Creep Resistant 10% Cr Steel with High Boron and Low Nitrogen Contents at Low Temperatures.

An advanced, high chromium, creep-resistant steel was subjected to the tensile tests and three-point bending tests of Charpy V-notch specimens at temperatures of −196 to 20 °C. The steel exhibited ductile fracture under tension tests at all of the temperatures studied. The mechanical properties, i.e., strength and uniform elongation, were enhanced with a decrease in temperature down to −140 °C. Transgranular, dimpled fracture remained the primary fracture mechanism under tension. On the other hand, the results obtained with Charpy V-notch specimens suggested the ductile–brittle transition (DBT). Full embrittlement was observed at temperatures of −60 °C and −150 °C upon impact tests and three-point bending tests, respectively, when the unstable crack started to propagate without remarkable plastic deformation. The DBT temperature of −27 °C for the present steel corresponded to the 28 J impact transition temperature, T28J, when the maximum impact stress matched the maximal true tensile stress.

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.

Influence of Chemical Composition on the Mechanical and Microstructural Properties of High Strength Steel Weld Metals Submitted to PWHT

The development of new filler metals for offshore industry is challenging because high strength and toughness levels are required after post-welding heat treatment (PWHT) is applied to reduce residual stresses. The higher chromium contents added to the chemical composition of welding consumables to attain the required ultimate tensile strength are usually associated with reduced toughness. This work presents a comparative analysis of the behavior of two high strength steel weld metals with different Cr–Mo and Mn–Ni additions that were submitted to PWHT. Weld metals obtained by shielded metal arc welding process were evaluated using mechanical tests (Charpy-V notch and Vickers microhardness) and microstructural analysis (scanning electron microscopy and electron backscattered diffraction) conducted in samples removed from the center line of weld deposits, both in the as-welded and heat-treated (600 °C for 1 h) conditions. Although similar results were obtained in the as-welded condition, different behaviors were observed after PWHT. While impact toughness for Cr–Mo additions was reduced by carbide precipitation, as predicted by thermo-calc simulation, weld metal based on Ni–Mn addition exhibited improvements after PWHT because the predominantly martensite microstructure was tempered. This makes the latter more appropriate for weldments submitted to PWHT.

Experimental investigation of fatigue crack initiation from notch for 2024 T351 Al-alloy

In this investigation, fatigue criterion was established to predict crack-initiation at the tip of a notch in aged hardening aluminum alloy. This criterion was used to predict the residual lifetime in aeronautical structures using concept of local stress at notch and subjected to constant amplitude loading characterized by mean stress. Charpy V-notch specimens were taken from sheet plate and the notch radius accurately machined at value of 0.2mm. The local stresses were determined numerical and validated analytically. In experimental investigation, the V-notched specimens were loaded in four point bending fatigue with a load ratio R=0.1 and variation in amplitude loading.

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.

Mechanical and microstructural behavior of C-Mn steel weld deposits with varying titanium contents

The effect of micro-alloying on the microstructure of C-Mn steel weld metals has been extensively studied in the past and the effect of titanium, in particular, has motivated several investigations, due to its essential contribution to the occurrence of acicular ferrite. However, there are cases where a substantial amount of acicular ferrite present in the last pass is not directly related to higher impact energy, since other parameters such as the proportion of the transformed phases by reheating, occurrence of micro-phases and non-metallic inclusions also influence the toughness of weld metal. Based on this scenario, the current work complements previous investigation and proposes an alternative methodology for evaluation of the behaviour of microstructure and impact toughness of C-Mn steel weld metals containing varying titanium contents, in the range of 6 and 255 ppm, based on comparison with the microstructural characteristics observed at the Charpy-V notch, where impact toughness was measured.The weld metal microstructure was characterized at the top bead and at the mid-thickness by Optical (OM), Electron Microscopy (SEM) and electron backscattered diffraction (EBSD). Vickers microhardness tests were performed at the same positions where metallographic examination was done. Charpy-V impact tests employing test samples removed from the center line and at mid-thickness of the weld deposits were carried out in order to obtain test temperatures corresponding to impact energy of 100 joules.The results evidenced that the increase of titanium content promoted a substantial amount of acicular ferrite and the refinement of microstructure at the top bead. At the Charpy-V notch location, the same behavior was observed, the difference being relative to the lower amount of acicular ferrite. Optimum impact toughness was attained for 28 ppm Ti, although the metallographic examination revealed that the higher amount of refined acicular ferrite presenting a lower effective grain size and increased frequency of high angle boundaries was observed for 255 ppm Ti. From these findings, a methodology involving OM, SEM and EBSD for an adequate characterization of the microstructure at the Charpy-V notch location is applied, in order to evaluate the influence of reheating, microstructure, effective grain size, frequency of high angle boundaries, presence of MA constituents and inclusions. The analysis revealed that the behavior of impact toughness is associated with the presence of the M-A constituents occurring at the Charpy-V notch for 255 ppm Ti.The obtained results indicate the need for a more stringent procedure for an explanation of the behaviour of the impact properties of C-Mn steel weld metals, due to the role of the thermal cycle build-up of multi-layer weldments.

Improvement of the toughness and ductility of the weld beads by inducing growth of acicular ferrite with TiO2-nanoparticles during submerged arc welding

This work was focused to relate the mechanical properties of weld beads with size and superficial density of the acicular ferrite (AF) needles changing by impregnating two different sizes of TiO2-nanoparticles at the bevel before the SAW process. Microstructure of weld metal with addition of TiO2-nanoparticles with 21 and 127 nm sizes, respectively, shows an increase of the density (61.3 and 75.4%) and length of AF needles formed (156.4 and 68.8%), in comparison to the weld metal without TiO2-nanoparticles, namely the WB-0. Moreover, chemical analyses at weld metal and inclusions of weld beads with TiO2-nanoparticles reveal and increase on their Ti contents respect to those in the WB-0. Tensile and yield strength of the weld bead fabricated with the smallest TiO2-nanoparticles diminished in comparison to other weld beads. Besides, it was found that the average Vickers microhardness of weld metal decreased by adding TiO2-nanoparticles. On the other hand, their Charpy V-notch toughness values were found to be 30.1 and 102.6% bigger respect to those of the WB-0, by adding TiO2-nanoparticles with 21 and 127 nm sizes, respectively. In general, it was found that by adding TiO2-nanoparticles before the SAW process, the density and dimensions of the AF needles increase, which could be associated with the improvement of toughness and ductility of weld beads.

A-TIG welding of dissimilar P92 steel and 304H austenitic stainless steel: Mechanisms, microstructure and mechanical properties

The present study focuses on the dissimilar joining of P92 steel and 304H austenitic stainless steel plates of thickness 8 mm using A-TIG welding with aim to select a proper flux for through thickness penetration. The effect of four different oxide fluxes named: Cr2O3, MoO3, SiO2 and TiO2 on weld bead geometry was studied. A detailed investigation on possible mechanism for increase in depth of penetration has been carried out. Out of four oxide fluxes, TiO2 provided through thickness penetration. Further, the integrity of weld joint developed using TiO2 flux was assessed in light of microscopy, hardness test, tensile test and impact toughness test. Microstructure study of the weld joint showed the formation of untempered martensite, polygonal austenite grains and ferrite stringers in different zones of weld joint. In tensile testing, the specimens were failed from 304H austenitic stainless steel base metal. The tensile properties were found as tensile strength 688.6 MPa, yield strength 562.2 MPa and elongation 37.4%. The V-notch Charpy impact toughness of the weld joint (30 J) was quite lower than the base metals due to the formation of untempered martensitic structure.

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.

Detection of embrittlement in low alloy steels due to thermal aging by small punch test

The applicability of miniaturised small punch tests (SPT) to capture thermal aging degradation of two commonly used low alloy steels (LAS) viz. Mn-Ni-Mo and Cr-Mo-V steels, was investigated. This was done by comparing the SPT results to the results of the conventional Charpy V-notch (CVN) tests, fractographic analysis and results of chemical etching test. Both the steels were subjected to accelerated thermal aging at 450 °C (8400 h for Mn-Ni-Mo steel and 5600 h for Cr-Mo-V steel). For both LAS, conventional CVN tests showed reduction in the impact energy and fractographic analysis indicated increase in the percentage brittle fracture with thermal aging. Fractographs indicated intergranular (IG) brittle fracture for the Mn-Ni-Mo steel and transgrannular (TG) brittle fracture for the Cr-Mo-V steel. This was corroborated by the chemical etching tests that showed preferential attack at prior austenitic grain boundaries (PAGBs) for Mn-Ni-Mo steel and conspicuous absence of such IG attack for Cr-Mo-V steel. SPT could clearly capture the embrittlement due to thermal aging for both the LAS as investigated by the conventional CVN testing. However, SPT could not establish the ductile to brittle transition temperature (TSP) as the lower shelf energy was not reached even at the lowest possible test temperature (-196 °C). SPT has been successful in capturing the indications of embrittlement due to thermal aging for both the LAS i.e. it is able to show the loss of ductility and increase in the fraction of brittle fracture. However, the use of SPT, at least in the present form, as an alternative to the most commonly used CVN test, for the materials studied is not recommended.

Effect of Cooling Medium on Microstructure, Impact and Hardness Properties of Al–15Sn Alloy

In the present study, effect of quenching/cooling conditions on microstructure, impact and hardness properties of custom-made Al–15Sn alloy were investigated. Heat-treated Charpy V-notch rectangular bar specimens were suddenly quenched in normal water (28 °C), cold water (6 °C) and ice bath (0 °C). Further, experiments were repeated and allowed to cool in an open atmosphere (air) and furnace to achieve different cooling conditions. Impact and hardness properties were assessed for the alloy quenched/cooled in different conditions. Microstructures revealed that alloy quenched in cold water showed finest (4.9 µm) Sn phase particles compared to other cooling conditions. Specimen quenched in cold water exhibited highest impact energy (77 Joules) and hardness number (RHN 90.7) whereas as-received sample showed least impact strength (32 J) and hardness (RHN 73.5).