Charpy Specimens Research Articles

Influence of Textures of Pipeline Steels after Operation on Their Brittle Fracture Resistance

The difference between the brittle fracture resistances of ferrite-pearlite steels in the as-received state and after operation is determined, depending on the orientation of the specimens, by the impact toughness testing of Charpy specimens cut out in different directions relative to the axis of the pipe. The longitudinal specimens exhibit the highest level of impact toughness, the transverse specimens have a somewhat lower level impact toughness, and the radial specimens whose fracture plane is parallel to the rolling plane revealed the lowest impact toughness. The long-term operation of steels significantly decreases their brittle fracture resistance. In this case, the impact toughness of radial specimens was, as a rule, lower than the normative values specified in the industry.

Significance evaluation of effect of operational changes in material on impact strength of various zones in leaf spring at low temperature

Significance of the effect of structural changes in spring steel arising during operation in various zones of the leaf springs on zonal resistance of the steel to a brittle fracture at temperatures of a climatic range of a permafrost region is studied. Structural damage was determined at the temperatures of 20, –20 and –60 °C and impact strength of the different zones of the leaf spring failed during the operation was estimated. Features of a fine structure of fracture surfaces of Charpy specimens and micromechanisms of their failure are investigated. To determine variability of the impact strength in the zones of the spring, analysis of variance (ANOVA) of test data was carried out. It was found that at temperatures of 20 and –20 °C the effect of the zone damage of a material on the impact strength is insignificant, and at –60 °C the effect of the zonal damage on the resistance to the brittle fracture becomes significant. The fractographic features of the fine structure of the fracture surfaces at the various test temperatures are discussed. Results can be used in a prediction of a life time of elastic elements in vehicle suspension: it is necessary to take into account zoning of the operational damage of the steel under conditions of the low climatic temperatures.

Study of the Breakage Region of High Ductility Steel 08G2B Charpy Specimen Fracture

The structure, microhardness, and orientation of individual areas of the breakage region of steel 08G2B Charpy specimens with an ultrafine ferrite-bainite-martensite structure are studied. The contribution of plastic deformation energy in the breakage region to the total energy of Charpy specimen fracture is assessed.

Structure and properties of NR- and XNBR-toughened polypropylene produced with latex by melt compounding

Binary blends composed of polypropylene (PP) and natural rubber (NR) or carboxylated acrylonitrile butadiene rubber (XNBR) were produced in batch-type melt compounding process. The rubbers used as toughening agents were introduced into PP in up to 20 wt% in their dry and latex forms. Structure-property relationships of the blends were studied. Both NR and XNBR were homogenously dispersed in the PP matrix: XNBR in microscale, NR in the submicronic scale. Fracture toughness and energy, determined on notched Charpy specimens, were improved by rubber toughening, however, at the cost of stiffness and strength, determined in tensile tests. NR toughened blends were superior to blends containing XNBR with respect to impact resistance. The introduction of rubbers in latex form resulted in slightly better mechanical performance compared to addition in dry form.

Relating Charpy energy to fracture toughness in the lower transition region using a Weibull stress dependent energy scaling model

Accurate prediction of fracture toughness from Charpy energy is challenging due to the differences between fracture toughness tests and Charpy tests, such as geometry (deep sharp crack vs. shallow blunt V-notch) and loading rate (quasi-static vs. dynamic). This paper uses experimental and numerical approaches to investigate whether the Weibull stress can be used to scale Charpy energy to fracture toughness in the lower transition region. A Weibull-stress based energy scaling model is proposed which is shown to accurately predict cleavage fracture at quasi-static loading rates. An extension of the energy scaling approach to dynamic loading conditions indicates that the Weibull modulus is independent of strain rate, and that quasi-statically derived moduli can be used to predict fracture toughness from a dynamically tested Charpy specimen. The paper concludes that the proposed energy scaling approach, and its extension to dynamic loading conditions, provides a suitable basis for relating Charpy energy to fracture toughness in the lower transition region.

Instrumented Impact Testing of Miniaturized Charpy Specimens of AM Ti-6Al-4V.

An investigation on the impact toughness properties of wrought and additively manufactured (AM) Ti-6Al-4V was conducted at National Institute for Standards and Technology (NIST) Boulder by means of instrumented impact tests on miniaturized Charpy specimens. Full transition curves for absorbed energy and lateral expansion were obtained by performing tests in the temperature range between -196°C and 700°C. The effect of various parameters was investigated for AM specimens, namely specimen orientation, hot isostatic pressing (HIPing), and notch configuration (printed or machined). Our results indicate that AM specimens exhibit equivalent or better impact toughness than wrought material after HIPing and that the material is more resistant to cracks growing in the plane perpendicular to the build direction than in the plane containing the build direction. HIPing has a significantly beneficial effect for the AM material, while no effect of notch configuration was observed from the results obtained.

Influence of the cutting direction of Charpy specimens from steel 10GN2MFA on the components of the fracture energy and the ductile-to-brittle transition temperature

Проведено ударні випробування стандартних зразків Шарпі зі сталі 10ГН2МФА, вирізаних у двох напрямках – поздовжньому та поперечному, за різних температур на інструментованому вертикальному копрі. За результатами випробувань побудовано температурні залежності повної енергії деформування, руйнування та її складових. За результатами фрактографічних досліджень отримано температурні залежності питомих енергій на різних ділянках поверхні зламу та проведено їх порівняння для двох груп зразків. Визначено критичні температури крихкості.

Ductile Fracture Behaviour of Hot Isostatically Pressed Inconel 690 Superalloy

Herein we assess the differences in Charpy impact behavior between Hot Isostatically Pressed and forged Inconel 690 alloy over the temperature range of 300 °C to − 196 °C. The impact toughness of forged 690 exhibited a relatively small temperature dependence, with a maximum difference of ca. 40 J measured between 300 °C and − 196 °C, whereas the HIP’d alloy exhibited a difference of approximately double that of the forged alloy over the same temperature range. We have conducted Charpy impact testing, tensile testing, and metallographic analyses on the as-received materials as well as fractography of the failed Charpy specimens in order to understand the mechanisms that cause the observed differences in material fracture properties. The work supports a recent series of studies which assess differences in fundamental fracture behavior between Hot Isostatically Pressed and forged austenitic stainless steel materials of equivalent grades, and the results obtained in this study are compared to those of the previous stainless steel investigations to paint a more general picture of the comparisons between HIP vs forged material fracture behavior. Inconel 690 was selected in this study since previous studies were unable to completely omit the effects of strain-induced martensitic transformation at the tip of the Chary V-notch from the fracture mechanism; Inconel 690 is unable to undergo strain-induced martensitic transformation due to the alloy’s high nickel content, thereby providing a sister study with the omission of any martensitic transformation effects on ductile fracture behavior.

Ductile fracture of advanced pipeline steels: study of stress states and energies in dynamic impact specimens - CVN and DWTT

The development of robust protocols for assessing the structural integrity of gas pipelines is of paramount relevance, since failures can lead to financial and human losses. In this scenario, the material’s ability to slow down the propagation of a running crack (crack arrest) becomes a design requirement. Several empirical models and criteria, calibrated by real pipeline burst tests, have been developed, being the Battele Two Curve Method (BTCM) one example of technique widely employed during decades. With the evolution of steels, there was a significant increase of ductility and toughness, in a way that such semi-empirical models usually based on the energy absorbed in the Charpy impact test (ISO 148-1, ASTM E-23) began to present unsatisfactory predictions. This may be explained by the fact that in current high-ductility and high-toughness materials (e.g.: API-5L X65, X80, X100), the dominant mechanism of fracture propagation is plastic collapse. Consequently, the energies involved in deforming and fracturing a laboratory specimen are remarkably altered and transferability to pipelines by means of the aforementioned models can be lost. Therefore, for a better phenomenological comprehension of the ductile fracture process under such circumstances, this work investigates Charpy and DWTT (ASTM E-436) dynamic tests assessing stress fields and respective energies involved in deformation and fracture. It is of great interest to evaluate the energy associated to steady state ductile fracture and thus try to characterize the energy available to slow down an ongoing fracture. Pipelines are references for the developments and support assumptions and some conclusions. Based on these golas, numerical analyses including damage models (XFEM and GTN) were implemented, including parameters’ calibration and sensitivity analyses. The methodology closely reproduced available experimental results. Besides that, stress fields and energies could be quantified for the studied geometries and such analyses indicated the potential and limitations of Charpy and DWTT specimens to characterize the energies required to describe steady state ductile crack propagation and crack arrestability. Results support further developments related to pipeline integrity assessments.

A Study of the Cold Resistance of Coiled Stock for Pipes Produced at Foundry-Rolling Works. Part 1

The mechanical properties, the microstructure, and the cold resistance of coiled strips from low-carbon manganese-and silicon-alloyed steels with different additions of niobium and titanium produced by direct rolling of thin slabs right after their continuous casting under the conditions of foundry-rolling works are studied. It is shown that the addition of 0.01% Ti into the steel alloyed with 0.04% niobium has a positive effect on the ductility, toughness and cold resistance of the rolled strips and a negative effect on the strength. A high (0.08%) content of niobium is favorable for the impact toughness and resistance to brittle fracture of Charpy specimens of the steels studied under negative temperatures of up to –80°C.

Temperature dependent crack initiation of 42CrMo4 steel at high loading rates

Dynamic crack initiation with crack tip loading rates K˙ of approximately 2 ‧ 106 MPa√ms− in high-strength 42CrMo4 steel was investigated. To this end, a recently developed split Hopkinson pressure bar with four-point bending was utilized. V-notched and precracked Charpy specimens were tested. The tests were performed at temperatures of –40 °C and 20 °C. The loading of the specimen was determined by analyzing the strain in the incident and transmission bars. Furthermore, strain gauges at the specimen’s surface were applied to measure the crack tip loading. High-speed photography complemented the analysis of the specimens loading and the detection of the crack initiation. Fracture surface analysis by means of scanning electron microscopy enabled the measurement of the fracture surface topography and, consequently, stretch zone height and width. Hence, the macroscopically measured dynamic crack initiation toughness was correlated with the toughness at microscopic scale. It was observed that the resistance against dynamic crack initiation decreased with decreasing temperature. Microscopically, a decrease in toughness was analogously observed. Non-metallic inclusions resulted in crack path deflection with localized shear zones. After a small stable crack extension, cleavage fracture was observed.

MICROMECHANISMS OF FERRITE-PEARLITE STEELS FRACTURE UNDER CYCLIC AND IMPACT LOADING

During operation, the railway axle is exposed to static, cyclic and dynamic loading that is accompanied by influence of corrosive environments. This causes the accumulation of structural and mechanical damages in the material, formation of corrosion and mechanical micro-defects that are potential sites of fatigue crack nucleation and growth. This requires the development of failure analysis methods that allow determining main mechanisms of ferrite-pearlite steels fracture on the micro scale level. The railway axle’s material – the OSL steel – main regularities of temperature influence on fracture mechanisms in ferrite-pearlite steel type are shown. Qualitative and quantitative analysis of fracture surface of Charpy specimens tested at 20°C and -40°C was performed on micro scale level using SEM investigation.

Fracture Toughness and Charpy CVN Data for A36 Steel with Wet Welding

This study presents KIC data obtained from KIC-CVN correlations from Charpy CVN values. For this study, T-welded connections were manufactured from ASTM A36 and E6013 electrodes in dry conditions. Then, a rectangular grinding at the weld toe was carried out and filled with wet welding. Charpy specimens were extracted to obtain CVN values. An exhaustive search through the literature of several authors was performed to collect experimental CVN data about wet welding being applied to A36 steel for comparison with CVN data obtained in this study. By using Charpy impact energy (CVN), KIC values could be predicted by KIC-CVN correlations. In addition, correlations were presented to obtain KIC values in the lower shelf, transition temperature zones and different zones for the energy-temperature curve of A36 steel. Of these correlations, Barsom’s equation was adopted, because he applied the stress yield (σYS) of the material and it can be applied in all zones for the energy-temperature curve. The results revealed that CVN values are proportionate to KIC, this data decreases as water depth increases. This took place because several discontinuities, such as, porosity, slag inclusion, non-metallic inclusion, cracking and microstructures are present in the wet welding.

Observation of micro-cracks beneath fracture surface during dynamic crack propagation

In this study, micro-cracks generated during dynamic crack propagation were investigated for an ESSO specimen and V-notched Charpy specimen. The ESSO test with temperature gradient and the Charpy impact test were conducted for steel for shipbuilding. The macroscopic roughness of the fracture surface increases with increasing temperature. The section beneath the fracture surface generated during dynamic brittle crack propagation was observed using a scanning electron microscope, after cutting with a focused ion beam or grinding. Many micro-cracks can be found beneath the brittle fracture surface in the V-notched Charpy specimen. In the ESSO specimen, which had a macroscopic flat surface, micro-cracks can be detected beneath the main crack propagated in the temperature range from −140°C to −100°C. On the other hand, there were a few micro-cracks beneath the main crack propagated at relatively high temperature, at which the fracture surface was macroscopically rough. The number of micro-cracks in the Charpy specimen is fewer than those in the ESSO specimen at the same temperature. The observations indicated that the correlations of Charpy impact properties with crack arrest performance for the ESSO specimen was inconvincible in terms of the differences in the micro-scale fracture behavior as well as lack of theoretical basis.

Effect of prior-austenite grain refinement on microstructure, mechanical properties and thermal embrittlement of 9Cr-1Mo-0.1C steel

The effect of ‘conventional normalizing and tempering’ (CNT) and ‘double austenitization based normalizing and tempering’ (DNT) heat treatments on the microstructure, tensile, creep and impact toughness properties of 9Cr-1Mo steel has been studied. The tempered martensite microstructure obtained through DNT treatment exhibited smaller sizes of prior-austenite grains/martensite packets (28 μm/11 μm) compared to the CNT treatment (44 μm/14 μm). The tempered martensite morphology was largely retained after long-term thermal aging at 550 °C/5000 h, while the M23C6 and M2(C,N) type of precipitates were found to act as nucleation sites for precipitation of brittle Fe2Mo Laves phase. The grain refinement by DNT was found to be beneficial for minimizing the ductile-to-brittle transition characteristics (25 °C lower ductile-to-brittle transition temperature and 70 J higher upper shelf energy) over the CNT. Thermal embrittlement occurred in both heated treated steels, but the transition temperature of aged DNT steel remained below room temperature. Fractured Charpy specimens revealed ductile failure by void coalescence for high temperature tests, and a quasi-cleavage fracture at low temperatures with few isolated occurrence of intergranular crack in thermal embrittled steel. The DNT treated steel resulted in similar or better tensile and creep properties, when compared to the CNT treatment. The homogeneous fine grained tempered martensite microstructure obtained by DNT treatment resulted in improved embrittlement resistance and mechanical properties over the conventional treatment.

Influence of Texture on Impact Toughness of Ferritic Fe-20Cr-5Al Oxide Dispersion Strengthened Steel.

Fe-based oxide dispersion strengthened (ODS) steels are oriented to applications where high operating temperatures and good corrosion resistance is paramount. However, their use is compromised by their fracture toughness, which is lower than other competing ferritic-martenstic steels. In addition, the route required in manufacturing these alloys generates texture in the material, which induces a strong anisotropy in properties. The V-notched Charpy tests carried out on these alloys, to evaluate their impact toughness, reveal that delaminations do not follow the path that would be expected. There are many hypotheses about what triggers these delaminations, but the most accepted is that the joint action of particles in the grain boundaries, texture induced in the manufacturing process, and the actual microstructure of these alloys are responsible. In this paper we focused on the actual role of crystallographic texture on impact toughness in these materials. A finite elements simulation is carried out to solely analyze the role of texture and eliminate other factors, such as grain boundaries and the dispersed particles. The work allows us to conclude that crystallographic texture plays an important role in the distribution of stresses in the Charpy specimens. The observed delaminations might be explained on the basis that the crack in the grain, causing the delamination, is directly related to the shear stresses τ12 on both sides of the grain boundary, while the main crack propagation is a consequence of the normal stress to the crack.

The Application of Miniaturized Three-Point-Bend Specimens for Determination of the Reference Temperature of A533 Cl.1 Steel

This contribution deals with the determination of the reference temperature of A533 Cl.1 steel using miniaturized specimens. The dimensions of the miniaturized specimens used were 3 × 4 × 27 mm (thickness × width × length). This specimen type allows utilizing a limited amount of test material or the broken halves of precracked Charpy or larger specimens. The test material comes from the broken halves of 0.5 T SE(B) specimens previously tested for the determination of the reference temperature at Ciemat, Madrid. The fracture toughness tests were performed in the transition region of the steel according to the recommendations of the standard ASTM E1921 and according to Wallin's recommended temperature range of miniaturized specimens. The reference temperature of the Master Curve was very similar to the ones obtained from three-point-bend specimens of sizes 0.2 T, 0.4 T, and 0.5 T. The results obtained confirm a necessity to conduct tests at low temperatures and to test a sufficient number of specimens in order to generate enough valid data for the determination of the reference temperature.

Influence of specimen dimensions on ductile-to-brittle transition temperature in Charpy impact test

This paper discusses the correlation between specimen dimensions and transition temperature. Notch toughness properties of Standard Charpy-V specimens are compared to samples with lower width (7.5 mm, 5 mm, 2.5 mm) and sub-size Charpy specimens with cross section 3×4. In this study transition curves are correlated with lateral ductile part of fracture related ones for 5 considered geometries. Based on the results obtained, correlation procedure for transition temperature determination of full size specimens defined by fracture appearance of sub-sized specimens is proposed.

Tensile behavior and impact toughness of an AlSi3MgCr alloy

Abstract: Recently, an innovative AlSi3Mg alloy with Cr and Mn additions was developed for the production of truck wheels by means of a non-conventional hybrid technique, which combines features of both low pressure die casting and forging processes. The presence of both Cr and Mn leads to the formation of an intermetallic phase rich in Cr, Mn and Fe with a globular or dendritic morphology. Furthermore, proper solution treatments cause the formation of dispersoids in the aluminium matrix. These dispersoids are responsible of enhancing the alloy performance due to dispersion hardening mechanism. In the present work, the tensile properties and the impact toughness of the alloy in as-cast and different heat-treated conditions were studied. Moreover, tensile and impact strength tests were performed on A356 samples in T6 condition machined from traditional LPDC wheels, whose results were compared with the performance of the innovative alloy. Fracture surfaces of tensile and Charpy specimens were observed by Scanning Electron Microscopy (SEM) in order to identify the role of the Cr-Mn containing intermetallic particles in the failure mechanism and the influence of the heat treatment parameters. Considering the static properties, the innovative alloy showed remarkable values of tensile strength, while ductility was improved only after heat treatment optimization. Poor impact toughness values were measured and the microstructural analysis confirmed the presence of coarse intermetallic secondary phases, acting as crack initiation and propagation particles, on the fracture surfaces.

Determination of the Degree of Degradation of Steels of Steam Pipelines According to Their Impact Toughness on Specimens with Different Geometries of Notches

We analyze the characteristics of impact toughness KCU and KCV of 15Kh1M1F and 12Kh1MF heatresistant steels by using Menage and Charpy specimens, respectively, taken from the straight sections of the steam pipelines of a thermal power plant, their welded joints, and different zones of the bends. We reveal the specific features of their behavior depending on the period of operation and the number of shutdowns of the units. It is shown that, as a result of the in-service degradation of the metal, the KCU/KCV ratio significantly increases due to a more intense reduction of the crack-growth resistance as compared with the crack initiation resistance.