Showing papers on "Charpy impact test published in 2000"

The Mechanism of Brittle Fracture in a Microalloyed Steel: Part I. Inclusion-Induced Cleavage

Abstract: The cleavage resistance of two microalloyed steels (steels A and B) was studied using several tests, including the instrumented precracked Charpy and Charpy V-notch (CVN) techniques. Ductile-to-brittle transition temperatures were measured for the base-metal and simulated heat-affected zone (HAZ) microstructures. Steel B showed inferior cleavage resistance to steel A, and this could not be explained by differences in gross microstructure. Scanning electron fractography revealed that TiN inclusions were responsible for cleavage initiation in steel B. These inclusions were well bonded to the ferritic matrix. It is believed that a strong inclusion-matrix bond is a key factor in why TiN inclusions are potent cleavage initiators in steel. Strong bonding allows high stresses in a crack/notch-tip plastic zone to act on the inclusions without debonding the interface. Once an inclusion cleaves, the strong bond allows for transfer of the TiN crack into the ferritic matrix. It was estimated that only 0.0016 wt pct Ti was tied up in the offending inclusions in steel B. This indicates that extended times at high temperatures during the casting of such steels could produce TiN-related toughness deterioration at even modest Ti contents.

Toughness and microstructure of 13Cr4NiMo high-strength steel welds

Abstract: The microstructures and tensile, Charpy, and crack tip opening displacement (CTOD) properties of 13Cr4NiMo soft martensitic stainless steel flux cored are welding process (FCAW) weld metals have been studied through different applied postweld heat treatments (PWHT). Phases and microstructural characteristics have been analyzed by scanning electron microscopy (SEM) and x-ray diffraction. The effect of the tempering and double tempering, with and without previous solution annealing, on the impact and fracture toughness has been studied. The role of the retained austenite resulting from tempering has been recognized, and it is suggested that the austenite particles improve the toughness of the welds through their transformation by the transformation-induced plasticity (TRIP) mechanism.

Processing of steel for ultrafine ferrite grain structures

Abstract: The attainment of ultrafine ferrite grain structures in low carbon, low alloy steels is of interest because of the improvement in yield strength and Charpy impact transition temperature predicted by extrapolation of known data to very fine grain sizes. This paper presents a summary of research aimed at producing ultrafine ferrite in a niobium microalloyed, low carbon steel by three processing routes. Transformational grain refinement (TGR), in which extrafine austenite is hot rolled and cooled rapidly, has been shown to be capable of producing grain sizes of <1 µm in a surface layer, and 1.5 µm in the centre of 3 mm thick plate. Dynamic recrystallisation of ferrite during multipass warm rolling was shown to be neither complete nor uniform within the cross-section of the plate. Nevertheless, a partly recrystallised, partly recovered grain structure with an average grain size of 1.5 µm was obtained in the centre of 3 mm thick plate. Cold rolling and recrystallisation of ferrite that had been previou...

Effect of Mn and Ni on the variation of the microstructure and mechanical properties of low-carbon weld metals

Abstract: Low-carbon weld metals with various amounts of Mn and Ni addition were made using metal-cored wires and Ar-2%O 2 shielding gas, and their mechanical properties were evaluated. The objective of the research, aimed to develop welding consumables with better resistance to cold cracking, was to determine the optimum composition ranges of Mn and Ni, in the presence of carbon content less than 0.02 %. The hardness of weld metals were found to increase linearly with Mn and Ni, which was attributed mainly to solid solution strengthening and in part to formation of hard phases. Varying Ni content influenced Charpy impact energy, the extent of which depended on Mn content. For a low-Mn composition, Ni addition increased hardness without sacrificing impact toughness whereas for a high Mn composition, Ni deteriorated the impact toughness seriously and caused intergranular fracture. The fracture path followed columnar grain boundaries that are identical to prior austenite grain boundaries since no δ-ferrite phase formed during solidification. Accordingly, these boundaries without having ferrite phase were susceptible to cracking under dynamic loading. Based on hardness and impact resistance, the optimum levels of Mn and Ni were suggested to be 0.5-1 % and 4-5 %, respectively.

Cleavage initiation in Ti-V-N and V-N microalloyed ferritic-pearlitic forging steels

Abstract: The effects of silicon (053 and 105 wt%) and titanium (<0002 and 0022 wt%) on microstructure and mechanical properties of vanadium microalloyed medium carbon steels heat treated after rolling to simulate the thermal cycle of hot forging have been determined using room temperature tensile tests, impact tests, optical microscopy and scanning electron microscopy (SEM) Silicon was found to increase strength values whilst titanium had a strong refining action on prior austenite grain size Room temperature Charpy ‘U’ notch impact energies were all on the lower shelf; ductile–brittle transition temperatures, determined from fracture appearance in Hounsfield impact tests, ranged from 100 to 145°C, scaling with material strength Initiation in the Charpy tests was by microcracking of coarse (Ti,V)(C,N)-containing single or multi-phase inclusions except in the low strength, titanium-free case when the absence of a completely continuous grain boundary ferrite layer allowed matrix microstructure initiation by interfacing pearlite colonies to occur

Numerical modeling of the ductile-brittle transition

Abstract: Numerical studies of the ductile-brittle transition are described that are based on incorporating physically based models of the competing fracture mechanisms into the material's constitutive relation. An elastic-viscoplastic constitutive relation for a porous plastic solid is used to model ductile fracture by the nucleation and subsequent growth of voids to coalescence. Cleavage is modeled in terms of attaining a temperature and strain rate independent critical value of the maximum principal stress over a specified material region of the order of one or two grain sizes. Various analyses of ductile-brittle transitions carried out within this framework are discussed. The specimens considered include the Charpy V-notch test and cracked specimens under mode I or mode II loading conditions. The fracture mode transition emerges as a natural outcome of the initial-boundary value problem solution.

Prestressed Polymeric Composites Produced by Viscoelastically Strained Nylon 6,6 Fibre Reinforcement

Abstract: A method for improving impact resistance is reported. Polymeric fibres are stretched under a load which is released prior to moulding them into a matrix. On solidification of the matrix, compressive stresses imparted by the viscoelastically strained fibres impede crack propagation. The prestressing principle is illustrated through a polariscope study of nylon 6,6 monofilament embedded in transparent resin. Previous work demonstrated the method through Charpy tests on nylon 6,6 fibre-polyester resin composites: prestressed samples absorbed 25% more impact energy than their control (unstressed) counterparts. Further impact studies are reported in the present work to evaluate probable service life. Batches are tested from 12 to 96 hours after moulding and (through heat treatment to accelerate ageing) at estimated ages of one to 10 years. They show no discernible change in impact performance with age.

Estimation of mechanical properties of ferritic steel welds. Part 2: Elongation and Charpy toughness

Abstract: Previous work presented models which can be used to estimate the yield and ultimate tensile strengths of ferritic steel welds. The present paper deals with properties that are much more difficult to predict: the elongation and Charpy impact toughness. While the models are found to be useful and emulate expectations from current physical metallurgy principles, it is clear that much more systematic experimental data are needed before the predictability becomes as good as the strength models of Part 1.

Investigation into the feasibility of viscoelastically generated pre-stress in polymeric matrix composites

Abstract: A method of producing compressive stresses within a composite material, to improve resistance to crack propagation, is described. This involves stretching polymeric fibres under a load that is released prior to moulding the fibres into a matrix material. On solidification of the matrix, continuing viscoelastic recovery of the strained fibres imparts compressive stresses to the surrounding material. To evaluate the technique, batches of samples, comprising nylon 6,6 fibres (3–4% fill) in a polyester resin matrix, were produced for Charpy impact testing. Each batch consisted of five test and five control samples, the latter containing the fibres in unstressed condition. Of the 15 batches tested, test samples from 11 batches absorbed, on average, 25% more impact energy than their control counterparts. The remaining four batches showed little difference between test and control samples, and this may be attributed to inadequate fibre–matrix bonding. Batches were tested up to 3500 h after moulding, and there was no discernible deterioration in impact performance with age.

Toughening mechanisms in commercial thermoplastic polyolefin blends

Abstract: Impact fracture mechanisms in a variety of commercially available and experimental thermoplastic polyolefin (TPO) blends were studied using the double-notch four-point-bend Charpy impact test, followed by microscopy observations. It was shown that the failure mechanisms and the sizes of the subcritically formed crack-tip damage zone before fracture were quite different among the TPO systems investigated. The room temperature Izod impact strengths of the TPOs investigated were found to correlate qualitatively well with the sizes of the damage zone. At room temperature the main fracture mechanisms observed in TPOs include matrix crazing, particle-matrix debonding, rubber particle internal cavitation, and shear banding. At low temperature (-40°C), the operative fracture mechanisms in TPOs are limited only to crazing, particle cavitation, and debonding. A strategy for improving impact strength without sacrificing scratch/mar resistance of TPOs is discussed.

Temper embrittlement of a CrMo low-alloy steel evaluated by means of small punch testing

Abstract: Temper embrittlement of a 2.25Cr1Mo steel is evaluated by use of small punch testing with disc specimens. Clearly, a ductile-to-brittle transition temperature of the steel may be determined by this test, but the value obtained is much lower than that determined by the standard Charpy test. The steel exhibits some temper embrittlement when tempered at 400°C for 86 days.

An investigation of the effect of fatigue deformation on the residual mechanical properties of Ti-6Al-4V ELI

Abstract: Tensile properties, hardness, and Charpy impact toughness of Ti-6Al-4V extralow interstitial (ELI) with equiaxed α and Widmanstatten α structures at various stages of fatigue were investigated. Fatigue crack initiation characteristics of the same alloy were also investigated in this study. In the equiaxed α structure, fatigue cracks initiated mainly at the interface between primary-α grains, while in the Widmanstatten α structure, they initiated across α plates at an angle of around 45 deg to the stress axis. Specimens with the Widmanstatten α structure fractured before adequate fatigue hardening was achieved because a multitude of microcracks readily formed. Specimens with the equiaxed α structure fractured after adequate fatigue hardening developed. Tensile strength, 0.2 pct proof stress, and hardness increased clearly with increasing stress cycles and fatigue steps, particulary in the low-cycle fatigue (LCF) region, while impact toughness and elongation showed a reverse trend. It is suggested, therefore, that the dislocation density multiplies more rapidly near the specimen surface during the early stages of fatigue, while during the later stages of fatigue, dislocation density increases near the center of the specimen. Also, the dislocation multiplication will continue until saturation of the entire specimen has occurred.

Review of ctoa as a measure of ductile fracture toughness

Abstract: The ductile fracture toughness of steel is used to assess the ability of a pipeline to resist long running ductile fractures in a burst event. With the introduction of modern low carbon clean steels with ultra high toughness, conventional measures of ductile fracture toughness (standard Charpy and DWTT energy) are under review, and alternatives are being studied. The crack tip opening angle (CTOA) was investigated to evaluate its appropriateness as a measure of modern pipeline steel ductile fracture toughness. At first, fracture mechanics tests at quasi-static rate were analyzed to examine the constancy of CTOA with crack growth. The results of this initial review are based on four pipeline steels with a range of ductile fracture toughness. The CTOA values are also compared with appropriate parameters from conventional tests to examine potential relationships that may be used to indicate the relative resistance of pipeline steels to ductile fracture propagation. The final objective is to compare CTOA values determined by the simple two specimen method and those developed through a formal fracture mechanics based technique.

Modelling the impact energy in the ductile/brittle transition region of C-Mn multi-run welds

Abstract: Contrary to a commonly made assumption, the statistical analysis of Charpy impact tests of C-Mn multi-run welds revealed that the impact energy at a specified test temperature is not normally distributed. Test results from specimens with a notch located in different microstructural zones (e.g. as-deposited and reheated) are characterised by distinct distributions of the impact energy values. The resultant distribution of the impact energy is a mixture of several distributions scaled by the probabilities of sampling the existing microstructural zones. Equations regarding the variance of the mixed impact energy distribution have been derived and a weighted regression model has been applied to fit the systematic variation of the mean impact energy in the transition region of C-Mn multi-run welds. The central zone, the bulk of the as-deposited zone and the reheated zone are characterised by distinct variations of the mean impact energy.