Showing papers on "Charpy impact test published in 1991"

Microstructure and local brittle zone phenomena in high-strength low-alloy steel welds

Abstract: This study is concerned with a correlation between the microstructure and the local brittle zone (LBZ) phenomena in high-strength low-alloy (HSLA) steel welds The influence of the LBZ on toughness was investigated by means of simulated heat-affected zone (HAZ) tests as well as welded joint tests Micromechanical processes involved in microvoid and cleavage microcrack formation were also identified using notched round tensile tests and subsequent scanning electron microscopy (SEM) analyses The LBZ in the HAZ of a mUltipass welded joint is the intercritically reheated coarse-grained HAZ whose properties are strongly influenced by metallurgical factors such as an effective grain size and high-carbon martensitic islands: The experimental results indicated that Charpy energy was found to decrease monotonically with increasing the amount of martensitic islands, confirming that the martensitic island is the major microstructural factor controlling the HAZ toughness In addition, microvoids and microcracks were found to initiate at the interface between the martensitic island and the ferrite matrix, thereby causing the reduction in toughness These findings suggest that the LBZ phenomena in the coarse-grained HAZ can be explained by the morphology and the amount of martensitic islands

Low temperature aging behavior of type 308 stainless steel weld metal

Abstract: The aging behavior of welded type 308 stainless steel was evaluated by mechanical property testing and microstructural examination. Aging was carried out at 475°C for up to 20,000 h. The initial material consisted of austenite with approximately 10% ferrite. Upon aging, the ferrite hardness increased up to 100%. This hardening was accompanied by a noticeable increase in the ductile—brittle transition temperature and a drop in the upper shelf energy, as measured by Charpy impact tests, and a degradation in fracture toughness, as determined by J-integral test. Tensile properties did not change significantly with aging. Microstructural analysis indicated that the ferrite decomposed spinodally into iron-rich α and chromium-enriched α′. In addition, abundant precipitation of nickel- and silicon-rich G-phase was found within the ferrite and M23C6 carbide formed along the austenite-ferrite interface. These effects are similar to the aging behavior of cast stainless steels. Occasionally, large G-phase or α precipitates were also found along the austenite-ferrite interface after aging more than 1000 h. After comparison of the mechanical property changes with the microstructural features, it was concluded that both spinodal decomposition as well as G-phase formation contribute to ferrite hardening. Spinodal decomposition results in embrittlement of the weld insofar as the ductile-brittle transition temperature is raised. G-phase formation and carbide precipitation are associated with a degradation in the ductile fracture properties, as shown by a drop in the upper shelf energy and a decrease in the fracture toughness.

Zn-al Matrix Composites - Investigation of the Thermal-expansion, Creep Resistance and Fracture-toughness

Abstract: Composites of Zn-A1 alloys (ZA8 and ZA27) reinforced by fibres of alumina, carbon or steel have been prepared by squeeze casting. Microstructural observations reveal the segregation of the zinc-rich phase around the fibres. The composites exhibit a greatly reduced creep rate in comparison with the matrix alloys. The thermal expansion coefficient is also significantly decreased. The impact toughness has been studied by Charpy tests.

Microstructurally toughened particulate-reinforced aluminum matrix composites

Abstract: Silicon carbide particulate-reinforced aluminum matrix composites with dramatically higher energy absorption capability and damage tolerance have been demonstrated. The approach, referred to as microstructurally toughened composites, consists of segregating the composite into particulate-reinforced regions and continuous ductile toughening regions. Composites consisting of silicon carbide particulate-reinforced 6061 Al (SiCp/6061) with monolithic 6061 and commercially pure (CP) titanium toughening regions were fabricated. As much as an order of magnitude increase in notched Charpy impact energy absorption capability was demonstrated relative to conventional SiCp/6061 composites, with the higher values being associated with those samples that deflected the crack front more extensively during failure. The longitudinal tensile strength of the composites was shown to be independent of the scale of the microstructure or the magnitude of the toughening/reinforced region’s interfacial strength. The high impact energy 6061 toughened composites displayed low transverse tensile strength values, while the CP titanium toughened composites simultaneously displayed high energy absorption and high transverse tensile strength. A model was also developed to predict the minimum and maximum energy absorption capability of the composites, as well as provide a quantitative estimate of the composite energy absorption based on the measured crack front deflection length in the failed impact samples.

Analysis of the Hopkinson Pressure Bar loaded Instrumented Charpy Test using an inertial modelling technique

Abstract: A one-dimensional mass spring model of the Hopkinson Pressure Bar loaded Instrumented Charpy Test is developed and validated for the lower shelf behaviour of an alloy steel, En24 (817M40). The assessment of component stiffnesses and masses, in the inertial model, is discussed. It is shown that the linear elastic components of contact stiffness values do not vary markedly with strain rate and temperature. It is also shown that plastic components of contact stiffnesses become important at intermediate force values. The developed inertial model is then used to derive values of dynamic initiation fracture toughness, K Id .

NiAl-Based microstructurally toughened composites

Abstract: Intermetallic NiAl-based composites with dramatically higher energy absorption capability and damage tolerance have been demonstrated. The approach consisted of incorporating continuous tubular 304 stainless steel toughening regions throughout the majority phase NiAl matrix. To compensate for the increase in density resulting from the 304 stainless steel, B4C particulate was added to the NiAl so that the overall composite density was within 5 pct of the value for monolithic NiAl. The notched Charpy impact energy absorption of the B4C/NiAl/304 stainless steel composites was in the range of 15 to 90 J/cm2, compared to a value of 0.8 J/cm2 for NiAl. The higher energies were measured on samples that deflected the crack front more extensively during failure. A model has been developed that is consistent with the energy absorption values measured during notched impact testing of the composites. Finally, significant room-temperature tensile strains (20 to 35 pct) were achieved due to constrained yielding of the 304 stainless steel, which prevented composite failure after the NiAl regions had cracked.

Charpy impact tests near absolute zero

Abstract: We review Charpy impact testing at extreme cryogenic temperatures, especially at liquid helium temperature (4 K), considering methods of testing and calibration, thermal behavior during the various stages of testing, and correlations between Charpy absorbed energy and quantitative toughness parameters. Because of the very low specific heats of metals near absolute zero, any surface condensation of gases, convective or conductive heat transfer, or plastic deformation during a test will cause the specimen temperature to rise rapidly. Consequently, valid impact tests of alloys at 4 K can not be performed according to the procedure outlined in ASTM Methods E 23-88. During Charpy tests, the temperature of austenitic steel specimens, initially at or near 4 K, may in fact rise outside the cryogenic regime. Fracture does not occur at the intended temperature, but at an uncontrolled temperature, since materials with different work hardening rates heat differently. In view of the temperature rise variability and scatter in measurements and property correlations, we conclude that it is not possible to accurately estimate the 4 K fracture toughness of ductile steels, or rank them properly, using Charpy tests.

BEHAVIOUR OF A 1Cr-1Mo-0.25V STEEL AFTER LONG-TERM EXPOSURE–I. CHARPY IMPACT TOUGHNESS AND CREEP PROPERTIES

Abstract: This paper deals with the effect of casting conditions and long-term in-service exposure on the Charpy-V notch and creep properties of a 1Cr-lMo-O.25V steel. The material is investigated under two conditions: (i) as-cast in a thick part which produces a mixed ferrite-bainite microstructure and (ii) as-cast as a small ingot which produces a fully bainitic microstructure. The material from the thick part was taken from either the cold part (-200°C) or the hot part (u54oOC) of a steam power casing which operated for about 150,000 h. A small shift in the ductile-brittle transition temperature is shown after the long exposure time. This shift is related to segregation of phosphorus impurities along the grain boundaries as shown by Auger electron spectrometry. The bainitic microstructure exhibits a much better creep resistance compared to the thick part component material. Moreover, in the latter case, it is shown that long-term in-service exposure induces an increase in creep strain rate and a decrease in creep ductility at least for relatively short times to failure (8 1000 h). These variations in mechanical properties are briefly discussed in terms of microstructural changes. Scanning electron microscopy confirmed that grain boundary cavitation was responsible for decreasing the creep ductility at large times to failure (2 10,000 h).

Effects of vanadium and processing parameters on the structures and properties of a direct-quenched low-carbon Mo-B steel

Abstract: The structures and mechanical properties of a series of thermomechanically processed, direct-quenched martensitic 0.1C-1.4Mn-0.5Mo-B steels containing from 0 to 0.24 wt pct va have been investigated and compared to those obtained after a conventional austenitizing-and-quenching treatment. For all processing conditions, vanadium additions to the base composition are found to increase hardenability (ideal critical parameter,D,); the largest effects (up to a 90 pct increase inDI) are noted when samples are hot-rolled prior to direct quenching. Vanadium additions are also observed to provide significant strengthening in the quenched-and-tempered condition as the result of the precipitation of fine V-Mo carbides. The strengthening increment due to these precipitates is approximately 100 MPa/0.1 wt pct V over the range of vanadium additions examined. At the same time, however, these precipitates reduce notch toughness; on the average, the 20 J transition temperature increases by about 4 °C for each 10 MPa increment in yield strength. For the conditions examined, the best balance of strength and toughness is obtained in direct-quenched samples which are control-rolled(i.e., rolling is completed below the austenite recrystallization temperature) prior to quenching.

Low- Temperature improvement of mechanical properties of aisi 4340 steel through high-temperature thermomechanical treatment

Abstract: The low-temperature mechanical properties of AISI 4340 ultrahigh-strength steel can be dramatically improved by high-temperature thermomechanical treatment (HTMT). A comparison was made with the mechanical properties developed by the conventional heat-treatment (CHT). When the steel was hot forged by 50 pct reduction at 1177 K followed by direct oil quenching and subsequent tempering at 423 K, the slow-bending fracture energy of fatigue-precracked steels was significantly improved, with notably increased strength, owing to a moderate increase in ductility over the temperature range of 123 to 293 K, and the ductile-to-brittle transition temperature (DBTT) in the Charpy impact test was remarkably lowered. The marked development was achieved in the slow-bending fracture energy, with moderate improvement in strength, owing to significantly increased ductility over the temperature range of 123 to 293 K, when the steel was deformed by 50 pct reduction at 1473 K followed by direct oil quenching and subsequent tempering at 453 K. The treatment also improved the shelf energy and DBTT in the Charpy impact test. The improved mechanical properties are attributed to the subcell structure introduced in austenite by processing HTMT and inherited by martensite. The present work shows that an HTMT steel is attractive for low-temperature ultrahigh-strength applications when suitable combinations of deformation temperatures with tempering conditions are applied to the steels.

Measurement of Impact Load by using an Inverse Analysis Technique : Comparison of Methods for Estimating the Transfer Function and its Application to the Instrumented Charpy Impact Test

Abstract: This paper is concerned with a method for measuring an impact load acting on a body of arbitrary shape. The impact load is determined from the strain response of the body by the deconvolution procedure which is performed in the frequency domain by using Laplace transformation. The transfer function is estimated from the result of the calibration experiment. The deconvolution is one of the ill-posed problems, and it is usually difficult to obtain a good result. In particular, high- frequency noise involved in the measured signal is often expanded to obscure the result. For this problem, we used five estimators of the transfer function and investigated which of them gives the most suitable transfer function for the deconvolution of experimental data. The efficiency of the method is demonstrated by applying it to the instrumentation of the Charpy impact testing machine.

Comparison of effects of aging on fracture of 9Cr–1Mo and 2·25Cr–1Mo steel Part 1 Quenched and tempered material

Abstract: A 9Cr–1Mo (wt- %) steel and a 2·25Cr–1Mo steel have been investigated in the quenched and tempered condition and after subsequent aging at 550°C for either 1000 or 5000 h. Transmission electron microscopy (TEM) was used to characterise the microstructures produced by each heat treatment. The ductility and fracture resistance were measured using ambient temperature tensile tests and Charpy tests at various temperatures, respectively. TEM was also used to monitor void nucleation during tensile deformation. Differences in the ductility of the steels before aging and their different responses to aging, which decreases the ductility of the 9Cr–1Mo steel while leaving that of the 2·25Cr–1Mo steel unaffected, are attributed to the different void nucleation characteristics of the precipitates in each steel. Differences in ductile–brittle transition temperature (DBTT) before aging are discussed in terms of differences in the lower shelf fracture mode of each steel and the possibility of crack nucleation at...

Comparison of effects of aging on fracture of 9Cr–1 Mo and 2·25Cr–1 Mo steel Part 2 Non-standard microstructures

Abstract: Samples of a 9Cr–1Mo and a 2·25Cr–1Mo steel (both wt-%) have been heat treated to produce microstructures that are likely to form in the heat affected zone of a weld. Two treatments were employed for this purpose, involving quenching (i) from an artificially high austenitising temperature and (ii) from an abnormally low temperature, within the intercritical region. Some of the samples in each condition were given aging treatments at 550°C, which were designed to simulate exposure to typical in-service temperatures. The microstructures produced by each heat treatment schedule were characterised using transmission electron microscopy (TEM). The effects of each treatment on the mechanical properties of the steels were assessed using ambient temperature tensile testing and Charpy tests at various temperatures. The results were compared with those obtained from samples for which standard austenitising temperatures were employed. The higher austenitising temperature was found to have very little effect ...

Mechanical and physical properties of squeeze-cast aluminium steel matrix composites containing 5%–30% Saffil

Abstract: Saffil short alumina fibre preforms are amongst the most widely available reinforcing materials suitable for metal matrix composites. Although much work has been reported on specific composite couples of this type, little work has been conducted covering a range of reinforced loadings in a single matrix alloy. In the present research work Saffil preforms have been squeeze infiltrated to produce metal matrix composites (MMCs) having Saffil volume fractions from 0.05 to 0.30 in 0.05 volume fraction increments. Pure aluminium was used as the matrix material so as to avoid variable contributions to resultant properties due to alloying element interactions with the preform system. Charpy impact and fracture toughness values were obtained from specimens of the as-cast material. Impact values were generally very low, decreasing marginally with increasing volume fraction: the fracture toughness decreases precipitately with reinforcement volume fractions of 0.05 and above. The measured thermal expansion coefficients exhibit an elastic response in the lower temperature range and a plastic response at higher temperatures. The critical temperature between the two responses decreases linearly with volume fraction reinforcement, and for any given volume fraction loading the critical temperature decreases with decreasing heating rate. On repeated thermal cycling the MMC specimens undergo progressive decreases in length until a steady state cycle is reached after approximately 10 cycles.

Thermal aging of cast stainless steels in LWR systems: Estimation of mechanical properties

Abstract: A procedure and correlations are presented for predicting Charpy-impact energy, tensile flow stress, fracture toughness J-R curve, and J{sub IC} of aged cast stainless steels from known material information. The ``saturation`` impact strength and fracture toughness of a specific cast stainless steel, i.e., the minimum value that would be achieved for the material after long-term service, is estimated from the chemical composition of the steel. Mechanical properties as a function of time and temperature of reactor service are estimated from impact energy and flow stress of the unaged material and the kinetics of embrittlement, which are also determined from chemical composition. The J{sub IC} values are determined from the estimated J-R curve and flow stress. Examples of estimating mechanical properties of cast stainless steel components during reactor service are presented. A common ``lower-bound`` J-R curve for cast stainless steels of unknown chemical composition is also defined for a given grade of steel, ferrite content, and temperature.

Application of the J‐integral concept for the description of toughness properties of fiber reinforced polyethylene composites

Abstract: For the determination of toughness properties of HDPE/glass fiber and HDPE/cotton fiber composites, an instrumented Charpy impact test has been used. The interpretation of impact load-deflection curves has been carried out with several concepts of fracture mechanics. Here the limits of linear elastic fracture mechanic (LEFM) have been shown. The change of toughness properties with increasing fiber volume can be described for short fiber reinforced composites with the help of the J-integral concept in a suitable mode. An application of the conventional Charpy impact test will result in an overestimation of material behavior because of the energy of crack propagation. With the help of a micromechanical model to describe failure processes, taking account of energy dissipative processes, it is possible to calculate fracture mechanical material parameters. Because of the peculiarities of deformation and fracture behavior, the application of elastic-plastic fracture mechanic (EPFM)-concepts for fiber reinforced polymers is required.

Effects of delta ferrite morphology on low‐temperature fracture toughness of austenitic stainless steel weld metal

Abstract: The effect of δ ferrite morphology and grain boundary property on the low temperature toughness of austenitic stainless steel SUS304L weld metal has been studied The types of δ ferrite were classified into three categories : vermicular, lacy, and globular When Charpy test were performed at low temperature, brittle fracture easily occurred in a vermicular type δ ferrite because the longitudinal direction of this δ perallel to [100]δ, that is a cleavage plainOn the other hand, brittle fracture scarcely occurred in both a globular type and a lacy type δ ferrite, because the the stress concentration must be low at globular type δ and the Kurdjumov-Sachs (K-S) relationship holds between lacy type δ and γ phase Grain boundary holding the K-S are coherence to grains and hardly cause the decohesionThe present data indicate that changing the δ ferrite morphology from a vermicular type to lacy type prevents the cleavage fracture and increases the fracture toughness at low temperature

Neutron effects on borated stainless steel

Abstract: In this paper neutron effects on the mechanical properties and the microstructures of borated stainless steel are studied by irradiating three borated stainless steel batches to different radiation levels (from 1 {times} 10{sup 13} to 1 {times} 10{sup 17} n/cm{sup 2}) Each batch includes samples varying in boron content from 025 to 201 wt% and manufactured by two different processes: a powder metallurgical and a conventional wrought technique, which meet the requirements of American Society of Testing and Materials Standard A-887 grades A and B, respectively A total of 50 tensile specimens, 81 Charpy V-notch samples, and 17 metallographic specimens are used for this purpose In general, the mechanical properties of samples manufactured by both the powder metallurgy and the wrought techniques show almost no change in mechanical properties with fluence In addition, no evidence of helium effects are observed during the investigation Further studies on helium formation in this material during irradiation are ongoing

Processing of particulate reinforced metals and intermetallics for improved damage tolerance

Abstract: A fabrication approach has been developed that substantially improves the damage tolerance of materials that would normally exhibit failure at room temperature with minimal absorption of energy. The approach, termed microstructurally toughened (MT SM ) composites, consists of controlling the composite microstructure such that continuous ductile toughening regions are incorporated throughout the composite microstructure. Results will be presented on the tensile and impact behavior of both ductile and brittle matrix composites. The ductile matrix composites consist of silicon-carbide-particulate-reinforced aluminum alloy 6061 with commercially pure titanium toughening regions. The brittle matrix composites consist of B 4 C-particulate-reinforced NiAl with type 304 stainless steel toughening regions. Order-of-magnitude increases in the notched Charpy impact energy absorption capability of MT composites have been measured relative to the baseline composites.

Multivariable modeling of pressure vessel and piping J-R data

Abstract: Multivariable models were developed for predicting J-R curves from available data, such as material chemistry, radiation exposure, temperature, and Charpy V-notch energy. The present work involved collection of public test data, application of advanced pattern recognition tools, and calibration of improved multivariable models. Separate models were fitted for different material groups, including RPV welds, Linde 80 welds, RPV base metals, piping welds, piping base metals, and the combined database. Three different types of models were developed, involving different combinations of variables that might be available for applications: a Charpy model, a preirradiation Charpy model, and a copper-fluence model. In general, the best results were obtained with the preirradiation Charpy model. The copper-fluence model is recommended only if Charpy data are unavailable, and then only for Linde 80 welds. Relatively good fits were obtained, capable of predicting the values of J for pressure vessel steels to with a standard deviation of 13--18% over the range of test data. The models were qualified for predictive purposes by demonstrating their ability to predict validation data not used for fitting. 20 refs., 45 figs., 16 tabs.