Showing papers on "Charpy impact test published in 2011"

Microstructural characteristics and toughness of the simulated coarse grained heat affected zone of high strength low carbon bainitic steel

Abstract: The correlation of microstructural characteristics and toughness of the simulated coarse grained heat affected zone (CGHAZ) of low carbon bainitic steel was investigated in this study. The toughness of simulated specimens was examined by using an instrumented Charpy impact tester after the simulation welding test was conducted with different cooling times. Microstructure observation and crystallographic feature analysis were conducted by means of optical microscope and scanning electron microscope equipped with electron back scattered diffraction (EBSD) system, respectively. The main microstructure of simulated specimen changes from lath martensite to coarse bainite with the increase in cooling time. The deterioration of its toughness occurs when the cooling time ranges from 10 to 50 s compared with base metal toughness, and the toughness becomes even worse when the cooling time increases to 90 s or more. The MA (martensite–austenite) constituent is primary responsible for the low toughness of simulated CGHAZ with high values of cooling time because the large MA constituent reduces the crack initiation energy significantly. For crack propagation energy, the small effective grain size of lath martensite plays an important role in improving the crack propagation energy. By contrast, high misorientation packet boundary in coarse bainite seems to have few contributions to the improvement of the toughness because cleavage fracture micromechanism of coarse bainite is mainly controlled by crack initiation.

Effects of acicular ferrite on charpy impact properties in heat affected zones of oxide-containing API X80 linepipe steels

Abstract: This study was concerned with effects of acicular ferrite on Charpy impact properties in heat affected zones (HAZs) of two API X80 linepipe steels containing oxides. In the one steel, Mg and O 2 were additionally added to form a larger amount of oxides than the other steel, which was a conventional X80 steel containing a considerable amount of Al and Ti. Various HAZ microstructures were obtained by conducting HAZ simulation tests under different heat inputs of 35 kJ cm −1 and 60 kJ cm −1 . Oxides present in the API X80 linepipe steels were complex oxides whose average size was 1–2 μm, and the number of oxides increased with increasing amount of Mg and O 2 . The volume fraction of acicular ferrite present in the steel HAZs increased with increasing number of oxides, and decreased with increasing heat input. When the volume fraction of acicular in the HAZ was higher than 20%, Charpy impact energy at −20 °C was higher than 100 J as the ductile fracture mode was dominant. Particularly in the steel HAZs having a larger amount of oxides, Charpy impact properties were excellent because oxides worked as nucleation sites of acicular ferrite during welding. Charpy impact properties of the HAZs could be well correlated with the volume fraction of acicular ferrite and number of oxides under different heat input conditions.

Direct metal deposition (DMD) of H13 tool steel on copper alloy substrate: evaluation of mechanical properties

Abstract: H13 tool steel powder was clad on copper alloy substrate both directly and using 41C stainless steel (high Ni steel) powder as a buffer layer by direct metal deposition (DMD). Cu–steel bimetallic die casting and injection molding tools are of high interest for reduction of cycle time by efficient heat extraction due to high thermal conductivity of copper. The mechanical properties of these bimetallic structures were investigated in terms of bond strength, impact energy and fracture toughness. The bond interfaces of these claddings showed porous and crack free transition regions. The bond strength was higher in the directly clad H13 tool steel compared to the H13 tool steel clad with 41C stainless steel as buffer layer. The fracture morphology in tensile test specimens showed ductile dimple fracture. Presence of necking just below the interface depicted the softening of substrate in heat affected zone (HAZ) during cladding. The Charpy impact energy is little higher in the 41C stainless steel buffered specimens compared to the directly clad H13 tool steel specimens but the fracture toughness results showed reduction of fracture toughness in the 41C stainless steel buffered specimens due to the low strength in the tensile test. However the fracture toughness value was in the ductile region for both deposits.

Microstructure and toughness of coarse grain heat-affected zone of domestic X70 pipeline steel during in-service welding

Abstract: The microstructures and mechanical properties of coarse grain heat-affected zone (CGHAZ) of domestic X70 pipeline were investigated. The weld CGHAZ thermal cycles having different cooling time Δt 8/5 were simulated with the Gleeble-1500 thermal/mechanical simulator. The Charpy impact absorbed energy for toughness was measured, and the corresponding fractographs, optical micrographs, and electron micrographs were systematically investigated to study the effect of cooling time on microstructure, impact toughness, and fracture morphology in the CGHAZ of domestic X70 pipeline steel during in-service welding. The results of simulated experiment show that the microstructure of CGHAZ of domestic X70 pipeline steel during in-service welding mainly consists of granular bainite and lath bainite. Martensite–austenite (M–A) constituents are observed at the lath boundaries. With increase in cooling time, the M–A constituents change from elongated shape to massive shape. The reduction of toughness may be affected by not only the M–A constituents but also the coarse bainite sheaves. Accelerating cooling with cooling time Δt 8/5 of 8 s can be chosen in the field in-service welding X70 pipeline to control microstructures and improve toughness.

Thermo-mechanical characterization of epoxy/clay nanocomposites as matrices for carbon/nanoclay/epoxy laminates

Abstract: Microstructural and thermo-mechanical characterization were performed on epoxy–clay nanocomposites, to be used as matrix for continuous carbon fiber reinforced composites, containing various amounts of clays having different hydrophilicity. XRD tests displayed that the dispersion degree of the clay lamellae was strictly correlated to their hydrophilicity, while DSC tests revealed that the crosslinking degree was negatively affected by the presence of most hydrophilic clays. Therefore, the balance between polymer–filler interaction and crosslinking degree influenced the final properties of the resulting composites. The mechanical behaviour, both under quasi-static and impact conditions, was positively affected by resin nanomodification. Fracture toughness and threshold to crack initiation under cyclic loading were also interestingly improved. While the effect of nanoclay introduction on the quasi-static tensile properties of the carbon/epoxy laminates was negligible, Charpy impact tests on nanomodified epoxy/carbon fiber cross-ply laminates evidenced slight enhancements of the elastic modulus and of the energy adsorption capacity with respect to the unfilled epoxy–carbon composites. Moreover, drop weight test on laminates evidenced improvements in energy absorption capacity due to resin nanomodification.

Correlation of austenite stability and ductile-to-brittle transition behavior of high-nitrogen 18Cr–10Mn austenitic steels

Abstract: Ductile-to-brittle transition behavior of high-nitrogen 18Cr–10Mn austenitic steels containing different contents of Ni, Mo, Cu as well as nitrogen is discussed in terms of austenite stability and associated deformation-induced martensitic transformation (DIMT). Electron back-scattered diffraction and transmission electron microscopy analyses of cross-sectional area of the Charpy impact specimens fractured at −196 °C indicated that the brittle fracture planes were almost parallel to one of {1 1 1} slip planes and some metastable austenites near the fracture surface were transformed to α′-martensite by localized plastic deformation occurring during crack propagation. Quantitative evaluation of deformation-induced martensite together with characteristics of true stress–strain and load–displacement curves obtained from tensile and Charpy impact tests, respectively, supported that DIMT might take place in high-nitrogen austenitic steels with relatively low austenite stability. The occurrence of DIMT decreased low-temperature toughness and thus increased largely ductile-to-brittle transition temperature (DBTT), as compared to that predicted by empirical equations strongly depending on nitrogen content. As a result, the increased DBTT could be reasonably correlated with austenite stability against DIMT.

Effect of Tempering on Microstructure and Mechanical Properties of Boron Containing 10%Cr Steel

Abstract: The effect of heat treatment on the microstructure and the mechanical properties of a 10%Cr steel with 0.008% boron was examined. The microstructure and the mechanical properties of this steel subjected to the normalizing were studied after tempering under different conditions. The layers of retained austenite are located along the lath boundaries. The formation of M23(B·C)6 phase having film-like shape takes place on interface boundaries of retained austenite/martensite during tempering at 525°C. As a result, the steel exhibits brittle fracture with a low value of Charpy V-notch impact toughness of 6 J/cm2. Particles of the M23(B·C)6 phase are highly resistant against the spheroidizing. The tempering at 770°C only leads to the coagulation of these particles; when the fraction of M23(B·C)6 phase significantly decreases while the fraction of M23C6 carbides increases. The tempered at 770°C steel exhibits a high value of Charpy V-notch impact toughness of 260 J/cm2. The effect of boron additives on the phase composition and the brittleness of high-chromium steels is discussed.

Effects of Sr-modification, iron-based intermetallics and aging treatment on the impact toughness of 356 Al–Si–Mg alloy

Abstract: Impact toughness as a property has been acquiring increased importance in recent years, since data regarding this property can provide a means for assessing alloy ductility under high rates of deformation. The main objective of this study is to investigate the effects of Sr-modification, Fe-based intermetallic phases and aging conditions on the impact toughness of widely used 356 alloys. The total absorbed energy was measured using a computer-aided instrumented Instron Charpy impact testing machine. Increasing the level of iron additions decreases the impact energy values of 356 alloys to a noticeable degree (~35–57%). The addition of 0.1 wt% Mn to non-modified 356 alloys seems to have no observable effect on the impact energy, while increasing the Mn addition to 0.4 wt% produces a slight improvement in the impact energy values for non-modified and Sr-modified 356 alloys compared to that of those containing only iron under the same conditions. The application of solution heat treatment in combination with Sr-modification was found to significantly improve the impact energy of as-cast 356 alloys, particularly at low iron additions. Artificial aging of non-modified and Sr-modified 356 alloys at 180 °C diminishes the impact energy values with an increase in the aging time up to 8 h compared to those obtained under the solution heat-treated conditions. On the other hand, aging at 220 °C for 12 h increases the impact energy values of Sr-modified 356 alloy containing 0.12 wt% Fe and combined 0.2 wt% Fe–0.1 wt% Mn to about 20 and 18 J, respectively. The fracture behavior of non-modified 356 alloys containing 0.12 wt% Fe is mainly controlled by the acicular eutectic Si particles whereas β-iron platelets act as crack initiation sites and provide further path for final crack propagation in non-modified 356 alloys containing 0.9 wt% Fe. The β-iron platelets and π-iron phase particles contribute largely to crack initiation and propagation in the Sr-modified 356 alloys containing 0.9 wt% Fe.

Effect of simulated welding thermal cycle on microstructure and mechanical properties of X90 pipeline steel

Abstract: The effect of welding thermal cycle simulation on the microstructure and mechanical properties of X90 pipeline steel was investigated by means of microstructure analysis, tensile- and Charpy impact-tests. At the heat input of 15 kJ/cm, the microstructure of coarse-grained heat affected zone is mainly composed of lath bainite and granular bainite, resulting in excellent strength and toughness. At 25 kJ/cm with two thermal cycles, however, strength and impact toughness decrease due to the formation of more polygonal ferrite with coarser grains.

Effects of carbon equivalent and cooling rate on tensile and Charpy impact properties of high-strength bainitic steels

Abstract: The effects of carbon equivalent and cooling rateon tensile and Charpy impact properties of high-strength bainitic steels were investigated. Eight steel plates were fabricated with varying C, Cr, and Nb additions under two different cooling rates, and their microstructures, tensile, and Charpy impact properties were evaluated. Volume fractions of microstructural components present in the steels increased in the order of granular bainite, acicular ferrite, bainitic ferrite, and martensite as the carbon equivalent or cooling rate increased, which resulted in decreased ductility and upper shelf energy and increased energy transition temperature in spite of increased strength. In the steels containing about 50 vol.% of bainitic ferrite and martensite, the tensile strength was about 900 MPa, while the elongation and upper shelf energy were about 20% and 200 J, respectively. In order to achieve the best combination of tensile strength, ductility, and upper shelf energy, e.g., 860–900 MPa, 20%, and 200 J, respectively, granular bainite, and acicular ferrite were produced by controlling the carbon equivalent and cooling rate, while about 50 vol.% of bainitic ferrite and martensite were maintained to keep the high strength.

Effect of direct quenching on the microstructure and mechanical properties of the lean-chemistry HSLA-100 steel plates

Abstract: The influence of direct quenching on structure-property behavior of lean chemistry HSLA-100 steels was studied. Two laboratory heats, one containing Cu and Nb (C:0.052, Mn:0.99, Cu:1.08, Nb:0.043, Cr:0.57, Ni:1.76, Mo:0.55 pct) and the other containing Cu, Nb and B (C:0.04, Mn:1.02, Cu:1.06, Nb:0.036, Cr:0.87, Ni:1.32, Mo:0.41, B:0.002 percent) were hot-rolled into 25 and 12.5 mm thick plates by varying finish-rolling temperatures. The plates were heat-treated by conventional reheat quenching and tempering (RQT), as well as by direct quenching and tempering (DQT) techniques. In general, direct-quench and tempered plates of Nb–Cu heat exhibited good strength (yield strength ∼ 900 MPa) and low-temperature impact toughness (average: 74 J at −85 °C); the Charpy V-notch impact energies were marginally lower than conventional HSLA-100 steel. In Nb–Cu–B heat, impact toughness at low-temperature was inferior owing to boron segregation at grain boundaries. Transmission electron microscopy (TEM) and scanning auger microprobe (SAM) analysis confirmed existence of borocarbides at grain boundaries in this steel. In general, for both the steels, the mechanical properties of the direct-quench and tempered plates were found to be superior to reheat quench and tempered plates. A detailed transmission electron microscopy study revealed presence of fine Cu and Nb (C, N) precipitates in these steels. It was also observed that smaller martensite inter-lath spacing, finer grains and precipitates in direct-quench and tempered plates compared to the reheat quench and tempered plates resulted in their superior strength and good impact toughness.

Effect of Heat Treatment on Microstructure, Mechanical Properties and Fracture Behaviour of Ship and Dual Phase Steels

Abstract: Grade A (GA) and high strength steel DH36 ship steels possessing different chemical compositions were used, and strength properties of GA steel and DH36 steel were compared. Additionally, 4 types of dual phase (DP) steels with different martensite volume fractions (MVFs) were produced from GA steel by means of heat treatment and they were compared with other steels through conducting microstructure, microhardness, tensile and impact tests. The fracture surfaces of specimens (DH36, GA and DP steels) exposed to tensile and Charpy impact tests were investigated by scanning electron microscope. Furthermore, it was found that the specimens quenched from 800 and 900 °C had better strength than DH36 steel. The tensile test results indicated that the tensile strength of DP steel water quenched from 900 °C was 3 times that of GA steel and twice that of DH36 steel.

Martensitic stainless steel AISI 420—mechanical properties, creep and fracture toughness

Abstract: In this paper some experimental results and analyses regarding the behavior of AISI 420 martensitic stainless steel under different environmental conditions are presented. That way, mechanical properties like ultimate tensile strength and 0.2 percent offset yield strength at lowered and elevated temperatures as well as short-time creep behavior for selected stress levels at selected elevated temperatures of mentioned material are shown. The temperature effect on mentioned mechanical properties is also presented. Fracture toughness was calculated on the basis of Charpy impact energy. Experimentally obtained results can be of importance for structure designers.