Microstructure and property of a medium carbon steel processed by equal channel angular pressing
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Abstract:
Equal channel angular pressing (ECAP) has been widely applied for grain refinement in both ultra-low and low carbon steels. However, ECAP is significantly more difficult in stronger medium carbon steels. In the present study, the microstructures resulting from ECAP at 600°C for up to four passes in a medium carbon steel with ∼0.4 wt% C were examined. The original lamellar cementite was deformed and broken during ECAP, especially after 4 passes, into nano-sized particles. The steel was hardened with increasing number of ECAP passes, and the spheroidisation process was significantly accelerated during subsequent annealing. © Institute of Materials Engineering Australasia Ltd.Keywords:
Carbon steel
Carbon fibers
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The properties of ultra-fine grained materials are superior to those of corresponding conventional coarse grained materials, being significantly improved as a result of grain refinement. Equal channel angular pressing (ECAP) is an efficient method for modifying the microstructure by refining grain size via severe plastic deformation (SPD) in producing ultra-fine grained materials (UFG) and nanomaterials (NM). The grain sizes produced by ECAP processing are typically in the submicrometer range and this leads to high strength at ambient temperatures. ECAP is performed by pressing test samples through a die containing two channels, equal in cross-section and intersecting at a certain angle. The billet experiences simple shear deformation at the intersection, without any precipitous change in the cross-section area because the die prevents lateral expansion and therefore the billet can be pressed more than once and it can be rotated around its pressing axis during subsequent passes. After ECAP significant grain refinement occurs together with dislocation strengthening, resulting in a considerable enhancement in the strength of the alloys. A commercial AlMgSi alloy (AA6063) was investigated in this study. The specimens were processed for a number of passes up to nine, using a die channel angle of 110°, applying the ECAP route B C . After ECAP, samples were cut from each specimen and prepared for metallographic analysis. The microstructure of the ECAP-ed and as-received material was investigated using optical (OLYMPUS – BX60M) and SEM microscopy (TESCAN VEGA II – XMU). It was determined that for the as-received material the microstructure shows a rough appearance, with large grains of dendritic or seaweed aspect and with a secondary phase at grain boundaries (continuous casting structure). For the ECAP processed samples, the microstructure shows a finished aspect, with refined, elongated grains, also with crumbled and uniformly distributed second phase particles after a typical ECAP texture.
Severe Plastic Deformation
Texture (cosmology)
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Elongation
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Cast and rolled Al were deformed by equal channel angular pressing (ECAP).Microstructure before and after ECAP were observed and hardness of the materials were measured.The grains in the materials could be remarkably refined by ECAP and there is no obvious difference between the deformed microstructure of specimens which had different initial states.The Vickers hardness of the materials increased after 1st pass during ECAP process and the increasing trend become slower in the following passes.The initial states have no obvious impact on the microstructure and mechanical properties of the materials during ECAP process.
Vickers hardness test
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This paper examines the effect of equal-channel angular pressing (ECAP) on creep behaviour of pure aluminium, binary Al-0.2wt.%Sc alloy and ternary Al-3wt.%Mg-0.2wt.%Sc alloy. The ECAP was conducted at room temperature with a die that had a 90° angle between the channels and 8 repetitive ECAP passes followed route BC. Constant stress compression creep tests were performed at 473 K and stresses ranging between 16 to 80 MPa on ECAP materials and, for comparison purposes, on the initial coarse-grained materials. The results showed that the creep resistance of the ECAP processed Al-Sc and Al-Mg-Sc alloys was markedly deteriorated with respect to unpressed coarse-grained materials.
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An I-phase containing Mg-8Sn-6Zn-2Al (wt %; TZA862) alloy was fabricated and subjected to different number of passes of equal channel angular pressing (ECAP) processing at 300 °C. The results showed that the alloys exhibited a bimodal microstructure, which consisted of fine dynamically recrystallized (DRX) grains and coarse non-DRX grains. When increasing the number of ECAP passes from 2 to 6, the fraction of DRX grains and the dispersed second phase particles subsequently increase. However, the fraction and particles then decrease once the number of ECAP passes increases to 8. After 6 ECAP passes, remarkable grain refinement was achieved and increasing the number of passes to 8 cannot further refine the microstructure. Furthermore, the alloys having undergone ECAP exhibited a strong ED-tilted texture, the intensity of which increased with an increase in the number of ECAP passes. The ultimate tensile strength (UTS; 338 MPa) and elongation (El.; 14.2%) of the alloy processed with 6 ECAP passes were considerably higher compared to those of the other materials that had undergone ECAP. These significant enhancements were attributed to extensive grain boundary strengthening, precipitation strengthening and a higher work-hardening capacity.
Elongation
Work hardening
Hardening (computing)
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Al-0.6%Cu alloy processed by equal channel angular pressing (ECAP) was investigated in this study. The samples processed with 1, 2 and 4 ECAP passes were annealed at different temperatures. And the microstructures of the Al-0.63%Cu alloy samples were observed. The results show that the grains of the alloy are refined to sub-micron level after multipass ECAP. Moreover, it is found that the recrystallization temperature of the Al-0.6%Cu alloy samples decreases with increasing the number of ECAP passes. Based on the study above, effect of cold rolling reduction and ECAP passes on recrystallization temperature of materials should be further investigated.
Recrystallization (geology)
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Elongation
Ductility (Earth science)
Severe Plastic Deformation
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Equal channel angular pressing was successfully carried out on the solution-annealed steel with two passes at room temperature through the route C. Influences of ECAP multi-passes on microstructure and mechanical properties of T250 maraging steel were studied with metalloscope, TEM and mechanical property test. The results show that a well grain refinement can be achieved by ECAP forming. The martensite lath is elongated to more narrow bands, and the width is decreased from 0.8 μm to 0.2 μm after two passes ECAP deformation. Density of dislocations in lath is increased and distortions are serious. After two passes ECAP deformation,the microhardness is enhanced about 24.7 % from 336.3 HV to 419.8HV, and the growth range of one pass is significantly greater than two pass.
Lath
Maraging steel
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A commercial AZ31 Mg alloy was subjected to ECAP (equal-channel angular pressing) and the tensile properties were examined at room temperature. It is shown that the microstructure before ECAP significantly affects the performance of ECAP without breaking samples. When the initial structure before ECAP is homogeneous with equiaxed grains, subsequent ECAP is feasible at lower temperatures. The grain refinement is achieved more effectively as the temperature for ECAP is lowered and the number of ECAP pass is increased. The tensile ductility was reduced after 1 pass of ECAP at 473 K or 498 K. However, the ductility was improved with an increasing number of ECAP pass or by annealing at 473 K or 523 K after ECAP. The results demonstrate that a homogeneous distribution of fine equiaxed grains is important for ductility improvement in the AZ31 Mg alloy.
Equiaxed crystals
Ductility (Earth science)
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Acceleration of spheroidisation in a medium carbon steel processed by equal channel angular pressing
A medium carbon alloy steel was processed using equal channel angular pressing (ECAP) with a view to accelerating the softening process during subsequent annealing. The steel was pressed at 600°C for up to four passes. The original well-aligned lamellar cementite became bent and discontinuous with increasing number of passes. Complete spheroidisation was achieved after annealing at 720°C for only 30 minutes, resulting in the same hardness as that obtained after 72 hours without ECAP. This can potentially lead to significant cost reduction and environmental benefit for the industry.
Carbon steel
Severe Plastic Deformation
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