Papers by Keyword: Grain Refinement

Abstract: Grain refinement should increase strength of metallic materials in a predictable manner. However, in applications of severe plastic deformations for this purpose, limits have been observed due to self-recovery and strain-induced precipitation assisted by generation of adiabatic heat. Pure metals and single-phase alloys have not been the best candidates for achieving ultrafine-grained microstructures therefore more often precipitation-hardening multi-phase alloys have been used in SPD experiments. To generate ultrafine-grained microstructures by accumulated multiple compressive strains executed at various strain rates during programmed thermal cycles the MaxStrainTM device was developed for GleebleTM physical simulator. This paper deals with processing of Al-6061 wrought alloy and Al-319 cast alloy by the MaxStrain device, and describes obtained microstructures.

Abstract: This study evaluated the microstructure and mechanical properties enhancement of cross roll rolled Ni-10Cr alloy, comparing with conventional rolled material. Cold rolling was carried out to 90% thickness reduction and subsequently annealed at 700 °C for 30 min to obtain the fully recrystallized microstructure. Particularly, cross roll rolling was carried out at a tilted roll mill condition of 5º from the transverse direction in RD – TD plane. Application of cross roll rolling on Ni-10Cr alloy contributed to the notable grain refinement due to higher effective strain than that of conventional rolling, consequently, average grain size was refined from 135 µm in initial material to 4.2 µm in cross roll rolled material. Furthermore, //ND texture in CRR material was well developed than that of CR material, which contributed to the mechanical properties and formability enhancement.

Abstract: Aiming at the disadvantages of low utilization ratio of steel ingot, uneven microstructure properties and long production period in the solid steel ingot forging process of heavy cylinder forgings such as reactor pressure vessel, a new shortened process using hollow steel ingot was proposed. By means of modeling of lead sample and DEFORM-3D numerical simulation, the deformation law and grain refinement behavior for 162 ton hollow steel ingot upsetting at different reduction ratios, pressing speeds and friction factors were investigated, and the formation rule of inner-wall defects in upsetting of hollow steel ingots with different shape factors was further analyzed. Simulation results show that the severest deformation occurs in the shear zone of meridian plane in the upsetting process of hollow steel ingot, and the average grain size in the shear zone is the smallest. As pressing speed increases, the forming load gradually increases and the deformation uniformity gets worse, while the average grain size decreases. An increase in friction factor can increase the peak value of effective strain, but it significantly reduces the deformation uniformity, increases the forming load and goes against grain refinement. Moreover, the four kinds of defects on the inner wall of steel ingot can be eliminated effectively by referring to the plotted defect control curve for hollow steel ingot during high temperature upsetting.

Abstract: In current research the effect of the strain-induced melt activation (SIMA) process on the microstructure of 6061 Al alloy has been investigated. The optimum amount of Ti for proper grain refining was found to be 0.03 wt.% in the alloy. In SIMA process, after hydraulic pressing, recrystallization and partial melting (RAP) were employed to obtain a fine globular microstructure. Certain amount of strain (40%), heat treatment time (30 min) and temperatures (605-645^°C) were employed to find an optimized fine globular microstructure of the alloy. A microstructural study which was carried out by optical microscopy, exhibited the uniform equiaxed recrystallized grain structure.

Abstract: The effects of forced convection on grain refinement of S32205 duplex stainless steel (DSS) were investigated using a continuous casting simulation setup. The sample, which was called slab simulating unit here, was solidified in the continuous casting simulation setup, whose temperature was controlled according to the cooling curve of a DSS slab center, with the bottom cooled by running water to simulate the solidification process of slab. Forced convection was generated by square-wave mechanical stirring. The CET (columnar to equiaxed transition) was gradually promoted by forced convection when the rotation of the crucible speeded up, the microstructure was refined significantly. The average columnar zone length was reduced from 8.2 cm to 4.8 cm, the equiaxed grain average size decreased from 9.5 mm to 1.7 mm, and the microstructure was refined from Widmänstatten plates to island-shaped pieces.

Abstract: Plates, 6.3 mm thick, of 2205 duplex stainless steel (DSS) were gas metal arc welded (GMAW) under the application of an axial magnetic field (0 to 15 mT) with an ER-2209 filler wire using a gas mixture of 98%Ar + 2%O₂. Microstructural characterization of the welds revealed that electromagnetic stirring (EMS) increases the content of austenite in both weld center and high temperature heat affected zone (HTHAZ). It induced a grain refining effect during freezing of the ferritic matrix which in turn enabled more sites for nucleation of austenite. This mechanism of solidification was reflected in an increase in the mechanical strength of the welds. Besides, the extent of the HTHAZ was reduced and its microhardness increased when applying the external magnetic field. It is believed that the vibration induced in this region favored the regeneration of austenite in the ferritized HTHAZ enhancing the balance of phases.

Abstract: The grain size, recrystallization, phase transformation and mechanical properties of a cold-rolled high-strength steel (HSS) are studied after annealing with high (~140°C/s) and ultra-high (~1500°C/s) reheating rate, followed by subsequent water quenching without isothermal soaking. By monitoring the hardness and microstructure, it was shown that the increase of the reheating rate from 140°C/s to 1500°C/s causes grain refinement from 5 µm to 1 µm in diameter and the final ferrite grain size depends significantly on the reheating temperature and reheating rate. It was observed that after an extreme reheating rate of ~1500°C/s the α-γ phase transformation starts before the completion of recrystallization in the recovered matrix. The crystallographic texture of the ultrafast reheated and water-quenched high-strength steel inherits the cold-rolled deformation texture with well pronounced RD and ND texture fibres, even after the α-γ-α′ phase transformations. It was found that the ultrafast reheating results in a very fine non-equilibrium ferrite-martensite structure with an excellent ultimate tensile strength of ~1400 MPa and an acceptable elongation at fracture. The observed data are very promising from industrial application point of view and open up possibilities for further structural refinement and alternative texture control.

Abstract: Single pass and double-pass friction stir processing was carried out on commercially pure aluminium at a rotation speed of 640 rpm and traverse speed of 150 mm/min and a detailed electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) analysis was carried out to understand the microstructure developed. The grain size was refined substantially after the first pass whereas there was no significant change in the grain size after the second pass. This indicates that the final grain size after friction stir processing does not depend on the starting grain size. The equiaxed fine grains were formed by dynamic recrystallization process as revealed by EBSD analysis. TEM observations showed banded contrast across the grain boundaries indicating grain boundaries to be in equilibrium. Free dislocations observed inside grains after the first pass were well arranged into subgrain boundaries after the second pass. EBSD also revealed some variation in microstructural features such as grain size, texture index, grain orientation spread and grain average misorientation across the surface and also in the cross section of the stir zone both after single and double pass.

Abstract: The Strain-Induced Dynamic Transformation (SIDT) is an efficient way to overcome the limitation of grain refinement during the conventional thermomechanical controlled processing (TMCP) of steels. The present study deals with the effects of austenite morphology on the SIDT in microalloyed and IF steels. The discussion of the processing route in terms of chemical composition, deformation schedules, heating and cooling conditions is carried out by the means of torsion tests with deformation in metastable conditions. As it was expected, the microstructure of microalloyed steel was clearly controlled by the microalloying elements what, in turn, directly affectedthe SIDT products. Examination of water-quenched microstructures, just after deformation, revealed the morphology of "strain-induced ferrite". The kinetics of SIDT were observed and analyzed using the strain-stress curves. It is shown that presence of strain-induced precipitations in microalloyed steels accelerates kinetic of SIDT - by reducing the amount of Nb in solution. The changes in ferrite refinement of the experimental steels were explained from the view of the austenite morphology and processing parameters.

Abstract: The aim of the present study was to understand recrystallisation behaviour of a cold rolled AISI 316L austenitic stainless steel when annealed repetitively for short durations. The results were compared with isothermal annealing process. The evolving microstructure was examined by electron microscope. Electron backscattered diffraction was performed in a FEI NANO SEM 430 Field Emission Gun scanning electron microscope using an EDAX/TSL high-speed Digiview camera for Kikuchi pattern acquisition. Also progress of recrystallisation was assessed by hardness measurements at different annealing conditions. The study revealed distinct differences in the progress of recrystallisation of repeated and isothermally annealed specimens. Also the formation of ultrafine grained microstructure by repeated annealing process was noted.