Papers by Keyword: Warm Deformation

Abstract: The microstructure evolution and grain-refining mechanism in the process of ultrafine grain preparation with combination of different heat treatments and warm deformation were studied. 5140 steel，medium carbon low alloy steel, was used in this experimental and divided into three groups, which is one time quenching state, three times cyclic quenching state and original state without any heart treatment as the control group. All the samples of above three groups hold at 600°C for 5 min, then warm deformation was carried out immediately in YJ-450 Four-Pillar Hydraulic Machine. The deformation amount of above samples is controlled beyond 50%. The result shows that microstructure of the medium carbon steel samples after once quenching treatment and three times cyclic quenching treatments was significantly refined after warm deformation. The grain-refining mechanism of supersaturated single-phase ferrite in warm deformation is that new dislocation boundaries are produced continuously through dislocation cross-slip, which segments and refines the original grains. The grain size of three times quenching state after warm deformation is obviously finer than the grain size of one time quenching state after warm deformation under the same condition. And quenching times is a very significant factor to the extent of fining grain after warm deformation. Through three times cyclic quenching treatments and warm deformation, the size of grains has reached nano-scale.

Abstract: Effect of annealing temperature on microstructures and properties of warmly deformed SCRAM (Super-clean Reduced Activation Martensitic) steel on Gleeble-3500 thermo-simulation machine was investigated. The results showed that an increase in the annealing temperature can result in increasing the martensitic lath width from 0.48 um to 0.65 um and decreasing the dislocation density from 6.4×10¹⁵ m^-2 to 2.8×10¹⁵ m^-2 in SCRAM steel. The specimen exhibited high reduction of area and total elongations when the annealing temperature is up to 600 oC. The tensile fracture surface observation indicated that dimples became more uniform and deeper and cleavage fracture traces disappeared with the annealing temperature increasing. The irradiation-induced helium bubbles and hardening were observed in all the specimens after helium implantation to 1e + 17/cm² at 450 oC. The helium bubbles became larger but less when the annealing temperature increased. The optimal annealing temperature is 450 oC in this experiment.

Abstract: The SCRAM steel was processed by warm deformation on Gleeble-3500 thermo-simulation machine. The effect of strain on the microstructures and mechanical properties of SCRAM steel was investigated. The results show that an increase in the strain can result in refining the martensitic laths, increasing the volume fraction of precipitates and the dislocation density in SCRAM steel. The martensitic lath width decreases from 0.83 μm to 0.48 μm and the dislocation density increases from 1.3 × 10¹⁵ m^-2 to 6.4 × 10¹⁵ m^-2 in SCRAM steel. The specimen exhibits high ultimate tensile strength and yield strength but low reduction of area and total elongations when the strain (ε) is up to 0.5. The tensile fracture surface observation indicates that dimples become smaller and shallower while tear ridges drastically grow up with the strain increasing.

Abstract: Combining phase transformation with deformation effectively, a new preparation process of ultra-fine grain medium-carbon steel using martensitic steel was developed. In this research, quenched steel was heated to 500~600 °C quickly, and tempered for an appropriate time to retain a certain degree of supersaturation. Then quick warm deformation was implemented. By the action of twinning, dislocation decomposition, recrystallization, etc., which realizes dislocation motion on slip planes to form dislocation forests or cross-slip, supersaturated ferrites are refined quickly. Strain-induced cementite particles precipitate dispersively in grain boundaries or sub-grain boundaries, and uniform and stable structures are obtained.

Abstract: The warm compression experiment of medium carbon steel was conducted using the Gleeble-3500 thermal/mechanical simulator system. By the experiment, the warm deformation of medium carbon steel was studied within the temperature (500~700°C) and the strain rate (0.001~10s-1). The results indicate that the flow stress was increasing with the lowering temperature and the higher strain rate. And the stress-strain curves could be divided into four parts, including four stage of the Strain-Hardening, the First Softening, the Strong Softening, and the Steady Deformation. Dynamic recovery softening has little effect on the flow stress. The peak stress was caused by kink and fracture of the lamellar cementite. Strong softening stage was longer than other one, while its softening influence was stronger compared with hot deformation.

Abstract: Microstructure evolution and mechanical behavior of alpha/beta Ti-6Al-4V titanium alloy with initial α-colony microstructure during uniaxial compression at 600 and 800°C to a height strain of 70% were studied. It was shown that decrease in deformation temperature considerably influences on the kinetics of globularization of a lamellar microstructure. At the lower temperature stages of strengthening and softening extend that associates with inhibition of globularization. Deformation at 600°C is also associated with a smaller fraction of high-angle boundaries during deformation, smaller fraction of globular grains, increased contribution of shear deformation and more intensive rotation of α-lamellae towards the metal flow direction. In contrast to 800°C, the rate of thinning of α-lamellae at the lower temperature is noticeably higher. The results obtained are related to the change of the type of dislocation slip in α-lamellae due to inhibition of dynamic recovery with decreasing deformation temperature.

Abstract: To Understand the Mechanisms of Accelerated Dynamic Recrystallization Behavior during the Warm Deformation of Martensites, the Tempered Lath Martensite of 0.4C Steel (Fe-0.399%C-1.96%Mn in Mass %) Was Deformed at 650 °C in Compression to Different Reductions, and Microstructural Evolution Was Investigated. During the Deformation, an Initial Lath Martensite Structure with a Complicated Morphology Was Gradually Changed into More Equiaxed Structure. After 50% Reduction and above, an Equiaxed, Fine Grained Structure Mainly Surrounded by High-Angle Boundaries Was Uniformly Formed with Dislocation Substructures, where the Dislocation Density in the Grains Is Relatively Low. Since there Was No Significant Boundary Migration during this Process, this Microstructural Evolution Can Be Termed as Continuous Dynamic Recrystallization.

Abstract: In the current study, a novel approach was employed to produce a unique combination of ultrafine ferrite grains and low temperature bainite in a low carbon steel with a high hardenability. The thermomechanical route included warm deformation of supercooled austenite followed by reheating in the ferrite region and then cooling to bainitic transformation regime (i.e. 400-250°C). The resultant microstructure was ultrafine ferrite grains (i.e. <4μm) and very fine bainite consisting of bainitic ferrite laths with high dislocation density and retained austenite films. This microstructure offers a unique combination of ultimate tensile strength and elongation due to the presence of ductile fine ferrite grains and hard low temperature bainitic ferrite laths with retained austenite films. The microstructural characteristics of bainite were studied using optical microscopy in conjunction with scanning and transmission electron microscopy techniques.

Abstract: The warm deformation behavior of as-quenched and as-annealed ASTM 1045 steel was studied by isothermal compression testing on a Gleeble3500 machine. The temperature range was 550-700°C and the strain rate range 0.001-0.1s^-1. Transmission electron microscopy (TEM) was used to study the microstructures associated with the observed deformation phenomenons. The results show that the flow stress of quenched specimens is higher than that of annealed ones at 550°C when strain rates are greater than 0.001s^-1. However, at 600-700°C and strain rate of 0.001s^-1, the whole flow curves of quenched specimens are below that of annealed ones. Under the rest conditions, the flow stress of quenched specimens is higher at the beginning of compression and then the opposite is true after the strain is greater than a critical value. The microstructure examination proves that the dynamic recrystallization easily occurs in quenched specimens during warm compression, which results in the above phenomenons. Keywords: warm deformation, flow stress, steel, quenching, annealing

Abstract: In order to produce nano-structured carbon steel, a 0.45%C steel was quenched and warm-compressed on a Gleeble 3500 Machine. The microstructural evolution during the process was studied by using an optical microscope and a transmission electron microscopy. The starting microstructure was lath martensite with a small amount of flake martensite. A lot of high-angle boundaries between martensite laths were induced after 50% reduction compression at 350°C. The microstructure of the specimen compressed at 600-650°C was nano-carbides + equiaxed ultrafine ferrite grains. The mechanism for grain refinement is incomplete dynamic recrystallization.