Continuous Cooling Heat Treatment Research Articles

Microstructural engineering by heat treatments of multi-principal element alloys via spinodal mediated phase transformation pathways

Nanoscale multi-phase microstructures observed in multi-principal element alloys (MPEAs) such as AlMo0.5NbTa0.5TiZr, Al0.5NbTa0.8Ti1.5V0.2Zr, TiZrNbTa, AlCoCrFeNi and Fe15Co15Ni20Mn20Cu30 that exhibit promising mechanical or functional properties may have evolved through spinodal-mediated phase transformation pathways (PTPs). The microstructures in such MPEA systems could be further engineered for targeted applications by appropriately designing the alloy composition and heat-treatment schedule. In this study, we investigate systematically how different heat treatment schedules such as single-step isothermal aging, two-step isothermal aging and continuous cooling alter the interplay among the various factors associated with alloy composition, such as volume fraction of individual phases, lattice misfit and modulus mismatch between the co-existing phases. We have determined the degree to which these factors influence significantly the spinodal-mediated PTPs and the corresponding microstructures by use of high-throughput phase-field simulations. In particular, we demonstrate that the microstructural topology (i.e., which phase forms the continuous matrix and which phase forms discrete precipitates) in the same MPEA having an asymmetric miscibility gap could be inverted simply by a continuous cooling heat treatment. Further, we reveal a rich variety of novel hierarchical microstructures that could be designed using two-step isothermal aging heat treatments in MPEA systems with symmetric or asymmetric miscibility gaps. These simulation results may shed light on novel microstructure design and engineering for the above-mentioned MPEA systems.

Two-Step Continuous Cooling Heat Treatment Applied in a Low Carbon Bainitic Steel

Thermo-mechanical treatments using continuous cooling after forging are an established method for producing bainitic steels, mainly because of the elimination of energy intensive additional heat treatment processes. The cooling is usually employed in an uncontrolled manner in the industrial sector, which can be detrimental to the resulting microstructural morphology and, consequently, to the final product properties. In this study, a new controlled two-step cooling route based on the principles of bainitic displacive growth was designed and applied in a 0.18C (wt-%) steel. Inverse finite element method was used on the cooling data to obtain the evolution of temperatures for the samples during cooling, allowing to assess point to point cooling rates. Investigations via X-ray diffraction, optical microscopy analysis and hardness testing revealed a variation of bainitic morphology, namely, the transition from granular bainite to lath-like bainite with relatively high hardness and constituents/phase boundaries than the pre-treated microstructure.

Development of Dual Phase Microstructure in Medium Carbon Low Alloy Steel by Continuous Cooling Heat Treatment

Development of Dual Phase Microstructure in Medium Carbon Low Alloy Steel by Continuous Cooling Heat Treatment

Effect of Mn content on low-cycle fatigue behaviors of low-carbon bainitic steel

The microstructures and low-cycle fatigue behaviors of steels with different Mn contents subjected to continuous cooling heat treatment processes (from Ms +10°C to Ms −20°C) are examined. Phase transformation testing results show that the steel without Mn exhibits short transformation time and short incubation period. When Mn content increases from 1.8% to 3.2%, bainite transformation time is prolonged, especially incubation period. The microstructure of the steel without Mn is mainly grain boundary allotriomorphic ferrite, while the other three steels with Mn consist of bainitic ferrite plate and retained austenite. Mn is an essential element used to obtain lower bainite. The low-cycle fatigue of the steels undergoes three stages, namely, cyclic hardening, saturation or cyclic softening, and fracturing. Higher elongation of the steel without Mn is beneficial to improve fatigue life under plastic strain amplitudes. The Coffin–Manson formula and damage hysteresis model are then used to evaluate the fatigue performances of the steels. It shows that the steel with 2.3% Mn exhibits higher fatigue damage capacity (W0) with a considerable damage transition exponent (β) than the other three steels. This result attributes to the largest distribution of high-angle misorientation of in the bainitic steel with 2.3% Mn.

Heat treatment analysis of multiphase steels through the use of a coupled phase field and finite element model methodology

Steel is widely used due to its remarkable mechanical properties. Heat treatments are applied in steels to enhance these properties, affecting the microstructure. Since little is known concerning phase interactions during heat treatments, a model of the microstructural evolution of steel is considered. The austenite to pearlite phase transformation in eutectoid steel when subject to non-isothermal continuous cooling was simulated. To predict the thermomechanical behaviour of each phase during continuous cooling, a coupled multiphase field and finite element models are used. The information from each phase of the microstructure is then used in a finite element commercial code, where the materials’ thermoelastoplastic constitutive equations are applied though a user routine for each microstructure constituent. Periodic boundary conditions are used in the representative unit cell. The thermomechanical behaviour of each phase is numerically taken into account. The evolution of von Mises stress and strain is analysed for each phase. Quantitative results for the thermomechanical behaviour of the microstructure during continuous cooling heat treatment are achieved and this study provides an understanding of the relation between microstructure and thermal and mechanical behaviour of the different phases of the microstructure.

Effect of continuous cooling heat treatment on interface characteristics of S45C/copper compound casting

The bonding of an S45C steel insert to copper during cast welding and continuous cooling treatment (including: furnace cooling; air cooling; oil quenching and water quenching) was investigated. The interface shear strength was determined using a push-out test. A cast welding layer formed between S45C steel and copper. After continuous cooling heat treatment, a cast welding layer was present near the S45C steel, an irregular layer near the copper matrix, and a middle layer between these two. X-ray diffraction analysis was used to determine that the interface layer consisted of carbon and CuFeO2.Electron probe microanalysis (EPMA) demonstrated that most iron atoms and carbons diffused into the copper matrix. The interface shear strength of the compound casting under furnace-cooling was the largest and that of the compound casting formed by water-quenching was the smallest, and all fractures in the cast welding layer occurred near the S45C steel matrix.

The Effect of Continuous Cooling Heat Treatment on Interface Properties of SK3/Copper Compound Casting

The bonding of an SK3 steel insert to copper during cast welding and heat treatment was studied. The interface shear strength was made with a push-out test. After cast welding, a cast welding layer formed between SK3 steel and copper. After a continuous cooling heat treatment, there was a cast welding layer near the SK3 steel matrix, an irregular layer near the copper matrix, and a middle layer between them. X-ray diffraction analysis was used to determine that the interface layer consisted of carbon and CuFeO2. Through electron probe x-ray microanalysis (EPMA), it was shown that mainly iron atoms and carbons diffused into the copper matrix. The interface shear strengths of the compound casting while with water quenched, oil quenched, air cooled, and furnace cooled, were 23.42, 18.74, 12.29, 13.43, and 8.33 MPa, all of heat treatment fractures in the cast welding layer near the SK3 steel matrix.