Papers by Keyword: Cooling Rate

Abstract: ER70-Ti is a high strength gas shielded welding wire steel, which is suitable for ships, bridges and other structures, and can be used for thick plate welding with high current. In the welding wire industry, ER70-Ti is a high-tech deep-processing product with high added value. In this study, the thermal expansion experiment of ER70-Ti wire rod was carried out. The critical temperature of ER70-Ti phase transformation was measured and the continuous cooling transformation curve (CCT curve) of undercooled austenite was drawn. The microstructure and hardness of the samples under different cooling rates were observed. The results show that A_c1 temperature of ER70-Ti sample was 690 °C, A_c3 temperature was 877°C, and B_s temperature was 575°C. When the cooling rate was low (0.1°C/s~2.5°C/s), the phase transformation products of ER70-Ti were equiaxed polygonal ferrite and granular bainite. With the increase of cooling rate, the grain size of ER70-Ti sample was refined and the bainite content increased from 53% to 85%. When the cooling rate was higher than 5°C/s, all the phase transformation products were bainite. The Vickers microhardness also increased with the increase of cooling rate, from 185HV to 325HV.

Abstract: Controlled cooling rate is essential in steel production in order to obtain the desired grades for specific mechanical properties. Optimal control of cooling process parameters is important to obtain the desired cooling rate. The system level uncertainty around the cooling process, the model form error around the generative model for the cooling process as well as the measurement noise make the problem of optimal cooling even more challenging. Machine learning approaches have been used in the recent past to solve optimization and optimal control problems. The present study sets out to design an optimal and robust cooling rate controller using a data-driven approach within a machine learning framework which accounts for the uncertainties inherent in the system. A Gaussian process regression model is developed to predict the cooling rate using temperate-time data and two simulated latent parameters with a suitable confidence interval. The experiments have been undertaken on a laboratory scale Run Out Table setup. The results show the suitability of the proposed approach to obtain a robust response surface of the cooling rate with the process parameters.

Abstract: Due to the temperature and concentration determine the kinetic undercooling of interface growth and nucleation undercooling inside the melt, they play an important role in the solidification microstructure of the alloy. In this paper, the effect of temperature gradient and cooling rate on the dynamic undercooling was studied and the mechanism of the concentration at the solid-liquid interface on the kinetic undercooling during the continuous cooling process was analyzed. A calculation method for the coupling of temperature and concentration during Inconel 718 alloy solidification was developed, which can solve the problem that the concentration and temperature are difficult to be calculated at the same time in the numerical calculation.

Abstract: After solid solution treatment at 1335°C for 4 hours and cooling to room temperature at different rate, the nickel-based single crystal superalloy were made into three kinds of nickel-based single crystal superalloy materials containing different size γ′ phases, respectively. The tensile test of I-shaped specimens was carried out at 980°C, and their effect of γ′ phase microstructure on the tensile properties was studied. The results show that the yielding strength of the material air-cooled to room temperature was lower than that with cooling rate at 0.15°C/s, but both of them were lower than the yielding strength of original material. Little difference was found on the elastic modulus of I-shaped specimens made of three kinds of materials. When the cubic degree of the γ′ phase is higher and the size is larger, the tensile properties of the material is better, which can be attributed to the larger size and narrower channel of the matrix phase that lead to higher dislocation resistance.

Abstract: Titanium nickelide (nitinol) is of great applied interest in various industries due to unique combination of its physical and mechanical characteristics. In the present work, we consider the possibility of obtaining nitinol with mesoporous structure by rapidly cooling the molten sample to room temperature. Based on molecular dynamics simulation data, it was shown that the rapid cooling of the nitinol melt leads to formation of a porous structure. It was shown that the inner pore wall is formed mainly by titanium atoms, which provide biocompatibility of nitinol. It was found that the porosity of nitinol weakly depends on the cooling rate, while the porosity increases linearly with decreasing melt density.

Abstract: Fe-Ni based superalloys have been widely used in land-base gas turbine application. The turbine blade was in service for 50,000 h at high temperature and stresses. When subjected to long-term exposure at high temperature, the microstructure lost its best mechanical properties due to the microstructural instability. The aim of this research is to understand the effect of cooling rate on gamma (γ) grain size and gamma prime (γ’) particle size, morphology, and its volume fraction in rejuvenated Fe-Ni based superalloys. The alloys were solutionized above the γ’ solvus temperature at 1125 °C for 2 h for homogenization and cooling to room temperature at different cooling rates. The alloys were experienced with furnace cooling, air cooling, oil quenching, and water quenching. Microstructural analyses were investigated. Grain size, morphology, volume fraction of γ’ precipitates were investigated. Preliminary mechanical properties such as microhardness was conducted.

Abstract: In metastable beta Ti alloys, microstructural features can be varied over a wide range of length scales by changing different heat treatment parameters. Effect of cooling methods on microstructure and mechanical properties of Ti-3537 alloy after solution treatment was investigated. The result shows that with the decrease of cooling rate, the Vickers hardness of the alloy gradually increases. Among the three cooling methods of OQ, AC and FC, Ti-3573 alloy has the best shape and moderate yield strength, but tensile strength. The fractography of the β-substrate specimens showed that the fracture mode was ductile fracture. In the FC state, the α phase precipitates in a large amount in the Ti-3573 alloy, the yield strength and the tensile strength are greatly increased and the elongation is remarkably lowered. The tensile fracture shows a shallow fracture dip with low toughness.

Abstract: The effect of cooling rates during a double stage solution treatment (DSST) on the volume fraction of the massive phase (α_m) in Ti-6Al-4V alloy was successfully confirmed in the present study. The morphology of Ti-6Al-4V alloy depends on the cooling rates during the cooling from the β region. The α_m, which has a transformation characteristic between martensite (α′) and α diffusion, is reported to be a potential method for obtaining a fine lamellar α/β by thermal decomposition. The different fraction of α_m was found after DSST with the first stage was conducted above the β-transus temperature at 1050 °C, followed by second annealing at different temperatures in the α+β region. It was found that the formation of α_m exists in a specific temperature region. A longer period in this region, which was calculated based on different cooling rates during DSST, will increase the fraction of α_m in the specimen. All specimens after DSST contain α_m with the α width of approximately 1μm and white-dot particles, which is predicted to be V-enriched precipitates. The DSST can be a potential method for producing a high fraction of α_m, which can be thermally decomposed into a fine lamellar α/β, introducing a Ti-6Al-4V alloy with superior mechanical properties.

Abstract: The alteration in phase morphology of Ti-6Al-4V alloy fabricated using directed energy deposition (DED) was investigated in this study. Owing to the fast cooling rate during DED, the specimen exhibited the diffusionless transformation products of martensite (α′) and massive (α_m) phases. In the top layer, the α′ exhibited a needle-like morphology with the width of approximately 0.94 μm. Meanwhile, the α_m presented a lamellar structure with α thickness of nearly 0.98 μm. In contrast, the morphology of α′ and α_m started to decompose into α+β phase in the bottom layer. Furthermore, the hardness values increased with higher deposition layers. These phenomena could be explained by the effect of repetitive heating, as the nature of DED method during the depositing of new layers. Moreover, it was observed the α thickness of α_m in the bottom layer was finer than that in the top layer due to the higher cooling rate.

Abstract: High demand for new asphalt pavements often requires that paving is to be done in an unfavourable condition such as low air temperatures, high wind speeds, and night construction that will affect the Time Available for Compaction (TAC). In local practice, the asphalt paving compaction control mechanisms quoted from the locally used specifications are normally based on the limits of the delivery and laying completion temperatures. This study was conducted which aim to determine the appropriate TAC of Warm Mix Asphalt (WMA) with the effect of different wind velocity using laboratory tests. It focused on WMA Asphaltic Concrete (AC)14 of mix type for wearing course and AC28 of mix type for binder course. Sample of WMA was prepared in a slab mould instead of typical round mould, which was used to resemble the shape of actual pavement. The different wind velocity was simulated by the fan located at different distance from the sample. The results indicated that the cooling rate of WMA is significantly affected by the wind velocity thus influencing the Time Available for Compaction (TAC), which decrease in average by 10-50% during windy days.