Critical cooling rate on carbide precipitation during quenching of austenitic manganese steel

Critical cooling rate to avoid carbide precipitation during quenching of austenitic manganese steel was investigated by means of optical microscopy,image analyzer and numerical analysis.An efficient heat treatment analysis program including temperature-dependent material properties was developed for the prediction of cooling rate and probability of carbide precipitation during quenching by finite difference method.Time-dependent heat transfer coefficient was adopted to achieve more precise results.Area ratio of carbide precipitation was measured by image analyzer to determine the critical point of carbide precipitation.Temperature-dependent critical cooling rate at that point was calculated by the developed numerical program.Finally,the probability of carbide precipitation on the whole area of specimen can be predicted by the proposed numerical program and the numerical result of a specimen was compared with the experimental result.

Simulation of critical cooling rate and process conditions for metallic glasses in vertical type twin-roll casting

Critical cooling rates for producing metallic glasses were evaluated based on a calculated continuous cooling transformation(CCT)diagram.Temperature distributions of the melt in molten pool in the vertical type twin-roll casting(VTRC)process of metallic glasses were simulated,and cooling rates under different casting conditions were calculated with the simulated results.By comparing the results obtained by CCT diagrams and simulation,the possibility of producing metallic glasses by the VTRC method and influences of casting conditions on cooling rate were discussed.The results reveal that cooling rate with3or4orders of magnitude by the VTRC process can be attained in producing Mg-based metallic glasses,which is faster than the critical cooling rate calculated by the CCT diagram.One side pouring mode can improve the temperature distributions of casting pool.VTRC process has a good ability in continuous casting metallic glassy thin strips.

Critical Cooling Rate for the Glass Formation of Ferromagnetic Fe_(80)P_(13)C_7 Alloy

In this paper the critical cooling rate, Rc, for the glass formation of Fe80P13C7 alloy has been determined using both Uhlmann＇s and Barandiaran-Colmenero＇s method. In Uhlmann＇s method, all kinds of the expres-sions of △G^l-s （T） and η/（T） determined using the different modes and methods had been investigated. It is indicated that the Rc for the glass formation of FesoP13C7 alloy can be estimated to be 349 K/s by Uhlmann＇s method based on the appropriate expressions of △G^l-s（T） and η/（T）. The calculated result accords with our experimental result. The Rc for the glass formation of Fe80P13C7 alloy has also been determined to be 0.49 K/s using Barandiaran-Colmenero＇s method. This resultant Rc is unreasonable low and it indicates that Barandiaran-Colmenero＇s method does not suit to Fe-based alloy.

Construction and Application of Quenching Critical Cooling Rate Model

Based on exploring quenching mechanism, three types of quenching critical cooling rate models of mediumcarbon steel and low-carbon steel and low-alloy construction steel were established, as modified Maynier model, Eldis model and isothermal curve model, respectively. During construction process, regression analysis and Newton interpolation method were used for higher calculated precision. Through comparing and analyzing, the superior one was selected to evaluate the full hardening thickness after quenching. According to hardness analyzing, the deviation between predicted hardness distribution and the experimental value is within 6%.

Continuous cooling precipitation diagram of cast aluminium alloy Al-7Si-0.3Mg

The precipitation behaviour during quenching of cast Al-7Si-0.3Mg aluminium alloy was investigated by DSC in the cooling rate range of 0.01 K/s to 3 K/s and by quenching dilatometry for higher rates. Two main precipitation reactions were observed during cooling, a high temperature reaction starting almost directly with quenching from 540℃ and a low temperature reaction starting at about 400℃. Quenching with 3 K/s already significantly suppresses precipitation during quenching. Hardness after T6 ageing increases with increasing quenching rate, due to the increasing content of supersaturated solid solution. By dilatometry and hardness results the critical cooling rate can be estimated as about 60 K/s. Quenched Al-7Si-0.3Mg microstructures have been investigated by light microscopy. The microstructures consist of an aluminium-silicon eutectic structure, aluminium solid solution dendrites and precipitates inside the aluminium dendrites, depending on quenching rate.

Effect of Actual Cooling Rate of Ladle Stream on Persistent Metallurgical Performance of a Given Tundish

To evaluate the effect of actual cooling rate of liquid steel in the ladle on the metallurgical performances of a tundish, a transient and coupled computational model was developed to reveal the flow fields, temperature fields, residence time distribution of the molten steel and the inclusion removal efficiency in a typical single-strand tundish with given geometry and process parameters. The results showed that, with the decrease of the ladle stream cooling rate, the temperature difference of bulk flow at the outlet of tundish over a normal casting period decreased from 11.3 to 2.6 K, and the dead volume fraction of the tundish decreased from 17.58% to 14. 35%, while the inclusion removal efficiency was increased especially for the inclusions with the diameter less than 50 μm, whose removal ratio could be increased by 20.62%. When the cooling rate was less than 0.3 K · min-1 , however, the variation rates of the three evaluation criterions above declined significantly, which suggested that a critical value existed for the effect of the cooling rate of ladle stream on the tundish performances. The establishment of the critical ladle stream cooling rate should be very important to achieve persistent metallurgical properties of tundish over the whole casting stage, together with the reasonable ladle insulation design.

Glass formation for iron-based alloys by combining kinetic and thermodynamic parameters

The glass formation was intensively studied for Fe-based alloys. Parameters defining kinetics and thermodynamic behavior of crystallization were calculated using calorimetric measurements and physical properties of constituent elements. It is found that the critical cooling rate Rc estimated by combining kinetic and thermodynamic parameters highly correlates with measured Rc found in literatures with correlation coefficient R2=0.944, and alloy compositions with high melting enthalpy AHm can easily form glass even without high undercooling and high value of the ,β-parameter of Tumbull＇s theory, revealing that the glass formation in this group of alloys is mostly controlled by growth limitation. This combination of kinetic and thermodynamic parameters can be used to determine alloy composition with good glass forming ability in Fe-based alloys just using physical properties of alloying elements and calorimetric measurements.

Surface vitrification of carbon by laser treatment: Insights of glass formation from molecular dynamics

Microstructural modification of carbon materials,such as carbon fibers(Cf)and pyrolytically deposited carbon,is important for engineering applications.However,the regulation of these materials is not an effortless task.To understand the impacts of thermal spikes from pulsed laser processing on the structural adaptation of amorphous carbon(a-C),we performed melt quenching by molecular dynamics(MD)simulations.Our results confirm that the vitrification behavior of carbon can be tuned by adjusting the cooling rate(R),which is controlled by the thermal spikes of laser processing.Moreover,we set up a two-step way to locate the critical cooling rate(R_(c))of monoatomic carbon,which is refined by the sharp change in the environmental similarity parameter.Using this novel technique,we demonstrate that the ordering degree and the local atomic motif can be largely modified by going across a bar of 100 K/ps,which is extracted as the critical cooling rate to ensure the complete amorphization of carbon.This approach provides a criterion for both experimentally processing and theoretically simulating a-C structures.Therefore,this work provides guidelines on how to tune the amorphous carbon structures of engineering materials and provides an outlook for the wonderland of amorphous carbon materials.

CCT and TTT Diagrams to Characterize Crystallization Behavior of Mold Fluxes

The isothermal and non-isothermal experiments were performed to construct the continuous cooling transformation （CCT） and temperature time transformation （TTT） diagrams of four industrial mold fluxes through visual observations in an experimental apparatus based on the single hot thermocouple technique （SHTT）. The results of the CCT diagrams indicate that ① the crystallization temperature of mold fluxes lowers as the cooling rate increases, ② the mold fluxes have larger critical cooling rate, higher crystallization temperature, and less onset time of crystallization when the basicity increases or the viscosity decreases, ③ the influences of the melting points of the mold fluxes on their crystallization tendency are not significant. Isothermal tests show that the onset time of crystallization decreases at first, and then increases, and finally represents a ＂C＂ shape with increasing isothermal temperature. The TTT diagrams of four industrial mold fluxes were divided into two separate ＂C＂ shape regions. The crystal phase of C20A selected was analyzed by X-ray diffraction, which is cuspidine （Ca4 Si2 O7F2 ） over I 100 ℃ and calcium silicon oxide fluoride （Ca2SiO2F2） below 1 100℃. When compared with the TTT diagram, the CCT diagram can provide a more realistic estimate of the critical cooling rate of the mold fluxes. Thus, both the CCT and TTT diagrams can unambiguously describe the crystallization phenomena of the mold fluxes.