Determination of surface heat transfer coefficient during quenching of steel plate (2)
September 17 07:42:52, 2025
The data has been used to calculate thermal stress and strain. The surface heat transfer coefficient was determined using an equation combined with the cooling curve, yielding a result of 9. Two experiments with detectable vapor films during early quenching showed that the temperature dropped to a low value before 650°C. In other experiments, the h value increased steadily from a minimum of 850 until reaching a maximum at 350°C.
In each type of quenching, the maximum h value is consistent because the temperature that produces it remains the same. The conductivity coefficient can be derived from the cooling curve in a chrysanthemum test sequence for a single sample, which helps determine the surface heat transfer coefficient when quenching thick steel plates. This requires Equation 2, where the depth of the measured temperature below the surface must be accurately known. Even a small error in Equation 2 can lead to significant errors in the calculated h value.
For instance, if there's a 0.2% error and the actual value is 18, the resulting value could produce unacceptable inaccuracies. Therefore, when measuring the cooling curve, steel plates are cut along the plane containing the thermocouple junctions, and their depth is measured to the nearest ±0.1 mm.
A freshly ground steel plate was quenched in water, producing five cooling curves from which the h value was calculated. Although all curves showed clear maximum values in the boiling stage, the dispersion was large, with the maximum h value ranging between 2000 and 8500 W/m²·K. This range is much greater than the values obtained from 3.3 steel bars. The temperature range corresponding to the maximum h value lies between 280°C and 510°C. Since the steam film does not completely disappear once the temperature drops below 700°C, the steam film either does not exist or persists for a very short time.
The average h value obtained from the surface temperatures was 1013 W/m²·K. These results from quenching unoxidized samples show consistency, but they differ significantly from those obtained from oxidized oil-quenched samples. The surface heat transfer coefficient during quenching varies depending on the medium used. For example, water quenching resulted in higher h values compared to polymer quenching. The boiling temperature range varied from 750°C to 150°C, similar to pure water, but the maximum h value was lower than under water quenching conditions, with two distinct peaks observed.
The h value between the two peaks at a certain temperature was much higher than the h value in the vapor film stage. When quenching in oil, the surface temperature and heat transfer coefficient relationship showed a limited boiling stage with a very low peak, approximately 2000 W/m²·K. Results from quenching with different oxidation levels on the steel surface were quite consistent, but they differed significantly from those of water quenching.
After the temperature changes across the steel plate, the calculated temperature gradient was compared with the experimental value. Although the temperature gradient measured later during water quenching was slightly higher than the calculated value, the comparison between water and oil quenching showed that the predicted heat transfer coefficient distribution was reasonable. This relates to several quenching problems and can be useful in future studies on thermal stress and strain.
According to the accuracy of the thermal stress and strain calculation, although there are still significant differences in the boiling stage, the formation of a surface oxide layer increases the stability of the steam film stage. Moreaux et al. proposed that the presence of a thermal insulation layer on the steel surface and a high cooling rate are related, which contradicts the findings here. However, even though the h value calculated from Equation 1 for 3.3 steel bars is relatively small, the results are only slightly less than those obtained by a more accurate method for thick sections. Thus, despite some assumptions being inaccurate, this simple method can yield acceptable results.
The results do not clearly indicate that the 25% water quenched 1250 polymerization exaggerates the initial temperature gradient. In oil quenching, although the deviation between the calculated and experimental temperature gradients decreases in the later stages, the calculated value is still overestimated.
The temperature distribution values used in the calculations were provided by the 85-316 laboratory. Data from similar materials were also used. The constant used in the calculation of 13 was averaged from the values in 4. It can be used as a variable to slightly improve the correlation between calculations and experiments, but it greatly increases the computational cost. Calculating the temperature distribution takes 4 units, which is the minimum required to obtain accurate results.
The quenching fluid, particularly polymer solutions, has a significant effect on the cooling curve. Higher polymer concentrations lead to lower heat transfer coefficients in the boiling stage, though only slightly lower than the minimum values seen in water quenching. Additionally, the reduction in the heat transfer coefficient due to polymer quenching may be caused by polymer precipitation at the steam film stage on the steel surface. The h value in the steam film stage is much lower than that in water quenching. Moreover, it is easier to form a stable vapor film in the 25% water-polymer solution compared to water alone.
The surface heat transfer coefficient obtained through oil quenching is actually lower than that from the previous two media. The h value in the boiling stage does not exceed 2000 W/m²·K, and the enhanced heat transfer period is very short. The heat dissipation rate during the steam film stage in oil quenching is slow until the sample temperature drops below 520°C. Considering the variability in the cooling curves, it is reasonable to assume that the measured data aligns with the predicted temperature distribution.
In conclusion, the presence of an insulating oxide film on the workpiece surface during the initial stage of water quenching helps form a stable vapor film. During the steam film stage, the surface heat transfer coefficient is around 21 W/m²·K. The average maximum h value in the boiling stage of water quenching ranges from 3500 to 18500 W/m²·K. The heat transfer coefficient relative to pure water quenching is much lower when an oxide film is present, and achieving a lower coefficient requires using a 1250 polymer quenching medium. The boiling stage is very short, and the maximum h value is low.
The surface heat transfer coefficient determined using Equation 1 is much lower than the value obtained via the explicit difference method at any given temperature. However, the accuracy of this simple method is sufficient for some applications. To calculate residual stress, it is important to use the most accurate data possible. Therefore, in this case, it is recommended to use the finite difference method to obtain the surface heat transfer coefficient.
The meanings of the symbols used in the text are as follows: h = heat transfer coefficient; n = number of nodes in the half-section of the steel plate; t = number of time intervals; m = mass of the sample; T = temperature at the junction after a time interval; Tâ‚€ = initial temperature of the sample; Tâ‚ = temperature of the sample at a specific moment; ΔT = temperature change; α = thermal diffusivity; T_q = quenching liquid temperature; Ï = density; σ = standard deviation; subscripts denote experimental values.
Conventional and welding processes show that the microstructure of the 1.1203 weld seam forms a thin boundary film between alumina and the active welding material. In contrast, the boundary structure between welded alumina and the welding material is continuous with a thick reaction layer.
In other words, when the sample is heated, the dielectric loss of alumina increases, leading to direct coupling with microwave radiation.
4 Conclusion
This review highlights the great potential of using microwave radiation for material treatment, including sintering and welding. It has been shown that selecting appropriate microwave frequencies, such as 2.8 GHz and 2.45 GHz, can significantly enhance the sintering capability of ceramics. Progress has demonstrated that microwave radiation can be focused into a thin beam, making it convenient for material processing without the need for additional heat sources. Finally, it was noted that microwave radiation can generate high-density plasma, enabling ultra-fast sintering of ceramics onto metals.