Determination of surface heat transfer coefficient during quenching of steel plate (2)
September 17 07:47:25, 2025
The data was used to calculate thermal stress and strain. The surface heat transfer coefficient can be determined by combining the cooling curve with Equation 5, resulting in a value of 9. Two experiments showed that when a detectable vapor film was present during early quenching, the temperature dropped rapidly below 650°C. In other experiments, the heat transfer coefficient increased steadily from a minimum of 850 up to a maximum recorded at 350. In each type of quenching, the maximum value of the heat transfer coefficient is consistent because it occurs at the same temperature.
The determination of the heat transfer coefficient requires Equation 2, which depends on the depth of the measured temperature below the surface. Any small error in this equation can lead to significant errors in the calculated value. For instance, if there is an error of 0.2 and the actual value is 18, the result would produce unacceptable inaccuracies. Therefore, during cooling curve measurements, steel plates are cut along the plane containing the thermocouple junctions, and their depth is measured to the nearest ±0.1 mm.
After quenching a freshly ground steel plate in water, five cooling curves were obtained, and the heat transfer coefficient was calculated. Although all curves exhibited clear maximum values in the boiling stage, the dispersion was large, with the maximum value ranging between 2000 and 8500 W/m²·K. This range is much larger than the values obtained from 3.3 steel bars. The temperature range corresponding to the maximum value is between 280°C and 510°C. Since the vapor film does not completely disappear once the temperature drops below 700°C, it either does not exist or persists for only a short time.
The average heat transfer coefficient obtained from the surface temperatures was 1013 W/m²·K. These results, based on quenching unoxidized samples, show consistency, while results from oxidized oil-quenched samples vary significantly. The surface heat transfer coefficient during quenching is influenced by the material’s surface condition and quenching medium. For example, water quenching yields higher coefficients compared to polymer or oil quenching. The boiling temperature range varies between 750°C and 150°C, similar to pure water, but the maximum value is lower than that under water quenching conditions, with two distinct peaks observed.
The h value between the two peaks at T is much higher than the value in the vapor film stage. When quenched in oil, the surface heat transfer coefficient shows a limited boiling stage with a low peak, approximately 2000 W/m²·K. The results from different oxidation levels on the steel surface are relatively consistent, but they differ significantly from those obtained through water quenching.
After analyzing the temperature changes across the steel plate, the calculated temperature gradient was compared with the experimental values. Although the later stages of water quenching showed slightly higher gradients, the overall comparison between water and oil quenching aligns with the predicted heat transfer coefficient distribution. These findings are relevant to future studies on thermal stress and strain in structural components.
According to the calculation of thermal stress and strain, although some differences remain in the boiling stage, the formation of a surface oxide layer enhances the stability of the steam film. This effect influences both the temperature gradient and the heat transfer process. Moreaux et al. suggested that a thermal insulation layer on the steel surface correlates with a high cooling rate, which contradicts the observations here. However, despite some assumptions being less accurate, the simple method described provides acceptable results for practical applications.
The results do not clearly indicate whether the 25% water quenched 1250 polymer solution exaggerates the initial temperature gradient. In oil quenching, the deviation between calculated and experimental values decreases in the later stages, but the calculated values are still somewhat overestimated.
The temperature distribution calculations rely on data from the 85-316 laboratory and similar materials. The constant used in the calculation of 13 is averaged from the values in 4. While using this as a variable could improve the accuracy of the model, it significantly increases computational time. It takes four units to calculate the temperature distribution, which is the minimum number required to obtain accurate results.
Polymer quenching fluids have a more pronounced effect on the cooling curve compared to water, especially at higher concentrations. Even at maximum concentration, the heat transfer coefficient in the boiling stage is only slightly lower than the lower limit observed in water quenching. Additionally, the reduction in heat transfer coefficient during polymer quenching may be due to polymer precipitation at the steam film stage. The heat transfer coefficient in the steam film stage is much lower than that in water quenching. Moreover, a stable vapor film is easier to form in the 25% water-polymer solution compared to water alone.
The heat transfer coefficient from 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 very slow until the sample temperature drops below 520°C. Considering the variability in the cooling curves, the test results suggest that the temperature distribution and gradient predictions are reliable throughout the quenching process.
In conclusion, the presence of an insulating oxide film on the workpiece surface during the initial stage of water quenching promotes the formation of a stable vapor film. The average maximum heat transfer coefficient in the boiling stage of water quenching ranges from 3500 to 18500 W/m²·K. The heat transfer coefficient in the presence of an oxide film is significantly lower than that in pure water quenching. To effectively reduce the coefficient, a polymer quenching medium is recommended. Oil quenching produces a much lower heat transfer coefficient, with a short boiling stage and limited enhancement.
The approximate method using Equation 1 gives a lower heat transfer coefficient than the explicit difference method at any given temperature. However, its accuracy is sufficient for many practical applications. For calculating residual stress, it is essential to use the most accurate data available. Therefore, the finite difference method is recommended for determining the surface heat transfer coefficient.
The following symbols are used in the text: 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; Δt = time interval; x = distance between points; α = thermal diffusivity; T_q = quenching liquid temperature; T_0 = initial temperature of the sample; T_j = temperature of the sample at the junction after a time interval; T_i = temperature of the sample at time i; T_h = temperature at the hot end of the thermocouple after the time interval; k = thermal conductivity; Ï = density; σ = standard deviation; subscripts: exp = experimental value.
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