The cooling rate varies according to the hardness of a steel under the influence of two important parameters: one heat transfer from inside to the surface of the steel sample and the other heat transfer from the surface of the piece through the cooling medium. The heat transfer capability of a steel is determined by its thermal diffusion parameter or its thermal conductivity to specific volumetric heat. The thermal penetration of austenite metamorphic products increases with decreasing temperature. For a fast cooling environment, thermal diffusivity determines the temperature distribution in the fast cooling zone at any given moment. The following figure shows the cooling tracks of different points in a 1 inch steel piece when cooled in a fast cooling environment with H = 1. Slower cooling tracks in far-off and near-center locations provide more time for intrusive transformation. In fact, it is this condition that results in the low hardness of rod centers; this phenomenon is especially evident in rods of large diameter. Since the thermal properties of the steel are practically impossible to control, the most important method is to control the cooling rate of a piece by selecting the appropriate cooling medium.

Heat transfer in the common season of steel with its cooling environment is a relatively complex issue. This phenomenon depends on the intensity of heat radiation from the steel surface and convection currents inside the cooling medium. This removes heat from the metal-environment interface. The best way to address this is to study the cooling pattern of a steel sample in the environment. To draw this diagram, thermocouples are installed at various points in the sample and they record time-lapse temperature using a highly sensitive recorder. The following is an example of these diagrams for the surface and center of a small steel cylinder when exposed to a cold liquid from high temperatures.

In this figure, four stages of cooling are shown:

Stage ‘A: Initial liquid contact stage

This stage shows the first effect of immersion of the hot sample in a cold liquid and involves the formation of steam bubbles on the surface of the sample, which results in the formation of a vapor layer (stage A). Stage ‘A’ takes only about 2.5 seconds and has very little hemi in evaluating the temperature reduction mechanism. It is only if step ‘A’ can be realized that very precise and sensitive equipment can be used to draw a temperature-reduction curve over time. In addition, if the coolant has a high viscosity, contains gas, or is used at temperatures close to boiling, the above step is eliminated.

Step A: Vapor blanket cooling stage

This step involves forming a stable vapor layer around the piece and thus separating it from the cold environment around it. We will have this step when the heat from the sample surface exceeds the amount of heat needed to vaporize the liquid per unit area. Since this part of the cooling is accomplished by heat radiation through the stable vapor layer and in addition to the vapor layer acting as an insulator, this step is one of the slow cooling steps. This phase is not found in non-volatile solutions such as potassium chloride, lithium chloride, sodium hydroxide and sulfuric acid (at concentrations of about 1%). Cooling diagrams in the above environments begin immediately from stage B. If from such materials

(2) saturated solutions of barium hydroxide, calcium hydroxide (or other similar substances that are poorly soluble in water)
(1) Solutions containing very fine and suspended solids
(1) Colloidal solutions in water
To be used, in step A, a layer of the above material is deposited on the surface of the piece and prolongs steps A and C. The above conditions will usually result in step B performance intensification. Solutions containing gelatinous substances, such as polyvinyl alcohol, gelatin, soap, and starch, form a gelatinous layer around the vapor layer formed in step A, thus prolonging the various cooling stages.

Step B: Vapor transport cooling stage

This phase has the highest heat transfer rate between the various stages and starts when the surface temperature of the metal decreases so much that it causes the vapor layer to become unstable and thereby convert it to liquid droplets. These droplets immediately vaporize upon contact with the metal and rapidly cool down as a result of absorbing the latent heat of the piece. The duration and rate of cooling at this stage are controlled by various parameters such as the boiling point of the liquid, the size and shape of the vapor bubbles.

Step C: Liquid Cooling Stage

The cooling rate at this stage is lower than steps A and B. Step C begins when the surface temperature of the metal drops below the boiling point temperature of the coolant. Below this temperature, the boiling of the liquid is stopped and therefore the liquid is occupied around the piece. Hence, heat dissipation from this moment onwards is accomplished by conduction and convection. One of the effective parameters on cooling rate at this stage is liquid viscosity and temperature difference between ambient temperature and boiling point.
The effects of different parameters on the cooling rate of a piece in a given environment can be investigated with the help of cooling curves. For example, creating turbulence in the cooling environment or moving the sample in it will reduce the stability of the vapor layer in step A and the formation of smaller, separate bubbles in phase B. The effect of ambient turbulence in step C is to mechanically eliminate the solid and gelatinous layers formed on the surface of the fragment or in the interface between the vapor layer and the medium, thereby increasing the cooling rate in this step Find out. In addition, turbulence in the environment causes the liquid to cool, rapidly replacing the heated fluid, and to come into direct contact with the component.

Although the surface and center cooling curves in the figure above relate exclusively to one experimental sample, it should be noted that the cooling mechanism is exactly the same as the cooling mechanism of a real sample during heat treatment. Also, although the cooling curve obtained depends on the size and sex of the sample tested, the position of the thermocouples and the coolant conditions, nevertheless using heat transfer formulas obtained under specified conditions from the cooling curves It is possible to deduce the cooling curves under other conditions. The above information can also be used to determine the intensity or the cooling power of different nutrients.