What Is a Cooling Curve?

Cooling curve is the curve of the relationship between the cooling temperature and time after the workpiece is heated in the metal heat treatment process. Can be plotted in equidistant, single-log, or double-log coordinates.

The cooling curve is also called step cooling curve , which is an important basis for drawing the phase diagram of the condensation system by thermal analysis. The plateau and turning point on the step cooling curve represent the information of phase change at a certain temperature. The phase diagram of the binary condensation system can be drawn based on the step cooling curve . Conventional manual drawing methods are not only cumbersome but also unavoidably introduce human error. With the application of computer technology in data processing, computer editing can be used.

On this type of curve, the conditions of the phase change and the name of the material are generally indicated; using this type of curve, the structure and performance of the required material can be obtained by controlling the amount of deformation, temperature, and cooling rate. The temperature and time when the material undergoes a phase change under extreme heating and cooling rates, or under the condition that deformation exists at the same time, is called the dynamic phase change point. With the help of specialized equipment, phase transitions under extreme conditions that are generally difficult to measure with conventional methods can be obtained.

The meaning of the dynamic phase transition point is: (1) It is advised to obtain a structure that is difficult to obtain under conventional heating or cooling speed under high-speed heating or cooling speed. Figure 2 shows the continuous heating curves of low carbon steel at different heating rates. The figure shows. When the heating speed rises to 100 ° C / s, the A and A points increase, and after this speed, the Al and Al alloys decrease as the speed increases;

(2) As the deformation progresses, the phase transition point also changes. The reason is that the deformation energy, the interface energy, and the surface energy of the material have changed. ^[1]

The crystallization of pure metals is carried out at a certain temperature and is usually used

Observing the cooling curve of a material with a fixed composition can obtain a lot of information about the crystallization process of the material. The so-called cooling curve is a cooling cycle curve drawn by using temperature as a function of time, and phase transition points appear as characteristic points on the curve.

For lead-tin alloy systems, six different compositional cooling curves are shown in the figure. Curve I indicates that pure lead begins to cool from the liquid state, and a smooth curve is observed in the liquid state. When the crystallization point a is reached, crystallization starts. Due to the release of latent heat of crystallization, an a-a temperature maintaining line appears. Starting from a, the newly formed solid phase continues to cool, and the temperature drops along a smooth curve. This curve is characterized by the common characteristics of pure metals and substances with defined melting points, such as the pure tin cooling curve shown in curve VI.

Equilibrium phase diagram drawn on the cooling curve of lead-tin alloy system

For the 61.9% Sn alloy, a melting point can be observed as determined by curve IV in the figure. The difference from pure metal is that the mechanical mixture formed by the solid phase of two components and different structures is crystallized from the alloy liquid at the same time. This process is called eutectic reaction, that is,

L _{E C} + _D

It should be noted that the eutectic reaction occurs at a fixed temperature, and in the reaction, the two solid phase components of the alloy liquid and after crystallization are fixed.

Curve III in the figure is the cooling curve of 40% Sn alloy. The alloy solution was continuously cooled to the point k, and a solid solution of Sn in Pb ( ) And releases energy, so the slope of the curve suddenly decreases. The formation of these small grains causes the Sn concentration in the remaining alloy solution to increase, thereby reducing the crystallization point. In order to continue to form a new solid phase, cooling must continue. More solid phases continue to be formed, resulting in a higher Sn concentration in the remaining alloy solution, which further reduces the crystallization point. Therefore, a solidification temperature range appears. The corresponding upper characteristic temperature is the initial crystallization temperature and the lower characteristic temperature is the final crystallization temperature. When the temperature reaches the k 'point, the remaining alloy liquid composition reaches the point E (61.9% Sn) composition. Therefore, at this time, the eutectic reaction occurs, and the remaining alloy liquid crystallizes into a two-component mechanical mixture formed by a solid phase with different structure, and is cooled An isothermal holding line can also be observed on the curve. Subsequently, the solid phase mixture continued to cool. The cooling curve V in the figure is similar to III, and the analysis method is the same. The difference is that the solid solution of Pb in Sn is first precipitated. .

Curve II in the figure is the cooling curve of 13% Sn alloy. The curve h and above are similar to curve III, and the slope of the curve suddenly drops from the beginning of white h. This is because the secondary phase is precipitated from the solid solution starting from h . The solid solution is caused by the simultaneous release of energy.

Now you can transfer the phase transition points to the temperature-component coordinates at the same temperature on the cooling curve, and connect the characteristic points with the same physical meaning into a curve to get the lead-tin phase diagram shown in the figure above. The AEB in the figure is the lowest temperature when all alloys are in the liquid state, called the liquidus; the ACEDB line is the highest temperature when all alloys are in the solid state, called the solidus. The liquid and solid phases coexist between the liquid and solid phase lines, which is called the liquid-solid two-phase region. ^[3]