Kinetic characteristics of pearlite transformation:

　　There is an incubation period before the transformation begins; at a certain temperature, the transformation rate has a maximum value as time increases; as the transformation temperature decreases, the incubation period of the P transformation has a minimum value, at which point the transformation is fastest; the influence of alloying elements is significant.

　　Kinetic study of pearlite transformation:

　　The nucleation rate is determined by three factors:

　　One is the volume free energy, i.e., the phase transformation driving force △G*.

　　Two is the activation energy Q for atomic migration.

　　Three is the degree of undercooling ΔT.

　　

 

　　Relationship between nucleation rate N and transformation time:

　　When the transformation temperature is constant, the nucleation of pearlite transformation has a certain relationship with the isothermal time. As the transformation time increases, the nucleation gradually increases. When it reaches a certain level, it drops sharply to zero, which is the so-called positional saturation.

　　Relationship between growth rate G and transformation time:

　　Studies have shown that the isothermal holding time has no significant effect on the growth rate of pearlite.

　　Factors affecting the kinetics of pearlite transformation

　　

 

　　The farther the carbon concentration is from the eutectoid composition, the faster the transformation of austenite to pearlite.

　　Compared with alloying elements, the influence of carbon content on pearlite transformation is relatively small.

　　2) Influence of alloying elements

　　① Rules of the influence of alloying elements on pearlite transformation

　　Except for Co and Al greater than 2.5%, all commonly used alloying elements shift the pearlite nose to the right, and the proeutectoid ferrite nose to the right.

　　Except for Ni, all commonly used alloying elements shift these two noses to the high-temperature region.

　　

 

　　Among the commonly used alloying elements, the order of the effect of delaying the pearlite transformation is:

　　Mo > Mn > W > Ni > Si

　　In addition, strong carbide-forming elements such as V, Ti, Zr, Nb, and Ta form insoluble carbides in steel. If these elements can dissolve into austenite during heating, the stability of undercooled austenite will be increased. However, even if heated to a very high temperature, these carbides can hardly be completely dissolved into austenite. Therefore, when strongly carbide-forming elements are added to the steel and the austenite temperature is not very high, the stability of the undercooled austenite may not be increased, but even decreased.

　　Boron element B is special: It is generally believed that adding a small amount of B to steel can significantly reduce the rate of proeutectoid ferrite precipitation in hypoeutectoid steel, and also has an inhibitory effect on pearlite formation. As the carbon content in steel increases, the effect of B increasing the stability of austenite gradually decreases.

　　Reasons why adding a small amount of B to steel can reduce the transformation rate of proeutectoid ferrite and pearlite:

　　This is mainly because the relative size of the atomic radius of B is not suitable for forming interstitial solid solutions or substitutional solid solutions, so it has a strong tendency to enrich at grain boundaries. B adsorbs on the austenite grain boundaries, reducing the grain boundary energy and thus reducing the nucleation rate of proeutectoid ferrite and pearlite.

　　Reasons for the influence of alloying elements on pearlite transformation

　　The reasons for the influence of alloying elements on pearlite transformation are still not fully understood. In summary, it can be considered from the following aspects.

　　A) Influence of alloy element self-diffusion

　　In addition to carbon diffusion, alloy element diffusion redistribution is also required for the eutectoid decomposition of alloy austenite.

　　Due to the lower diffusion rate of alloying elements, their diffusion coefficient is 3-5 orders of magnitude lower than that of C in austenite, thus increasing the incubation period of the transformation of undercooled austenite into pearlite and reducing the formation rate.

　　B) Influence of alloying elements on carbon atom diffusion

　　The influence of alloying elements on pearlite transformation can also be achieved by reducing the diffusion coefficient of C in austenite. Except for Co and Ni less than 3%, all alloying elements increase the diffusion activation energy of carbon in austenite, thus reducing the diffusion coefficient of C in A.

　　The reduction of the diffusion coefficient of C by alloying elements will increase the incubation period of pearlite transformation and thus reduce the transformation rate.

　　C) Influence of alloying elements on iron atom diffusion

　　The addition of alloying elements can also affect the diffusion of Fe atoms, and affect the pearlite transformation by affecting the process of γ→α allotropic transformation.

　　For example, the addition of Ni, Mn, and Cr increases the self-diffusion activation energy of Fe, thus reducing the diffusion coefficient of Fe atoms in austenite, and then affecting the reconstruction of the crystal structure and reducing the pearlite transformation rate.

　　D) Alloying elements change the position of the eutectoid point

　　The addition of alloying elements will change the position of the eutectoid point, thus affecting the critical transformation temperature and affecting the pearlite transformation.

　　It mainly affects the pearlite transformation by affecting the free energy of the system and the undercooling of the transformation.

　　

 

　　E) Drag effect of alloying elements on the γ/α interface

　　The addition of alloying elements will have a drag effect on the γ/α interface, thus reducing the moving speed of the γ/α interface and further reducing the formation rate of pearlite.

　　For example, strong carbide-forming elements such as V, Ti, Zr, Nb, and Ta form insoluble carbides in steel. Even if heated to a very high temperature, these carbides cannot be completely dissolved into austenite.

　　Therefore, when strongly carbide-forming elements are added to the steel and the austenite temperature is not very high, these dispersed insoluble carbide particles will hinder the migration of the γ/α interface, thus reducing the formation rate of pearlite.

　　3) Influence of austenite composition homogeneity and excess phase dissolution

　　Under actual heating conditions, austenite in steel components is often in a non-uniform state, and sometimes a small amount of carbide particles may remain.

　　

 

　　4) Influence of Austenite Grain Size

　　Due to differences in steel chemical composition, deoxidizers, etc., the austenite grain size obtained under the same heating conditions varies.

　　

 

　　Similarly, fine austenite grains will also promote the precipitation of proeutectoid ferrite and carbide.

 

　　External Influencing Factors

　　⑴ Influence of Heating Temperature and Holding Time

　　The heating temperature and holding time of the steel directly affect the uniformity and grain size of the austenite.

　　

 

　　⑵ Influence of Stress and Plastic Deformation

　　Under tensile stress or plastic deformation in the austenite state, there is an accelerating effect on the pearlite transformation. Compressive stress has the opposite effect.