In this post, I will share with you the four factors that affect the austenite formation rate.
I believe that through the introduction of this post, you will have new views and understanding on some issues.
Factor 1: effect of heating temperature
Generally speaking, with the increase of heating temperature, the austenite formation rate is faster.
Moreover, with the increase of heating temperature, the nucleation rate I and growth rate G of austenite increase, but the increase rate of I is higher than that of G (as shown in Table 1).
Table 1 Relationship between austenite nucleation rate (I), growth rate (G) and temperature
|Transition temperature / ℃||Nucleation rate I (1 / mm3S)||Growth speed G (mm / s)||Time required for half conversion/s|
Therefore, the higher the austenite formation temperature, the smaller the initial grain size obtained.
At the same time, with the increase of heating temperature, the ratio of the phase interface transition velocity from austenite to ferrite to that from austenite to cementite increases.
For example, when the temperature is 780 ℃, the ratio of the two is 14.9, and when the temperature rises to 800 ℃, the ratio of the two increases to 19.1.
Therefore, when the austenite formation temperature rises, the amount of residual carburizing increases and the average carbon content of the newly formed austenite decreases at the moment when the ferrite phase in pearlite disappears (i.e., all of it is transformed into austenite) (Table 2).
Table 2 Effect of Austenite Formation Temperature on matrix carbon content
|Austenite formation temperature / ℃||735||760||780||850||900|
|Matrix carbon content（ When α -phase disappears) /%||0.77||0.69||0.61||0.51||0.46|
Therefore, the larger the heating speed (or superheat degree) during the actual heat treatment, the more carbides may remain in the steel.
With the increase of austenite formation temperature, the initial grain of austenite is refined;
At the same time, the disequilibrium degree of phase transformation increases.
At the moment when the ferrite phase disappears, the amount of residual carburizing increases, and the average carbon content of austenite matrix decreases.
These two factors are beneficial to improve the toughness of quenched steel, especially quenched high carbon tool steel.
Factor 2: influence of carbon content
The higher the carbon content in the steel, the faster the austenite formation.
As the carbon content increases, the number of carbides increases, and the interface area between ferrite and cementite increases, thus increasing the nucleation site of austenite and increasing the nucleation rate.
At the same time, when the number of carbides increases, the diffusion distance of carbon decreases, and the diffusion coefficient of carbon and iron atoms increases with the increase of carbon content in austenite.
All these factors accelerate the formation of austenite.
However, in hypereutectoid steel, due to the excessive amount of carbides, the time for dissolution of residual carbides and homogenization of austenite will be prolonged with the increase of carbon content.
Factor 3: influence of original tissue
In the case of the same steel composition, the larger the dispersion of carbides in the original structure, the more the phase interfaces and the larger the nucleation rate.
At the same time, due to the decrease of pearlite spacing and the increase of carbon concentration gradient in austenite, the diffusion speed of carbon atoms is accelerated, and the diffusion distance of carbon atoms is also reduced, which all increase the growth speed of austenite.
Therefore, the finer the original structure of steel, the faster the formation of austenite.
For example, when the austenite formation temperature is 760 ° C and the lamellar spacing of pearlite is reduced from 0.5μm to 0.1μm, the growth rate of austenite increases by about 7 times.
The shape of carbides in the original structure also has some influence on the formation rate of austenite.
Compared with granular pearlite, because the phase interface of lamellar pearlite is large, the cementite is thin and easy to dissolve, so austenite is easy to form when heated.
Factor 4: effect of alloying elements
The addition of alloying elements to steel does not affect the transformation mechanism of pearlite to austenite, but affects the stability of carbides and the diffusion coefficient of carbon in austenite.
Moreover, the distribution of most alloying elements between carbides and matrix is uneven.
Therefore, alloying elements will affect the nucleation and growth of austenite, the dissolution of carbides and the homogenization speed of austenite.
Strong carbide forming elements such as Mo, W, Cr, etc. reduce the diffusion coefficient of carbon in austenite, and form special carbides which are not easy to dissolve, thus significantly slowing down the formation speed of austenite.
Non carbide forming elements Co and Ni increase the diffusion coefficient of carbon in austenite and accelerate the formation of austenite.
Si and al have little effect on the diffusion of carbon in austenite, so they have no significant effect on the formation rate of austenite.
The addition of alloying elements to steel may change the positions of transformation critical points A1, A3 and Acm, that is, change the superheat during transformation, thus affecting the formation rate of austenite.
For example, Ni, Mn, Cu, etc. reduce A1 point and relatively increase superheat, thus increasing the formation rate of austenite;
Cr, Mo, Ti, Si, Al, W, V, etc. increase the A1 point and relatively reduce the degree of superheat, thus slowing down the formation speed of austenite.
The addition of alloying elements to steel can also affect the pearlite lamellar spacing and the solubility of carbon in austenite, thus affecting the concentration difference at the phase interface, the concentration gradient and nucleation in austenite, and thus affecting the formation rate of austenite.
The results show that the distribution of alloying elements in the original microstructure is not uniform.
In the annealed state, carbide forming elements (such as Mo, W, V, Ti, Cr, etc.) are mainly concentrated in the carbide phase, while non carbide forming elements (such as CO, Ni, Si, etc.) are mainly concentrated in the ferrite phase.
This non-uniform distribution of alloying elements remains significantly in austenite until the carbides are completely dissolved.
Therefore, the austenitic homogenization process of alloy steel includes the homogenization of alloying elements in addition to the homogenization of carbon.
Since the diffusion coefficient of alloy elements is about 1000-100000 times smaller than that of carbon atoms, and the carbide forming elements also reduce the diffusion coefficient of carbon atoms in austenite, it is more difficult to dissolve if special carbides (such as VC, tic, etc.) are formed.
Therefore, the austenite homogenization process of alloy steel is much longer than that of carbon steel.
In view of the above reasons, in order to homogenize austenite during quenching and heating of alloy steel, it is necessary to heat it to a higher temperature and hold it for a longer time.
Through the above introduction, I believe you have a better understanding of several factors affecting the austenite formation rate.
If you want to remember better, you still need to combine practice to complement each other.