Understanding Austenite Formation: 4 Key Influencing Factors

In this post, I will share with you the four factors that impact the rate of austenite formation.

I hope that after reading this post, you will have a new perspective and increased understanding on the topic.

Factor 1: effect of heating temperature

Generally, as the heating temperature increases, the formation rate of austenite speeds up.

In addition, as the heating temperature increases, both the nucleation rate (I) and growth rate (G) of austenite increase. However, 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

Therefore, the initial grain size obtained decreases as the austenite formation temperature increases.

At the same time, as the heating temperature increases, the ratio of the transition velocity from austenite to ferrite to that from austenite to cementite also increases.

For instance, when the temperature is 780°C, the ratio is 14.9, and when the temperature rises to 800°C, the ratio increases to 19.1.

As a result, when the austenite formation temperature increases, the residual carburizing amount increases and the average carbon content of the newly formed austenite decreases when the ferrite phase in pearlite disappears and all of it is transformed into austenite (as shown in Table 2).

Table 2 Effect of Austenite Formation Temperature on matrix carbon content

Therefore, the larger the heating speed (or superheat degree) during the actual heat treatment, the more carbides may remain in the steel.

In summary:

With the rise in temperature for forming austenite, the starting grain size of the austenite becomes finer.

Simultaneously, the level of imbalanced phase transformation also increases.

When the ferrite phase vanishes, the amount of residual carburizing increases and the average carbon content within the austenite matrix decreases.

These two factors positively impact the toughness of quenched steel, particularly high carbon tool steel that has been quenched.

Factor 2: influence of carbon content

The speed of austenite formation increases as the carbon content in steel increases.

As the carbon content rises, the number of carbides also increases, which leads to an increase in the interface area between ferrite and cementite. This in turn results in more nucleation sites for austenite and a faster nucleation rate.

Additionally, as the number of carbides grows, the diffusion distance of carbon decreases and the diffusion coefficient of carbon and iron atoms increases with the rise in carbon content in austenite.

All these factors combined contribute to the acceleration of austenite formation.

However, in hypereutectoid steel, the excessive presence of carbides can lead to a prolonged time for the dissolution of residual carbides and homogenization of austenite, as the carbon content increases.

Factor 3: influence of original tissue

The larger the dispersion of carbides in the original steel structure, the more phase interfaces exist and the higher the nucleation rate will be, given the same steel composition.

Furthermore, a decrease in pearlite spacing and an increase in carbon concentration gradient in austenite lead to an acceleration in carbon atom diffusion speed and a reduction in diffusion distance, both of which contribute to an increase in the growth rate of austenite.

Consequently, the finer the original structure of steel, the quicker the formation of austenite.

For instance, when the austenite formation temperature is 760°C and the pearlite lamellar spacing is reduced from 0.5μm to 0.1μm, the growth rate of austenite increases by approximately seven times.

The shape of carbides in the original structure also plays a role in influencing the formation rate of austenite. Compared to granular pearlite, austenite is more easily formed when heated due to the large phase interface and thin, easily dissolvable cementite in lamellar pearlite.

Factor 4: effect of alloying elements

The addition of alloying elements to steel does not impact the transformation mechanism of pearlite to austenite, but it does affect the stability of carbides and the diffusion of carbon in austenite. Additionally, the distribution of most alloying elements between carbides and the matrix is uneven.

Consequently, the presence of alloying elements affects the nucleation and growth of austenite, the dissolution of carbides, and the homogenization speed of austenite.

Elements that form strong carbides, such as Mo, W, and Cr, reduce the diffusion of carbon in austenite and form carbides that are not easily dissolved, thus significantly slowing down the formation rate of austenite. On the other hand, non-carbide forming elements such as Co and Ni increase the diffusion of carbon in austenite and accelerate the formation of austenite.

Si and Al have little effect on the diffusion of carbon in austenite and therefore have no significant impact on the formation rate of austenite.

The addition of alloying elements to steel may also change the positions of the transformation critical points A1, A3, and Acm, which affects the superheat during transformation and the formation rate of austenite. For instance, elements such as Ni, Mn, and Cu reduce the A1 point and increase the superheat, thereby increasing the formation rate of austenite. Meanwhile, elements such as Cr, Mo, Ti, Si, Al, W, and V increase the A1 point and decrease the degree of superheat, thus slowing down the formation speed of austenite.

Finally, the addition of alloying elements to steel can affect the pearlite lamellar spacing and the solubility of carbon in austenite, altering the concentration difference at the phase interface, the concentration gradient, and nucleation in austenite, and ultimately affecting the formation rate of austenite.

The results indicate 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, and Cr are primarily found in the carbide phase, while non-carbide-forming elements like CO, Ni, and Si are primarily found in the ferrite phase.

This non-uniform distribution remains substantial in austenite until the carbides are fully dissolved. As a result, the homogenization process of alloy steel in the austenitic state requires the homogenization of alloying elements in addition to the homogenization of carbon.

However, the diffusion coefficient of alloy elements is around 1000 to 100,000 times smaller than that of carbon atoms, and carbide-forming elements also reduce the diffusion coefficient of carbon atoms in austenite. This makes it more challenging to dissolve special carbides like VC and TIC.

Therefore, the austenite homogenization process of alloy steel takes much longer than that of carbon steel. To achieve homogenization of austenite during the quenching and heating of alloy steel, it is necessary to heat it to a higher temperature and hold it for an extended period.

Conslusion

Through the above introduction, I hope that you have gained a better understanding of the various factors that impact the formation rate of austenite. To ensure that you remember this information, it is recommended that you combine practice with your learning for maximum retention.