1. Effect of chemical composition
Generally speaking, the Ms point mainly depends on the chemical composition of the steel, of which the carbon content has the most significant effect.
With increasing carbon content in steel, the martensitic transformation temperature range decreases, as shown in Figure 1.
Fig. 1 Effect of carbon content on Ms and Mf
With the increase in carbon content, the changes of the Ms point and Mf point are not completely consistent, and the Ms point shows a relatively uniform continuous decline;
When the carbon content is less than 0.6%, the Mf point decreases more significantly than the Ms point, thus expanding the martensitic transformation temperature range (Ms Mf).
However, when the carbon content is greater than 0.6%, the Mf point slowly decreases, and because the Mf point has dropped below 0 ℃, there is more residual austenite in the structure at room temperature after quenching.
The effect of N at point M is similar to that of C.
Just like C, N forms interstitial solid solution in steel, which has a strengthening effect on the solid solution in the γ phase and the α phase, but especially in the α phase, thus increasing the shear strength of the martensitic transformation and increasing the driving force of the transformation .
At the same time, C and N are also elements that stabilize a phase.
They reduce the equilibrium temperature T0 of the γ → α' phase transition, therefore strongly reduce the Ms point.
Common alloying elements in steel can reduce the Ms point, but the effect is not as significant as that of carbon.
Only Al and Co increase the Ms point (as shown in Fig. 2).
Fig. 2 Effect of alloying elements on the Ms point of the ferroalloy
The elements that reduce the Ms point are organized in order of intensity of influence: Mn, Cr, Ni, Mo, Cu, W, V, Ti.
Among them, W, V, TI and other strong carbide-forming elements mainly exist in the form of carbides in steel and are rarely dissolved in austenite during quenching and heating, so they have little effect on the Ms point.
The influence of alloying elements on the Ms point depends mainly on their influence on the equilibrium temperature T 0 and the strengthening effect on austenite.
All elements (such as C) that drastically reduce T 0 temperature and strengthen austenite drastically reduce the Ms point.
Mn, Cr, Ni, etc. not only reduce the T 0 temperature, but also slightly increase the austenitic strength, so they also reduce the Ms point.
Al, Co, Si, Mo, W, V, Ti, etc. all increase the T 0 temperature, but also increase the strength of the austenite by several degrees.
Then,
① If the former plays a greater role, the Ms point will increase, like Al and Co;
② If the latter has a greater effect, the Ms point will be reduced, such as Mo, W, V, Ti;
③ When the two functions are approximately equivalent, it has little effect on the point Ms such as Si.
In fact, the interaction between alloy elements in steel is very complex, and the Ms point of steel mainly depends on the test.
It is generally believed that all alloying elements that reduce the Ms point also reduce the Mf point.
2. Effect of deformation and stress
As mentioned previously, martensitic transformation will be induced when austenite is plastically deformed between Md Ms.
Similarly, plastic deformation between Ms Mf can also promote martensitic transformation and enhance martensitic transformation.
In general, the larger the deformation and the lower the deformation temperature, the more martensite transformation variables induced by deformation.
Since martensite transformation will inevitably produce volume expansion, multidirectional compressive stress will prevent martensite formation, thereby reducing the Ms point.
However, tensile stress or unidirectional compressive stress often leads to the formation of martensite, which causes the Ms point to rise.
3. Effect of austenitizing conditions
The influence of heating temperature and retention time on the Ms point is complex.
Increasing the heating temperature and extending the retention time leads to the further dissolution of carbon and alloy elements in austenite, which will reduce the Ms point, but at the same time will cause the growth of austenite grains, reduce its crystal defects, and reduce the shear strength during martensite formation, thus increasing the Ms point.
In general, if there is no change in the chemical composition, that is, in the condition of complete austenitization, increasing the heating temperature and prolonging the holding time will increase the Ms point;
Under the condition of incomplete heating, increasing the temperature or prolonging the time will increase the content of carbon and alloy elements in austenite, leading to the decrease of the Ms point.
Under the condition that the composition of austenite is constant, the strength of austenite will increase and the shear strength of martensitic transformation will increase when the grain is refined, which will reduce the Ms point.
However, when grain refinement does not significantly affect shear strength, it has little effect on the Ms point.
4. Effect of Quench Cooling Rate
The influence of the quench cooling rate on the Ms point is shown in Fig.
Fig. 3 Effect of quenching speed on the Ms point of Fe-0.5% C-2.05% NI steel
When the quenching speed is low, the Ms point remains constant, forming a lower step, which is equivalent to the nominal Ms point of the steel.
When the quenching speed is very high, another stage occurs where the Ms point remains constant.
Between the above two quenching speeds, the Ms point increases with increasing quenching speed.
The above phenomena can be explained as follows:
It is assumed that the distribution of C in austenite during phase transformation is uneven, and segregation occurs at defects such as dislocations, forming “atomic air mass C”.
The size of this “air mass” is related to the temperature.
Under high temperature, the atomic diffusion capacity is strong and the segregation tendency of C atom is small, so the size of the “air mass” is also small.
However, as the temperature decreases, atomic diffusivity decreases, the tendency of C atoms to segregate increases, and the size of the internal “air mass” increases with decreasing temperature.
Under normal quenching conditions, these “air masses” can reach sufficient size to strengthen the austenite.
However, the extremely fast quenching speed inhibits the formation of “air mass”, which leads to the weakening of austenite and the reduction of shear strength during martensitic transformation, thus raising the Ms point.
However, when the cooling rate is high enough, the bending of the “air mass” is restricted and the Ms point no longer increases with increasing quenching rate.
5. Effect of magnetic field
The test shows that when steel is quenched and cooled in the magnetic field, the applied magnetic field will induce martensite transformation.
Compared with that without the magnetic field, the Ms point increases and the transformation of martensite at the same temperature increases.
However, the external magnetic field only makes the Ms point rise, but has no effect on the phase transition behavior below the Ms point.
Fig. 4 Effect of external magnetic field on the martensite transformation process
As shown in Fig. 4, the applied magnetic field increases Ms to Ms' during quenching and cooling, but the increasing trend of the rotational variable is basically consistent with that without magnetic field.
When the applied magnetic field is removed before the phase transformation ends, the phase transformation will immediately return to the state in which the magnetic field is not applied, and the final amount of martensite transformation will not change.
The reason why the external magnetic field affects the transformation of martensite is that the external magnetic field makes the martensite phase with the maximum magnetic saturation strength more stable.
Fig. 5 Thermodynamic diagram of the increase in the Ms point caused by the external magnetic field
As shown in Figure 5, the free energy of martensite decreases in the magnetic field, while the magnetic field has little effect on the free energy of non-ferromagnetic austenite.
Therefore, the two-phase equilibrium temperature T0 increases and the point Ms also increases. It can also be considered that the external magnetic field actually compensates part of the chemical driving force with magnetic energy, and martensitic transformation can occur above the Ms point due to magnetic induction.
This phenomenon is very similar to strain-induced martensitic transformation from a thermodynamic point of view.
6. Conclusion
Through the introduction of this question, we should have clarity about the five factors that affect Mrs.
Of course, regularly reviewing these knowledge points will also play a beneficial role in our understanding of the knowledge points.
