Q2. How can a high volume fraction of acicular ferrite be obtained?
3. Lower cooling rates cause the development of other phases such as allotriomorphic ferrite or pearlite but a high volume fraction of acicular ferrite can be achieved using two stage continuous cooling cycles.
Q3. What is the composition of the microstructure at 550°C?
The microstructure is mainly composed of poligonal ferrite and pearlite, with acicular ferrite, which is obtained at lower temperatures.
Q4. What is the main interest of the paper?
Its main interest lies in the good combination of mechanical properties that presents as compared with bainite and above all with ferritic-perlitic microstructures.
Q5. What is the effect of the low cooling rate on the formation of acicular ferrite?
2. The observed inhibition of grain boundary nucleation of bainite enables the formation of fully acicular microstructures, although the high cooling rates (30°C/s) needed to completely avoid the formation of allotriomorphic ferrite reduce its industrial applicability.
Q6. What is the nature of acicular ferrite?
It is accepted that nucleation of acicular ferrite takes place inside the austenite grains at non metallic inclusions, whose nature and number can determine the transition from an acicular ferrite microstructure to a bainitic one.
Q7. How does the ferrite microstructure be characterized?
3The analysis of the microstructures obtained with isothermal treatments has shown the need for using temperatures below 500°C to ensure a high volume fraction of acicular ferrite plates.
Q8. What is the cooling rate required to avoid the formation of martensite?
The results have shown that if the cooling rates required to avoid the formation of a high volume fraction of allotriomorphic ferrite and pearlite are maintained until room temperature, the development of martensite is inevitable.
Q9. What is the result of the acicular ferrite?
The result is an acicular microstructure where the high disorientation and chaotic arrangement does not occur between individual ferrite plates but between these intragranular sheaves.
Q10. What temperature is the microstructure obtained at 550°C?
Lowering the treatment temperature to 450°C results in a significant increase of the number of ferrite plates at the expense of poligonal ferrite and pearlite, and the resultant microstructure has a clearly acicular appearance, see Fig. 1.d.
Q11. What is the cooling rate required to avoid the formation of a high volume fraction of allo?
In the present case, this phase is not desired in the microstructure and, in order to elude its formation, the cooling rates have to be lowered after reaching the range of temperatures where acicular ferrite develops.
Q12. How does the CCT diagram show the acicular ferrite?
The CCT diagram presented in Fig. 2 shows that the non-isothermal austenite-to-acicular ferrite transformation in this steel occurs at cooling rates ranging from 25°C/s to 1°C/s.
Q13. What is the heat treatment for acicular ferrite?
As a result, a two stage continuous cooling cycle is proposed as the best heat treatment to produce acicular ferrite with a low volume fraction of other phases such as alotriomorphic ferrite, pearlite, and/or martensite.
Q14. What is the cooling rate of the acicular ferrite?
The former is avoided cooling down at about 7°C/s and the latter is suppressed if cooling is carried out at 30°C/s. Nevertheless, as can be seen in the micrograph of Fig. 3.a, when the samples are continuously cooled down at 30°C/s, the obtained microstructure is martensite, which is not a desired phase due to its detrimental effect in toughness.
Q15. What is the effect of cooling down the austenite?
The decomposition of the austenite is not completed before reaching the martensite start temperature, which causes the appearance of this phase at the end of the cooling.
Q16. What is the temperature of the cooling?
Micrographs of the isothermal treatments presented in Fig. 1 indicate that the formation of a great number of acicular ferrite plates takes place under 500°C, which has motivated the choice of this temperature, below the acicular ferrite start temperature for all the cooling rates tested in the CCT, as the temperature where the cooling rate is lowered.