*3.2. Ferrite Hardening Variation*

The alteration of ferrite hardening as a function of SQ holding time in conjunction with the changes in ferrite volume fraction, are illustrated in Figure 3. Figure 3a illustrates that the progress of ferrite formation can be characterized with a typical S-shaped curve indicating that the phase transformation of prior austenite to ferrite can be rationalized by a general diffusional nature of nucleation and growth for ferrite formation during SQ holding at 720 ◦C. The variation of ferrite grain size against SQ holding time is indicated in Figure 3b. The abnormal trend in ferrite hardness as a function of SQ holding time is illustrated in Figure 3c. Accordingly, the ferrite microhardness initially decreased from 352 to 217HV5g with the increase in SQ holding time from 1 to 15 min, beyond which it surprisingly increased to 245HV5g with further increase in SQ holding time to 30 min. It is interesting to emphasize that the minimum ferrite microhardness occurred in the ferrite-martensite DP specimens SQ15 consisting of 13% ferrite volume fraction with remainder phase fraction as martensite, and subsequent increase in ferrite hardness was realized for the SQ30 samples with essentially ferrite-pearlite microstructures. The minimum ferrite hardening response can be fully supported by the verity that the SQ heat treatment at longer holding time can be related to the more ferrite formation, in addition to the occurrence of significantly thermally activated relaxation, which decreases the accumulation of transformational residual stresses causing a lower hardness level in ferrite. In addition to the occurrence of these ferrite softening phenomena during longer holding times at 720 ◦C, the abnormal higher ferrite hardness realized in SQ30 samples suggests that another ferrite hardening mechanism must be operational, such as solid solution hardening effects caused by diffusion of substitutional alloying elements leading to simultaneous hardening of ferrite during the progress of its formation.

To examine the hardening alteration of ferrite grains more precisely, the microhardness test has been accomplished at different locations within certain ferrite grains using various SQ heat-treated samples. Typical light optical micrograph in Figure 4 shows an example of microhardness method followed, indicating indentation impressions taken from various positions (the central regions of ferrite grains and the ferrite regions adjacent to the ferriteprior austenite interfaces) of ferrite grains using SQ5 heat-treated samples. It is obvious that the minimum ferrite microhardness has been associated to the central location of ferrite grains, and it increases as the microhardness test location is moved towards ferrite-prior austenite interfaces, thus clarifying that the deformation resistance of ferrite is related to the position of microhardness testing location. Therefore, as the microhardness location is moved from the central location of ferrite grains towards the ferrite area adjacent to the ferrite-prior austenite interfaces, the average ferrite microhardness increased from 122 to 145HV1g for the SQ5 specimens.

**Figure 3.** Changes in: (**a**) volume fraction of ferrite as a function of SQ holding time; (**b**) ferrite grain size against SQ holding time; and (**c**) ferrite microhardness versus SQ holding time. An abnormal trend in respect of ferrite hardening occurred during the progress of ferrite formation in the SQ30 heat-treated samples. The ferrite microhardness measurements are related with a fixed loading force of 5 g.

**Figure 4.** Typical light micrograph with superimposed locations at which ferrite microhardness tests were carried out and the associated ferrite microhardness data showed the alteration of ferrite hardening within the specific ferrite grains recorded on heat-treated SQ5 ferrite-martensite DP specimens. A greater ferrite hardening response was realized in the areas adjacent to the martensite in contrast to the central areas of ferrite grains. The ferrite microhardness data are measured with a fixed loading force of 1 g. F: Ferrite; M: Martensite [25].
