*3.3.1 Hardness profile analysis*

**Figure 6** shows the hardness profiles of different types of steel rebar samples (along the cross-sectional diameter). It has been reported earlier that the hardness value of TMT rebar samples becomes maximum at the periphery due to the presence of tempered martensite, which gradually decreases towards the

### **Figure 5.** *SEM micrographs of plain rebar sample showing (a) no transition zone and (b) pearlite at higher magnification.*

centre due to the formation of ferrite-pearlite mixed microstructure thereby exhibiting the classical U-shaped profiles [13, 23, 45]. It has been observed that the hardness value of Fe 600 and galvanized rebar samples in the periphery or outer rim region is around ≈285 HV, which is an indication of the presence of the tempered martensite or bainite phase [23, 45]. It has been also observed that the hardness value of the transition zone is ≈220 HV for both TMT rebar samples due to the presence of lower bainite, whereas for the core region or ferrite-pearlite region, the hardness value is ≈200 HV for Fe 600 rebar and ≈180 HV for galvanized rebar, respectively.

**Figures 7(a)** and **(b)** displays the comparative hardness variations for stainless steel and plain rebar samples, respectively. It is evident from **Figure 7** that the stainless steel rebar shows higher values of hardness (≈195 HV to 260 HV) than the plain rebar (≈145 HV to 205 HV). Previous studies have confirmed that the hardness value in steel samples predominantly depends on the carbon equivalent value (CEV) [23]. It has been also reported that the weldability primarily depends on CEV for carbon steel; that is, higher CEV leads to a hard and brittle heat-affected zone (HAZ). The microstructure in the HAZ zone has an important role in the mechanical properties of the weldment; therefore, CEV is an important parameter for rebars [46]. **Table 2** summarizes the estimated carbon equivalent values of different rebar samples that were calculated by using Eq. (1). It can be seen in **Table 2** that the plain rebar sample shows the lowest CEV (≈0.2163), thereby showing the lowest value of hardness among all the samples. It is imperative to mention here that although the stainless steel rebar consists of only equiaxed ferrite microstructure (**Figure 4**), due to high carbon equivalent value and the presence of other alloying elements such as Mn, Si, Ni (**Table 1**), and it shows a higher hardness value. It cannot be domineered that Fe 600 steel rebar has a higher value of CEV (≈0.3383) than that of galvanized rebar (≈0.3227), which results in a higher core hardness value (≈196 HV) (**Figure 6**) when compared with that of galvanized rebar sample (≈184 HV). The aforesaid two TMT rebars with CEV < 0.42 according to IS 1786:2008 standard will exhibit superior weldability. However, the

**Figure 6.** *A comparison of hardness data among all the rebar specimens.*

*Study on the Perspective of Mechanical Properties and Corrosion Behaviour of Stainless… DOI: http://dx.doi.org/10.5772/intechopen.101388*


**Table 2.**

*Estimated carbon equivalent value (CEV) for different rebar samples.*

stainless steel rebar (ferritic grade) shows higher CEV (≈2.7323) because of higher Cr content but it can be welded in accordance with IS 16651:2017.

$$\text{CEV} = \text{C} + \text{Mn}/\text{6} + \{(\text{Cr} + \text{Mo} + \text{V})/\text{5}\} + \left\{\frac{\text{Cu} + \text{Ni}}{\text{15}}\right\} \tag{1}$$

(Where all the elemental values are expressed in wt.%)
