**2. Materials and Methods**

In the present investigation, a commercial grade of medium silicon low alloy 35CHGSA steel was used with the chemical composition indicated in Table 1. The heat treatment schedules were designed to achieve multiphase microstructures including various volume fractions of ferrite, pearlite, martensite and retained austenite microconstituents. The practical heat treatment processes practical the following sequential stages: (a) normalizing after austenitising at 900 ◦C for 15 min, (b) re-austenitising at 900 ◦C for 15 min to get fully homogenous prior austenite grains, (c) step-quenching (SQ) in a salt bath at 720 ◦C for 1–30 min to achieve different volume fractions of ferrite, and (d) rapid water quenching to transform all the remaining prior austenite to martensite. For each set of ferrite–martensite and ferrite–pearlite DP microstructure, three specimens were heat-treated in order to confirm the reproducibility of DP microstructures with respect to volume fractions of ferrite, pearlite, and martensite. In this way, the related specimens were marked as SQ1, SQ5, SQ15, and SQ30, representing the SQ holding times of 1, 5, 15 and 30 min, respectively, in salt bath maintained at 720 ◦C prior to water quenching. The applied heat treatment cycles are indicated schematically in Figure 1.

The metallography of heat-treated specimens was carried out on the transverse section surfaces relative to hot rolling direction of as-received strap specimens according to ASTM E 3 standard [20]. The polished specimens were etched with a 2% Nital solution (2 mL HNO3 and 98 mL C2H5OH) [21] to reveal various microstructural features with good contrasting resolution. The ferrite, pearlite, and martensite volume fractions were measured according to the ASTM E562-02 standard [22]. The microstructural observations were carried out using a Olympus-PMG3 light microscope and a TESCAN-MIRA 3-XMU field-emission scanning electron microscope (FE-SEM) operating at an accelerated voltage of 15 kV. Both the spot and line scan energy dispersive X-ray spectroscopy (EDS) analysis techniques were largely applied to identify the carbon, Si, Cr and Mn concentrations in different locations of ferrite. Microhardness tests were conducted within ferrite, pearlite, and martensite microphases using a load of 5 g for ferrite grain and a somewhat higher load of 10 g for pearlite and martensite areas, applied for 20 s duration using a FM700 Future Tech microhardness tester, and the related data were presented as Vickers hardness numbers (VHNs). In addition, microhardness tests were also carried out at different locations of select ferrite grains, but with a small load of 1 g applied for 20s duration loading time using a Future Tech microhardness tester machine model FM700. Two different locations of each step-quenched heat-treated samples were considered for conducting microhardness tests: (1) the central area of ferrite grain, and (2) the ferrite region close to the ferrite-martensite interfaces. The microhardness data were collected from at least five readings per sample, and the data were reported as Vickers hardness numbers (VHNs).

**Table 1.** The chemical composition of researched low alloy, medium silicon commercial grade of 35CHGSA steel (in wt%).


**Figure 1.** The schematic diagrams indicating heat treatment cycles used to achieve microstructures involving different volume fractions of ferrite, pearlite and martensite microphases. WQ: water quench; SQ: step quench [10].

The X-ray diffraction analysis was also carried out on a AW-DX300 Asenware X-ray diffractometer using copper Kα radiation (1.54184 Å) and angular step size (in 2θ) of 0.05 degrees with a time step of 1 s. The residual stresses were quantified using the Sin2Ψ analysis technique [23,24]. The following expression was used to calculate the amount of residual stress within various SQ specimens [24]:

$$\frac{d\_{\Phi\Psi} - d\_0}{d\_0} = \frac{1+v}{E} \,\sigma\_{\Phi} \sin^2 \Psi - \frac{v}{E} (\sigma\_{11} - \sigma\_{22}) \tag{1}$$

where *d*φ<sup>Ψ</sup> is the d-spacing of diffracted planes oriented at an angle Ψ from the surface normal, *d*<sup>0</sup> is the d-spacing of (hkl) planes at Ψ = 0 deg, *v* is poisson's ratio, *E* is young's modulus, *σ*<sup>11</sup> and *σ*<sup>22</sup> are principal stresses parallel to the surface, and *σ*<sup>φ</sup> is the stress acting in a chosen direction of surface at an angle φ from σ11. The *σ*<sup>φ</sup> factor accounts for differences in specimen orientation, and is related to *σ*<sup>11</sup> and *σ*<sup>22</sup> parameters. The term (*σ*<sup>11</sup> − *σ*22) is constant for a given specimen, and Equation (1) shows that a linear variation between *<sup>d</sup>*φ<sup>Ψ</sup> and sin<sup>2</sup> <sup>Ψ</sup> is expected [23]. The term (*d*φ<sup>Ψ</sup> <sup>−</sup> *<sup>d</sup>*0)/*d*<sup>0</sup> can be plotted against sin2 Ψ for several Ψ angles for a particular diffracted plane, and, the stress *σ*<sup>φ</sup> can be calculated from the slope. When, the deviation of *d* with sin2 Ψ data occurs, the residual stress can be determined using additional mathematical and experimental treatments.
