*2.1.5 Cooling rate and solidification time*

Cooling rate '*CR*' is the heat loss during welding per unit time. *CR* plays an important role in determining the final solidification microstructure and its properties. Merchant Samir [27] studied the influence of welding current, arc voltage and welding speed on cooling rate, solidification time and hardness value of mild steel welded by MMAW process. It was found that the cooling rate decreased with increasing the welding current, while the solidification time increased for samples welded using different current and voltage values. The cooling rate increased with increasing


#### **Table 2.**

*Tensile test results [24].*

**Figure 13.** *Optical micrographs of weld samples (a) 316 L electrode, (b) 308 L electrode and (c) 310 electrode [24].*

welding speed, while the solidification time decreased. The best result for *HRN* is obtained in HAZ of all samples. Besides, Rahul Kumar et al. [28] examined the effect of welding variables and cooling rate on the mechanical behavior of mild steel welded by SAW. They found that the cooling rate and hardness increased with reducing the

**Figure 14.** *Microhardness profile across the weld specimens [24].*

**Figure 15.** *Potentiodynamic polarization plots of various sample [24].*


#### **Table 3.**

*Microstructural details of welding [24].*

**Figure 16.**

*Effect of welding current on (a) cooling time (b) solidification time of 316SS welding [26].*

heat input. Whereas a finer grain size was formed at higher cooling rate and lower heat input.

Effect of cooling rate on solidification and segregation characteristics of SASS was studied [9]. The grain size was refined more with increasing cooling rate. Dendrite arm spacing decreased at welding begin, then decreased slowly with increasing cooling rate. Transition cooling rate was 20°C/sec. Also, the effect of heat input on cooling rate and PREN in SDSS welds was studied [14]. Grain size and cooling rate increased with increasing heat input. Best results for PREN were obtained at an intermediate heat input value of 1.4 kJ/mm. Besides, Ahmed et al. [29] examined the effect of heat input and shielding gas on the Performance of 316SS welded by GTAW. They found that the heat input, cooling time, solidification time, grain size and nugget area increase with increasing the welding current. Besides, the cooling rate decreases with increasing the welding current. Whereas the UTS, YS and EL% decrease with increasing heat input, and the addition of 2%N2 to Ar shielding gas increases the mechanical properties of 316 stainless steel weld joints. The best mechanical properties are obtained at welding current 80 amp with Ar-2%N2. The hardness is lower in the weld zone than in the heat affected zone and base metal, and the addition of 2% N2 to shielding gas increases it. Moreover, the hardness decreases with increasing heat input (**Figures 19**–**26**).

#### **Figure 17.**

*Mechanical test results of TIG welded joints (a) tensile strength, (b) yield strength, and (c) percentage elongation [26].*

The cooling rate in the temperature range 800–500°C is important for phase transformation of stainless steel. It determines the final solidification mode or microstructure of the weld metal and its properties [30]. The cooling rate and cooling time [26, 31] can be calculated using Eq. (4) and Eq. (5), respectively.

$$\left(\frac{\partial \mathbf{T}}{\partial \mathbf{t}}\right)\_{\mathbf{x}} = \left(\frac{\partial \mathbf{T}}{\partial \mathbf{x}}\right)\_{\mathbf{t}} \* \left(\frac{\partial \mathbf{x}}{\partial \mathbf{t}}\right)\_{\mathbf{x}\mathbf{T}} = -2\pi \mathbf{K} \* \left(\frac{(\mathbf{T} - \mathbf{T}\_{\mathrm{o}})^2}{\mathbf{H}\_{\mathrm{net}}}\right) \tag{4}$$

$$\mathbf{t}\_{\\$\xi} = \frac{\text{HI}}{2\pi\lambda} \ast \left(\frac{\mathbf{1}}{500 - \mathbf{T}\_{\text{o}}} - \frac{\mathbf{1}}{800 - \mathbf{T}\_{\text{o}}}\right) \tag{5}$$

where, (∂T/∂t)*<sup>x</sup>* is the cooling rate '°C/sec', *K* or λ is the thermal conductivity (W/mmK)*,* and *T* is the temperature near the pearlite nose on TTT diagram '550°C' and To is the initial temperature of the plate '20°C'.

The solidification time 'St**'** of welding joint depends on the cooling rate and heat input. The St time is important as it affects the microstructure and properties, and can be calculated using Eq. (6) [26, 31]:

$$\text{St}\left(\text{sec}\right) = \text{L Hnet} / 2\pi \text{Kpc}\left(\text{T}\_{\text{m}} - \text{T}\_{\text{o}}\right)^2\tag{6}$$

*Effect of Welding Variables on the Quality of Weldments DOI: http://dx.doi.org/10.5772/intechopen.103175*

#### **Figure 18.**

*Vickers hardness profiles of 316SS TIG joint cross sections for different values of welding current using (a) ER309L, (b) ER316 L and (c) ERNiCrMo-3 as filler rods [26].*

#### **Figure 19.**

*Microstructure of weldments using various welding currents 80, 100 and 130 amp and pure argon as shielding gas, at different locations (a) and (b) [31].*

#### **Figure 20.**

*Microstructure of weldments using various welding currents 80, 100 and 130 Amp and Ar-2%N2 as shielding gas, at different locations (a) and (b) [31].*

#### **Figure 21.**

*Microstructure of weldments using various welding currents 80, 100 and 130 amp with/without N2 [31].*

## *2.1.6 Weld bead geometry*

The effect of weld bead area on mechanical properties was investigated [26, 31], and it was found that the nugget area 'Na' increases with increasing weld current and *Effect of Welding Variables on the Quality of Weldments DOI: http://dx.doi.org/10.5772/intechopen.103175*

#### **Figure 22.**

*Ultimate tensile strength of GTAW welded AISI 316SS using various welding currents 80, 100 and 130 Amp and Ar-2%N2 as shielding gas [31].*

#### **Figure 23.**

*Yield stress of GTAW welded AISI 316SS using various welding currents 80, 100 and 130 Amp and Ar-2%N2 as shielding gas [31].*

**Figure 24.** *Percentage elongation of 316SS welding specimens [31].*

**Figure 25.** *Hardness profiles of 316SS GTAW joint cross sections of weld joints using pure argon as shielding gas [31].*

**Figure 26.** *Hardness profiles of 316SS GTAW joint cross sections of weld joints using Ar-2% N2 as shielding gas [31].*

arc voltage, but decreases with increasing welding speed, and can be calculated with Eq. (7) (**Table 4**).

$$\text{Na} \,\left(\text{mm}^2\right) = \text{33312} \ast \text{10}^{-6} \ast \left[\text{A1.55/S0.903}\right] \tag{7}$$

where, Na is nugget area 'mm<sup>2</sup> ', A is the welding current in 'amp', and *S* is the welding speed 'mm/sec'.
