**3. Results and discussion**

#### **3.1 Microstructure**

#### *3.1.1 Base metal*

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

using the following empirical formula [30]:

were used to estimate the lattice parameter.

tensile testing m/c (Instron 8862).

calculated by using the formula:

X-ray diffraction (XRD) analysis was performed to estimate the volume fraction of retained austenite and its carbon content using anode Co-Kα radiation in 1.79026 targets with 24 kV and tube current was 40 mA. The specified 2*θ* range was varied from 30 to 110° with a step size of 0.2°/min. Detailed XRD analysis was performed using integrated intensities of the positions and the integrated intensities for the {1 1 1}, {2 2 0} and {3 1 1} planes of FCC austenite as well as the {1 1 0} and {2 1 1} planes of BCC ferrite. The volume fraction of retained austenite was calculated

*<sup>X</sup>*<sup>γ</sup> <sup>=</sup> \_\_\_\_\_\_\_\_\_\_\_\_\_ *<sup>I</sup>*γ/*R*γ (*<sup>I</sup>*γ/*R*) <sup>+</sup> (*I*α/*R*α) (1)

Where *Iγ* and *Iα* are the integrated intensities and *Rγ* and *Rα* are the theoretical relative intensity for the austenite and ferrite, respectively, and Bainitic ferrite was

*X*<sup>γ</sup> + *X*<sup>α</sup> + *Xg* = 1 (2)

Where, *Xγ* and *Xα* and *Xg* represent the volume percentage of retained austenite, volume percentage of bainitic ferrite and volume percentage of graphite. The carbon concentration of the austenite was determined using the equation [30].

*a*<sup>γ</sup> = 0.3548 + 0.0044*C*<sup>γ</sup> (3)

Vickers microhardness test of the weld metals was performed at room temperature using Leco Vickers microhardness tester (Model LM 248SAT) with 100 gf load at 10 s holding. The hardness values were taken from six different positions of each

Tensile properties such as ultimate tensile strength (UTS), yield strength (YS) and % elongation of the welded joints were evaluated using transverse tensile specimen keeping the weld metal at the center of the gauge length. The tests were performed under uniaxial loading at a crosshead speed of 5 mm/min in universal

Sub-size (55 × 10 × 3.3 mm) and without notch transverse Charpy impact test of the ADI welded joints were performed at room temperature according to ASTM E-23 [31]. Four samples were tested at each austempering condition (300 and 350°C

High cycle fatigue (HCF) test of transverse weld samples as per ASTM E466-15 [32] (**Figure 3**) were performed using Rumul resonant testing machine to determine

Where *aγ* is the lattice parameter of austenite (in nm) and *Cγ* is the carbon content of austenite (in wt.%). The {1 1 1}, {2 2 0} and {3 1 1} planes of austenite

weld specimens and the average of the six values considered the final one.

for the 2 h holding time) and an average of four values has been reported.

**132**

**Figure 3.**

*Schematic view of transverse high cycle fatigue sample as per ASTM 606.*

**Figure 4** shows the optical microstructure of as-cast DI (base metal). The microstructure shows graphite nodules surrounded with ferrite matrix. The average nodularity shows 90% with 130 nodules per unit area (mm2 ) and average nodule size is r = 18.5 μm.

#### *3.1.2 As-welded microstructure*

The optical microstructures of weld metals using two selected coated electrodes containing without and with Ce is shown in **Figure 5**. In **Figure 5a** and **b**, the microstructure shows ledeburitic carbide (LC), alloyed pearlite (AP) and graphite nodules (G). In both the weld metal microstructure shows small amount of graphite nodules with smaller in size due to higher cooling rate experienced in weld metal. Although both the as-weld microstructure shows similar microstructural appearance, a close look into the microstructure reveals difference in grain size and volume percentage of ledeburitic carbide and alloyed pearlite. The presence of Ce in weld metal has caused the structure finer (the finer the dendritic structure), lesser ledeburitic carbide, higher amount of alloyed pearlite and increasing the graphite volume percentage and nodularity.

It has been shown that cerium reduces both primary [33] and secondary [34] dendritic arm spacing as well as inhibit the development of columnar crystal.

**Figure 4.** *Optical microstructure of as-cast ductile iron.*

**Figure 5.** *Optical microstructure of as-welded weld metal (a) Trial 4 (b) Trial 7.*

Also, the degree of supercooling for rare earth treated steel has been reported to be smaller than rare earth free steel [35]. The refined microstructure (**Figure 5b**) that has been observed for Ce treated weld metal is presumably due to the fine primary austenite dendrite and suppression of columnar grain growth during solidification of the weld pool. Furthermore, it is believed that smaller degree of super cooling associated with Ce treated weld metal has caused reduction in ledeburitic carbide.

Ce acts as a modifying element on DI as a form of deoxidization and desulfuration [36]. Ce reacts with oxygen and sulfur to form Ce-rich oxides, Ce-rich sulfides or Ce-rich oxide-sulfides formed in DI welds and act as a heterogeneous nuclei of primary carbides, according to the principle of crystallography so that the nuclei of primary carbides can form and grow everywhere in molten metal [37] and refine the structure. Furthermore, cerium content present in the carbide as a form of Ce2S3 and CeO2 (measured by X-RD analysis) and increase the solidification rate to refine the structure [37].

#### *3.1.3 Austempered microstructure*

After austempering heat treatment the weld metal microstructure consists of bainitic ferrite (BF) and retained austenite (RA) matrix with graphite nodules (G). **Figure 6a** and **b** illustrate the weld metal microstructure after austempering at 300 and 350°C for 2 h holding time using Trial 4 electrode. Similarly, **Figure 6c** and **d** illustrate weld metal structure after austempering at 300 and 350°C for 2 h holding time using Trial 7 electrode. In both the weld metals austempering at 300°C, the microstructure (**Figure 6a** and **c**) shows needle shape bainitic ferrite, retained austenite and graphite nodules. Whereas at 350°C (**Figure 6b** and **d**) shows feathery shape (lath type) bainitic ferrite with retained austenite and graphite nodules.

For better clarity, the microstructures of heat treated weld metals after austempering at 300 and 350°C for 2 h holding time were studied under SEM and the structures are shown in **Figure 7** for without and with Ce containing weld metals respectively.

Interestingly, both the weld metal shows the same microstructural appearance at respective austempering conditions. But the structures were varied in morphology, amount, shape and size of bainitic ferrite, amount of retained austenite, nodule size and nodularity with changing the austempering conditions and type of electrode used (without and with Ce containing).

However, at 350°C more amount of retained austenite and lesser amount of bainitic ferrite was observed; but the opposite trend in microstructural constituents has been revealed at 300°C i.e. lower amount of retained austenite and higher amount bainitic ferrite. The nodularity also varied with varying the austempering temperatures and higher nodularity is observed at 350°C at both the weld metals. The microstructural constituents also changed with changing the austempering

**135**

**Figure 7.**

**Figure 6.**

*Analysis of Mechanical Properties of Austempered Ductile Iron Weld Joints Using Developed…*

holding time at a given temperature [9]. However, for both the austempering temperatures the variation of the microstructural constituent is similar i.e. with changing the holding time from 1.5 to 2 h the amount of retained austenite was increased and the amount of bainitic ferrite was decreased also refine the bainitic

*and (c) 300°C (d) 350°C for 2 h holding time using Trial 7 coated electrode.*

*Optical microstructure of weld metal austempered at (a) 300°C (b) 350°C for 2 h holding time using Trial 4* 

*SEM microstructure of weld metal austempered at (a) 300°C (b) 350°C for 2 h holding time using Trial 4 and* 

*(c) 300°C (d) 350°C for 2 h holding time using Trial 7 coated electrodes.*

*DOI: http://dx.doi.org/10.5772/intechopen.84763*

*Analysis of Mechanical Properties of Austempered Ductile Iron Weld Joints Using Developed… DOI: http://dx.doi.org/10.5772/intechopen.84763*

**Figure 6.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

*Optical microstructure of as-welded weld metal (a) Trial 4 (b) Trial 7.*

Also, the degree of supercooling for rare earth treated steel has been reported to be smaller than rare earth free steel [35]. The refined microstructure (**Figure 5b**) that has been observed for Ce treated weld metal is presumably due to the fine primary austenite dendrite and suppression of columnar grain growth during solidification of the weld pool. Furthermore, it is believed that smaller degree of super cooling associated with Ce treated weld metal has caused reduction in ledeburitic carbide. Ce acts as a modifying element on DI as a form of deoxidization and desulfuration

[36]. Ce reacts with oxygen and sulfur to form Ce-rich oxides, Ce-rich sulfides or Ce-rich oxide-sulfides formed in DI welds and act as a heterogeneous nuclei of primary carbides, according to the principle of crystallography so that the nuclei of primary carbides can form and grow everywhere in molten metal [37] and refine the structure. Furthermore, cerium content present in the carbide as a form of Ce2S3 and CeO2 (measured by X-RD analysis) and increase the solidification rate to refine the structure [37].

After austempering heat treatment the weld metal microstructure consists of bainitic ferrite (BF) and retained austenite (RA) matrix with graphite nodules (G). **Figure 6a** and **b** illustrate the weld metal microstructure after austempering at 300 and 350°C for 2 h holding time using Trial 4 electrode. Similarly, **Figure 6c** and **d** illustrate weld metal structure after austempering at 300 and 350°C for 2 h holding time using Trial 7 electrode. In both the weld metals austempering at 300°C, the microstructure (**Figure 6a** and **c**) shows needle shape bainitic ferrite, retained austenite and graphite nodules. Whereas at 350°C (**Figure 6b** and **d**) shows feathery shape (lath type) bainitic ferrite with retained austenite and graphite nodules.

For better clarity, the microstructures of heat treated weld metals after austempering at 300 and 350°C for 2 h holding time were studied under SEM and the structures are shown in **Figure 7** for without and with Ce containing weld metals respectively. Interestingly, both the weld metal shows the same microstructural appearance at respective austempering conditions. But the structures were varied in morphology, amount, shape and size of bainitic ferrite, amount of retained austenite, nodule size and nodularity with changing the austempering conditions and type of electrode

However, at 350°C more amount of retained austenite and lesser amount of bainitic ferrite was observed; but the opposite trend in microstructural constituents has been revealed at 300°C i.e. lower amount of retained austenite and higher amount bainitic ferrite. The nodularity also varied with varying the austempering temperatures and higher nodularity is observed at 350°C at both the weld metals. The microstructural constituents also changed with changing the austempering

*3.1.3 Austempered microstructure*

**Figure 5.**

used (without and with Ce containing).

**134**

*Optical microstructure of weld metal austempered at (a) 300°C (b) 350°C for 2 h holding time using Trial 4 and (c) 300°C (d) 350°C for 2 h holding time using Trial 7 coated electrode.*

holding time at a given temperature [9]. However, for both the austempering temperatures the variation of the microstructural constituent is similar i.e. with changing the holding time from 1.5 to 2 h the amount of retained austenite was increased and the amount of bainitic ferrite was decreased also refine the bainitic

#### **Figure 7.**

*SEM microstructure of weld metal austempered at (a) 300°C (b) 350°C for 2 h holding time using Trial 4 and (c) 300°C (d) 350°C for 2 h holding time using Trial 7 coated electrodes.*

ferrite shape and size. Further increasing the austempering holding time from 2 to 2.5 h the amount of retained austenite was decreased and the amount of bainitic ferrite was increased. Interestingly, at both 300 and 350°C austempering temperature for 2 h holding time the carbon enrichment in austenite is maximum (**Figure 9**) which has caused to stabilize more amount of retained austenite (**Figure 8**) at both the austempering conditions after cooling to room temperature. However, at higher holding time (2.5 h) untransformed austenite transformed to carbides (ε carbide) and bainitic ferrite leading to decrease the amount of retained austenite content [1].

The observed finer and homogeneous structure along with increasing the amount of retained austenite (**Figure 8**) with the presence of Ce content in weld metal. At both 300 and 350°C austempering temperature microstructure attributed finer the bainitic ferrite size, higher amount of carbon enriched retained austenite and higher graphite nodularity was observed with presence of Ce in weld metal. The carbon enrichment of austenite will be faster in Ce treated weld metal due to lesser diffusion distance for carbon, which diffuses from fine cementite lamellae of pearlite. Also, smaller the nodule size having more surface area to volume ratio will favor carbon diffusion from graphite [38]. Thus, with the increase of carbon content of initial austenite the driving force of stage I reaction become slow and delay the transformation rate of bainitic ferrite due to drag effects of Ce. As a result more amount of carbon was diffused to the surrounding austenite and austenite become more stable.

#### *3.1.4 Volume percentage of retained austenite and its carbon content*

The volume percentage of retained austenite and its carbon content of both the weld metals after austempering at 300 and 350°C for different holding times have been calculated from X-RD analysis. The variation of retained austenite with changing the holding time (1.5, 2 and 2.5 h) at 300 and 350°C austempering temperature has been plotted in **Figure 8**. In **Figure 8**, it is seen that at both 300 and 350°C temperature both the weld metal (without and with Ce containing) the amount of retained austenite was changed with changing the austempering holding time. Holding time changed from 1.5 to 2 h the amount of retained austenite was increased with decreasing the amount of bainitic ferrite. With further increases the holding time from 2 to 2.5 h the amount of retained austenite was decreased. Although the nature of change the amount of retained austenite is same at both

#### **Figure 8.**

*Volume percentage of retained austenite content at different holding time of weld metal austempering at (a) 300°C and (b) 350°C using Trial 4 and Trial 7 electrodes.*

**137**

**Figure 9.**

*using Trial 4 and Trial 7 electrodes.*

*Analysis of Mechanical Properties of Austempered Ductile Iron Weld Joints Using Developed…*

the austempering temperature of both the weld metals, austempering at 350°C shows the higher amount of retained austenite compared to 300°C at each holding time. This could be due to the higher diffusion rate of carbon during bainitic ferrite

Bainitic ferrite formation is a growth process; during austempering process bainitic ferrite is transformed from the existing austenite (residual austenite). During the transformation of bainitic ferrite, carbon was diffused from the bainitic ferrite to the surrounding austenite to make the austenite stable, and this austenite is called untransformed austenite or retained austenite. At lower austempering temperature, due to higher super cooling the transformation rate of bainitic ferrite is high and diffused less amount of carbon to the surrounding austenite, as a result formed less amount of retained austenite in weld metals. Furthermore, at higher austempering temperatures, due to lower supper cooling the growth of bainitic ferrite is high and diffused higher amount of carbon content to the surrounding

austenite and shows higher amount of retained austenite in weld metals.

microstructure to indicate higher toughness and longer fatigue life.

austenite at 350°C for 2 h holding time.

Ce is a modifying alloying element and act as a nodularizing and austempering element during austempering process. The presence of Ce in weld metal to slow the austempering kinetics and prolongs to the stage I reaction. As a result higher amount of carbon was diffused during the transformation of bainitic ferrite to the surrounding austenite and show a higher amount of retained austenite at both the austempering temperature than without Ce content weld metal also refine the

Furthermore, the carbon content of retained austenite in two weld metals (without and with Ce containing) at 300 and 350°C for 1.5, 2 and 2.5 h holding time have been determined using the empirical formula (2). The calculated results of the amount of carbon present in austenite at 300 and 350°C for different holding times have been plotted in **Figure 9**. In **Figure 9**, it is seen that, in both the weld metals, the trend in variation of carbon content in retained austenite with changing the austempering holding at 300 and 350°C is similar to the variation of retained austenite (**Figure 8**). The maximum amount of carbon content was achieved at 2 h holding time irrespective of the austempering temperature and electrode composition. Weld metal containing Ce show the highest carbon content (2.2 wt.%) in retained

During austempering transformation, bainitic ferrites are nucleated out of austenite (residual austenite) to refusing the carbon content to the surrounding austenite

*Austenitic carbon content of weld metal at different holding time austempering at (a) 300°C and (b) 350°C* 

*DOI: http://dx.doi.org/10.5772/intechopen.84763*

transformation at higher austempering temperature [39].

#### *Analysis of Mechanical Properties of Austempered Ductile Iron Weld Joints Using Developed… DOI: http://dx.doi.org/10.5772/intechopen.84763*

the austempering temperature of both the weld metals, austempering at 350°C shows the higher amount of retained austenite compared to 300°C at each holding time. This could be due to the higher diffusion rate of carbon during bainitic ferrite transformation at higher austempering temperature [39].

Bainitic ferrite formation is a growth process; during austempering process bainitic ferrite is transformed from the existing austenite (residual austenite). During the transformation of bainitic ferrite, carbon was diffused from the bainitic ferrite to the surrounding austenite to make the austenite stable, and this austenite is called untransformed austenite or retained austenite. At lower austempering temperature, due to higher super cooling the transformation rate of bainitic ferrite is high and diffused less amount of carbon to the surrounding austenite, as a result formed less amount of retained austenite in weld metals. Furthermore, at higher austempering temperatures, due to lower supper cooling the growth of bainitic ferrite is high and diffused higher amount of carbon content to the surrounding austenite and shows higher amount of retained austenite in weld metals.

Ce is a modifying alloying element and act as a nodularizing and austempering element during austempering process. The presence of Ce in weld metal to slow the austempering kinetics and prolongs to the stage I reaction. As a result higher amount of carbon was diffused during the transformation of bainitic ferrite to the surrounding austenite and show a higher amount of retained austenite at both the austempering temperature than without Ce content weld metal also refine the microstructure to indicate higher toughness and longer fatigue life.

Furthermore, the carbon content of retained austenite in two weld metals (without and with Ce containing) at 300 and 350°C for 1.5, 2 and 2.5 h holding time have been determined using the empirical formula (2). The calculated results of the amount of carbon present in austenite at 300 and 350°C for different holding times have been plotted in **Figure 9**. In **Figure 9**, it is seen that, in both the weld metals, the trend in variation of carbon content in retained austenite with changing the austempering holding at 300 and 350°C is similar to the variation of retained austenite (**Figure 8**). The maximum amount of carbon content was achieved at 2 h holding time irrespective of the austempering temperature and electrode composition. Weld metal containing Ce show the highest carbon content (2.2 wt.%) in retained austenite at 350°C for 2 h holding time.

During austempering transformation, bainitic ferrites are nucleated out of austenite (residual austenite) to refusing the carbon content to the surrounding austenite

#### **Figure 9.**

*Austenitic carbon content of weld metal at different holding time austempering at (a) 300°C and (b) 350°C using Trial 4 and Trial 7 electrodes.*

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*3.1.4 Volume percentage of retained austenite and its carbon content*

The volume percentage of retained austenite and its carbon content of both the weld metals after austempering at 300 and 350°C for different holding times have been calculated from X-RD analysis. The variation of retained austenite with changing the holding time (1.5, 2 and 2.5 h) at 300 and 350°C austempering temperature has been plotted in **Figure 8**. In **Figure 8**, it is seen that at both 300 and 350°C temperature both the weld metal (without and with Ce containing) the amount of retained austenite was changed with changing the austempering holding time. Holding time changed from 1.5 to 2 h the amount of retained austenite was increased with decreasing the amount of bainitic ferrite. With further increases the holding time from 2 to 2.5 h the amount of retained austenite was decreased. Although the nature of change the amount of retained austenite is same at both

ferrite shape and size. Further increasing the austempering holding time from 2 to 2.5 h the amount of retained austenite was decreased and the amount of bainitic ferrite was increased. Interestingly, at both 300 and 350°C austempering temperature for 2 h holding time the carbon enrichment in austenite is maximum (**Figure 9**) which has caused to stabilize more amount of retained austenite (**Figure 8**) at both the austempering conditions after cooling to room temperature. However, at higher holding time (2.5 h) untransformed austenite transformed to carbides (ε carbide) and bainitic ferrite leading to decrease the amount of retained austenite content [1]. The observed finer and homogeneous structure along with increasing the amount of retained austenite (**Figure 8**) with the presence of Ce content in weld metal. At both 300 and 350°C austempering temperature microstructure attributed finer the bainitic ferrite size, higher amount of carbon enriched retained austenite and higher graphite nodularity was observed with presence of Ce in weld metal. The carbon enrichment of austenite will be faster in Ce treated weld metal due to lesser diffusion distance for carbon, which diffuses from fine cementite lamellae of pearlite. Also, smaller the nodule size having more surface area to volume ratio will favor carbon diffusion from graphite [38]. Thus, with the increase of carbon content of initial austenite the driving force of stage I reaction become slow and delay the transformation rate of bainitic ferrite due to drag effects of Ce. As a result more amount of carbon was diffused to the surrounding austenite and austenite

**136**

**Figure 8.**

become more stable.

*Volume percentage of retained austenite content at different holding time of weld metal austempering at* 

*(a) 300°C and (b) 350°C using Trial 4 and Trial 7 electrodes.*

and austenite become stabilized. Thus, maximum stability of retained austenite should possess at 2 h holding time, irrespective of austempering temperature of both the weld metals.
