**1. Introduction**

Austempered ductile iron (ADI) is a new family member of engineering materials. It has recently received significant attention owing to the excellent combination of mechanical properties such as high strength together with good ductility, good wear resistance, and higher fatigue strength to make the material as a successful substitute for forge steels or aluminum alloys [1–4]. The remarkable properties of the ADI are attributed with the unique microstructural constituents such as bainitic ferrite and high carbon enriched retained austenite.

The low production cost and production advantage of ADI, it has been used in many structural applications and many critical parts of automobiles such as crankshaft, steering knuckles, hypoid rear axle gears, camshafts and disk-brake calipers etc. in which fatigue resistance is an important requirement [5, 6].

ADI shows higher fatigue life than as-cast ductile iron (DI) and determined by generating stress-life (S-N) curves [7]. The fatigue life of ADI strongly depends on the austempering temperature, austempering holding time, austempering kinetics, the amount of retained austenite, shape and size of bainitic ferrite and graphite nodules [1, 3].

The chemical composition of ADI is similar to conventional DI. However, some alloying elements such as Ni, Cu and Mo are usually added to increase the austemperability, to delay the austenite decomposition [8, 9] to pearlite and ferrite upon cooling. DI converts to ADI with the help of two steps isothermal heat treatment process. Austenitization was done at 850–910°C for 30 min–2 h holding time and austempering at 250–450°C for 5 min–4 h holding time and finally cooled at room temperature in the open air [10]. The total heat treatment process depends on the chemical composition and thickness of the as-cast DI. Austempering temperature and holding time shows significant effects on the microstructure and mechanical properties of ADI. During austempering two-stage reactions have been done, at the initial stage (stage I) austenite (residual austenite) decomposed into bainitic ferrite and high carbon austenite (retained austenite). Increasing holding time the stage II reaction was start and high carbon austenite further decomposed into bainitic ferrite and carbide (ɛ carbide). The time periods between the two stage reactions is called the process windows, obtained optimum combination of microstructural and mechanical properties. Presence of alloying elements such as Ni, Cu and Mn to delay the austempering reaction and increased the process windows [11].

At a lower austempering temperature ADI shows needle shaped bainitic ferrite with a lower amount of retained austenite and graphite nodules, which in turns increased the tensile strength and hardness to decrease the elongation and toughness. However, at higher austempering temperature shows coarser bainitic ferrite with higher amount of retained austenite and graphite nodules, as a result to decrease the tensile strength and hardness; increased toughness that illustrate higher fatigue strength [6, 10, 12].

It is reported, the fatigue strength of ADI is not only depended on tensile strength and hardness like as steel [13]. However, in ADI the amount of retained austenite and its carbon content, size of graphite nodules, nodularity, shape and size of bainitic ferrite plays an important role in the high cycle fatigue performance and higher fatigue limit [1, 13–15]. Bahmani et al. [16] illustrate a relationship between the microstructure and fatigue strength of ADI and obtained, the fatigue strength depended on the amount of retained austenite and its carbon content. The fatigue strength was increased as increasing the amount of retained austenite and its carbon content. Graphite nodularity and its size show significant effects in fatigue life. In ADI graphite working as a shrinkage cavities. During fatigue test, micro crack was initiated around the graphite and then formed to macro crack which leads to the final failure of the sample [17]. Sofue et al. [18] reported, with increasing the graphite size to decrease the fatigue strength and the fatigue strength was optimized by decreasing the graphite nodule size.

Further, rare earth metal such as cerium has a beneficial effect on the microstructure and properties of ADI. However, the optimum rare earth content varies significantly according to different investigators. For example, researchers [19] reported that the presence of Ce content from 0.005 to 0.014% the nodularity was increased with refining the size of the nodules but further increasing Ce content up to 0.018 or 0.020%, the nodularity decreased and formed some non-spherical graphite with coarsening the nodule size to decreased the fatigue strength. Choi et al. [20] observed that DI castings with 0.3% rare earth content attributed improved graphite nodules, lower tensile strength and hardness, higher elongation to indicate the higher fatigue strength than that of DI castings without rare earth.

However, in spite of high strength, reasonable ductility and higher fatigue life, the application of ADI is somehow limited due to non-availability of a suitable electrode which has inhibited the joining of such high potential material.

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**Table 1.**

shown in **Figure 2**.

*Chemical composition of as-cast ductile iron.*

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

Furthermore, many a time ductile iron castings for converting ADI need to repair welding which also demands welding consumables compatible with DI base materials. Commercially available coated electrodes for welding conventional DI include pure nickel (90–97%) [21], stainless steel and iron-nickel [22] which are first of all not suitable for converting ADI from DI weld due to poor austemperability and also not cost-effective [21, 22]. Recent authors successfully developed a coated electrode for welding DI [23] and convert to ADI after heat treatment, obtained higher tensile strength and toughness. Further improvement of the weldability of DI and welding of higher grade DI to obtain higher toughness and fatigue strength, coated

electrode was developed to introduce nano-CeO2 as a flux ingredient [24].

the fatigue properties of ADI weld metal to use in commercial application.

However, all the previous literature discussed about the fatigue strength or fatigue life only on DI or ADI base metal [1, 14, 25, 26]. But there is no such a literature to discuss about the fatigue life of ADI weld joints. Owing to the importance of the ADI especially on structural and automobile application, it is needed to find out

The present work thus aims is to study the mechanical properties of ADI weld joints and co-relation between as-cast and heat treated microstructure, using two developed electrodes. Microstructural studies were done using optical microscopy (OM) and scanning electron microscopy (SEM). Phase analysis was performed utilizing XRD analysis and mechanical properties of the weld joints performed under microhardness, transverse tensile, room temperature Charpy impact and high cycle fatigue (HCF) test. Fatigue crack growth and fatigue fracture surface

As cast DI was collected from the local foundry and used as a base metal for this experimental purpose. The details chemical composition of the as-cast DI is given

Two developed electrodes containing without and with Ce such as Trial 4 and Trial 7 selected for the experimental purpose. Among the two electrodes, Trial 4 contents with without Ce and Trial 7 contents with Ce content (0.1%). The details

Modified U groove (**Figure 1**) weld was performed on the DI base plate using both Trial 4 and Trial 7 electrodes applied shielded metal arc welding (SMAW) process. Preheat was applied at 300°C for 1 h and post weld heat treatment (PWHT) at 300°C for 1 h immediate after welding using the constant welding parameters [27, 28]. The details welding parameters are given in **Table 3** and the

The welded DI specimens are converted to ADI with the help of two steps isothermal heat treatment process. Austenitization was done at 900°C for 2 h holding time and then samples are immediately transferred to salt bath for austempering process. Austempering was done at 300 and 350°C for 1.5, 2 and 2.5 h holding time than air cooled to room temperature. Typical isothermal heat treatment cycle is

**Element C Si Mn S Cr Mg P Fe** Wt.% 3.60 2.92 0.22 0.019 0.028 0.041 0.01 93.16

chemical composition of the two developed electrodes is given in **Table 2**.

defect free weld was considered as per AWS (D11) [29].

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

were investigated under SEM studies.

**2. Experimental procedure**

in **Table 1**.

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

Furthermore, many a time ductile iron castings for converting ADI need to repair welding which also demands welding consumables compatible with DI base materials. Commercially available coated electrodes for welding conventional DI include pure nickel (90–97%) [21], stainless steel and iron-nickel [22] which are first of all not suitable for converting ADI from DI weld due to poor austemperability and also not cost-effective [21, 22]. Recent authors successfully developed a coated electrode for welding DI [23] and convert to ADI after heat treatment, obtained higher tensile strength and toughness. Further improvement of the weldability of DI and welding of higher grade DI to obtain higher toughness and fatigue strength, coated electrode was developed to introduce nano-CeO2 as a flux ingredient [24].

However, all the previous literature discussed about the fatigue strength or fatigue life only on DI or ADI base metal [1, 14, 25, 26]. But there is no such a literature to discuss about the fatigue life of ADI weld joints. Owing to the importance of the ADI especially on structural and automobile application, it is needed to find out the fatigue properties of ADI weld metal to use in commercial application.

The present work thus aims is to study the mechanical properties of ADI weld joints and co-relation between as-cast and heat treated microstructure, using two developed electrodes. Microstructural studies were done using optical microscopy (OM) and scanning electron microscopy (SEM). Phase analysis was performed utilizing XRD analysis and mechanical properties of the weld joints performed under microhardness, transverse tensile, room temperature Charpy impact and high cycle fatigue (HCF) test. Fatigue crack growth and fatigue fracture surface were investigated under SEM studies.

### **2. Experimental procedure**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

nodules [1, 3].

the amount of retained austenite, shape and size of bainitic ferrite and graphite

delay the austempering reaction and increased the process windows [11].

higher fatigue strength [6, 10, 12].

was optimized by decreasing the graphite nodule size.

At a lower austempering temperature ADI shows needle shaped bainitic ferrite with a lower amount of retained austenite and graphite nodules, which in turns increased the tensile strength and hardness to decrease the elongation and toughness. However, at higher austempering temperature shows coarser bainitic ferrite with higher amount of retained austenite and graphite nodules, as a result to decrease the tensile strength and hardness; increased toughness that illustrate

It is reported, the fatigue strength of ADI is not only depended on tensile strength and hardness like as steel [13]. However, in ADI the amount of retained austenite and its carbon content, size of graphite nodules, nodularity, shape and size of bainitic ferrite plays an important role in the high cycle fatigue performance and higher fatigue limit [1, 13–15]. Bahmani et al. [16] illustrate a relationship between the microstructure and fatigue strength of ADI and obtained, the fatigue strength depended on the amount of retained austenite and its carbon content. The fatigue strength was increased as increasing the amount of retained austenite and its carbon content. Graphite nodularity and its size show significant effects in fatigue life. In ADI graphite working as a shrinkage cavities. During fatigue test, micro crack was initiated around the graphite and then formed to macro crack which leads to the final failure of the sample [17]. Sofue et al. [18] reported, with increasing the graphite size to decrease the fatigue strength and the fatigue strength

Further, rare earth metal such as cerium has a beneficial effect on the microstructure and properties of ADI. However, the optimum rare earth content varies significantly according to different investigators. For example, researchers [19] reported that the presence of Ce content from 0.005 to 0.014% the nodularity was increased with refining the size of the nodules but further increasing Ce content up to 0.018 or 0.020%, the nodularity decreased and formed some non-spherical graphite with coarsening the nodule size to decreased the fatigue strength. Choi et al. [20] observed that DI castings with 0.3% rare earth content attributed improved graphite nodules, lower tensile strength and hardness, higher elongation to indicate the higher fatigue strength than that of DI castings without rare earth. However, in spite of high strength, reasonable ductility and higher fatigue life, the application of ADI is somehow limited due to non-availability of a suitable electrode which has inhibited the joining of such high potential material.

The chemical composition of ADI is similar to conventional DI. However, some alloying elements such as Ni, Cu and Mo are usually added to increase the austemperability, to delay the austenite decomposition [8, 9] to pearlite and ferrite upon cooling. DI converts to ADI with the help of two steps isothermal heat treatment process. Austenitization was done at 850–910°C for 30 min–2 h holding time and austempering at 250–450°C for 5 min–4 h holding time and finally cooled at room temperature in the open air [10]. The total heat treatment process depends on the chemical composition and thickness of the as-cast DI. Austempering temperature and holding time shows significant effects on the microstructure and mechanical properties of ADI. During austempering two-stage reactions have been done, at the initial stage (stage I) austenite (residual austenite) decomposed into bainitic ferrite and high carbon austenite (retained austenite). Increasing holding time the stage II reaction was start and high carbon austenite further decomposed into bainitic ferrite and carbide (ɛ carbide). The time periods between the two stage reactions is called the process windows, obtained optimum combination of microstructural and mechanical properties. Presence of alloying elements such as Ni, Cu and Mn to

**128**

As cast DI was collected from the local foundry and used as a base metal for this experimental purpose. The details chemical composition of the as-cast DI is given in **Table 1**.

Two developed electrodes containing without and with Ce such as Trial 4 and Trial 7 selected for the experimental purpose. Among the two electrodes, Trial 4 contents with without Ce and Trial 7 contents with Ce content (0.1%). The details chemical composition of the two developed electrodes is given in **Table 2**.

Modified U groove (**Figure 1**) weld was performed on the DI base plate using both Trial 4 and Trial 7 electrodes applied shielded metal arc welding (SMAW) process. Preheat was applied at 300°C for 1 h and post weld heat treatment (PWHT) at 300°C for 1 h immediate after welding using the constant welding parameters [27, 28]. The details welding parameters are given in **Table 3** and the defect free weld was considered as per AWS (D11) [29].

The welded DI specimens are converted to ADI with the help of two steps isothermal heat treatment process. Austenitization was done at 900°C for 2 h holding time and then samples are immediately transferred to salt bath for austempering process. Austempering was done at 300 and 350°C for 1.5, 2 and 2.5 h holding time than air cooled to room temperature. Typical isothermal heat treatment cycle is shown in **Figure 2**.


#### **Table 1.**

*Chemical composition of as-cast ductile iron.*

