NOMENCLATURE
CHAPTER 5 Results and discussions
B. Results of experimental investigation
5.9 Effect of process parameters on microstructure and mechanical properties
transient thermal history of bead on plate welding with a large range of process parameters with error within 12%.
The generalised form of the avocado-configuration heat source is consists of limited number of model parameters which can provide a true non-uniform shape and it can represent the moving heat source of fusion welding.
Weld bead shape and dimensions were also predicted using the FE simulation applying the proposed heat source. The predicted weld bead shapes are compared with popular double ellipsoidal heat source model. It is observed that the proposed heat source model is flexible enough for the application in other fusion welding processes provided the proper applicability of the process parameters.
different levels as shown in Table 5.21. Square butt joints were prepared using two plates of dimension 200 × 200 × 8 mm. The pates were tack welded using shielded metal arc welding (SMAW) before final welding. The experimental results are shown in Table 5.28.
Table 5.21 Welding parameters and their levels
Variable parameters Fixed parameters
Parameters Values in each level Parameters Values
L1 L2 L3 L4
Voltage (V) 27 27.5 28 28.5 Root gap (mm) 2.5
Current (A) 470 475 480 485 Wire feed Automatic
Welding speed (mm/s) 5.5 6 6.5 7 Welding polarity (DCEP) - Stick-out (mm) 22 24 26 28 Plate thickness (mm) 8
5.9.1 Micro-hardness
Micro-hardness test were carried out in the Vickers micro-hardness tester. The maximum magnitude of hardness was seen in the fusion zone. The hardness of the heat affected zone is lower than the fusion zone but more than the base metal due to grain refinement. The Figure 5.74 explains the variations of hardness values with the distance perpendicular to the weld line of a welded sample.
Figure 5.74 Micro-hardness variation with distance
5.9.2 Effect of rate of heat input on tensile properties
Tensile testing was carried out using Instron 8801 universal testing machine with 100 kN load cell at a strain rate of 1.0 mm/min. It was observed that none of the sample failed in the welding zone. The tensile failure occurred between heat affected zone and base metal. Yield strength and ultimate tensile strength of base metal are 388 MPa and 519 MPa respectively. The complete input parameters and results from the tensile and hardness test is given the Table 5.28. The weld quality is highly influenced by the rate of heat input. Rate of heat input can be defined as a measure of the energy transferred per unit length of weld [189]. The thermal efficiency of the SAW process was considered as 95% [156].
1750 1925 2100 2275 2450
250 300 350 400 450 500
550 Ultimate tensile strength (MPa)
Yield strength (MPa)
Stress
Rate of heat input (J/mm)
Figure 5.75 Effect of heat input on stresses
To study the combined effect of welding parameters, rate of heat input was considered. It was observed that with increase in rate of heat input the tensile strength of the weld metal increases as the amount of energy to weld more metal increases. It can also be observed that for getting full penetration in the single sided single pass butt welding of 8 mm mild steel plates without edge preparation the rate of heat input can be kept in the range of 1.71-2.35 kJ/mm. Figure 5.75 shows the effect of heat input on yield stress (YS) and ultimate tensile strength (UTS). It can be seen from the figure that the UTS and YS increases with that rate of heat input.
5.9.3 Effect of process parameters on microstructure
The welded samples were polished using emery papers of varying grades from 120 to 2000.
Final polishing was done using velvet cloth and alumina paste. Etching was done in 2% Nital solution (i.e. 2% nitric acid and 98% ethyl alcohol). Microstructure was studied in the Carl Zeiss upright optical microscope at various magnifications. METZER-M stereo-microscope was used to capture the macro-graph of welded specimen which showed the different weld zones. Figure 5.76 shows the three different microstructure zones of the welded sample obtained by METZER-M stereo-microscope. Figures 5.77 (a) to (d) show the microstructure of different weld zones of a welded sample.
Figure 5.76 Different weld zones (macrograph)
Figure 5.77 Optical micrograph of (a) Base metal (b) HAZ (c) weld zone (d) HAZ-Weld zone
The microstructure of the base metal (Figure 5.77 (a)) contains ferrite (white) and pearlite (black). Fine grain structure was found in the HAZ (Figure 5.77 b) due to grain refinement and
grain coarsening was observed in the weld zone. Combination of both fine and coarse grain can be seen in Figure 5.77 d which represents the transition between coarse and find grain structure i.e. this is the intermediate zone between fusion zone and HAZ. As the peak temperature at HAZ region reached above A3 temperature therefore upon cooling austenite decomposes into finer pearlite (black) and ferrite (white). The boundary between HAZ and weld zone is also clearly visible in Figure 5.77 (d). Figure 5.78 shows one of the fractured surface of the tensile samples which was captured by using a FESEM.
(a) (b)
(c)
Figure 5.78 FESEM image of fractured surface of tensile samples
The Figure 5.78 (a) shows FESEM image at the fracture location of the tensile samples. The ball socket shape/dimple shape of the fractured surfaces (Figure 5.78 (b) & (c)) indicates ductile
failure. During tensile testing of the welded samples, failure occurred in between base metal and heat affected zone, which was away from the weld line, which indicates a superior weld quality.
5.9.4 Summary
The summary of the preceding study can be stated as below:
Maximum hardness was observed in the weld zone and which decreases gradually away from the weld line towards base material. The hardness in the weld zone was more due to the greater carbon content in the filler wire.
None of the tensile samples failed in the weld zone, therefore the strength of the welding was more than the base metal, which confirms a sound welding joint.Based on UTS the maximum joint efficiency was 96%.