## 4 Results and Discussion
### Traverse Stage and Model Validation  
#### 4.2.1 Temperature Distribution and Plastic Strain  
Experimental and simulation work are conducted using five different tool rotations: 250, 350, 450, 550 and 650 rpm. Figures 7-10 show the experimental macrostructures for all welded conditions and the corresponding temperature distribution (Node Temperature NT) and equivalent plastic strain (PEEQVAVG) results from the CEL model. For all tool rotations, the higher peak temperatures are found in the SZ, with slightly higher values at the retreating side (RS). [PERSON] et al. [48] used FSW to join DH36 steel and reported that the peak temperature was found near the FSW tool, i.e., inside the SZ. In addition, the temperatures measured at the RS were higher than the temperatures measured at the advancing side (AS) by 25 \({}^{\circ}\)C [48]. The peak temperature increases with an increase in  
Figure 6: Equivalent plastic strain distribution at plunge and dwell stages. (**a**) Top view at plunge, (**b**) top view at dwell, (**c**) cross section view at plunge, and (**d**) cross section view at dwell. the tool rotation rate. At the SZ of joints welded at 250, 350, 450 and 550 rpm, the peak temperatures are 1128, 1181, 1254 and 1300 \({}^{\circ}\)C, respectively. Figure 8: Temperature and equivalent plastic strain distribution of welded joint at 350 rpm: (**a**) temperature distribution in \({}^{\circ}\)C, (**b**) experimental micrograph, and (**c**) equivalent plastic strain distribution. (AS on the left, counter-clockwise tool rotation). Figure 7: Temperature and equivalent plastic strain distribution of welded joint at 250 rpm: (**a**) temperature distribution in \({}^{\circ}\)C, (**b**) experimental micrograph, and (**c**) equivalent plastic strain distribution. (AS on the left, counter-clockwise tool rotation). The experimental weld cross section is characterized by four metallurgical zones, including the base metal (BM), HAZ, thermo-mechanical affected zone (TMAZ) and SZ. The HAZ can be divided into three zones--HAZI, II and III--according to their color under etching and temperature distribution. To understand the difference between the three HAZs, the thermal cycles are recorded for the three HAZs in the AS and RS. Figure 11 shows the location of the selected nodal points in each zone. Figure 10: Temperature and equivalent plastic strain distribution of welded joint at 550 rpm: (**a**) temperature distribution in \({}^{\circ}\)C, (**b**) experimental micrograph, and (**c**) equivalent plastic strain distribution. (AS on the left, counter-clockwise tool rotation). Figure 9: Temperature and equivalent plastic strain distribution of welded joint at 450 rpm:(**a**) temperature distribution in \({}^{\circ}\)C, (**b**) experimental micrograph, and (**c**) equivalent plastic strain distribution. (AS on the left, counter-clockwise tool rotation). Figures 12 and 13 exhibit the thermal cycles at the selected nodal points in Figure 11 for the joints welded at 250 and 550 rpm, respectively. As the plunge and dwell stages are the same for all welds, the thermal cycles until the dwell stage, i.e., until the second 6.25, for all respective points are nearly the same. The peak temperatures in the RS are slightly higher than those in the AS. In addition, the cooling rate in the RS is slower than the cooling rate in the AS. For the joint welded at 250 rpm, the peak temperatures at HAZI, II and III are 830, 600 and 415 \({}^{\circ}\)C, respectively. Likewise, the peak temperatures at the same HAZs in the joint welded at 550 rpm are 880, 635 and 423 \({}^{\circ}\)C. [PERSON] et al. [49] studied the effect of tempering temperature on the microstructure of 13 wt. % Cr- type martensitic stainless steel. They reported that the resultant microstructure of the sample tempered at 300 \({}^{\circ}\)C is tempered  
Figure 11: Schematic location of nodes on the weld cross section for measuring the thermal cycle. Figure 12: Heat affected zone’s (HAZ) thermal cycle for the FSW welded joint at 250 rpm.