<sup>5</sup> Over-Tempering Zone

Due to the OTZ being far away from the WM, its lower peak temperature of the welding thermal cycle is not more than AC1 and its cooling speed for welding is less. Therefore, the base metal in the over tempered zone does not undergo austenitizing transformation in the welding thermal cycle; it seems to be double tempered at higher temperature than that of the quenching and tempering. The higher temper temperature is beneficial for the carbide particles to aggregate and grow up in this area, which results in more formations of polygonal ferrite phases, which weaken the microhardness. Therefore, the OTZ has a significant responsibility for the softening of the welded joint.

From the discussion of the OTZ in the welded joint, it can be observed that there are more granular carbides dispersed on the ferrite matrix; these granular carbides probably originate from the sorbite phase carbides in the base metal, where they aggregate and grow up and are distributed on the ferrite grain boundary after double tempering, as shown in Figure 7. The area with more coarse granular carbides is black, while the polygonal ferrite phase is white. In the OTZ, there are more carbides precipitated, grown, and coarsened, which causes the hardness of the OTZ to decrease. Therefore, it is the most softened zone in the whole welded joint.

In order to figure out the phase constituents of the joint softening zone, EBSD was used and the results are shown in Figure 8. The EBSD results show that the Fe3C fraction of the phase constituents is 0.77% and much higher, as shown in Table 2. The reason is that there is no post-weld heat treatment after welding, which results in some residual stress in the welded joint; the lower resolution rate is 75.23%. The face-centered cubic (FCC) austenite fraction of the phase constituents is only 0.01%, which means that the amount of retained austenite in the laser-welded joint is very small, and the body-centered cubic (BCC) ferrite fraction of the phase constituents is 74.44%. According to the previous analysis, this zone consists of some polygonal ferrite and few sorbite phases. These polygonal ferrite phases are responsible for the hardness decreasing.

**Figure 7.** Microstructure of the OTZ of the LBW joint (**a**) Microstructure in the OTZ (**b**) SEM in the OTZ (**c**) Polygonal ferrite (**d**) Carbides on ferrite grain boundaries. F: ferrite.

**Figure 8.** Results of EBSD in the OTZ of the LBW joint (**a**) Location of EBSD (**b**) Phase distribution. **Table 2.** Phase content in the OTZ of the LBW joint.


### *3.2. SMAW Weld Microstructure*

Figure 2b shows the microstructure of the whole arc welded joint. It can be seen that 21 passes of SMAW were need to fill the V-groove for 16-mm thick weldment. The heat input was higher than 16.4 kJ/cm and the maximum bead width was approximately 30 mm. The joint morphology was good and the maximum bead width was about 30 mm; the average width of HAZ between the BM and the WM was nearly 2.5 mm; while the width of HAZ in the inner WM was only 0.5 mm. This can be explained by the fact that the HAZ is produced by reheating the previous part of the weld with the following weld in the area where the welds overlap with each other. The formation of HAZ leads to the change in microstructure, which in turn has an impact on the performance of the joint. Finally, the typical solidification structure, such as the columnar crystal and the equiaxed crystal, appears in each layer and in each pass; most of them are the column crystal.

The microstructure in the SMAW joint is almost similar to that in the LBW joint overall. However, if a higher heat input (16.4 kJ/cm) and slower cooling rate were used, the microstructure in the SMAW joint would show some differences that the LBW joint would not have. The following discussion focuses on the microstructure differences of the WM and HAZ.

In the multi-layer multi-pass SMAW, there are many HAZs that are heated many times in the WM, which results in obvious secondary HAZs existing among each layer and each pass of the WM, as shown in Figure 9a. The reason for this is similar to the formation of the black microstructures in the TPZ of the LBW HAZ. Due to the higher heat input and slower cooling rate, carbides in the secondary HAZ have sufficient time to precipitate and grow up among each layer of the WM, forming complex phases of ferrite and cementite with finer microstructures and a darker color. Figure 9c is a local magnification photo of Figure 9a. The typical secondary HAZ and WM can be easily seen in the 2-mm weld pass. The WM is mainly composed of equiaxed crystal and columnar crystal phases, and the equiaxed crystal phases are composed of some acicular and polygonal ferrite and bainite. The columnar crystal is mainly made up proeutectoid ferrite and bainite ferrite, as shown in Figure 9c. The size of the equiaxed crystal phase is around several microns, and the width of the columnar crystal phase is 150 μm; the width of the black secondary HAZ is almost 250 μm. Typical bainite ferrite clusters can be seen in the equiaxed crystal phase in Figure 10b; the length of ferrite is 70 μm and the width is 3 μm.

**Figure 9.** Microstructure of SMAW joint (**a**) Macro-photo in weld metal (**b**) Bainite ferrite in weld metal (**c**) Equiaxial and columnar crystals (**d**) Equiaxial crystals. F: ferrite; B: Bainite.

The HAZ of the SMAW is also composed of a CGHAZ, a FGHAZ, a TPZ, and an OTZ. The grain size of the CGHAZ is nearly 100 μm and consists of coarse martensite and bainite. Because of the higher heat input and slower cooling rate, most phases of the CGHAZ are bainite with a few polygonal ferrite. In Figure 10, the grain size in the FGHAZ is nearly 10 μm. The TPZ consists of some duplex microstructures and ferrite phases. The more the HAZ is away from the WM, the lower the decrease in the peak temperature and cooling rate of the thermal cycle, and the less carbide precipitate. The precipitated carbide is discontinuously distributed and becomes a tempered sorbite phase when it reaches the TPZ. In view of the post-weld heat treatment, it is not easy to find black and white striped phases of the TPZ in SMAW.

**Figure 10.** Microstructure in HAZ of SMAW joint (**a**) Macro-photo in HAZ (**b**) Chain-distribution of carbides in HAZ (**c**) OTZ (**d**) OTZ close to BM. F: ferrite; M: martensite; B: bainite.

The total heat input of the SMAW with filler wire was greater than that of the LBW; the difference in microstructure resulted in different performances of their mechanical properties. The comparison results are shown in Table 3.

**Table 3.** Microstructure comparison between two processes.


Meanwhile, the differences in microstructure can be described through the content of phases and the width of each zone in particular. In order to find the difference between each welding, the abovementioned average statistical method was used and the statistical results are shown in Figure 11.

The results show that significant differences between two joints are as follows:

The joint weld by multi-layer multi-pass SMAW results in more complex microstructures and double-HAZ, which are composed of some black duplex phases, like those of the two-phase zone, and there are much more proeutectoid ferrite phases in the WM. The average bead widths of the LBW joint and the SMAW joint are 4.2 mm and 15.4 mm, respectively. The average bead width of the LBW is significantly smaller than that of the SMAW. The performance of mechanical properties in the WM is always poorest in the whole joint. The decrease in bead width in LBW is helpful in order to improve the performance of the whole welded joint.

There is minor difference in the width of the CGHAZ and the FGHAZ for the two joints. The difference is that there is more bainite phases in the CGHAZ and the FGHAZ in the SMAW joint, and several chained carbide particles along the grain boundary; the most probably reason for this is the larger heat input used.

The TPZ is not obvious and its width is 0.8 mm in the SMAW joint, while the width in the LBW joint can reach 1.5 mm, or even more than 2.5 mm, and the performance of the TPZ is slightly lower than that of the base metal. Therefore, the performance of the LBW joint could worsen due to its wider TPZ.

The width of the OTZ in the two joints are almost same and equal to 0.4 mm. The microstructure has no obvious change and it has improved to a degree because of the overheating treatment in the SMAW. However, it is not good for the performance of the OTZ to improve the LBW without any post-weld heat treatment.

### *3.3. Mechanical Properties*

• Microhardness test results

In Figure 12a, the microhardness variations across the middle of the SMAW joint are presented. The hardness of the BM is about 360 HV0.3, which is similar to that of the LBW, indicating that post-weld heat treatment had little effect on the hardness of the BM. Because of multi-layer and multi-pass welding, the transverse hardness fluctuation of the arc-welded joint is obvious. It can be seen that the whole hardness spot experienced four weld bead ups and downs, and each weld bead had some degree of hardening and softening, but due to the effect of post-weld heat treatment, the hardening and softening effect was not obvious. The highest hardness is 430 HV0.3 in the HAZ between each

pass, which is about 30 HV0.3 lower than that of the LBW joint, which is the result of the post-weld heat treatment.

**Figure 12.** Vickers microhardness distributions (**a**) Hardness area of the SMAW joint (**b**) Hardness area of the LBW joint.

Figure 12b depicts the Vickers microhardness variations across the middle of the LBW joint. The hardness of the BM is about 360 HV0.22, and the WM are formed by the remelting of the BM. Because the WM had not undergone any post-weld heat treatment, its hardness is higher than that of the BM, which is about 400 HV0.22. There is a certain degree of softening in the OTZ; the lowest hardness is only 334 HV0.22, and the width of the whole softening zone is about 1.2 mm.

• Tensile test results

The stress–strain curves for the LBW and SMAW joints are shown in Figure 13, and the average values of the tensile tests for the LBW and SMAW joints are summarized in Table 4. The average yield strength *R*p0.2, the average ultimate tensile strength *R*m, and the average elongation *A* of LBW joints are 898 MPa, 973.5 MPa, and 10%, respectively. Compared to those of the LBW joints, the decline in the average yield strength (896 MPa) and the average ultimate tensile strength (951 MPa) in the SMAW joints is not glaring, except that the average elongation (2.9%) dramatically decreases. It could be that increasingly more weld microstructures are involved in the tensile behavior from the root of the weld to the top of the SMAW joint, resulting in a smaller elongation of the tensile coupon. In Table 4, it can be found that the fracture of most of specimens occurred in the BM for the LBW and SMAW joints; only one fracture occurred in the WM of LBW joint due to weld defects.


**Table 4.** Results of tensile test.

**Figure 13.** Stress–strain curves of the joints (**a**) the SMAW (**b**) the LBW.
