*3.4. Microstructure Analysis*

The microstructure analysis gives relevant information about the behavior of the different zones of the welds' cross-section. Thus, each sample was carefully analyzed, allowing a deep understanding of the phenomena produced by the different thermal cycles applied to the samples. A vast number of micrographs were collected, sorted, and analyzed, and the selection of the most representative micrographs regarding each sample and area of analysis are presented in Figure 7.

All the micrographs present microstructures consisting of tempered martensite, which gives the joint very good properties, excluding the P\_APP\_T00 sample. This presents a microstructure with martensite, but it is not tempered, because the PWHT was not performed. Samples P\_APP\_T01, P\_APP\_T02, and P\_APP\_T03 present very similar microstructures with identical grain sizes. Precipitates are found along the grain boundaries. The typical grain size in the HAZ is about 20–30 μm, and in the MZ it appears larger, with grains of 60–70 μm. In sample P\_APP\_T01, the grains appear more defined, and the martensite areas appear tempered as well. The sample P\_APP\_T08 presents a regular HAZ; however, the MZ presents an atypical structure relative to the other samples used in this work. This

reveals that the creep behavior can limit the application of this kind of joint in classic applications of this kind of material, but this expectation needs to be confirmed.

**Figure 7.** Micrographs of the different cross-section areas for samples subjected to different thermal cycles. Blue arrows highlight grain boundaries and green arrows highlight precipitates.

*3.5. Tensile Tests at Room and High Temperature*

The ASME IX: 2013–QW 150 [41] code refers to 585 MPa as the minimum yield strength to be achieved in welds of this kind of material; this is the value that is also required as a minimum for the base material. Regarding yield strength and elongation, the code does not specify any minimum value. ASME code B31.3—Process Piping [37] refers to minimum yield stress 413 MPa and ultimate tensile

strength 586 MPa. Analyzing the rupture results, it was found that it always occurred in the base material at a considerable distance from the welding area, as can be seen in Figure 8. Moreover, all ruptures presented a ductile look, even when the elongation was relatively low. The values obtained in the tensile tests can be observed in Table 9, namely the yield strength, ultimate tensile strength, and elongation at room and high temperature. The results can be observed in Figures 9 and 10. The highest values are presented by sample P\_APP\_T00, because they were not subjected to heat treatments and PWHT. Thus, the yield strength and ultimate strength are the highest values, but the elongation presents the lowest values, although these were not significantly different from samples P\_APP\_T02, P\_APP\_T03, and P\_APP\_T08. Sample P\_APP\_T03 presents the lowest ultimate strength value at room temperature, and P\_APP\_T02 presents the lowest ultimate strength value at high temperature.

**Figure 8.** Aspect of the samples used in tensile tests after rupture. Line 2 indicates the welding zone.


**Table 9.** Yield strength, ultimate tensile strength, and elongation at room and high temperature.

**Figure 9.** Results of the tensile tests performed at room temperature.

**Figure 10.** Results of the tensile tests performed at elevated temperature (600 ◦C).

Regarding the results obtained at elevated temperature, ASME B31.3—Process Piping code refers to the minimum yield strength of 71 MPa at a temperature of 600 ◦C [37]. In some codes, this value may be higher, requiring a minimum of 215 MPa. In any case, the results presented are above these minimum values for both codes cited. As in the tests carried out at room temperature, the samples also presented a ductile rupture clearly out of the MZ or HAZ.

Regarding Table 9, and excluding the sample P\_APP\_T00 which presents high risks of cracking in service due to its structure in the HAZ, and considering the sample P\_APP\_T01 as the model usually recommended by the main steel manufacturers and construction codes, the sample that presents the closest yield and ultimate strength both at room and elevated temperatures is sample P\_APP\_T08. The main drawback presented by this sample is the low elongation which, being higher than that in the samples P\_APP\_T02 and P\_APP\_T03, is relatively lower than sample P\_APP\_T01. Indeed, P\_APP\_T01 is the sample that presents a better relation between mechanical strength and ductility, mainly at room temperature. However, at elevated temperature, the elongation decreases substantially and reaches similar values to the other samples. Sample P\_APP\_T03 is the one that presents the best elongation at elevated temperature, but its mechanical resistance decreases more than that of the P\_APP\_T01 and P\_APP\_T08 samples. At this stage, samples P\_APP\_T02 and P\_APP\_T03 can be discarded, because they present a considerable drop in the mechanical resistance relative to the P\_APP\_T01 sample.

#### *3.6. Bending Tests*

After the bending tests have been completed, the samples were initially observed with the naked eye and subsequently subjected to penetrant liquid tests (Figure 11) to identify the possible existence of microcracks. In fact, no cracks or microcracks were found in any of the samples in either MZ, HAZ, or BM. Penetrant testing is not usual in industrial terms to verify this kind of joint, as cracks usually develop quite rapidly and are perfectly noticable to the naked eye. However, the penetrant liquid technique was used to confirm the absence of any kind of cracking in this work. All specimens described a perfect curvature, except for the sample P\_APP\_T00. These samples showed an atypical curvature, where it can be seen that the MZ offered such high bending resistance that the curvature took place in the BM. This effect is clearly depicted in Figure 12. Anyway, this sample does not present any crack or discontinuity. Given that the P\_APP\_T00 sample presented high mechanical strength and low elongation, and that its microstructure consisted essentially of non-tempered martensite, it would be expected that this samples would exhibit cracks when subjected to more intense deformation stresses. However, this was not the case. Thus, as a summary of these tests, it can be stated that all the samples are approved, but the P\_APP\_T00 sample shows greater resistance to deformation in the weld bead zone, transfering the deformation to an adjacent zone (HAZ and BM).

**Figure 11.** Samples subjected to bending tests: (**a**) samples bent in face and root surfaces, and (**b**) liquid penetrant technique applied to bent samples.

**Figure 12.** Bending tests corresponding to samples (**a**) P\_APP\_T08 and (**b**) P\_APP\_T00.
