3.1.1. Light Microscopy Investigation

No lack of fusion or bond, cracks, or other defects are observed. The individual runs are recognized. The first weld overlay pass has a thickness of 1.2 to 2 mm, while the second pass has a thickness

from 2 to 2.5 mm (Figure 2). The dendritic structure of the weld overlay (second pass) is clearly shown in Figure 3. A detail of the coarse-grained heat-affected zone (CGHAZ) is shown in Figure 4. The heat-affected zone (HAZ), 1.0 to 1.5 mm thick, can be clearly distinguished in the low-alloy steel after etching (Figure 5). Additionally, the coarse-grained heat-affected zone is revealed between adjacent runs of the first weld overlay, close to the fusion line. *Metals* **2020**, *10*, x FOR PEER REVIEW 5 of 14 clearly shown in Figure 3. A detail of the coarse-grained heat-affected zone (CGHAZ) is shown in Figure 4.The heat-affected zone (HAZ), 1.0 to 1.5 mm thick, can be clearly distinguished in the lowalloy steel after etching (Figure 5). Additionally, the coarse-grained heat-affected zone is revealed between adjacent runs of the first weld overlay, close to the fusion line. *Metals* **2020**, *10*, x FOR PEER REVIEW 5 of 14 clearly shown in Figure 3. A detail of the coarse-grained heat-affected zone (CGHAZ) is shown in Figure 4.The heat-affected zone (HAZ), 1.0 to 1.5 mm thick, can be clearly distinguished in the lowalloy steel after etching (Figure 5). Additionally, the coarse-grained heat-affected zone is revealed between adjacent runs of the first weld overlay, close to the fusion line. *Metals* **2020**, *10*, x FOR PEER REVIEW 5 of 14 clearly shown in Figure of the coarse-grained is Figure 4.The heat-affected zone (HAZ), 1.0 to 1.5 mm thick, can be clearly distinguished in the lowalloy etching (Figure 5). Additionally, coarse-grained heat-affected between adjacent runs of the first weld overlay, close to the fusion line.

**Figure 2.** As-clad material (polished section). **Figure 2.** As-clad material (polished section). **Figure 2.** As-clad material (polished section).

**Figure 3.** AISI 316L weld overlay (etching: 50% HNO3, and 50% H2O). **Figure 3.** AISI 316L weld overlay (etching: 50% HNO3, and 50% H2O). **Figure 3.** AISI 316L weld overlay (etching: 50% HNO<sup>3</sup> , and 50% H2O).

**Figure 4.** Q235 substrate (Q and T material). **Figure 4.** Q235 substrate (Q and T material). (Q and **Figure 4.** Q235 substrate (Q and T material).

*Metals* **2020**, *10*, x FOR PEER REVIEW 6 of 14

*Metals* **2020**, *10*, x FOR PEER REVIEW 6 of 14

**Figure 5.** Detail of in the coarse-grained heat-affected zone (CGHAZ) (2% Nital etching). **Figure 5.** Detail of in the coarse-grained heat-affected zone (CGHAZ) (2% Nital etching). **Figure 5.** Detail of in the coarse-grained heat-affected zone (CGHAZ) (2% Nital etching).

#### 3.1.2. .Hardness Profiles 3.1.2. Hardness Profiles 3.1.2. .Hardness Profiles

Examples of indentation array used to measure HV10 hardness are shown in Figure 6. Three indentation profiles acquired in three different positions are shown in Figure 7 (profiles 1–3). Figure 7 shows that the hardness peaks (e.g., 250 to 270 HV10) are detected in the Q235 steel close to the fusion line in the CGHAZ. Examples of indentation array used to measure HV<sup>10</sup> hardness are shown in Figure 6. Three indentation profiles acquired in three different positions are shown in Figure 7 (profiles 1–3). Figure 7 shows that the hardness peaks (e.g., 250 to 270 HV10) are detected in the Q235 steel close to the fusion line in the CGHAZ. Examples of indentation array used to measure HV10 hardness are shown in Figure 6. Three indentation profiles acquired in three different positions are shown in Figure 7 (profiles 1–3). Figure 7 shows that the hardness peaks (e.g., 250 to 270 HV10) are detected in the Q235 steel close to the fusion line in the CGHAZ.

**Figure 6.** Examples of indentation across the Q235 and AISI 316L interface. **Figure 6. Figure 6.** Examples of indentation across Examples of indentation across the Q235 and AISI 316L interface. the Q235 and AISI 316L interface.

*Metals* **2020**, *10*, x FOR PEER REVIEW 7 of 14

*Metals* **2020**, *10*, x FOR PEER REVIEW 7 of 14

**Figure 7.** Hardness profiles across the Q235-AISI 316L interface. **Figure 7.** Hardness profiles across the Q235-AISI 316L interface.  **Cr % Ni % Mn % Mo % Fe, %** 

#### 3.1.3. SEM-EDS Investigation 3.1.3. SEM-EDS Investigation Zone IV (CGHAZ) 0.05 0.1 0.9 0.05 98.9 Zone III 13.2 5.0 1.0 0.50 80.3

On the basis of the light microscopy results, four zones are selected (Figure 8) and examined by SEM-EDS. The average values of the EDS area analysis are shown in Table 4 for the various zones. On the basis of the light microscopy results, four zones are selected (Figure 8) and examined by SEM-EDS. The average values of the EDS area analysis are shown in Table 4 for the various zones. Zone II 18.8 7.7 0.9 1.1 71.5 Zone I 17.9 7.8 1.0 1.2 72.1

**Figure 8.** Identification at the light microscope of the zones examined by SEM-EDS. **Figure 8.** Identification at the light microscope of the zones examined by SEM-EDS.

**Table 4.** Quantitative SEM-EDS microanalysis (mass, %).


Zone I is chosen as the external layer zone, zone II as the interface between the two-layer passes, zone III as second layer pass, and zone IV as the carbon steel-stainless steel interface. Zone I is chosen as the external layer zone, zone II as the interface between the two-layer passes, zone III as second layer pass, and zone IV as the carbon steel-stainless steel interface.

Fe increases and is detected in the weld overlay. Due to the dilution phenomena, iron is detected

Fe increases and is detected in the weld overlay. Due to the dilution phenomena, iron is detected about 80% closer to the microalloyed steel (first pass, zone III) and about 72% closer to the second overlay pass (surface to be in contact with sour fluid, zone I), in the AISI 316L weld. In zone IV, the CGHAZ is observed (Figure 9). Austenite grains reached a size greater than 50 µm. The hardness of the peaks is attributed to increased local hardenability caused by grain-coarsening. *Metals* **2020**, *10*, x FOR PEER REVIEW 8 of 14 overlay pass (surface to be in contact with sour fluid, zone I), in the AISI 316L weld. In zone IV, the CGHAZ is observed (Figure 9). Austenite grains reached a size greater than 50 μm. The hardness of the peaks is attributed to increased local hardenability caused by grain-coarsening.

**Figure 9.** Detail of the CGHAZ at the Q235-AISI 316L interface. **Figure 9.** Detail of the CGHAZ at the Q235-AISI 316L interface.

#### 3.1.4. Corrosion Resistance of Clad Layer 3.1.4. Corrosion Resistance of Clad Layer

Corrosion tests on the as-received cladding, i.e., determination of CPT by the ASTM G-48 test, were not promising (Table 5). This is because at 10 °C, severe pitting corrosion was exhibited on one face when the weld overlay specimen was machined considering all its thickness (both first and second welding pass). This behavior was likely due to excessive Fe content (>15%) in the corrosionresistant alloy layer. Also, the Huey (ASTM A262 Type C) immersion test to evaluate the intergranular corrosion resistance gave unsatisfactory results with corrosion rates greater than 60 mm/yr in the first immersion. Later, after the third immersion, when the cladding with the lowest iron content remained, the corrosion rate decreased to 2.6 mm/year. When cladding coupons were predominantly sampled from the second overlay pass, the corrosion resistance significantly improved (Table 4) with CPT > 10 °C, and the corrosion rate in the Huey solution was about 2.5 mm/yr, although slightly below that expected for standard AISI 316L. Corrosion tests on the as-received cladding, i.e., determination of CPT by the ASTM G-48 test, were not promising (Table 5). This is because at 10 ◦C, severe pitting corrosion was exhibited on one face when the weld overlay specimen was machined considering all its thickness (both first and second welding pass). This behavior was likely due to excessive Fe content (>15%) in the corrosion-resistant alloy layer. Also, the Huey (ASTM A262 Type C) immersion test to evaluate the intergranular corrosion resistance gave unsatisfactory results with corrosion rates greater than 60 mm/yr in the first immersion. Later, after the third immersion, when the cladding with the lowest iron content remained, the corrosion rate decreased to 2.6 mm/year. When cladding coupons were predominantly sampled from the second overlay pass, the corrosion resistance significantly improved (Table 4) with CPT > 10 ◦C, and the corrosion rate in the Huey solution was about 2.5 mm/yr, although slightly below that expected for standard AISI 316L.

**Table 5.** The corrosion resistance of the clad layer. **Table 5.** The corrosion resistance of the clad layer.

