3.1.5. Chemical Composition Profiles

affected by diffusion from Q235 steel.

3.1.5. Chemical Composition Profiles In order to have a more quantitative assessment of the iron content in the cladding, a throughthickness profile is measured by XRF on the clad material, following the procedure described in Section 2.4, for a total depth of 3.5 mm (Figure 10). The first 2 mm thick layer of the cladding shows a uniform composition of about 70%. However, depths greater than 2 mm give an iron content >70%, with values that increase almost linearly, reaching 80% at a 3.0 mm depth (Figure 9). At this position, In order to have a more quantitative assessment of the iron content in the cladding, a through-thickness profile is measured by XRF on the clad material, following the procedure described in Section 2.4, for a total depth of 3.5 mm (Figure 10). The first 2 mm thick layer of the cladding shows a uniform composition of about 70%. However, depths greater than 2 mm give an iron content >70%, with values that increase almost linearly, reaching 80% at a 3.0 mm depth (Figure 9). At this position, Cr and Mo contents are about 13.1% and 0.9%, respectively. Of course, given this

Cr and Mo contents are about 13.1% and 0.9%, respectively. Of course, given this through-thickness

through-thickness profile of iron, it is very difficult to take corrosion coupons from cladding having Fe content not a *Metals* **2020** ffected by diffusion from Q235 steel. , *10*, x FOR PEER REVIEW 9 of 14 *Metals* **2020**, *10*, x FOR PEER REVIEW 9 of 14

**Figure 10.** Chemical composition profiles (mass pct) through the thickness of the AISI 316L overlay by X-ray fluorescence (XRF) measurements. **Figure 10.** Chemical composition profiles (mass pct) through the thickness of the AISI 316L overlay by X-ray fluorescence (XRF) measurements. **Figure 10.** Chemical composition profiles (mass pct) through the thickness of the AISI 316L overlay by X-ray fluorescence (XRF) measurements.

#### *3.2. Stress-Relieving E*ff*ect 3.2. Stress-Relieving Effect*

at 640 °C for a 2 h holding time.

*3.2. Stress-Relieving Effect*  Because stress-relieving treatments do not give significant microstructural changes that can be revealed by light microscopy and SEM, only results in terms of hardness will be shown. The hardness profiles performed on the clad material after stress relieving at 640 and 660 °C for a holding time of 2 h are shown in Figures 11 and 12, respectively. The hardness peaks (e.g., 255 to 260 HV10) still remain in the CGHAZ of the microalloyed steel, close to the fusion line (0.3 mm distance), although slightly reduced compared to the as-clad material. Because stress-relieving treatments do not give significant microstructural changes that can be revealed by light microscopy and SEM, only results in terms of hardness will be shown. The hardness profiles performed on the clad material after stress relieving at 640 and 660 ◦C for a holding time of 2 h are shown in Figures 11 and 12, respectively. The hardness peaks (e.g., 255 to 260 HV10) still remain in the CGHAZ of the microalloyed steel, close to the fusion line (0.3 mm distance), although slightly reduced compared to the as-clad material. Because stress-relieving treatments do not give significant microstructural changes that can be revealed by light microscopy and SEM, only results in terms of hardness will be shown. The hardness profiles performed on the clad material after stress relieving at 640 and 660 °C for a holding time of 2 h are shown in Figures 11 and 12, respectively. The hardness peaks (e.g., 255 to 260 HV10) still remain in the CGHAZ of the microalloyed steel, close to the fusion line (0.3 mm distance), although slightly reduced compared to the as-clad material.

**Figure 11.** Hardness profiles across the Q235-AISI 316L interface after a stress-relieving heat treatment **Figure 11.** Hardness profiles across the Q235-AISI 316L interface after a stress-relieving heat treatment at 640 °C for a 2 h holding time. **Figure 11.** Hardness profiles across the Q235-AISI 316L interface after a stress-relieving heat treatment at 640 ◦C for a 2 h holding time.

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

**Figure 12.** Hardness profiles across the Q235-AISI 316L interface after a stress-relieving heat treatment at 660 °C for a 2 h holding time. **Figure 12.** Hardness profiles across the Q235-AISI 316L interface after a stress-relieving heat treatment at 660 ◦C for a 2 h holding time. **Figure 12.** Hardness profiles across the Q235-AISI 316L interface after a stress-relieving heat treatment at 660 °C for a 2 h holding time.

This means that stress-relieving treatments are not sufficient to avoid hardness on the higher peaks at the interface. A possible alternative in the production route is to use the as-rolled (green) plates, which are clad and submitted to Q and T treatment later. In order to investigate the possibility to follow such a route, specimens were treated considering three austenitizing temperatures, 920 °C, 980 °C, and 1000 °C, respectively, and one tempering condition (670 °C × 2 h). The lower austenitizing temperature is typical of standard (unclad) Q235 plates, while the other temperatures were selected because they were recommended for the heat treatment of AISI 316L, which is still practiced in the present industrial furnaces. The hardness profiles performed on the clad material after the Q and T laboratory treatments are shown in Figure 13. This means that stress-relieving treatments are not sufficient to avoid hardness on the higher peaks at the interface. A possible alternative in the production route is to use the as-rolled (green) plates, which are clad and submitted to Q and T treatment later. In order to investigate the possibility to follow such a route, specimens were treated considering three austenitizing temperatures, 920 ◦C, 980 ◦C, and 1000 ◦C, respectively, and one tempering condition (670 ◦C × 2 h). The lower austenitizing temperature is typical of standard (unclad) Q235 plates, while the other temperatures were selected because they were recommended for the heat treatment of AISI 316L, which is still practiced in the present industrial furnaces. The hardness profiles performed on the clad material after the Q and T laboratory treatments are shown in Figure 13. This means that stress-relieving treatments are not sufficient to avoid hardness on the higher peaks at the interface. A possible alternative in the production route is to use the as-rolled (green) plates, which are clad and submitted to Q and T treatment later. In order to investigate the possibility to follow such a route, specimens were treated considering three austenitizing temperatures, 920 °C, 980 °C, and 1000 °C, respectively, and one tempering condition (670 °C × 2 h). The lower austenitizing temperature is typical of standard (unclad) Q235 plates, while the other temperatures were selected because they were recommended for the heat treatment of AISI 316L, which is still practiced in the present industrial furnaces. The hardness profiles performed on the clad material after the Q and T laboratory treatments are shown in Figure 13.

**Figure 13.** *Cont*.

*Metals* **2020**, *10*, x FOR PEER REVIEW 11 of 14

**Figure 13.** Hardness profiles across the Q235-AISI 316L interface after stress--relieving heat treatment at 920 °C × 1 h + 670 °C × 2 h (**a**), 980 °C × 1 h + 670 °C × 2 h and (**b**) 1000 °C × 1 h + 670 °C × 2 h (**c**). **Figure 13.** Hardness profiles across the Q235-AISI 316L interface after stress–relieving heat treatment at 920 ◦C × 1 h + 670 ◦C × 2 h (**a**), 980 ◦C × 1 h + 670 ◦C × 2 h and (**b**) 1000 ◦C × 1 h + 670 ◦C × 2 h (**c**).

The hardness peaks in the CGHAZ of the microalloyed steel, close to the fusion line (0.3 mm distance), disappeared; they composed all values lower than 220 HV10. Reaustenitizing acts at the interface microstructure by refining austenite grain size with respect to the as-clad material (Figure 14), thus lowering local hardenability, with consequent lower hardness values at the interface. No significant effects are found depending on austenitization temperature variation in the range of 920– 1000 °C. The hardness of the base material after Q and T appears slightly decreased compared to the as-clad and Q and T materials. However, this is not a critical aspect and can be balanced using a suitable tempering temperature. The hardness peaks in the CGHAZ of the microalloyed steel, close to the fusion line (0.3 mm distance), disappeared; they composed all values lower than 220 HV10. Reaustenitizing acts at the interface microstructure by refining austenite grain size with respect to the as-clad material (Figure 14), thus lowering local hardenability, with consequent lower hardness values at the interface. No significant effects are found depending on austenitization temperature variation in the range of 920–1000 ◦C. The hardness of the base material after Q and T appears slightly decreased compared to the as-clad and Q and T materials. However, this is not a critical aspect and can be balanced using a suitable tempering temperature.

*Metals* **2020**, *10*, x FOR PEER REVIEW 12 of 14

**Figure 14.** Q and T effect on the microstructure at the interface. (**a**) As-received material, (**b**) 920 °C × 1 h + 670 °C × 2 h, (**c**) 980 °C × 1 h + 670 °C × 2 h, and (**d**) 1000 °C × 1 h + 670 °C × 2 h. **Figure 14.** Q and T effect on the microstructure at the interface. (**a**) As-received material, (**b**) 920 ◦C × 1 h + 670 ◦C × 2 h, (**c**) 980 ◦C × 1 h + 670 ◦C × 2 h, and (**d**) 1000 ◦C × 1 h + 670 ◦C × 2 h.

#### **4. Conclusions 4. Conclusions**

The following conclusions can be drawn from the above results: The following conclusions can be drawn from the above results:


**Funding:** This research received no external funding. **Author Contributions:** Conceptualization, A.D.S. and C.T.; methodology, A.D.S. and C.T.; formal analysis, A.D.S. and C.T. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
