Corrosion Behavior and Mechanical Properties of AISI 316 Stainless Steel Clad Q235 Plate

: This paper deals with carbon steel and stainless steel clad-plate properties. Cladding is performed by the submerged-arc welding (SAW) overlay process. Due to element di ﬀ usion (Fe, Cr, Ni, and Mn), a 1.5 mm wide di ﬀ usion layer is formed between the stainless steel and carbon steel interface of the cladded plate a ﬀ ecting corrosion resistance. Pitting resistance is evaluated by measuring the critical-pitting temperature (CPT), as described in the American Society for Testing and Materials (ASTM) G-48 standard test. Additionally, Huey immersion tests, in accordance with ASTM A262, Type C, are carried out to evaluate the intergranular corrosion resistance. Some hardness peaks are detected in microalloyed steel close to the molten interface line in the coarse-grained heat-a ﬀ ected zone (CGHAZ). Results show that stress-relieving treatments are not su ﬃ cient to avoid hardness peaks. The hardness peaks in the CGHAZ of the microalloyed steel disappear after quenching and tempering (Q and T).


Introduction
Metal-clad plates for different applications have been developed in many industrial sectors [1][2][3]. Among them, carbon-steel plates cladded by stainless steel are one of the most widely used products since they allow one to combine the mechanical properties of carbon steel with the excellent corrosion resistance of the adopted clad.
In fact, following their peculiar corrosion-resistance properties, stainless steels are nowadays used in many applications that are high targets of corrosion resistance [4,5]. In particular, they are employed in automotive [6], construction and building [7,8], energy [9][10][11], aeronautics [12], food [13][14][15], and three-dimensional (3D) printing [16] applications. Their poor mechanical resistance does not allow them to be directly applied in structural applications. The combination of carbon steel adopted as a substrate coated by stainless steel is, therefore, well-suited for several applications calling for high-mechanical targets coupled with good-corrosion resistance (especially in petroleum, petrochemical, shipbuilding, and pressure-vessel industries). In particular, in the case of stainless steel clad plates, the cladding material satisfies the corrosion resistance requirement. In this case, the cladding material thickness just corresponds to about 10-20% of the clad plate. Therefore, stainless steel clad plates show a chromium and nickel content reduction if compared to plates fully manufactured by stainless steel. This reduces about 30-50% of the costs following the carbon steel low cost [17,18]. Many technologies are utilized to produce metal-clad plates, for instance, explosion welding [19], roll bonding, and welding overlay [20][21][22]. Among the welding overlay methods, currently, several welding technologies have been adopted in the welding of stainless steel clad plates. Shielded-metal-arc welding (SMAW) [23], submerged-arc welding (SAW) [24], tungsten-inert-gas welding (TIG) [25],

Materials and Methods
The stainless steel clad plate was manufactured by weld overlaying with the submerged-arc strip cladding method, which utilizes an arc that runs back and forth at high speeds along the strip. Two feed hoppers were necessary to guarantee the complete protection of the electrical arc. A simplified scheme of the adopted overlay welding process is shown in Figure 1. The process was conducted by means of a protective slag. Q235 steel was chosen as a substrate, and the AISI 316 strip was chosen as cladding material (strip of 30 mm wide and thickness of 0.5 mm). Q235 steel was chosen as a substrate. Q235 steel samples were received in the form of 16 mm thick plates after hot-rolling and quenching and tempering (Q and T) at an industrial scale. Such steel was chosen since it is characterized by good plasticity, toughness, weldability, and good cold-bending performance, making it widely used in construction and engineering welding structures. AISI 316L was chosen as the clad for its excellent corrosion performances. The two actual steel chemical composition are shown in Table 1. Plates sizing 500 mm × 500 mm × 16 mm were clad by an external supplier. The welding supplier declared the plate manufacturing process parameters as weld metal dilution of 40%, 7 kg/h deposition, and a deposit efficiency of 95%. Two layers were deposited, and the main welding parameters are shown in Table 2.

Microstructure and Hardness
Longitudinal and transverse metallographic specimens taken from plates were prepared by a standard procedure. In particular, the steel-corrosion resistant alloy interface was examined by light microscopy (LM, Nikon) and scanning electron microscopy (SEM). The low-alloy steel was etched by Nital 2%, while the microstructure of the AISI 316L weld overlay revealed etching with 50% HNO3 + 50% H2O. In order to assess that the distribution was of various chemical elements (in particular Ni, Cr, Fe, and Mo) in the through-thickness direction, quantitative energy dispersive spectrometry (EDS) was performed by the scanning electron microscope field emission gun (SEM-FEG), LEO 1550 (Zeiss, Oberkochen, Germany). Hardness through-thickness profiles were measured by means of 10 kg Vickers hardness indenter (HV10) using steps of 0.3 and 0.5 mm at various locations on longitudinal sections.

Corrosion Testing
Pitting resistance was evaluated by measuring the critical-pitting temperature (CPT) by the American Society for Testing and Materials (ASTM) G-48 test. Additionally, Huey immersion tests in accordance with ASTM A262, Type C, were carried out to evaluate the intergranular corrosion resistance.
• ASTM G-48 test (method C). Coupons of size 20 mm × 30 mm × 1.5-2.5 mm thickness were taken from the AISI 316L clad layer. The carbon steel was machined and removed in order to obtain a flat surface. The other surface of the stainless steel coupons was practically left in the as-received Plates sizing 500 mm × 500 mm × 16 mm were clad by an external supplier. The welding supplier declared the plate manufacturing process parameters as weld metal dilution of 40%, 7 kg/h deposition, and a deposit efficiency of 95%. Two layers were deposited, and the main welding parameters are shown in Table 2.

Microstructure and Hardness
Longitudinal and transverse metallographic specimens taken from plates were prepared by a standard procedure. In particular, the steel-corrosion resistant alloy interface was examined by light microscopy (LM, Nikon) and scanning electron microscopy (SEM). The low-alloy steel was etched by Nital 2%, while the microstructure of the AISI 316L weld overlay revealed etching with 50% HNO 3 + 50% H 2 O. In order to assess that the distribution was of various chemical elements (in particular Ni, Cr, Fe, and Mo) in the through-thickness direction, quantitative energy dispersive spectrometry (EDS) was performed by the scanning electron microscope field emission gun (SEM-FEG), LEO 1550 (Zeiss, Oberkochen, Germany). Hardness through-thickness profiles were measured by means of 10 kg Vickers hardness indenter (HV 10 ) using steps of 0.3 and 0.5 mm at various locations on longitudinal sections.

Corrosion Testing
Pitting resistance was evaluated by measuring the critical-pitting temperature (CPT) by the American Society for Testing and Materials (ASTM) G-48 test. Additionally, Huey immersion tests in accordance with ASTM A262, Type C, were carried out to evaluate the intergranular corrosion resistance.

•
ASTM G-48 test (method C). Coupons of size 20 mm × 30 mm × 1.5-2.5 mm thickness were taken from the AISI 316L clad layer. The carbon steel was machined and removed in order to obtain a flat surface. The other surface of the stainless steel coupons was practically left in the as-received condition, except for slight grinding and polishing. Duplicate specimens were considered. The ASTM G-48 test (method C) consisted of the determination of the CPT in a highly acidic media, e.g., ferric chloride. CPT temperature is defined as the lowest temperature at which pitting corrosion occurs. Test samples were exposed for 72 h to 6% ferric chloride solution. If no pitting corrosion was detected, the testing temperature was increased. The test performed was duplicated. • ASTM A262 test (method C, Huey). This test consisted of a cyclic exposure to nitric acid of coupons (30 mm × 20 mm), taken from the AISI 316L clad layer, for 5 periods of 48 h each. The test performed was duplicated.

Heat Treatments
Specimens (100 mm × 20 mm) were taken from the clad plate and heat-treated at laboratory scale by means of an FM77H (chamber size 250 mm × 190 mm × 500 mm) muffle able to maintain 20 • C difference for 2 h. The effect of final heat-treatment conditions on the properties of both the welded layer and substrate were investigated by performing stress-relieving treatments on specimens taken from the clad plate. The original specimens were also fully retreated to simulate a Q and T treatment after cladding of the as-rolled plates. The heat-treated samples were examined in terms of microstructure and hardness indentation profiles. The following cases were considered (temperatures and holding times are indicated) ( Table 3). Table 3. Heat treatment conditions on the performed tests.

Hardness
Microstructure The 640-660 • C stress-relieving temperature range was chosen high enough (higher than the standard one for such class of material) to investigate its effect on the eventual hardness of peaks at the interface. Three different fully Q and T cycles (including austenitization step) were chosen to simulate the possibility of cladding the as-rolled plate before submitting it to a Q and T treatment.

Chemical Composition by X-ray Fluorescence (XRF)
In order to have a more quantitative assessment of the iron content in the cladding material, a through-thickness profile was measured by X-ray fluorescence (XRF), starting from the second pass layer and moving towards the base steel on 30 mm × 20 mm specimens. Because a flat surface was required for the analysis, a section perpendicular to the plate was prepared by grinding the first 0.5 mm layer at the external clad surface. After performing the chemical analysis, the other 0.5 mm was ground and a chemical analysis carried out again. This procedure was repeated for a total depth of 3.5 mm.

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 Metals 2020, 10, 552 5 of 14 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 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.

.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.

Hardness Profiles
Examples of indentation array used to measure HV 10 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 HV 10 ) are detected in the Q235 steel close to the fusion line in the CGHAZ.

.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.   Table 4 for the various zones.  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 about 80% closer to the microalloyed steel (first pass, zone III) and about 72% closer to the second  Table 4 for the various zones.  Table 4 for the various zones.  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 about 80% closer to the microalloyed steel (first pass, zone III) and about 72% closer to the second  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 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.

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. Passed at 10 °C -

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, Cr and Mo contents are about 13.1% and 0.9%, respectively. Of course, given this through-thickness profile of iron, it is very difficult to take corrosion coupons from cladding having Fe content not affected by diffusion from Q235 steel.

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 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.

Chemical Composition Profiles
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

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.

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 HV 10 ) 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.

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.   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. 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 HV 10 . 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.

Conclusions
The following conclusions can be drawn from the above results: 1) Corrosion tests on the clad and Q and T material, i.e., the determination of CPT by the ASTM G-48 test and Huey (ASTM A262 Type C) immersion tests to evaluate the intergranular corrosion resistance, were not promising when almost all weld overlays were sampled, likely due to excessive Fe content in the CRA layer. When cladding coupons were predominantly sampled from the second overlay pass, the corrosion resistance was significantly improved, although it remained slightly below what was expected for the standard AISI 316L steel. 2) The above results are related to the interface chemical composition. In particular, the results showed that the first 2 mm thick layer of the cladding showed a uniform composition at about 70% Fe content. However, depths greater than 2 mm gave an iron content >70%, with values that increased almost linearly, reaching 80% at 3.0 mm depth due to dilution phenomena. 3) Some hardness peaks (e.g., 250 to 270 HV10) were detected in the microalloyed steel close to the fusion line in the CGHAZ. 4) The hardness peaks in the CGHAZ of the microalloyed steel, close to the fusion line (0.3 mm distance), disappeared after Q and T; they composed all values lower than 220 HV10. This means that the reaustenitizing treatment, at temperatures below those experienced at 0.3 mm from the fusion line, produces a refinement of austenite grains, sufficient to decrease hardenability and hardness after tempering.

Conclusions
The following conclusions can be drawn from the above results: (1) Corrosion tests on the clad and Q and T material, i.e., the determination of CPT by the ASTM G-48 test and Huey (ASTM A262 Type C) immersion tests to evaluate the intergranular corrosion resistance, were not promising when almost all weld overlays were sampled, likely due to excessive Fe content in the CRA layer. When cladding coupons were predominantly sampled from the second overlay pass, the corrosion resistance was significantly improved, although it remained slightly below what was expected for the standard AISI 316L steel. (2) The above results are related to the interface chemical composition. In particular, the results showed that the first 2 mm thick layer of the cladding showed a uniform composition at about 70% Fe content. However, depths greater than 2 mm gave an iron content >70%, with values that increased almost linearly, reaching 80% at 3.0 mm depth due to dilution phenomena. (3) Some hardness peaks (e.g., 250 to 270 HV 10 ) were detected in the microalloyed steel close to the fusion line in the CGHAZ. (4) The hardness peaks in the CGHAZ of the microalloyed steel, close to the fusion line (0.3 mm distance), disappeared after Q and T; they composed all values lower than 220 HV 10 . This means that the reaustenitizing treatment, at temperatures below those experienced at 0.3 mm from the fusion line, produces a refinement of austenite grains, sufficient to decrease hardenability and hardness after tempering.