**1. Introduction**

Metal-clad plates for different applications have been developed in many industrial sectors [1–3]. 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 [9–11], aeronautics [12], food [13–15], 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 [20–22]. 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],

CO<sup>2</sup> arc welding [26,27], and laser welding [28] were successfully applied in the case of AISI 304 stainless steel adopted as the clad for Q235 steel, thus improving the corrosion resistance of the original carbon steel plate. However, under some conditions, stainless steel could also undergo corrosion [29]. It is, for example, known that carbide precipitates on the grain boundaries of AISI 304 steel, forming a chromium-depleted region if it is exposed to the temperature range of 550–850 ◦C. Following this formation, the steel becomes susceptible to attack in a corrosive medium. Similarly, stainless and carbon steel bimetal plates are no exception [30]. Moreover, for the stainless and carbon steel bimetal plate, the effect of element diffusion should also be taken into consideration in the study of its corrosion-resistance evolution. Many previous works have shown that there is a diffusion area between stainless steel and carbon steel that makes the stainless steel and carbon steel bond well together [31]. In particular, the composite mechanism and interface microstructure of metal-clad plates have been largely analyzed [32]. It was found that element diffusion occurring near the interface of the clad plate appears to be the key to the bonding of metal-clad plates [33,34]. This strongly depends on the cladding process that also affects the interface microstructure and hardness. Compared to carbon and stainless steel, the interface layer retains different microstructures and mechanical properties, which can affect the mechanical performance of the clad plate. In particular, it was found that due to element diffusion (Fe, Cr, Ni, and Mn), a 20 µm thick diffusion layer forms between AISI 304 stainless steel and carbon steel clad plates. The diffusion layer is characterized by stable mechanical performance, and that microstructure does not show any grain growth [35], with internal mechanical properties showing a gradual change in the thickness direction [36]. Therefore, such a transition layer appears to be beneficial to a strong bond between stainless steel and carbon steel, also guaranteeing a stable transition of mechanical performance in the thickness direction. Carburization of the stainless steel with a thickness of 150 µm is found, and decarburization carbon steel with a thickness of 80 µm is formed on the carbon steel side [37]. Many efforts have been made in order to assess the corrosion rate at the inner surface of ultrasupercritical boilers [38]. Moreover, often a hard peak is detected at stainless and carbon steel interfaces following austenite grain growth in the carbon steel side during welding [39]. Such peaks should be carefully taken and kept as low as possible. Since it is often difficult to avoid such peaks during the welding process, proper heat treatments are necessary in order to optimize interface microstructure and hardness values [40,41].

In this paper, the microstructure characterization of stainless steel clad as obtained by the welding overlay is shown. The postwelding overlay heat treatment effect is analyzed on the interface of in terms of microstructure and hardness.

#### **2. 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.

**Table 1.** Main chemical composition of the considered materials (mass, %).


parameters are shown in Table 2.

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

**Figure 1.** Schematic view of the adopted overlay process. **Figure 1.** Schematic view of the adopted overlay process.

**Table 2.** Adopted main welding parameters. **Welding Parameters Values**  Welding Current 450–550 A Arc Voltage 28–33 V 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.



#### 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, *2.1. Microstructure and Hardness*

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. *2.2. 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 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<sup>3</sup> + 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.

#### resistance. • ASTM G-48 test (method C). Coupons of size 20 mm × 30 mm × 1.5–2.5 mm thickness were taken *2.2. Corrosion Testing*

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 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.
