Introduction

Metallic clad plates, especially stainless steel clad plates, have a widespread application in nuclear power plant for safety injection tank, which realize the possibility of combining the weldability, formability of carbon steel substrates with the corrosion resistance of stainless steel layer [1]. Meanwhile, multi-or bi-layered composites have been initially designed to enhance the damage tolerance, toughness, ductility, weldability and bending formability due to the presence of interfaces [1,2,3,4,5]. Ohashi et al. [6] proposed that superior bending ductility can be obtained in the laminated steel-brass composites without interfacial debonding. Yanagimoto et al. [3, 7] reported that multilayered steel possess remarkable tensile ductility and bending formability by delaying necking of brittle layer.

In order to develop stainless steel clad plates with high toughness and corrosion resistant, Jing et al. have reported the interfacial characteristics and shear behavior of stainless steel clad plates in detail [8, 9]. However, the topics on the tensile deformation process and fracture modes of stainless steel clad plates have yet to be studied systematically. In this paper, it is really interesting to further investigate the tensile behavior and fracture characteristics of stainless steel clad plates, which is useful to further comprehend the relationship between structure and performance, and guide the microstructure design for property improvement of bi-layered metal composites.

Experimental Procedures

The chemical composition of raw stainless steel and carbon steel is listed in Table 1. A stainless steel sheet with a thickness of 12 mm and carbon steel with a thickness of 60 mm were stacked together and subjected to vacuum hot rolling for eight passes at the temperature of 1100 °C followed by air cooling. The total thickness of stainless steel clad assemble is 145 mm with a 1 mm thick insulating cloth. Finally, it was rolled to 14.5 mm, and the total rolling reduction ratio is 90%. Herein, the layer thicknesses of stainless steel cladding and carbon steel substrate are about 1.2 and 6.0 mm, respectively. The fabrication parameters during the hot roll bonding process are listed in Table 2.

Table 1 The chemical composition of the stainless steel and carbon steel (wt%)
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Table 2 Fabrication parameters during the hot roll bonding process
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Samples were cut from the stainless steel clad plates and prepared using metallographic corrosion and electrical polishing technologies. The microstructures and fracture characteristics are investigated by optical microscope (OM) and scanning electron microscopic (SEM), respectively. The longitudinal tensile tests were carried out using an Instron-5569 universal testing machine at a constant crosshead speed of 2 mm/min, and a total of five samples were tested for each material. Tensile dog bone samples have dimensions of 110 mm × 20 mm × 2.4 mm.

Results and Discussions

As shown in Fig. 1, five different microstructural layers are located in the stainless steel clad plate: carbon steel layer, decarburized layer, interface, carburized layer and stainless steel layer. The carbon steel layer contains ferrite phase and dark pearite phase. During the hot rolling process, the decarburized layer and carburized layer are caused by the diffusion of carbon element from carbon steel substrate into the stainless steel layer. It can be found that the thickness of purity ferrite decarburized layer and carburized layer are about 50–60 μm and 20–30 μm, respectively. Herein, many equiaxed polygon grains with size of 10–15 μm were well-distributed in the carburized layer, and the clear grain boundary may be pinned by Cr23C6 carbides precipitates [10, 11]. Many rolling twinning were decorated in the stainless steel layer caused by severe rolling deformation rate. In addition, complete interface with slight undulation realizes the metallurgical bonding as shown in Fig. 1a. However, from the high magnified view shown in Fig. 1b, a small amount of very fine pores (diameter < 1.5 μm) were distributed at the interface.

Fig. 1
figure 1

Microstructures of stainless steel clad plate fabricated by vacuum hot rolling at 1100 °C, a a low magnificent; b a high magnificent

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Figure 2 shows the macro fracture characteristics of the stainless steel clad plate. The clad plate has displayed a high uniform plastic deformation capacity and slight localized necking until failure as shown in Fig. 2a. It is interesting to note that severe warping resilience and interfacial delamination phenomena are shown in Fig. 2b. There is serious localized necking and interfacial delamination phenomena existed at the fracture zone, and the depth of interfacial delamination crack is about 800 μm as shown in Fig. 2c.

Fig. 2
figure 2

The macro fracture characteristics of stainless steel clad plate, a after tension; b warping resilience and delamination phenomena; c localized necking and interfacial delamination crack

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Figure 3 shows the micro fracture morphologies of the stainless steel clad plate. The moderate interfacial bonding strength results into a deep interfacial delamination crack as shown in Fig. 3a, which may be related to the fine interfacial pores. Figure 3b shows many intergranular cracks in the carburized layer. The Cr23C6 carbides often precipitate at the grain boundary of stainless steel due to the diffusion carbon element, leading to the high brittleness of grain boundary. Therefore, many cracks propagate along the grain boundary and form intergranular tunnel cracks [11]. According to elastic plastic deformation model [12]:

Fig. 3
figure 3

Micro fracture characteristics of stainless steel clad plate; a interfacial delamination; b carburized layer; c decarburized layer; d carbon steel layer; e stainless steel layer

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$$ t \le \frac{{2\sqrt 3 \mathop K\nolimits_{IC}^{2} \mathop \sigma \nolimits_{s} }}{{\mathop \sigma \nolimits^{3} }} = \mathop t\nolimits_{critical} $$
(1)

where \( t \) and \( \mathop K\nolimits_{IC} \) are the thickness and fracture toughness of carburized layer, respectively. \( t_{critical} \) is the critical tunnel crack size, \( \sigma_{s} \) is the yielding strength of stainless steel layer, and σ is the stress imposed on the carburized layer. It is revealed that when the size of tunnel crack is lower than the critical crack size, the tunnel crack will be blunted by the adjacent stainless steel layer. Therefore, the thinner carburized layer can result into more intergranular cracks and higher fracture toughness.

Figure 3c shows the fracture characteristics of decarburized layer. There are many transgranular fracture characteristics accompanying with river-like patterns and cleavage fracture. It is because that the plastic deformation of decarburized layers was constrained by carbon steel substrate and moderate interface, forming super-high triaxial tensile stress and resulting into ductile brittle transition of decarburized layer, which is similar with that of the laminated Al-2024Al composites [13]. As shown in Figs. 3d, e, it is observed that both carbon steel layer and stainless steel layer show ductile fracture surface. However, the size of dimples in carbon steel layer and stainless steel layer are about 20–30 and 2–5 μm, respectively.

The tensile stress-displacement curves of raw materials and stainless steel clad plate are shown in Fig. 4, and the corresponding tensile properties are listed in Table 3. The stainless steel clad plate demonstrates a “rule of mixture effect” i.e., an intermediate yield strength (298 MPa), ultimate strength (578 MPa) and fracture elongation (46%) between carbon steel and stainless steel. The prediction of the true tensile stress-strain relationship on the clad plate requires the tensile behavior of individual layers. During the longitudinal tensile deformation stage, the deformation behavior of stainless steel layer and carbon steel layer is fitting to the iso-strain condition. One class of metals considered is a pure power-law hardening (Hollomon) materials, finite strain J2 deformation theory solid with hardening exponent (N). In plane strain tension, the true stress-strain relation is expressed as follows [14]:

Fig. 4
figure 4

Tensile behavior of stainless steel clad plate. A, B, C values show the elastic recovery displacement of carbon steel, stainless steel clad plates and stainless steel, respectively

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Table 3 Tensile properties of stainless steel clad plates and raw materials
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$$ \sigma = K\mathop \varepsilon \nolimits^{N} $$
(2)

where \( K \) is the strength coefficient. Therefore, the stress-strain curve of stainless steel clad plates can be computed by the following expression:

$$ \sigma = (1 - \mathop f\nolimits_{ss} )\mathop \sigma \nolimits_{cs} + \mathop f\nolimits_{ss} \mathop \sigma \nolimits_{ss} = (1 - \mathop f\nolimits_{ss} )\mathop K\nolimits_{cs} \mathop \varepsilon \nolimits^{{\mathop N\nolimits_{cs} }} + \mathop f\nolimits_{ss} \mathop K\nolimits_{ss} \mathop \varepsilon \nolimits^{{\mathop N\nolimits_{ss} }} $$
(3)

where \( \mathop f\nolimits_{ss} \) is the volume fraction of stainless steel layer, and the “cs” and “ss” represent the carbon steel and stainless steel. Therefore, the tensile stress-displacement curve of stainless steel clad plates is located in between carbon steel and stainless steel.

Based on the Consider criterion, the condition of diffuse necking, which leads to non-uniform plastic deformation even fracture failure defined by the following equations:

$$ \frac{d\sigma }{d\varepsilon } = \sigma $$
(4)
$$ \frac{d\sigma }{d\varepsilon } = KN\mathop \varepsilon \nolimits^{N - 1} = K\mathop \varepsilon \nolimits^{N} $$
(5)
$$ \mathop \varepsilon \nolimits_{necking} = N $$
(6)

Actually, the uniform plastic deformation up to necking strain is mainly depended on the strain hardening coefficient \( N \). Since the stainless steel experiences a longer deformation stage and higher fracture elongation than the carbon steel. Substituting Eq. (4) into (3), the diffuse necking strain of clad plates can be expressed as follows:

$$ N_{ss} > N_{cs} $$
(7)
$$ \frac{{\mathop K\nolimits_{ss} \mathop f\nolimits_{ss} }}{{\mathop K\nolimits_{cs} \left( {1 - \mathop f\nolimits_{ss} } \right)}} = \frac{{\mathop N\nolimits_{cs} \mathop \varepsilon \nolimits_{necking(L)}^{{\mathop N\nolimits_{cs} - 1}} - \mathop \varepsilon \nolimits_{necking(L)}^{{\mathop N\nolimits_{cs} }} }}{{\mathop \varepsilon \nolimits_{necking(L)}^{{\mathop N\nolimits_{ss} }} - \mathop N\nolimits_{ss} \mathop \varepsilon \nolimits_{necking(L)}^{{\mathop N\nolimits_{ss} - 1}} }} $$
(8)

where \( \mathop \varepsilon \nolimits_{necking(L)} \) is the clad plates limit strain at the onset of necking. Serror et al. [15] reported that the necking strain of laminates is bounded by the necking strain of carbon steel as a lower bound and that of stainless steel as an upper bound, respectively. Therefore, the stainless steel clad plate reveals a intermediate fracture elongation.

In addition, severe warping resilience phenomenon is attributed to the different deformation behavior of carbon steel and stainless steel. The stainless steel clad plates will experience an elastic recovery stage after fracture. Because that the elastic modulus is lower, whereas the stress of stainless steel is higher than that of carbon steel as shown in Table 3, the elastic recovery displacement of stainless steel layer is longer than that the carbon steel layer as shown in Fig. 4. Therefore, residual compression stresses are distributed in the stainless steel layer, whereas residual tensile stresses in the carbon steel layer, resulting into apparent warping resilience phenomenon.

Summaries

  1. (1)

    The tensile behavior of stainless steel clad plates reveals “rule of mixture effect” i.e., an intermediate tensile strength, ultimate strength and fracture elongation locate in between carbon steel and stainless steel.

  2. (2)

    The fracture characteristic of clad plate reveals an obvious interfacial delamination crack and many intergranular tunnel cracks, which are attributed to the moderate interfacial bonding and brittleness of carburized layer, respectively.

  3. (3)

    The stainless steel clad plates displays severe warping resilience phenomenon, which is attributed to the elastic modulus and stress mismatch between carbon steel layer and stainless steel layer.