Mechanical bonding properties and interfacial morphologies of austenitic stainless steel clad plates

Author’s Accepted Manuscript Mechanical bonding properties and interfacial morphologies of A283/316 clad composite Zina Dhib, Noamen Guermazi, Ahmed Ktari, Monique Gasperini, Nader Haddar www.elsevier.com/locate/msea

PII: DOI: Reference:

S0921-5093(17)30552-X http://dx.doi.org/10.1016/j.msea.2017.04.080 MSA34981

To appear in: Materials Science & Engineering A Received date: 6 January 2017 Revised date: 18 April 2017 Accepted date: 20 April 2017 Cite this article as: Zina Dhib, Noamen Guermazi, Ahmed Ktari, Monique Gasperini and Nader Haddar, Mechanical bonding properties and interfacial morphologies of A283/316 clad composite, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.04.080 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Mechanical bonding properties and interfacial morphologies of A283/316 clad composite Zina DHIBa, Noamen GUERMAZIa,b*, Ahmed KTARIa, Monique GASPERINIc, Nader HADDARa a

Laboratoire Génie des Matériaux et Environnement (LGME), Ecole Nationale d'Ingénieurs de Sfax (ENIS), BP 1173-3038, Sfax, Université de Sfax, Tunisie

b

Institut Supérieur des Sciences Appliquées et de Technologie de Kasserine, Université de Kairouan, BP 471 - 1200 Kasserine, Tunisie. c

Université Paris 13, Sorbonne Paris Cité, Laboratoire des Sciences des Procédés et des

Matériaux (LSPM), CNRS(UPR3407), 99 av. J.B. Clément, F-93430 Villetaneuse, France *

[email protected]

Abstract: The current paper focuses on the correlation between microstructure and mechanical properties in low-carbon steel / austenitic stainless steel clad composite fabricated by hot-roll bonding. For this reason, the morphology and interfacial characteristics, tensile properties shear strength and fracture toughness of the cladded material were evaluated. From the main results, it was found that carbon element diffusion caused the forming of a decarburized ferrite zone (DFZ) of the parent metal and a carburized austenite zone (CAZ) of the clad layer, and between these two area, a thin diffusion layer with rapid element component change are formed in the hot-roll cladding process. Stress-strain curves obtained from tensile testing of parent metal and clad layer can predict the bi-material tensile behavior. The shear test proved that the stainless steel clad plate presents an acceptable shear bond strength at the interface joint. Impact test toughness results confirm that fracture took place only in the parent metal side of cladded specimens; the clad layer was bent but without obvious fracture. Fractography was carried out using scanning electron microscope in the tensile, shear bond test specimens and Charpy impact ones. It reveals the presence of predominantly dimpled fracture. Charpy impact specimens of the interface failed in mixed mode while impact specimens of the base plate failed in ductile mode. Keywords: Stainless steel clad plate; Carbon diffusion; Diffusion layer; Tensile shear test; Charpy impact test; Fracture mode.

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1. Introduction Nowadays, along with the development of science and technologies, the challenge is progressively changing from the optimization of novel bulk materials to the production of compounds that contain metallurgical bond joints [1]. Since it is difficult to meet a large variety of requirements such as superior mechanical, physical and thermal properties for a single material, clad metals, consisting of two or more metals, have been invented and industrialized because of their unique properties [2-5]. In recent years, there has been a great demand in the industry for a structure which can simultaneously satisfy the combination of ductility and corrosion resistance of structure. As a substitute for the whole stainless material (in bulk), the stainless steel clad plates have become increasingly popular for engineering applications and have been widely used in many fields, such as transferring pipes, reservoirs, vessels, heat exchangers, kitchen utensils and decorative trade. Many kinds of technologies are used to manufacture stainless clad plates, such as explosion welding [1, 6-9], diffusion bonding [10], weld overlay cladding [11] and roll bonding [12-14]. Hot roll cladding is a bonding process in which plates are joined under high temperature as described in the literature [3, 16, 17, 22]. The bond is metallurgical in nature and usually as strong as or stronger than the weaker parent metal. There have been some reported works on the roll bonding of clad plates. The most widely fabricated commercial / industrial clad plates products are Al/ferritic stainless steel [22], austenitic stainless steel/Al/Cu [2], and Ti/ferritic stainless steel [19], stainless steel/ Carbon steel [20,22,23]. The stainless clad plate is a kind of layered structures generally fabricated by bonding a stainless steel plate to another metal such as carbon steel or low-alloy steel plate. It has been largely used because of its ideal combination of mechanical, physical and corrosion properties, and the advantage over common engineering materials at price [5]. The bonding of the interface between two metals has an important influence on the quality of the clad plate. Some researchers have involved themselves in the study of stainless steel cladding either with explosive welding or hotrolling processes. Kaya and Kahraman [9] have investigated the bonding ability of Grade A ship steel (parent plate) and AISI 316L austenitic stainless steel (clad plate) plates with explosive cladding using different ratios of the explosive (R). Kacar and Acarer [6] have focused on the microstructure, mechanical properties of duplex stainless steel/vessel steel explosive cladding. They have found that although some regions of interfacial melting were observed; in general, the 2

interface had the characteristics of the solid-state nature; also, elongation of grains near the interface parallel to the impact direction was related to a high degree of plastic deformation. They also [7] have emphasized on Charpy impact test of 316L stainless steel in DIN-P355GH-grade steel cladding. Zhu and co-workers [23] have investigated the bonding ability of cladded steel through shear tensile test and they proved that cladding improves the ductility of materials. The later have also shown that there was a diffusion phenomenon of chemical elements (C, Cr, Ni, Mo,..) around the interface line and resulted in the appearance of brittle intermetallic diffusion layer [22-25]. In order to explain the different behaviors of the cladded metals many researchers [16, 22, 26, 27] have focused on tensile test of cladded metal and prove that rule of mixture can be applied elsewhere. It has been shown in recent work [16] that the tensile strength and total elongation of samples in rolling direction (RD) are superior to those of samples in transverse direction (TD) after the same treatments. This phenomenon results from the grain anisotropy induced by the large plastic deformation. The effects of composite structures of cladded materials on the fracture toughness have been previously investigated using crack type opening displacement (CTOD) bending test by Khadadad et al. [22]. The experimental study has been adopted by some researchers because it can credibly estimate the causal relationship metallurgical and mechanical properties. Our previous research [16] deals with the study of the mechanical and metallurgical properties of the welded-cladded steel (A283/316) applied to acid tank to reduce the amount of corrosion products in the coolant. For this reason, three weld samples with different configurations where the type of chamfering and the order of welding passes of the three filler metals (A283, 309L and 316) versus their location during the welding process are different. These welded joints which were prepared from cladded materials were studied in detail and at the end one configuration was selected as the most efficient joint in service. However, in the present investigation, it is really interesting to further evaluate the joining ability of A283 low carbon steel and 316 austenitic stainless steel with hot-roll cladding. To our knowledge, few studies in the literature reports the interfacial morphologies and mechanical behavior of this stainless steel clad plates. Therefore, in order to understand these features in as 3

clad condition, mechanical properties (tensile test, shear strength, and toughness test), scanning electron microstructures and also energy-dispersive spectroscopy (EDS) analysis were investigated in detail. This current contribution will be useful to better discern the relationship between structure and performance and as a result, it could act as a guide in the production of A283/316 bimetal joints in the industry.

2. Experimental procedures The bi-material clad plates used through this study were already described in previous work of Dhib and co-workers [28]. The cladded pates were prepared by hot-roll bonding and consist of low-carbon steel (ASTM A283 grade C) as parent metal stainless steel (ASTM A240 type 316) as clad layer. A schematic view of the hot rolling process is shown in Fig.1. The chemical composition of the two steels is given in Table 1. The bonding between the two materials is obtained under the combination of an important applied pressure and a high temperature during an acceptable time. The industrial thermal cycle is summarized as follows: a) Slow increase in temperature until 1230 °C for 16 hours. b) Maintaining clad plate at 1230 ° C for 5 hours. c) Rolling between 1230 °C and 850 °C. d) Cooling in air. e) Annealing at 920–950 °C followed by cooling in air. It is worth noting that before the hot-roll cladding; the two materials surfaces are prepared with an appropriate mechanical polishing and chemical etching to remove organic matter and surfaces oxides. According to literature [29], during the bonding process, high reduction in thickness of the materials is achieved under high pressure at the roller. The high reduction generates a great amount of heat and creates virgin surfaces on the materials being bonded. The fresh, virgin surfaces along the bond interface are in a self-enclosed environment, where oxidation cannot occur, and therefore do not have bond-impeding oxide barriers. A bond (normally a mechanical bond) in the layered composite is thus obtained through interfacial mechanical locking and atomic affinity between the two metals. After roll bonding, an annealing treatment is necessary, not only, to increase the adhesion of the plate, but also, to restore different metallurgical structures of components .Then, the carbon steel plate and stainless steel one are stacked together

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against the clean surfaces, and the leading heads were spot-welded to enter smoothly into the roll gap. As it was previously mentioned in our published work [28], microstructural analyses of hot-roll cladded plates were deeply studied. Among others, it was shown the presence of heterogeneous microstructure with appearance of new areas, around the bond interface, having different microstructures from those of parent metal and clad layer. These new zones were localized from both the two sides of the interface line and their presence may be due to the carbon diffusion and others substitutional elements. In order to highlight this phenomenon of diffusion of chemical elements, the area adjacent to the bi-material interface was scanned for distribution of chemical components such as carbon, chromium and nickel elements by scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy method (EDS). To estimate the mechanical bonding properties and the fracture behavior of cladded plates, different mechanical tests were carried out in this study. In order to investigate the tensile properties of each material of cladded steel, uniaxial tensile tests were conducted to both parent metal and clad layer each one separately from the other before being bonded together by hot-roll cladding. To find the local variation of tensile properties from the clad layer down the bi-material interface and then the parent metal, micro flat tensile specimens (with a thickness of 1 mm) were extracted by Spark Erosion Cutting (SEC) in the rolling direction as shown in Fig. 2. Tensile specimens consisting of both clad layer and parent metal) with a total thickness of 6 mm (thickness of parent metal = 3 mm and thickness of clad layer = 3 mm) were also tested at room temperature. For the tension test, an universal testing machine with capacity of 50 kN was used and data were obtained from three samples for the same conditions. During the test, the crosshead speed was fixed at 1 mm/min. In order to evaluate the bond quality of cladded plates, shear and charpy impact tests were retained. The shear tests were conducted to determine the bonding qualities according to the standard ASTM A264-09 [30]. Accordingly, the shear test is to be performed as indicated in Fig.3 and it was carried out at room temperature on universal testing machine in the compression direction with a shearing speed of 1 mm/min. Three samples were tested and average values are considered. According to the standard ASTM A264-09, shear test specimen must be fixed between two shear block, which shall be bolted firmly together against the filler piece which provides a space 0.13 5

wider than the ‘‘t’’ of the specimen (t refer to the thickness of parent metal in cladded steel; t = 12 mm) as it is shown in Fig. 4. Shear test has been monitored by a single monochrome camera with high resolution of (1624×1236) and 4 µm×4 µm as pixel size. On the other hand, in order to investigate the toughness of the cladded metals, Charpy impact test was retained. Standard Charpy V-Notch (CVN) specimens with a (10×10 mm2) section, a central 45° V-notch of 2 mm depth, were used. Therefore, notch has been machined on the parent metal side of cladded steel. The CVN test specimen is shown schematically in Fig. 5 and all tests were carried out at room temperature. Fractography studies were carried out on all the fractured mechanical test specimens using FEG SUPRA 40VP-SEM. Microstructural features of cross sections of the fractured surfaces of shear and impact tests were investigated using a LEICA type metallographic microscope.

3. Results and discussions 3.1. Interfacial morphologies In our previous study [28], microstructural examinations of the cladded steel revealed that parent metal is a carbon steel with ferritic-pearlitic structure and the clad layer is a stainless steel with austenitic structure. The bond interface between the two materials of cladded steel shows a flat or straight boundary. This linear bond interface is a characteristic of hot-roll cladding process, which was different from the wave-like explosive clad interface originated from the concentrated plastic deformation localized in the contact zone [1,7,8]. Microstructural investigations of the clad steel plate, as shown in Fig. 6, revealed the existence of a carbon enriched area around the interface line in the side of austenitic clad layer; this area is called carburized austenite zone (CAZ). However, in the other side of parent metal near the interface line; there is a coarse grained ferritic structure as a bond along the clad interface with almost no evidence of pearlitic structure; this area will be called in this paper decarburized ferrite zone (DFZ). Distributions of alloy elements across interface are evaluated in two selected areas around the clad interface: selected area 1 and 2 refer to clad layer and parent metal, respectively as shown in Fig. 6. According to the Energy Dispersive Spectroscopy (EDS) results displayed in Figs. 7-a and 7-b, the elements analyzed were those present in varying quantities in the carbon and 6

stainless steels used in this study such as iron, molybdenum, silicuim, chromium and nickel elements. EDS result by SEM of chemical components in selected areas near the interface is consistent with the chemical analysis of parent metal (A283) and clad layer (316) [28]. In fact, Fig. 7-a shows that Cr and Ni appear in greater quantities in the selected area 1 for the clad layer side near the CAZ, whereas Fig. 7-b indicates that the parent metal side (selected area 2) presents a lower Cr and Ni content. Appearance of these areas (CAZ and DFZ) can be explained by migration of carbon from parent metal forwards clad layer across the bi-material interface. So, there is creation of carbon depleted area and a carbon enriched area in the two sides of the bond interface. As mentioned previously [28], parent metal had a 100 µm wide DFZ adjacent to the interface, and ferritic- pearlitic structure away from the interface. The stainless steel exhibited a 100 µm wide CAZ near the interface line. In fact, the interface was exposed to higher temperature for longer duration and hence grain coarsening as well as decarburization occurred at the interface. In fact, because of high temperature tempering, carbide precipitation takes place in the austenitic structure [22]. The same trend was also recently observed by Zhu and co-workers in their research about shear properties of hot-rolled stainless steel clad plate [23]. Before hot-rolling, the assembly of a bi-material plate usually heats up to 1200 °C for five hours. During this period, due to the compositional gradients for carbon, chromium and nickel between the parent metal (0.23% C, 0.024% Cr and 0.029% Ni) and clad layer (0.08% C, 16.18% Cr and 10.55% Ni) and sufficient time and temperature, the required driving force for diffusion of the elements in the bi-material can be easily provided. Therefore, there is a phenomenon of carbon diffusion from parent metal to clad layer in the surrounding area of bi-material interface. Thus, carbon diffusion can be easily pronounced because of its small atomic radius (0.077nm), which offers it a great mobility. In the present study, in order to highlight this phenomenon of diffusion and taking into account the presence of two new areas around the bond interface: DFZ and CAZ, an elemental distribution of carbon across the bond interface is performed. This qualification is done to prove the phenomenon of migration of this element. Regarding to Fig.8, it is clearly seen that there is fluctuation of carbon profile through the interface line. The element transfer result shows that element content of carbon decreases when moving from DFZ to CAZ. In fact, the carbon migrates from parent metal towards the bi-material interface and then accumulates in the clad layer. This carbon diffusion phenomenon induces the appearance of decarburized area in the 7

parent metal and carburized zone in clad layer near the interface line. In accordance with Mendes et al [8] and Zhang and co-workers [31] findings, the formation of these interfacial zones is expected to be created with materials resulted from the mixing of both the clad layer and the parent material stainless steel and carbon steel,

even if those contributions may be not

uniformed. In recent work, Nambu et al [32] prove that martensitic steel was sandwiched by 304 stainless steel and Nickel sheet was inserted between parent metal and clad in order to avoid carbon diffusion during hot-roll cladding process. On the other hand, chromium and Nickel elements can also move, but in the direction opposite to carbon, i.e from the clad layer to the bi-material interface and then to the parent metal. This is deduced from the distribution of Cr and Ni across the bi-material interface. As shown in Fig.9, it was observed that Cr and Ni elements diffused significantly from the stainless steel side to the carbon steel side with diffusion distances of ~13µm and ~9µm, respectively. According to the literature [23,24], diffusion coefficients of Ni and Cr in ɣ-Fe at the rolling temperature of 1180°C were calculated to be approximately 5.7×10-11 and 4.6×10-11 Cm2/s, respectively. The bond interface is expected to reach such high temperature (1200°C) during hot-roll cladding process, and hence these diffusion coefficients are considered valid for the present study. Although Ni has a little higher diffusion coefficient, the diffusion distance of Ni was lower than that of the Cr. This should be attributed to the obviously different concentration gradients for Cr and Ni as reported in Ref. [23]. Similar findings are also found by Zhu and co-workers [23] in their recent work when they have shown that the elevated composition gradient of alloying elements, which takes place between HSLA steel and stainless steel, could result in remarkable elements changes near the interface of clad plates during the high temperature diffusion process. In addition, they prove that the element contents of Cr and Ni decrease from stainless steel to carbon steel. Concentration gradients and diffusion coefficients difference between Cr and Ni elements led to the diffusion distances of 12-13 µm and 3-4 µm, respectively. Carbon element in CS was supposed to diffuse into SS, but we could see the concentration phenomenon of C element in CAZ near the interface. The compositional profile of C element can be ascribed to the uphill diffusion effect under the produce conditions. Overall, the diffusion layer of the alloying elements near the interface of the bi-metallic sheet was relatively thin. These significant fluctuations in Cr and Ni contents near the clad interface with a small width producing may be due to the higher temperature that reached the material during hot8

rolling process. Khadadad et al. [22] prove that Cr and Ni elements also moved, in their turn, but in the direction opposite to carbon i.e. from the clad layer towards the bi-material interface and then the base metal. Therefore, these gradients of elements lead to graded interface with different microstructures. It is important to note that diffusion of Cr and Ni elements have no significant effect on the microstructure of the decarburized ferritic zone (DFZ). Whereas, carbon diffusion induces a deep modification in microstructure since it causes the appearance of CAZ and DFZ.

3.2. Mechanical bonding properties 3.2.1. Tensile test of A283, 316 and cladded steel It is worth noting that the tensile properties of a hot-rolled clad interface can be with an important consideration when structural components are subjected to tensile loading in service. Availability of clad tensile strength test data would strengthen the design analysis, and may lead to extend the range of applications of hot-roll cladded materials. In our case, the tensile behavior of pure parent metal, pure clad layer and bi-material were studied. The obtained stress-strain curves are shown in Fig.10. The corresponding tensile properties are listed in Table 2. It is to note that the rule of mixture for composite materials was applied in our case. Here, the theoretical values are compared with those evaluated from the experimental curves. As shown in Table 2, the theoretical values correlate well the experimental ones and the stainless steel clad plate as a composite material demonstrates a “rule of mixture effect” i.e., an intermediate yield strength (332 MPa), ultimate strength (530 MPa) and elastic modulus (200 GPa). The rule of mixture for sandwich structure can accurately predict the tensile behavior of bi-material clad plates. Khadadad et al. [22] have found the same tendency and they have evaluated the effect of relative thickness of the bi-material cladded system on the type of stress-strain curves. By defining clad thickness:

, they show that the higher value of , the higher is the obtained

stress level due to the high strength clad. So, for specimens fully made of the clad material (

the stress-strain curve is highest and for specimens fully made in parent metal (

the stress-strain curve is the lowest. In our current study,

,

and the stress-strain curve was

found to be the intermediate curve: between the two tensile curves of pure clad layer and pure 9

parent metal. It is noticeable that the shape of tensile curve changes from one with a yield plateau (for the parent metal) to one with continuous yielding (for the clad layer). By considering the bi-material plate as a whole, the rule of mixture was found to be accurate for excellent prediction of the yield and ultimate tensile strengths and elastic modulus. Prediction of the yield and tensile strengths and elastic modulus is found not too far from the real experimental data for the bi-material cladded steel (Table 2). In order to study the fracture behavior of parent metal and clad layer, fractography observations are carried out on the surface of the tested samples (Fig. 11). Figs. 11-a and 11-b reveal typical characteristic features of ductile fracture, including a large amount of equiaxed dimples. A high fractography resolution in position A shows that broken surfaces of parent metal A283 spherical dimples which correspond to the micro-voids characteristic of monotonic ductile fracture [33]. Clad metal exhibits a ductile behavior with the occurrence of dimpled fractography as shown in position B in Fig. 11-d. The fractography of clad layer (Figs. 11-c and 11-d) evidence the presence of many fines dimples with different sizes oriented in the fracture direction. Indeed, it can be found that yield stress and ultimate strength of clad layer are higher than parent metal. The large difference between the yield stress and ultimate strength in the case of clad layer (464 and 697 MPa, respectively) underlines the ability of this material to undergo a significant amount of work hardening during monotonic deformation. The difference of monotonic behavior between clad layer and parent metal was also observed in the work of Khadadad et al. [22]. The latter have found that clad layer have a higher yield stress compared to parent metal with a difference of ~250 MPa. Concerning the clad layer, we can also observe a dimpled structure revealing ductile fracture more pronounced than parent metal. The interfacial morphologies of fractured surfaces of cladded material were examined by scanning electron microscopy. Fig.12 presents the SEM image of interfacial fractography after uniaxial tensile test. With regards to this figure, it is important to note that the fractured surfaces of cladded plates consist of a certain amount of small scale dimples. As shown, a large amount of small-scale dimples in clad layer side and a small amount of large-scale dimples in parent metal side can be observed in the fracture surface, which is correlated with the improved plasticity upon the hot rolling. Interestingly, between clad layer and parent metal, the brittle intermetallic compound layer CAZ with thickness of about 100 µm is shown. This zone would play a key role in the interface cracking. Therefore, this brittle zone (CAZ), created after interfacial reaction in 10

bonding process, has a great effect on the fracture mode. These results are in good agreement with previous studies realized on mechanical properties of austenitic stainless steels [22, 23, 27]. Results from tensile testing demonstrate also that cladding improves the ductility of the stainless steel clad plates. 3.2.2. Shear tensile test In order to discuss the joint interface strength, shear test was conducted under the condition that shear direction corresponds to the hot-roll direction i.e. parallel to the interface line. To determine the shear strength of the clad plate, tensile shear specimens are submitted to compression loading. The load "P" is applied on the top of the shear tensile test specimens which are cut off along the bonding interface at the end of the test. Then, the load "P" will be divided by the shearing area of the specimen. The value of shear strength is measured and the average of three tests is presented. The shear strength test specimen (Fig. 13-a) was found to be fractured at the bond line. It is well known that metallic materials exhibit a shear strength which is more than 50% of their tensile strength [23]. In the current case, the average shear strength of the clad plate was found about 282±5 MPa which corresponds to 59% of its tensile strength. As mentioned elsewhere [6,9,22,25], this value of tensile shear strength was found to be much higher than the minimum of 140 MPa as given in ASTM A-264 specifications [30]. This is in agreement with the study of Rao and co-workers about cladding low alloy steel with austenitic stainless steel [23]. These authors have found that shear strength of the parent metal was found to be around 399 MPa. The shear bond strength of the cladded steel plates was 488 MPa which is 22% higher when compared to the shear strength of the parent metal. The failure location and fractural surface morphology after shear test were presented in Fig.13-b. At the end of shear test, it is interesting to note that total separation between parent metal and clad layer occurred and led then to an interfacial delamination. To more highlight the location of fracture interface, tensile shear test has been monitored by CCD camera and some images describing the progress during the test have been taken. It can be seen from Fig.14 that for the bi-material interface, where the transition zones between the two materials (CAZ and DFZ) were relatively wide (approximately 200 µm), the initial shear crack is initiated at the bond interface, the shear crack firstly tears from the bond interface and then propagates in DFZ in the parent metal side (CS) of which the strength was lower that CAZ in the 11

clad layer side. Fracture occurred in form of total disbonding between parent metal (CS) and clad layer (SS)). The main cracks propagated in the direction parallel to the loading action and they did not include secondary cracks. The crack was developed in the parent metal but it did not reached the interface line. This finding is consistent with those reported by Zhu et al. [23] when they have found that the shear strength of 316L stainless steel clad plates reaches 420 MPa and they prove that for the majority of the stainless steel clad plate, the interface exhibited a sharp transition and good bonding quality. Their results show that the main fracture section was in the weaker DFZ in the parent metal side which may act as alternative crack propagation path in shear test. A SEM study was undertaken to clarify the rupture mechanisms at the interface of cladded plates. The fracture surface of shear test specimen is shown in Fig.15. It is probably seen from the fracture surface that the fracture site occurred in the carbon steel side (Fig.15-b). Moreover, there is a region of tearing strip that occurs at the carbon steel side marked by arrows (Fig.15-c) around the interface and may show an interfacial delamination. The tearing strips of carbon steel parent metal contains many fine dimples as shown in Fig.15-d, which can play an important role in toughening the interface of clad plate [28]. As already shown in Fig.14, the initial shear notch is always cut to across the interface, the shear crack firstly tears from carbon steel and then propagates along the interface. Fig.15-c shows a Y-shaped crack in the carbon steel fracture surface. Fig.15-b shows three regions with distinct microscopic morphologies. The upper area (detail A) exhibited quasicleavage fracture morphology, the lower area (detail B) exhibited dimple morphology, and small dimples were distributed throughout the area between these two regions (see Fig.15-d). The Yshaped cracks may be related to the concentrated stress that easily occurred at the intersection of three different structures (see Fig.15-c). As seen in Figs.15-d (1) and 15-d (2), the fracture surface consisted of the fine dimple rupture. The failure is predominantly dimpled rupture. In order to better understand the damage mechanism of cladded steel, cross-sections are carried out on the fractured surfaces of shear tensile specimens (parent metal side and clad layer side). Optical micrographs in Fig.16 shows that fracture occurred in the parent metal side and not at the interface. Therefore, these optical micrographs indicate perfect bonding quality fabricated by hotroll cladding process that can be safely used in service. 3.2.3. Charpy impact tests 12

Charpy V-notch impact tests were carried out to evaluate the toughness of stainless steel clad plates. The main results of impact tests are given in Fig.17. The samples were broken only on the parent metal side of cladded materials. However, the clad layer was bended but it did not present a clear fracture (Fig.17-b). As it can be seen, notch on the specimen presents non-significant effect on the impact toughness of austenitic stainless steel (clad layer side). So, clad layer is more ductile than parent metal which presents a ductile fracture less pronounced. This is consistent with the earlier studies [9, 17, 23]. The average value of impact energy of the cladded materials (> 300 J) exceeds the capacity of the machine at 25 °C. The results can be considered as successful when compared with base plate impact energy which reaches only 27 J at 25 °C [28]. The toughness measured at all tests was much higher than that of the parent metal due to the effect of the austenitic stainless steel clad layer. Indeed, in previous study [34], Kchaou et al. have shown that the fracture energy of 316 stainless steel presents higher value because of the presence of  2 % of Mo in the austenitic stainless steel. The higher notch toughness value (> 300 J) can be attributed to cladding that made a positive effect on the base material. In order to better understand the damage mechanism of cladded steel, cross-sections are carried out on the fractured surfaces of CVN tests in the RD. The metallographic observations as shown in Fig.17-c indicate perfect bonding between the parent metal and clad layer and the fracture occurred only in the parent metal side adjacent to the bi-material interface. As mentioned earlier, delamination takes place in the interface line which acts as a barrier to the crack propagation. Examples of the fracture surface appearance of test samples are shown in Fig.18. As mentioned earlier, only the parent plate of the cladded metals was fractured in the Charpy impact tests. As seen in Fig.18-a, the parent plate shows a combination of transgranular fracture and microvoids coalescence (MVC) surface in fractured samples. Cleavage is well shown in Fig. 18-c and it led to brittle failure in the carbon steel side. Meanwhile, the fracture surface of clad layer (stainless steel side) of cladded materials displays predominantly a ductile fracture appearance due to presence of fcc phase in 316 stainless steel [23]. Fig.18-d shows that the SEM micrographs of fracture surface of clad layer and the global appearance are similar to the one already observed on the fracture surface after tensile test (Fig. 18-d). A ductile mode of fracture is evidenced by the presence of dimples. In previous studies [6,9], it was found the same fracture mode with the same approximately impact energy at room temperature for an austenitic stainless steel. The ductility of the clad layer combined with the high toughness of the stainless steel cladding resulted in the 13

fact that cladded steel absorbed a good amount of impact energy (>300J). However, in some grains near the interface, brittle fracture was observed due to high degree of shock hardening that may cause brittle fracture in ductile materials as shown in Refs. [9,22]. This was also supported by microhardness test results in which hardness of parent plate was found approximately as 500HV near the interface [28]. Because of high plastic deformation resulting in shock hardening, the metallic materials show brittle behavior. It also probably causes a reduction in fracture toughness.

4. Conclusions A283 Grade C (parent metal) and AISI 316 austenitic stainless steel (clad layer) were successfully cladded through hot-roll bonding process. Therefore, a focus was done on their interfacial morphologies and mechanical properties. From the main results, the following conclusions can be drawn: a. The clad interface was flat in nature and around it chemical composition was a mixture of alloying elements per-taining to both carbon steel and stainless steel. Therefore, two new areas adjacent to the bond interface appeared: DFZ in the parent metal side and CAZ in the clad layer side due to the material mismatching between inner stainless steel layer and outer low alloy steel layer. b. The microstructural changes at the interface correlated with EDS analysis prove a phenomenon of carbon diffusion and others chemical elements (Cr, Ni). c. All the mechanical tests indicate that the cladded plates produced by hot-roll bonding present good mechanical properties in service. d. The tensile behavior of clad plates was found in accordance with the rule of mixture i.e., intermediate values of yield tensile strength, ultimate strength and elastic modulus were measured for the composite structure. e. Shear tests exhibit a strong bond since fracture occurred in the parent metal and the average shear strength reaches 282 MPa. f. Fractography of shear strength/ CVN test specimens revealed mixed mode of cleavage and dimpled rupture. Finally, throughout this investigation, it can be discerned that a good explanation of relationship between interfacial morphologies and mechanical properties could be obtained by studying the 14

fracture mode. Correlation between the interfacial morphologies and the mechanical properties could be used for bonding quality assessment.

Acknowledgments The authors would like to thank Dr. Ovidui BRINZA (LSPM-PARIS13), for his technical assistance and contribution to this research.

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Table 1. Chemical composition of clad plate (in wt%) Elements Wt

C

Si

Mn

P

Cr

Ni

Fe

316

0.08

0.53

1.17

0.053

16.18

10.55

Balance

A283

0.23

0.171

0.40

0.032

0.024

0.02

balance

(%)

Table 2.Tensile properties of the parent metal, clad layer and the bi-material clad plate and comparison with the rule of mixture Yield tensile Ultimate tensile Elastic Modulus Material strength (MPa) strength (MPa) (GPa) Parent metal

211

414

220

Clad layer

464

697

186

322

530

200

337.5

555.5

203

Measured Cladded steel Rule of mixture

18

Figures captions Figure 1. Schematic representation of the hot roll bonding system .............................................................. 2 Figure 2. Geometrical details of tensile specimens: ..................................................................................... 3 Figure 3. Test specimen and the method of shear testing of clad plates according to the ASTM A 264 standard. t: Thickness of parent metal an a: thickness of clad layer [29] .................................................... 4 Figure 4. Experimental set of shear tensile test of cladded steel .................................................................. 5 Figure 5. Standard Charpy impact test specimen .......................................................................................... 6 Figure 6. SEM image of the straight interface of austenitic stainless steel clad plate: (a) chemical etching revealing DFZ in A283 Grade C (selected area 1) and (b) electrolytic etching revealing CAZ in 316 (selected area 2) ............................................................................................................................................ 7 Figure 7. EDS analysis results carried out on (a) the clad layer (selected area 1) and (b) the parent metal (selected area 2) ............................................................................................................................................ 8 Figure 8. Compositional profiles via EDS analysis of carbon and iron in the bi-material clad interface .... 9 Figure 9. EDS quantitative analysis data of Cr and Ni elements – diffusion layer widths.......................... 10 Figure 10. Stress-strain curve of the bi-material system for parent metal, clad layer and cladded steel ... 11 Figure 11. (a) Macro-fractography of parent metal after the tensile test, (b) high-resolution fractography of position A, (c) macro-fractography of clad layer after tensile test ,(d) ) high-resolution fractography of position B..................................................................................................................................................... 12 Figure 12. SEM micrograph of fractured surface after tensile ................................................................... 13 Figure 13. Tensile shear test specimens (a) before and (b) after shear strength test .................................. 14 Figure 14.(a)Stress-strain curve from the shear test and (b) Macro-fractography of shear tensile specimen: shear cracking paths of the clad interface (CS: A283 carbon steel; SS: 316 stainless steel) RD: rolling direction........................................................................................................................................... 16 Figure 15. SEM fractograph of shear strength test. .................................................................................... 17 Figure 16. Optical micrographs (OM) of cross sections of fractured surfaces after shear tensile test (a)Fractured surface of clad layer side (b) cross section clad layer electrolytically etched (c) cross section of clad layer chemically etched (d) Fractured surface of parent metal side (e) cross section parent metal chemically etched ........................................................................................................................................ 18 Figure 17. Charpy impact test results: CVN (a) before and (b) after test and (c)....................................... 19 Figure 18. SEM fractograph of charpy impact test. .................................................................................... 20

1

Figure 1. Schematic representation of the hot roll bonding system

2

Parent metal Clad layer

Micro- flat tensile specimen (a)

Parent metal (b)

Parent metal Clad layer Clad layer

(c) Figure 2. Geometrical details of tensile specimens: (a) Parent metal, (b) clad layer and (c) tensile specimen of cladded steel

3

Figure 3. Test specimen and the method of shear testing of clad plates according to the ASTM A 264 standard. t: Thickness of parent metal an a: thickness of clad layer [29]

4

Driven plate

Shear test specimen

Shear blocks

Fixed plate Figure 4. Experimental set of shear tensile test of cladded steel

5

Figure 5. Standard Charpy impact test specimen

6

DFZ CAZ

Figure 6. SEM image of the straight interface of austenitic stainless steel clad plate: (a) chemical etching revealing DFZ in A283 Grade C (selected area 1) and (b) electrolytic etching revealing CAZ in 316 (selected area 2)

7

(a)

(b)

Figure 7. EDS analysis results carried out on (a) the clad layer (selected area 1) and (b) the parent metal (selected area 2)

8

CAZ

DFZ

Figure 8. Compositional profiles via EDS analysis of carbon and iron in the bi-material clad interface

9

Figure 9. EDS quantitative analysis data of Cr and Ni elements – diffusion layer widths.

10

800

Engineering stress (MPa)

700 600 500

Clad layer Cladded steel

400

Parent metal

300 200 100 0 0

10

20 30 Engineering strain (%)

40

50

Figure 10. Stress-strain curve of the bi-material system for parent metal, clad layer and cladded steel

11

(a)

(b)

Position A

10µm

500µm

(c)

(d) Position B

10µm

1mm

Figure 11. (a) Macro-fractography of parent metal after the tensile test, (b) high-resolution fractography of position A, (c) macro-fractography of clad layer after tensile test ,(d) ) highresolution fractography of position B

12

Figure 12. SEM micrograph of fractured surface after tensile

13

10mm

10mm

Figure 13. Tensile shear test specimens (a) before and (b) after shear strength test

14

Shear stress (MPa)

300

C B

250 200 150

100 50

A

0 0

10

20

30

40

Shear strain (%)

15

50

60

Shear direction

A: Crack initiation

B: Crack propagation

RD

C: Delamination of the bonding area Figure 14.(a)Stress-strain curve from the shear test and (b) Macro-fractography of shear tensile specimen: shear cracking paths of the clad interface (CS: A283 carbon steel; SS: 316 stainless steel) RD: rolling direction

16

Detail A

Detail B

Figure 15. SEM fractograph of shear strength test.

17

A-A

(b)

(a) SS

A

A-A

A

CAZ

Interface line CS DFZ

(d) B

B

CS

Figure 16. Optical micrographs (OM) of cross sections of fractured surfaces after shear tensile test (a)Fractured surface of clad layer side (b) cross section clad layer electrolytically etched (c) cross section of clad layer chemically etched (d) Fractured surface of parent metal side (e) cross section parent metal chemically etched

18

(a)

10mm (b)

Kv>300J

(c)

Resin Figure 17. Charpy impact test results: CVN (a) before and (b) after test and (c) Optical microscopy observation of cross section of CVN fractured surfaces

19

Figure 18. SEM fractograph of charpy impact test.

20