Double-layer materials clad with corrosion-resistant steels are increasingly being used as materials for hull structures operating in aggressive environments. Two-layer materials began to be used as hull material in the second half of the 20th century  to ensure high corrosion and erosion resistance of atomic icebreakers. Currently, the development of the Arctic regions, the implementation of large-scale projects in northern latitudes, the use of the northern sea route lead to the necessity the use of more powerful icebreakers, which leads to greater ice loads and, as a consequence, the need to use advanced corrosion-resistant materials, which also have high strength characteristics [1-4].
To ensure safety operation of products made of laminated materials, as well as to assess the residual life of parts and structural elements, it is necessary to diagnose changes that occur during operation both in the layered material as a whole and in its components. At present, there are no adequate methods for non-destructive testing of the current state of individual layers in bimetallic materials under the conditions of current stresses.
In this work, we investigated the structure, phase composition, mechanical and magnetic properties of structural steel, clad with corrosion-resistant steels with varying degrees of austenite stability in order to determine the possibility of using magnetic methods to diagnose the current state of such materials.
2. Results and Disscustion
Two two-layered materials were obtained: “constructional steel – high-nitrogen containing steel” and “constructional steel – AISI 304”. The chemical composition of investigated materials is presented in Table 1. After explosion welding, the samples were subjected to cold pack rolling with varying degrees of rolling in order to investigate the possibility of using magnetic diagnostic methods to assess the changes occurring during plastic deformation in the structure, phase composition, and physical-mechanical properties of the two-layered materials under the study. Also for the completeness, individual components of the layered materials were subjected to the same processing methods.
To assess the strength joint of the connection of the layers, shear tests were carried out. The appearance of the samples is shown in Fig. 1. According to the test results, it was found that the shear resistance of both materials is 560 MPa.
Microstructural investigations devoted to the study of the boundary between the layers during explosion welding, show that increasing the degree of cold rolling of layered materials leads to a decrease in the wave-like boundary obtained during explosion welding [5-8]. It was also found that, with an increase in the degree of cold rolling, grain refinement occurs in all deformable materials.
Metallographic and X-ray phase analyzes (Fig.2) carried out on all the studied samples confirmed the purely austenitic structure in both corrosion-resistant steels in the initial state, also after deformation by cold rolling in the high-nitrogen containing steel, which indicates the stability of the structural state of this steel during plastic deformation. From fig.2a it is also clearly seen that the structure of AISI 304 steel is approximately equiaxial austenite grains of 25-40 microns in size. The structure is also characterized by the presence of twins.
According to optical microscopy data, the structure of high-nitrogen containing steel consists of anisomerous austenite grains of 10–40 µm in size with twins. Investigations of the fine structure of high0nitrogen containing steel at the initial state by electron transmission microscopy revealed a heterogeneous structure (Fig. 3.) of this steel, which is characteristic of the state formed during hot rolling . There are areas with a band structure consisting of austenite subgrains with predominantly in one direction with different, including low, dislocation density (Fig. 3a). The austenitic structure also contains numerous particles with different sizes and shapes (Fig. 3a – c). From small (up to 50-100 nm in size) particles of nitrides V, Cr, Nb, fixing dislocations, on diffraction patterns one can see blurred reflexes - arms (see. Fig. 3c). There are also large individual particles with a size of 500–700 μm, for example, in Fig. 3d presents a similar particle with a facet and an interplanar distance of 2.52 Å, which corresponds to the niobium nitride (or a complex nitride with a predominance of niobium).
During cold rolling in AISI 304 steel have undergone the phase transformation and the martensite deformation occurs, which is an undesirable ferromagnetic phase and leads to a decrease in resistance to intergranular corrosion. Moreover, with an increase in the degree of deformation by cold rolling, its amount, both in a separate component and in the composition of a bimetal, increased (Fig. 4).
Figure 4b shows the field dependence of the differential magnetic permeability for metastable AISI 304 steel. It is clearly seen that with increasing degree of deformation by rolling, the height of the peak increases, the field of localization of the peak, as is known , approximately coincides with the values of the coercive force of the material. The increase in the peak height is due to the formation of a new ferromagnetic phase, α/-martensite deformation. Also, the diagnosis of the formation of a magnetically ordered phase in these steels could be performed by measuring the maximum magnetization in the maximum applied field (saturation magnetization), the maximum magnetic permeability and the root-mean-square values of the Barkhausen magnetic noise voltage, which vary monotonically with the degree of the deformation.
In fig. 5 it is shows the field dependences of the differential magnetic permeability of the bimetal “constructional steel – AISI 304” with an increase in the degree of deformation by rolling. On these curves with a deformation of more than 0.30, it could be clearly seen that two peaks form on these dependences: the first — in weaker fields, but higher in intensity — characterizes the ferromagnetic phase (ferrite) in the component of constructional hull steel. The second peak, which is localized in stronger fields and belongs to the newly formed as a result of rolling phase – martensite deformation in a component of AISI 304 steel. Thus, with the help of magnetic methods, it is possible to diagnose changes occurring in layered materials, and in its individual components.
The values of the maxima in the field dependences of the differential magnetic permeability of layered materials could serve as parameters for diagnosing the formation of a new magnetically ordered phase and its quantity formed during plastic and elastoplastic deformation. The field values at which maxima are formed in the dependences of the differential magnetic permeability of layered materials and their individual components could be used to estimate the degree of deformation by rolling and uniaxial stretching of these materials.
The results obtained are important and relevant for the subsequent work on the development of methods for non-destructive evaluation of changes occurring in layered high-strength materials during the operation cycle.