Proportionality limit and defect structure of laminated steel composite material after severe plastic deformation

The article considers the issues of increasing the strength and changing the defect structure in a layered steel composite obtained by pressure welding (forge welding) after severe plastic deformation (SPD) using the equal-channel angular pressing (ECAP) scheme. The aim of the work is to investigate the defect structure of the layered steel composite material after SPD. The layered steel composite material was obtained as a result of forge welding of eight plates of low-alloy steel 09G2S and alloyed carbon steel 40Kh with alternating layers. ECA pressing of the layered composite 09G2S + 40Kh in one pass was performed parallel to the plane of the layers at an intersection angle of the channels of 120° and a temperature of 823 K. The microstructure of the composite was examined using a JSM-7800F scanning electron microscope. The proportional limit of the material was determined based on the results of uniaxial compression tests using a Zwick/Roell-Z600 universal testing machine. X-ray diffraction analysis (XRD) of the composite was performed using a high-precision Ultima IV (Rigaku) powder X-ray diffractometer in CoKα radiation (wavelength λ = 1.790255 Å). An increase in the proportionality limit of the composite is shown, caused by grain refinement, a change in the crystal lattice parameter, and an increase in the dislocation density. Forge welding resulted in strain hardening in the intermediate layer (transitional between 09G2S and 40Kh steels) caused by grain boundary diffusion processes due to distortion of the crystal lattice. It was found that after forge welding and ECAP, the transition layer with a width of 4.3 ± 0.4 μm consists of deformed ferrite grains with an average size of 5.0 ± 0.5 μm. The transition layer contains impurities — manganese sulfide particles. ECAP affects the defectiveness of different layers of the joint differently: in the 09G2S layer and in the intermediate layer, the dislocation density increases significantly, while in the 40Kh layer, these values decrease slightly. In the intermediate layer, as a result of plastic deformation at the substructural (mesoscopic) level, the significant increase in dislocation density acts as a micromechanism for the formation of a fine-grained structure.

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