Structure and properties of hot-forged powder steel–bronze bimetal with SiC additives

Abstract

One of the main problems in the production of bimetals is associated with the difference in the physico-mechanical and structural properties of the materials being joined. Both solid-phase and liquid-phase methods are used to obtain bimetals. The main technological task is to create conditions for the formation of a transition zone between the working layer and adhesively bound substrate. We analyzed the known methods for producing compact and powder bimetals (insert molding, diffusion welding in the solid phase, infiltration, hot isostatic pressing, etc.). The bonding strength of bimetal layers is evaluated according to the results of mecha­nical shear or pull tests; however, such an assessment does not enable to determine if the product can be operated in the mode of frequent thermal cycles. The above method, which involves joint hot repressing of previously separately cold-pressed and sintered blanks of the working layer and substrates, is promising in terms of improving the mechanical and tribotechnical properties, reducing the risk of structural degradation of particles of hardening additives, as well as enhancing the quality of the connection of steel–bronze bimetal layers. In this case, the working layer is heated through heat transfer from the side of the substrate warmed up to a higher temperature. We studied the impact of technological conditions for obtaining hot-forged powder steel–bronze bimetal on the structure, features of thermal fatigue failure and tribological properties and presented the research results. For structural analy­sis, thermal fatigue and tribotechnical tests, the bimetal samples with vertical and horizontal arrangement of layers were obtained. The atomized iron powder PZhRV 3.200.28 was used as a base for fabricating the substrate from PK40 steel. Graphite powder GK-3 (GOST 4404-78) was used as a carbonaceous additive. The working layer was fabricated from BrO10 bronze powder obtained by atomizing. To improve the tribotechnical characteristics of the working layer, bronze powder was mixed with superfine grinding micropowder F1000 of black silicon carbide 53S. The quality of bonding of bimetal layers was assessed based on the thermal shock test results. Tribotechnical tests were carried out in the dry friction mode according to the “shaft–block” scheme. We proposed the technique for producing hot-forged powder bimetal “PK40 steel–BrO10 bronze”, which includes the following independent procedures: cold pressing of the substrate and working layer blanks, their sintering in a reducing environment, pre-deformation heating of the substrate and working layer at temperatures that ensure their satisfactory formability, assembly of heated substrate and working layer blanks in the mold and subsequent joint hot repressing. The resulting bimetal is characterized by increased values of thermal fatigue and wear resistance in comparison with the control samples manufactured using the traditional technology of hot repressing of the cold-pressed bimetallic blank.