ANALYSIS OF PROCESSES OCCURRING DURING SOLID-PHASE BORIDING OF STEELS

PYBRES

Abstract

When developing boron-containing compositions for solid-phase boriding, it is necessary to clearly understand the physicochemical features of the processes occurring in the reaction space at the operating temperature. This paper presents the results of an analysis of the chemical processes occurring during boriding in powder mixtures based on B_amorphous - Na₂B₄O₇ and B₄C - NH₄Cl with an additive in the form of spent ceramic welding flux powder of the aluminate-basic type. In the course of experimental studies, solid-phase boriding of construction carbon steel in the developed compositions was carried out at a temperature of 900°C for 2 hours and a metallographic analysis of the obtained diffusion layers was carried out. As a result of thermodynamic analysis, reactions were determined that have no thermodynamic limitations. It is shown that boron activation in all the studied compositions occurs mainly due to the decomposition of boron fluoride, which is synthesized in the reaction space. Boron fluoride is formed by reactions involving halides of B, Fe, Ca (in B₄C - NH₄Cl mixtures) and Na, Ca (in B_amorphous - Na₂B₄O₇ mixtures). In B₄C - NH₄Cl compositions, a greater amount of active boron is synthesized due to a greater number of thermodynamically preferred chemical transformations leading to the formation of BF₃. As a result of metallographic analysis, it was shown that the depth of the diffusion layer during boriding in B₄C - NH₄Cl-based compositions is more than 2 times greater than the depth obtained using B_amorphous - Na₂B₄O₇ -based compositions, and the microhardness is 30% higher. The possibility of using spent ceramic welding flux as a component of borating compositions, which simultaneously acts as an inert additive (SiO₂, MnO, CaO, MgO, Al₂O₃, FeO, TiO₂, ZrO₂) and an activator of the borating process (CaF₂), has been theoretically proven and experimentally confirmed.