Investigating Growth of Iron Borides with the Formation of Monolithic Fe2B Layer on AISI 304 Stainless Steel via Cathodic Reduction and Thermal Diffusion-Based Boriding

Abstract

This study is focused on the determination of the effects of processing time and temperature on the thickness, morphology, and hardness of boride layers grown on AISI 304L stainless steels. For boriding, a new molten salt electrolysis method called as CRTD-Bor (Cathodic Reduction and Thermal Diffusion-based boriding) was chosen due to its fast and green nature. CRTD-Bor of AISI 304L substrates was carried out in a borax-based molten electrolyte at temperatures ranging from 950 to 1050 °C for periods of 15 to 60 min at a constant current density of 200 mA/cm 2 . The x-ray diffraction analyses revealed the mixed iron boride phases including Fe 2 B, FeB. Moreover, cross-sectional scanning electron microscopy examinations confirmed the growth of these phases. Additionally, a phase homogenization (PH) step was adapted into CRTD-Bor to eliminate brittle FeB layer. It was founded that after 70 min of treatment at 1000 °C (15 min of CRTD-Bor + 55 min of PH) it is possible to grow % 40-lm-thick Fe 2 B layer exhibiting 1700 ± 100 HV on the surface with excellent adhesion to the substrate (HF1). Besides, kinetic calculations showed the activation energy (Q) of boride layer growth as 181.45 kJ/mol.

Growth Kinetics, Microstructure Evolution, and Some Mechanical Properties of Boride Layers Produced on X165CrV12 Tool Steel

“…This situation brought about lowering of the boron activation energy of the treated AISI 316 steel. In other research work, Arslan et al [39] used a new thermochemical process, called CRTD-Bor (cathodic reduction and thermal diffusion-based boriding), for boronizing AISI 314 L steel between 950 and 1050 • C. The boriding medium was composed of 90 wt.% Na 2 B 4 O 7 and 10% Na 2 CO 3 under a constant current density of 200 mA cm −2 for 0.25 to 1 h. The key benefit of a such process was the generation of thick boride layers in shorter times. Furthermore, the determined boron activation was 181.45 kJ mol −1 for AISI 314 L steel.…”

“…The estimated activation energies were diminished compared to those of conventional powder-pack boriding, due to effect of the electrical field that promoted the diffusion of boron atoms by producing thicker layers for shorter times (less or equal to 2 h). Keddam et al [38] boronized AISI 316 steel by plasma-paste boriding with 100% B 2 O 3 in the temperature interval of 970-1073 K. The estimated boron activation energy for this steel was 118.12 kJ mol −1 , which was the lowest value of those [27,[33][34][35][36][37][38][39][40] listed in Table 5 owing to the activation of plasma energy. This situation brought about lowering of the boron activation energy of the treated AISI 316 steel.…”