https://dr.ntu.edu.sg/bitstream/10356/95598/1/4.%20FeBFe2B%20Phase%20Transformation%20during%20SPS%20Pack-Boriding%20Boride%20Layer%20Growth%20Kinetics.pdf

FeB/Fe2B phase transformation during SPS pack-boriding: Boride layer growth kinetics

Boriding kinetics of H13 steel

In present study, kinetic studies on borided AISI 5140, AISI 4340 and AISI D2 steels are reported. Steels were borided in a salt bath consisting of borax, boric acid and ferro-silicon between 1073 and 1273 K for 2, 4, 6 and 8 h. The morphology and types of borides formed on the surface of steel substrates were confirmed by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. Boride layer thickness formed on the borided steels ranged from 21 to 238 μm depending on process temperature, treatment time and alloying elements of the substrates. The hardness of borides formed on the samples changes between 1077 and 2140 HV0.1 according to treatment time and temperature. Layer growth kinetics were analyzed by measuring the extent of penetration of FeB and Fe2B sublayers as a function of boronizing time and temperature in the range of 1123–1273 K. The depth of the tips of the most deeply penetrated FeB and Fe2B needles are taken as measures for diffusion in the growth directions. The kinetics of the reaction, K=Ko exp (−Q/RT), have also been determined by varying the boriding temperature and time. The results showed that K increase with boriding temperature. Activation energies (Q) of borided AISI 5140, AISI 4340 and AISI D2 steels at present study were determined as 223, 234 and 170 kJ/mol, respectively. Moreover, an attempt was made to investigate the possibility of predicting the contour diagrams of boride layers variation and to establish some empirical relationships between process parameters and boride layer thicknesses.

An approach to kinetic study of borided steels

A technologically interesting characteristic of boriding is the production of a hard, wear-resistant coating layer on the steel substrate. In this study, case properties of borided AISI W1 steel has been investigated by conducting a series of experiments in Ekabor-I powders at the process temperature of 1123–1323 K at 50 K intervals for periods of 1–8 h. The presence of borides FeB and Fe 2 B formed on the surface of steel substrate was confirmed by optical microscopy and X-ray diffraction. Cross-sectional observation in the optical microscope revealed smooth and compact morphology of the borided layer. The distribution of alloy elements from the surface to the interior was confirmed by energy dispersive X-ray spectroscopy. The hardness of the boride layer formed on the surface of the steel substrate was higher than 1500 HV. It was concluded that the optimum temperature for AISI W1 steel borided in Ekabor-I powders was approximately 1223 K for hardness in 10 μm depth, and the hardness change with boriding temperature was related to the grain size of the treated steel. The kinetics of boriding show a parabolic relationship between layer thickness and process time, and the activation energy for the process is 171.2±16.6 kJ mol −1 . Moreover, an attempt was made to investigate the possibility of predicting the iso-thickness of boride layer variation and to establish an empirical relationship between process parameters of boriding and boride layer.

Kinetics of boriding of AISI W1 steel

Vanadium carbide coatings on AISI H13 steel were prepared by thermo-reactive deposition/diffusion process (TRD) in molten salt bath for 1 to 6 h at 920 °C and 1000 °C, respectively. The obtained coatings were characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction analysis (XRD). Equiaxed grains were observed throughout the coatings. The grain size gradually increased from the coating/substrate interface to the top surface. The coatings were composed of ordered state V6C5 phase and disordered state VCx (x = 0.83–0.88) phases and had a preferential orientation of (111) and (200) planes. The values of nano-indentation hardness and elastic modulus of the coating are 28.1 ± 0.7 GPa and 421 ± 14 GPa, respectively. The growth of the vanadium carbide coating by the TRD process followed a parabolic kinetics with an activation energy of 199.3 kJ/mol. The variation of the coating thickness on the AISI H13 steel with treating time and temperature can be determined.

Evaluation of vanadium carbide coatings on AISI H13 obtained by thermo-reactive deposition/diffusion technique

A multi-component diffusion study of boron, chromium, aluminum and silicon on an AISI 1045 steel substrate has been carried out by pack cementation A commercial boriding powder (Ekabor-3) and an Fe–Cr alloy powder plus NH 4 Cl activator make up 30 wt% of the pack, and 70 wt% of Al 2 O 3 serves as an inert diluent and supply source of aluminum atoms The structure and constituent changes of the coated layers and their thickness and hardness, as influenced by different pack compositions, have been investigated at 1000°C The kinetics of the reaction, K = K 0  exp(− Q / RT ), have also been determined in this work by varying the NH 4 Cl additions The results show that Q and K 0 decrease with increasing amount of NH 4 Cl addition at temperatures between 900°C and 1000°C

The kinetics and mechanism of multi-component diffusion on AISI 1045 steel

Heat resistance casting alloy ACI HP was aluminized or silicon-aluminized for investigating their carburization resistance in a high-carbon potential atmospherized-fluidized-bed which simulated the carburizing environment of the pyrolysis tubing in a ethylene production line. The results showed that the coating layer prepared by the high-activity aluminizing pack was superior to that of the low-activity one. On the contrary, the coating layer provided by the low-activity silicon-aluminizing pack was superior to that of the high-activity one. Experimental results evidenced that silicon could effectively retard the degeneration rate of NiAl and reduce the formation of porous band, which located at the interface between aluminide layer and interdiffusion zone. Moreover, the kinetics of the reaction, D = D exp (− Q /RT) , was empirically determined by the method of Al pack and Al–Si pack. The results showed that, in a low-activity Al–Si pack, Si can effectively reduce both Q and D 0 . As a consequence, it increased the formation rate of aluminide layer, in contrast, it reduced the formation rate of interdiffusion zone.

The aluminizing and Al-Si codeposition on AISI HP alloy and the evaluation of their carburizing resistance

The present work is concerned with super-carburization treatment on low alloy steel AISI 4135 and plain carbon steel AISI 1020 by fluidized-bed furnaces. Super-carburization is a repetition of carburization and precipitation heat treatment processes to produce a dense spheroidized carbide layer near to the steel surface. Carburization was conducted at 930°C and 800°C in an endothermic atmosphere with a carbon potential higher than 1.3% generated by the direct boosting of propane and air into the bed. The precipitation of carbides in steels was conducted at 600°C in a nitrogen-fluidized bed. The rapid cooling rate associated with the great undercooling of the fluidized bed effectively suppressed the formation of network cementite when carburized steels were cooled from the austenitizing temperature to the precipitation temperature. This phenomenon has made it possible for even a plain carbon steel to be super-carburized without difficulty.

Kuo-Liang Wang

Papers

The kinetics and mechanism of multi-component diffusion on AISI 1045 steel

The aluminizing and Al-Si codeposition on AISI HP alloy and the evaluation of their carburizing resistance

Super-carburization of low alloy steel and low carbon steel by fluidized-bed furnaces