Alternating current field assisted pack boriding to Fe 2 B coating

Introduction

Boride coatings by pack boriding can greatly enhance ferrous and many non-ferrous metallic materials’ hardness, wear and corrosion resistance.1 ^– 4 The coatings on steels are generally composed of either a single phase Fe₂B layer or a dual phase layer with an outer FeB zone and an inner Fe₂B zone. The type of coating depends on the boron potential at the steel surface, the boriding temperature and the composition of the substrate.5 ^– 7 A single phase Fe₂B coating is generally preferred in industrial applications for its lower brittleness under thermal or mechanical shock than the dual phase coating.6,8 Single Fe₂B phase coatings can be produced by lowering the boron potential of the boriding media,9,10 but the boriding speed is decreased. A dual phase FeB–Fe₂B coating can be changed to a single Fe₂B phase coating by a vacuum or salt bath treatment for several hours above 800°C after normal boriding.11 However, production cost will be increased because of the prolonged heat treating time. Kartal et al.12 introduced a method ‘phase homogenisation in electrochemical boriding’ for forming a single phase Fe₂B coating on steel substrates. FeB formed by electrochemical boriding was transformed to Fe₂B by a subsequent homogenisation treatment in the same electrolyte for an additional time. Employing direct current field can greatly accelerate pack boriding as well as alumunising with energy saving,13 ^– 16 but the boride coating is a dual phase one. However, no report has been found that produces a single phase Fe₂B coating directly by pack boriding in a quick and economic way.

The aim of the present work was an attempt to use an alternating current field (ACF) during pack boriding to enhance diffusion in the treated samples to obtain a thick Fe₂B coating at a moderate temperature and within a shorter time. The goal was to create a new engineering process that is both economic and efficient. A mechanism to the enhancing effect of the ACF was proposed.

Experimental

The experimental ACF enhanced pack boriding (ACFPB) apparatus is schematically shown in Fig. 1. All parts of the apparatus were heated in a chamber furnace during the treatment except for the adjustable ac supplier (part 6) and some of the conducting lines (part 7). Conventional pack boriding (CPB) for comparison was carried out in a separate container heated in the same furnace used for ACFPB. Samples subjected to boriding were made of normalised AISI 1045 steel with dimensions of 10×10×5 mm. The samples were ground and cleaned to remove surface contamination before boriding. Alternating current field enhanced pack boriding was realised by applying a 50 Hz ACF with a designated current to the electrodes once the soaking temperature (800°C) was reached. Detailed parameters for the treating are given in Table 1. After cooling in the furnace to the room temperature, the samples were removed from the container for further study.

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Figure 1

1: sample; 2, 10: electrodes; 3: container; 4: powder boriding medium; 5, 9: clay sealing; 6: adjustable ac supplier; 7: conducting wire; 8: lid

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Table 1

Boriding parameters

				ACF parameters
Process code	Boriding media formulation/wt-%	Furnace temperature/°C	Soaking duration/h	Voltage/V	Current/A
ACFPB11 to ACFPB19	5B–Fe+5KBF4+2 charcoal+balance SiC	800	4	15–40	1, 2, 3, 4, 5, 6, 7, 8, 9, respectively
CPB1				0	0
ACFPB21 to ACFPB24	10B–Fe+5KBF4+2charcoal+balance SiC	800	1, 2, 4, 6 respectively	30–50	4
CPB21 to CPB25			0, 1, 2, 4, 6 respectively	0	0

Microstructures of the treated samples were evaluated in cross-sections by optical microscopy (OM). Coating phases were investigated by both OM and X-ray diffraction (XRD, Cu K_α radiation, 100 mA and 40 kV). The boride coating thickness, as seen in the OM, was obtained by averaging the distance from the surface to the tip of the finger shaped boride. Borides measured were always at the centre of the sample.

Results

Coating thickness

Figure 2 gives the change of boride coating thickness with boriding time for CPB and ACFPB at an ACF current of 4 A. The boride coating thickness versus boriding time shows a parabolic character, which proves that diffusion is the main controlling factor in the growth of the coating for both ACFPB and CPB. In this case, the ACFPB coating is about twice as thick as the CPB coating as a function of different boriding times.

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Figure 2

Boride coating thickness versus boriding time

The influence of current on the ACFPB boride coating thickness after 4 h boriding is shown in Fig. 3. As the current increased to 9 A, the coating thickness increased in a linear manner to more than 2·5 times the thickness of the CPB coating. As an example, a Fe₂B coating more than 140 μm thick was produced by 4 h ACFPB treatment with an ACF current of 8 A, but the boride coating thickness of the corresponding CPB sample was only ∼60 μm.

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Figure 3

Boride coating thickness versus ACF current

Microstructures

Investigation through OM found no FeB in the boride coatings of all ACF samples. Single Fe₂B phase coatings comprised the coating of samples borided at 800°C not only with the pack media containing 5 wt-% masteralloy of FeB, but also with the one containing 10 wt-%FeB masteralloy. However, dual phase coating was clearly revealed for all CPB samples, even with the lower boron potential of the boriding media containing 5 wt-%FeB masteralloy. Figure 4 shows microstructures in cross-sections of some typical CPB and ACFPB samples. FeB phase appeared in the 0 h soaking sample (process CPB21) indicated that FeB phase had formed in the surface before the designated boriding temperature (800°C) was reached.

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Figure 4

Microstructures in cross-sections of borided samples: a process ACFPB23; b process ACFPB21; c process ACFPB14; d process CPB1; e process CPB21

Phases

Surface XRD patterns on typical CPB and ACFPB samples are given in Fig. 5. Only FeB was detected for the 4 h CPB sample. Although 100 mA X-ray was used during XRD, the inner Fe₂B phase could not be detected for the CPB sample because of the outer thick FeB layer. Both FeB and Fe₂B were identified for the 0 h soaking sample CPB21. Only Fe₂B was detected for all ACFPB samples, which indicates that no FeB phase exists in the heat treated samples. The XRD analysis further verified the types and amounts of boride phases observed through OM.

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Figure 5

1: process CPB21; 2: process ACFPB23; 3: process ACFPB14; 4: process CPB1

Discussions

During pack boriding, Fe₂B forms first by a reaction of active boron atoms with iron atoms in the sample surface.14,17 When the concentration of boron at the exterior of the Fe₂B reaches ∼16%, FeB forms and grows on top of the Fe₂B layer by a reaction of boron with the Fe₂B. A two-phase boride coating is thus produced.18 Once the compact boride coating has formed on the surface, its further growth takes place at the common interface of the FeB zone with the Fe₂B zone. The FeB zone grows by the reaction of the inner Fe₂B with boron atoms diffusing from the surface across the outer FeB zone, while the Fe₂B zone grows by the reaction of the outer FeB zone with iron atoms diffusing from the substrate across the inner Fe₂B zone.17 When the formation rate of the new Fe₂B is faster than that of the new FeB and the boriding time is long enough, a single phase Fe₂B coating is produced. As the iron atom is larger than boron atom, the iron atom's diffusion depends a lot on vacancies in the matrix and is relatively slow in CPB.

It was reported that a direct current increases vacancy concentrations and their mobility in some metallic materials.19,20 Therefore, it is reasonable to believe that alternating current flowing through the steel sample will also increase vacancy concentrations. The vacancies will greatly promote the diffusion of iron atoms for the formation of the new Fe₂B. The treated sample was positioned between the electrodes (see Fig. 1). Alternating current field induced an alternating current in the treated sample. The induced alternating current distributed itself within the sample with the current density being largest near the sample's surface, decreasing from the surface at greater depths. This is called skin effect. The concentration of vacancy produced by the alternating current should thus be highest near the sample's surface. Increasing ACF current correspondingly increased vacancy concentration. With the high concentration vacancies, much more iron atoms than those in CPB diffused from the substrate across the inner Fe₂B layer to react with the outer FeB. The formation rate of the new Fe₂B was therefore faster than that of the new FeB, and a thick single phase Fe₂B coating would be obtained. Increasing ACF current increased alternating current induced in the treated sample. Correspondingly, the vacancy concentration was increased in the near surface region of the treated sample, and the coating thickness was therefore increased (see Fig. 3).

Alternating current field should also intensify chemical reactions and diffusions in the boriding media with the energy from the ACF and the ACF's electromagnetic stirring effect. Therefore, more active boron containing species moved to the sample's surface to promote the formation of borides.

Conclusion