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ADVANCED MATERIALS & PROCESSES •

NOVEMBER-DECEMBER 2014

HTPRO

formed at the surface. However, the dif-

fusion treatment on the piece was in-

sufficient to reduce all of the FeB

present. The near surface has fully re-

verted from FeB (darker teeth) into

B (lighter teeth), but there is still

some FeB present at depths between

0.001 and 0.002 in. that was not con-

verted into Fe

B. It is crucial in this

process to ensure that spalling does not

occur between the boost and diffuse

steps and that the diffuse process is suf-

ficient to ensure that all FeB is reduced

into Fe

B by the end of the process.

(2). The entire boriding process can be

carried out at lower boron potentials so

FeB never forms. This requires the devel-

opment of a customized boriding powder

different from most of the standard com-

mercially available boriding powders.

With many different boron sources, acti-

vators, and diluents available to choose

from, a wide array of different results are

possible through the development of cus-

tomized boriding powders. However, each

grade of material will boride differently as

they all have different boron diffusivity

rates necessitating a different customized

powder for each material. Another draw-

back of boriding at lower boron potentials

is that the boron enters the parts very

slowly, requiring longer cycle times. Sub-

stantial knowledge, testing, experimenta-

tion, and research are required to develop

customized deep-case boriding powders

for different materials and desired boride

layer depths.

(3). A method where a boost-diffuse type

process comparable to gas carburizing

and gas nitriding, such as electrolytic

salt-bath boriding, could be used. The

method involves immersing parts into a

borax salt bath held at austenitizing tem-

peratures. Parts are connected to a dc

power supply (cathodes) and graphite

plates are connected to the powder sup-

ply and immersed into the salt bath (an-

odes). Boriding occurs as electric current

flows through the system with borax re-

acting with both parts and anodes to free

up boron atoms that diffuse into the part

surfaces. The boost-diffuse process is

possible because boriding only occurs as

electric current is applied. When current

is off, boron continues to diffuse into the

parts at austenitizing temperature. Thus,

a deeper boride layer forms while reduc-

ing the boron concentration at the sur-

face of the parts. The process, called

phase homogenization in electrochemi-

cal boriding (PHEB), was developed by

G. Kartal, S. Timur, V. Sista, O.L. Eryil-

maz, and A. Erdemir

[2]

The process has not yet gained wide-

spread industrial acceptance due to var-

ious problems such as salt residues being

difficult to remove from parts, difficul-

ties in building and maintaining a borax

salt-bath furnace, and questions about

reliability and safety of high electrical

current flowing through a high-tempera-

ture borax salt bath. Another concern is

the ability to maintain uniform electrical

connectivity of the parts to the fixtures

as the fixture material degrades.

(4). Another boost-diffuse boriding

process could use boriding gases in an at-

mosphere furnace for a portion of a cycle

and then replace the boriding gases with

an inert gas, or adjust the gases to reduce

the boriding potential during later stages

of the process. Bluewater Thermal Solu-

tions is developing this type of process,

but details are proprietary and are not

discussed at length here.

Results of deep-case boriding

Bluewater Thermal Solutions has devel-

oped several deep-case boriding processes,

which are currently being performed com-

mercially. Parts treated using these

processes outperform conventionally

borided parts with shallower boride layers

in several applications including oilfield

drilling equipment, industrial pumps, high

wear agricultural machinery parts, and

ground-engaging tools, all subject to harsh

abrasive and erosive wear conditions.

Materials including 1018 plain carbon

steel, 4140 alloy steel, and O1, A2, D2,

and S7 tool steels have been deep case

borided with a single-phase Fe

B layer

with no spalling problems. Steels like

1018 and 4140, traditionally borided to a

depth of 0.003 to 0.005 in., are now able

to be deep case borided to a depth of

0.010 to 0.020 in. Tool steels like O1, A2,

and S7, traditionally borided to a depth

of 0.001 to 0.003 in., can be deep case

borided to a depth of 0.008 to 0.012 in.

Martensitic and PH grade stainless steels,

traditionally borided to a depth of 0.001

in., can be borided to a depth of 0.004 in.

with no spalling problems. Figure 6

shows a photomicrograph of 4140 alloy

steel deep case borided to a depth of

0.015 in. where the structure is predom-

Fig. 4 —

AISI 4140 alloy steel deep case

borided to a depth of 0.020 in. using

commercially available boriding powder. The

boride layer consists of about 50% FeB

(dark colored teeth near surface) and 50%

B (lighter teeth) below the FeB layer, an

undesirable condition prone to spalling.

Fig. 5 —

AISI 4140 alloy steel deep case

borided to a depth of 0.011 in. followed by a

vacuum diffusion treatment. Some FeB near

the surface converted back to single-phase

B (light colored teeth) while some FeB

(darker teeth) is still present at slightly

deeper depths. Single-phase Fe

B is

present below.

Fig. 6 —

AISI 4140 alloy steel deep case

borided to a depth of 0.015 in. consisting of

nearly 100% single-phase Fe

B with only

minor traces of FeB (darker colored teeth)

near the surface.