NovDec_AMP_Digital

Boriding (also known as boronizing) is a

diffusion-based case-hardening process

for metals that creates an ultrahigh hard-

ness case (1500–2200 HV) below the

surface of the parts being treated. It pro-

duces exceptional wear resistance for

metal parts that operate in severely abra-

sive and erosive operating conditions

and also improves anti-galling proper-

ties. Boriding typically more than triples

the service life of high wear parts

compared with other traditional heat

treatments such as carburizing, carboni-

triding, nitriding, nitrocarburizing, thin

PVD coatings, and platings like hard

chrome. Boriding creates a wear layer

with higher hardness than many wear

resistant thermal spray coatings, such as

tungsten carbide and chrome carbide. It

is not mechanically bonded to the sur-

face, but instead is diffused below the

surface of the metal, making it less prone

to peeling and breaking off treated parts.

Boriding is performed by diffusing

boron atoms into a metal surface and al-

lowing the boron to react with elements

present in the substrate to form metal-

boride compounds. Upon diffusing

enough boron into the surface, a metal-

boride compound layer with high hard-

ness precipitates and grows below the

surface of the part. Different types of

metal-boride layers are possible, the

composition of which depends on the

material being treated. For example,

iron-boride layers form in steels and cast

irons and nickel-boride layers form in

nickel-base alloys. Other compounds

possible include cobalt and titanium

borides. This article discusses only

boriding of ferrous alloys.

Two iron-boride compounds formed

when boriding steel are Fe

B (at lower

boron concentrations) and FeB (at higher

boron concentrations), each having dif-

ferent structures, densities, and mechan-

ical properties as shown in Table 1

[1]

Single-phase Fe

B layers

. As boron dif-

fuses into a steel surface, concentration

at the surface starts out at zero and be-

gins to increase until the concentration

just below the surface reaches a level

where Fe

B compounds begin to pre-

cipitate and grow. Fe

B is the first

phase to form, as it contains a lower

boron concentration (33%) than FeB,

which requires 50% boron concentra-

tion. A solid continuous single-phase

B layer forms below the surface and

grows deeper over time as boron con-

centration increases. An Fe

B single-

phase layer is the ideal result and is the

desired microstructure after boriding

ferrous materials.

Dual-phase Fe

B-FeB layers

. Boriding the

part for longer times to create a deeper

case makes it more likely the boron con-

centration just below the surface will ex-

ceed 33%, which leads to the formation of

the more boron-rich FeB compound in

addition to the already formed Fe

Powder pack cementation is the most

commonly used method for boriding. A

drawback of the process is that the

boron activity of the powder pack is

fixed; it cannot be varied during the

process. This makes it impossible to per-

form a diffusion process similar to the

boost-diffuse methods used in carburiz-

ing and two-stage Floe nitriding. In

these processes, carbon and nitrogen

potentials of the furnace atmosphere can

be reduced to lower values during later

stages of the process to avoid oversatu-

rating the surface with excessive carbon

or nitrogen, which are responsible for

carbide networking, excessive retained

austenite, and/or nitride networking.

Therefore, boriding to deeper case

depths with a fixed high boron activity

during the entire process eventually

oversaturates the surface with boron,

forming a second FeB layer on the out-

side of the original Fe

B layer.

Most commercially available boriding

powders create high boron activity lev-

els, and are good for quickly forming

high-quality thin boride layers with

short cycle times. However, there is a

limit to the depth of the single-phase

layer that can be formed with longer

cycle times before undesirable FeB be-

gins to precipitate resulting in a dual-

phase FeB-Fe

B layer. The limit on depth

depends on the material as each one has

a different boron diffusivity rate.

Plain carbon and lower alloy steels gen-

erally have higher boron diffusivity rates

and enable rapid diffusion to deeper

depths without a buildup of high boron

concentration at the surface. More

highly alloyed steels have lower boron

diffusivity rates, which tends to trap

boron near the surface. This increases

surface boron concentration, forming

ADVANCED MATERIALS & PROCESSES •

NOVEMBER-DECEMBER 2014

DEEP CASE BORIDING

FOR EXTREME WEAR RESISTANCE

HTPRO

THE ABILITY TO PRODUCE DEEPER CASE BORIDE LAYERS THAT ARE NOT PRONE TO SPALLING

MAKES THE BORIDING PROCESS COMPETITIVE WITH SOME CONVENTIONAL DEEP-CASE

AND THICK-COATING PROCESSES.

Craig Zimmerman* and Nick Bugliarello-Wondrich,*

Bluewater Thermal Solutions

*Member of ASM Heat Treating Society

TABLE 1 — PROPERTIES AND CHARACTERISTICS

OF IRON-BORIDE COMPOUNDS

FeB

Boron content, wt%

16.23

8.83

Structure

Rhombic

Tetragonal

Residual stress on cooling

Tensile

Compressive

Coefficient of linear thermal expansion,

-6

7.9–9.2

Hardness (HV0.1)

1900–2100 1800–2000

Density, g/cm

(lb/in.

)

6.75 (0.244)

7.43 (0.268)