Practical Heat Treatment
Training programs on video
by the ASM International
SURFACE TREATMENT
Exercise2
Read the following text of the video film and collect 20 of
the most important engineering expressions!
1
01:13
01:46
There are many reasons to perform surface treatments on metal parts. Some of the main
reasons are to add stiffness to the part, increase wear resistance, increase fatigue strength,
prevent galling; correct a prior surface problem, increase corrosion resistance, increase
load-carrying ability, replace lost alloying elements, and simply to increase overall
strength. There are four categories of surface treatments.(movie1)
They are
 workhardening,
 metal alloy build-up,
 phase change hardening and
 alloy diffusion.
Work Hardening
Now, let's talk first about work hardening. Work hardening does several things; it increases
the fatigue strength, it minimizes stress corrosion, it increases bending strength, and it
increases wear resistance. It does this by imparting compressive stresses and hardness to the
surface.
02:08
A very common method of work hardening the surface is shot peening.(Fig.1) In shot
peening, hardened steel shot is forced or thrown against the part. This is usually done with
the shot being carried in an air blast. Both the size of the shot and its velocity determine
how deep the affected layer will be. Typically, the depth of work hardening is between five
to ten thousandths of an inch. It is important that the part is not stress relieved and that no
grinding or other machining is performed after shot peening.
A common misconception about shot peening is that it is a stress relieving process, but it is
actually just the opposite. Shot peening is a stress inducing process. It work hardens the
surface, which results in compressive stresses. This increases fatigue resistance and
minimizes susceptibility to stress corrosion. These are two major reasons for shot peening.
The process is most effective on parts that have been prehardened.
03:09
Coining (movie2) tends to be more of a volume work-hardening method than a surface
hardening method, but it does harden the surface to a greater extent. In coining, an area of
the part is compressed. As we have noted, compressive stresses increase the fatigue strength
and the material's resistance to stress corrosion cracking. (Fig.2) Stress corrosion cracking
generally occurs when the surface of a part, which is exposed to a specific corrosive
medium, is placed in tension.
03:32
Another work hardening method is burnishing. (Fig.3) There are two main types of
burnishing; ball burnishing and roll burnishing. Ball burnishing is a process that is
performed on crankshafts, where the bearing area joins the counter-weights or where it
joins the throws. This area is ball burnished. A hardened ball is forced against the radius
and it work hardens the surface in that area. As with other processes, burnishing leaves the
treated surface with a net compressive stress. Ball burnishing is generally done in localized
areas, to increase the fatigue strength at specific locations of shafts and bolts in materials
which are subject to fatigue failure. Roll burnishing has the same effect, but covers a much
larger area.
To review, work hardening by shot peening, coining or burnishing is applied to the part
after all heat treatments have are completed. Parts are not stress relieved or ground
2
afterwards. The processes are used to give the part surface compressive stresses, which
improve the resistance of the part to fatigue or stress corrosion damage.
04:40
Metal Alloy Build-Up
The second surface treatment we will discuss is Metal Alloy Build-Up. Again, this type of
surface finishing is not directly performed during the heat treating process. Metal Alloy
Build-Up is used to increase the corrosion resistance of the material, increase its wear
resistance and sometimes just to increase the size of the part.
05:00
One such method is called Braze Build-Up. (Fig.4) In this process, an alloy is added to the
surface of a part by brazing, and it alone provides the desired properties. Occasionally, the
braze alloy is mixed with a ceramic, abrasive resistant material to improve the wear
resistance of the layer. This type of build-up is used on low alloy hardenable steels. If the
part was heat treated first, the heat from the brazing operation would tend to oversoften the
quenched and tempered steel.
05:33
A very similar method is Weld Build- Up. (Fig.5) The difference between the two is that in
a weld build-up there is melting of both the weld material and the base metal. In braze
build-up only the build-up is melted. An example of weld build-up is a stainless steel
welded surface on an alloy steel backing. The stainless layer is added to impart corrosion
and oxidation resistance to the underlying alloy steel. Some parts may be hardened and
tempered after this treatment is performed, to restore certain properties to the alloy.
06:05
Flame Spraying (Fig.6) is yet another way to build up or add metal to the surface of the
part. Here, the metal, either in powder or wire form, is fed into an oxyacetylene or plasma
torch. The powder or wire is melted in the gas of the torch and is sprayed onto the part by
the force of the gas exiting the nozzle. The liquid metal drops, frees and bonds to the part
on contact.
Compressed air can be used in some processes to accelerate the particles. This process is
often used to build up worn areas of the part. The parts are generally not heat treated after
this process. If they are, the flame-sprayed material tends to spall off. Today, there is more
and more use of inert gas and vacuum sprayed plasma coatings. These provide better
adherence, less oxidation of the coating particles, and a wide variety of coating alternatives.
07:00
Another metal alloy build-up process is Plating. Plating is used generally for three reasons:
to change the size of the part, increase the corrosion resistance, or to change the
characteristics of the surface. Four different methods of plating are liquid
Electrochemical plating, vapor plating, plasma plating and mechanical plating. We will
briefly discuss each of these methods.
07:25
Liquid bath electrochemical plating is performed on all types of parts. Chromium, zinc,
copper and nickel are metals that are commonly plated on parts for increasing wear and
corrosion resistance. Chromium (Fig. 7) is used on heat treated parts to increase their wear
resistance in certain areas. Typical uses are bearings and landing gear parts. Nickel (Fig. 8)
may be plated on parts to increase corrosion resistance, improve wear, or to act as a base for
brazing stainless steels, providing a good joint filler and wetting the surface to be brazed.
Zinc (Fig. 9) is plated to provide sacrificial corrosion protection, while copper (Fig. 10) is
used to improve the efficiency and performance of electrical connectors, as well as being a
primary masking material used in the carburizing and nitriding processes.
3
08:12

09:12
Vacuum or vapor plating (Fig. 11) is one of several physical vapor deposition systems
utilizing vacuum technology to vaporize metals and subsequently deposit the vaporized
metal onto the substrate or part to be plated. The metal vapor can be obtained by
different methods, depending on the metal to be deposited. Evaporation can be
accomplished by resistance heating of the metal to its boiling point, by part evaporation,
or by electron beam melting. The parts to be plated are fixtured on racks, which are then
loaded into the vacuum chamber for processing. Typical materials that are plated are
chromium, aluminum and cadmium. Common applications of vacuum or vapor plating,
sometimes referred to as vacuum metallising, include the coating of plastic automotive
trim with chrome for decorative purposes, aluminum coatings of headlight reflectors
(Fig. 12), and cadmium plating of components for corrosion protection without
hydrogen embrittlement.
Plasma plating, utilizing glow discharge techniques, can be viewed as a modified vapor
plating process, wherein the plasma or glow discharge is used to accelerate and improve the
efficiency of the plating. This is achieved in several ways; by vaporizing the material to be
plated, accelerating the deposition rate, improving the substrate cleaning prior to plating,
and by improving the uniformity of the plated layer. One emerging example of this
technique is Ion Vapor Deposition of aluminum. The ion vapor deposition process (movie3)
is similar in sequence to conventional plating operations, requiring preparation, processing
and finishing operations, in order to correctly process parts to be coated. The preparation
stage consists of a degreasing operation to remove gross contaminants prior to an aluminum
oxide blast treatment, which textures the part surface and removes any solid contaminants.
The preparation stage would also include any masking operations required. Suitable low out
gassing masking tapes, or metal foils are typical materials used. Part racking or barrel
loading operations precede the charging of loads to the IVD vacuum chamber. (movie4)
The vacuum vessel is evacuated to a pressure of 8x10-6 Torr to purge the system, prior
to vac filling with argon, to 2x10-2 Torr. At this pressure, the parts are subjected to a
flow discharge cleaning or sputtering operation. A high negative potential is applied
between the parts being coated and the evaporation source. The argon gas in the
chamber ionizes and creates a glow discharge around the parts, bombarding them with
positively charged ions. (Fig. 13) This ion bombardment of the part surface effects a
final cleaning operation prior to coating.
The evaporated boat system contained within the vacuum chamber is heated and
continuously fed with aluminum wire. The aluminum evaporates and passes through the
glow discharge, where it combines with the ionized argon and is transported to the part
surface. In this way, pure aluminum is plated on the part surface, providing a uniform,
dense, adherent coating.
Following the coating operation (Fig.14), parts are removed from the process chamber,
and post-treated operations are applied. In order to densify the aluminum coating, the
parts are glass bead peened (Fig. 15) using a number ten glass bead. This operation also
serves to polish the coating and cosmetically improve its appearance. The primary
purpose of this peening operation is to improve the corrosion resistant performance of
the coating. It should be noted, however, that the glass bead peening operation also
serves as a one hundred percent quality control check on the coated part; any flaw in
coating adhesion being highlighted by this operation. The final post-treatment operation
4
is the application of a conventional chromate conversion coating-, which prepares the
aluminum surface for finishing and improves the corrosion performance of the coating.
The IVD aluminum coating equipment consists of a steel vacuum chamber, a pumping
system, a parts racking or barrel tumbling device, an evaporation source and a high voltage
power supply. (Fig. 16) The system utilizes conventional vacuum technology components,
controlled by a microprocessor-based control system, capable of monitoring system
sequencing, system alarms, process timing and datarecording functions.
Another example of this type of process is the coating of cutting tools with titanium
nitride, to provide a hard, adherent surface coating. Two basic types of processes are
available for deposition of titanium nitride coatings: Chemical Vapor Deposition, or
CVD, and Physical Vapor Deposition.
CVD (Fig.17) requires temperatures in the areas of 1850 oF. In this process, a gaseous
metal chloride reacts with hydrogen and nitrogen to form titanium nitride; because of
the high temperature of the reaction, and the need to reheat tooling after coating to
develop the necessary substrate hardness, some distortion is inevitable.
On the other hand, Physical Vapor Deposition, or PVD, can be performed at less than
930'F, a temperature well below that required for the tempering of high speed steels.
Several types of PVD systems are available:
 Evaporative Electron Beam Melting,
 Cathodic Arc Deposition and
 Sputter Ion Plating, or SIP.
S
I
P
i
s
a
p
a
t
e
n
t
e
d
p
r
o
c
e
s
s
,
5
d
e
r
i
v
e
d
f
r
o
m
t
h
e
c
o
a
t
i
n
g
m
e
t
h
o
d
s
u
s
e
d
f
o
r
m
a
n
y
y
e
a
6
r
s
i
n
t
h
e
s
e
m
i
c
o
n
d
u
c
t
o
r
i
n
d
u
s
t
r
y
.
T
h
e
a
d
v
a
n
t
a
g
e
s
o
f
7
S
p
u
t
t
e
r
I
o
n
P
l
a
t
i
n
g
c
a
n
b
e
s
t
b
e
s
u
m
m
a
r
i
z
e
d
a
s
f
o
l
l
8
o
w
s
:
g
o
o
d
p
r
o
d
u
c
t
q
u
a
l
i
t
y
,
e
a
s
e
o
f
e
q
u
i
p
m
e
n
t
m
a
i
n
t
e
9
n
a
n
c
e
,
a
n
d
h
i
g
h
p
r
o
d
u
c
t
i
v
i
t
y
.
S
I
P
t
i
t
a
n
i
u
m
n
i
t
r
i
d
e
10
c
o
a
t
i
n
g
s
(
F
i
g
.
1
8
)
d
e
m
o
n
s
t
r
a
t
e
g
o
o
d
a
d
h
e
r
e
n
c
e
,
h
i
g
h
11
i
n
t
e
g
r
i
t
y
,
h
i
g
h
h
a
r
d
n
e
s
s
,
a
n
d
a
u
n
i
f
o
r
m
m
i
c
r
o
c
r
y
s
t
a
12
l
l
i
n
e
s
t
r
u
c
t
u
r
e
.
S
I
P
s
y
s
t
e
m
s
a
l
s
o
p
r
o
v
i
d
e
r
e
p
l
i
c
a
t
i
13
o
n
o
f
t
h
e
s
u
b
s
t
r
a
t
e
s
u
r
f
a
c
e
f
i
n
i
s
h
a
n
d
a
n
e
x
c
e
l
l
e
n
t
14
g
o
l
d
c
o
l
o
r
.
B
e
c
a
u
s
e
o
f
t
h
e
s
i
m
p
l
i
c
i
t
y
o
f
c
o
n
s
t
r
u
c
t
15
i
o
n
,
S
I
P
e
q
u
i
p
m
e
n
t
t
e
n
d
s
t
o
o
f
f
e
r
a
h
i
g
h
u
p
t
i
m
e
a
n
d
16
m
i
n
i
m
u
m
m
a
i
n
t
e
n
a
n
c
e
.
P
e
r
h
a
p
s
t
h
e
m
o
s
t
i
m
p
o
r
t
a
n
t
;
i
17
t
p
r
o
v
i
d
e
s
u
n
i
f
o
r
m
c
o
a
t
i
n
g
o
f
c
o
m
p
o
n
e
n
t
s
o
f
d
i
f
f
e
r
e
18
n
t
s
i
z
e
s
a
n
d
s
h
a
p
e
s
i
n
a
s
i
n
g
l
e
w
o
r
k
l
o
a
d
.
Th
e
SI
P
pr
oc
es
s
19
(Fi
g.
19
)
uti
liz
es
a
gl
o
w
dis
ch
ar
ge
to
fre
e
tit
an
iu
m
fro
m
a
lar
ge
tit
an
iu
m
tar
ge
t.
Tit
an
iu
m
at
o
ms
re
act
wi
th
nit
ro
ge
n
to
for
20
m
tit
an
iu
m
nit
rid
e.
Tit
an
iu
m
nit
rid
e
is
att
ra
cte
d
to
th
e
w
or
kp
iec
es,
w
hi
ch
ar
e
po
sit
iv
el
y
ch
ar
ge
d.
Th
e
pr
oc
es
s
is
pe
rfo
21
rm
ed
in
a
ch
a
m
be
r
un
de
ra
so
ft
va
cu
u
m
of
nit
ro
ge
n
an
d
ar
go
n,
an
d
ab
ou
t
10
0
mi
cr
on
s
pr
es
su
re.
Th
e
va
cu
u
m
is
ac
22
hi
ev
ed
usi
ng
m
ec
ha
ni
cal
pu
m
ps,
eli
mi
na
tin
g
th
e
ne
ed
for
dif
fu
sio
n
pu
m
ps,
co
ldtra
ps
or
cr
yo
ge
ni
c
pu
m
pi
ng
sy
ste
ms
.
Th
e
23
w
or
k
to
be
co
ate
d
is
fir
st
cle
an
ed
usi
ng
de
gr
ea
ser
s,
aci
d
an
d
ba
se,
rin
se
s
an
d
ult
ras
on
ic
dr
yi
ng
,
ta
nk
s.
(Fi
g.
20
)
Th
e
w
or
24
k
su
rfa
ce
m
ust
be
ox
id
efre
e
for
go
od
co
ati
ng
ad
he
re
nc
e.
W
he
n
cle
an
ed
,
pa
rts
ar
e
lo
ad
ed
on
a
sta
tio
na
ry
fix
tur
e,
w
hi
ch
is
lo
25
we
re
d
int
o
th
e
co
ati
ng
ch
a
m
be
r.
(Fi
g.
21
)
W
ith
th
e
w
or
kl
oa
d
in
pl
ac
e,
th
e
co
ati
ng
ch
a
m
be
r
is
ev
ac
ua
te
d
an
d
a
26
lo
w
he
ate
d
to
ap
pr
ox
im
ate
ly
50
0'
wi
th
ex
ter
na
l
he
ate
rs.
Th
is
ste
p
en
su
res
re
m
ov
al
of
an
y
m
ois
tur
e
or
tra
ce
hy
dr
oc
ar
bo
ns
th
27
at
mi
gh
t
be
pi
ck
ed
up
by
th
e
fix
tur
e
or
pa
rts
aft
er
cle
an
in
g.
(m
ov
ie
5)
In
th
e
ne
xt
ste
p,
ar
go
n
is
int
ro
du
ce
d
int
o
th
e
ch
a
28
m
be
r
an
d
th
e
w
or
k
is
ne
ga
tiv
el
y
ch
ar
ge
d
to
50
0
V
olt
s.
Io
ni
ze
d
ar
go
n
bo
m
ba
rd
s
th
e
w
or
kp
iec
es,
sp
utt
er
cle
an
in
29
g
th
e
su
rfa
ce
of
th
e
pa
rts
to
be
co
ate
d.
Th
is
ste
p
re
m
ov
es
ox
id
es
or
co
nt
a
mi
na
tio
n,
as
su
rin
g
a
go
od
bo
nd
.
O
nc
e
th
e
co
30
ati
ng
te
m
pe
rat
ur
e
is
re
ac
he
d,
th
e
bi
as
on
th
e
w
or
kp
iec
e
is
re
ve
rse
d,
so
th
at
th
e
gl
o
w
dis
ch
ar
ge
oc
cu
rs
on
th
e
tit
an
iu
31
m
so
ur
ce.
Th
e
ins
id
e
wa
lls
of
th
e
ve
ss
el
ar
e
th
or
ou
gh
ly
lin
ed
wi
th
tit
an
iu
m
to
pr
ov
id
ea
lar
ge
,
un
ifo
rm
so
ur
ce
for
co
ati
ng
.
32
Ar
go
n
io
ns
bo
m
ba
rd
th
e
tit
an
iu
m
lin
in
g
to
rel
ea
se
tit
an
iu
m
at
o
ms
,
w
hi
ch
ar
e
th
en
de
po
sit
ed
on
to
th
e
po
sit
iv
el
y
ch
33
ar
ge
d
w
or
kp
iec
es.
(Fi
g.
22
)
W
he
n
nit
ro
ge
n
is
ad
de
d,
a
tit
an
iu
m
nit
rid
e
co
ati
ng
is
for
m
ed
.
16:50

Lastly, there is Mechanical Plating. Mechanical Plating is a method of plating parts by
tumbling or burnishing the parts in an environment where the plating material, in
powder or plate form, is mechanically forced against the part. In this way, by repeated
contact between the parts and the plating material, the plating material mechanically
bonds to the part.
34
17:13
Phase Change Hardening
Hardening of the surface can also be done by locally heating just the surface. This is called Phase
Change Hardening. (movie 6) Phase Change Hardening increases the hardness of the surface to a
controlled depth below the surface. Carbon steels, alloy steels, cast iron and some tool steels are
often treated by this method. What is Phase Change Hardening? It is a method by which the metal is
heated through the surface to form austenite at a specific depth, and then cooled to form martensite.
The four common methods of heating for Phase Change Hardening are:
 induction,
 flame,
 electron beam, and
 laser beam.
Let's look at these processes more closely:
17:55
Induction hardening (movie7) is a non-contact hardening process. The work is heated by causing an
electrical current to flow in the surface of the part by close proximity to a high frequency induction
field. The lower the frequency, the deeper the heating. The higher the frequency, the more shallow
the heating. Heating is very rapid (Fig.23) and tends to follow the surface of the part. Often, a
round coil can be placed around a small gear that has teeth on the outer diameter and the heating
will follow the contour of the teeth. The length of time that the current is induced in the part, plus
the power density used, determines how high a temperature the part reaches.
To develop shallow cases, for example, five to ten thousandths of an inch deep, the base material
may act enough as a heat sink to fully harden the material to martensite. For deeper cases, such as
18 to 20 thousandths of an inch or more, it is common to quench the part in oil, polymer solution, or
water. Heating times are very short, and usually measured in seconds. As a result, pieces can be
heated one at a time very efficiently.
Induction hardening has been used, for example, for hardening automobile axles and camshafts.
Here's an example (movie8) of how camshaft lobes are selectively hardened with induction heating.
The camshaft is first loaded into a specially designed induction coil. Then, the part is rotated for
heat uniformity. (Fig.24) Notice how the lobes slowly heat up to a reddish-orange color. Finally, the
camshaft is unloaded with the aid of a robotic ripper, is placed in a quench chamber, a hood is
lowered to collect and contain fumes generated by the quenching operation, and the camshaft is
quenched to attain the desired hardness in the lobe sections.
In this next scene, a bar is loaded into the induction scanner. Watch as the coil moves along the bar,
progressively heating and then quenching the bar. Notice how the bar is rotated for heat uniformity
and is supported to help control distortion. To rapidly heat a specific area of a steel part to
austenitizing temperature, flux concentrators (movie9) are attached to the inductor. These are made
from ferritic powder or silicon electrical steel. The flux concentrator compresses and directs the
magnetic flux lines through the surface of the parts to be heated. Concentrators can be used to tailor
the heating pattern to a particular part geometry. These are often used in combination with coils,
specifically conformed to a gear profile. (Fig.25)
20:37
In flame hardening (Fig.26), the part surface is heated with a flame-head burner. The flame is
produced by oxygen, mixed with either acetylene mixed gas, or hydrogen. The surface is brought to
a high enough temperature to form austenite and then the part is quenched by a liquid spray, or air
hardened if the material is an air hardening metal. Flame hardening is one of the oldest and simplest
methods for hardening specific areas of a particular part. For instance, this crane wheel (movie10) is
flame hardened where it comes into contact with the rail. A part this size would be very difficult and
expensive to harden any other way. Occasionally, the inside diameter may be hardened to provide a
bearing surface. Flame hardening can also be used to harden the inside diameter of a part if the bore
is large enough to accept the flame heads. Forged wheels are annealed and pre-hardened and
tempered prior to flame hardening.
35
21:39
Another method of surface phase change hardening of steel uses an electron beam (Fig.27) to heat
the surface by a high energy stream of electrons. A beam of high speed electrons, very similar in
principle to the beam of electrons in a TV set, but with much higher energy, is focused on the
surface of the part. This beam focuses several kilowatts of power in a spot less than one quarter of
an inch in diameter. As a result, the surface at that spot is heated almost instantly to the austenitizing
temperature. (Fig.28) The beam is moved electronically in a pattern to heat a broad area of the
surface. Because the surface is heated to only a few thousandths of an inch deep, the unheated
material underneath cools the surface fast enough to harden the part as the beam moves.
Although the surface can be quickly heated in a very fine and controlled pattern, the system must be
programmed to avoid overheating. Also, electrons are very easily scattered at normal atmospheric
pressure. Because of this, electron beams must be close to the part and the part must be normally in
a high vacuum chamber. In spite of this, processing times can be very short, with high precision.
One example is the hardening of the roll on an automatic typewriter. The roll is placed in a chamber
and rotated under the beam. The beam hardens the surface of the roll in a spiral pattern with no
distortion.
23:09
Laser hardening is similar in principle to electron beam, except that the beam is high energy light.
There is no special chamber required. The light energy heats the surface to the austenitising range,
and the part self quenches. The unique part of the process is that the energy beam can be directed
into almost totally inaccessible areas using mirrors. (Fig.29) By rotating the mirror and controlling
the beam it is possible to harden areas of a part that would be very difficult to harden any other way.
Since it is light energy, shiny surfaces must be coated black to absorb the energy for good
efficiency. One example would be a clutch on an automatic transmission that is laser hardened on
the back of the clutch.
There are many variations of these local heating processes. Alloys can be made by diffusing
coatings with the heat source, for example. The key is that heat is very locally applied to heat only a
portion of the outer surface. The heat source does not need to be a point source of beam. For
example, in rotation hardening, the rim of a part or the teeth of a gear are hardened by placing the
part into the salt bath only as deep as it needs to be hardened, rotating it in the bath. (Fig.30) As the
part is rotated, the rim is heated and can be hardened by quenching. This process is similar to flame
hardening, and is used to give the part wear- or load-carrying capabilities. (movie11) The rest of the
part can be left in a softer condition for further machining, or for toughness and ductility of the
overall part.
24.45
To summarize, we have discussed four different methods of phase change hardening:
 Flame hardening is used for deep hardening. The process uses simple equipment, is portable,
and requires a lot of experience.
 The second method is induction hardening. It can rapidly provide a contoured case, is energy
efficient, case properties are easily controlled, and the process can automatically treat large
numbers of similar parts in sequence.
 Electron beam hardening, as a third method, can be used to provide a very intricate surface
hardened pattern, even on very tiny parts.
 The fourth method, laser hardening, is similar to electron beam, except that it can be done in air,
and the beam can be directed by mirrors.
 Both electron beam and laser hardening can produce hard surfaces with no distortion.
36
25:38
Alloy Diffusion
The next category of surface treatment we will discuss is surface hardening by alloy diffusion. The
three most common alloy diffusion surface treatments are nitriding, carburizing and carbonitriding.
All of these involve adding nitrogen, or carbon, or both, to aid in the hardening of the surface.
25:
58
Nitriding involves absorption of nitrogen into the surface of the steel. The steel is heated to a
temperature range between 800 oF and 1050 oF. Today, there are three methods used to
nitride. These are salt bath, gas, or ion, also known as plasma nitriding. Most materials
nitrided are alloy steels, however, tool steels, nickel-based alloys, stainless steels, and even
titanium are successfully nitrided. The key to successful nitriding is the conversion of
nitrogen to a form that can be absorbed into the metal. Air is 80% nitrogen. However, that is
molecular nitrogen, also known by its chemical formula, N2; that is, two atoms of nitrogen,
combined to form a molecule. Nitrogen in this form is not readily soluble in iron or other
materials.
When
the
nitroge
n
molec
ule is
split
into its
two
indepe
ndent
atoms,
it can
be
absorb
ed. In
the
atomic
form,
it
is
also
called
"nasce
nt"
nitroge
n.
Atomi
c, or
nascen
t,
nitroge
n
is
obtain
ed by
decom
positio
n
of
37
ammo
nia
gas,
NH3,
at the
surfac
e
of
the
work,
by the
follow
ing
reactio
n: two
molec
ules of
ammo
nia,
plus
heat,
gives
two
atoms
of
nitroge
n, plus
three
molec
ules of
hydrog
en.
(Fig.3
1) The
nitroge
n, in
this
atomic
form,
diffuse
s into
the
surfac
e
of
the
steel.
Nitrog
en
atoms,
that do
not
diffuse
, join
to
form
nitroge
38
n
molec
ules,
and
are
swept
away
in the
effluen
t gas.
The
depth
that is
achiev
ed
during
the
proces
s at a
consta
nt
temper
ature
is
depen
dent
upon
time.
The
depth
of the
case is
related
to
some
consta
nt for
the
temper
ature
used,
times
the
square
root of
time.
Now,
what
this
means
is that
if you
can get
five
39
thousa
ndths
of an
inch
case in
four
hours,
with
everyt
hing
else
equal,
you
will
get
approx
imatel
y ten
thousa
ndths
of an
inch in
sixteen
hours.
To
double
the
case,
you
have
to go
four
times
the
length
of
time.
The
hardne
ss of
the
case
(Fig.3
2)
obtain
ed
during
the
nitridi
ng
proces
s is
depen
dent
upon
40
the
micros
tructur
e that
you
start
with,
and
the
alloy
that
you're
nitridi
ng.
Materi
als that
contai
n
alumin
um
and
chromi
um
form
very
hard
nitride
s. Iron
forms
fairly
hard
nitride
s, but
iron
nitride
s are
very
brittle,
and
tend to
spall,
or
chip,
off.
When
nitroge
n
is
absorb
ed by
steel,
two
distinc
t zones
can be
41
created
in the
case.
(Fig.3
3) At
the
surfac
e, iron
and
other
strong
nitride
formin
g
eleme
nts,
such
as
chromi
um,
readily
form
high
nitroge
n
compo
unds
in
a
very
thin
layer
at the
surfac
e. This
layer
has
been
called
the
white
layer,
or
compo
und
zone,
and is
normal
ly
betwee
n two
ten
thousa
ndths
and
42
one
thousa
ndth of
an
inch
thick.
The
second
re-ion
is the
diffusi
on
zone,
which
contai
ns less
nitroge
n. The
depth
of this
layer
contin
ues to
build
as the
metal
is held
at
temper
ature.
We
will
revisit
this
subject
shortly
, but
first
let's
discus
s the
differe
nces in
the
three
nitridi
ng
proces
ses.
The
three
main
nitridi
ng
43
proces
ses;
salt
bath,
gas,
and
ion,
differ
in the
way
that
they
create
atomic
nitroge
n, but
all
share
the
fact
that
the
purpos
e is to
make
nitroge
n
availa
ble for
absorp
tion in
the
metal.
29:27
In salt nitriding, the parts are immersed in a bath containing nitrogen compounds. These chemically
decompose and release nitrogen to the surface. In earlier days, the compounds were cyanide salts.
Today, formulations without cyanide have replaced the older compounds. The salt bath compounds
also contain some carbon, which is also transferred to the surface.
29:54
The salt and gas treatments which provide carbon in addition to nitrogen form a special class of
nitriding, known as Nitrocarburising. The gas nitriding process uses ammonia gas to release atomic
nitrogen to the metal. The surface of the metal, especially iron, acts as a catalyst to break ammonia
into its component parts. Some of the atomic nitrogen is absorbed, but most reforms into molecular
nitrogen.
This process is relatively simple in principle, but is rather difficult to control in practice. This is
because the atmosphere does not become stable relative to the work. It also is affected by the
surface area, or number of parts, put in a given vessel. To make it simple, a large excess of ammonia
is used. This results in excess nitrogen absorbed, with an excessive white layer. This white layer
must then be removed because of its brittleness. Some improvements have been made over years of
experience to control this build-up. In the flow process, the concentration of ammonia is reduced in
the second half to two thirds of the cycle. This is usually done by increasing the amount of alreadydissociated ammonia. When this is done, the nitrogen diffuses into the case, reducing the white
layer. However, if the white layer is to be completely removed, it must be done by grinding the part
after nitriding.
44
31:25
The third major nitriding process is called ion nitriding or plasma nitriding. (Fig.34) These names
were given to the process because of the way atomic nitrogen is Generated and transferred to the
part surface. Ion nitriding is carried out in a vacuum chamber. (Fig.35) Ordinary nitrogen is
introduced and the pressure is held at between one and ten Torr. Remember, normal atmospheric
pressure is 760 Torr. A voltage potential of about 450 Volts is applied between the inside of the
vessel and the work support on which the parts are placed. This causes the parts to be negatively
charged. They become the cathode in the electrical circuit formed. The high voltage causes the
nitrogen to be positively ionized, which allows them to be absorbed into the part surface.
Nitrogen ions move to the negative part surface, pick an electron, and diffuse into the surface. The
ionization process and electron recombination cause a re-ion in the gas near the part surface to glow
with a bluish-purple color (Fig.36), which is why the process is also known as a glow discharge
process. This ionization process is very efficient. The nitrogen in the -as is readily transferred to the
part, meaning that very little nitrogen is required for nitriding.
In addition, the surface is cleaned during nitriding, by a process known as sputtering. This means
that metal, such as stainless steel, can easily be nitrided. The oxide, which is always present on
stainless steels, is very stable at nitriding temperatures. In ion nitriding, this oxide is removed by
sputtering, allowing the nitrogen to penetrate the surface. In gas, or soft, nitriding the oxide would
need to be chemically removed, or mechanically blasted.
Also, because the process is electrical in nature, the white layer composition and thickness can be
easily controlled and even eliminated. The ion nitriding process is growing in importance for other
reasons, the major ones of which are summarized (Fig.37) here:
 all steels are nitrided in the hardened condition;
 most materials can be used directly after nitriding with only minimum finishing;
 these materials usually do not need quenching, therefore, and are gas- or air-cooled from the
nitriding temperature.
Other comparisons will be made after we have finished discussing carburizing.
34:00
Carburizing, which, as one might guess, is the process of adding carbon to metal, is also a
diffusion process. It differs from nitriding in two basic ways:
First, the temperature is in the austenite field, between 1500 and 1900 oF. The higher the
temperature, the faster the carbon diffuses into the metal. The longer the time in the furnace, and the
higher the temperature, the deeper the case. These relations are well known and are usually plotted
as straight lines on a logarithmic plot, as shown here. (Fig.38) The graph says, for example, that if
we hold steel at 1700 oF for ten hours, we can predict the depth that carbon will be diffused into the
steel.
The second difference from nitriding is re ate to the process parameters. Since carburizing is carried
out at the austenitizing temperature, the part can be quenched to hard martensite. The maximum
attainable hardness of steel parts is a function of the carbon content, reaching a maximum at about
0.8% carbon. The depth below the surface at which the carbon is about 0.3 to 0.4%, hardens to
approximately 50 Rockwell C hardness (HRC). This depth is commonly referred to as the affected
case depth.
The carbon profile is controlled by the concentration of the carbon at the surface and the rate of
diffusion at the temperature used. A typical profile is shown (Fig.39), along with the hardness that
resulted after quenching. Carburizing is a very attractive process. The hardened depth can be
reached much faster than in nitriding. The atmosphere is also very close to equilibrium. This means
that the carbon at the surface can be predicted by measuring the composition of the gas, as we will
discuss shortly.
45
Just as in nitriding, carburizing can be done in different ways. These all relate to the method of
getting the carbon to transfer to the surface of the part. There are five main commercial processes.
These are: liquid, pack, gas, vacuum, and ion or plasma carburizing.
36:25
Liquid carburizing is done in a molten salt bath containing salts that liberate carbon. Usually
chlorides are added. Other metal salts are there to control the fluidity and fume in the bath. Here the
released carbon, and a little bit of nitrogen, is absorbed in the steel part during this process.
36:44
In pack carburizing (Fig.40), the parts are surrounded by a carbon-containing material in a suitable
container. Typically, charcoal and coke are used, and mixed in with the charcoal and coke is an
activator such as barium carbide, which releases carbon dioxide when heated. Carbon dioxide reacts
with carbon in the charcoal and coke to form carbon monoxide. Carbon gets from the solid charcoal
and coke to the surface of the part via the carbon monoxide. Two molecules of carbon monoxide
release one carbon atom to the surface of the face centered cubic austenite, and the carbon atoms
diff-use inward to form the case. The pack carburizing process was the first carburizing process
developed. Because it is labor intensive and dirty, much pack carburizing has been replaced by gas
carburizing.
37:35
In gas carburizing, the parts are heated to austenite in an atmosphere that contains carbon
monoxide, carbon dioxide, nitrogen, hydrogen, water vapor, and enriching gases such methane or
propane. In almost all cases, gas carburizing, is done with an endothermic atmosphere as the carrier
gas (Fig.41). This is a generated atmosphere that is approximately 38% nitrogen, 40% hydrogen,
20% carbon monoxide, 1% carbon dioxide and 1% water vapor maximum, and extremely minute
amounts of oxygen. A similar type of endothermic atmosphere can be generated in the furnace by
cracking methanol and adding nitrogen. It is the carbon monoxide to carbon dioxide ratio. and the
hydrogen to water vapor ratio, that determine the carbon potential in the atmosphere. The higher
these ratios, the more carbon that will be absorbed into the steel, until the carbon's saturated level of
austenite is reached, which varies with temperature.
Here is a practical application of carburizing and hardening an AISI 8620 steel shaft, 1.25 inch in
diameter by 8 inches long (Fig.42). The drawing specifies that only a 1.5 inch length, at the center
of the bar should be carburized, to a depth of one tenth of an inch. After carburizing, the shafts are
to be quenched in oil. To prevent carburization in other sections. the 1.5 inch length is masked, and
the remainder of the bar is copper plated to a thickness of 5 thousandths of an inch. Final surface
carbon content is specified to be in the range of 0.80 to 1%. Due to the large depth of case required,
1700 oF is selected as the optimum carburizing temperature. Sixteen hours at this temperature will
produce the required case depth of approximately one tenth of an inch. (Fig.43)
Parts are properly spaced and stacked vertically in the baskets. Baskets are then placed on trays for
processing through a pusher carburizing furnace (movie12). A schematic of a typical pusher furnace
is shown here (Fig.44), including loading vestibule, carburizing section, and quench chamber. The
next figure (Fig.45) depicts the zones of temperature and carburizing potential atmospheres which
will provide the case depth and approximate surface carbon percent. The dew point temperature for
the mixing of endothermic carrier gas and enriching gas should be approximately 16 oF inside the
furnace. Slight modifications are made at the charge and discharge ends of the furnace, including
temperature and atmosphere, to provide an optimum carbon percent gradient from the part surface
to the core.
The carbon is very sensitive to the changes in the carbon dioxide, water vapor and oxygen in the
atmosphere. These are also related to each other through the chemical reactions going on in the
atmosphere. As a result, the surface carbon can be controlled very closely by measuring one or more
of these variables. Carbon dioxide can be measured with high accuracy using an infrared analyzer.
Water vapor is measured by a dew point analyzer. Oxygen is measured by a solid state oxygen
probe inserted into the hot furnace. All of these can be used to generate an electrical signal for
controlling the carbon potential in the atmosphere.
46
41:19
Similar to nitrocarburizing and nitriding, carbonitriding is a modified gas carburizing process,
rather than a form of nitriding. The modification consists of introducing ammonia into the gas
carburizing atmosphere. This adds nitrogen to the carburized case as it is being produced. The
nitrogen serves several useful purposes:



First, nitrogen in the case lowers the austenite transformation temperature. Very low carbon
steels can be carbonitrided at temperatures as low as 1450 oF.
Second, nitrogen increases the hardenability of the case. A slower, oil, quench can be used
instead of water quenching. This minimizes distortion.
And, lastly, nitrogen retards softening during tempering. A hardened core can be softened
without excessive softening of the case.
One problem is that nitrogen tends to lower the martensite finish temperature, retaining austenite.
Ammonia additions, therefore, must be kept to a minimum. Gaseous carbonitriding has rapidly
replaced salt bath cyaniding. Cyanide is a toxic chemical, and is very difficult to dispose of under
EPA regulations.
42:37
Vacuum carburizing is a modified gas carburizing process, in which the carburizing is carried out at
pressures less than atmospheric. (Fig.46) A typical pressure is between 50 and 300 Torr. The
advantage of this method is that the surface of the metal stays very clean, due to the vacuum,
making it easier to transfer carbon to the surface (Fig.47). Natural gas, which is methane or CH4, or
propane C 3H 8, provides carbon by decomposing at the temperature of carburizing. These are not
equilibrium reactions as in gas carburizing (Fig.48). Instead, the surface is raised to the saturation
level of carbon in austenite very rapidly.
To achieve the proper surface hardness, the carburizing step is followed by a diffusion step. In this
step, the furnace is evacuated to remove the carburizing gas. Diffusion causes the carbon to diffuse
into the steel, reducing the surface concentration and increasing the case depth. By repeating the
steps of carburizing to saturation or boosting, then diffusing, any desired carbon profile can be
achieved. (Fig.49) Similar to the atmosphere carburizing process, curves have been developed to
relate total boost time, diffusion time, surface carbon and case depth for various temperatures
(Fig.50). To ensure a uniformity of carburizing, various methods of introducing the gas and causing
it to move about have been developed. In atmosphere carburizing, this is done with fans, when, in
vacuum processing, fans are generally not practical.
44:16
Another vacuum process for carburizing is ion or plasma carburizing (Fig.51). This process is, in
principle, identical to ion nitriding, except that the temperature is higher, and the ions consist of
carbon generated from ionizing propane or methane. Like in the other ion processes, the purpose of
the ionizing is to accelerate carbon transfer. The electrical attraction of the positive, carbon-bearing
ions to the negative workpieces, also promotes uniformity. These two factors result in an
improvement over vacuum carburizing and also over atmosphere, in that very uniform carbon
profiles can be achieved everywhere on the part at pressures even lower than in vacuum
carburizing,. The pressure used in ion carburizing is usually much less than 10 Torr, and typically
about 3 Torr.
The ion carburizing process uses a boost and diffuse method, similar to vacuum carburizing, to
generate case depth and control surface carbon. In addition, the throwing power of carbon into the
surface is enhanced by increasing the current density so this variable can be controlled. Pressure in
the vessel affects the thickness of the ionized layer, known as the glow scene, allowing the ions to
be made uniform around the contours of the part (Fig.52). As pressure is increased, the -low scenes
become thinner and closer to the part surface (Fig.53). As a result, holes and gear teeth can be
uniformly carburized by increasing the pressure.
45:
47
54
The questions may be asked: why carburize?; or, how does one determine which process to
use? There are many factors to consider in answering these. First there are the part properties
to be achieved. If one needs very high strength, it may only be possible to achieve this with
carburizing.
For
examp
le, this
curve
was
develo
ped to
compa
re
carburi
zing
and
nitridi
ng
case
depths
for a
highly
loaded
gear
(Fig.5
4).
Practic
ally
speaki
ng, if
case
depth
exceed
s about
40
thousa
ndths
of an
inch, it
is no
longer
possibl
e
to
use
nitridi
ng,
becaus
e
of
the
slow
diffusi
on of
nitroge
48
n. Of
course,
one
could
redesi
gn the
gear,
to
reduce
tooth
stresse
s,
allowi
ng to
some
extent
the use
of
shallo
wer
case
depths
which
might
then
permit
nitridi
ng.
Nitridi
ng,
becaus
e it
does
not
involv
e
quenc
hing
transfo
rmatio
n, is
capabl
e of
produc
ing
very
low
distorti
on. On
the
other
hand
nitridi
ng
49
usuall
y
require
s more
expens
ive
materi
als,
which
are
more
difficu
lt to
machi
ne or
grind.
In
summ
ary,
the
proces
s
selecti
on is a
series
of
interac
tive
steps,
involvi
ng
materi
als
selecti
on to
achiev
e
engine
ering
proper
ties,
includi
ng the
design
itself.
The
selecti
on
must
then
be
weigh
ed
against
50
the
produc
tion
consid
eration
s,
which
again
impact
upon
the
quality
issues
import
ant to
the
end
user of
the
part.
In closing (movie13), we tried to provide an overview of the many processes available to improve
the surface properties of parts. These fall into four major categories, which can be further
subdivided into specific processes. The choice of a process depends upon the properties required,
the production requirements, and the environment in an individual production plant. We have not
attempted, in this short program, to provide all the details of each process, but rather, have
attempted to give a clearer picture of the choices available, and their major difficulties.
(Fig.55)
51