ASM Handbook, Volume 4D, Heat Treating of Irons and Steels
J. Dossett and G.E. Totten, editors
Copyright # 2014 ASM InternationalW
All rights reserved
asminternational.org
Low-Temperature Carburization of
Austenitic Stainless Steels
S.R. Collins, P.C. Williams, and S.V. Marx, Swagelok Company
A. Heuer, F. Ernst, and H. Kahn, Case Western Reserve University
LOW-TEMPERATURE CARBURIZATION
is a gaseous carburization process performed
at atmospheric pressure, at temperatures where
the kinetics of substitutional diffusion are very
slow. Low-temperature carburization hardens
the surface of austenitic stainless steels through
the diffusion of interstitial carbon, without the
formation of carbides. The surface must be activated, by modification and removal of the naturally occurring chromia layer, for the process to
work. The process results in a hardened diffusional case on the surface, typically 20 to 35 mm
thick, with carbon content at the surface in
excess of 10 at.%. The process is conformal and
can be applied to finished components without
change in dimension. Property enhancements
include:
Increased surface hardness: Close to the
surface, a microhardness value of approximately HV 1200 is obtained, which corresponds to greater than 70 HRC.
Residual compressive stress: At the surface,
values in excess of 2 GPa (300 ksi) have
been measured by X-ray diffraction.
Retained ductility: Scanning electron microscopy of surfaces of tensile bars tested to failure show deformation characteristics typical
of austenite (i.e., intersecting slip bands) and
the absence of cracking.
This combination of increased surface hardness,
compressive surface stress, and retained ductility
leads to improved performance in applications
demanding resistance to wear, erosion, and
fatigue. Additionally, corrosion resistance to a
variety of chloride-containing media also has
shown dramatic improvement, due to carbonenhanced passivity of the resulting surface.
Overview
Case hardening is a widely used industrial
process for enhancing the surface hardness of
metal and alloy components. In a typical
commercial process, a carburizing gas contacts
the workpiece at elevated temperature and carbon atoms diffuse into the metal surface. Hardening occurs through the formation of carbide
precipitates. Gas carburization normally is
accomplished at 950 C (1700 F) or above,
because most steels must be heated to these
temperatures to transform to the high-temperature
austenitic (face-centered cubic, or fcc) structure
preferable for carbon infusion. This structure is
preferable because it enables higher diffusivity
and higher solubility limits than the ferritic
(body-centered cubic, or bcc) structure.
Austenitic stainless steels are among the
most ductile and most corrosion resistant of all
ferrous alloys. However, they have only modest
hardness compared to other steels, limiting
their performance in many applications. These
steels typically contain 10 to 18 wt% Cr and
8 to 14 wt% Ni; the chromium addition provides
corrosion resistance, while the nickel addition
stabilizes the desirable austenitic structure at
room temperature. These alloys are valued for
their combined corrosion resistance and ductility. In many technical applications, however,
their performance, lifetime, and aesthetic
appearance would be improved if their surfaces
were hardened. Unfortunately, hardening by
conventional carburization is carried out at elevated temperatures, where the formation of chromium-bearing carbides cannot be avoided, and
chromium depletion from the austenite matrix
can compromise corrosion resistance. Additionally, the levels of hardness that can be achieved
are limited by the resulting microstructure of
the case, a distribution of chromium-rich carbides in a chromium-depleted matrix. Hardness
values of 50 to 60 HRC are achieved under these
conditions, at the expense of corrosion resistance
of the alloy.
Low-temperature carburization hardens the
surface of chromium-containing austenitic alloys,
in particular austenitic stainless steels. The process involves activation of the surface followed
by a gas-phase carburization treatment, performed at temperatures low enough to avoid the
formation of carbides (350 to 550 C, or 660 to
1020 F), for a sufficient time (typically 20 to
60 h) to allow carbon diffusion to occur.
The result is a carbon-rich, uniform (singlephase) and highly conformal compositionally
graded surface layer ranging from 10 to 40 mm
thick, with a near-surface hardness that can
approach HV 1200 (over 70 HRC). Because the
treatment occurs at low temperature and phase
transformations are avoided, parts do not distort
or change dimensions. Carbon concentrations at
the surface of treated 316 stainless steel components have been verified by a variety of analytical
methods to be on the order of 12 to 15 at.%; up to
20 at.% has been demonstrated on treated superaustenitic steels. The case remains austenitic and
retains its ductility. Parts can be bulk handled,
enabling processing of high numbers of parts.
In this article, examples shown are results for
316 stainless steel, although a substantial body
of data has been generated for other industrially
important alloys, such as superaustenitic stainless steels, precipitation-hardening stainless
steels, duplex alloys, nickel-base alloys, and
cobalt-base alloys (Ref 1–3).
Background and Competing
Technologies
Methods for hardening stainless steel surfaces
have been researched since at least the 1980s
(Ref 4, 5). These methods include liquid sodium
and cyanide salt bath treatments, plasma nitridation and carburization (Ref 6, 7), ion implantation, and gaseous atmospheric heat treatments.
All of these methods will provide a hardened
diffusion case on stainless steels with varying
performance characteristics. However, commercial scale-up has been elusive for many of these
methods due to technological barriers. For example, liquid sodium activates and readily transfers
carbon to the stainless steel, but a scale-up from
the laboratory to a production process presents
a significant hurdle for safety and handling
reasons. Plasma and ion methods are line-of-sight
452 / Heat Treated High-Alloy Steels
methods. They effectively activate the surface
but are not able to provide a uniform case for
workpieces with complex shapes, especially in
batch processing.
Recent survey articles have reviewed the
worldwide state of low-temperature nitriding
and carburizing of austenitic stainless steels
(Ref 1, 8). In addition, several academic conferences have been held in the past two decades on
the topic of low-temperature thermochemical
surface treatments of stainless steels (Ref 9–13).
On the commercial front, several entities advertise low-temperature hardening processes for
stainless steels, as shown in Table 1. It should
be noted that only two of these processes
(SAT12 and NV Pionite) are known to be gasphase low-temperature carburization, while the
entity responsible for a third carbon-imparting
method (Kolsterising) is entirely silent on how
it is achieved. The process and results discussed
in this article are specific to the Swagelok
SAT12 treatment.
Commercial Application
Since 1999, low-temperature carburization
has been applied to the ferrules of the Swagelok
tube fitting, as shown in Fig. 1. In recognition
of the development and commercialization of
the surface-hardening technology in the tube fitting application, Swagelok received the ASM
International Engineering Materials Achievement Award in 2006 (Ref 14). Swagelok has a
captive heat treating facility that batch processes
and bulk handles millions of ferrules per year, in
nominal sizes ranging from 6 mm to 1 inch. The
process is performed in specialty-built carburization furnaces with enhanced gas handling capabilities, as shown in Fig. 2. Due to the use of
dry HCl and CO in the process, the furnaces must
be leak-tight to atmosphere.
Process Considerations
Activation
Stainless steels and other chromiumcontaining alloys obtain their “stainless” characteristic from the chromium-rich oxide that forms
on the exposed surface. This oxide layer forms
when a fresh surface is exposed to an oxygencontaining medium, such as air, and will form
after very short time exposures. The chromia
layer on the surface of stainless steel constitutes
a major hurdle to low-temperature carburization
because it blocks the inward diffusion of carbon.
In order to diffuse the carbon atoms into the
stainless steel from the surface, the chromium
oxide layer must be removed, or at least modified
to become carbon-transparent. This step generally is known as surface activation or depassivation. Several methods have been developed
(Ref 15–19) that eliminate the inhibiting effects
of the oxide and activate the surface for efficient
transport of carbon from a conventional carburizing atmosphere into the stainless steel matrix.
As noted earlier, some processes, such as plasma
or liquid sodium treatments, readily activate a
passive stainless steel surface by removing the
oxide by sputtering. In commercial production
processing, activation can be achieved by exposing the furnace load to a halide-containing gas
mixture such as nitrogen trifluoride or hydrogen
chloride and nitrogen at atmospheric pressure.
Hydrochloric acid is effective as an activating
gas for a wide range of chromium-containing
corrosion-resistant austenitic alloys, including
stainless steels and nickel-base alloys, such as
alloys 625 and 825.
Processing Temperature Ranges
and the Concept of Paraequilibrium
Successful low-temperature carburization of
stainless steels and other chromium-containing
alloys depends on the alignment of several
Table 1
processing parameters. These include activation
of the surface, proper surface preparation,
selection and condition of the alloy to be carburized, treatment temperature, and carburizing
atmosphere. Figure 3 shows a typical process
cycle.
Low-temperature carburization has a limited
temperature processing range, usually 350 to
550 C (1020 F). At the lower processing temperature, the process becomes very slow and
less economically feasible. Lower temperatures
mean longer times for the same case thickness.
At the upper end, it becomes increasingly difficult to avoid the formation of carbides, which
depend in part on the thermally-assisted mobility of sustitutional alloying elements, such as
chromium, in the iron matrix.
Traditionally, carburization is carried out at
high temperature to maximize the solubility and
the rate of interstitial solute diffusion. On cooling
to room temperature, however, a major part of
the solute atoms will precipitate as carbide
phases. Figure 4 shows the appropriate timetemperature-transformation (TTT) diagram for
the heat treatment of an austenitic stainless steel
with conventional carbon contents (Ref 20). After
high-temperature carburization (T > 950 C, or
1740 F), precipitation of carbides can be
avoided only by extremely high cooling rates
(path A). Furthermore, it is easy to exceed the
solubility limit at these temperatures and form
carbides during the carburization process itself.
At the cooling rates typical of industrial processes, however, the path will cross the carbide
“nose” and precipitation will occur (path B).
Similar problems are encountered in nitridation
of these steels. Carburization at low temperatures
proved capable of hardening the surface of austenitic stainless steel without forming phases that
would deplete the matrix of chromium, and
thereby degrade corrosion resistance. In this
process, the formation of carbides is kinetically
suppressed (path C). This results in an extremely
high or “colossal” carbon supersaturation, in
fact far higher than can be achieved by hightemperature carburization.
At moderate temperatures, substitutional
solutes such as chromium and nickel diffuse
much more slowly in austenitic steels than do
interstitial solutes such as carbon. At 450 to
500 C (840 to 930 F), the diffusivity of chromium is on the order of 10–21 m2/s (Ref 21),
whereas the carbon diffusivity at this temperature
is in the range 10–16 to 10–17 m2/s (Ref 22–24). This
factor of 104 to 105 difference in diffusion coefficients enables homogeneous carburization in
austenitic stainless steels (and other fcc alloys)
Commercially available processes for low-temperature nitriding and carburizing of stainless steels
Entity
University of Birmingham,
U.K.
Bodycote, U.K.
Nihon Parkerizing, Japan
Airwater Ltd., Japan
Swagelok Company, U.S.A.
Nitrex Metal Technologies,
Canada
Expanite A/S, Denmark
Process name
LTPN
LTPC
Kolsterising
Nivox2
Nivox4 and Nivox LH
Palsonite
NV Super Nitriding
NV Pionite
SAT12
Nitreg-S
Expanite
Interstitial hardening
element
N
C
C
N
C
N+C
N
C
C
N
N+C
Temperature
C
F
<450
<842
<550
<1022
Trade secret
<400
<752
<460
450–490
<860
842–914
300–400 572–752
<500
<932
380–550 716–1022
...
...
...
...
Method
Plasma
Plasma
Trade secret
Plasma
Plasma
Cyanide salt
bath
Gas
Gas
Gas
Gas
Gas
Applications
Comments
...
...
...
...
...
Hardware, watch cases
Control rods used in nuclear
Bodycote acquired Nutrivid
reactors
(France) 2010
...
...
Small bore weapon components
Flatware, hardware
Hardware, watch cases
Tube fitting ferrules, hardware
Piston rings, pitch gears
...
Fluoride activation
Fluoride activation
HCl activation
...
...
Low-Temperature Carburization of Austenitic Stainless Steels / 453
under conditions where the formation of
chromium-rich carbides is suppressed by insufficient diffusion kinetics for substitutional solutes.
In other words, carburization is possible below
the nose of the carbide curve on the TTT diagram
shown in Fig. 4. At the relevant temperatures, the
system is not able to approach a complete thermodynamic equilibrium, because local differences in the chemical potentials of the metal
atoms cannot be equilibrated by transport. Only
nonuniformities of the chemical potentials of
the interstitial solutes can be equilibrated. Such
limited equilibration of some, but not all, components in a metallic system is called “paraequilibrium.” This can occur at temperatures where
substitutional solutes are effectively immobile,
whereas interstitial solutes such as carbon can
diffuse over appreciable distances within reasonably short times. Paraequilibrium in austenitic
stainless steels allows for vastly increased solubility limits for carbon.
Fig. 1
Commercial application of low-temperature carburization: Swagelok Tube Fitting. Courtesy of the Swagelok
Company
Carburizing Atmosphere
Several carburizing gas species will impart
carbon to the active surface, with varying levels
of efficiency at the processing temperatures
required for low-temperature carburization.
These include acetylene and carbon monoxide,
among others. Pack carburization also has been
demonstrated. A side effect of the high carbon
potential needed for processing can be the generation of soot, which then has to be removed
after processing using standard aqueous cleaning methods.
Candidate Alloys for Treatment
Fig. 2
Pit furnaces for low-temperature carburization. Courtesy of the Swagelok Company
Several commercially-available corrosionresistant alloys can be treated by low-temperature
carburization. The process has been applied to
300 series stainless steels, in particular 316 stainless steel, with great effectiveness. Optimal treatment is obtained on single-phase fcc or fully
austenitic alloys containing high fractions of
chromium and nickel. Under paraequilibrium
conditions chromium, with its high affinity for
Temperature, °C
Low-temperature carburization
Co + H2 + N2, 470 °C (880 °F)
3h
3h
20–30 h
3h
Cooling
Surface activation
HCl + N2, 250 °C (480 °F)
Time, h
Fig. 3
Typical process cycle for low-temperature
carburization of austenitic stainless steels. It
takes up to three days to perform the carburization
process, in contrast to minutes with typical heat treating
processes.
Fig. 4
Time-temperature-transformation (TTT) diagram of carbide formation in austenitic stainless steels. Source: Ref 20
454 / Heat Treated High-Alloy Steels
Quality Control Methods for
Evaluating Treatment
The low-temperature carburized layer on austenitic steel has been characterized using optical
metallography. On a sectioned and etched specimen, the case appears as a relatively featureless,
etch-resistant surface layer of approximately
10 to 40 mm. This layer is a diffusion gradient
of carbon in solid solution in the austenite
matrix. The structure of the case has been evaluated by X-ray diffraction, and has been shown to
be expanded austenite. The lattice parameter of
the austenite expands from 0.360 to 0.372 nm
after treatment (Ref 30). Other researchers, particularly those working in low-temperature
nitriding processes, have called this structure
S-phase (Ref 31–33). However, there is no phase
transformation and the layer does not represent a
new phase. Expanded austenite is formed when
large amounts of either nitrogen or carbon
(or both) are dissolved in the surface of an austenitic stainless steel, forming a supersaturated
solid solution without the precipitation of chromium-containing nitrides or carbides. Data for
treated 316L stainless steel are shown in Fig. 5
(Ref 34). The surface was electropolished for different times to progressively remove the case.
The analysis used X-ray diffraction (XRD) to
determine the lattice parameter and evaluate
residual stresses, and to identify the possible
presence of carbides arising from carburization.
The shift in the peaks to lower 2y values for
lesser amounts removed (higher concentrations
of carbon) indicates the increasing lattice expansion. Carbon concentration profiles using
Auger electron spectroscopy (AES) and energydispersive spectrometry (EDS) show carbon
(111)γ
Carburized
surface
12
(200)γ
6 μm
0
10
20
30
40
50
1200
8
800
4
400
HV25
When parts are treated correctly, application of
the low-temperature carburization process should
be transparent to the end user. In other words, the
parts should visually appear as they did prior to
treatment. Because only a layer comprising the
first 25 mm or so below the surface is affected by
the treatment, it can be difficult to determine if a
component has been treated without resorting to
destructive methods and optical microscopy.
Two qualitative methods for examining treatment
have been used: scratch hardness using calibrated
files, and immersion in sodium hypochlorite solutions. These methods interrogate the case by
examining improvements in hardness and corrosion response to a chloride-containing environment. Although they can identify a difference
between a treated and nontreated surface, they
are not specific enough to be used for anything
but a screening test. For detailed evaluation of
case properties, quantitative methods providing
enhanced spatial resolution are preferable. These
include light optical microscopy of metallographic cross sections, depth profiles of nano- or
microhardness, and composition-depth profiles
obtained through glow-discharge optical emission
spectrometry (GD-OES) or calibrated Auger electron spectroscopy (AES) using a scanning Auger
microprobe (SAM) on a cross section. These
methods are destructive, in that a component must
be sectioned to evaluate the case. However, quantitative comparative data is obtained and can be
used to develop metrics for the case.
Xc, at.%
11 μm
0
19 μm
0
24 μm
Untreated
specimen
40
(a)
Fig. 5
42
44
46
48
2θ, degree
50
52
0
−1
145
−2
290
0
(b)
10
20
30
40
Depth from surface, μm
σ11 ,ksi
Low-temperature carburization has been successfully applied to several product forms,
including plate, sheet, foil, wire, machined components, castings, forgings, powder, powdered
metal (P/M) and metal injection molded (MIM)
components. Surfaces must be clean and free
of scale, oil, or other residues so that the activation step can proceed. The mechanical surface
condition needed for a successful treatment
has been evaluated based on performance
characteristics of the surface after treatment.
Acceptable surface roughness parameters were
determined from cyclic polarization tests performed on samples with different finishes.
A 120 grit finish or better hardly limits the corrosion response in cyclical polarization testing.
Rougher surfaces may show metastable pitting
(Ref 28). The 120-grit finish typically is much
rougher than the finish obtained through standard machining practices.
The activation step illustrated in Fig. 3 is interrupted with a short carburization cycle. This processing feature enables a deeper and more
uniform case, apparently because it provides better surface activation. Observations by researchers of corrosion in flue gases (Ref 29) suggest
that the presence of soot enhances activation of
passive surfaces. Additional research on the second carburization cycle showed that a ramped
carbon potential was needed to achieve the hardness and depth required, and to minimize the
Microstructure of the
Low-Temperature Carburized Layer
σ11, GPa
Product Forms and Surface Condition
formation of soot that would need to be removed
in postprocessing.
Intensity
carbon, enables a dramatic increase of the solubility limit of carbon (Ref 25, 26). Nickel, although
not a carbide-forming element, appears to
enhance the carbon levels that can be obtained
by suppressing carbide precipitation (Ref 27).
Austenitic stainless steels can contain less
desirable metastable phases, such as ferrite
(bcc) and strain-induced martensite (body-centered tetragonal, or bct). Ferrite can be retained
in austenitic stainless steels if not sufficiently
hot worked or homogenized. Strain-induced
martensite can occur on the surface of susceptible alloys that have been heavily worked or
machined. Carburization treatment of ferrite or
martensite at temperatures usually applied to
austenite results in a severe loss of corrosion
resistance. The mechanism appears to be the
creation of surface carbides in the ferrite phase.
Optimal treatment by low-temperature carburization occurs when these phases are not present; ferrite can be reduced or eliminated by
annealing, and strain-induced martensite can
be removed via electropolishing.
Other alloys have been treated with varying
levels of success; these include superaustenitic
stainless steels (AL6XN, 254SMO), precipitationhardening iron-base superalloys (A286), nickelbase corrosion-resistant alloys (alloy 625, alloy
718, alloy 825, Hastelloy C), precipitationhardening stainless steels (17-4PH, 15-5PH,
13-8Mo), and duplex stainless steels (2205 and
2507).
50
Microstructural evaluation of low-temperature carburized 316 stainless steel. (a) X-ray diffraction (XRD) of
different depths within the case, obtained by serial removal of the surface via electropolishing. Note the
peak shift to the left from nontreated specimen to carburized surface, indicating lattice expansion. No peaks
associated with carbides are evident in the case. (b) Microhardness profile as a function of depth. Measurements
were taken from multiple components from a single process run. Superimposed curves of carbon concentration
(XC, at.%) and residual compressive stress (s11, GPa) are obtained from the XRD spectra shown in (a). Source: Ref 34
Low-Temperature Carburization of Austenitic Stainless Steels / 455
levels in excess of 12 at.% in treated 316. It is
clear that the lattice parameter within the carburized case has expanded. No peaks associated
with carbides are evident. The process achieves
very high hardness values. Figure 5 also shows
a microhardness profile as a function of depth,
with measurements taken from multiple metallographic mounts of 316 stainless steel specimens
from a single process run. As shown, hardness
values exceeding 1200 HV (70 HRC) are
achieved at the surface, dropping to the value of
the base material (300 HV or 23 HRC) at a depth
of approximately 25 mm.
Twelve at.% carbon corresponds to 105 times
the room-temperature equilibrium solubility
limit of carbon in 316 stainless steel. Side
effects of this colossal supersaturation of carbon in austenite include residual compressive
stresses of at least 2 GPa (300 ksi), based on
lattice expansion as measured by XRD and the
carbon concentration. The hardness and stress
depth profiles show an excellent correlation
with the carbon concentration depth profile
obtained by XRD analysis of the variation of
plane spacings with depth. The layer remains
austenite and retains its ductility (Ref 25, 26,
30, 34). As shown in Fig. 6, scanning electron
microscopy of failed treated tensile specimens
shows slip bands and no decohesion of the
treated layer, verifying that the layer remains
ductile. This result can be counterintuitive; usually harder materials are less ductile and more
brittle. However, this desirable combination of
hardness and ductility is useful in enhancing
damage tolerance. Conventional surfacehardening methods applied to austenitic stainless steels usually show some loss of ductility
and corrosion resistance.
Figure 7 shows metallographic evidence of
improved corrosion resistance; the case is not
etched by the Kane’s reagent used to reveal
the microstructure of the core. This image also
reveals the conformal nature of the gas carburization process—treatment extends to the base
of the deep pits formed by removal of manganese sulfide stringers by electropolishing prior
to carburization (not a standard part of the process). These features are carburized with the
same efficacy as the flat planar external surface.
The expanded austenite case is a nonequilibrium structure and represents a condition far
from equilibrium. It should be noted that the temperature stability of this structure depends on
minimizing exposure to temperature, because
the carburized steel may undergo decarburization, or the carbon in solid solution will continue
to diffuse into the matrix at temperatures
approaching treatment temperatures of 350 to
550 C (660 to 1020 F). After extended exposure, the case may no longer be present.
Performance Properties of the
Low-Temperature Carburized Layer
As noted earlier, only the outer 25 mm or so in
the near surface of the treated component is
affected by the process. It is interesting to see
the effect that this surface treatment has on tensile
properties. Figure 8 shows stress-strain curves for
treated and nontreated 316 stainless steel specimens, using a standard flat dog-bone specimen
with a cross section of 0.635 mm (0.025 in.). For
the low-temperature carburized specimen, there
is a small increase in yield stress—from 650 to
670 MPa (94 to 97 ksi)—and a more noticeable
increase in ultimate tensile stress, from 690 to
750 MPa (100 to 109 ksi). A minor decrease in
strain-to-failure, from 45 to 40%, also is evidenced, indicating the treated material is still ductile. In combination with the increased surface
hardness and compressive surface stresses, these
minor changes in tensile properties have profound
effects on performance characteristics, in particular fatigue, wear, and erosion.
Fatigue Resistance
As anticipated, the extremely high surface
compressive stresses (>2 GPa or 300 ksi—
essentially the yield stress of interstitially hardened 316 stainless steel) enhance fatigue
response. It should be noted that the compressive
residual stresses obtained by low-temperature
carburization are higher and more uniform than
any that could be obtained by mechanical means,
such as shot peening. Figure 9 shows the results
of fatigue testing of standard round bar 316 stainless steel specimens, in tension-compression
cycling (R = –1) (Ref 35). The symbols represent
different sets of specimens, with nontreated specimens identified by triangles, treated specimens
by squares. The effects of the thermal cycle (heat
treatment without carburization) also were evaluated, and tests of these specimens are identified
by circles. The thermal cycle alone did not show
a significant effect on fatigue performance.
Fatigue life (number of cycles) for treated
316 stainless steel showed two orders of magnitude improvement versus nontreated 316 at
the same maximum stress level. In addition, the
maximum stress at 107 cycles (considered to be
the endurance stress for infinite life), improved
by 75%, from 200 to 350 MPa (28 to 49 ksi).
Low-temperature carburized components can
sustain higher stresses in alternating loading conditions, or at the same stress level, have a highcycle fatigue life that is approximately 100 times
longer than that of nontreated components.
Testing was performed to determine the effect
of low-temperature carburization on fatigue
crack growth, distinct from fatigue crack initiation (Ref 36). In this investigation, six specimens
of 316L stainless steel with machined V-notches
first were subjected to fatigue loading in order to
produce fatigue cracks 2.4 mm (0.09 in.) long.
Fig. 6
Scanning electron micrograph of plastically
deformed surface of a low-temperature
carburized 316 stainless steel tensile specimen. Because
the treated material retains its austenitic structure, it also
retains its ductile deformation characteristics, as shown
by the intersecting slip bands. Courtesy of F. Oba, Case
Western Reserve University
Fig. 7
Metallographically prepared specimen of treated austenitic stainless steel. The uniform and conformal nature
of the case has been revealed by etching with Kane’s reagent. An advantage of the gas phase treatment is the
ability to treat all exposed surfaces, even deep blind holes. Courtesy of A. Avishai, Case Western Reserve University
456 / Heat Treated High-Alloy Steels
Fracture toughness testing was used to evaluate effects of hydrogen charging on 316 stainless steel compact specimens in the nontreated
and treated conditions. Testing was performed
under load control (no cycling) with crack
length measured by the compliance method.
Cracks were observed in the hydrogen-charged
carburized layer. A minor decrease in the fracture toughness also was measured, although
the material remained ductile.
101.5
600
87.0
500
72.5
400
58.0
300
43.5
L-T carburized 316 SS
Nontreated 316 SS
200
100
0
0
Fig. 8
5
10
15
20
25 30
Engineering strain, %
29.0
14.5
35
40
0
45
Stress-strain curve for treated vs. nontreated
316 stainless steel tensile specimens. Source:
Ref 35
450
60
400
350
50
300
40
250
(Ref 37, 38). The effects of carburization on
wear resistance of 316L austenitic stainless
steel were investigated using the pin-on-disk
method. Sliding friction and reciprocating friction tests show wear rates of treated couples
(ball and disk) lowered by approximately 100
times compared to nontreated steel. An ASTM
standard continuous loop abrasion test (rotating
abrasive belt) showed a 30% reduction in wear
volume for treated versus nontreated 316 specimens (Ref 2, 39).
Dry sliding was tested against an Al2O3 ball,
using varying contact loads from 2 to 40 N, and
varying sliding velocity from 0.1 to 0.6 m/s
(4 to 24 in./s). The total sliding distance was kept
constant at 1 km (0.6 miles). The wear of the carburized specimens varied from 0 to 2.4 mm3 for
1 km of sliding, while the nontreated specimens
varied from 1.1 to 11 mm3 for 1 km of sliding,
as illustrated in the wear maps (Fig. 11). Carburization reduces wear by over a factor of four
under these conditions. In addition, the wear
debris was analyzed and used to show that the
nontreated material suffered from severe plastic
deformation during the test, while the carburized
material did not. The wear mechanism for
0.6
1.08
0.81
30
200
104
105
106
107
Number of cycles
108
Fig. 9
Results of fatigue testing of standard round bar
316 stainless steel specimens, in tensioncompression cycling (R = –1). Note two orders of
magnitude improvement for fatigue life of treated vs.
nontreated 316 specimens at a maximum stress of
350 MPa (50 ksi). Also, maximum stress at the endurance
limit (107 cycles) for treated vs. nontreated has improved
from 200 to 350 MPa (30 to 45 ksi). Source: Ref 35
0.48 0
Velocity, m/s
700
Nontreated
Heat treated 70
1x carburized
500
Maximum stress, ksi
116.0
Wear tests show significant enhancement in
wear properties, which is important for bearing
applications. Wear and frictional properties
were characterized using a universal microtribometer and standard ASTM test methods
Maximum stress, MPa
800
Wear Resistance
Engineering stress, ksi
Engineering stress, MPa
Two specimens were carburized, two were given
the same heat treatment as the carburization process, but with no carburizing gases flowing, and
the remaining two specimens were left nontreated. All six specimens were tested in a horizontal fatigue apparatus at 10 Hz, and the crack
growth rates (da/dN, where a denotes the crack
length and N the number of cycles) are plotted
versus the stress intensity factor change for each
stress cycle (DK), as shown in Fig. 10. For a
given DK, the crack growth rate for the carburized specimens is lower than for the nontreated
and heat treated specimens. The difference
becomes greater as DK decreases. So, for
small DK, which is experienced in typical applications, low-temperature carburization will
greatly increase the lifetimes of components.
More importantly, the threshold for fatigue crack
growth (DKth, defined as the DK for a crack
growth rate of 10–7 mm/cycle) is increased by
low-temperature carburization from 8 MPa m1/2
to 10 MPa m1/2. This indicates there are stress
states under which fatigue cracks will initiate
and grow in nontreated components, but not
in treated components.
0.35
0.54
1.62
2.16
0.23
1.89
0.27
0.1
1E–3
2
(a)
12
1.35
20
Load, N
0.6
2.43
30
40
2.2
Noncarburized: ΔKth = 8.2 MPa • m
4.4
1.1
1E–4
0.48
Carburized: ΔKth =
10.2 MPa • m1/2
Heat treated: ΔKth=
9.0 MPa • m1/2
1E–5
Velocity, m/s
da/dN, mm/cycle
1/2
6.6
0.35
8.8
0.23
11
3.3
0.1
1E–6
10
15
20
25
30
35
40
45
ΔK, MPa • m1/2
Fig. 10
Effect of low-temperature carburization on fatigue crack growth for 316 stainless steel. Low-temperature
carburization treatment slows the progress of a fatigue crack through a specimen. These lines of data and
their slopes indicate that a much larger stress must be applied in order to make a crack progress through a treated
specimen. Note also the increase in DKth, indicating that it is more difficult to initiate a fatigue crack in a lowtemperature carburized component. Source: Ref 36
(b)
2
Fig. 11
12
5.5
20
Load, N
7.7
30
9.9
40
Wear maps for (a) treated and (b) nontreated
316 stainless steel. T316 stainless steel:
treated by low-temperature carburization. NT316
stainless steel: nontreated (as received). Note the
significant improvement in wear rates for treated vs.
nontreated surfaces for the same loads and sliding
velocities. Source: Ref 40
Low-Temperature Carburization of Austenitic Stainless Steels / 457
nontreated material showed large plates, indicative of contact welding and stick-slip phenomenon, while carburized material showed minor
surface wear debris (Fig. 12).
The effects of carburization case depth on
wear resistance of 316L austenitic stainless steel
also were investigated. Dry sliding again was
tested against an Al2O3 ball, with a 40 N load
and a 0.1 m/s sliding velocity. Carburized 316L
specimens, with a nominal case depth of 30 mm,
were electropolished to remove 5, 10, 15, and
20 mm from the surface. While all the carburized
specimens performed better than the nontreated
specimen (case thickness = 0), the specimens
with case depths of 15 and 20 mm performed
slightly better than the specimens with 25 and
30 mm case depths. This effect could be due to
improvements in surface roughness with electropolishing, without a significant reduction in surface hardness.
A field service example of the effect of
enhanced wear resistance is shown in Fig. 13.
This 304 stainless steel wear plate for a ten horsepower pump, outside diameter of approximately
25 cm (10 in.), is used in a pulp and paper application. In regular service, it usually wears out in
three months (nontreated worn plate shown on
the left) and must be replaced to restore pump
efficiency. A treated plate is shown on the
right; this plate had been in service for over
30 months with no sign of wear. In addition
to the savings associated with avoidance of
replacement, maintenance, and repair, significant energy efficiencies also have been realized
due to the enhanced pump efficiency.
NT316
T316
300 mm
300 mm
•Severe wear
•Wear flakes >100 mm in size
•Mild wear
•Delamination wear flakes
•Oxidized wear particles
NT316
T316
100
100
80
80
60
60
40
40
20
0
0
y/mm 0
500
1000
1500
2000
2500
3000
x/mm
20
0
0
y/mm 0
»50 mm depth
500
1000
1500
2000
2500
3000
x/mm
»18 mm depth
Fig. 12
Wear debris and wear track trace for nontreated and treated 316 stainless steel. The wear track trace was
measured using atomic force microscopy (AFM); a deeper trace indicates more material removed. Source:
Ref 41
Fig. 13
Field service testing of ten horsepower pump wear plate. The plate on the left was nontreated and required
replacement due to wear after three months in service. The treated plate on the right had been in service for
over 30 months with no sign of wear. Source: Ref 2
Erosion Resistance
The combination of hardness and ductility
achieved by low-temperature carburization also
enhances resistance to erosion. Figure 14 shows
the results of cavitation testing of treated and
nontreated 316 stainless steel specimens. Cavitation tests were performed for up to six hours
in duration, with a vibratory horn and mercury
as the dense liquid medium. Results of these
tests showed significant increases in cavitation
resistance; an approximately eight-fold reduction in weight loss rate was realized for the
treated surfaces as compared to the nontreated
surfaces for type 316 stainless steel. These
results have been replicated in other alloys
and fluids, indicating that the diffusional nature
of the case provides a benefit in fluid applications where cavitation can occur.
Corrosion Resistance
Electrochemical polarization curves show a
600 to 800 mV increase in pitting potential in
treated (900 to 1000 mV) versus nontreated
(200 to 300 mV) type 316 in chloride solutions
(Ref 42). Two possible causes for the enhanced
corrosion resistance have been postulated: the
concentration of carbon at the surface, or
the enormous surface compressive stress. An
electrochemical polarization curve was
prepared for the plastically deformed gage
length of a pulled-to-failure treated tensile
specimen, to remove the effect of the residual
compressive stress. This curve showed the same
enhanced corrosion behavior, indicating that
the enhancement is due to the high carbon concentration (Ref 43, 44).
458 / Heat Treated High-Alloy Steels
ACKNOWLEDGMENTS
The authors acknowledge their colleagues at
Swagelok and Case Western Reserve University
(CWRU) who have supported and contributed to
the ongoing research effort. Gary Michal,
CWRU, was instrumental in developing kinetic
and thermodynamic models; his substantial
0.04
Cumulative weight loss, g
#2 316 untreated
contributions to this research effort over the last
15 years were pivotal and thought provoking.
Preparation of this article has been supported
by Swagelok Company, and the research
described has been supported by the Department
of Energy Industrial Materials of the Future Program, the Ohio Department of Development
Third Frontier Program, DARPA, and the Office
of Naval Research. The authors are grateful for
the contributions of research colleagues at Oak
Ridge National Labs: Vinod Sikka, Peter Blau,
Jun Qu, and Dane Wilson. Thanks are also
extended to research colleagues at the Naval
Research Laboratory—Farrel Martin, Paul
1400
Potential, mV vs. Ag-AgCl
1200
0.6 V/h
0.6M NaCl
1000
Carburized
800
600
400
Nontreated
200
0
–200
–400
10–3
(a)
10–1
103
10
105
Current density, µA/cm2
1500
1250
Potential, mV vs. Ag-AgCl
Figure 15 shows cyclic polarization scans of
treated and nontreated specimens of 316L in
both 0.6M NaCl and seawater. In each environment, pitting corrosion, normally the first indication of corrosion damage in this steel, has
been completely suppressed by carburization.
(The apparent pitting at high potentials actually
is decomposition of the NaCl solution—
Cl2 generation—as was confirmed by posttesting metallographic examination.)
On nontreated and treated 316 stainless steel
specimens, ASTM G150 critical pitting temperature (CPT) tests were performed (Ref 2). The
CPT for nontreated specimens was 16.9 ±
1.2 C (62.4 ± 2.2 F), while treated specimens
had a CPT of 79.1 ± 6.6 C (174.4 ± 11.9 F). This
CPT value closely approximates the CPT of more
highly alloyed corrosion-resistant alloys.
In crevice corrosion testing, treated 316 stainless steel compares favorably with alloy
625 (Ref 45), as shown in Fig. 16. This figure
shows three specimens at the end of a weeklong
crevice corrosion test in natural seawater, at
room temperature, under an applied potential of
300 mV. Nontreated 316 showed initiation of a
crevice in under one hour. At the end of the
week, a 1270 mm (0.050 in.) crevice cut with
significant material wastage was experienced.
Alloy 625 did not initiate crevice formation
until 60 h had passed; at the end of the week,
a 76.2 mm (0.003 in.) crevice cut was evident.
The treated 316 specimen, however, showed
no initiation in over 160 h, and no sign of etching after this weeklong test. The suppression of
crevice corrosion as a failure mode for 316 is
an unexpected but important result.
1000
750
Carburized
500
250
Nontreated
0
–250
–500 –5
10
(b)
Fig. 15
10–3
10–1
10
103
Current density, µA/cm2
105
Cyclic polarization curves of nontreated and
treated 316 stainless steel specimens in (a)
0.6M NaCl and (b) seawater. Courtesy of F.J. Martin,
Naval Research Laboratory (NRL)
Natishan, Bob Bayles, Roy Rayne, and Diane
Lysogorski—who verified some earlier findings
on corrosion and erosion response, and gave a
much better understanding of the mechanisms
underlying the unexpected enhancements in corrosion performance.
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