ISIJ International, Vol. 46 (2006), No. 1, pp. 111–120
Nitriding of Interstitial Free Steel in Potassium–Nitrate Salt Bath
Yin Zhong SHEN,1) Kyu Hwan OH1) and Dong Nyung LEE1)
1) School of Materials Science and Engineering, College of Engineering, Seoul National University, Seoul 151-744, Korea.
E-mail: [email protected], [email protected], [email protected]
(Received on June 15, 2005; accepted on October 31, 2005 )
A study has been made of nitriding of interstitial free (IF) steel in the potassium–nitrate salt bath at temperatures ranging from 400 to 650°C. The salt is decomposed to generate nitrogen and oxygen. Nitrogen
diffuses into steel, or steel is nitrided, while oxygen reacts on steel surface to form the oxide scale. The
oxide scale thickness is much smaller than the nitriding thickness. Most of nitrogen resides in steel as a
form of interstitial solid-solution. For nitriding at higher temperatures, nitride precipitates (g -Fe4N and z Fe2N) exist mostly in grain boundaries and partly in grains of the steel. The nitrate nitriding gives rise to
much larger nitriding depth than other nitriding methods at similar nitriding temperature and time. The nitrate nitriding of steel substantially increase its tensile strength as well as hardness, e.g., an IF steel specimen nitrided at 650°C for 1.5 h shows a tensile strength of 916 MPa, which is 2.2 times higher than that of
non-nitrided IF steel specimen, and an elongation of 20 % at 70°C. Severe serrations are observed in flow
curves of nitrided steel specimens, mainly due to dynamic strain aging that occurs because of interaction
between dissolved nitrogen and moving dislocations. The effective diffusion coefficient of nitrogen DN obtained from the nitriding data, DND0 exp(Q/RT ) with D03.789107 m2 · s1 and Q76.62 kJ mol1, is
approximately the same as that for diffusion of nitrogen in a -iron.
KEY WORDS: potasium nitrate; steel; nitriding; hardness; tensile strength; solid solution strengthening; diffusion coefficient; dynamic strain aging.
1.
monia, but the laser irradiation did not succeed in introducing a detectable amount of nitrogen into the metals when
conducted in 1 atm gas ambient.27) The white compound
layer formed during gas nitriding is related to the quantity
of nitrogen atoms in the interface between the gas medium
and workpiece surface, e.g., if the nitrogen supply exceeds
the absorbing capability of the workpiece, besides the diffusion zone, the nitrides layer is present.6)
Conventional industrial gas-nitriding is usually performed in NH3 gas at temperatures ranging from 450 to
650°C for tens of hours. Nevertheless, the nitrided layer
thickness is usually less than 0.5 mm. The gas nitriding can
result in a white brittle nitrites layer on the surface. In most
cases, this white layer must be removed by machining before use. Another nitriding method, which is most commonly used in industries, is salt-bath nitriding using liquid salts
consisting of cyanide and cyanate. In fact, this process is
actually a nitrocarburizing process, because the environment of molten salt contains both carbon and nitrogen, and
the two elements diffuse into steel parts, simultaneously. Its
major problem in industrial application is the toxicity involved in the cyanide–cyanate salt. To the best of our
knowledge, no nitriding for steel without simultaneous carburization has been realized with salt baths. Therefore, an
effort for finding a pure salt-bath nitriding of steel is necessary to be done.
Reported decomposition temperatures of potassium nitrate (KNO3) are about 527–567°C28) and 628°C29) obtained
by differential scanning calorimetry and differential ther-
Introduction
One of strengthening mechanisms of iron and its alloys is
well known to be solid-solution strengthening by interstitial
atoms such as carbon and nitrogen. Since the solubility of
nitrogen in a -Fe is two times and five times (at least)
greater than that of carbon at temperatures of 20 and
590°C, respectively,1) thereby decreasing the tendency for
precipitation at a given level of strengthening. Furthermore,
dissolved nitrogen is a more effective solid-solution
strengthener than carbon at (or below) room temperature in
austenitic stainless steels.2,3)
To improve the surface hardness, wear and corrosion resistance, fatigue life, as well as global mechanical performance of pure iron and simple iron alloys, nitriding has
been widely used in industries. Many nitriding processes
have been applied to pure iron and low carbon steels, for
instance, gas nitriding,4–10) salt bath nitriding,11) plasma
nitriding,10,12–16) plasma immersion ion implantation
(PIII),16–20) laser nitriding,21–25) and ion implantation nitriding.26) Although the above methods are well established,
some of them such as modern nitriding techniques still have
some disadvantages from an engineering viewpoint because
they require the use of rather complicated and expensive
apparatus, and cannot, in general, produce thick nitrided
layers.
A large amount of nitrogen can be introduced into the
surface layers of iron and titanium by laser irradiation of
the metals in liquid medium such as liquid nitrogen or am111
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ISIJ International, Vol. 46 (2006), No. 1
mal analyzer techniques, respectively. Potassium nitrate
may decompose by some of the following reactions28,30–33):
Table 1. Chemical composition (wt%) of IF steel
KNO3→KNO21/2 O2 ........................(1)
2KNO2→K2ONO2NO......................(2)
2KNO22NO→2KNO3N2....................(3)
KNO2NO2→KNO3NO .....................(4)
K2O2NO2→KNO2KNO3 ...................(5)
Reaction (2) has been observed at temperatures ranging
from 517 to 537°C.28) In addition, potassium nitrate may
also directly decompose into potassium oxide30,34,35) according to the following reaction (6) or (7):
2KNO3→K2O2NO21/2 O2 ..................(6)
2KNO3→K2O5/2 O2N2 .....................(7)
Thus, the thermal decomposition of KNO3 on heating can
liberate nascent nitrogen and the nascent nitrogen can diffuse into the steel coupon. Therefore, there is a possibility
of nitriding in the liquid KNO3 salt bath under ambient
pressure.
Interstitial-free (IF) steels are ultra low carbon and nitrogen steels manufactured by adding carbide, nitride and sulphide-forming elements such as titanium and aluminum to
form precipitates such as TiC, TiN, AlN, and TiS, leading
to an elimination of the nitrogen, carbon and sulphur elements from the solid solution like nearly pure iron. For this
reason, in order to investigate the effect of solid-solution
strengthening by diffusion of nitrogen to the interstitial sites
of a -Fe lattice when KNO3 salt bath is used as nitriding of
iron and steels, we chose to use a TiAl stabilized IF steel,
in which Ti and Al scavenge both carbon and nitrogen.
The purpose of this study is to investigate nitriding behavior of IF steel in the KNO3 salt-bath, with emphasis on
solid-solution hardening and nitriding kinetics.
2.
Fig. 1.
Optical and SEM micrographs were obtained from 3%
nital-etched samples using Olympus PMG3 microscope and
JSM-5600 scanning electron microscope (SEM) equipped
with OXFORD Link ISIS 300 Energy Dispersive X-ray
Spectrometer (EDS), respectively. Auger electron spectroscopy (AES) of the samples was conducted by using a
Scanning Auger Microprobe System (model 660).
The hot-rolled IF steel sheet of 4 mm in thickness was nitrided for 3 h in the KNO3 bath (the nitrate bath) at 650°C,
and the nitrogen and oxygen concentrations in a 0.4 mm
thick surface-layer (from 50 to 450 m m in depth) and a
0.5 mm thick center-layer of the nitrided sample were measured using a LECO TCH 600 Nitrogen/Oxygen/Hydrogen
Determinator (an experimental error of less than 0.1 ppm).
Powders taken from the surface of samples heated in the
nitrate bath were subjected to X-ray diffraction (XRD) with
the Cu-Ka 1 radiation (l 0.154056 nm) to measure their
lattice parameters and phases.
Tensile specimens of 1 mm6 mm25 mm in gauge dimensions, in accordance with ASTM E 8M-99, were sparkerosion-cut from steel sheets, with the tension axis parallel
to the rolling direction. They were ground and polished up
to a bright surface before tensile test. Tensions were conducted on an Instron-5582 testing machine at an initial
strain rate of 4.23105 s1. The stress drop magnitude in
serration, which was defined as the engineering stress difference between the maximum and minimum points of individual serrations, was calculated using a special computer
program edited for serration data analysis.
Experimental Procedures
A hot-rolled IF steel sheet of 4 mm in thickness was used
as the initial material. The chemical composition of the
steel is given in Table 1. The sheet was cold-rolled by 10,
30, and 75% in thickness under lubrication.
Extra pure potassium nitrate (99.0 wt% min. purity) with
impurities of chloride (Cl, 40 ppm max.), sulfate (SO4,
80 ppm max.), ammonium (NH4, 60 ppm max.), lead
(10 ppm max.), iron (8 ppm max.), and calcium (60 ppm
max.) was used for nitriding of steel. The nitrate nitriding
temperature ranged from 400 to 650°C. Samples to be nitrided were cleaned in hot 10% HCl solution to remove the
surface scales. Annealing of steel samples were carried out
in the 67%CaCl2–33%NaCl salt bath (the chloride bath) to
avoid any reaction between the samples and the bath. After
the heat treatments, all the samples were quenched in cold
water to remove surface scales. The scale-free specimens
were slightly ground on 2000-grit SiC emery paper to get a
bright surface before hardness test. Vickers-hardness tests
were performed with Instron Wolpert Tester 930 under
loads of 1 to 2 kg. The measured surface hardness values
are mean values of ten measurements.
© 2006 ISIJ
XRD spectrum of powder taken from surface of IF steel
heated in KNO3 bath at 650°C for 1.5 h.
3.
Results and Discussion
3.1. Formation of Oxide Scale
Figure 1 shows the XRD spectrum of the powder sample
taken from the surface of an IF steel sample heated for 1.5 h
in the nitrate bath at 650°C. The XRD result indicates that
it consists of magnetite (Fe3O4), hematite (Fe2O3), and
wustite (FeO). Therefore, oxygen must have been generated
in the nitrate bath.
It has been reported that magnetite and hematite form on
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Fig. 2.
Fig. 3.
Fig. 4.
Change in oxide thickness of full-annealed IF steel sample during heating in KNO3 salt bath. heating at 580 to
640 °C for 16 h; heating at 640 °C for 4 to 16 h.
Vickers hardness (load: 1 kg) of surface of 10 to 75%
cold-rolled IF steels after annealing in CaCl2–NaCl salt
bath at 650°C.
bath, which does not contain any nitrogen-generating components, at 650°C for 1 min to 4 h. The surface hardness
values of the chloride-bath annealed samples are shown in
Fig. 4. The hardness values decrease with increasing annealing time as expected. For the 10% cold-rolled sample,
the hardness decreases a little even for an annealing time of
3 h, whereas the 30% and 75% cold-rolled samples undergo drastic decreases in hardness. The differences in hardness values of the specimens are attributed to whether they
underwent recrystallization or not. The 10% cold-rolled
specimen did not undergo recrystallization, because of lack
of driving force for recrystallization, unlike the 30% and
75% rolled specimens. Figure 5 shows some optical micrographs of center part in thickness direction of 30% rolled
specimens annealed in the chloride-bath. The hardness data
in Fig. 4 and the microstructures in Fig. 5 indicate that recrystallization was completed within about 1 h for the 75%
rolled specimen and about 2 h for the 30% rolled specimen.
Therefore, the hardness increases in Fig. 3 must be related
to diffusion of nitrogen into the steel samples, or nitriding.
If nitriding takes place, a question arises why the hardness values decrease above about 620°C in Fig. 3. One possibility may be recrystallization occurring above the temperature of 620°C. Figure 6 shows SEM micrographs of
the cross-sections of 30 and 75% cold-rolled IF steel samples after heating for 1 h in the nitrate bath at 600 and
640°C, which are respectively 20°C lower and higher than
the temperature at which the hardness values are maximal.
We do not see any evidence of recrystallization in the specimens. This may be attributed to the nitrogen segregation in
grain boundaries and to interaction between dissolved nitrogen and dislocations inside of grains, which pin grain
boundaries and dislocations, resulting in prevention of recrystallization. Therefore, the decreases in hardness above
620°C in Fig. 3 cannot be due to recrystallization.
Figure 7 shows SEM micrographs of the full-annealed
IF steel after nitriding at various conditions. White and dark
particles are observed along the grain boundaries. The
amount and size of the particles tend to increase with increasing nitriding time (Figs. 7(a)–7(c)), and are larger in a
region near the surface than in the center region (Figs. 7(d),
7(e)) and at 640°C than at 620°C (Figs. 7(e), 7(f)). The
EDS and AES results indicate that the particles have higher
nitrogen concentrations than the matrices, as shown in Fig.
Vickers hardness (load: 2 kg) of surface of 10 to 75%
cold-rolled IF steels after heating for 1 h in KNO3 salt
bath at various temperatures.
the surface of cold-rolled low carbon steel after carbonitriding in the ABI salt bath consisting of MNO3, MNO2, and
M2CO3, where M refers to Na and K, at 400°C.11)
Figure 2 shows a change in surface-oxide thickness of a
full-annealed IF steel sample during heating for 16 h in the
nitrate bath at 580, 600, and 640°C as well as for 4, 8, and
16 h at 640°C. The full-annealed sample was made by 30%
cold rolling and subsequent annealing for 2.5 h in the chloride-bath at 650°C. The steel sample undergoes losses of
about 154 m m (77 m m on one side) and 35 m m (17.5 m m on
one side) in thickness after heating for 16 h at 640°C and
580°C, respectively.
3.2. Nitriding
Figure 3 shows the Vickers hardness (load: 2 kg) of the
surface of 10 to 75% cold-rolled IF steel samples after
heating for 1 h in the nitrate bath at temperatures of 400 to
650°C. The hardness increases with increasing reduction at
a given heating temperature because of strain hardening. As
the heating temperature increases, the hardness increases to
a maximum value and then decreases, with the increasing
rate being low up to about 550°C, above which the increasing rate becoming high. The 10, 30, and 75% cold-rolled
samples have their maximum hardness values at 600, 610,
and 620°C, respectively. This hardness increase during
heating may be associated with nitriding.
In order to see if the hardness increase has to do with nitriding, the same samples were annealed in the chloride
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ISIJ International, Vol. 46 (2006), No. 1
Fig. 5.
Optical microstructures of midthickness of 30% cold-rolled IF steel after annealing in 67%CaCl2–33%NaCl bath
at 650°C for (a) 0 min, (b) 5 min, (c) 15 min, (d) 1 h, (e) 2.5 h, and (f) 3 h.
Fig. 6.
SEM micrographs of cross-sections of cold-rolled IF steel after heating for 1 h in KNO3 salt-bath. (a) 30% coldrolled, heated at 600°C; (b) 30 % cold-rolled, heated at 640°C; (c) 75% cold-rolled, heated at 600°C; (d) 75%
cold-rolled, heated at 640°C.
g -Fe4N and z -Fe2N phases. The XRD result along with the
EDS and AES results in Fig. 8 indicate that the particles
distributed along the grain boundaries seem to be z -Fe2N
and g -Fe4N. In addition, it seems that there are fine nitrides
in the matrix of the sample nitrided at higher temperatures,
as shown in Figs. 7(d) and 7(e). Further studies such as
8, possibly due to higher diffusion rates along the grain
boundaries.
In order to identify the particles, XRD of the sample nitrided at 640°C for 16 h was carried out after grinding away
150 m m in depth from the surface to remove the surface
scale. The XRD result is represented in Fig. 9, which shows
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Fig. 7.
SEM micrographs of cross-sections of full-annealed IF steel after heating in KNO3 salt-bath. (a) 640°C for 1 h,
(b) 640°C for 4 h, (c) 640°C for 16 h, (d) 640°C for 16 h (near-surface), (e) 640°C for 16 h (center), (f) 620°C for
16 h.
Fig. 8.
(a) EDS and (b) AES results obtained from sample of Fig. 7(d).
TEM works are desirable for direct identification of these
precipitates. The formation of g -Fe4N is favorable for prolonged nitriding at high temperatures.36) Usually, g -Fe4N
forms a kink of coarse needle-like precipitates dispersed in
the ferritic matrix.13) For the conventional nitriding, g Fe4N and e -Fe23N are present in nitrided layers. The formation of z -Fe2N during nitriding is reported.16) The e Fe23N phase is found to form at the grain boundaries (or
junctions) of the ultrafine-grained a -Fe phase, when nitrided for 9 h at 300°C in ammonia gas.4) It follows from these
facts that the hardness drops above 620°C in Fig. 3 seem to
be due to g -Fe4N.
Chemical analyses of nitrogen and oxygen in the full-annealed IF steel after heating for 3 h in the nitrate bath at
650°C were performed. The analyses results are given in
Table 2, which indicate that the nitrogen concentration of
the surface layer is almost ten times higher than that of the
center layer. The oxygen concentration varies little with
depth, indicating that most oxygen is consumed to form oxides at the surface. It is quite clear that nitriding took place
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ISIJ International, Vol. 46 (2006), No. 1
Table 3. Identification of XRD peaks in Fig. 10 and calculated
lattice parameters a and mean values ā.
Fig. 9.
XRD spectrum of ground surface of full-annealed IF
steel after nitriding for 16 h in KNO3 salt bath at 640°C.
Table 2.
Concentration (wt%) of nitrogen and oxygen in fullannealed IF steel after heating for 3 h in KNO3 bath
at 650°C.
Fig. 11. Stress–strain curves of 75% cold rolled IF steel specimens after heating in salt baths at 650°C, with initial
tensile strain rate of 4.23105 s1 at 70°C.
ferences in peak intensities of the sample before and after
heating in Fig. 10 are due to a slight texture change after
heating.
We estimate the nitrogen concentration of the surface of
the heat-treated sample using the lattice parameter data
in Table 3. Let atomic radius and mass of element i be ri
and mi , respectively. Then, rFe0.124 nm,38) mFe55.85,39)
39)
rN0.071 nm,38)
– and mN14.01. The lattice parameter of
Fe is aFe(4/ø3)rFe. Since the interstitial positions in bcc Fe
are 1/2 00, 0 1/2 0, 00 1/2, the interstitial gap in a -Fe is calculated to be [a Fe2rFe0.03836 nm]. When one nitrogen
atom intrudes in the gap, a lattice-parameter expansion of
[2rN0.03836 nm0.10364 nm] takes place. The latticeparameter expansion after nitriding, (0.28711–0.28679) nm
from Table 3, is assumed to be caused by the nitrogen intrusion. Then, 0.10364 XN(0.28711–0.28679), with XN
being atomic fraction of nitrogen. We obtain XN0.003088
and XFe0.9969, from which the nitrogen concentration is
calculated to be 0.0776 wt%. According to Wriedt and
Zwell,40) the unit cell parameter of a iron increases linearly
by 3.2103 nm per wt% N dissolved. If the lattice-parameter expansion after nitriding, 3.2104 nm from Table 3,
is assumed to be caused by the nitrogen intrusion, then the
nitrogen concentration of the surface of the nitrate-bath
heated (NBH) sample is estimated to be about 0.1 wt%. It is
interesting to note that these estimated nitrogen concentrations are similar to the solubility of nitrogen in a -Fe at
650°C, about 0.09 wt%.41)
Tensile testing results of the chloride-bath annealed
(CBA) sample and the NBH samples reveal a pronounced
difference, as shown in Fig. 11. The flow stresses of the
NBH samples are more than two times higher than that of
the CBA specimen. The tensile strengths of the NBH speci-
Fig. 10. XRD spectra of 30 % cold-rolled IF steel (a) before and
(b) after nitriding in KNO3 bath at 650°C for 3 h.
in the sample.
Figure 10 shows XRD results of 30% cold-rolled samples before and after heating for 3 h in the nitrate bath at
650°C, which do not show any iron nitrides. This result
seems to contradict the result of Fig. 9. However, as the nitrogen concentration is small, and thus, if nitrides had been
formed, the volume fraction of nitrides would therefore also
be small, the XRD phase analysis may not be sensitive
enough to detect the presence of iron nitrides.
We have calculated the lattice parameters from the data
in Fig. 10, and the calculated results are given in Table 3.
The lattice parameter (0.28711 nm) of the heat-treated sample is slightly larger than that (0.28679 nm) of the nontreated one, indicating that nitrogen is in interstitial sites.
The XRD work by Walkowicz et al.37) indicates that the
average lattice parameter of a -Fe(N) phase formed in ISO
35CrMoV5 steel nitrided for 8 h at 540°C is 0.28694– nm.
The lattice constant a of a -Fe calculated from a(4/ø3)rFe,
with the radius of Fe atom, rFe0.124 nm, is 0.28637 nm,
and the increment of lattice constant, D ā, is about
5.7104 nm. In this study, the difference between the lattice parameters before and after the heat treatment,
D ā3.2104 nm, is comparable with their data. The dif© 2006 ISIJ
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Table 4. Serration characteristics of CBA and NBH specimens.
mens nitrided for 0.5, 1.5, and 3 h at 650°C are 1.7, 2.2, and
2.15 times higher than that of the CBA specimen, respectively. It is noted that the specimens (2) and (4) in Fig. 11
are heat-treated for the same period of time in different
baths at the same temperature. However the NBH sample
(2) shows a much higher tensile strength than the CBA
sample (4), indicating a pronounced solid-solution strengthening in NBH sample.
Furthermore, severe serrations occur in the flow curves
of NBH samples, whereas little serrations are observed in
the CBA specimen (Table 4). The serrations in the NBH
samples are characterized by fine serrations superimposed
coarse stress drops. The fine serrations are due to dynamic
strain aging (DSA), which is related to nitrogen dissolved
in the NBH specimen. The coarse stress drops may be associated with shearing of some nitride particles or by inhomogeneous nitrogen concentration.
The studies on DSA in iron and steel by Keh et al.42) indicate that the serration phenomena in a low carbon
rimmed steel (with alloying elements of 0.035% C,
0.36Mn, 0.006P, 0.016S, 0.005Si, 0.005Cu, 0.002Ni,
0.008Cr, 0.003N) (LCRS) appear at 68°C, and most severely occur at 162°C at a tensile strain rate of 4.23105 s1.
The carbon concentration in the present IF steel is much
lower than that of LCRS, but the amount of nitrogen is similar to LCRS. The weak serration behavior in the CBA
specimen may be caused by a little dissolved (unscavenged
by Ti and Al) carbon and nitrogen solutes.
According to the temperature dependence of diffusivities
of carbon and nitrogen, and the values of activation energy
for diffusion of carbon and nitrogen in a -Fe,1) the diffusion
coefficients of carbon and nitrogen at 70°C are calculated to
be 1.3161018 and 1.7601018 m2 s1, respectively, indicating that the diffusion velocity of nitrogen in a -Fe is
slightly higher than that of carbon. Since a substantial DSA
can occur at a carbon level of around 0.002 wt%, and as little as 0.001–0.002 wt% N can result in severe strain aging,1)
the serrated flow of the present IF steel at 70°C is caused by
nitrogen solute.
In contrast to the classical model of solute–dislocation
interactions, the serrated flow is also believed to be directly
caused by the shearing of precipitates,43) i.e., by the interactions of precipitates with mobile dislocations. In light of the
fact that the same amount of fine particles, such as TiN,
TiC, and AlN, are present in both the CBA and NBH specimens, the differences in serration amplitude between the
CBA and NBH specimens must be caused by dissolved nitrogen.
Since the recrystallization may be prevented in the NBH
specimens as discussed before, their strengths may not reflect a pure nitriding effect. Therefore, the hardness values
of 75% cold-rolled, 1 mm thick IF steels before and after
heating for 3 h in the nitrate and chloride baths at 650°C
were measured and compared in Fig. 12. The nitrided specimen has higher hardness than the as-rolled specimen.
Fig. 12. Vickers hardness values (load: 1 kg) of 75% cold rolled,
1 mm thick IF steel sheets before and after heating in
650°C KNO3 and CaCl2–NaCl baths for 3 h.
Fig. 13.
Vickers hardness (load: 2 kg) of surface of full-annealed
IF steel as a function of nitriding time in KNO3 salt bath
at various temperatures.
3.3. Nitriding Kinetics
Figure 13 shows the surface hardness of the full-annealed IF steel samples after nitriding for 1 to 16 h at temperatures of 560 to 640°C. The surface hardness increases
with nitriding time and temperature. However, for the samples nitrided at higher temperatures of 620 and 640°C, the
hardness increases to a peak value and decreases, with time
to reach the peak decreasing with increasing temperature.
These phenomena are attributable to increases in nitrogen
concentration and grain size with increasing nitriding temperature and time. The increase in interstitial-nitrogen concentration raises the hardness, while the formation of Fe4N
decreases the hardness.44) The decreases in the surface
hardness of the specimens nitrided at 620 and 640°C seem
to be caused by the formation of Fe4N as discussed in the
previous section.
Figure 14 shows the hardness profiles through the thickness of the full-annealed IF steel samples after nitriding for
1 to 16 h at 560 to 640°C. The hardness decreases with increasing distance from the surface and decreasing temperature. For the samples nitrided for 1 to 16 h at 640°C, the
overall hardness level increases with annealing time, especially for the samples nitrided for 8 and 16 h, and is much
higher than that of the samples nitrided at other temperatures, implying the higher nitrogen concentration throughout the samples. Therefore, we may achieve a relatively
uniform nitrogen-concentration profile through the thickness, if the nitriding time is sufficiently long.
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Fig. 15. Arrhenius-type plot for diffusion coefficient of nitrogen
in full-annealed IF steel, DN, during nitriding in KNO3
salt-bath. Activation energy is determined from slope of
straight line.
Fig. 14. Vickers hardness (load: 1 kg) as a function of distance
from surface for full-annealed IF steel after nitriding in
KNO3 salt bath. (1) 640°C for 16 h, (2) 640°C for 8 h,
(3) 640°C for 4 h, (4) 640°C for 2 h, (5) 640°C for 1 h,
(6) 620°C for 8 h, (7) 600°C for 8 h, (8) 580°C for 8 h,
(9) 560°C for 16 h, (10) 560°C for 8 h.
 Q 
DN D0 exp 
 .......................(10)
 RT 
where D0 is the temperature-independent pre-exponential
factor, Q is the activation energy for diffusion, R8.3143 (J
mol1 K1) is the ideal gas constant, and T is the absolute
temperature (K).
The DN values calculated using the nitriding depth, time,
and temperature data are plotted in Fig. 15. The DN–T data
fit the Arrhenius equation. It follows from Fig. 15 that
The data in Figs. 13 and 14 and results in Sec. 3.2 indicate that the hardness data are related to the nitrogen diffusion. Tong et al.4) found the coincidence in the profile
shapes between nitrogen concentration and hardness along
the nitrided depth. Simmons45) observed a linear relation
between the tensile strength and the nitrogen concentration
dissolved in a austenitic stainless steel. Therefore, we define a nitriding depth as the distance between the surface
and the position where the hardness value is equaled by
10% of the difference between the surface hardness and the
initial hardness of the center of sample plus the initial hardness of the center of sample.
From the hardness data in Fig. 14, we obtain the nitriding
depth data: 1.067, 1.261 and 1.385 mm for nitriding for 8 h
at 580, 600, and 620°C, respectively, and 1.374 mm for nitriding for 16 h at 560°C. The data at higher temperatures
are excluded in this kinetics of nitriding to avoid complexity due to the formation of nitrides and due to the limit of
specimen thickness.
Since the thickness of IF steel samples concerned is larger than the nitriding thickness, and the quantity of the nitrate bath is high enough for the generated nitrogen concentration to be constant, the nitrogen diffusion during nitriding can be described by the following equation46):

 x
C ( x , t ) Cs 1 erf 
 2 Dt

 76.62 
DN 3.789 107 exp 

 RT 
The activation energy, Q76.62 kJ mol1, is in good agreement with the activation energy for diffusion of nitrogen in
a -Fe, 77.8 kJ mol1, which Fast and Verrijp47) obtained at
temperatures of 500–600°C, and 76.2 kJ mol1, which
Wert48) obtained over a much smaller temperature range.
The pre-exponential factor, D03.789107 m2 s1, is similar to 3107 m2 s1 48) and a little lower than 6.6107 m2
s1.47) Therefore, we conclude that the diffusion behavior of
nitrogen for the KNO3-nitriding of the IF steel sample
is similar to that for the nitrogen diffusion in a -Fe at
temperatures below 620°C. It is also noted that Przyleski
and Maladazinski49) obtained the self-diffusion coefficient of nitrogen in a -Fe, DN(a )*6.6107 · exp (77.9/RT)
m2 s1.
The nitrogen concentration in a layer from 50 to 450 m m
in depth of the full-annealed steel specimen after nitriding
at 650°C for 3 h was measured to be 0.0556% N. We want
to calculate the nitrogen concentration using Eq. (8). The
diffusion coefficient of nitrogen in the sample at 650°C is
calculated from Eq. (11) to be 1.7481011 m2 s1. The
solubility of nitrogen in a -Fe at 650°C is about 0.09%.41)
This concentration is assumed to remain constant and
equivalent to Cs in Eq. (8). The average nitrogen concentration in the layer from 50 to 450 m m in depth is calculated
using the following trapezoidal rule.50)

  ..................(8)
 
where C(x, t) represents the concentration at depth x after
time t, Cs the concentration at the surface (x0),—and D the
diffusion coefficient. The expression erf(x/2øDt) is the
Gaussian error function. Based on the present definition of
nitriding depth, setting C(x, t)Cs/10, we obtain
x 2.32 DN t ..............................(9)
For nitriding, x, t, and DN are the nitriding depth and
time, and the effective diffusion coefficient of nitrogen, respectively. The DN value may obey an Arrhenius-type equation:
© 2006 ISIJ
(m 2 s1 ) ....(11)
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ISIJ International, Vol. 46 (2006), No. 1
Table 5.
Comparison between nitriding-thickness data measured in full-annealed IF steel and data from various
references.
processing optimization, along with simple apparatus,
cheap salt, and non-toxicity.
4.
C
∫
Potassium nitrate was successfully used for pure nitriding
of steel in salt bath, for the first time. Nitriding of IF steel
specimens in the KNO3 salt-bath at 400 to 650°C lead to
the following conclusions.
Most of nitrogen resides in steel as a form of interstitial
solid-solution. The hardness of the steel specimens increases with increasing nitriding time and temperature due to increasing nitrogen concentration. However, the nitriding
process with too high temperatures and longer time can decrease the surface hardness due to the formation of soft nitride. The hardness decreases with increasing distance from
the surface due to gradient profiles of the nitrogen concentration. The nitriding kinetics is controlled by diffusion of
nitrogen.
The KNO3 bath nitriding gives rise to a much thicker nitriding depth than existing nitriding methods at similar nitriding temperature and time. A pronounced solid-solutionstrengthening effect is therefore obtained, i.e., the 1 mmthick nitrided IF steel specimen shows a tensile strength of
916 MPa and an elongation of 20% at 70°C after nitriding
for 1.5 h at 650°C. Thus, the KNO3 bath can be used for nitriding of steels to achieve their solid-solution strengthening.
During nitriding of steel, the oxide scale forms, but the
oxide thickness is only a few percents of the nitriding thickness. For nitriding at high temperatures of 640°C, nitride
precipitates (g -Fe4N and z -Fe2N) seem to form mostly in
grain boundaries.
xn
c( x , t )dx
x0
nD x
Conclusions
1
[c( x0 , t )2c( x1 , t )
2n
2c( x2 , t )L2c( xn1 , t ) c( xn , t )] .......(12)
where the depth interval D x10 m m, the number of intervals n40, x00 m m, xn400 m m were used, and c(xi , t)
was calculated using Eq. (8) with Cs0.09%, D
1.7481011 m2 s1, t3 h. The average nitrogen concentrationis calculated to be 0.0674%, which is comparable
with the measured one, 0.0556%. This analysis implies that
Eq. (11) is applicable to nitriding of IF steel or a -iron.
It was pointed out in Sec. 3.1 that steel samples formed
oxide scales of about 154 m m (77 m m each side) and 35 m m
(17.5 m m each side) in thickness after nitriding for 16 h at
640°C and 580°C, respectively. Under the same nitriding
conditions, the nitriding thickness is calculated from Eqs.
(11) and (9) to be about 2.21 mm at 640°C and 1.55 mm at
580°C. The nitriding thicknesses are much larger than the
oxide scale thicknesses.
Various nitriding results are compared in Table 5. It can
be seen that the nitriding layer in this study saliently thicken compared to other methods. It should be noted that the
nitriding thickness from the references is the distance from
the surface to the location where the hardness value (or nitrogen concentration) reduces to the core hardness (or nitrogen concentration). In addition, bubbling of gaseous products, such as NO and NO2, was not observed during nitriding. Therefore, this nitriding method is environment-friendly.
From a technical viewpoint, therefore, the KNO3 saltbath nitriding can be as a pure nitriding of iron and steels
that can be used in industrial applications, because it is easy
to control the nitrogen balance in the steel coupon and the
Acknowledgement
One of the authors (Shen) acknowledges financial support from BK21 Materials Education and Research
Division, Seoul National University.
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