1996 GAVRILJUK Nitrogen in Iron and Steel ISIJ 360738

Prévia do material em texto

ISIJ International, Vol. 36 (1 996), No. 7, pp. 738-745
Nitrogen in lron and Steel
Valentin G. GAVRILJUK
Institute for Metal Physics, Kiev 252142, 36 Vernadsky av.. Ukralne
(Received on September29. l995, accepted in final form on March4. 1996)
A short review of beneficial effects of nitrogen in steel is given including mechanical and corrosion
properties, and the data of interatomic interactions and distribution of solute atoms in solid solutions are
discussed with the aim of explanation of the physical nature of nitrogen steels. The concept is presented
according to which alloying by nitrogen enhancesthe metallic componentof interatomic bondsand provides
more homogeneousdistribution of substitutionai solutes through short range ordering of nitrogen atoms
and strong chemical interaction between nitrogen and al]oying elements, which results in the high
thermodynamical stability of nitrogen austenitic steeis. Theopposite tendency to clustering andconcentration
inhomogeneity of austenitic steels dueto carbon is shown. Inheritance of the atomic distribution by martensite
is discussed in terms of short range atomic order and data on crystal structure of precipitations during
tempering of nitrogen martensite are presented as comparedto carbon and carbon+nitrogen martensites
KEYWORDS:nitrogen; carbon; austenite; martensite; toughness; strength; creep rate; fatigue; wear
resistance; corrosion; state density on the Fermi surface; short range ordering; clustering, tempering: nitrides;
carbides.
1. Introduction
Nitrogen as an alloying element in iron-based alloys
is knownsince the beginning of this century having been
profoundly studied during the last two decades (see, e.g.,
Refs. l)-3)). Nevertheless, nitrogen steels are so far not
widely used. The reason for the comparatively narrow
industrial applic~Ltion lies in the old customer skepticism
in relation to nitrogen as an element causing brittleness
in ferritic steels, sometechnical problems involved with
introducing nitrogen into steel and, Iast but not least,
the insufficient knowledge of the physical nature of
nitrogen effects in iron and its alloys. The purpose of
this paper is to give a short survey of nitrogens favoura-
ble influence on the properties of steel and analyse the
physical background of the striking difference between
nitrogen and carbon effects in steel.
2. WhichProperties CanBe Improved by Nitrogen?
Impact toughness wasperhaps oneof the first brilliant
examplesof nitrogen effects in austenitic steel making it
clear that nitrogen steels represent a newpromising class
of engineering materials. Bymean~of melting under high
pressure Frehser and Kubisch4) introduced more than
l o/o of nitrogen in the duplex X8 CrNiM0275 steel,
which provided a fully austenitic structure, andmeasure(r
mechanical and corrosion properties. Impact toughness
wasfound to increase simultaneously with yield strength
while the strengthening of austenitic steels by carbon is
usually accompanied by a decline in toughness. The
opposite effects of nitrogen and carbon on the fracture
C 1996 ISIJ 738
toughness of CrMn18 18 austenitic steels under simi-
lar strengthening behaviour were also dem:onstrated by
Speidel.5) It is important that, in contrast to carbon,
nitrogen austenitic steels retain high fracture toughness
at low temperatures: yield strengt~ at 0.2 o/o Plastic strain
more than 1200MPaand fracture toughness Klc more
than 200MPam~1/2 were obtained at 4K for nitrogen
containing JCSsteels.6) Thanks to this excellent com-
bination, nitrogen austenitic steels are successfully ap-
plied in the cryogenic industry.
The strengthening of austenite by nitrogen was an
object of extensive studies (see, e.g., Refs. 7)-11)). The
following topics are usually discussed in relation to
nitrogen effects: Iow temperature strength, strengthening
by grain boundaries, cold work hardening, creep, Iow
cycle fatigue and wear resistance.
The lower is the temperature the moreeffectiveiy (by
a factor of
-
I.4 at 4.2 K) nitrogen strengthens austen-
ite.7) The strengthening mechanismproposed by Owen
et al.8.9) for low temperatures is based on the existence
of Cr-N clusters and includes disordering of nitrogen
atoms by a moving dislocation over the area in the
slip plane. The planar dislocation arrays which consti-
tute a special feature in the substructure of nitrogen aus-
tenitic steels after plastic deformation (see, e.g., Ref. lO))
are consistent with this idea. The increase of the low
temperature strength of austenitic steels thanks to nitro-
gen observed in numerousstudies promotedsuggestions
that nitrogen assists the BCC-like temperature de-
pendenceof the yield stress, Rp,(T), at low temperatures.
However, precise measurementsperformed by Obst and
Nyilas I I )haveclearly confirmed Seeger's theory of plastic
ISIJ International, Vol.
deformation for the FCCcrystals with a low stacking
fault energy (SFE). A stair-like behaviour of Rp.(T)
occurs with the clear transition to the low temperature
range where the edge dislocation parts sustain the strain
rate because their mobility becomeshigher than that of
the screw dislocation components.
Nitrogen enhances the contribution of grain bound-
aries to yield strength, i.e., it increases the k coefficient
in the Hall-Petch equation Rp0.2
=
(T, +kd~ 1/2 wheredis
the grain size. This effect was studied in detail in Refs.
12)-1 5). Theplanarity of the dislocation slip is obviously
the reason for the grain boundaries becoming more
effective obstacles to the slip transfer from one grain to
the other, whatever the physical mechanism,short range
Cr-N ordering or the decrease of SFEby nitrogen, is
responsible for the planar slip.
The cold work hardening of stable austenitic steels is
markedly increased by nitrogen (see, e.g., Refs. 15)l8)),
and this effect is used in the production of high strength
stainless wire and ropes. Thehigh density of deformation
twins and the planar slip can be considered as strengthen-
ing factors, the latter preventing the development of a
cellular dislocation substructure which would cause a
comparatively low cold work hardening. The refinement
of twins at heavy deformation and the formation of the
new small twins within the already twinned volume
(second order twinningi9)) additionally contribute to
strength. Oneof the unsolved problems in the production
of cold drawn wire from austenitic steels is the low
plasticity during the torsion tests under loading, as
comparedto the high plasticity of wire niade of carbon
pearlitic steels.
The interaction between nitrogen atoms and disloca-
tions in austenite is stronger than that for carbon
atoms. 20,2 1) This effect is of practical significance because
strain ageing is used in the production of cold worked
springs etc.
Creeprate has beenshownto decrease with the increase
of nitrogen content in austenitic steels.22~25) Unlike
nitrogen-free austenitic steels, no subgrains were ob-
served after the high-temperature creep of nitrogen
austenite.25) Substitution of carbon with nitrogen results
in small Cr2NandFe2Moprecipitations instead of coarse
M23C6carbide particles.24,25) Solid solution strength-
ening by nitrogen is found to control the creep rate. A
remarkable feature is that, contrary to carbon, nitrogen
increases rupture strength without reducing rupture
ductility. The increase of the climb vector during creep
due to the large spacing betweenplanar dislocation arrays
is considered by Owen26)as the primary reason for the
reduction of creep rate by nitrogen. This idea is consistent
with the experimental data by Howell et al.27) indicating
that dislocation multipoles are formed during primary
creep on {1 11} planes in 316L steel containing 0.1 o/o of
Nand remain confined to these planes during steady-
state creep due to the pinning effect of nitrogen atoms.
Onehas to add that full or partialreplacement of
carbon by nitrogen in 12~/o Cr martensitic steels decreases
the creep rate too (see, e.g., Refs. 5), 28)). Thesmall size
of nitrides andcarbonitrides and their high temperature-
time stability as comparedto carbides causes the higher
739
36 (1996), No, 7
hot strength and creep resistance of nitrogen ferritic and
martensitic steels.
Nitrogen's beneficial effect on the low cycle fatigue of
austenitic steels is shownto result mainly from the planar
slip.29
~
31)
The replacement of carbon by nitrogen in 12010 Cr
martensitic steels increases the values of the cycle stress
but reduces fatigue strength because of the nitrogen
caused modification of the substructure: the absence of
soft ferrite domains which could accomodate plastic
strain.32)
Goodwear resistance is conferred by nitrogen in
austenitic steels as a result of high work hardening. It
exceeds the wear resistance of Hadfield's manganesesteel
and high carbon martensitic steels and the difference is
greater for the rounded than for the sharp abrasive.33)
Solid state nitriding of austenitic steels34) and surface
nitrogen implantation35) jmprove wear behaviour. It is
also shown35) that, despite the inhibition of the strain-
induced martensitic transformation in the 304 type steel,
nitrogen implantation increases wear resistance, which
provides evidence for the decisive role of austenite work
hardening on the wear behaviour. Surface nitriding of
austenitic steels at high temperatures is revealed to be an
effective tool to reach excellent wearproperties as well as
good cavitation resistance.36,37)
It is noteworthy that high nitrogen austenitic steels
produced by powdermetallurgy showless abrasive wear
than molten and forged steels, although HIPped ma-
terials are less ductile.38)
Corrosion properties of austenitic, martensitic and
duplex steels are strongly affected by nitrogen. Nitrogen
dissolved in austenite has no significant effect on general
corrosion in acids, but markedly improves resistance to
intergranular, pitting and crevice corrosions and stress
corrosion cracking (SCC).
The resistance of nitrogen austenitic steels to inter-
granular corrosion is clearly concerned with the retard-
ing of sensitization.39 ~41) Alloying by nitrogen prevents
precipitation and the growth of chromium carbides,
and increases the concentration of chromiumat grain
boundaries.
Nitrogen increases the pitting potential of austenitic
steels, particularly in the concentration range between
O, 15and O.3 o/o of N, and prevents crevice corrosion (see,
e.g., Refs. 42)~4)). A possible explanation is based on
the considerable rise of the pHwithin pits and crevices
due to the formation of ammoniumions, which facilitates
re passivation.
There is someuncertainty in the evaluations of nitro-
gen effect on the SCCof austenitic steels (see, e.g., Refs.
42), 45)~~8)). In water and nitrate solutions the CrMn
nitrogen austenitic steel is superior to the CrMncarbon
steel,48) It is also shownthat nitrogen assists the tran-
sition from the intergranular to the transgranular SCC.
Thepositive effect of nitrogen on SCCresistance can not
be derived from the polarization curves. Onecan sup-
pose that nitrogen effects on SCCand hydrogen em-
brittlement (HE) are similar in their nature.
Nitrogen improves the resistance of austenitic steels
to the HE.49~51) This effect in unstable 304 type steels
C 1996 ISIJ
ISIJ International, Vol.
is unambiguously attributed to the increase of austenite
stability thanks to nitrogen.49) Such an explanation is
not valid for stable austenitic steels which do not undergo
phase transformations at any cold work or cooling. If
one accepts that hydrogen-induced reversible y~8H
transformation is responsible for hydrogen failure (see,
e,g., Refs. 52), 53)), the data of nitrogen's suppressing
effect on eH formation54) can be useful for understanding
the improved mechanical behaviour of nitrogen austenit-
ic steels in the hydrogen environment. It seems,however,
that the coincidence of the cracking surface during
hydrogen-induced fracture with the eHhabit plane {II I}y,
which was revealed in Ref. 52), is not sufficient to draw
conclusions about the nature of the HEin austenitic
steels. Hydrogentransport in solid solution maybe more
slgnificant for cracking and the effect of nitrogen on the
hydrogen permeability of austenite has to be taken into
account.
36 (1996). No. 7
2.5
O
'E 2.0
p
> I .5RE
"'~o 1.0
~L
a 0.5
0.0
e
-
DFX 10~2
e o
'----
-- --o -
A- A
A c
e
3. WhichBasic Crystal Lattice Properties Are Affected
by Nitrogen in lron-based Solid Solutions?
Electronic Structure. The difference between nitro-
gen and carbon effects in austenite (martensite) can not
be understood in terms of elastic continuum because
nitrogen and carbon atoms cause the sameelastic dis-
tortions in solid solutions. That is why the study of
electron properties is expected to shed light on the
physical cause of nitrogen's ability to improve many
properties. The electron state density on the Fermi sur-
face, DF, i.e., the concentration of conduction electrons,
and its dependenceon nitrogen concentration seemsto
provide useful information on the chemical interstitial-
substitutional interaction in austenite.
Conduction electron spin resonance (CESR)wasused
in Refs. 55)-58) to study the effects of nitrogen and
carbon on the states of conduction electrons in austenite
and the interaction between conduction electrons and
localized spin moments, in other words, between inter-
stitial and substitutional solutes.
The values of the state density on the Fermi surface,
DF, wereobtained from the detailed analysis of the shape
of CESRsignal in metal. Aprocedure of the comparison
between the theory of the line shape and the signals
measuredis described in detail elsewhere,s6) Wedid not
do any approximations in CESRtheory and took into
account the contributions into the resonance absorption
from s- and d-electrons, the depth of the skin-1ayer, 8,
and the length of diffusion, ~*, of the conduction electron
from the skin-1ayer into the bulk for the given relaxa-
tion time. The temperature dependencesof an integral
intensity of CESR.I(T), a width of the resonance line,
AH(T), and a parameter of the signal asymmetry, R(T),
were calculated and then, from the comparison with the
experimental curves I(T). AH(T) and R(T), the values
of 8, 8. and the Pauli magnetic susceptibility of s-
electrons, X*, were obtained. From the formula x*=
l/292,t~Dp, relating the magnetic susceptibility of con-
duction electrons and their state density on the Fermi
surface (g is a factor of spectroscopic splitting equal
-
2
for conduction electrons, pB is the Bohr magneton), the
@1996 ISIJ 740
0.5 1.O 3.01.5 2.0 2.5
Nj' at. olo
Fig. l. Electron state density on the Fermi surface vs. nitrogen
(O) and carbon (A) concentrations in Crl 8Nil6Mnlo
austenite. i=N, C. e -data for the samealloy but
with 6"/o of Mn, 100 times reduced in the values.
values of DFWereobtained and presented in Fig. I as
functions of nitrogen and carbon concentrations in
Fe56Crl8Nil6Mnlo austenites. Each point in Fig, l
corresponds to one of the alloys studied and the pro-
cedure of measurementsI(T), AH(T) and R(T) and
comparison with the theory were performed independ-
ently for each sample.
The important conclusion derived from these data is
that nitrogen significantly increases the concentration of
conduction electrons in austenitic steel as comparedto
carbon. In terms of interatomic interaction it meansthat
nitrogen increases the metallic componentof interatomic
bonds. Oneof the possible practical consequencesis good
ductility and fracture toughness of nitrogen austenitic
steels at low temperatures. The concentration depen-
dence of DFis not monotonousfor nitrogen and has a
maximumat cN=1.5 ato/o(about 0.4masso/o of N). The
decrease of the conduction electrons concentration at a
higher nitrogen content, i.e., the increase of the cova-
lent contribution into the interatomic bond, is consistent
with the observed cases of brittle fracture in high nitro-
gen austenitic steels.59,60) Based on these data, we can
suppose that the optimal alloying of construction
austenitic steels by nitrogen, particularly for cryogenic
applications, Iies in the concentration range of
O.3-0.5 masso/o of N.
Note that the experimental data for Fe58Cr20Ni16Mn6
austenite containing less manganese(black circles in Fig.
l) fit the curve for Fe56Crl8Nil6Mnlo alloy after being
decreased by 100 times. According to this result, the
alloying by manganeseenhancesthe covalent interatomic
bond in FCCiron-based alloys. Thus, CESRmethod
seemsto provide an opportunity to evaluate the effect
of the substitutional solutes on the interatomic bond in
austenitic steels.
The interstitial-substitutional interaction wasstudied
on the basis of the data on magnetic susceptibility derived
from CESRspectra.58) The contributions to magnetic
susceptibility from three paramagnetic subsystems were
obtained: (1) X,o is the magnetic susceptibility of the free
ISIJ International. Vol. 36 (1 996), No. 7
electron systemwhich is independent of the temperature,
(2) Xdl(T)
=
CilT-the magnetic susceptibility of isolat-
ed localized d-electrons, which changes with tempera-
ture according to the Curie-Weiss law, and (3) Xd2=
C2(cotanh(O/T)
-
T/O)-the magnetic susceptibility of
superparamagnetic clusters, i.e., the clusters of substi-
tutional solutes, which obeys the Langevin law (O is
the magnetic energy of the cluster in the external field
Hand is proportional to the numberof paramagnetic
atoms in the clusters). Figure 2 shows, as an example,
the temperature dependenceof magnetlc susceptibility
for Fe56Crl8Nil6Mnlo austenites alloyed by 2,17ato/o
of nitrogen or I.84 ato/o of carbon, and the contribu-
tions of the three above mentioned electron subsystems
to magnetic susceptibility. Thenonlinearity of the curves
in Fig. 2proves the existence of clusters in the alloys
as, otherwise, X' 1(T) has to be a linear function of
temperature in the temperature range where xdl
Drastic difference is observed in the behaviour of X' 1(T)
for nitrogen and carbon austenites. The results of the
fitting of experimental data in Fig. 2are shownin Table I
.
The Ovalues in carbon austenite are several times
higher than in nitrogen austenite, i.e., nitrogen assists a
morehomogeneousdistribution of substitutional solutes
in austenite as comparedto carbon. Fromthe relation
O=M•HlkB, where Mis the cluster magnetic moment,H is the external magnetic field (H=0.3T in this ex-
periment), kB is the Boltzmann constant, one can ob-
tain the numberof atoms in the clusters, N~;0/2, and
their linear size, of about 2nm. Of course, the super-
paramagnetic clusters are not "pure" precipitations of
substitutional solute atomsand include someconcentra-
~~
2
tion of iron atoms too.
These data show that carbon in austenite assists the
clustering of substitutional solutes, while nitrogen en-
sures their morehomogeneousdistribution.
As follows from Table I
,
the Pauli magnetic sus-
ceptibility X,o is about ten times higher in nitrogen
austenite, while the Curie-Weiss magnetic susceptibility,
Xdl, is about ten times higher in carbon austenite. This
result meansthat the growth of the conduction electron
concentration in nitrogen austenite is caused by the
decrease of the localized d-electron density. Therefore,
again we can state that the replacement of carbon by
nitrogen decreases the covalent andenhancesthe metallic
componentsof the interatomic bond.
Interatomic Interactions. The energy of interaction
of about 0.05~0.1 eV between interstitial atoms oc-
cupying the neighbouring interstickes in the carbon
and nitrogen austenites was usually derived as a fittlng
parameter while comparing the theoretical equations for
thermodynamical activity with the data of the experi-
mental measurements,and the conclusion about a strong
repulsion of the carbon atoms preventing formation of
90' C-Cconfigurations (see Fig. 3) has been madebased
on the thermodynamical studies (see, e.g., Refs. 61)-63)).
This result is inconsistent with the results of M6ssbauer
studies on carbon distribution in FeNiC64,65) and
FeMnC66)austenites, which is a sign of inadequate
analysis of the solid solution accounting only for the
interaction in the Ist coordination sphere of the inter-
stickes sublattice. The occurrence of 180' N-N con-
1
o
-1
Table
2N.,
2.1 7at.oloN
IN
.eN
4at.o/oC
Fig. 2.
l. Magnetic characteristics of electron subsystems in
carbon and nitrogen Fe56Crl8Nil6Mnlo austen-
ites: (~ is proportional to the size of the clusters,
X*o, Xdlmagnetic susceptibilities of s- and d-
electron subsystemswhich are proportional to the
concentrations of free s-electrons and isolated
10calized c!-electrons.
Allo y (? (K) 10 X*o 105Xdl
Fe56Crl8Nil6Mnlo (I.58 ato/o N)
Fes6Crl 8Ni 16MnIo (I.44 ato/o C) 140
5.4IO O
715 80.00.8
20 40 60 80 100120140160180200
()temperatwe K
Temperaturedependenceof the relative magnetic sus-
=
x*/xd I = oc IX*o +x*oXd1
1+oe2Xd2X*oXd11ceptibiiity x, 1
for Crl8Nil6Mnlo austenites alloyed by nitrogen or
carbon. Three contributions from free electrons (lN),
isolated ci-electrons (2N) and superparamagnetic
clusters (3N) are shownby the dashed lines for the
nitrogen austenite. Deviation from the linear relation
is caused by the magnetic clusters of substitutional
solutes.
Fig. 3.
L
(
(
O : •-(). :
.•1F
o'edf~---
•
' o
-:)
.
---C
Fe2,9(
2,18
(])
90' and 180' ii atomic pairs in FCCiron-based
austenite (i=C, N).
741 C 1996 ISIJ
ISIJ International, Vol. 36 (1996), No. 7
figurations (Fig. 3) in FeNaustenite and their absence
(within the limits of M6ssbauerspectra resolution) in
FeCaustenite provided evidence for interactions in 2nd
and, probably, 3rd coordination spheres to be important
for the successful description of interstitial solid so-
lutions.
An interesting opportunity to derive the values of
interaction energies arises from the comparison of
M6ssbauerdata with the results of the Monte Carlo
simulation.67 ~69) M6ssbauerspectra of the carbon and
nitrogen austenites consist of the componentsbelonging
to the iron atoms with interstitial atoms as nearest
neighbours and those which have no interstitials in the
first coordination sphere. The areas under the spectra
components, being corrected on self-absorption of y
quanta, i.e., on the finite thickness of the samples, are
proportional to the fractions of different kinds of the
iron atoms, which allows one to get a knowledge of
distribution of carbon and nitrogen in the FCCiron.
Using MonteCarlo simulation of atomic distribution, it
is possible to determine the values of the interatomic
interaction energies which could cause the distribution
of the interstitials derived from M6ssbauerexperiment.
Shownin Fig. 4are the areas of possible values for the
N-NandC-Cinteractions energies in the first andsecond
coordination spheres satisfying the experimental data
available.64 ~68,70) Astrong repulsion of nitrogen atoms
in the nearest neighbouring interstickes and their weak
interaction as second neighbours corresponds to abund-
ances of the different types of iron sites obtained by
meansof M6ssbauerspectroscopy. The opposite situa-
tion, a weak interaction for interstitials in the first
coordination sphere andastrong repulsion for the second
sphere, takes place in FeCaustenite.
Nitrogen and Carbon Distribution. Oneof the first
important conclusions concerning the distribution of
0.30
0.25
0.20
>
,D
N 0.15
~
0.10
0.05
0.00
c
K. Odaet al 68)
A. Sozinov69)
N
o.o 0.1 0.2 0.3 0.4 0.5 o.e
W1
'
eV
Fig. 4. The values of i-i interaction energies in the first (wl)
and second (w2) coordination spheres for Fe-N and
Fe-C binary austenites satisfying M6ssbauer data
available.
nitrogen atoms in austenite wasmadeby Foct71) based
on the results of M6ssbauerstudies. It was the first time
when180' N-Npairs wereproven to exist and the Fe4N*
(x l) structure of the binary FeNaustenite was pre-
sumed. Suyazovet al.72) observed superstructure reflec-
tions in the electron diffraction patterns of FeNaus-
tenite and the short range ordering of nitrogen atoms
during the low temperature ageing of freshly formed FeN
martensite. Theconclusion about the clustering of carbon
atoms in FeNiCandFeMnCaustenites wasderived from
the data of M6ssbauerspectroscopy.6s,66) Theclustering
of carbon atomsand the short range ordering of nitrogen
atoms during the early stages of ageing in binary FeC
andFeNmartensites wasrevealed by Mittemejier et al.73)
using the dilatometric and calorimetric measurernents.
The analysis of available experimental data allows to
consider the short range ordering of nitrogen atomsand
the clustering of carbon atoms as features commonfor
both FCCand BCCiron-based solid solutions.
s-s, i-i and i-s interactions have to be taken into
account in order to analyse the atomic distribution in
carbon and nitrogen austenitic steels. The distribution
of solute atoms in substitutional solid solutions is
determined by s-s interaction. FeCr and FeMnFCC
binary alloys are prone to clustering, i,e., short range
decomposition74,75) because the Cr-Cr and Mn-Mn
interactions are stronger than those of Fe-Cr and
Fe-Mn. Spinodal decomposition with the formation of
nickel-rich submicroregions takes place in FeNi austeni-
tic alloys,76) although short range atomic ordering, i,e.,
Fe-Ni interaction prevails.
Considering the above mentioned difference in the
distribution of nitrogen and carbon in austenite, onecan
suggest the following mechanismof nitrogen's favourable
effect on the homogeneity of iron-based solid solution:
the ordering of nitrogen atoms prevents the clustering
of substitutional solutes by meansof strong i-s inter-
action. The clustering of carbon atoms enhances the
tendency of short range decomposition which exists in
iron-based CrNiMnsubstitutional solid solutions.
These peculiarities of atomic distribution in nitrogen
and carbon austenltes are the reason for the different
thermodynamical stabilities of interstitial solid solutions,
which results in different resistance to precipitation,
corrosion properties etc.
C 1996 ISIJ 742
4. Precipitation during the Tempering of Nitrogen
Martensite
Alloying by nitrogen improves the properties of
martensite, which has been used to develop newgrades
of corrosion-resistant martensitic steels (see, e.g., Refs.
77), 78)). Twomain peculiarities characterize the change
in the hardness of nitrogen and carbon martensites after
tempering: in agedand low tempered states the hardness
of the carbon martensite is higher, but in the temperature
range of "second hardening" (450-550'C) the hardness
of mtrogen martensrte prevails (see, e.g., Ref. 79)). The
first impression of the different precipitation phenomena
in nitrogen and carbon martensites is acquired by
comparing the dilatometric data (Fig. 5). Precipitation
ISIJ International, Vol. 36 (1996). No. 7
40
30
-20E
:L
-
Io
(D
O)
C050JCO 40
J~
~O)C 30
(DJ 20
10
O
l
/y r
/~/
I~A
l
3
V
3
2(RA~
N
200
Fig. 5.
400 600 800 1OOO
Temperature. K
Dilatometric curves of the Fe-15Cr-lMo-0.6C and
Fe-1 5Cr-lMo-0.6N (wto/*) martensites after quench-
ing from I 100'C. The heating rate is 2.5 K/min The
Ist, 2nd and 3rd stages of tempering are indicated,
which corresponds to precipitation of the metastable
e-carbide/nitride, decomposition of the retained aus-
tenite and precipitation of stable interstitial phases,
respectively. The third stage of transformation in the
nitrogen martensite is expandedover the broad tem-
perature range. Decomposition of the retained aus-
tenite (RA) in Cmartensite is shifted, as compared
to unalloyed carbon martensitic steels, to temperatures
higher than that of the 3rd stage. In Nmartensite RA
decomposesduring cooling and only partially (see the
data in detail in Refs. 79), 80)).
of carbides or nitrides is accompaniedwith a contraction
while decomposition of the retained austenite causes
dilation of the samples. It is clearly seen in Fig. 5that
the second temperature stage of contraction (so-called
3rd transformation during tempering), which is con-
cerned with formation of the stable chromiumcarbides
or nitrides, is shifted in the nitrogen martensite to higher
temperatures and expandedover the broad temperature
range. It provides an evidence for a delayed nucleation
and growth of the precipitates. Decomposition of the
retained austenite occurs during heating in the carbon
quenchedalloy while the nitrogen retained austenite is
decomposedduring cooling of the temperedsamples(and
only partially according to M6ssbauer data80)). The
expandedtemperature ranges of precipitation in nitrogen
and nitrogen +carbon martensites, as compared to
carbon martensite and a higher stability of the retained
austenite are signs of the higher resistance of the nitrogen
solid solution to tempering.
The following sequence of precipitations was dis-
covered by meansof TEMstudies79,8 1) for the tempering
(2 hours) of CrMo15, I (O.6C), (0.6N) and (0.29C+
0.35N) martensites quenched after solution treatment
at 1100'C:
743
CrMo15 1 (O.6C) undissolved Cr23C6globules exist
in as-quenched state
200'C 300'C 600'C
hexagonal orthorhombic hexagonal
e-carbide cementrte' (Cr,Fe)7C3
(Fe.Cr)3C
CrMo15 1(0.6N)
200'C 300-650'C 650'C
hexagonal orthorhombic orthorhombic
e-nitride (Fe,Cr)2N (Fe,Cr)2N
+hexagonal Cr2N
CrMo15 1(O.29C+0.35N)
-
undissolved cementite glob-
ules exist in as-quenched state
200'C 500-600'C 650'C
hexagonal orthorhombic orthorhombic
e-nitride cementite 'cemennte
+e-carbide +cubic (Cr,Fe)N +hexagonal Cr2N
As follows from the presented data, the solution
treatment of Cand (C +N) martensites at I 100'C does
not result in complete dissolution of carbon. Theabsence
of cubic CrNas a phase preceding the precipitation of
hexagonal Cr2Nis a feature of tempering of carbon-Iess
Nmartensite.
Tempering of (C+N) chromium martensite is
characterized by separate precipitation of carbides and
nitrides, which wasdiscovered earlier by Mittemejier et
al.82) for e-carbide and oc"-Fel6N2 in aged unalloyed
FeCNmartensite. This result makesit possible to as-
sumea need for someshort range redistribution of car-
bon and nitrogen atoms in (C+N) solid solutions
before precipitation, which can be the reason for the
higher thermodynamical stability of (C +N) austenite
and martensite.80.8 1)
According to TEMstudies, chromiumnitrides have
a significantly smallcr size as comparedto carbides in
the whole temperature range of tempering, which ex-
plains their larger contribution to hardness. Short-
range ordering in aged nitrogen martensite and the sim-
ilarity of the nearest neighbourhood for nitrogen atoms
in solid solution and low-temperature precipitations
cause smaller crystal lattice distortions at the coherent
matrix-precipitation interface and, as a result, the lesser
hardness andstrength of agedand low-tempered nitrogen
martensites as comparedto carbon ones.
It seems reasonable to suggest a relation between
atomic distribution in nitrogen and carbon martens-
ites and their different stability to precipitations. The
M6ssbauerstudy of short-range atomic order helps to
clarify the peculiarities of martensite decomposition
during tempering. Figure 6showsthe M6ssbauerspectra
of C, N, and (C +N) chromiummartensites after tem-
pering at 650'C. The fourcomponents in the spectra
denoted as Feo,
' ' '
,
Fe3 belong to the iron atomshaving
no, one, two and three Cr atoms as nearest neighbours.
Visual comparison already shows that the Feo cornpo-
nent is smaller in the Nmartensite, than in the Cmar-
tensite and is the smallest in the (C +N) one. In other
words, the numberof Fe-Cr interatomic bonds in high-
tempered Fe-Cr ce-solid solution increases in the direc-
@1996 ISIJ
ISIJ International, Vol. 36 (1996), No. 7
~
C:O
CD
~2
'E
CO
C:
CULH
4 5 6
VeIOCity, mm/s
Fig. 6. M6ssbauer spectra of Crl5Mol (0.6C), Crl5Mol
(0.6NO) and Crl5Mol (0.29C+0.35N) martensitic
steels after quenching from I 100'C and tempering at
650*C for 2h.
Fe3 Fe2 Fel Feo
i oo
~;
95
(a)
Fe3 Fe Fel Feo
1oo
~~~ l
95
(b)
Fe3 Fe2 Fe Feo
1oo ;~::
95 '. ,'*\
•
90
tion C->N-~(C+N)martensite. Onecan explain this
tendency towards homogeneouschromiumdistribution
considering the inheritance by martensite of the chromi-
umdistribution in austenite and, therefore, the correla-
tion between short-range ordering of nitrogen atoms
and chromiumdistribution.
Thus, as it was the case for austenite, short range
atomic ordering in the nitrogen martensite, is responsible
for delayed precipitation from the solid solution, a fine
precipitation structure and good mechanical and elec-
trochemical properties. It is clear that a smaller depletion
of the matrix in chromiumduring nitride precipitation
is the reason for the higher corrosion resistance of
nitrogen martensitic steels.
5. Summary
The nature of favourable effects of nitrogen on the
mechanical and electrochemical properties of steel lies
in the enhancementof the metallic componentof the
interatomic bond in solid solutions and the tendency of
nitrogen atoms towards short range ordering in contrast
to prevailing covalent bonds due to carbon in austenite
and the clustering of carbon atoms. Strong interstitial-
substitutional interaction causes, to a certain degree, the
distribution of substitutional solutes to follow that of
interstitials and this results in higher chemical homo-
geneity of nitrogen austenitic steels and a fine precipita-
tion structure of nitrogen martensitic steels.
The considerable contribution of nitrogen to the
strengthening of austenitic steels can be also traced from
short range ordering in solid solution, though effect of
nitrogen on the individual properties of dislocations and
interaction betweennitrogen atomsanddislocations have
to be studied in detail for a morecomplete description
of the mechanical properties.
Mixed nitrogen +carbon iron-based solid solutions
possess certain peculiarities of atomic distribution result-
ing in their enhancedthermodynamical stability and thus
bear practical significance for the design of nitrogen
steels.
Acknowledgments
Themain physical results discussed in this article were
obtained within the framework of INTASproject 93-
2471
.
The author is deeply grateful to Professor J. Foct
and Professor H. Berns for the helpful discussion and
valuable remarks.
l)
2)
3)
4)
5)
6)
7)
8)
9)
lO)
ll)
l2)
l3)
l4)
15)
l6)
l7)
18)
l9)
20)
21)
REFERENCES
High Nitrogen Steels, Proc. of Ist Int. Conf. High Nitrogen Steels,
ed. by J. Foct and A. Hendry, Inst. Met., London, (1989).
High Nitrogen Steels, Proc, of 2nd Int. Conf. High Nitrogen
Steels, ed, by G. Stein andH. Witulski, Stahl &Eisen, Dbsseldorf,(1990) .
High Nitrogen Steels, Proc, of 3rd Int. Conf. High Nitrogen
Steels, ed. by V. G. Gavriljuk and V. M. Nadutov, Inst. Met.
Phys., (1993).
J. Frehser and Ch. Kubisch: Berg Htittenmiinn. Monatsh., 108,
369.
M. O. Speidel: High Nitrogen Steels, Proc. of Ist Int. Conf. High
Nitrogen Steels, ed. by J. Foct and A. Hendry, Inst. Met., London,
(1989), 92.
H, Nakajima, K. Yoshida and S. Shimamoto:ISIJ 1,It., 30 (1990),
567.
R. P Reedand N. J. Simon: Adv. in Cryogenic Engineering, Vol.
30, PlenumPubl. Corp., (1984), 127.
M. L. G. Byrnes, M. Grujicic a~rd W. S. Owen:Acta Meta!1., 35
(1987), 1853.
M. Grujicic, J.-O. Nilsson, W.S. Owenand T. Thorwaldsson:
High Nitrogen Steels, Proc. of Ist Int. Conf. High Nitrogen Steels,
ed, by J. Foct and A. Hendry, Inst. Met., London, (1989), 151.
J. Sassen, A. J. Garrat-Reed and W. S. Owen: High Nitrogen
Steels, Proc. of Ist Int. Conf. High Nitrogen Steels, ed. by J. Foct
and A. Hendry, Inst. Met., London, (1989), 159.
B. Obst and A, Nyilas: Mater. Sci. Eng., A137(1992), 141,
L. Norstr6m: Mel. Sci., 17 (i977), 208.
E. Werner: Mater. Sci. Eng., AIOI (1988), 93.
P. J. Uggowitzer and M. Harzenmoser: High Nitrogen Steels.
Proc. of Ist Int. Conf. High Nitrogen Steels, ed. by J. Foct and
A. Hendry, Inst. Met., London, (1989), 174.
P. J. Uggowitzer and M. O. Speidel: High Nitrogen Steels, Proc.
of 2nd Int. Conf. High Nitrogen Steels, ed, by G. Stein and H.
Witulski, Stahl &Eisen, Dtisseldorf, (1990), 156.
V. G. Gavriljuk, V. A. Duz' and S. P. Yefimenko: High Nitrogen
Steels, Proc, of 2nd Int. Conf. High Nirogen Steels, ed by G.
Stein and H. Witulski, Stahl &Eisen, Dtisseldorf, (1990), 100.
N. D. Aphanas'ev, V. G. Gavriljuk, V. A. Duz', S. P. Yefimenko
and V. L. Svechnikov: Phys. Me!. Metallogr., 8(1990), 121.
N. Paulus, R. Magdowskiand M. O. Speidel: High Nitrogen
Steels, Proc, of 3rd Int. Conf. High Nitrogen Steels, ed. by V.
G Gavriljuk and V. M. Nadutov, Inst. Met.. Phys., (1993), 394.
P. Mtillner, C, Solenthaler and M. O. Speidel: Acta Metall.
Mater., 42 (1994), 1727.
V. G. Gavriljuk, V. A. Duz*, S. P. Yefimenko and O. G.
Kvasnewski: Phys. Me!. Meta!!og,'., 64 (1987), I132.
V. G. Gavriljuk, V. A. Duz' and S. P. Yefimenko: High Nitrogen
Steels, Proc, of Ist Int. Conf. High Nitrogen Steels, ed, by J. Foct
and A. Hendry, Inst. Met.. London, (1989), 447.
C 1996 ISIJ 744
ISIJ International, Vol. 36 (1 996), No. 7
22)
23)
24)
25)
26)
27)
28)
29)
30)
31)
32)
33)
34)
35)
36)
37)
38)
39)
40)
41)
42)
43)
44)
45)
46)
47)
48)
49)
5o)
M. Yu and R. Sandstr6m: Sca,Id. J. Metal!., 17 (1988), 156.
T. Matsuo, N. Morioka, S. Kaise, M. Kikuchi and R Tanaka:
High Nitrogen Steels. Proc, of Ist. Conf. High Nitrogen Steels,
ed. by J. Fcot and A. Hendry, Inst. Met., London, (1989), 213.
T. Nakazawa,H. Abo, N. Tanino, H Komatsu, T. Nishida and
M. Tashimo: High Nitrogen Steels, Proc of Ist Int. Conf. High
Nitrogen Steels, ed. by J Foct andA. Hendry, Inst. Met., London,
(1989), 218.
T. Matsuo, N. Fujita and M. Kikuchi: High Nitrogen Steels,
Proc. of 2nd Int. Conf. High Nitrogen Steels, ed. by G. Stein and
H. Witulski, Stahl &Eisen, Dtisseldorf, (1990), 182.
W. S. Owen:High Nitrogen Steels, Proc, of 2nd Int, Conf. Hlgh
Nitrogen Steels, ed. by G. Stein and H. Witulski, Stahl &Eisen,
Dtisseldorf, (1990), 42.
P. R. Howell, J. O, Nilsson, A. Horsewell and G. L. Dunlop: J.
Mate,'. Sci., 16 (1981). 2860.
H. Berns and F. Krafft: High Nitrogen Steels, Proc, of Ist Int.
Conf. High Nitrogen Steels, ed. by J. Foct and A. Hendry. Inst.
Met., London, (1989), 169.
S. Degalaix, J. I. Dickson and J. Foct: High Nitrogen Steels,
Proc, of Ist Int. Conf. High Nitrogen Steels, ed, by J. Foct and
A. Hendry, Inst. Met., London, (1989), 380.
R. Taillard and J. Foct: High Nitrogen Steels, Proc, of Ist Int.
Conf. High Nitrogen Steels, ed. by J. Fcot and A. Hendry, Inst.
Met., London, (1989), 387.
J.-B. Vogt. J. Foct and J.-O. Nilsson: Scand. J. Meta!!., 19 (1990),
273
.
J.-B. Vogt, C. Bigeon and J. Foct: Z. Metcl!!kd., 85 (1994), 92.
U. R. Lenel and B. R. Knott: Meta!l. Trans., 18A (1987), 847.
B. F. Campillo lllanes and A. D. Sarkar: J. Tribology, (Trans.
ASME),108 (1986), 334.
A. Cavalleri, L. Guzman,P. M. Ossi and I. Rossi: Scr. Meta!l.,
20 (1986), 37.
H. Berns: Stain!ess Stee! Eu,'ope, 4(1994), 56.
S. Siebert: Randaufsticken nichtrostender Stahle, Doctor thesis,Ruhr-Universitzit Bochum,(1994).
J. Romu, J. Tervo, H. Hanninen and J. Liimatainen: High
Nitrogen Steels, Proc. of 3rd Int. Conf. High Nitrogen Steels, ed.
by V. G. Gavriljuk and V. M, Nadutov. Inst. Met. Phys., (1993),
372.
T. A. Mozhi, W. A. T. Clark, K, Nishimoto, W. B. Johnsonand
D. D. Macdonald: Corrosion, 41 (1985), 555.
T. A. Mozhi, H. S. Betrabet, V. Jagannathan, B. E. Wilde and
W. A. T. Clark: Sc,'. Meta!!., 20 (1986), 723.
H. S. Betrabet, K. Nishimoto, B. E. Wilde and W. A. T. Clark:
Corrosion, 43 (1987), 77.
J. E. Truman: High Nitrogen Steels, Proc. of Ist Int. Conf. High
Nitrogen Steels, ed. by J. Foct andA. Hendry, Inst. Met., London,
(1989), 225.
R. F. A. Jargelius and T. Wallin: Proc, of lOth Scandinavian
Corrosion Cong., Stockholm, (1986), 161
.
J. R. Kearns: NewDevelopmentsin Stainless Steel Technology,
A. S. M., Detroit, (1985), I17.
M. Kowadaand T. Kudo: T,'ans. Jpn. Inst. Met., 16 (1975), 358.
T. A. Mozhi, K. Nishimoto, B. E. Wilde and W. A. T. Clark:
Co,'rosion, 42 (1986), 197.
Wen-TaTsai, B. Reynders, M. Stratmann and H. J. Grabke:
Co,','os. Sci., 34 (1993), 1647.
R. M. MagdowskiandM. O. Speidel: High Nitrogen Steels, Proc.
of Ist Int. Conf. High Nitrogen Steels, ed. by J. Foct and A.
Hendry, Inst. Met., London, (1989), 251.
S.-P. Hannula, H. Hanninen and S. T~htinen: Meta!!. T,'ans.,
15A (1984), 2205.
V. G. Gavriljuk and S. P. Yefimenko: High Nitrogen Steels, Proc.
of 2nd Int. Conf. High Nitrogen Steels, ed. by G, Stein and H.
51)
52)
53)
54)
55)
56)
57)
58)
59)
60)
61)
62)
63)
64)
65)
66)
67)
68)
69)
70)
71)
72)
73)
74)
75)
76)
77)
78)
79)
80)
81)
82)
Witulski, Stahl & Eisen, Dtisseldorf, (1990), I I.
P. Rozenak: J. Mater. Sci., 25 (1990), 2532.
A. Inoue. Y. Hosoyaand T. Matsumoto: T,'ans. Iron Steel Inst.
Jpn., 19 (1979), 171.
N. Narita, C. J. Altstetter and H. K. Birnbaum: Meta!l. Trans.,
13A (1982), 1355.
V. G. Gavriljuk, H. Hanninen, A. S. TereshchenkoandK. Ullako:
Scr. Metall. Mater., 28 (1993), 247.
N. P. Baran, V. G. Gavriljuk. V. M. Maksimenko,Ye. E Smouk.
S, Yu. Smoukand B. D. Shanina: Solid State Commun.,81
(1992), 55.
V. G. Gavriljuk, S. P. Yefimenko, Ye. E. Smouk,S. Yu. Smouk,
B. D. Shanina, N. P. Baran and V. M. Maksimenko:Pllys. Rev.
B, 48 (1993), 3224.
B. D. Shanina. S. P. Kolesnik. A. A. Konchyts, V. G. Gavriljuk,
S. Yu. Smoukand A. V. Tarasenko: Solid Sta!e Commun.,90
(1994), 109.
B. D. Shanina. V. G. Gavriljuk, A. A. Konchyts, S. P. Kolesnik
and A. V. Tarasenko: Phys. Status Solidi (a), 149 (1995), 71 l,
J. Foct and N. Akdut: Sc,'. Metal!. Mater., 29 (1993), 153.
P. Mtillner, C. Solenthaller, P. J. Uggowitzer and M. O. Speidel:
Acta Meta!1. Maier., 42 (1994), 221 l.
P. T. Gallagher, J. A. Lambert and W. A. Oates: T,'ans Meta!!.
Soc. AlME, 245 (1969), 887.
K. Alex and R. B. McLellan: Acta Meta!l., 19 (1971), 439.
G. E. Murchand R. J. Thorn: Acta Metall., 27 (1979), 201.
V. G. Gavriljuk and V.M. Nadutov: Phys. Met. Metallog,'., 55
(1983), 520,
V. N. Bugaev, V. G. Gavriljuk. V. M. Nadutov and V. A.
Tatarenko: Acta Meta!l., 31 (1983), 407.
V. N. Bugaev. V. G. Gavriljuk. V. M. Nadutov and V. A.
Tatarenko: Rep. Acad. Sci. USSR,288 (1986), 362,
K. Oda,H. Fujimura, K. MaeandH. Ino: Hype,fine Inte,'actions,
69 (1991), 533.
K. Oda, H. Fujimura and H. Ino: J. Phys., Condens. Mattei', 6
(1994), 679.
A. L. Sozinov and A. G. Balanyuk: Private communication
Institute for Metal Physics. Kiev, (1994).
V. G. Gavriljuk. V. M. Nadutov and O. G. Gladun: Phys. Met.
Metallogr., 3(1990), 128.
J. Foct: C.R. Acad. Sci,, 273 (1973), I159.
A. V. Suyazov. M. P. Usikov and B. M. Mogutnov: Phys. Met.
Metallog,'., 42 (1976), No. 4, 69.
E. I. Mittemejier, Liu Cheng, P. J, van der Schaaf, C. M.
BrackmanandB. M.Korevaar: Metall. Trans., 19A(1988), 925.
V. S. Litvinov, M. E. Poptsov and L. D. Chumakova:Phys. Met.
Metallogr., 58 (1984), 1037.
G G. Amigudand V. S. Litvinov: Phys. Mel. Metallog,'., 56
(1983), I132.
K. C. Russell and F.A. Garner: Physical Metallography of
Controlled Expansion Invar-Type Alloys, ed. by K.C. Russell
and D.F. Smith, TMS,(1990), 25.
H. Berns and J. Lueg: NeueHatte. 36 (1991), 13.
H. Berns: Stee/ Res., 63 (1992), 343.
H. Berns, S. N. Bugaychuk, V. A. Duz', R. Ehrhardr. V. G.
Gavriljuk, Yu. N. Petrov and I. A. Yakubzov: Steel Res., 65
(1994), 444.
H. Berns. R. Ehrhardt. V. A. Duz', V. G. Gavriljuk, and A. V.
Tarasenko: Meta! Physics Adv. Technol., 17 (1995), 19.
H. Berns. V. A. Duz', R. Ehrhardt. V. G. Gavriljuk. Yu. N.
Petrov. A. V. Tarasenko and L. N. Trofimova: to be sent to Z.
Metal !kd
.
M. J, vanCenderen,A, B6ttger, R. J. Cernik andE. J. Mittemejier:
Metall. T,'ans., 24A (1993), 1965.
745 C 1996 ISIJ