Nitrogen Containing Austenitic Stainless Steels

Mat-wiss. u. Werkstoiftech. 2006, 37, No. 10
DOI: 10.10O2/mawe.20O6O0O68
Nitrogen Containing Austenitic Stainless Steels
Austenitische rostfreie Stahle mit Stickstoff
M. O. Speidel
Dedicated to Prof. Dr.-lng. Christina Berger on the occasion of her 60th birthday
Nickel and nitrogen are the two most widely used alloying elements which can impart the face-centered-cubic crystal lattice to
stainless steels. With the recent price increases and the price volatility of nickel, nitrogen is ever more important as an alloying element for a number of reasons. First, nitrogen is easily available
everywhere and thus is not subject to speculation at the Metal Exchange. Second, in addition to making stainless steels austenitic,
nitrogen can also make them stronger and more corrosion resistant
It is also a well and clearly established fact since many years, that
nitrogen in solid solution makes austenitic stainless steels more
wear resistant and more fatigue resistant.
Austenitic stainless steel alloy design with nitrogen has for many
years now taken account of the role of carbon. This is not only because carbon is just a useful austenite former, but also because nitrogen reduces the temperature where carbides begin to form. Thus
there is always an optimum carbon to nitrogen ratio. Finally it is
now well established that carbon in solid solution helps to increase
the strength, the corrosion resistance and the wear resistance of austenitic stainless steels.
A number of quantitative correlations between alloy composition and materials properties are presented and their useful role in
alloy design is pointed out. This will further help to lower the nickel
content in austenitic stainless steels or even replace nickel altogether.
Key words: nitrogen steels, stainless steels, austenitic steels,
strength, corrosion resistance, wear resistance, carbon in solid solution.
1 Austenitic stainless steels and the
nickel price
From 1909 to 1912, Strauss and Maurer showed in their
publications for the first time that the combination of about
19 percent chromium and about 9 percent nickel in iron results
in a stainless steel with the face-centered cubic crystal lattice.
[1]. Steels with this crystal structure are called austenites and
can have excellent ductility and toughness, combined with relative low strength and thus excellent formability. Moreover,
such steels are not ferromagnetic.
Because of this combination of desirable properties, in the
decades leading up to the year 2000, the worlds production of
stainless steels consisted typically of 70 percent or more austenites containing 8 to 11 percent nickel, typified by the commercial steels 1.4301, or X5CrNil8-10 or AISI 304.
This dominant role of the austenitic stainless steels has been
loosing ground in the last three years for economic reasons.
For a long time, nickel has been the single most important cost
factor in the production of austenitic stainless steels. The high
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Stickstoffrialtige austenitische rostfreie Stahle faaben jflngst
nochmals an wirtschaftricher Bedeutung gewonnen durch die starke
Erhohung dee Nickelpreises und des Molybdanpreises. Dies liegt
daran, dass Stickstoff durch seine austeniusierende Wirkung Nickel
in austenitischen Stahlen ersetzen kann und zugleich korrosionshemmend wirkt wie MolybdSn. Die vorliegende Arbeit zeigt diese
Wirkungen and Einfllisse quantitativ. Insbesondere wird gezeigt
wie der Widerstand gegen Locbfrasskorrosion und Spaltkorrosion
liber die Wirksumme MARC quantitativ von der Legierungszusammensetzung abhSngt. DarUber Mnaus wird gezeigt, wie Stickstoff
die Streckgrenze, Zugfestigkeit und Harte erhoht und ebenso den
Widerstand gegen ErmUduag , Korrosionsermtidung und Verschleiss.
Schlu&selworte: Austenit Stickstoff-Stahl.Legierungskosten.
Lochfrasskorrosion. Spaltkorrosion.Festigkeit.MARC.Verschleiss.
HSrte.
above the gamma border line. This is, of course, to use as little
nickel as possible, because the main role, and often the only
role of the expensive nickel is to make the stainless steels austenitic. From die formulation of the nickel equivalent in Figure 2, one can see how nitrogen can partially or even fully take
over the role of nickel as an austenite former.
The austenite borderline, with the chromium and nickel
equivalents used in Figure 2 is determined as follows:
Nickel Equivalent = 1.2 Chromium Equivalent minus 13.
(equation 1)
3 Adding nitrogen
Figure 2. Alloy composition and the borderline of austenite at
HOCC
Bild2. Legierungszusammensetzung und Austenitgrenze bei
HOO'C.
nickel price and its recent increase and volatility, Fig.l) have
now become major driving forces to substitute austenitic
stainless steels containing 8 to 11 percent nickel with either
one of three alternatives:
ferritic stainless steels, containing no nickel, [2], [3]
duplex stainless steels, containing 0 to 5 percent nickel, [4],
[5]
austenitic stainless steels containing 0 or 1 to 4 percent nickel., [6] to [13].
The present paper is concerned with the third alternative
only, because of die favorable combination of properties
the face-centered cubic crystal lattice imparts to the austenitic
steels and also because of the high solubility of nitrogen in this
austenitic solid solution which in turn permits the achievement of very desirable properties.
2 Alloy composition and the austenite
boundary
For many years, the Scheffler diagram was used to mark the
limits of the austenite region in terms of alloy composition
represented by a nickel equivalent and a chromium equivalent The Scheffler diagram was originally intended only to
characterize weld microstructures, and there has also been
some controversy over how to formulate the nickel equivalent
and the chromium equivalent. An excellent review of the situation is found in [14].
Our Figure 2 presents the most advanced state of knowledge in this field. It gives the austenite borderline at 1100
°C, based on three independent considerations: I.)ttiermodynamic calculations 2.) the observed microstructure in stainless
steels quenched rapidly from 1100°C, and 3.) data from [14].
The nickel- equivalent and the chromium equivalent are those
of [6], [14] and [15].It is little surprise to see that most commercial austenitic stainless steels indicated in Figure 2 lie just
876
M. O. Speidel
>
The solubility of nitrogen in stainless steels depends, for
practical steelmaking purposes, on three major influences:
temperature, pressure and alloy composition. In the following,
we fix the nitrogen partial pressure to one atmosphere (or
slightly below) and die temperature range of the liquid steel
under consideration to 1460 - 1500 °C (this being close to the
lower end of me temperature range of AOD for many stainless
steels). With this, we measure the nitrogen concentration after
saturation equilibrium and obtain the data shown in Figure 3.
The line corresponds to the following correlation equation for
the solubility of nitrogen in weight-percent:
%N = 0.067 %Cr + 0.02 %Mn + 0.04 %Mo - 0.01 %Ni
minus 1.0
(equation 2)
As seen from Figure 3, it is possible to calculate me nitrogen solubility with this handy equation for stainless steel melts
of both low and of high alloy content. The experimental basis
for equation 2 is primarily consisting of alloys high in chromium and manganese, but low in nickel and molybdenum, as
the present world market price situation would favor. We emphasize also that solubilities significantly below 0.2 weight
percent nitrogen should not be calculated this way, because
this could lead out of die range of applicability of the correlation equation 2.
It is also immediately evident from Figure 3 that nitrogen
concentrations higher than 1,2 weight-percent can be reached
in stainless steels at atmospheric pressure if the steel melt has
an appropriate composition, for example a high enough chromium content. In this way, we have made steels in 20 kg quantities with up to 2.5 weight percent nitrogen under atmospheric pressure, [16].
4 Alloy composition and corrosion
resistance
While nickel dominates the cost of austenitic stainless
steels, it does NOT dominate their corrosion resistance. Traditionally, me resistance to localized corrosion, such as pitting
corrosion and crevice corrosion in aqueous chloride solutions,
is described as being controlled by the "pitting corrosion equivalent " PREN = %Cr+ 3.3 %Mo + 16 %N, [10]. In those traditional assessments of corrosion resistance, nickel does not
even figure, and it appears tiiat all the money spent on nickel is
just to make the crystal lattice face centered cubic.
(This view is sometimes tempered by the claim that nickel
additions might have a beneficial effect, not on the initiation,
Mat-wiss. a. Werkstofftech. 2006, 37, No. 10
I
0
0.2
0.4
0.«
0.8
1
1.2
1.4
IS
calculated nltrogan concentration [weight-percant]
Figure 3. Measured and calculated maximum nitrogen content
Bild 3. Gemessener und bcrechneter maximaler Stickstoffgehalt
im Stahl bci Erschmelzung ohne Druck.
-10
but on the growth rate of crevice corrosion. A parallel and
matching observation is that the addition of a few percent nickel can reduce the corrosion rate of stainless steels in hot
acidic solutions).
The widest data base known so far, relating the alloy content of austenitic stainless steels to their localized corrosion
resistance in chloride solutions, [7,8,9,14] takes into account
not only Cr, Mo and N, but also C, Mn and Ni:
MARC = % Cr + 3.3 Mo + 20 N + 20 C - 0.5 Mn - 0.25 Ni.
(equation 3)
0
10
n
30
40
SO
SO
70
80
MARC = Cr+3.3Mo+20C+20N-0.5Mn-0.25Ni
Fig. 5. Alloy composition and crevice corrosion resistance
Bild 5. Legierungszusammensetzung und Widerstand gegen Spaltkorrosion.
MARC stands for " Measure of Alloying for Resistance to
Corrosion" and it is the sum of the alloy additions in weight
-percent. Per definition it applies only to alloy elements in
solid solution and it is seen that carbon plays a beneficial
role while manganese and nickel have a negative influence
on the corrosion resistance. Obviously some highly important
alloying elements, such as silicon and copper have so far not
yet been studied and not yet been incorporated into the MARC
formula. Moreover, with a widening data base, some of the
factors in the MARC formula might have to be adjusted. After
each significant future widening of the data base and the corresponding adjustment of the factors, there might be consensus on future MARC 2, MARC 3 .... formulations. Already
the MARC formula has been successfully applied to nickelbasis and chromium-basis austenites, [17,18], It has also been
independently confirmed for carbon-rich austenites, [19] and
it has been shown to be superior to the PREN formula especially for very highly alloyed high-nitrogen steels, [14] Figures 4 and 5 present examples of the data base for commercial
stainless steels concerning both, pitting corrosion resistance in
22 % NaCl solutions and crevice corrosion resistance in FeC13
solutions [8].
Figure 6 includes additionally many experimental alloys in
the data base.
5 The cost of corrosion resistance
Suppose we would like to know the cost to improve the corrosion resistance of an austenitic stainless steel by one MARC
unit. To do this, we would have to increase the alloy content of
chromium or molybdenum or nitrogen, according to equation
3, and then to add further alloying elements to stay above the
Stainless Steels
877
0
10
20
30
40
50
60
70
SO
M A R C = Cr+3.3Mo+2OC+20N-0.5Mn-0.25Ni
Fig. 6. MARC controls both, pitting corrosion and crevice corrosion in stainless steels.
BiW 6. Die Wirksumme MARC korreliert gut mit dem Widerstand
gegen Lochfrass und Spaltkorrosion.
austenite border, according to equation 1. Obviously, we can
achieve this increase by one MARC unit with different combinations of alloy additions. Once we have determined suitable combinations of alloying additions, we can determine the
cost of these if we know the cost of adding one weight-percent
of those alloying additions. This is not a fixed number over
any length in time, as seen from Figure 7. In order to do
any calculation of cost and to derive metallurgical inspiration
from it, we assume three price levels for each important alloying element: low, medium and high. The LOW price level is
the one we assume when the huge price increases seen in Figure 7 should fall back to the much lower levels prevalent in the
years before 2000. The HIGH price level is assumed for the
case that raw materials have found a more permanently high
price and just oscillate a little about it. A medium price chosen
between these two levels appears to be realistic.
These assumed low, medium and high price levels are
shown in Table 1 for the important alloying elements.
It is obvious from Table 1 that nickel and molybdenum are
not only the most expensive additions, but also those which
fluctuate most. The question mark behind N means that we d«
not really know this cost, partly because N can be added in
Table X. Three price levels assumed for the alloy cost to add one weight percent of each alloying element to austenitic stainless steel. (USD
/ton).
Tabetic 1. Niedrige, mittlere, oder hohe Kosten fiir jedes Legierungselement, urn dessen Gehalt in austenitisch rostfreiem Stahl um ein
Gewichtsprozent zu erhohen
Price
Cr
Mn
Ni
Mo
N
low
9
4.5
60
100
10?
medium
13
6.5
100
400
10?
high
16
14.5
160
800
10?
Table 2. Cost to increase the corrosion resistance by one MARC. (USD / ton)
Tabelle 2. Niedrige, mittlere oder hohe Kosten fiir jedes Legierungselement, um den Korrosionswiderstand um eine MARC - Einheit zu
erhfihen.
Mo + 1.8Ni
N + 15Cr
N + 25Mo
cost
Ci+ 1.2 Ni
low
106
63
4?
24
medium
175
175
6?
98
nigh
273
330
7?
195
878
M. O. Speidel
Mat.-wiss. u. Werkstofftech. 2006, 37, No. 10
totally different ways. Even if the nitrogen price shown in Table 1 is unrealistic on an absolute scale, the essential message
remains: nitrogen prices are not subject to large fluctuations,
particularly when nitrogen is added as gas, for example in
AOD.
With the alloy element cost given in Table 1 we can now
calculate the cost to increase the corrosion resistance by 1
MARC unit by taking into account how strongly each element
increases MARC (equation 3) and what other additions will be
necessary to stay above the austenite borderline (equation 1).
The result is shown in Table 2.
It turns out, as expected, that nitrogen-chromium additions
are the most economic way to increase the corrosion resistance of typical austenitic stainless steels. The chromium
here is necessary to increase the nitrogen solubility in the
steel, according to equation 2.
Thus, if nitrogen containing, low nickel austenitic stainless
steels of the 200 series were an economically meaningful
choice for applications in the year 2000, when the alloy costs
were "low" according to Figure 7 and according to Tables 1
and 2, then this choice would make even more sense in the
year 2006, when the alloy costs were" high".
trogen containing austenitic stainless steels, as has been discussed in detail in [9,20,21,22], It is thus meaningless to discuss strengthening theories of polycrystals without taking the
grain size effect ( and its temperature dependence!) into account. [17,18]. To a first approximation, however, the hardness increases linearly with the nitrogen content, as seen in
Figure 9. In the same linear fashion, the wear resistance
>
x
» *x>
6 Strength, wear resistance and fatigue
resistance
The yield strength and tensile strength increase with nitrogen in solid solution, as shown in Figure 8, [8].
The wide variations in strength seen in Figure 8 for any given nitrogen content result from the fact that this is a collection of data from steels which did not all have the same grain
size. The grain size is highly important for the strength of ni-
0.3
M
0.0
1.3
1J0
1.4
1.1
nltrogen content, [weight-percent]
Figure 9. Effect of nitrogen in austenitic solid solution on hardness
and wear resistance of stainless steels.
Bild 9. Einfluss von Stickstoff in austenitisch fester Ldsung auf
Ha'rtfi und Verschleiss.
1
—i
1
1
1
1
1
r
fatigue) strength of ausMnttic stainless sushi
solution annealed, lest In air, ambtanMempcnlura
R~-t,fe50Ht,N*10r
•rr
R,=2SI)J-250VC+N
01
c
3
Rpej * yield strength
ft„ x ultimate tensile strength
Q • commercial steels
O • experimental stasis
tests In Ringsrt solution, 37°C
J
100
0
0.2
i
1
I
I
1
1
1—
0.4
0.6
0.1
1.0
1.2
1.4
IS
0
18
nitrogen content, [weight - percent]
Figure 8. Effect of nitrogen in austenitic solid solution on yield
strength and tensile strength.
Bild 8. Einfluss von Stickstoff in austenitisch fester Losung auf
Streckgrenze und Zugfestigkeit.
Mat.-wiss. u. Werkstofftech. 2006, 37, No. 10
0.1
0-2
I
I
I
1
1
0.3
0.4
0.5
0.S
0.7
1—
0.S
0.8
interstitial content, C+N, weight-percent
Figure 10. Nitrogen and carbon in austenitic solid solution increase the fatigue resistance and the corrosion fatigue resistance .
Abb. 10. Stickstoff und Konlenstoff in austenitisch fester Ldsung
erhoht die Ermiidungsbestandigkeit und die SchwingungsrisskorrosionsbestSndigkeit
Stainless Steels
879
- a l s o i m p r o v e s ^ seen'by the reduction of the volume loss in
Figure 9, [8,16).
There are many other useful improvements of mechanical
properties possible through nitrogen in austenitic solid solution. Ons further example is shown in Figim JO. where it is
seen that the fatigue resistance of the steels is clearly inv
proved in air environment as well as in potentially corrosive
body fluids "Ringer solution" [23].
7 Conclusions
Nitrogen in austenitic solid solution is an enormously useful element with respect to austeoite stability, corrosion resistance and raechanic.il properties. It is also economically useful
because it is not as subject to price volatility as are nickel aud
molybdenum. It is therefore to be expected that nitrogen-containing; austenitk; stainless steels with little or no nickel (the
so-called 200 Series austenitic stainless steels) will be more
widely used in the foreseeable future.
8 References
1.
2.
3.
4.
B. Strauss, B. Maurer, Kmpp Monaahcfte 1920,1. 129.
KIM Kwangyuk, et al, BAOSTEEL BAC 20116 3, 228.
Fan Guangwel. BAOSTEEL BAC MM, 3. 310.
M.O. Speide], /.Wang, PJ. Uggowitzer, PRJCM 3. Honolulu
1998, TMS, Wanmidato. PA, USA, J, 161-166.
1 J. Wang, PJ. Uggowitzer, R. Magdowskt, M.O. Speidel, Scripla Matertalia 1999.40. No.l. 123.
6. P. J. Uggowitzer, R. Magdowalu. M. O. Speidel, ISIJlmcmatloiial 1996, 36, No.7, 901.
"t "Si. O. Speidei.Ytainicss Steel World2M1 Ka publishing BV.
8. M. O. Speidel et al., Tram. bid. Ins) Ma. June 2003,56, No.3,
281.
9. M. O. Speidel, M. Zheng-Cui, HNS 2003. High Nitrogen
Steels, vdf Hochschulverlag, Zurich, Switzerland, 6 3 - 7 3 .
10. J. Charles, BAOSTEEL BAC 2006, 3,211.
11. Jindal Stainless: 200 Series Austenitk: Stainless Steels, New
Delhi 2006.
12. "New 200-series steels" ISSF, November 2005.
13. "Development of Type 204 Cu Stairless, A Low Cost Alternate lb type 304,Carpentcr Technology , Reading, PA.
USA. January 2 N L
14. G. Salter et al.. High Nitrogen Steels 2004, Steel Crips 2(2004),
283-292.
15. M.O.Speidel, EJ. Uggowitzer, Proc. Int. Conf. High Manganese Austenitic Steels, 1993, Chicago, 135-142. .
16. M. O. Speidel, HNS 2003, High Nitrogen Steels, vdf
HochschulverJag. Zurich, Switzerland,pp J - S .
17. H.J C Speidel. Markus O.Speidel, HNS 2003, vdTHochschol
verlag, Zurich, Switzerland, pp.101 -112.
18. HJ.CSpeidel, M. O.Speidel, Materials and Manufacturing
Procters 1004,19, No.l, 95.
19. J. Bemaucr, G.Saller. MO .Speidel, High Nitrogen Steels 2004,
Steel drips (2004), 529-537.
20. M. O-Spaklel, Z Utiallkd. 2003, 943, 719.
21. M. O.Speidel, H. JX.Speids!, 2 MtlallU 2004, »S. 7, 596.
22. M. O-Spcidel. H. J.Speidel, BAOSTEEL BAC 2006, 3, 224.
23. M. Diener, M. O.Speidel, HNS 2003, vdf Hochschulverlag, Zurich, Switzerland, 211 -216.
Prof. Dr. rer. nar. Markus O. Speide], Swiss Academy of Materials
Science, Birmcnstorf, Switzerland, E-mail: srjeid^Omatcrialsucademy.com
Received in filial form: luly 25, 2006
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1 Einleitung
Zur Erfassuog dea Einflusses einer mehrach:
spruchung anfdas Festigkcitsverhslten sind zahln
teifshypothesen entwickelt worden. Vorausgese
dtesen Hypothesen, dass sich das Festigkeiusvt
mehrachsiger Beanspruchung mil Hirfe einer Vcr;
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den sind [ 3 , 4 , 5 ] , FUr korapjexe mehnziale Abis
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880
M. O. Speidel
Mat.-wiss. a. Werkstoffteeh. 2006. 37, No. 10
© 2006 WILEY-VCH Verlag GmbH & Co. KCaA. \