Nitrogen Containing Austenitic Stainless Steels
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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 ele- Stickstoffrialtige austenitische rostfreie Stahle faaben jflngst
ments which can impart the face-centered-cubic crystal lattice to nochmals an wirtschaftricher Bedeutung gewonnen durch die starke
stainless steels. With the recent price increases and the price vola- Erhohung dee Nickelpreises und des Molybdanpreises. Dies liegt
tility of nickel, nitrogen is ever more important as an alloying ele- daran, dass Stickstoff durch seine austeniusierende Wirkung Nickel
ment for a number of reasons. First, nitrogen is easily available in austenitischen Stahlen ersetzen kann und zugleich korrosions-
everywhere and thus is not subject to speculation at the Metal Ex- hemmend wirkt wie MolybdSn. Die vorliegende Arbeit zeigt diese
change. Second, in addition to making stainless steels austenitic, Wirkungen and Einfllisse quantitativ. Insbesondere wird gezeigt
nitrogen can also make them stronger and more corrosion resistant wie der Widerstand gegen Locbfrasskorrosion und Spaltkorrosion
It is also a well and clearly established fact since many years, that liber die Wirksumme MARC quantitativ von der Legierungszusam-
nitrogen in solid solution makes austenitic stainless steels more mensetzung abhSngt. DarUber Mnaus wird gezeigt, wie Stickstoff
wear resistant and more fatigue resistant. die Streckgrenze, Zugfestigkeit und Harte erhoht und ebenso den
Austenitic stainless steel alloy design with nitrogen has for many Widerstand gegen ErmUduag , Korrosionsermtidung und Ver-
years now taken account of the role of carbon. This is not only be- schleiss.
cause carbon is just a useful austenite former, but also because ni- Schlu&selworte: Austenit Stickstoff-Stahl.Legierungskosten.
trogen reduces the temperature where carbides begin to form. Thus Lochfrasskorrosion. Spaltkorrosion.Festigkeit.MARC.Verschleiss.
there is always an optimum carbon to nitrogen ratio. Finally it is HSrte.
now well established that carbon in solid solution helps to increase
the strength, the corrosion resistance and the wear resistance of aus-
tenitic stainless steels.
A number of quantitative correlations between alloy composi-
tion 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 alto-
gether.
Key words: nitrogen steels, stainless steels, austenitic steels,
strength, corrosion resistance, wear resistance, carbon in solid so-
lution.
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 re-
lative 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 aus-
tenites containing 8 to 11 percent nickel, typified by the com-
mercial 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, Weinheimabove 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 aus-
tenitic. From die formulation of the nickel equivalent in Fig-
ure 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 >
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
Figure 2. Alloy composition and the borderline of austenite at under consideration to 1460 - 1500 °C (this being close to the
HOCC lower end of me temperature range of AOD for many stainless
Bild2. Legierungszusammensetzung und Austenitgrenze bei steels). With this, we measure the nitrogen concentration after
HOO'C. 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:
nickel price and its recent increase and volatility, Fig.l) have
now become major driving forces to substitute austenitic %N = 0.067 %Cr + 0.02 %Mn + 0.04 %Mo - 0.01 %Ni
stainless steels containing 8 to 11 percent nickel with either minus 1.0 (equation 2)
one of three alternatives:
ferritic stainless steels, containing no nickel, [2], [3] As seen from Figure 3, it is possible to calculate me nitro-
duplex stainless steels, containing 0 to 5 percent nickel, [4], gen solubility with this handy equation for stainless steel melts
[5] of both low and of high alloy content. The experimental basis
austenitic stainless steels containing 0 or 1 to 4 percent ni- for equation 2 is primarily consisting of alloys high in chro-
ckel., [6] to [13]. mium and manganese, but low in nickel and molybdenum, as
The present paper is concerned with the third alternative the present world market price situation would favor. We em-
only, because of die favorable combination of properties phasize also that solubilities significantly below 0.2 weight
the face-centered cubic crystal lattice imparts to the austenitic percent nitrogen should not be calculated this way, because
steels and also because of the high solubility of nitrogen in this this could lead out of die range of applicability of the correla-
austenitic solid solution which in turn permits the achieve- tion equation 2.
ment of very desirable properties. 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 chro-
mium content. In this way, we have made steels in 20 kg quan-
2 Alloy composition and the austenite tities with up to 2.5 weight percent nitrogen under atmo-
boundary spheric pressure, [16].
For many years, the Scheffler diagram was used to mark the
limits of the austenite region in terms of alloy composition 4 Alloy composition and corrosion
represented by a nickel equivalent and a chromium equiva-
lent The Scheffler diagram was originally intended only to resistance
characterize weld microstructures, and there has also been
some controversy over how to formulate the nickel equivalent While nickel dominates the cost of austenitic stainless
and the chromium equivalent. An excellent review of the si- steels, it does NOT dominate their corrosion resistance. Tra-
tuation is found in [14]. ditionally, me resistance to localized corrosion, such as pitting
Our Figure 2 presents the most advanced state of knowl- corrosion and crevice corrosion in aqueous chloride solutions,
edge in this field. It gives the austenite borderline at 1100 is described as being controlled by the "pitting corrosion equi-
°C, based on three independent considerations: I.)ttiermody- valent " PREN = %Cr+ 3.3 %Mo + 16 %N, [10]. In those tra-
namic calculations 2.) the observed microstructure in stainless ditional assessments of corrosion resistance, nickel does not
steels quenched rapidly from 1100°C, and 3.) data from [14]. even figure, and it appears tiiat all the money spent on nickel is
The nickel- equivalent and the chromium equivalent are those just to make the crystal lattice face centered cubic.
of [6], [14] and [15].It is little surprise to see that most com- (This view is sometimes tempered by the claim that nickel
mercial austenitic stainless steels indicated in Figure 2 lie just additions might have a beneficial effect, not on the initiation,
876 M. O. Speidel Mat-wiss. a. Werkstofftech. 2006, 37, No. 10I 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 0 10 n 30 40 SO SO 70 80
matching observation is that the addition of a few percent ni- MARC = Cr+3.3Mo+20C+20N-0.5Mn-0.25Ni
ckel can reduce the corrosion rate of stainless steels in hot Fig. 5. Alloy composition and crevice corrosion resistance
acidic solutions). Bild 5. Legierungszusammensetzung und Widerstand gegen Spalt-
The widest data base known so far, relating the alloy con- korrosion.
tent 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 stands for " Measure of Alloying for Resistance to
Corrosion" and it is the sum of the alloy additions in weight
MARC = % Cr + 3.3 Mo + 20 N + 20 C - 0.5 Mn - 0.25 Ni. -percent. Per definition it applies only to alloy elements in
(equation 3) 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 cor-
responding adjustment of the factors, there might be consen-
sus on future MARC 2, MARC 3 .... formulations. Already
the MARC formula has been successfully applied to nickel-
basis 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 espe-
cially for very highly alloyed high-nitrogen steels, [14] Fig-
ures 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 cor-
rosion 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 8770 10 20 30 40 50 60 70 SO
M A R C = Cr+3.3Mo+2OC+20N-0.5Mn-0.25Ni any calculation of cost and to derive metallurgical inspiration
Fig. 6. MARC controls both, pitting corrosion and crevice corro- from it, we assume three price levels for each important alloy-
sion in stainless steels. ing element: low, medium and high. The LOW price level is
BiW 6. Die Wirksumme MARC korreliert gut mit dem Widerstand the one we assume when the huge price increases seen in Fig-
gegen Lochfrass und Spaltkorrosion. ure 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
austenite border, according to equation 1. Obviously, we can between these two levels appears to be realistic.
achieve this increase by one MARC unit with different com- These assumed low, medium and high price levels are
binations of alloy additions. Once we have determined suita- shown in Table 1 for the important alloying elements.
ble combinations of alloying additions, we can determine the It is obvious from Table 1 that nickel and molybdenum are
cost of these if we know the cost of adding one weight-percent not only the most expensive additions, but also those which
of those alloying additions. This is not a fixed number over fluctuate most. The question mark behind N means that we d«
any length in time, as seen from Figure 7. In order to do 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.
cost Ci+ 1.2 Ni Mo + 1.8Ni N + 15Cr N + 25Mo
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. 10totally different ways. Even if the nitrogen price shown in Ta- trogen containing austenitic stainless steels, as has been dis-
ble 1 is unrealistic on an absolute scale, the essential message cussed in detail in [9,20,21,22], It is thus meaningless to dis-
remains: nitrogen prices are not subject to large fluctuations, cuss strengthening theories of polycrystals without taking the
particularly when nitrogen is added as gas, for example in grain size effect ( and its temperature dependence!) into ac-
AOD. count. [17,18]. To a first approximation, however, the hard-
With the alloy element cost given in Table 1 we can now ness increases linearly with the nitrogen content, as seen in
calculate the cost to increase the corrosion resistance by 1 Figure 9. In the same linear fashion, the wear resistance
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 resist-
ance 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 x
steels of the 200 series were an economically meaningful » *x>
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".
6 Strength, wear resistance and fatigue
resistance 0.3 M 0.0 1J0 1.3 1.4 1.1
The yield strength and tensile strength increase with nitro- nltrogen content, [weight-percent]
gen in solid solution, as shown in Figure 8, [8]. Figure 9. Effect of nitrogen in austenitic solid solution on hardness
The wide variations in strength seen in Figure 8 for any gi- and wear resistance of stainless steels.
ven nitrogen content result from the fact that this is a collec- Bild 9. Einfluss von Stickstoff in austenitisch fester Ldsung auf
tion of data from steels which did not all have the same grain Ha'rtfi und Verschleiss.
size. The grain size is highly important for the strength of ni-
—i 1 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 tests In Ringsrt solution, 37°C
O • experimental stasis
100 J I I I 1 1 1—
i 1 I I 1 1 1— 0 0.1 0-2 0.3 0.4 0.5 0.S 0.7 0.S 0.8
0 0.2 0.4 0.6 0.1 1.0 1.2 1.4 IS 18
interstitial content, C+N, weight-percent
nitrogen content, [weight - percent]
Figure 10. Nitrogen and carbon in austenitic solid solution in-
Figure 8. Effect of nitrogen in austenitic solid solution on yield crease the fatigue resistance and the corrosion fatigue resistance .
strength and tensile strength. Abb. 10. Stickstoff und Konlenstoff in austenitisch fester Ldsung
Bild 8. Einfluss von Stickstoff in austenitisch fester Losung auf erhoht die Ermiidungsbestandigkeit und die Schwingungsrisskorro-
Streckgrenze und Zugfestigkeit. sionsbestSndigkeit
Mat.-wiss. u. Werkstofftech. 2006, 37, No. 10 Stainless Steels 879- a l s o i m p r o v e s ^ seen'by the reduction of the volume loss in "t "Si. O. Speidei.Ytainicss Steel World2M1 Ka publishing BV.
Figure 9, [8,16). 8. M. O. Speidel et al., Tram. bid. Ins) Ma. June 2003,56, No.3,
There are many other useful improvements of mechanical 281.
properties possible through nitrogen in austenitic solid solu- 9. M. O. Speidel, M. Zheng-Cui, HNS 2003. High Nitrogen
tion. Ons further example is shown in Figim JO. where it is Steels, vdf Hochschulverlag, Zurich, Switzerland, 6 3 - 7 3 .
10. J. Charles, BAOSTEEL BAC 2006, 3,211.
seen that the fatigue resistance of the steels is clearly inv
11. Jindal Stainless: 200 Series Austenitk: Stainless Steels, New
proved in air environment as well as in potentially corrosive Delhi 2006.
body fluids "Ringer solution" [23]. 12. "New 200-series steels" ISSF, November 2005.
13. "Development of Type 204 Cu Stairless, A Low Cost Alter-
nate lb type 304,Carpentcr Technology , Reading, PA.
USA. January 2 N L
7 Conclusions 14. G. Salter et al.. High Nitrogen Steels 2004, Steel Crips 2(2004),
283-292.
Nitrogen in austenitic solid solution is an enormously use- 15. M.O.Speidel, EJ. Uggowitzer, Proc. Int. Conf. High Manga-
ful element with respect to austeoite stability, corrosion resist- nese Austenitic Steels, 1993, Chicago, 135-142. .
ance and raechanic.il properties. It is also economically useful 16. M. O. Speidel, HNS 2003, High Nitrogen Steels, vdf Untersucbt wird ein ejrrfacbar ebener Spaniiung
HochschulverJag. Zurich, Switzerland,pp J - S . ainer schwingenden Norraalspaanung uud zwe: su
because it is not as subject to price volatility as are nickel aud
17. H.J C Speidel. Markus O.Speidel, HNS 2003, vdTHochschol malspannungen. Dicscr Spaniwngszustand kaw Z.B
molybdenum. It is therefore to be expected that nitrogen-con- verlag, Zurich, Switzerland, pp.101 -112.
taining; austenitk; stainless steels with little or no nickel (the luidcm daigestellt warden, die durch Inaen- Oder
18. HJ.CSpeidel, M. O.Speidel, Materials and Manufacturing schwingend und in axiaier ItxchUdg statisch beonspi
so-called 200 Series austenitic stainless steels) will be more Procters 1004,19, No.l, 95. Eine besonaeiE Bedeutung hat dieser Spanttungszus
widely used in the foreseeable future. 19. J. Bemaucr, G.Saller. MO .Speidel, High Nitrogen Steels 2004, die BeurkUung der WirJctMg von Eigenspannungen.
Steel drips (2004), 529-537. Es wird gezeigt wie die Scbiibspannungainlcnsi
20. M. O-Spaklel, Z Utiallkd. 2003, 943, 719. S1H dea Binfluss eines biaxialen Spannunganutan
21. M. O.Speidel, H. JX.Speids!, 2 MtlallU 2004, »S. 7, 596. lire Beurteiliing weicht teilweise erbebbch ab von de
8 References 22. M. O-Spcidel. H. J.Speidel, BAOSTEEL BAC 2006, 3, 224.
23. M. Diener, M. O.Speidel, HNS 2003, vdf Hochschulverlag, Zu-
deret Hypotsssen. So wirken sich z. B. holie Druckin
gen negativ auf die ertragbare Spannugtamplitiide.
1. B. Strauss, B. Maurer, Kmpp Monaahcfte 1920,1. 129. rich, Switzerland, 211 -216. lidierUDg der Rechnung erfolgt mil Vnsuchsergebnii«
2. KIM Kwangyuk, et al, BAOSTEEL BAC 20116 3, 228. teralur fur unlegierto und niedriglegicrte SUhle im
3. Fan Guangwel. BAOSTEEL BAC MM, 3. 310. Prof. Dr. rer. nar. Markus O. Speide], Swiss Academy of Materials MPa < Rpjj < 940 MPs. Oabei zcigt sich eine gutc
4. M.O. Speide], /.Wang, PJ. Uggowitzer, PRJCM 3. Honolulu Science, Birmcnstorf, Switzerland, E-mail: srjeid^Omatcrialsuca- mung zwuchea Recbnung und Vaisueh.
1998, TMS, Wanmidato. PA, USA, J, 161-166. demy.com Schlttsselworte: DauerschwingfeMigkeit, biaxiale
1 J. Wang, PJ. Uggowitzer, R. Magdowskt, M.O. Speidel, Scrip- naagen, Schubapannarja^intenaitatahrpothese
la Matertalia 1999.40. No.l. 123. Received in filial form: luly 25, 2006 [T68]
6. P. J. Uggowitzer, R. Magdowalu. M. O. Speidel, ISIJlmcma-
tloiial 1996, 36, No.7, 901.
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;
nung auf das VetfuUten bei einachsiger Beanspi
riJclcfubjco stunt. Bei sehwingender Beanspruch
tangent bekannt, dass konventionelle Hypothec
Sehubspamungs- und die Oestattlndertingsenerg
(van Mises) nur bei propottioitalcr Beonspruchur
det werden dUtfen (1, 2J. FUr sichlproportionai<
chungen, bei denen sich in der Regel die Hauptspu
tiiag wiihrend ci::cs Sdivr^tjjsais!: ir.dert, versaj
ventionellen Hypothesen. Hier giht es eine Reihe
zen, die aU Mcthoden der kritischeu Schnittebene,
len Ansbeogung und der Energicumwandlung bei
den sind [ 3 , 4 , 5 ] , FUr korapjexe mehnziale Abis
im Betrieb haufig aufneten, besleht noch ein crhet
retischer und experimenteller Forschungsbcdarf
vetlassigere Besclueibung des Einflusses der Men
[6j.
In der vorliegenden Unlcrsuchung soil ein set
Beanpnichnngsfall betrachtet werden, der bcinah
mutet: Ein ebener Spannungszustaod, bed dem ei
spanmtng a„ schwingend und zwei Normalspaa
und Gym statisch aufuxten, Bilri J. Da keine Schi
880 M. O. Speidel Mat.-wiss. a. Werkstoffteeh. 2006. 37, No. 10 © 2006 WILEY-VCH Verlag GmbH & Co. KCaA. \You can also read
