Nitrogen Containing Austenitic Stainless Steels
Transcrição
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 Untersucbt wird ein ejrrfacbar ebener Spaniiung ainer schwingenden Norraalspaanung uud zwe: su malspannungen. Dicscr Spaniwngszustand kaw Z.B luidcm daigestellt warden, die durch Inaen- Oder schwingend und in axiaier ItxchUdg statisch beonspi Eine besonaeiE Bedeutung hat dieser Spanttungszus die BeurkUung der WirJctMg von Eigenspannungen. Es wird gezeigt wie die Scbiibspannungainlcnsi S1H dea Binfluss eines biaxialen Spannunganutan lire Beurteiliing weicht teilweise erbebbch ab von de deret Hypotsssen. So wirken sich z. B. holie Druckin gen negativ auf die ertragbare Spannugtamplitiide. lidierUDg der Rechnung erfolgt mil Vnsuchsergebnii« teralur fur unlegierto und niedriglegicrte SUhle im MPa < Rpjj < 940 MPs. Oabei zcigt sich eine gutc mung zwuchea Recbnung und Vaisueh. Schlttsselworte: DauerschwingfeMigkeit, biaxiale naagen, Schubapannarja^intenaitatahrpothese [T68] 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. \