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BHM (2015) Vol. 160 (9): 406–412 DOI 10.1007/s00501-015-0410-1 © Springer-Verlag Wien 2015
New Stainless Steel Alloys for Low Temperature Surface Hardening? Thomas L. Christiansen, Kristian V. Dahl and Marcel A. J. Somers Department of Mechanical Engineering, Technical University of Denmark (DTU), Kongens Lyngby, Dänemark
Received August 12, 2015; accepted August 13, 2015; published online August 27, 2015 Abstract: The present contribution showcases the possibility for developing new surface hardenable stainless steels containing strong nitride/carbide forming elements (SNCFE). Nitriding of the commercial alloys, austenitic A286, and ferritic AISI 409 illustrates the beneficial effect of having SNCFE present in the stainless steel alloys. The presented computational approach for alloy design enables “screening” of hundreds of thousands hypothetical alloy systems by use of Thermo-Calc. Promising compositions for new stainless steel alloys can be selected based on imposed criteria, i.e. facilitating easy selection of candidate alloys designed for low temperature surface hardening. Keywords: Nitriding, Expanded austenite, Alloy design, Surface hardening, Strong nitride/carbide formers Neue Edelstahllegierungen für Oberflächenhärtung bei Niedrigtemperaturen? Zusammenfassung: Der vorliegende Beitrag zeigt die Möglichkeiten für die Entwicklung neuer Oberflächen von härtbaren rostfreien Stählen, die stark Nitrid/Carbid bildende Elemente (SNCFE) enthalten. Das Nitrieren der kommerziellen Legierungen (austenitische A286 und ferritische AISI 409) veranschaulicht die vorteilhafte Wirkung von SNCFE in rostfreihen Stahllegierungen. Der in diesem Beitrag vorgestellte Rechenansatz für Legierungsdesign ermöglicht ein „Screening“ von Hunderttausenden hypothetisch möglichen Legierungs-
T. L. Christiansen () Department of Mechanical Engineering, Technical University of Denmark (DTU), B425, Produktionstorvet, 2800 Kongens Lyngby, Dänemark e-mail:
[email protected]
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systemen mit Hilfe von Thermo-Calc. Vielversprechende Kompositionen für neue Edelstahl-Legierungen können auf Basis der auferlegten Kriterien ausgewählt werden, d. h. eine einfache Auswahl von Legierungen zu ermöglichen, die für das Oberflächenhärten bei Niedrigtemperaturen entwickelt wurden. Schlüsselwörter: Nitrierung, Ausgedehntes Austenit, Legierungs-Design, Oberflächenhärten, starke Nitrid/Karbidbildner
1. Introduction Low temperature surface hardening of stainless steel was first shown possible in the mid-1980s by Kolster [1] (carburizing) and by Zhang and Bell [2] (nitriding). Today low temperature surface hardening of stainless steel is coming of age and is commercially available from several providers. The microstructural feature that develops upon introduction of nitrogen and/or carbon at low temperatures is the so-called expanded austenite, which essentially is a solid solution of large quantities of nitrogen/carbon in FCC austenite. Expanded austenite is metastable and will tend to decompose into the thermodynamically more stable chromium carbides or nitrides, depending on the nature of the interstitial. As a consequence, the maximum nitriding/ nitrocarburising and carburizing temperatures are below, say, 450 °C and 520 °C, respectively. This maximum treatment temperature poses restrictions on the case depth which can be obtained; usually not more than 30 µm can be achieved by low temperature surface hardening. The restriction on process temperature is a direct consequence of the alloy composition of the stainless steel. Stainless steels are not intended for surface hardening, but were developed for other purposes, viz. excellent corrosion resistance in combination with formability or strength. The concept of specialty steels designed for surface harden-
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ing is known from the world of non-stainless steels, where “nitriding steels” and “carburizing steels” have been developed and are widely applied. Hitherto, this concept has not been applied for stainless steels. The present work explores possibilities for developing stainless steel alloys designed to give optimal response to low temperature surface hardening without loss of corrosion resistance. Actually, a previously, fragmentarily investigated concept is revisited: dispersion strengthening of austenitic stainless steel alloys with 2 wt% Ti by high temperature nitriding in nitrogen under controlled conditions without Cr-nitrides forming—only TiN was formed [3–6]. As the hardening response was moderate, due to the relatively coarse TiN precipitates formed as a consequence of the relatively high temperatures used (appr. 1000 °C), the concept was abandoned. Nitriding in the low temperature range where expanded austenite forms, of a Fe-Ni-Cr stainless steel containing strong nitride/carbide forming elements, hereafter denoted SNCFE, has so far not been investigated. The SNCFE in a stainless steel matrix will “control” the behavior of the interstitials, e.g. SNCFE nitrides/carbides will form preferentially over chromium nitrides/carbides. The surface hardening effect can derive from either (1) expanded austenite containing SNCFE hindering formation of unwanted chromium carbide/nitride or (2) controlled formation of (hard) SNCFE nitrides/carbides leaving chromium in solid solution and maintaining the favorable corrosion properties of the stainless steel. These concepts are presented in the first part of the article for the commercially available stainless alloys containing SNCFE, namely ferritic AISI 409 and austenitic precipitation hardening A286. The possibility to design new stainless steel alloys based along the concepts of the presence of SNCFE as active alloying element with respect to (low temperature) surface hardening is addressed in the second part of the article, where computational alloy design is applied. The computational design derives from an approach previously applied for design and development of nickel based superalloys [7, 8] and stainless marageing steels [9, 10]. The approach entails computational screening of a large number of (hypothetical) alloys; a composition range for selected relevant elements is chosen and all elements are systematically varied; for each composition relevant thermodynamic properties are then calculated by modelling software such as Thermo-Calc. The output from the modelling software is subsequently used either directly or in submodels to evaluate which compositions fulfil a number of optimization criteria (thermodynamic but also kinetic and mechanical) (e.g. see [10]). To illustrate the computational concepts, calculations on alloying of SNCFEs, Ti and Al, to austenitic stainless steel alloys is presented here, i.e. by variational analysis of SNCFE and the master alloy system.
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TABLE 1:
Nominal composition of A286 and AISI 409 (in wt%) C
Cr
Ni
Mo
Mn
Si
Ti
A286
0.08
13.5– 16
24.0– 1.0– 27.0 1.5
2.0
1.0
1.9– 0.1– 2.3 0.5
0.35 Bal.
AISI 409
0.01
11.5
0.1
1.0
1.0
0.2
-
-
V
-
Al
Fe
Bal.
TABLE 2:
Composition of AISI316L used in the calculations (wt%); balance is Fe Element
C
N
Mn
Si
Cr
Ni
Mo
Al
Ti
AISI 316L
0.02
0.03
1.8
0.5
12–18
10–20
0–3
0–3
0–3
2. Experimental Austenitic precipitation hardening stainless steel A286 and ferritic stainless steel AISI 409 (EN 1.4512) were subjected to gaseous nitriding; the compositions are given in Tab. 1. The A286 was solution heat treated at 900 ºC for 2 hours in argon protective atmosphere; AISI 409 was used in the as-delivered annealed condition. All samples were ground and polished to a mirror like surface finish prior to gaseous nitriding. Gaseous nitriding was conducted in a Netzsch STA 449 thermal analyzer. A286 was nitrided at 500 °C for 14 hours and AISI 409 was nitrided at 430 °C for 16 hours, the applied atmosphere for both treatments consisted of 15 %NH3− 77 %H2− 8 %N2. Activation of the samples to remove the protective oxide layer was carried out in-situ prior to nitriding; all details of this treatment are proprietary. Ageing of nitrided A286 was performed in a tube furnace in an atmosphere of 100 % N2 at 720 °C for 16 hours. Cross sections of the nitrided samples were prepared by conventional metallographic techniques. Optical microscopy was carried out on the cross sections using a Leica optical microscope after etching the samples with Kalling’s reagent no. 1. The microhardness profiles were obtained using a Future Tech model FM-700 hardness tester on the mounted cross sections applying a load of 5 g and a dwell time of 10 s. For the phase analysis, X-ray diffraction was performed on the surface of the samples using a Bruker D8 Discovery diffractometer with Cr-Kα radiation. The scans were obtained with step size of 0.025 ° and counting time of 3 s per step.
3. Computational Model Thermo-Calc [11] was used to evaluate the effect of addition of Al and Ti to austenitic steel type AISI316L. The Thermo-Calc steel database TCFE 7.01 [12] was used to obtain thermodynamic data for calculations. Phase fraction diagrams were calculated for different additions of Al and Ti. The composition used for calculations is shown in Tab. 2.
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Systematic variation of chemical elements was done in order to identify candidate compositions for obtaining a stable austenitic steel that can be heat treated (austenitized) and which contains high amounts of Al and Ti. Systematic variation was done using a C++ program that accesses Thermo-Calc through the Thermo-Calc application programming interface. Cr and Mo were varied in steps of 0.5 wt%, Ni was varied in steps of 1 wt%, Al and Ti were varied in steps of 0.25 wt%. Other elements, C, N, Mn, Si, were kept constant and taken as for “AISI316L” in Tab. 1; the balance was Fe. This composition range and the chosen composition steps yielded 169169 composition sets/candidate alloy compositions. For each alloy composition an equilibrium calculation was performed at 900 °C and 1100 °C and equilibrium phase fractions of BCC, FCC, and intermetallics, i.e. Laves phase, were calculated.
4. Low Temperature Nitriding 4.1 Austenitic A286 The austenitic A286 is an austenitic preciptation hardening stainless steel characterised by a high content of SNCFE (viz. Ti); the system can be used to illustrate the concept of alloying Cr-Ni-Fe austenitic stainless steel with SNCFE. To this end, the material was solution treated but not aged, in order to have all SNCFE elements in solid solution in the FCC host matrix. The material is normally not intended to be used in this condition. Fig. 1a shows the nitrided case. The treatment temperature of 500 °C would normally be considered too high and would, for the conventional austenitic stainless steel grades, e.g. AISI 316, result in abundant formation of CrN and, concomitantly, loss of corrosion resistance. This was not the case for A286 containing SNCFE, where a featureless expanded austenite layer is observed. X-ray diffraction analysis (Fig. 2) confirms that expanded austenite (γN) has developed without the presence of CrN. The nitrided material shown in Fig. 1a was then subjected to a normal ageing procedure in order to see the influence of highly elevated temperatures on the expanded austenite zone; the resulting microstructure is shown in Fig. 1b. Clearly, nitrogen was redistributed and the original zone of expanded austenite appears at the outermost surface as a slightly darker zone. Interestingly, only minor formation of CrN at grain boundaries (sensitization) in the surface-adjacent (darker) zone is observed. Overall the morphology is largely unaffected by the thermal treatment. The diffusion zone that has developed near the substrate is featureless, indicating that no CrN has formed. X-ray diffraction analysis (Fig. 2) shows that expanded austenite is no longer present, instead broadened austenite peaks appear together with broad peaks of MN-type nitrides. This is interpreted as caused by the presence of TiN and possibly (Ti, Cr)N type nitrides in austenite. The zone that has developed underneath the original expanded austenite zone is formed by diffusion of nitrogen from expanded austenite deeper into the material. The peak broadening of austenite reflections indicates that these nitrides are very
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Fig. 1: Nitriding and subsequent ageing of austenitic A286. (a) Nitriding: 500 °C/14h/15 %NH3− 77 %H2− 8 %N2. (b) Nitriding (see a) and ageing; ageing 720 °C/16h/100 %N2. (c) Hardness depth profiles of the samples shown in a & b
small and induce misfit strains in the austenite lattice. The absence of strong etching effects is taken as an indication that the Cr-content in solid solution in the austenite is sufficiently high to maintain the stainless properties. Hardness profiles of the nitrided and nitrided + aged A286 are shown in Fig. 1c. Expanded austenite, which forms after nitriding at 500 °C, has a hardness of more than 1200 HV; the substrate has a hardness of around 200 HV as the temperature is too low for a significant hardening
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Fig. 2: X-ray diffractograms of nitrided and aged A286. Treatment conditions see Fig. 1
effect in the bulk (ageing). After treatment of the nitrided sample at 720 °C, still a very high surface hardness is obtained and almost 1000 HV is reached. The hardness profile reaches significantly deeper and exhibits a smooth decrease in the direction of the substrate. Clearly a significant hardening effect is obtained by formation of MN nitrides in austenite at this temperature. Interestingly, the bulk hardness is increased to around 320 HV by precipitation hardening (ageing).
4.2 Ferritic AISI 409 The low-temperature nitriding response of Ti-containing ferritic AISI 409 is shown in Fig. 3. Nitriding at 430 °C for 16 hours resulted in the microstructure seen in Fig. 3a. The microstructure resembles an untreated material; there are no visual signs of a surface hardened zone, i.e. the surface zone is unaffected by the etching reagent suggesting similar corrosion resistance in both surface and bulk. However, hardness measurements (Fig. 3b) show that a massive increase in hardness is obtained, up to 1200 HV in the surface and with a smooth transition to the substrate. The nitriding/hardening depth is around 40 µm, which is appreciably deeper than for austenitic grades at the same temperature and gas conditions. The main hardening effect is attributed to the presence of Cr, but it is hypothesized that the relatively low Ti-content also plays a significant role. Fig. 3: Nitriding of ferritic AISI 409. (a) micrograph and (b) hardness depth profile. Nitriding conditions: 430 °C/16 h/15 %NH3− 77 %H2− 8 %N2
5. Designing New Stainless Steels— Computational Approach
5.1 Alloying SNCFE to “AISI 316L”
The experiments above illustrate the strong influence of SNCFE in both austenitic and ferritic stainless steels. In the following it is showcased how new optimized SNCFE containing stainless steel alloys can be designed by a computational approach. BHM, 160. Jg. (2015), Heft 9
In Fig. 4 the influence of adding Ti to a master alloy based on AISI 316L is shown. It is clear that Ti is an effective ferrite stabilizer when added to AISI 316L (Fig. 4a, b), which is why an alloy like
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A286 (cf. Table 1) contains a high Ni content. When the content exceeds aproximately 1 wt% Ti, the material can no longer be considered an austenitic stainless steel, i.e. the conventional heat treatment temperature range does not result in a fully austenitic structure. Furthermore, Ti promotes formation of Laves phases (Fig. 4c). The calculations indicate that addition of Ti to AISI 316L is not a viable route for obtaining a fully austenitic alloy as even minor contents of SNCFE will stabilize Laves phases and BCC. The main effect of adding Al to AISI 316L (Fig. 5) is that ferrite is stabilized at high temperature; a maximum of around 1 wt% is allowed in order to maintain a fully austenitic structure in the conventional austenitization temperature range. As for Ti, only limited amounts of Al are allowed in austenitic AISI 316L if a fully austenitic structure is desired.
5.2 Composition Variation Analysis As shown in the previous section, only minor fractions of SNCFE (viz. Al & Ti shown here) can be alloyed to a standard austenitic stainless steel AISI 316L. This implies that, in order to reach higher contents of SNCFE, the composition of the “master alloy” has to be adjusted accordingly. Below the master alloy composition and contents of Al & Ti are varied within predefined composition intervals and in well-defined steps (cf. above). In Figs. 6 and 7, the calculated results for all investigated alloy compositions (169169 alloys) are presented showing the volume fraction of austenite, ferrite, and Laves phase
at 900 °C or 1100 °C, where each data point in the plots represents one alloy composition. The plots thus show the possible alloy response within the chosen composition range. On the abscissa the total amount of SNCFE (Al + Ti) is shown. Each color represents a fraction of 10 % of the total number of alloys for a given value of Al + Ti. This method of color-mapping is convenient because the variational approach implies that the number of alloy compositions shown for each value of Al + Ti varies from 1001 alloys for Al + Ti = 6 wt% to more than 10,000 for Al + Ti = 3 wt%. A candidate alloy should be fully austenitic in a fairly broad temperature range, e.g. 900–1100 °C. The bright green color represents the 10 % of the alloys having the largest possible fraction of FCC and the lowest fractions of BCC and Laves, whereas the deep blue represents the 10 % of the alloys having the most unfavorable compositions leading to the lowest possible fraction of FCC and high fractions of BCC and Laves. Figure 6a shows that many alloy compositions yield a fully austenitic structure at 900 °C and only for SNCFE contents higher than approximately 4 wt% a fully austenitic structure is no longer possible. Higher contents of SNCFE generally favor ferrite and Laves phase formation. At 1100 °C a fully austenitic structure can be obtained up to SNCFE contents of around 5 wt% (Fig. 7a). Formation of Laves phase is not as pronounced at this temperature (effect of Ti). The conventional austenitization temperature for austenitic stainless steel is usually between the temperatures 900 °C and 1100 °C, and Figs. 6 and 7 represent the limits of this range. From the data set, candidate alloy composi-
Fig. 4: Thermodynamic calculations showing the influence of adding Ti, listed in wt%, to AISI 316L: Phase fraction by volume of (a) FCC, (b) BCC and (c) Laves phase
Fig. 5: Thermodynamic calculations showing the influence of adding Al, listed in wt%, to AISI 316L: Phase fraction by volume of (a) FCC, (b) BCC
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Fig. 6: Composition variation analysis performed at 900 °C. Each figure contains 169169 different alloy compositions; each color code corresponds to 10 % fraction of the total number of alloys for a given total SNCFE content. Volume fraction of (a) FCC, (b) BCC and (c) Laves phase as a function of total SNCFE content (Al + Ti)
Fig. 7: Composition variation analysis performed at 1100 °C. See caption in Fig. 6
tions can be identified by extracting the compositions with the highest possible amount of Al + Ti, while still having a fully austenitic structure at both 900 and 1100 °C. The applied composition range for the “austenitic” alloy may not result in a fully austenitic structure upon cooling due to a possible martensitic transformation. The martensitic start temperature, Ms, calculated based on ref. [13], is presented in Fig. 8 for all alloys. The full data set falls within a range of approximately 230 °C; it should be noticed that both Al and Ti will raise Ms. Obviously, in order to end up having an austenitic stainless steel at room temperature, Ms should not be too high. The graph shows that many of the alloy compositions will effectively be partially or fully martensitic at room temperature. The presence of Al and Ti, which are effective elements for precipitation hardening in marageing steels, will actually make many of these alloy compositions precipitation hardening stainless steel. Compositions can also be chosen, which produces a stable austenitic stainless steel at room temperature and a martensitic stainless steel by sub-zero thermal treatment.
6. Discussion The presence of SNCFE is beneficial for the low temperature nitriding response of stainless steel as significantly higher treatment temperatures can be applied without impairing the corrosion performance through CrN formation. This provides the basis for tailoring steels that can be BHM, 160. Jg. (2015), Heft 9
Fig. 8: Martensite start temperature, Ms, for the entire data set of alloy systems. Ms according to ref. [13]
treated at higher temperatures and thus enables a much thicker case depth or makes it possible to expose surface hardened steels at higher temperatures as compared to the conventional stainless steels. Elements like Ti and Al are stronger nitride formers than Cr, and their presence effectively stabilizes expanded austenite, i.e. postpones or hinders formation of CrN. It is anticipated that the SNCFE will form very small SNCFE nitrides or that short range ordering between nitrogen and SNCFE will occur. Hence, hardening can derive from solid solution hardening as in expanded austenite or by precipitation/dispersion harden-
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ing from small misfitting SNCFE-nitrides(/carbides), where Cr is maintained in solid solution in the matrix phase (cf. Figs. 1 and 2). SNCFE-nitrides/carbides in both host matrices of FCC and BCC are most effective in yielding increased hardness; the same SNCFE elements, in relatively small amounts, are also known to provide high hardness in the non-stainless nitriding steels. The commercially available stainless steels will typically not contain SNCFE except for some specialty grades like some precipitation hardening (PH) stainless steel (e.g. 17/7 PH, A286) or in steels grades where they function as a carbon getter (AISI 409). The experimental nitriding results strongly suggest that new stainless steels could be designed to benefit from the presence of SNCFE with respect to surface hardening. This could entail both austenitic (main focus here) but also ferritic and martensitic precipitation hardening stainless steel. The use of some SNCFEs in PH stainless steels is well established and the possibility of performing simultaneous bulk and surface hardening of these steels grades has previously been shown [14]. The modelling approach presented here, where more than one hundred thousand alloys systems with SNCFE were “screened”, can be an effective tool for selecting new candidate alloys suitable for surface hardening. The results presented herein are merely intended to show the magnitude and possibilities of the computational method—not specific alloy compositions. The key point is that a large array of hypothetical alloys can be investigated with respect to different properties, e.g. phase stability at given temperatures as a function of different alloying elements. The challenging task is to impose relevant constraints on the large number of alloys that are handled. To give an example: such a constraint could be that the material should be 99.5 % austenite in a temperature range 900–1100 °C, that Ms should be below 0 °C, that the SNCFE content should be minimum 2.5 wt%, and that the Ni content should be lower than 12 wt%. A list of possible candidate alloys that fulfill these requirements can easily be extracted from the dataset resulting from the modelling. This approach makes an excellent input for choosing actual model alloys for production and further investigation, viz. low temperature surface hardening. The computational approach could also be used for modelling the actual surface hardening of alloys containing SNCFE. The driving force for precipitation of SNCFE-nitrides(/carbides) versus formation of chromium nitrides(/carbides) for various alloy compositions can be calculated together with imposed nitrogen activities in the solid state; i.e. effectively establishing conditions that would lead to preferential formation of SNCFE-nitrides over CrN.
to be beneficial with respect to low temperature surface hardening. Nitriding of commercially available austenitic A286 and ferritic AISI 409 show that SNCFE has a major influence on the nitriding response. The presence of SNCFE will enable the use of significantly higher temperatures than hitherto considered possible. An effective computational approach for designing alloys containing SNCFE was presented. Several hundred thousand hypothetical alloy systems can be handled and selection criteria can be imposed yielding relevant candidate alloys. It is envisaged that new “surface hardenable stainless steels” can be developed based on this method.
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7. Conclusions The presence of strong nitride/carbide formers (SNCFE) in a stainless steel host matrix (Cr-Ni-Fe) has been shown
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