FERRITIC ALLOY

Abstract

A ferritic alloy comprising the following elements in weight % C 0.01 to 0.1; N: 0.001 to 0.1; 0: <0.2; Cr 4 to 15; Al 2 to 6; Si 0.5 to 3; Mn: ≤0.4; Mo+W≤4; Y≤1.0; Sc, Ce, La and/or Yb≤0.2; Zr≤0.40; RE≤3.0; balance Fe and normal occurring impurities and also fulfilling the following equation has to be fulfilled: 0.014≤(Al+0.5SQ (Cr+10Si+0.1)≤0.022.

Description

TECHNICAL FIELD

The present disclosure relates to a ferritic alloy according to the preamble of claim 1. The present disclosure further relates to use of the ferritic alloy and to objects or coatings manufactured thereof.

BACKGROUND AND INTRODUCTION

Ferritic alloys, such as FeCrAl-alloys comprising chromium (Cr) levels of 15 to 25 wt % and aluminium (Al) levels from 3 to 6 wt % are well known for their ability to form protective α-alumina (Al₂O₃), aluminium oxide, scales when exposed to temperatures between 900 and 1300° C. The lower limit of Al content to form and maintain the alumina scale varies with exposure conditions. However, the effect of a too low Al level at higher temperatures is that the selective oxidation of Al will fail and less stable and less protective scales based on chromium and iron will be formed.

It is commonly agreed that FeCrAl alloys will normally not form the protective α-alumina layer if exposed to temperatures below about 900° C. There have been attempts to optimize the compositions of FeCrAl alloys so that they will form the protective α-alumina at temperature below about 900° C. However, in general, these attempts have not been very successful because the diffusion of oxygen and aluminium to the oxide-metal interface will be relatively slow at lower temperatures and thereby the rate of formation of the alumina scale will be low, which means that there will be a risk of severe corrosion attacks and formation of less stable oxides.

Another problem arising at lower temperature, i.e. temperatures below 900° C., is a long term embrittlement phenomena arising from a low temperature miscibility gap for Cr in the FeCrAl alloy system. The miscibility gap exists for Cr levels above approximately 12 wt % at 550° C. Recently, alloys with lower Cr levels of about 10 to 12 wt % Cr have been developed in order to avoid this phenomenon. This group of alloys has been found to work very well in molten lead at controlled and low pressure O₂.

EP 0 475 420 relates to a rapidly solidified ferritic alloy foil essentially consisting of Cr, Al, Si, about 1.5 to 3 wt % and REM (Y, Ce, La, Pr, Nd, the balance being Fe and impurities. The foil may further contain about 0.001 to 0.5 wt % of at least one element selected from the group consisting of Ti, Nb, Zr and V. The foil has a grain size of not more than about 10 μm. EP 075 420 discusses Si additions in order to improve the flow properties of the alloy melt but the success was limited due to reduced ductility.

EP 0091 526 relates to thermal cyclic oxidation resistant and hot workable alloys, more particularly, to iron-chromium-aluminium alloys with rare earth additions. In oxidation, the alloys will produce a whisker-textured oxide that is desirable on catalytic converter surfaces. However, the obtained alloys did not provide a high temperature resistance.

Hence, there is still a need to further improve the corrosion resistance of ferritic alloys so that they can be used in corrosive environments during high temperature conditions. The aspects of the present disclosure are to solve or at least reduce the above-mentioned problems.

SUMMARY OF THE DISCLOSURE

The present disclosure therefore relates to a ferritic alloy, which will provide a combination of good oxidation resistance and an excellent ductility, comprising the following composition in weight % (wt %):

• • C 0.01 to 0.1;
  • N: 0.001-0.1;
  • O: 0.2;
  • Cr 4 to 15;
  • Al 2 to 6;
  • Si 0.5 to 3;
  • Mn: ≤0.4;
  • Mo+W≤4;
  • Y≤1.0;
  • Sc, Ce, La and/or Yb≤0.2;
  • Zr≤0.40;
  • RE≤3.0;
  • balance Fe and normal occurring impurities and also fulfilling the following equation has to be fulfilled:

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022.

Thus, there exists a relationship between the content of Cr and Si and Al in the alloy according to the present disclosure, which if fulfilled will provide an alloy having excellent oxidation resistance and ductility and also a reduced brittleness in combination with increased high temperature corrosion resistance.

The present disclosure also relates to an object and/or a coating comprising the ferritic alloy according to the present disclosure. Additionally, the present disclosure also relates to the use of the ferritic alloy as defined hereinabove or hereinafter for manufacturing an object and/or a coating.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1a and FIG. 1b disclose the phases in Fe-10% Cr-5% Al vs. Si level (FIG. 1a) and Fe-20% Cr-5% Al vs. Si level (FIG. 1b). The diagram has been made by using Database TCFE7 and Thermocalc software.

FIGS. 2a to e disclose polished sections of two alloys according to the present disclosure compared to three reference alloys after exposure to 50 times 1 hour cycles at 850° C. exposed to biomass (wood pellets) ash containing large amounts of potassium.

DETAILED DESCRIPTION OF THE DISCLOSURE

As already stated above, the present disclosure provides a ferritic alloy comprising in weight % (wt %):

• • C 0.01 to 0.1;
  • N: 0.001-0.1;
  • O: ≤0.2;
  • Cr 4 to 15;
  • Al 2 to 6;
  • Si 0.5 to 3;
  • Mn: ≤0.4;
  • Mo+W≤4;
  • Y≤1.0;
  • Sc, Ce, La and/or Yb≤0.2;
  • Zr≤0.40;
  • RE≤3.0;
  • balance Fe and normal occurring impurities and also fulfilling the following equation has to be fulfilled:

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022.

It has surprisingly been found that an alloy as defined hereinabove or hereinafter, i.e. containing the alloying elements and in the ranges mentioned herein, unexpectedly will form a protective surface layer containing aluminium rich oxide even at chromium levels as low as 4 wt %. This is very important both for the workability and for the long term phase stability of the alloy as the undesirable brittle σ-phase, after exposure for long time in the herein mentioned temperature range, will be reduced or even avoided. Thus, the interaction between Si and Al and Cr will enhance the formation of a stable and continuous protective surface layer containing aluminium rich oxide, and by using the above equation, it will be possible to add Si and still obtain a ferritic alloy which will be possible both to produce and to form into different objects. The inventor has surprisingly found that if the amounts of Si and Al and Cr are balanced so that the following condition is fulfilled (all the numbers of the elements are in weight fractions):

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022,

the obtained alloy will have a combination of excellent oxidation resistance and workability and formability within the Cr range of the present disclosure. According to one embodiment, 0.015≤(Al+0.5Si)(Cr+10Si+0.1)≤0.021, such as 0.016≤(Al+0.5Si)(Cr+10Si+0.1)≤0.020, such as 0.017≤(Al+0.5Si)(Cr+10Si+0.1)≤0.019.

The ferritic alloy of the present disclosure is especially useful at temperatures below about 900° C. since a protective surface layer containing aluminium rich oxide will be formed on an object and/or a coating made of said alloy, which will prevent corrosion, oxidation and embrittlement of the object and/or the coating. Furthermore, the present ferritic alloy may provide protection against corrosion, oxidation and embrittlement at temperatures as low as 400° C. as a protective surface layer containing aluminium rich oxide will be formed on the surface of the object and/or coating manufactured thereof. Additionally, the alloy according to the present disclosure will also work excellent at temperatures up to about 1100° C. and it will show a reduced tendency for long-term embrittlement in the temperature range of 400 to 600° C.

The present alloy may be used in the form of a coating. Additionally, an object may also comprise the present alloy. According to the present disclosure, the term “coating” is intended to refer to embodiments in which the ferritic alloy according to the present disclosure is present in form of a layer exposed to a corrosive environment that is in contact with a base material, regardless of the means and methods to accomplish it, and regardless of the relative thickness relation between the layer and the base material. Hence, examples of this but not limited to is a PVD coating, a cladding or a compound or composite material. The aim of the alloy is that is should protect the material underneath from both corrosion and oxidation. Examples, but not limited to, of suitable objects is a compound tube, a tube, a boiler, a gas turbine component and a steam turbine component. Other examples include a superheater, a water wall in a power plant, a component in a vessel or a heat exchanger (for example for reforming or other processing of hydrocarbons or gases containing CO/CO₂), a component used in connection with industrial heat treatment of steel and aluminium, powder metallurgy processes, gas and electric heating elements.

Furthermore, the alloy according to the disclosure is suitable to be used in environments having corrosive conditions. Examples of such environments include but are not limited exposure to salts, liquid lead and other metals, exposures to ash or high carbon content deposits, combustion atmospheres, atmospheres with low pO₂ and/or high N₂ and/or high carbon activity environments.

Additionally, the present ferritic alloy may be manufactured by using normally occurring solidification rates ranging from conventional metallurgy to rapid solidification. The present alloy will also be suitable for manufacturing all types of objects both forged and extruded, such as a wire, a strip, a bar and a plate. The amount of hot and cold plastic deformation as well as grain structure and grain size will, as the person skilled in the art know vary between the forms of the objects and the production route.

The functions and effects of essential alloying elements of the alloy defined hereinabove and hereinafter will be presented in the following paragraphs. The listing of functions and effects of the respective alloying elements is not to be seen as complete as there may be further functions and effects of said alloying elements.

Carbon (C)

Carbon may be present as an unavoidable impurity resulting from the production process. Carbon may also be included in the ferritic alloy as defined hereinabove or hereinafter to increase strength by precipitation hardening. To have a noticeable effect on the strength in the alloy, carbon should be present in an amount of at least 0.01 wt %. At too high levels, carbon may result in difficulties to form the material and also a negative effect on the corrosion resistance. Therefore, the maximum amount of carbon is 0.1 wt %. For example, the content of carbon is 0.02-0.09 wt %, such as 0.02-0.08 wt %, such as 0.02-0.07 wt % such as 0.02-0.06 wt % such as 0.02-0.05 wt %, such as 0.01-0.04 wt %.

Nitrogen (N)

Nitrogen may be present as an unavoidable impurity resulting from the production process. Nitrogen may also be included in the ferritic alloy as defined hereinabove or hereinafter to increase strength by precipitation hardening, in particular when a powder metallurgical process route is applied. At too high levels, nitrogen may result in difficulties to form the alloy and also have a negative effect on the corrosion resistance. Therefore, the maximum amount of nitrogen is 0.1 wt %. Suitable ranges of nitrogen are for example 0.001-0.08 wt %, such as 0.001-0.05 wt %, such as 0.001-0.04 wt %, such as 0.001-0.03 wt %, such as 0.001-0.02 wt %.

Oxygen (O)

Oxygen may exist in the alloy as defined hereinabove or hereinafter as an impurity resulting from the production process. In that case, the amount of oxygen may be up to 0.02 wt %, such as up to 0.005 wt %. If oxygen is added deliberately to provide strength by dispersion strengthening, as when manufacturing the alloy through a powder metallurgical process route, the alloy as defined hereinabove or hereinafter, comprises up to or equal to 0.2 wt % oxygen.

Chromium (Cr)

Chromium is present in the present alloy primarily as a matrix solid solution element. Chromium promotes the formation of the aluminium oxide layer on the alloy through the so-called third element effect, i.e. by formation of chromium oxide in the transient oxidation stage. Chromium shall be present in the alloy as defined hereinabove or hereinafter in an amount of at least 4 wt % to fulfill this purpose. In the present inventive alloy, Cr also enhances the susceptibility to form brittle σ phase and Cr₃Si. This effect emerges at around 12 wt % and is enhanced at levels above 15 wt %, therefore the limit of Cr is 15 wt %. Also from oxidation point of view, higher levels than 15 wt % will result in an undesirable contribution of Cr into the protective oxide scales. According to one embodiment, the content of Cr is 5 to 13 wt %, such as 5 to 12 wt %, such as 6 to 12 wt %, such as 7 to 11 wt %, such as 8 to 10 wt %.

Aluminium (Al)

Aluminium is an important element in the alloy as defined hereinabove or hereinafter. Aluminium, when exposed to oxygen at high temperature, will form the dense and thin oxide, Al₂O₃, through selective oxidation, which will protect the underlying alloy surface from further oxidation. The amount of aluminium should be at least 2 wt % to ensure that a protective surface layer containing aluminium rich oxide is formed and also to ensure that sufficient aluminium is present to heal the protective surface layer when damaged. However, aluminium has a negative impact on the formability and high amounts of aluminium may result in the formation of cracks in the alloy during mechanical working thereof. Consequently, the amount of aluminium should not exceed 6 wt %. For example, aluminium may be 3-5 wt %, such as 2.5-4.5 wt %, such as 3 to 4 wt %.

Silicon (Si)

In commercial FeCrAl alloys, silicon is often present in levels of up to 0.4 wt %. In ferritic alloys as defined hereinabove or hereinafter, Si will play a very important role as it has been found to have a great effect on improving the oxidation and corrosion resistance. The upper limit of Si is set by the loss of workability in hot and cold condition and increasing susceptibility to formation of brittle Cr₃Si and σ phase during long term exposure. Additions of Si therefore have to be performed in relation to the content of Al and Cr. The amount of Si is therefore between 0.5 to 3 wt %, such as 1 to 3 wt %, such as 1 to 2.5 wt %, such as 1.5 to 2.5 wt %.

Manganese (Mn)

Manganese may be present as an impurity in the alloy as defined hereinabove or hereinafter up to 0.4 wt %, such as from 0 to 0.3 wt %.

Yttrium (Y)

In melt metallurgy, yttrium may be added in an amount up to 0.3 wt % to improve the adherence of the protective surface layer. Furthermore, in powder metallurgy, if yttrium is added to create a dispersion of together with oxygen and/or nitrogen, the yttrium content is in an amount of at least 0.04 wt %, in order to accomplish the desired dispersion hardening effect by oxides and/or nitrides. The maximum amount of yttrium in dispersion hardened alloys in the form of oxygen containing Y compounds may be up to 1.0 wt %.

Scandium (Sc), Cerium (Ce) and Lanthanum (La), Ytterbium (Yb)

Scandium, Cerium, Lanthanum, and Ytterbium are interchangeable elements and may be added individually or in combination in a total amount of up to 0.2 wt % to improve oxidation properties, self-healing of the aluminium oxide (Al₂O₃) layer or the adhesion between the alloy and the Al₂O₃ layer.

Molybdenum (Mo) and Tungsten (W)

Both molybdenum and tungsten have positive effects on the hot-strength of the alloy as defined hereinabove or hereinafter. Mo has also a positive effect on the wet corrosion properties. They may be added individually or in combination in an amount up to 4.0 wt %, such as from 0 to 2.0 wt %.

Reactive Elements (RE)

Per definition, the reactive elements are highly reactive with carbon, nitrogen and oxygen. Titanium (Ti), Niobium (Nb), Vanadium (V), Hafnium (Hf), Tantalum (Ta) and Thorium (Th) are reactive elements in the sense that they have high affinity to carbon, thus being strong carbide formers. These elements are added in order to improve the oxidation properties of the alloy. The total amount of the elements is up to 3.0 wt %, such more than 1.0 wt %, such as 1.5 to 2.5 wt %.

The maximum amounts of respective reactive element will depend mainly on tendency of the element to form adverse intermetallic phases.

Zirconium (Zr)

Zirconium is often referred to as a reactive element as since it is very reactive towards oxygen, nitrogen and carbon. In the present alloy, it has been found that Zr has a double role as it will be present in the protective surface layer containing aluminium rich oxide thereby improving the oxidation resistance and it will also form carbides and nitrides. Thus, in order to achieve the best properties of the protective surface layer containing aluminium rich oxide, it is advantageous to include Zr in the alloy.

However, Zr-levels above 0.40 wt % will have an effect on the oxidation due to the formation of Zr rich intermetallic inclusions and levels below 0.05 wt % will be too small to fulfill the dual purpose, regardless of the C and N content. Thus, if Zr is present, the range is between 0.05 to 0.40 wt %, such as 0.10 to 0.35.

Furthermore, it has also been found that the relationship between Zr and N and C may be important in order to achieve even better oxidation resistance of the protective surface layer, i.e. the alumina scale. Thus, the inventor has surprisingly found that if Zr is added to the alloy and the alloy also comprises N and C and if the following condition (the element content given in weight %) is fulfilled:

$-0.15\le \mathrm{Zr}-\frac{4.7C+4N}{0.62}\le 0.15,$

such as

$-0.15\le \mathrm{Zr}-\frac{4.7C+4N}{0.62}\le 0.10,$

such as

$-0.05\le \mathrm{Zr}-\frac{4.7C+4N}{0.62}\le 0.10,$

the obtained alloy will achieve a good oxidation resistance.

The balance in the ferritic alloy as defined hereinabove or hereinafter is Fe and unavoidable impurities. Examples of unavoidable impurities are elements and compounds which have not been added on purpose, but cannot be fully avoided as they normally occur as impurities in e.g. the material used for manufacturing the ferritic alloy.

FIG. 1a and FIG. 1b shows that higher Cr in a Si-containing ferritic alloy is prone to form Si₃Cr inclusions and at 20% Cr also to promote undesirable brittle σ-phase after exposure for long time in the focus temperature area. Although diagrams are only shown for two Cr levels, 10 and 20%, the trend of embrittling phases increasing with higher Cr is clearly demonstrated Note the absence of σ-phase at 10% Cr and the increasing amount of Cr₃Si phase at higher Si content at both Cr levels. Hence, these figures show that there will be problems when using Cr levels around 20%.

When the terms “≤” or “less than or equal to” are used in the following context: “element≤number”, the skilled person knows that the lower limit of the range is 0 wt % unless another number is specifically stated. Further, the undefined article “a” does not exclude a plurality.

The present disclosure is further illustrated by the following non-limiting examples.

EXAMPLES

Test melts were produced in a vacuum melting furnace. The compositions of the test melts are shown in table 1.

The obtained samples were hot rolled and machined to flat rods with a cross section of 2×10 mm. They were then cut into 20 mm long coupons and ground with SiC paper to 800 mesh for exposure to air and combustion conditions. Some of the rods were cut to 200 mm long×3×12 mm rods for tensile testing at room temperature in a Zwick/Roell Z100 tensile test apparatus.

The results from exposure and tensile tests are shown in table 1.

The samples were tested for yield and rupture stress as well as elongation to rupture in a standard tensile test machine and the result giving >3% elongation to rupture is designated “x” in “Workable” column of the table. The “x” therefore designates an alloy that is easily hot rolled and that shows ductile behavior at room temperature. In the “Oxidation” column, the “x” designates that the alloy forms a protective alumina rich oxide scale at 950° C. in air and at 850° C. with biomass ash deposit.

TABLE 1
Composition of the melts and the results of testing workability and
oxidation an (x) designates a value between 3 and 6% elongation.
Composition/							Work-	Oxi-
Melt-number	Cr	Al	Si	C	N	Zr	able	dation
4785	5.2	4.0	0.03	0.020	0.012	0.296	x	No
Comparative
4784	5.2	6.0	0.02	0.025	0.012	0.297	x	No
Comparative
4783	5.2	3.9	1.96	0.021	0.010	0.292	x	X
(disclosure)
4782	10.0	2.0	0.02	0.025	0.014	0.273	x	No
Comparative
4781	10.0	3.0	0.03	0.025	0.021	0.296	x	No
Comparative
4780	10.1	4.0	0.02	0.021	0.015	0.296	x	No
Comparative
4779	10.1	4.0	1.91	0.022	0.013	0.296	x	X
(disclosure)
4778	10.2	5.9	0.11	0.018	0.012	0.294	x	No
Comparative
4777	20.0	4.0	0.02	0.018	0.020	0.295	Failed	No
Comparative							in
							rolling
4776	20.1	4.0	0.04		0.014	0.296	x	No
Comparative
4774	20.2	5.1	0.05	0.014	0.009	<0.01	x	No
Comparative
4773	19.7	4.8	0.02	0.004	<0.01	<0.01	x	No
Comparative
4772	12.2	3.6	2.5	0.003	<0.01	0.237	Failed	No
comparative							in
							rolling
4799	20.0	2.8	1.87	0.023	0.017	0.281	x	No
Comparative
4800	14.9	3.0	1.9	0.022	0.013	0.296	x	x
(disclosure)
4855	10.1	3.8	1.96	0.019	0.012	0.279	x	x
(disclosure)
4856	10.0	5.0	2.0	0.015	0.012	0.285	Failed	No
Comparative							in
							rolling
4857	10.0	3.1	1.97	0.025	0.015	0.297	x	x
(disclosure)
4858	14.7	3.9	2.01	0.022	0.015	0.292	x	x
(disclosure)
4859	12.1	4.0	2	0.024	0.014	0.289	X	x
(disclosure)
4860	12.0	3.1	1.98	0.016	0.014	0.284	X	x
(disclosure)
4861	10.0	4.0	1.99	0.015	0.015	0.29	X	x
(disclosure)

Thus, as can be seen from the table above, the alloys of the present disclosure shows good workability and good oxidation performance.

FIGS. 2 a) to e) disclose samples which are polished sections of of the present disclosure (FIGS. 2a) 4783 and 2b) 4779) compared to three comparative alloys after exposure to 50 times 1 hour cycles at 850° C. exposed to biomass (wood pellets) ash containing large amounts of potassium. The micrographs are taken in a JEOL FEG SEM at 1000 times magnification and show a clear advantage in behavior between the alloys of the present disclosure and reference materials. As can be seen, on the alloys of present disclosure, a 3-4 gm thin and protective alumina scale (aluminium oxide layer) has been formed, whereas a thicker and less protective chromia (chromium oxide) rich scale is formed on the stainless steel (2c—11Ni, 21Cr, N, Ce, Fe bal.) and Ni-base alloy (2e—Inconel 625: 58Ni, 21Cr, 0.4Al, 0.5Si, Mo, Nb, Fe), and a relatively porous and not as protective alumina scale forms on the comparative FeCrAl alloy (alloy 4776) (FIGS. 2d—20Cr, 5Al, 0.04 Si, Fe bal).

As can be seen from FIGS. 2a-e, the addition of Si, Al and Cr according to the ranges according to the present disclosure will promote alumina scale formation at Al levels as low as about 2 wt % and at chromium levels as low as 5 wt %.

Claims

1. A ferritic alloy comprising the following elements in weight % (wt %): wherein the following equation has to be is fulfilled (elements in weight fraction):

C 0.01 to 0.1;

N: 0.001 to 0.1;

O: 0.2;

Cr 4 to 15;

Al 2 to 6;

Si 0.5 to 3;

Mn: ≤0.4;

Mo+W≤4;

Y≤1.0;

Sc, Ce, La and/or Yb≤0.2;

Zr 0.40;

RE≤3.0;

balance Fe and normal occurring impurities, and

0.014≤(Al+0.5Si)(Cr+10Si+0.1)≤0.022.

2. The ferritic alloy according to claim 1, wherein (elements in weight fractions)

0.015≤(Al+0.5Si)(Cr+10Si+0.1)≤0.021.

3. The ferritic alloy according to claim 1, wherein Zr is of from 0.05 to 0.40 weight %.

4. The ferritic alloy according to claim 1, wherein Cr is of from 5 to 13 weight %.

5. The ferritic alloy according to claim 1, wherein RE is more than 1.0 to 3.0 weight %.

6. The ferritic alloy according to claim 1, wherein Al is of from 2.5 to 4.5 weight % or from 3 to 5 weight %.

7. The ferritic alloy according to claim 1, wherein Al is of from 3 to 4 weight %.

8. The ferritic alloy according to claim 1, wherein Si is of from 1.0 to 3 weight %.

9. The ferritic alloy according to claim 1, wherein Si is of from 1.5 to 2.5 weight %.

10. The ferritic alloy according to claim 1, wherein Zr is of from 0.10 to 0.35 weight %.

11. The ferritic alloy according to claim 1, wherein the amount of C, N and Zr fulfills the following equation: - 0.15 ≤ Zr - 4.7  C + 4   N 0.62 ≤ 0.15.

12. A coating comprising the ferritic alloy according to claim 1.

13. An object comprising the ferritic alloy according to claim 1.

14. Use of the ferritic alloy according to claim 1 for manufacturing a coating and/or a cladding and/or an object.

15. Use of the ferritic alloy according to claim 1 for manufacturing an object or a coating to be used in corrosive environments.

16. Use of the ferritic alloy according to claim 1 for manufacturing an object or a coating to be used in a furnace or as a heating element.

17. Use of the ferritic alloy according to claim 1 in environments wherein the ferritic alloy is exposed to salts, liquid lead and other metals, exposed to ash or high carbon content deposits, combustion atmospheres, atmospheres with low pO2 and/or high N2 and/or high carbon activity.