AN OBJECT COMPRISING A PRE-OXIDIZED NICKEL-BASED ALLOY

Abstract

The present application relates to an object comprising a pre-oxidized nickel based alloy comprising in percent by weight (wt-%) C 0.05-0.2; Si max 1.5; Mn max 0.5; Cr 15-20; Al 4-6; Fe 15-25; Co max 5; N 0.03-0.15; O max 0.5; one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb 0.25-2.2; one or more elements selected from the group consisting of REM max 0.5; balance Ni and normally occurring impurities an also to the use of said object wherein the use is in an environment comprised of high concentration of nitrogen, low partial pressure of oxygen and high temperature.

Description

TECHNICAL FIELD

The present disclosure relates to an object comprising a pre-oxidized nickel-based alloy and to the use thereof in environments where the temperature is high and the atmosphere surrounding the object comprises a high concentration of nitrogen and a low oxygen partial pressure. These environments exist in e.g. sintering furnaces and muffle furnaces.

BACKGROUND

Nickel-based alloys comprise aluminium are used in a variety of high temperature applications, such as in heat treatment furnaces, since they will form a stable and protective aluminium oxide on the surface of objects made thereof. The formed aluminium oxide has a very good adhesion and does not tend to spall or fall off the surface. Furthermore, the aluminium oxide will have a low growth rate even at high temperatures.

However, it has been found that in applications wherein the gas atmosphere comprises high nitrogen content and low oxygen content nickel-based alloys comprising aluminium will form aluminium nitrides on the surface instead of the protective aluminium oxide. The formation of aluminium nitrides will penetrate into the metal alloy rapidly and will also have a negative effect on the ability of the alloy to form a protective surface oxide. Furthermore, the mechanical properties, such as ductility and creep strength will due to this be reduced.

The aspect of the present disclosure is to overcome the above-mentioned problems.

SUMMARY OF THE DISCLOSURE

The present disclosure therefore relates to an object comprising a pre-oxidized nickel-based alloy comprising by weight (wt-%)

C  0.05-0.2;
Si max 1.5;
Mn max 0.5;
Cr 15-20;
Al 4-6;
Fe 15-25;
Co max 5;
N  0.03-0.15;
O  max 0.5;

• • one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb 0.25-2.2;
  • one or more elements selected from the group consisting of the rare earth metals (REM) max 0.5;
  • balance Ni and normally occurring impurities.

The present disclosure also relates to the use of the object as defined hereinabove or hereinafter in environments having a high nitrogen concentration and a low oxygen partial pressure and high temperature. Examples of where such environments exist are in sintering furnaces and muffle furnaces.

Examples of objects, but not limited to, are mesh-belts, rollers (such as furnace rollers), tubes (such as radiation tubes and thermocouple protection tubes), fixtures and heating elements.

The alloy and the objects made thereof may be manufactured from a powder.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and b discloses mass gain curves of the different nickel-based alloys at different temperatures.

FIG. 2a to c discloses the surface of objects formed by the nickel-based alloy as defined hereinabove or hereinafter and another alloy, which objects have been exposed to the conditions of high nitrogen concentration and a low oxygen partial pressure and high temperature.

DETAILED DESCRIPTION

It has surprisingly been shown that an object comprising a pre-oxidized nickel-based alloy with the following composition in weight % (wt %):

C  0.05-0.2;
Si max 1.5;
Mn max 0.5;
Cr 15-20;
Al 4-6;
Fe 15-25;
Co max 5;
N  0.03-0.15;
O  max 0.5;

• • one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb 0.25-2.2;
  • one or more elements selected from the group consisting of the rare earth metals (REM) max 0.5;
  • balance Ni and normally occurring impurities
    will have very good nitridation resistance in environments having a high concentration of nitrogen and high temperature. The nickel-based alloy is described in EP 2617858 A1 and it is known to be used in applications where there is a need for high oxidation resistance and good creep resistance. However, as stated above, it is very surprising that this pre-oxidized nickel-based alloy is very resistant against nitridation in environments having a high concentration of nitrogen and high temperature as it is alloyed with aluminium. Thus, normally aluminium nitrides would be formed on the surface instead of the protective aluminium oxide and also just below the surface but for this pre-oxidised alloy as defined hereinabove or hereinafter, a protective aluminium oxide is formed on the surface.

The alloy is pre-oxidated before being made into an object, thus the object comprises a pre-oxidated nickel based alloy. The object may also be peroxided after being made into an object. The pre-oxidation is performed by exposing the material to a high temperature (above 900° C.) and to atmosphere comprising oxygen (e.g. air).

Examples of objects are manufacturing parts which are exposed to environment having a high concentration of nitrogen and a low concentration of oxygen at high temperature (more than about 900° C.). Other examples are mesh-belts, furnace rollers, radiation tubes, fixtures, heating elements, and thermocouple protection tubes

The term “high temperature” is intended to mean temperatures above or equal to 900° C., However, the highest possible temperature is 1300° C., such as about 1250° C.

According to the present disclosure, the phrase “high nitrogen content” is intended to mean that the nitrogen concentration of more than or equal to 25 vol % N₂, such as more than or equal to 50 vol % N₂, such as more than or equal to 75 vol % N₂, such as more than or equal to 95 vol % N₂, such as more than or equal to 98 vol % N₂. Additionally, the phrase “low oxygen pressure” is intended to mean an oxygen content of less than or equal to 1000 ppm.

The elementary composition of the nickel-based alloy is generally as defined hereinabove or hereinafter and the function of each alloying element is further described below. However, the listing of functions and effects of the respective alloying elements is not to be seen as complete, but there may be further functions and effects of said alloying elements. The terms weight %, wt % and % are used interchangeably.

Carbon

Carbon in free form will take interstitial locations in the crystal structure and thereby lock the mobility of dislocations at temperatures up to approximately 400-500° C. Carbon also forms carbides with other elements in the alloy such as Ta, Ti, Hf, Zr and Nb. In a microstructure with finely dispersed carbides, these carbides provide obstacles for the dislocation movement and have effect even at higher temperatures. Carbon is an essential element to improve the alloy's creep strength since the dislocation mobility is the mechanism that generates creep elongation. Too high contents of C will however lead to the alloy becoming difficult to cold work due to deteriorated ductility at lower temperatures, such as below 300° C. The alloy therefore comprises 0.05-0.2% C.

Silicon

Silicon can be present in the alloy in contents up to 1.5%. Silicon in too high contents can in nickel based alloys lead to increased risk for precipitations of nickel silicides, which have an embritteling effect on this type of alloy. Results from creep testing of similar alloys have shown that the creep life time, i.e. the time to creep fracture, is reduced with Si contents close to 1.5%. The reason for this is however not known. Because of this, the Si content should preferably be maximally 1%. According to one embodiment, the alloy as defined hereinabove or hereinafter only comprises impurity content of Si, i.e. up to 0.3%.

Manganese

Manganese is present in the alloy as an impurity. It is likely that up to 0.5% can be allowed without negatively influencing the properties of the alloy whereby the alloy comprises maximally 0.5% Mn. According to a one embodiment, the alloy as defined hereinabove or hereinafter only comprises impurity content of Mn, i.e. up to 0.2%.

Chromium

Chromium is an element which for a long period of time has been the leading element when it comes to creating a dense and protective oxide scale. Less than 15% Cr in an austenitic structure tends to render an oxide which is not entirely covering the surface and which is not dense and consequently render an insufficient oxidation resistance to the alloy. There is also a risk that the material closest to the oxide is depleted of Cr such that possible damages to the oxide cannot heal since there is not sufficient Cr to form new oxide.

A nickel based alloy comprising 4% Al should however not comprise more than about 20% Cr as higher contents increase the risk of formation of γ′ and β phases. Therefore, in order to minimise the presence of the γ′ and β phases, the alloy as defined hereinabove or hereinafter comprises max 20% Cr. There may also be a risk of formation of other unwanted phases, such as σ-phase and chromium rich ferrite, at too high Cr contents. Moreover, Cr may also at high contents stabilise nickel aluminides. Thus, the alloy as defined hereinabove or hereinafter comprises 15-20% Cr, such as 17-20% Cr, such as 17-19% Cr.

Aluminium

Aluminium is an element that generates a much denser and more protective oxide scale compared to Cr. Aluminium can however not replace Cr since the formation of the aluminium oxide is slower than the chromium oxide at lower temperatures. The alloy comprises at least 4% Al, such as more than 4% Al, which will ensure a sufficient oxidation resistance at high temperatures and that the oxide covers the surface entirely. The relatively high content of Al provides excellent oxidation resistance even at temperatures of about 1100° C. At Al contents above 6%, there is a risk of formation of such an amount of intermetallic phases in a nickel based matrix that the ductility of the material is considerably deteriorated (this will also be discussed below with reference to FIG. 3). The alloy should therefore comprise 4-6% Al, such as >4-5.5%, such as >4-5.2% Al.

Iron

It has been shown in accordance with the present invention that relatively high contents of Fe in an aluminium oxide forming nickel based alloy can have positive effects. Additions of Fe generate a metallic structure which is energetically unfavourable for the formation of embritteling γ′, which in turn leads to the risk of the alloy becoming hard and brittle reducing considerably. Consequently, the workability is improved. Therefore, the alloy comprises at least 15% Fe. High contents of iron may however lead to formation of unwanted phases. Therefore, the alloy shall not comprise more than 25% Fe.

Moreover, at Fe contents over approximately 21-22%, the risk of formation of a β-phase (NiAl), which in some cases can be embritteling, increases. The alloy should therefore comprise 16-21.5% Fe. According to a preferred embodiment, the alloy comprises 17-21% Fe.

Nickel

The alloy according to the invention is a nickel based alloy. Nickel is an element which stabilises an austenitic structure in alloys and thereby counteracts formation of some brittle intermetallic phases, such as σ-phase. The austenitic structure of the alloy is beneficial for example when it comes to welding. The austenitic structure also contributes to the good creep strength of the alloy at high temperatures. This could be a result of that the diffusion rate is lower in an austenitic structure than for example in a ferritic. According to one embodiment, the alloy comprises 52-62% Ni, such as 52-60% Ni.

Cobalt

In some commercial alloys, a part of the Ni is substituted with Co in order to increase the mechanical strength of the alloy which may also be done in the alloy according to the invention. A part of the Ni of the alloy can be replaced with an equal amount of Co. This Co addition must however be balanced against the oxidation properties since the presence of NiAl will reduce the activity of Al and thereby deteriorate the ability to form aluminium oxide. According to one embodiment of the present invention, nickel is partly substituted with Co. The Co content shall, however, not exceed 5%.

Nitrogen

In the same way as C, free N takes interstitial locations in the crystal structure and thereby locks the dislocation mobility at temperatures up to approximately 400-500° C. Nitrogen also forms nitrides and/or carbon nitrides with other elements in the alloy such as Ta, Ti, Hf, Zr and Nb. In a microstructure where these particles are finely dispersed they confer obstacles for the dislocation mobility, especially at higher temperatures. Therefore, N is added in order to improve the creep strength of the alloy. However, when adding N to aluminium alloyed alloys the risk is high for formation of secondary aluminium nitrides and the present nickel based alloy therefore has a very limited N content. The alloy comprises 0.03-0.15% N, such as 0.05-0.15% N, such as 0.05-0.10% N.

Oxygen

Oxygen may be present in the present alloy either in the form of an impurity, or as an active addition up to 0.5%. Oxygen may contribute to increasing the creep strength of the alloy by forming small oxide dispersions together with Zr, Hf, Ta and Ti, which, when they are finely distributed in the alloy, improves its creep strength. These oxide dispersions have higher dissolution temperature than corresponding carbides and nitrides, whereby oxygen is a preferred addition for use at high temperatures. Oxygen may also form dispersions with Al, the elements in group 3 of the periodic table, Sc, Y and La as well as the fourteen lanthanides, and in the same manner as with the above identified elements thereby contribute to higher creep strength of the alloy. According to a preferred embodiment, the alloy comprises 200-2000 ppm O, such as 400-1000 ppm O.

Tantalum, Hafnium, Zirconium, Titanium and Niobium

The elements in the group consisting of Ta, Hf and Zr forms very small and stable particles with carbon and nitrogen. It is these particles which, if they are finely dispersed in the structure, help to lock dislocation movement and thereby increase the creep strength, i.e. provides the dispersion strengthening. It is also possible to accomplish this effect with addition of Ti. Additions of Ti can, however, sometimes lead to problems, especially during powder metallurgical production of the alloy, since it forms carbides and nitrides already in the melt before atomisation, which in turn may clog the orifice during the atomisation.

Niobium also forms stable dispersions with C and or N and can therefore suitably be added to the alloy according to the invention.

The alloy comprises one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb in an amount of 0.25-2.2%, such as 0.3-1.5%, such as 0.6-1.5%.

The alloy may also comprise such an amount of the elements Ta, Zr, Hf, Ti and Nb that essentially all C and N is bound to these elements. This ensures that for example the risk of formation of chromium carbides during high temperature use of the alloy is significantly reduced.

According to a preferred embodiment, the alloy as defined hereinabove or hereinafter comprises 0.1-0.5% Hf. According to another embodiment, the alloy comprises 0.05-0.35% Zr. According to yet another embodiment, the alloy comprises 0.05-0.5% Ta. According to yet another embodiment, the alloy comprises 0.05-0.4% Ti. According to yet another embodiment, the alloy comprises 0.1-0.8% Nb.

Rare Earth Metals (REM)

Rare earth metals (REM) relates in this context to the elements of group three of the periodic table, Sc, Y, and La as well as the fourteen lanthanides. REM affects the oxidation properties by doping of the formed oxide. Excess alloying of these elements often gives an oxide which tends to spall of the surface and a too low addition of these elements tends to give an oxide with weaker adhesion to the metal surface. The alloy may comprise one or more elements from the group consisting of REM in a content of up to 0.5% in total, such as 0.05-0.25%. According to a one embodiment, yttrium is added to the alloy as defined hereinabove or hereinafter in an amount of 0.05-0.25%.

The nickel based alloy as defined hereinabove or hereinafter may also comprise normally occurring impurities as a result of the raw material used or the selected manufacturing process. Examples of impurities are Ca, S and P. Furthermore, other alloying elements, which will not affect the properties of the alloy may optionally be added in amounts up to 1%.

When the term “max” is used, the skilled person knows that the lower limit of the range is 0 wt % unless another number is specifically stated.

The prenickel-based alloy as defined hereinabove or hereinafter may be manufactured according to conventional methods, i.e. casting followed by hot working and/or cold working and optional additional heat treatment. The nickel-based alloy as defined hereinabove or hereinafter may also be used produced as a powder product by for example hot isostatic pressure process (HIP).

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

EXAMPLES

Two alloys were used in these examples. The compositions of the alloys are shown in table 1. Alloy 1 is an alloy according to the present disclosure, and Alloy 2 is an austenitic nickel-chromium-iron alloy of the standard UNS N06600.

The alloys were exposed in an atmosphere containing 5% H₂ and 95% N₂, with a dew point below −40° C., reminiscent of the environment in a sintering furnace. Two exposure temperatures were used; 900° C. and 1150° C. The effect of pre-oxidation was investigated.

Sample coupons with the dimensions 10×15×2 mm with one corner cut off were machined and ground with successively finer grinding paper, ending at 600 grit. After grinding, the dimensions of the samples were measured and identification numbers were punched into the edges of the samples. Prior to exposure, the samples were cleaned and degreased in ethanol and acetone and the mass of each sample was recorded using a Sartorius microbalance with microgram resolution. The samples were mounted in cylindrical crucibles and exposed in horizontal tube furnaces. Half of the samples were pre-oxidized at 1150° C. for 20 minutes prior to the exposure to the nitriding atmosphere. The parameters for the pre-oxidation were selected to resemble the final hot step of the production of tubes.

TABLE 1
The compositions of the tested alloys (wt %)
Alloy	C	Si	Mn	Cr	Ni	Mo	Al	Fe
1	0.16	—	—	17.70	Bal.	—	4.81	18.60
2	<0.05	0.4	0.8	16.5	72.5	—	—	<10.0
comperative

Exposures were performed at 900° C. and 1150° C. The atmosphere consisted of 95% nitrogen and 5% hydrogen. The dew point was kept below −40° C. and continuously monitored using hygrometers. The exposure times were 200, 500 and 1500 hours at both temperatures. The exposures were isothermal, with each sample being exposed once only.

Analysis

After exposure, the mass changes of the samples were recorded (see FIGS. 1a and b) and selected samples were cut in half parallel to the longest axis and mounted in polyfast conductive plastic, and polished with a 1 μm diamond suspension to produce flat cross sections for microscopy.

The microstructural analysis was done using two different microscopes. One was a Zeiss EVO 50 variable pressure scanning electron microscope (VP-SEM), and the other was a Zeiss Sigma VP-SEM. An acceleration voltage of 20 kV was used for imaging and chemical analysis by energy dispersive spectroscopy (EDS). Back scattered (BSE) were used for imaging. FIG. 2 - - - discloses examples of the microscope studie.

Result

The mass changes at 900° C. for all materials are shown in FIG. 1a). As can be seen for FIG. 1, the alloy of the disclosure both the pre-oxidized and the non-oxidized had the lowest mass change. The lowest mass change was exhibited by the pre-oxidized alloy of the present disclosure samples, while the corresponding samples that were not pre-oxidized had the second lowest mass changes. The mass changes of sample of alloy 2 were higher.

In FIG. 1b), the mass changes at 1150° C. are shown. As can be seen from figure, the mass gains all of the samples are low. Thus, these results indicate that the samples of the alloy of the present invention will not gain mass by forming nitrides. Even though alloy 2 had the lowest mass change, FIG. 2c shows that nitrides are formed, to be compared with FIG. 2b (the present alloy) wherein no nitrides are formed. Thus alloy 2 is not suitable to be used in the conditions defined herein even though it had the lowest mass gain.

Hence, the result as shown in FIG. 1a and FIG. 1b shows that the samples of the alloy of the present disclosure has very little mass gain thus indicating that almost no nitrides are formed.

As can be seen from FIGS. 2a and b, the alloy according to the disclosure shows a nitridation resistance in 5% H₂-95% N₂. At 1150° C., there is no sign of nitridation on and at 900° C., only modest nitridation is seen on samples of the present nickel based alloy that have not been pre-oxidized. Without being bound to any theory, it is believed that it is possible this may be due to formation of transient alumina.

Further, as can be seen from the FIG. 2c, nitrides are formed on the surface of alloy 2, which makes it unsuitable for the conditions defined herein.

Hence, both FIG. 1a and FIG. 1b and the photos, an alloy of the present disclosure can be used in nitriding environments, especially at higher temperatures as the alloy will almost form no nitrides which keep the aluminum oxide layer undamaged and thereby preventing corrosion.

Claims

1. An object comprising a pre-oxidized nickel based alloy comprising in percent by weight (wt-%) C 0.05-0.2; Si max 1.5; Mn max 0.5; Cr   15-20; Al   4-6; Fe   15-25; Co max 5; N 0.03-0.15; O max 0.5;

one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb 0.25-2.2;

one or more elements selected from the group consisting of REM max 0.5;

balance Ni and normally occurring impurities.

2. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises 16-21.5 wt-% Fe.

3. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises 17-20 wt-% Cr.

4. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises max 0.3 wt-% Si.

5. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises max 1 wt-% Co.

6. The object according to claim 1, wherein the pre-oxidized nickel-based alloy one or more elements selected from the group consisting of REM in a total content of 0.05-0.25 wt-%.

7. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and Nb in a total content of 0.3-1.5 wt-%.

8. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises >4-5.5 wt-% Al.

9. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises 200-2000 ppm O.

10. The object according to claim 1, wherein the pre-oxidized nickel-based alloy comprises 52-62 wt-% Ni.

11. The object according to claim 1, wherein the pre-oxidized nickel-based alloy is oxidized before use.

12. Manufacture of an object comprising the pre-oxidized nickel-based alloy according to claim 1, wherein said manufacture comprises a step of preoxidation.

13. Use of an object comprising the pre-oxidized nickel-based alloy according to claim 1, wherein said use is in an atmosphere comprising a high concentration of nitrogen and a low oxygen partial pressure.

14. The use according to claim 13, wherein said use also comprises a high temperature.

15. The object according to claim 2, wherein the pre-oxidized nickel-based alloy comprises 17-20 wt-% Cr.

16. The object according to claim 1, wherein the object is a mesh belt, a furnace roller, a radiation tube, a thermocouple protection tube, or a heating element.