Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium
  • C22C19/058—Alloys based on nickel or cobalt based on nickel with chromium without Mo and W
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
  • C22C19/03—Alloys based on nickel or cobalt based on nickel
  • C22C19/05—Alloys based on nickel or cobalt based on nickel with chromium

Definitions

• 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.
• 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.
• 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.
• the mechanical properties, such as ductility and creep strength will due to this be reduced.
• the present disclosure therefore relates to an object comprising a pre-oxidized nickel- based alloy comprising by weight (wt-%)
• one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and b 0.25-2.2;
• REM rare earth metals
• 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.
• Figure 1 a and b discloses mass gain curves of the different nickel-based alloys at different temperatures.
• Figure 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.
• one or more elements selected from the group consisting of Ta, Zr, Hf, Ti and b 0.25-2.2 ;
• REM rare earth metals
• nickel-based alloy is described in EP 2617858 Al and it is known to be used in applications where there is a need for high oxidation resistance and good creep resistance.
• 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.
• 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
• 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.
• the phrase "high nitrogen content” is intended to mean that the nitrogen concentration of more than or equal to 25 vol% N 2 , such as more than or equal to 50 vol% N 2 , such as more than or equal to 75 vol% N 2 , such as more than or equal to 95 vol% N 2 , such as more than or equal to 98 vol% N 2 .
• the phrase “low oxygen pressure” is intended to mean an oxygen content of less than or equal to lOOO 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.
• 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 micro structure 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 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.
• the Si content should preferably be maximally 1 %.
• the alloy as defined hereinabove or hereinafter only comprises impurity content of Si, i.e. up to 0.3 %.
• 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 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 ⁇ ' and ⁇ phases.
• the alloy as defined hereinabove or hereinafter comprises max 20 % Cr.
• 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.
• the alloy should therefore comprise 4-6 % Al, such as >4-5.5 %, such as >4 - 5.2 % A1.
• 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 ⁇ -phase ( iAl), 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.
• 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 ⁇ -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.
• the alloy comprises 52-62 % Ni, such as 52-60% Ni.
• 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.
• nickel is partly substituted with Co. The Co content shall, however, not exceed 5%. Nitrogen
• 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.
• the alloy comprises 0.03-0.15 % N, such as 0.05-0.15 % N, such as 0.05-0.10 % N.
• 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.
• 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 b 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.
• the alloy as defined hereinabove or hereinafter comprises 0.1 -0.5% Hf.
• the alloy comprises 0.05-0.35 % Zr.
• the alloy comprises 0.05-0.5 % Ta.
• 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
• Rare earth metals 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.
• impurities are Ca, S and P.
• other alloying elements, which will not affect the properties of the alloy may optionally be added in amounts up to 1 %.
• max 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).
• HIP hot isostatic pressure process
• Alloy 1 is an alloy according to the present disclosure
• Alloy 2 is an austenitic nickel-chromium-iron alloy of the standard U S N06600.
• the alloys were exposed in an atmosphere containing 5% 3 ⁇ 4 and 95% N 2 , 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 x 15 x 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%)
• 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
• the micro structural 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.
• Figure 2 discloses examples of the microscope studie.
• the alloy according to the disclosure shows a nitridation resistance in 5% H 2 - 95% N 2 .
• the alloy according to the disclosure shows a nitridation resistance in 5% H 2 - 95% N 2 .
• 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.
• nitrides are formed on the surface of alloy 2, which makes it unsuitable for the conditions defined herein.
• 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.

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• 2017
  • 2017-05-19 WO PCT/EP2017/062136 patent/WO2017198831A1/en unknown
  • 2017-05-19 EP EP17723722.9A patent/EP3458620A1/de not_active Withdrawn
  • 2017-05-19 CN CN201780031213.2A patent/CN109154038A/zh active Pending
  • 2017-05-19 US US16/302,788 patent/US20190292631A1/en not_active Abandoned
  • 2017-05-19 JP JP2018560765A patent/JP2019519677A/ja active Pending

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