OXIDATION RESISTANT ALLOY

Abstract

The present invention relates to alloys used to prepare steel pipes i.e. tubes for use in chemical engineering applications. In particular, the invention relates to low carbon aluminium steel alloys and pipes made from such alloys. They may be used in plant such as ethylene cracker furnaces that need to be able to withstand elevated temperatures oxidation and carburisation for extended periods of time, the alloy been able to develop a pure, stable and continuous aluminium oxide layer on it surface when in service which is protective and anti-coking

Claims

1.-14. (canceled)

15. A steel pipe made from an alloy consisting of: from 0.15 to 0.35 wt % carbon, from 2.5 to 5.0 wt % aluminium, from 40 to 45 wt % nickel, from 25 to 35 wt % chromium, from 0.50 to 1.50 wt % niobium and/or vanadium, from 0.01 wt % to 0.25 wt % yttrium, from 0.01 wt % to 0.25 wt % tungsten and/or tantalum, from 0.01 wt % to 0.25 wt % in total of one or more of titanium and zirconium and hafnium, up to 0.9 wt % manganese, up to 0.9 wt % silicon, and up to 0.10 wt % nitrogen, with the balance of the composition being iron and incidental impurities; wherein the steel pipe has a substantially continuous layer of alumina on its internal surface.

16. The steel pipe of claim 15, wherein the substantially continuous layer of alumina contains no nickel, no chromium, and no iron.

17. The steel pipe of claim 15, wherein the alloy consists of: from 0.15 to 0.35 wt % carbon, from 2.5 to 5.0 wt % aluminium, from 40 to 45 wt % nickel, from 25 to 35 wt % chromium, from 0.50 to 1.50 wt % niobium, and/or vanadium, from 0.01 wt % to 0.05 wt % yttrium, from 0.05 wt % to 0.25 wt % of tungsten and/or tantalum, from 0.04 wt % to 0.15 wt % in total of one or more of titanium and zirconium and hafnium, an amount of up to 0.9 wt % manganese, an amount of up to 0.6 wt % silicon, and an amount of up to 0.10 wt % nitrogen, with the balance of the composition being iron and incidental impurities.

18. The steel pipe of claim 15, wherein carbon is present in an amount of from 0.20 wt % to 0.35 wt %.

19. The steel pipe of claim 15, wherein aluminium is present in an amount of from 3.5 wt % to 4.5 wt %.

20. The steel pipe of claim 15, wherein nickel is present in an amount of from 42 wt % to 45 wt %.

21. The steel pipe of claim 15, wherein chromium is present in an amount of from 28 wt % to 30 wt %.

22. The steel pipe of claim 15, wherein niobium is present in an amount of from 0.80 wt % to 1.50 wt %.

23. The steel pipe of claim 15, wherein silicon is present in an amount of from 0.3 wt % to 0.6 wt %.

24. The steel pipe of claim 15, wherein manganese is present in an amount of from 0.4 wt % to 0.8 wt %.

25. The steel pipe of claim 15, wherein tungsten is present in an amount of from 0.05 wt % to 0.15 wt %.

26. The steel pipe of claim 15, wherein titanium is present in an amount of from 0.08 wt % to 0.15 wt %.

27. The steel pipe of claim 15, wherein yttrium is present in an amount of from 0.01 wt % to 0.03 wt %.

28. The steel pipe of claim 15, wherein nitrogen is present in an amount of from 0.03 wt % to 0.06 wt %.

29. A steel pipe capable of forming a substantially continuous layer of alumina on its internal surface, the steel pipe being made from an alloy consisting of: from 0.15 to 0.35 wt % carbon, from 2.5 to 5.0 wt % aluminium, from 40 to 45 wt % nickel, from 25 to 35 wt % chromium, from 0.50 to 1.50 wt % niobium and/or vanadium, from 0.01 wt % to 0.25 wt % yttrium, from 0.01 wt % to 0.25 wt % tungsten and/or tantalum, from 0.01 wt % to 0.25 wt % in total of one or more of titanium and zirconium and hafnium, up to 0.9 wt % manganese, up to 0.9 wt % silicon, and up to 0.10 wt % nitrogen, with the balance of the composition being iron and incidental impurities.

30. The steel pipe of claim 29, wherein the steel pipe is capable of forming the substantially continuous layer of alumina on its internal surface when heated to above 900° C.

31. The steel pipe of claim 29, wherein the alloy consists of: from 0.15 to 0.35 wt % carbon, from 2.5 to 5.0 wt % aluminium, from 40 to 45 wt % nickel, from 25 to 35 wt % chromium, from 0.50 to 1.50 wt % niobium, and/or vanadium, from 0.01 wt % to 0.05 wt % yttrium, from 0.05 wt % to 0.25 wt % of tungsten and/or tantalum, from 0.04 wt % to 0.15 wt % in total of one or more of titanium and zirconium and hafnium, an amount of up to 0.9 wt % manganese, an amount of up to 0.6 wt % silicon, and an amount of up to 0.10 wt % nitrogen, with the balance of the composition being iron and incidental impurities.

Description

[0105] Some of the key technical benefits of the invention can be seen from the following Figures in which:

[0106] FIG. 1 shows an alloy according to the invention which has been subjected to EDX cross section surface mapping at the relatively low magnitude of 500λ.

[0107] It can be observed from FIG. 1 that the aluminium and oxygen appear in the alloy sample according to the invention as one very bright and continuous line. This indicates two things. Firstly, the aluminium and oxygen are present in a very high concentration, as alumina, at the exact same position where the surface oxide layer is observed by SEM photo (Scanning Electron Microscope). Secondly, there is a continuous layer of alumina at the surface of the material.

[0108] This analysis demonstrates that there is a dense, adherent, thin, continuous alumina (aluminium oxide) layer at the surface of the alloy even after pack-carburisation at 1100 C over a period of 200 hrs. It can also be seen that the alumina layer stays very stable and therefore does not get transformed in aluminium carbide, and acts as a protective layer i.e. a barrier against carburisation

[0109] FIG. 2 shows EDX surface mapping at the higher magnification of 2000λ for an alloy according to the invention having the composition shown in Table 1. In FIG. 2, it can be seen that the aluminium and oxygen appear as the brightest components, and hence are present in very high concentration. In addition, it can be seen that these elements appear at the exact same position where the surface oxide layer is observed by SEM. A further important feature is the dark areas in the Figure which indicate the absence of elements such as nickel, iron and chromium. FIG. 2 also illustrates the dense, adherent, thin, continuous alumina surface layer and indicates that it is pure, and free of highly catalytic elements as nickel and iron. The continuous surface layer and the absence of catalytic elements contribute to the anti-coking effect of the alloy when exposed to an external source of carbon. It can be seen from FIG. 2 that the alumina layer is anti-coking, and does not bond or react catalytically with carbon during pack-carburisation at 1100 C over a period of 200 hrs.

[0110] FIG. 3 shows is a High Magnification SEM Photo of the carburised surface of the alloy according to the invention having the composition of Table 1 (an alloy containing 3.8 wt. % Aluminium). The photograph clearly illustrates a continuous protective alumina layer at the surface. The thickness of the alumina layer was determined to be 4.28±1.21 μm on the basis of 100 measurements.

[0111] It can therefore be seen that the alloys of the invention demonstrate superior properties in terms of the carburisation resistance and also exhibit resistance to oxidation on account of the alumina layer which can be formed in situ on the alloys in use.

[0112] Alloys of the present invention can be tested using a thermal oxidation cycle test. This test involves cycling the alloy through high and low temperatures over an extended time period in order to expose the alloy to oxidative stress. This test is frequently used for studying the oxidation characteristics of materials such as the superalloys/superalloys coating employed in gas turbines which are intended for high temperature use and/or use under extreme condition.

[0113] The test machine consists of a furnace brought to the test temperature and a sample handling apparatus which is able to introduce and remove the sample from the furnace very quickly in order to facilitate rapid heating and rapid cooling. The alloys of the present invention were tested at 1150° C. The higher the temperature, the faster growth of the oxide layer, so a thick oxide layer is likely to be formed.

[0114] The sample handling apparatus is a mechanism for bringing the test sample quickly in and out the furnace. The higher the furnace temperature, more extreme the thermal shock on the sample as it is introduced into and removed from the furnace. Repeated exposure of the sample under these conditions leads to the potential spallation of the oxide layer. The adhesion and thickness of the resulting oxide layer on the surface can then be investigated and used as a basis for establishing the performance of the alloys of the invention relative to those of the prior art.

[0115] The alloys of the invention were tested by repeating the heat-shock exposure through 50 cycles. FIG. 4 of the drawings illustrates the temperature profile in the thermal oxidation test. The samples were pre-oxidised at 875° C. over a period of 48 hours. Subsequently, the samples were exposed to 50 cycles in and out of the furnace and the temperature profile is shown in FIG. 4. Each cycle consists of heating the sample under test from room temperature by introducing it into the furnace at 1150° C., keeping sample in the furnace for 45 minutes at that temperature in an atmosphere of air, and then removing the sample from the furnace and cooling it to room temperature over a period of 15 minutes. In the graph of FIG. 4, the vertical axis represents both the temperature of the sample in degrees centigrade and also the air pressure in millibars. It can be seen from the graph that the air pressure in millibars is just slightly less than 1000 millibars (approximately 975 mbar). It can also be seen from the graph that the maximum temperature achieved in the heating cycles is 1150° C. and that the minimum temperature, represented by the inverse peaks in the graph, is effectively room temperature. The horizontal axis in the graph indicates the time in hours throughout the test.

[0116] The repetitive nature of the test effectively exposes the sample to both mechanical fatigue and thermal fatigue so that the effects at the interface between bulk sample and its oxide surface can be investigated. The rate of growth of the oxide layer is measured as the sample is exposed to high temperature. The amount of stress and spalling of the oxide layer can therefore be viewed as the oxide layer develops.

[0117] The mass change of the sample is recorded before and after the test in order to assess the stability of the oxide layer. It is expected that the mass of any sample exposed to these harsh conditions will change because the test is very severe. However, a smaller variation in the mass before and after the test is indicative of a more stable and adherent oxide layer. This in turn is indicative of stability of the alloy over an extended period such as during long term service. The results achieved using alloys according to the invention are very impressive.

[0118] FIG. 5 shows the compositions of three alloys according to the invention which were subjected to the thermal oxidation test.

[0119] FIG. 6 shows photographs of the three samples after the Thermo-oxidation test and the variation of mass for each of the samples. It can be seen from FIG. 6 that the variation of mass after test is negligible (all show a mass change of less than 1.15 mg) and consequently that the alloys of the invention demonstrate excellent stability under these conditions. This also demonstrates that the alloys will be stable during an extended period in use.

[0120] FIG. 7 illustrates the cross section of the surface of Sample C of FIG. 5 after 50 thermo-oxidation cycles. This is investigated by X-ray spectroscopy using copper Kα radiation. It can be seen that there is a continuous oxide layer which is present across the majority of the sample. The oxide thickness is 3.61 μm±0.61 μm (based on 137 measurements). FIG. 7 also highlights the scale of the aluminium and oxygen components of the alloy, as well as the scale of the nickel plating in the alloy.

[0121] The thermo-oxidation test results demonstrate that the alloys of the invention have a good resistance to oxidation under extreme conditions. These results also show that the alloys will have a long service lifetime.