Examples of applications for tubes according to this invention are radiant tubes for the treatment, melting and heating of metallic and ceramic materials, tubes for heat exchangers and recuperators as well as thermocouple protection tubes for the measurements of high temperatures using thermocouples.

Radiant tubes are used in industrial furnaces, which are used for many different purposes and with varying atmospheres in the furnaces. Radiant tubes in furnaces are constantly exposed to corrosive atmospheres of different kinds. Oxidizing environments are common as well as strongly carburizing and sulphur containing environments. The temperature conditions can be unsuitable for the service life of the tubes. Both a very high temperature as well as specific temperature intervals below the highest temperature of application, can cause corrosion damages as well as other damages to quickly appear on the tubes. Both temperature conditions as well as other conditions can vary in a furnace, in continous furnaces are, for example, often the temperature lower in the inlet part of the furnace.

It is common to use radiant tubes in NiCr-and NiCrFe-alloys, however, these have limited service lives. NiCr-and FeNiCr-alloys are alloyed with Si (silicon) to receive a surface layer of Si02 as protection to attacks from a corrosive atmosphere.

Recently, also FeCrAl-alloys have been used for radiant tubes, among other things due to the advantageous properties of these alloys in strongly carburizing and sulphur containing atmospheres. The FeCrAl-alloys contain aluminium, which forms a layer of a thin aluminium oxide, which has a good resistance in most of the furnace atmospheres that are common today. The protecting aluminium oxide layer which forms on the surface do efficiently protect the base material from most corrosive environments. The FeCrAl-alloys spontanously form aluminium oxide if the temperature is high enough and if the atmosphere contains oxygen. Despite the protecting aluminium oxide layer, there are applications where this is not enough and as a result not even FeCrAl-tubes display enough good properties. The formation and the preservation of the protecting layer also require that the radiant tubes are surrounded by an atmosphere containing a high enough oxygen content.

NiCr-, FeNiCr-and FeCrAI-alloys can be used in many applications and in different atmospheres, but in certain applications, a satisfactorily service life of tubes in these alloys are not achieved.

The present invention makes it possible to avoid quick failures in furnaces and other applications with, for other tubes, an unsuitable atmosphere and/or temperature. This is also of increasing interest in continous heat treatment processes, where a production stop is of great disadvantage in planning and costs. This invention makes it possible to work with longer periods between production stops for maintenance works.

The purpose of the present invention is that during certain special conditions concerning for example atmospheres or temperature ranges, to achieve a prolonged service life for tubes. This purpose is achieved by the present invention, which concerns tubes with a coating which reinforces the protective oxide layers with, primarily, aluminium oxide. The invention concerns also ways of manufacturing such tubes as well as the use of such tubes.

According to the present invention, a surface layer of aluminium and/or aluminium oxide is brought onto the tube, whereafter the tube is heat treated at about 1050 C at enough long time, whereby the aluminium oxide layer on the FeCrAl-alloys are improved and the NiCr-and FeNiCr-alloys are given an aluminium oxide layer as protection in corrosive environments. The purpose of the coating is to significantly increase the service life for critical applications and at increased service temperatures for tubes in these alloys. The good properties of the aluminium oxide are in this way utilized in most atmospheres where these alloys earlier could not be used.

Tubes according to this invention give advantages in many different areas of applications, among which the following examples can be given: - In continous furnaces, where one wants to prolong the service life of the radiant tubes, and avoid unexpected stops in production.

- In contious hardening furnaces, where a low temperature in combination with a corrosive environment, especially in the inlet part, make both FeNiCr-alloys and FeCrAl-alloys to fail after a very short time.

- In continous furnaces with high temperatures, 1000-1050 C, in carburizing atmospheres with high carbon potentials, where the presence of oxygen is low, why no spontanous aluminium oxide formation occurs.

- In dosing furnaces for aluminium-zink melts, an exteme furnace environment, in which degassing and raffination take place, and where splashes and mechanical damage strongly reduse the service life of the radiant tubes.

- In holding furnaces for aluminium-zink melts with aggressive slag covers and additions of alloying materials.

- Protection tubes for thermoelements used to measure the temperature of metal melts of aluminium and zinc. The tubes are lowered into the melt for a shorter or a longer period of time.

- Recuperators within the glass industry, which work in corrosive environments, for example crystal glass, where large additions of lead into the furnaces are made.

The coating is performed with a thick aluminium oxide layer followed by a preoxidation. By coating the tubes, both NiCr-, FeNiCr-and FeCrAl-tubes, with a thick aluminium oxide layer and perform a preoxidation afterwards, the protection and therefore also the service life, are significantly improved.

The purpose of this invention is to improve the aluminium oxide layer of the FeCrAl-alloys and to give the NiCr-and FeNiCr-alloys an aluminium oxide layer, as protection in a corrosive environment. The purpose of the coating is to significantly increase the service life in critical applications as well as increased service temperatures for the NiCr-and FeNiCr-alloys.

In the coating procedure according to this invention, tubes for the use at high temperatures or any other aggressive environment, are coated with a coating of a layer of aluminium and aluminium oxide with a thickness of at least 0,3 mm, on the outside of the tube. In many applications it is suitable to coat the tube with many layers and perform an oxidation of every coated layer before next layer is put on, usually the coating is made with 2 or 3 layers to a layer thickness of 0,3-0, 7 mm, preferable ca 0,5 mm. Suitable methods for coating are flame spraying, high velocity spraying, plasma spraying or other ways of thermal spraying or a combination of these methods. The material which is put on is aluminium or possibly a mixture of aluminium and aluminium oxide. With aluminium also means alloys of aluminium with small amounts of alloy additions, preferably less than 5 weight percent. At the high temperatures which are prevailing during the coating process, a substantial amount of the coating material oxidizes, and the coating often contains a large amount of aluminium oxide. After the layer or the layers and possible also between the coating of two layers, a heat treatment is performed, which causes the coating material to completely transform into aluminium oxide. This treatment means heating to ca 1050 C during at least four hours. This procedure gives an aluminium oxide layer, which is not sensitive to mechanical damage and which stays on also at low temperatures, where one cannot expect spontanous aluminium oxide formation to occur.

One example of the use of a tube according the invention is shown in figures 1-2, which showns a dosing furnace for melted aluminium.

Figure 1 is a side view of the cross section of the furnace.

Figure 2 shows the furnace from above with the cover removed.

In the furnace there is melted aluminium, 1, which is hold at temperature using three elements, placed horisontally above the surface of aluminium.

Every element includes an electric resistance element, 3, which is surrounded by the tube, 2. The tube consists, according to the invention, by a high temperature alloy, on which a 0,5 mm thick surface layer has been put on, distributed on two separate layers, according to the desription above.