GB2117415A

GB2117415A - Process for coating a heat- resistant alloy base

Info

Publication number: GB2117415A
Application number: GB08308223A
Authority: GB
Inventors: Takanobu Shinohara; Toshio Anzai; Yutaka Kita; Susumu Taira
Original assignee: Toyo Engineering Corp
Current assignee: Toyo Engineering Corp
Priority date: 1982-03-26
Filing date: 1983-03-25
Publication date: 1983-10-12
Also published as: FR2524005A1; GB8308223D0; BR8301525A; DE3310750A1; JPS58167764A; JPH0124223B2

Abstract

A first coating alloy layer consisting of 20-80 wt.% chromium, 20-80 Wt.% nickel and up to 3 wt.% of another element is formed on the surface of a heat-resistant alloy base consisting of 18-35 wt.% chromium, 18-50 wt.% nickel, up to 0.6 wt.% carbon and the balance is essentially iron, and then forming a second layer comprising 70-30 wt.% of the foregoing first coating alloy and 30- 70 wt.% of aluminium oxide on the surface of the first layer, both coating layers being formed by the plasma flame spraying method. The product is useful as construction material e.g. for interior metal surface of furnaces using low grade fuels in distillation, reforming or cracking of hydrocarbons, refining petroleum because of its corrosion resistance land peeling resistance.

Description

SPECIFICATION Process for coating a heat-resistant alloy base The present invention relates to a process for forming coating layers having excellent heat resistance and corrosion resistance on the surface of a heat-resistant ferroalloy base which is used mainly in a low grade fuel-burning atmosphere.

Light fuels, such as natural gas, petroleum gas and naphtha, have been used in heating furnaces used for the distillation, reforming or cracking of materials, such as hydrocarbons, in petroleum refining or chemical plants. A principal reason therefor is that if a fuel other than the above-mentioned light fuels, such as a heavy oil containing a distillation residue obtained in a petroleum refining step, coal or a mixture thereof is used, the problem of high-temperature corrosion of the metallic material of the heating furnace arises.More particularly, if a low grade fuel is burnt, compounds of heavy metals, such as vanadium, sulfur compounds, sodium chloride, or the like contained therein, deposit on the interior metal surface of the furnace as so-called fuel ash, whereby to corrode or erode the same at a rate abnormally higher than the corrosion rate of the metal material when it is exposed to a high temperature in the ambient air. This phenomenon is called "hot corrosion".

Previously known principles for preventing hot corrosion may be roughly classified as follows: 1) the resistance of the metal material per se to high temperature oxidation is improved; 2) the corroding action of the heavy oil combustion gas is reduced: and 3) the metal material is surface-treated so as to improve its resistance to high temperature corrosion.

As a concrete example of the item 1), an oxidation-resistant material having a high chromium content has been developed as an alloy constituent. The oxidation resistance of the alloy is clearly improved by increasing the chromium content thereof. However, if the chromium content is increased above a certain level, the metal material becomes brittle and its strength at ambient and high temperatures is reduced, thereby lowering the practical value thereof as a structural material.

At present, a metal material of this kind which is most excellent as a practical structural material is an alloy comprising chromium and nickel in substantially equal amounts and a very small amount of Nb. However, even if this alloy is used, hot corrosion occurs when the metal surface temperature exceeds 9000C in the combustion atmosphere of heavy fuel and the metal becomes useless. Further, this material is quite expensive, as will be understood from its composition. The cost of this material is at least several times higher than that of ferroalloys generally used for making reforming furnace pipes, such as an alloy comprising 25 wt.% of chromium, 35 wt.% of nickel and the balance is essentially iron.

Thus, the measures wherein the oxidation resistance of the material per se is improved have the disadvantages that the furnace temperatures which can be employed for low grade fuel combustion are limited and the material is quite expensive.

As a concrete example of the item 2), calcium oxide, magnesium oxide, magnesium carbonate or the like is added to the low grade fuel so as to form calcium vanadate, magnesium vanadate or the like and prevent the melting temperature from lowering. The lowering of the melting temperature takes place by formation of eutectic mixtures of vanadium pentoxide from combustion of organic compounds of vanadium which have been contained in the low grade fuel with a thin layer of heavy metal oxides, that is, nickel oxides, chromium oxides and the like. These metal oxides cover closely the metal surface and protect effectively the metal surface from the common corrosion, because the eutectic mixture of vanadium pentoxide with a thin layer of heavy metal oxides has no performance for the protection of the metal surface.

The measures have many problems in that the melting point of the fuel ash is elevated, its amount is increased, and the ash deposits non-uniformly on the metal surface, and, consequently, heat transfer is inhibited and uniform heating becomes impossible. In addition, it is difficult to mix the above-mentioned additive homogeneously into the low grade fuel, and an increase of the fuel cost is inevitable.

As a concrete example of the above item 3), the metal material surface is coated with a substance having high heat and corrosion resistance by a surface treatment technique. Various coating materials and methods have been proposed.

However, the coating layers still have insufficient corrosion and peeling resistance when the coated bases are exposed repeatedly to temperature fluctuations. Such repeated heating and cooling is hereinafter referred to as heat history.

After intensive investigations and experiments made for the purpose of improving the corrosion and peeling resistance of coating films, which have as yet been insufficient in the prior art, the present inventors have attained the present invention. The coating process of the present invention will be described first.

The present invention provides a process for coating the surface of a heat-resistant alloy base with two or three protective layers. The base has a strength at high temperatures sufficient for practical use as a construction material for various furnace parts. The base comprises 1 8 to 35 wt.% of chromium, 18 to 50 wt.% of nickel, up to 0.6 wt.% of carbon and the balance is essentially iron. In particular, the balance of said heat-resisting alloy base consists essentially of iron and further may optionally contain a small amount of one or more of molybdenum, tungsten and niobium. The first coating layer comprises an alloy of 20 to 80 wt.% of chromium, 20 to 80 wt.% of nickel and up to 3 wt.% of another element. The first coating layer is formed on the surface of said base by the plasma flame spraying method, also known as plasma torch spraying.Then, the second coating layer, which comprises 70 to 30 wt.% of materials having the same composition as the alloy used in the first layer and 30 to 70 wt.% of aluminum oxide, is formed on the upper surface of the first layer, again by the plasma flame spraying method.

The third coating layer, which is optional, is made of aluminum oxide, it has a thickness of up to 100,u, and it is formed on the upper surface of the second layer by the plasma flame spraying method, whereby further to improve the coating effects.

According to the above-described process of the present invention, there is obtained a coated, heat-resistant, alloy base having improved peeling and corrosion resistance compared to those obtained by the above-mentioned known processes. Another advantage of the process of the present invention is that defects, such as acceleration of corrosion and deformation of the coating films, can be avoided in high-temperature heat treatments carried out in an oxidizing atmosphere as disclosed in some of the foregoing specifications.

Although the detailed reasons why the coating layer formed by the process of the present invention has excellent peeling and corrosion resistance have not yet been elucidated, these reasons are believed to be as follows. As described above, it is well known that the high-temperature corrosion resistance of an alloy increases as its chromium content increases. However, if the coating layer of an alloy having such a high chromium content is formed on a base having a thickness greater than that of the coating layer and having a high-temperature resistance higher than that of the coating layer, and it is exposed to a heat history, as defined above, the coating layer of high chromium content is inclined to crack, because an alloy having a high chromium content is brittle.However, if a base having the abovementioned composition, according to the invention, is used and the coating layer formed directly thereon is an alloy of the present invention comprising 20 to 80 wt.% of chromium, 20 to 80 wt.% nickel and up to 3 wt.% of another element, the thermal expansion difference between the base and the coating layer, which is observed when heat history is applied to the coating layer side, is reduced, and the thermal stress caused in the coating layer is also reduced, This is considered to be a reason why excellent peeling resistance can be obtained even when an alloy having such a high chromium content is used for forming the first layer.

The above-mentioned conventional processes are different from the present invention in that the difference in the metal composition between the first layer and the base is excessive in the conventional heat-resistant coated alloy bases. If an aluminum oxide layer insoluble in the alloy phase is used in the first layer in the form of a two-phase mixture, as disclosed in some of the prior techniques, the brittleness of the first layer is further increased thereby to cause the first layer to exhibit an effect opposite to the desired effect of preventing cracking of the first layer. The alloy used to form the first layer in the present invention, which has a high chromium content, has a high corrosion resistance, even though its corrosion resistance is still insufficient. Further, its moldability and other properties required of a structural material are insufficient.Although this alloy is thus hardly usable as the base, it can be used to form a dense first coating layer on a base, which base has sufficient moldability and the other properties required of a structural material, so as to prevent cracking and peeling and the corrosion resistance of the base can thereby be improved remarkably. Accordingly, the problem remaining unsolved is to prevent the components of the combustion gas which corrode the first layer from coming in contact with this layer. For this purpose, the second layer and, optionally, the third layer are coated successively on the first layer in the present invention.

The second layer in the present invention is a coating layer comprising a mixture of (1) an alloy having a composition within the same ranges as the first layer and (2) aluminum oxide. One reason why there is used in the second layer an alloy having a composition within the same ranges as the alloy used in the first layer is to minimize the formation of cracks in the second layer as described above with reference to the first layer. Aluminum oxide is incorporated in the second layer to increase the corrosion resistance of the second layer and, when a third coating layer of aluminum oxide is formed thereon, to improve the adhesion of the third layer to the second layer. If the second layer is exposed to heat history, as defined above, some cracks are formed, because this second layer contains aluminum oxide which is insoluble in the alloy (metal) phase.However, the second layer does not peel off from the first layer even if cracks are formed, because the composition of the alloy phase of the second layer is similar to that of the first layer. Thus, even though very fine cracks may be formed mainly at the interface between the aluminum oxide phase and the alloy phase in the second layer, the contact of corroding components with the first layer is greatly inhibited. The technique of forming a second layer comprising a mixture of (1) an alloy composition similar to the first layer and (2) aluminum oxide, is not found in the prior art. The present invention is different from the conventional processes in this respect.

A third coating layer made of aluminum oxide can be formed on the second layer according to the present invention. As compared with coated bases wherein an aluminum oxide layer is formed on an aluminum oxide-free layer, according to conventional processes, the third coating layer of aluminum oxide formed according to the process of the present invention has a quite excellent adhesion to the second layer, and the third layer does not substantially peel off from the second layer by exposure to heat history. Remarkable effects are obtained when the alloy phase formed on the surface of the second layer is prevented from coming in direct contact with the corroding components.The thickness of the third layer made of aluminum oxide is preferably up to 100,u. If the thickness of the third layer is thicker than 100 Su, it can be easily peeled off. The aluminum oxide content of the second layer is suitably 30 to 70 wt.%. According to the present invention, wherein the compositions of the three coating layers are specially controlled, far more excellent effects are obtained compared with those obtained by the conventional processes. These effects are obtained by the synergistic interaction of the combination of the two or three coating layers and the base.

It is desirable that the respective layers have dense, fine structures having only low porosities. If the fine structures are highly porous, a satisfactory performance cannot be expected. The highchromium alloy used for forming the first coating layer and the metal alloy component of the second coating layer has a high melting point. In order to form a coating layer having a dense, fine structure using a chromium alloy of a high melting point, by the plasma flame spraying method, there are required a high flame-spray temperature and pressure, that is, a high impact speed of the fine particles of the molten alloy and aluminum oxide flame-sprayed against the base. Therefore, a plasma flame spraying method in which a particularly high flame-spray temperature is attained is preferred to other known flame spraying methods for forming the coating layers in the process of the invention.It is also effective, for forming a coating layer having a high degree of adhesion to the surface of a substrate, to carry out the flame spraying while an oxidized film that may be formed on the substrate is reduced to the metal by incorporating hydrogen in the inert gas used in the plasma flame spraying.

The process according to the invention can be practically conducted in the following conditions.

The temperature is from 3 000 to 50 0000C, preferably 10 000 to 30 0000 C. The flow rate of the gas is from 1/4 of the sonic rate to the sonic rate, preferably 1/3 of the sonic rate to the sonic rate. The obtained coated base, in general, has a porosity range between 0.01 and 5 volume percent, preferably 0.01 and 1 volume percent.

The preferred thickness of the first and the second layers is 50 to 1 ,u, and that of the third layer is up to 1 00 in the present invention. The total thickness of the two layers or the three layers is desirably up to 300 iu. The process of the present invention can be employed for the treatment of heatresistant, high-chromium, high-nickel ferroalloy castings, rolling materials and welded products thereof having various shapes.

Reference will be made below to the accompanying drawings, in which: Figure 1 shows the results of corrosion tests.

Figures 2 and 3 show the results of peeling tests.

Figure 4 shows the shape of the test pieces used in the examples.

Examples Numerous test specimens were formed by cutting said specimens from a pipe formed by a centrifugal casting method from an HK-40 steel alloy comprising 25 wt.% chromium, 20 wt.% nickel, up to 0.4 wt% of carbon and the balance is iron, said pipe having an outer diameter of 120 mm and an inner diameter of 100 mm. The test specimens were partial cylinders as shown in Figure 4 having a wall thickness a of 5 mm, a chord length b of 30 mm and a length c of 100 mm. The outer surfaces A of each partial cylinder was the as-cast surface of the above-mentioned pipe. The test specimens were coated by various methods and then subjected to corrosion resistance and coating layer peeling tests.

For coating the test specimens, a commercially available plasma flame spray machine was used. The spraying gas used was a mixture of 60 vol.% of argon and 40 vol.% of helium. The spraying conditions were: direct current, 40 V, and a discharging current of 800 A. Coating layers of an alloy having the composition described below, a mixture of this alloy and alumina, or alumina alone, were applied to the entire surfaces of the respective test specimens in coating layer thicknesses of 50-1 50 4. The thuscoated test specimens were used as test pieces.

A powdery mixture comprising 85 wt.% of vanadium pentoxide and 1 5 wt.% of anhydrous sodium sulfate was kneaded together with acetone to form a soft paste. The soft paste was applied to the surface of the test pieces as an artificial fuel ash in an amount of 20 mg of solids per square centimeter of the test piece surface. The acetone was evaporated to dry the paste by air-drying at ambient temperature.

In the corrosion test, the test pieces to which the artificial fuel ash had been applied were heated to 9000C in an electric furnace for 3 hours. Then, the test pieces were taken out of the electric furnace and allowed to cool to room temperature over a period of 5 hours, and were then brushed in water to wash away the artificial fuel ash, and then they were dried and weighed. The weight was compared with the weight of the test pieces measured before the application of the artificial fuel ash. Figure 1 shows the results of the corrosion tests.

In Figure 1, the ordinates indicate values obtained by dividing the weight change of the test piece after the test by the surface area of the test piece. Ordinates on the positive side indicate that the weight of the test piece was increased by the test and ordinates on the negative side indicate that the weight of the test piece was reduced by the test. The numbers 1, 2, 3 and 4 shown with the bars in this figure refer to the respectively numbered coated bases having the structures given in Table 1.

The test results shown in Table 1 suggest that when a first alloy coating layer comprising 20 wt.% of chromium and 80 wt.% of nickel and a second coating layer comprising a mixture of (1) 70 parts by weight of the same alloy as the one used in the first layer and (2) 30 parts by weight of alumina, are formed (Test No. 2), the corrosion re"-2nce is improved but is still insufficient. However, when a third coating layer made of alumina (thickness: 1O0,u) is further formed on the second layer (Test No. 4), or the first layer is made of an alloy comprising 50 wt.% of chromium and 50 wt.% of nickel and the second layer comprises a mixture of (1) the same alloy as the one used in the first layer and (2) at least 30 wt.% of alumina, the product can be used in practice unless the layers are peeled off.

Table 1

Third layer First layer Second layer (layer (layer closest (layer formed formed on to the base on the first the second Test No. surface) layer) layer) 1 Not formed. Not formed. Not formed.

(comparison) 2 Coating of an Coating of a Not formed.

(invention) alloy of 20 wt. mixture of 70 % chromium and parts of the 80 wt.% nickel alloy of the having a thick- first layer and ness of 100 ,u. 30 parts of alumina having a thickness of 100,u.

3 Coating of an Coating of a Not formed.

(invention) alloy of 50 wt. mixture of 70 % chromium and parts of the 50 wt.% nickel alloy of the having a thick- first layer and ness of 100 y. 30 parts of alumina having a thickness of 1008.

4 The same as the The same as the Coating of (invention) first layer in second layer in alumina Test No. 2 Test No. 2. having a thickness of 50,u.

5 Coating of an Coating of-a Not formed.

(invention) alloy of 20 wt. mixture of 70 % chromium and parts of alloy 80 wt.% nickel of 50 wt.% having a thick- chromium and 50 ness of 100 u. wt% nickel and 30 parts of alumina having a thickness of 1008.

Table I (contd.)

Third layer First layer Second layer (layer (layer closest (layer formed formed on to the base on the first the second Test No. surface) layer) layer) 6 The same as the Coating of a Not formed.

(comparison) first layer in mixture of 20 Test No. 3 parts of the alloy of the first layer in Test No. 3 and 80 parts of alumina having a thickness of 100 7 Coating of an Coating of a Not formed.

(comparison) alloy of 15 wt. mixture of 70 % chromium and parts of alloy 85 wt.% nickel of 50 wt.% having a thick- chromium and 50 ness of 100 u. wt.% nickel and 30 parts of alumina having a thickness of 100,u.

8 Coating of an Coating of a Not formed.

(comparison) alloy of 20 wt. mixture of 70 %chromium, 10 parts of alloy wt.% silicon of 50 wt.% and 70 wt.% chromium and nickel (JIS Z- 50 wt.% nickel 3265-B Ni-5 and 30 parts of Nickel brazing alumina having filler metal) a thickness of having a thick- 100 ju.

ness of 100 U 9 Coating of The same as the Not formed.

(comparison) alloy of 85 wt. second layer in % chromium and Test No. 7.

1 5 wit.% nickel having a thick ness of 100 y.

10 The same as the The same as the Coating of (invention) first layer in second layer in alumina Test No. 3. Test No. 3 but having a having a thick- thickness ness of 150 dtz of 100 11 The same as the The same as the Coating of (comparison) first layer in second layer in alumina Test No. 10. Test No. 10. having a thickness of 150y.

The following additional tests were carried out for determining the degree of peeling caused by exposure to heat history, as defined above. The temperature of the test pieces prepared as described above was elevated from ambient temperature to 10000C at a constant rate over 10 hours. After the temperature reached 10000C, the test pieces were maintained at 10000C for 1 hour and then they were cooled to ambient temperature at a constant rate over a period of 10 hours. This procedure, comprising the foregoing steps, is designated one cycle. After the repetition of 5 cycles in total, the test pieces were weighed. After weighing, the same test pieces were again subjected to the same treatment repeated for 5 cycles and then weighed.The weight determination at intervals of 5 cycles was repeated and changes from the original weights were determined. The weight gains or losses per square centimeter of the test pieces determined for 5, 10 and 1 5 cycles are shown in Figure 2.

The numbers applied to the graph lines in Figures 2 and 3 refer to the test pieces identified in Table 1.

In Figure 2, the ordinates show the weight gain or weight loss per square centimeter of surface area of the test pieces. Negative values suggest that the coating layers were peeled off in the test to reduce the weight of the sample, and positive values suggest that the weight was increased by an oxidation reaction or the like. Figure 2 shows that the coating layers are peeled off to an impractically large extent when the chromium content of the base is greatly different from that of the first layer (Test 9), when the first layer contains a substance having properties different from those of metals, such as silicon (Test 8), or when the alumina content of the second layer is excessive (Test 6). Figures 2 and 3 indicate, by contrast, that the coated layer structures of tests 3 and 5 of the invention in Table 1 are suitable for practical use.

Figure 3 shows the results of peeling tests, which are the same as the peeling tests of the examples of Figure 2, except that the thickness of the third alumina coating layer was varied. The structures of the coated base materials are as given in Table 1 (Tests 10 and 1 1). The results shown in Figure 3 indicate that alumina layers having a thickness of greater than 100 U are easily peeled off even if the structures of the first and the second layers are suitable.

Claims

1. A process for coating a heat-resistant base made of a base alloy consisting essentially of from 18 to 35 wt.% of chromium, 18 to 50 wt.% of nickel, up to 0.6 wt.% of carbon and the balance is essentially iron, comprising the steps of coating onto said heat-resistant base, by plasma flame spraying, a first coating layer made of a first coating alloy consisting essentially of 20 to 80 wt.% of chromium, 20 to 80 wt.% of nickel and up to 3 wt.% of elements other than chromium and nickel; and then coating onto said first coating layer, by plasma flame spraying, a second coating layer consisting essentially of a mixture of (1) 70 to 30 wt.% of a second coating alloy consisting essentially of 20 to 80 wt.% chromium, 20 to 80 wt.% nickel, and up to 3 wt.% of elements other than chromium and nickel, and (2) 30 to 70 wt.% of aluminium oxide.

2. A process as claimed in claim 1, further comprising coating onto said second coating layer, by plasma flame spraying a third coating layer consisting of aluminium oxide, said third coating layer having a thickness of up to 100 microns.

3. A process as claimed in claim 1, wherein said first coating layer has a thickness in the range of 50 to 1 5C microns, and said second coating layer has a thickness in the range of 50 to 1 50 microns.

4. A process as claimed in claim 2, wherein the total thickness of said first, second and third coating layers is not greater than 300 microns.

5. A process as claimed in claim 3, wherein the difference between the chromium content in weight percent of said base and the chromium content in weight percent of said first coating layer is not greater than 25 weight percent.

6. A process as claimed in any one of claims 1 to 5, wherein said plasma flame spraying is carried out utilizing a sprayed gaseous mixture consisting essentially of hydrogen and an inert gas.

7. A process as claimed in claim 3, wherein said second coating layer consists of about 70 wt.% of said second coating alloy and 30 wt.% of aluminium oxide and said second coating alloy is the same as said first coating alloy.

8. A process as claimed in claim 1 or claim 2, wherein said second coating alloy is the same as said first coating alloy.

9. A process as claimed in any one of claims 1 to 8, wherein said base alloy contains an effective amount of at least one element selected from molybdenum, tungsten and niobium.

10. A process for coating a heat-resistant base made of a base alloy consisting essentially of 1 8 to 35 wt.% chromium, 18 to 50 wt.% nickel, 0 to 0.6 wt.% carbon and the balance is essentially iron, comprising the steps of: forming a first coating layer by plasma flame spraying directly on said base, said first coating layer consisting of a first coating alloy consisting essentially of 20 to 50 wt.% chromium, 50 to 80 wt.% nickel, and up to 3 wt.% of elements other than chromium and nickel, said first coating layer having a thickness in the range of 50 to 1 50 microns; then forming a second coating layer by plasma flame spraying directly on said first coating layer, said second coating layer consisting essentially of a mixture of (1) 30 to 70 wt.% of a second coating alloy consisting essentially of 20 to 50 wt.% chromium, 50 to 80 wt.% nickel, and up to 3 wt.% of elements other than chromium and nickel, and (2) 70 to 30 wt.% of aluminium oxide, said second coating layer having a thickness in the range of 50 to 1 50 microns.

11. A process as claimed in claim 10, further comprising a step of forming by plasma flame spraying a third coating layer consisting of aluminium oxide directly on said second coating layer, said third coating layer having a thickness of up to 100 microns, the total thickness of said first, second and third coating layers being not greater than 300 microns.

12. A process as claimed in claim 10 or claim 11, wherein said second coating alloy is the same as said first coating alloy.

13. A heat-resistant coated base prepared by the process claimed in any one of claims 1 to 9.

1.4. A heat-resistant coated base prepared by the process claimed in any one of claims 10 to 1 2.

15. A process as claimed in claim 1 and substantially as hereinbefore described with reference to any one of Tests Nos. 2 to 5 and 10 of the Examples.

1 6. A heat-resistant coated base prepared by the process claimed in claim 1 5.