Title:
CHROMIZING PROCESS
United States Patent 3728149


Abstract:
In a method of forming a corrosion resistant coating on steel strip, the strip is coated on at least one side with a uniform distribution of a chromium-containing metal powder and a considerably less amount of an alkali or alkaline earth metal halide. The powder, containing the halide, is compacted on the strip. The strip is coiled, preferably in a tight coil, and heated in a heating zone in a protective atmosphere for sufficient time and at a temperature to cause diffusion between the metal strip and metal powder, and to thus form a protective stainless steel coating on the strip.



Inventors:
Forand Jr., James L. (Bethlehem, PA)
Shin, Paik Woo (Coopersburg, PA)
Application Number:
05/061817
Publication Date:
04/17/1973
Filing Date:
08/06/1970
Assignee:
BETHLEHEM STEEL CORP,US
Primary Class:
Other Classes:
427/191, 427/252, 427/295, 427/377
International Classes:
C23C10/02; C23C10/32; (IPC1-7): C23C9/00
Field of Search:
117/107.2
View Patent Images:
US Patent References:



Foreign References:
ZA690769A
Primary Examiner:
Leavitt, Alfred L.
Assistant Examiner:
Ball, Wm E.
Claims:
We claim

1. A method of forming a stainless steel coating on steel strip which inhibits sticking between adjacent strip faces when coiled, which method comprises

2. A method according to claim 1 wherein the protective atmosphere is essentially a hydrogen atmosphere.

3. A method according to claim 2 wherein the halide is an alkali metal halide.

4. A method according to claim 3 wherein the alkali metal halide is a sodium halide.

5. A method according to claim 4 wherein the sodium halide is sodium chloride.

6. A method according to claim 5 wherein chlorine gas in the amount of 0.10 to 1.0 percent by volume of the total protective atmosphere is added thereto during the heating up period.

7. A method according to claim 3 wherein the alkali metal halide is a potassium halide.

8. A method according to claim 2 wherein the powder is ferrochrome powder.

9. A method according to claim 2 wherein the powder is applied to one side of the strip.

10. A method according to claim 2 wherein the powder is applied to both sides of the strip.

11. A method according to claim 1 wherein the halide is an alkaline earth metal halide.

12. A method according to claim 11 wherein the alkaline earth metal halide is a magnesium halide.

13. A method according to claim 11 wherein the alkaline earth metal halide is a calcium halide.

14. A method of forming a stainless steel coating on steel sheet which inhibits sticking between adjacent sheet faces when heat treated in a pack of sheets, which method comprises

15. A method according to claim 14 wherein the halide is an alkaline earth metal halide.

16. A method according to claim 15 wherein the alkaline earth halide is a magnesium halide.

17. A method according to claim 15 wherein the alkaline earth halide is a calcium halide.

18. A method according to claim 14 wherein the halide is an alkali metal halide.

19. A method according to claim 14 wherein the powder is applied to one side of the strip.

20. A method according to claim 14 wherein the powder is applied to both sides of the strip.

21. A method according to claim 14 wherein the halide is a sodium halide.

22. A method according to claim 14 wherein the halide is a potassium halide.

23. A method of preventing sticking between adjacent thin leaves of ferrous base metal in a chromizing operation wherein

24. A method of preventing sticking between adjacent thin leaves of ferrous base metal in a chromizing operation according to claim 23 wherein the halide is a sodium halide.

25. A method of preventing sticking between adjacent thin leaves of ferrous base metal in a chromizing operation according to claim 23 wherein the halide is a potassium halide.

26. A method of preventing sticking between adjacent thin leaves of ferrous base metal in a chromizing operation according to claim 23 wherein the halide is a magnesium halide.

27. A method of preventing sticking between adjacent thin leaves of ferrous base metal in a chromizing operation according to claim 23 wherein the halide is a calcium halide.

Description:
BACKGROUND OF THE INVENTION

This invention relates to an improvement in the production of chromized coatings, and more particularly to those coatings formed by application of powder to the substrate.

In the formation of chromized coatings on continuous steel strip, wherein a chromium-containing powder is applied to the strip, and the strip is then coiled and heated to permit diffusion of the chromium of the powder into the steel strip and diffusion of iron in the strip outwardly into the powder-coated surface, problems must often be overcome to assure rapid diffusion, and uniform distribution of the chromized coating. These problems are more prevalent when attempting to diffuse with the strip in the form of a tightly wound coil. When the strip is in the form of an open-wound coil, i.e., a coil which has voids between strip convolutions, there is no problem in unwinding after the diffusion treatment preparatory to any finishing operations and rewinding. The open coil may, however, have the disadvantage of a possible scouring effect on the applied powder due to gas currents set up during and leading up to the diffusion treatment. Scouring can be eliminated by heating the strip in the form of a tightly wound, or closed coil. Other advantages of operating with a closed coil reside in the use of lighter gage strip and greater coil length for a given coil diameter. However, closed coils do tend to stick during uncoiling following diffusion treatment. This sticking is due to a welding action which takes place between opposed iron surfaces under the pressure of the tightly wound coil and the elevated temperature of diffusion treating.

Accordingly, it is an object of this invention to provide a method for chromizing a strip in tight-coil formation, wherein diffusion takes place rapidly.

Another object is to produce a uniform coating by tight coil diffusion treating.

A further object is to form a chromized coating by tight-coil diffusion treating without sticking or welding of the coil laps upon uncoiling.

SUMMARY OF THE INVENTION

This invention can be described briefly as one in which a chromium-containing powder, such as ferrochrome, is applied as a first step to at least one side of a steel strip or sheet which has, preferably, been coated previously with a volatile liquid having sufficient tackiness characteristics to act as a temporary bonding agent for the powder. A minor proportion of an alkali metal or alkaline earth metal halide is added to the metal powder. The powder coated strip is subjected to a roll compacting operation, or an equivalent means of densification, to develop a more adherent bond between the powder and the strip. The coated strip is heated in a furnace, such as an annealing furnace, in a protective environment, usually a non-oxidizing gas such as hydrogen, for a time and at a temperature sufficient to produce an adherent iron-chromium alloy on the surface of the strip. During the heating up, diffusion treating and cooling cycle, the strip may be in the form of a tight coil.

The amount of "effective carbon," hereinafter defined, in both the steel strip substrate and the applied powder, should be kept below certain predetermined limits in order to develop coatings on the strip which are ductile and which have satisfactory corrosion resistance.

DETAILED DESCRIPTION

In one specific embodiment of the invention, a 3-ton coil of 36-inch wide, 20 gage strip of titanium-stabilized steel, having a total carbon content of 0.06 percent, was unwound, and a thin film of tridecyl alcohol was applied to both sides of the strip by means of rubber rolls. The filmed strip was then passed horizontally such that the under side of the strip contacted a vertically lifted stream of finely divided ferrochrome powder (-150 mesh, U. S. Standard Sieve Series), which had been previously mixed homogeneously with about 7 percent by weight of sodium chloride, the powder-salt mixture being applied uniformly on the underside of the strip. After coating one side of the strip, the strip was passed upwardly, and then horizontally in reverse direction such that the other side of the strip contacted another vertically lifted stream of similar powder-salt mixture. In this manner, the previously uncoated side of the strip was coated with the ferrochrome powder-sodium chloride mixture. The sodium chloride in this mixture was of commercial grade. The ferrochrome powder had the following analysis:

Chromium 69% Silicon 1.75% Carbon 0.035% Iron balance

The coated strip was run through a two-high temper mill to compact the powder mixture on the strip. The pressure of the rolls during compacting was sufficient to produce an elongation in the strip of about 1 percent. After leaving the temper rolls, the strip was wound on a take-up reel in tight coil formation.

The tightly wound coil was placed on edge in a high temperature annealing facility and diffusion treated. Before treating, the furnace system was purged with NH gas containing 4 percent H2. The diffusion treatment was performed in an atmosphere of hydrogen into which a small amount of chlorine was introduced, as the coil temperature was raised to a diffusion treating (sintering) temperature of about 1,650° F. The chlorine represented about 0.5 percent by volume of the total protective atmosphere during the heating up period. During this period, chlorine was introduced at a rate of 4.5 cu. ft. per hour. When the temperature reached 1,650° F., the amount of chlorine in the furnace atmosphere was reduced to 1.5 cu. ft. per hour. About 6 hours were required to bring the coil to the diffusion temperature of approximately 1,650° F. After fifteen hours at 1,650° F., the chlorine was shut off, and the temperature was maintained at 1,650° F. for an additional 12 hours with hydrogen flow maintained at 900 cu. ft. per hour. During the 15-hour period in which the coil was permitted to soak at 1,650° F. in the hydrogen-chlorine atmosphere, interdiffusion between the chromium of the powder and iron from the steel substrate produced a stainless iron-chromium alloy on both surfaces of the strip.

Upon discontinuance of chlorine addition after 15 hours at diffusion treatment temperature of 1,650° F., the additional twelve hour soak period at 1,650° F. was conducted in an atmosphere of 100 percent hydrogen. Chlorine should be exhausted from the treatment atmosphere prior to the point at which the outer furnace is removed to prevent condensation of chlorine compounds at the coating surface during the cooling period.

After cooling the strip in a hydrogen atmosphere, the system was purged with NH gas and the inner cover was removed from the coil. The coiled strip was uncoiled and washed with dilute nitric acid and brushed to remove any loose powder or reaction products retained on the coated surface. The strip was then given a temper roll on polished rolls equivalent to about 1.0 percent elongation.

A continuous, pore-free, stainless steel coating was produced on the strip. The coating was adherent, ductile and corrosion resistant.

As the coil of this example was tightly wound during the entire treatment cycle, very little gas escaped from or entered the area between convolutions of the coil. In those interior portions of the coil, away from the edges, where sodium chloride is entrapped, sodium from the salt is believed to react with oxides which may be present in the powder, while the chlorine from the salt is free to react with iron or chromium, and to thus aid in initiation of the diffusion process. The chlorine gas added to the protective atmosphere enhances the diffusion in the areas near the edges of the coil.

It will be apparent that many alternative means and materials may be made use of in the various operating steps of the example.

In place of sodium chloride as the halide used with the powder, any alkali metal or alkaline earth metal halide may be used. Compounds such as calcium or magnesium halides, which may be hygroscopic, should be incorporated with the metal powder in a container having an atmosphere of low humidity so that the mixture, after application, can be compacted or run over rolls in a relatively dry condition. Generally, sodium chloride, the cheapest and most abundant of the halides, will be preferred. The percent by weight of halide in the blended mixture of halide and metal powder should range between about 5 percent to 20 percent.

Besides mixing halide with the metal powder before powder application, as shown in the example, the halide may be applied to powder which has been compacted on the strip. However, this procedure results in a less satisfactory coating.

It is not necessary that the strip be diffusion heat treated in coil form, although treatment of a coil is the most efficient method from the standpoint of handling and utilization of furnace space. After compacting the powder on the strip, the strip can be cut into sheets of convenient length. The sheets can be stacked, one upon the other, and sintered in the furnace in this manner.

In selecting a particular liquid as the powder-retaining medium, care should be taken to select a volatile material which will not leave any substantial carbon deposit in the powder coating, which in turn could cause brittleness in the resultant alloy coating. Kerosene, transformer oil or straw oil are examples of liquids which may be used in place of tridecyl alcohol.

Metal powder which passes a 150 mesh screen has been found to be very satisfactory, although larger particles may be used. It is desirable to have the powder particles in a granular or angular shape, rather than flattened or spherical, to obtain the best control of powder coating weight and adherence of the compacted powder layer to the strip.

If the powder is applied to the strip in an amount of from 8 to 10 grams per square foot of backing strip surface, quite satisfactory results are obtained, and this amount of powder is held on the strip readily by a very light film of tridecyl alcohol. Heavier or lighter applications of powder may be used, depending to some extent on the desired distribution of chromium and the thickness of the diffused coating. By using a heavier alcohol film and/or coarse powder, the amount of powder applied can be increased by approximately twice the 10 grams per sq. ft. figure given above. As the powder coating is somewhat porous before diffusion treatment (sintering), the volatile powder-retaining liquid will evaporate during the heating up period and be removed completely from the coated article.

While application of powder to the strip is accomplished satisfactorily by contacting the strip with vertically lifted streams of powder, alternative methods for this application include use of a vibrator dispenser, electrophoretic deposition and electrostatic spray technique.

The different types of powder contemplated for use in this invention are those containing chromium, or iron and chromium. Metal powders answering this description are iron-chromium alloy, a mixture of iron and chromium, and commercial grade chromium powder. In the case of alloy powders, it will be apparent that iron or chromium powder may be added if desired. Small amounts of metallic impurities, which do not affect the resultant coating, can be tolerated in the powder.

We have found that excellent results are obtained with powder of the type given in the process example, i.e., ferrochrome powder containing approximately 70 percent chromium with the balance substantially iron. This type of powder produces, upon heating and diffusion, a stainless steel type coating, which coating, when continuous and pore-free, will resist a boiling 20 volume percent aqueous solution of nitric acid (based on 100% HNO3). While a relatively high amount of chromium in the powder is preferred for efficient formation of the coating, a stainless steel coating can be obtained when the chromium in the powder represents considerably less than 70 percent, but not less than about 20 percent, of the total metal powder. Both the amount of powder applied and the amount of chromium required in the powder depend on the desired thickness and chromium composition of the coating.

The temperature during diffusion (sintering) treatment should range preferably between approximately 1,550° F. and 1,900° F. for not less than about 12 hours, although considerably longer times may be desirable, depending on the amount of alloying required. Actually, there is no upper limit for diffusion temperature other than that which may be dictated by practical considerations. At temperatures above 1,550° F., the minimum time required will be reduced in an inverse manner.

The coating on each side of the strip, resulting after diffusion treatment, will generally have a thickness of from about 0.001 to 0.003 inch. This coating will contain not less than about 12 percent chromium throughout, and will be characterized by a sharp interface between the alloy coating and the metal substrate. Beneath the interface, the chromium content of the steel drops rapidly to zero.

Preferably, the coating has an average chromium content ranging from about 15 to 25 percent. Higher chromium contents may be used, but there would probably be little or no added benefit from the standpoint of corrosion resistance.

In this invention, "effective carbon," previously referred to, is that carbon, either in the base steel or in the applied powder, which by diffusion is free to combine with the chromium to form deleterious chromium carbides in the coating. Stated differently, it is that carbon which has a greater affinity for chromium at the diffusion temperature than for other elements in the substrate or coating. If a considerable amount of chromium carbide is present in the coating of the finished product, the coating is embrittled, and formability of the coated product is limited. Furthermore, a chromium alloy coating, containing considerable chromium carbide, has lower corrosion resistance than a coating substantially free of carbide.

Most metal powder will contain some carbon, and this carbon must be held to a value which will not produce the deleterious chromium carbides in the ultimate alloy coating. The amount of carbon which may be introduced into the coated article by the powder should be not more than about 0.25 percent by weight of the powder used.

There is no limitation on the type of steel which may be used as base material in our invention, as long as the effective carbon content of the base material is maintained at a figure no greater than 0.01 percent during diffusion treatment.

Maintaining the low value for effective carbon in the base steel during diffusion may be accomplished in various ways. The steel strip or sheet, for example a rimmed steel of 0.06 percent carbon, may be decarburized to below 0.01 percent carbon before any of the processing steps of the invention are applied.

Another procedure for obtaining the low effective carbon value in the base steel during diffusion is to decarburize the powder-coated strip in the treatment furnace during heating of the strip up to the temperature at which diffusion begins. Successful decarburizing can be performed in this manner by introducing a moist hydrogen atmosphere (dew point, +95° F., or 5.5 percent by volume) into the furnace during the heating-up period, then, when the temperature reaches about 1,250° F., holding at that temperature for about 5 hours. At the end of the 5-hour period, the furnace is purged of the moist hydrogen atmosphere, and dry hydrogen is introduced.

A third means, by which the effective carbon can be maintained at or below 0.01 percent during diffusion treatment, is that shown in the specific detailed example of the process, wherein a titanium-stabilized steel is used as the substrate. Titanium is a carbide-former having considerable affinity for carbon, and acts as a carbon-sequestering agent, and in this manner carbon is tied up and is not free to migrate to the chromium in the coating. Examples of other sequestering agents are zirconium and columbium. When carbide-formers, or sequestering agents, are used in the strip base metal to tie up carbon in this invention, it is still essential that any unbound, effective carbon in solution in the strip, that which is free to react with chromium in the powder coating, be held to a quantity not in excess of 0.01 percent.

When the carbon in the base steel is greater than 0.01 percent, and titanium is used to combine with the excess carbon, the amount of titanium necessary will, of course, depend on the amount of carbon to be sequestered. As a practical matter, when using a titanium-stabilized steel, it is desirable to maintain the titanium in an amount ranging from 0.2 percent to 0.5 percent, preferably in the range of from 0.25 to 0.35 percent, and always in an amount at least four times the amount by weight of carbon it is necessary to sequester.

In the specific process example of this invention, the base strip had an analysis of 0.30 percent titanium and 0.06 percent carbon. This amount of titanium combines with substantially all of the carbon to form a stable titanium carbide. An advantage of using titanium-stabilized steel strip is in the fact that it has the strength characteristics of a low-carbon steel.

Regardless of the source of carbon, the coating of the chromized product should preferably contain not more than 0.10 percent carbon. This refers especially to the main body of the coating. Carbides at the coating surface only may or may not be detrimental, depending on the ultimate use of the product.

The diffusion treatment must be performed in a protective atmosphere or environment substantially free of carbon, oxygen or nitrogen. To this end, any one of the noble gases may be used as a surrounding atmosphere, although hydrogen, preferably 100 percent pure, is the most practical atmosphere, as it has the added advantage of being able to remove oxygen from oxides which may have formed during processing.

In large scale operations, certain impurities, chiefly oxygen may enter the furnace atmosphere through leaks in the system from furnace walls, or from other portions of the equipment. At diffusion temperature, and even considerably below such temperature, the highly reactive chromium powder reacts with any small amount of oxygen present in the atmosphere. Oxide formation on the powder hinders diffusion, and, while not mandatory, it has been found desirable, in the practice of this invention, to add a small amount of a halogen-containing gas to the furnace atmosphere to promote rapid diffusion of the chromium into the base metal. Hydrogen chloride gas or chlorine are especially useful for this purpose, although any of the other halogens or their hydrogen compounds may be used. The amount of halogen-containing gas entering the furnace may be as low as 0.10 percent by volume of the furnace atmosphere, preferably about 0.5 percent (halogen content). As chlorine and hydrogen are known to produce an explosive mixture when the chlorine content is above 11.0 percent, care must be exercised not to exceed this amount. From any practical standpoint, the chlorine content of the furnace atmosphere would not exceed about 1.0 percent. Because of the poisonous and corrosive nature of the halogens, proper precautions should be taken to prevent escape of these materials into the ambient atmosphere. Furthermore, the halogen-containing gas should be exhausted from the diffusion treating furnace before the furnace cools to the point where halides can deposit on the surface of the steel coil, although in a tight coil such deposition would occur chiefly at the extreme edge of the coil. If any halides do form on the coil after sintering, they should be removed promptly by washing the coil in warm dilute nitric acid.

By the use of this invention, wherein an alkali metal or alkaline earth metal halide is used in conjunction with the chromium-containing powder, certain distinct advantages are obtained over prior practices. The halide incorporated with the metal powder, not to be confused with the aforementioned halogen-containing gas used chiefly as a scavenger, not only aids in promoting diffusion in a tightly wound coil, but also prevents sticking of the coil laps upon unwinding of the coil after removal from the furnace. Sodium cannot diffuse into either the stainless diffusion coating or the base steel because of its large atomic diameter. Therefore, it combines with oxygen and/or other elements to form one or more compounds which remain at the free surface of the diffusion coating and inhibit sticking or welding of adjacent coil laps.

By using a tightly wound coil, full hard steel strip may be used, thus eliminating an anneal prior to sintering. With open-coil diffusion treating, it is preferable to anneal prior to diffusion treating, in order to obtain maximum adherence of the powder particles to the strip surface so that they will not be removed during insertion of the spacers between the laps and the early stages of the diffusion treatment before sintering has taken place.

A further advantage in the use of a tight coil in the diffusion step stems from the fact that lighter gage strip may be used in tight-coil diffusion treatment. With open-coil diffusion treatment, strip gages lighter than 26 gage are impractical, due to lack of physical support during heat treatment and consequent sagging or collapse of the coil. In tight-coil diffusion treatment, gages as light as 30 gage can be used.

The tight coil also permits treatment of considerably more strip for a given furnace volume. For example, in an open coil of 20 gage, 36-inch wide strip having an outside diameter of 72 inches and an inside diameter of 24 inches, the coil weight is approximately 10 tons, while a tight coil of the same type strip of the same outside and inside diameters weighs approximately 18 tons.

Referring again to the example, portions were cut from various parts of the 3-ton chromized coil for test purposes. One of the more significant tests made on chromized steel is that known as the "Muffler Condensate Test." This test has been described in U.S. Pat. No. 3,343,930 to Borzillo et al. Six samples were subjected to the muffler test for three 1-day cycles, three cycles being the accepted standard for evaluating corrosion resistance of chromized specimens in this test. The results with specimens taken from the coil were excellent. No corrosion appeared in specimens during the test period.

An Olsen test was made on each of three specimens selected from different locations in the coil. The Olsen test procedure is described in "The Making, Shaping and Treating of Steel," 7th Ed. (1957) at pages 923-924. The average Olsen button (average of all tests) was 0.375 in.

Thickness of the continuous integral solid portion of the coating, determined by metallographic examination, was approximately 0.002 inch. The grain size of the base steel, determined metallographically at 100 magnifications, was 55 to 65 grains per square inch.

X-ray analysis of the coating disclosed no formation of sigma phase, silicon dioxide (SiO2), chromium carbide (Cr23 C6) or other harmful compounds.

As another example of chromizing by the method of this invention, a 15-ton coil of strip having the same analysis as the 3-ton coil of the first example was prepared and chromized under conditions identical with those used for the first coil, with one exception. In this second run, no chlorine gas was added to the furnace atmosphere. Surface appearance and test results for the area between edges of the coil were consistent with those obtained for the coil of the first example. The only difference noted between the two chromized coils was in the fact that the 15-ton coil showed an inferior coating at the edges to a depth of about two inches. While the edges of the coil can be trimmed to remove the inferior edge area, it would be preferable to avoid this step by addition of a small amount of chlorine to the furnace atmosphere, as in the first example.