US3449159A - Process for forming metal coatings - Google Patents

Description

June 10, 1969 A. L. BALD! 3,449,159

PROCESS FOR FORMING METAL COATINGS Filed Feb. 14, 1966 Sheet of 2 ATTORNEYS June 10, 1969 A. L. BALDI 3,449,159

PROCESS FOR FQRMING METAL COATINGS Filed Feb. 14, 1966 Sheet 3 of 2 IN VENTORL ATTORNEYS United States Patent 3,449,159 PROCESS FOR FORMING METAL COATINGS Alfonso L. Baldi, Drexel Hill, Pa., assignor to Alloy Surfaces Company, Inc., Wilmington, Del., a corporation of Delaware Filed Feb. 14, 1966, Ser. No. 527,148 Int. Cl. C23c 13/02 U.S. Cl. 117107.2 17 Claims ABSTRACT OF THE DISCLOSURE The present invention in summary relates to the creation of deposits on the surface of ferrous metal containing carbon. In the broadest aspect of the invention the ferrous metal is brought into contact with a granular oxide of a metal which forms a carbide having a substantial negative free energy of formation, in the presence of a reducing gas such as hydrogen at a temperature of 1,500 to 2,000 F. This creates a deposit of extreme thinness on the surface of the ferrous metal, the deposit predisposing the surface to form a chromized layer which will be smooth rather than rough. The ferrous metal may be steel which has been decarburized and has a carbon content between 0.001 and 0.010%. It may also be a steel having a carbon content between 0.001 and 0.20% which has been stabilized by introducing titanium, columbium, vanadium or tungsten in an appropriate ratio to the carbon content.

Further in summary, the surface of the stabilized steel having a carbon content between 0.001 and 0.20% can be predisposed to take a smooth chromized layer, without the previously mentioned annealing, in the presence of a granular oxide, by regulating the content of stabilizer within a narrow low range with respect to the carbon content.

DISCLOSURE OF INVENTION A purpose of the invention is to permit the formation of thin deposits on ferrous metal containing carbon which will include a carbide of a metal which forms a carbide having a substantial negative free energy of formation.

A further purpose is to create a preferential layer on ferrous metal which will predispose it to form a smooth chromized layer by deposition rather than etching and substitution.

A further purpose is to provide a surface on ferrous metal containing carbon by annealing at a temperature of 1,500 to 2,000 F. in contact with a granular oxide of a metal forming a carbide with substantial negative free energy of formation, so as to predispose the surface to form a smooth chromized layer.

A further purpose is to produce a greater case depth by chromizing in a given time or to reduce the time required to produce a given case depth.

A further purpose is to reduce the consumption of chromium in chromizing and minimizing the extent of contamination of the source of chromium with iron during chromizing.

A further purpose is to obtain a desposition reaction rather than an interchange reaction in chromizing.

A further purpose is to minimize the extent of adherence of the source of chromium to chromized work in pack or contact chromizing, reducing the labor required to clean or clean and polish the chromized product after chromizing.

A further purpose is to accomplish annealing and the creation of a thin deposit on ferrous metal work by blowing the granular oxide into contact with the work during annealing and permissibly while the work is advancing through an annealing chamber.

Further purposes appear in the specification and in the claims.

In the drawings I have chosen to illustrate a few only of the numerous embodiments in which the invention may appear, selecting the forms shown from the standpoints of convenience in illustration, satisfactory operation and clear demonstration of the principles involved.

FIGURE 1 is a diagrammatic central vertical section of an open coil annealing furnace for use in the present invention.

FIGURE 2 is a diagrammatic vertical section of a continuous annealing furnace according to the present invention.

In the prior art, both in gaseous chromizing by circulating a halogen-containing gas and a carrier gas through a source of chromium such as ferrochrome and then in contact with the work according to Samuel and Bell U.S. Patent No. 3,222,212, granted December 7, 1965, for Process of Chromizing, and also according to the pack technique in which a source of chromium such as ferrochrome, coated or impregnated with a suitable halogen chromizing catalyst such as bromine, chlorine, fluorine or iodine, suitably in the form of a chromous halide, is brought into contact with or adjacent to ferrous metal work, the results have remained up to the present time rather unpredictable. Two lots of steel which superficially seem to be the same produce quite different results from the standpoint of the chromized case, even though the chromizing technique is the same. One lot of steel undergoes a chromizing interchange reaction, iron leaving its surface in the form of ferrous halide, and depositing on the source of chromium, and chromium forming chromous halide and interchanging with the iron of the work, producing on the work surface a very bright but very rough chromized case. In this instance slight dimensional loss in the work or no dimensional gain occurs in chromizing. In this type of chromizing I find that the quantity of deposition in a given time is rather moderate. There is a great tendency for the ferrochrome or other source of chromium to adhere to the work if a chromizing pack is in contact with the work. As a consequence, a great deal of labor must be expended by way of cleaning 01f adhering particles and also burnishing the product to remove the roughness.

Furthermore, in this type of chromizing the source of chromium becomes contaminated with iron more extensively than in the other type to be discussed, and must be replaced when iron contamination is excessive.

Unpredictably in the prior art some steel specimens, on the other hand, produce dull, smooth chromized layers to which the particles of ferrochrome, or other pack, if present, have less tendency to adhere. They can be very quickly and inexpensively brightened by burnishing. Instead of an interchange reaction, in this instance the chromium simply deposits and there is very little loss of iron so that the source of chromium is kept free from contamination. Instead of losing dimension, or maintaining dimension, as in an interchange or interchange plus etching reaction, there is simply a dimensional change approximately equal to one-half the case depth.

I have conducted an extensive experimental program in an effort to determine the factors which lead to one type of chromizing rather than another.

ANNEALING I first attempted to determine whether grain size or shape is a factor. I was unable to find any correlation between grain size and grain characteristics in reasonably fine grained steels which would account for the two different types of chromizing.

As a result of annealing experiments, however, I discovered with some surprise that when I annealed relatively low carbon steels in the presence of certain granular oxides, and in reducing atmosphere, notwithstanding the free energy considerations suggesting the oxides were of great stability, something happened to the surface of the steel which changed its characteristics and made it uniformly form smooth, dull chromized layers in later chromizing, rather than rough, bright chromized layers. Furthermore, this led to a deposition rather than an interchange reaction, with dimensional growth rather than loss or no change in dimension, and freedom from tendency of the pack, if any, to adhere to the chromized case.

When reference is made to granular oxide, it is meant that the oxide is finely divided, in the form of individual particles, which will preferably be a powder. The oxide should be at least fine enough to pass through a screen of 20 mesh per linear inch, and preferably fine enough to pass through a screen of 60 mesh per linear inch, although in most cases a much finer particle size of oxide will be used, suitably through 100 mesh per linear inch, or preferably through 325 mesh per linear inch. When mesh are referred to herein it is intended to designate Tyler standard mesh per linear inch. When the granular material is required to be circulated by the gas to bring it into contact with the work, it will of course be understood that it should be fine enough so that the gas can carry it under the particular gas velocity conditions, for example through 100 mesh per linear inch.

I first found this effect when annealing in a hydrogen atmosphere with a very stable oxide, alumina and later by extending my experiments I found that a similar effect occurs when annealing in the presence of other granular oxides which have a substantial negative free energy of formation of the corresponding carbide.

It certainly would not be supposed that hydrogen would reduce alumina to form aluminum, but contrary to expectation I find that a small but important quantity of aluminum is reduced under these conditions. It appears to form on the surface of the iron a layer of aluminum carbide by reaction with carbon in the steel. This definitely improves the surface and makes it harder and more favorable to receive chromized deposits.

Trying other oxides, I find that similar efiects occur for example when the ferrous metal is annealed in the presence of titania or chromic oxide.

It seems that the effect is attributable to the fact that the particular element forms a carbide which has a high or substantial negative free energy of formation, and therefore is highly stable when applied to the surface of the steel. Elements of this class which can be used as oxides are aluminum, boron, calcium, columbium (niobium), chromium, molybdenum, silicon, strontium, tantalum, thorium, titanium, tungsten, uranium, vanadium, and zirconium.

The temperature of annealing will suitably be in the range of 1,500 to 2,000 F., preferably 1,700 to 1,850 F.

The oxides and the hydrogen or other reducing gas are preferably dry.

The gas can simply be hydrogen or cracked ammonia, or a mixture of nitrogen, carbon monoxide and hydrogen. The gas is preferably flowed or circulated through the annealing chamber and can conveniently be burned at an exit point. Hydrogen is decidedly preferable because of its strong reducing action. The gas is preferably substantially free from elemental halogen and from hydrogen halide.

The granular oxide can be packed around the work, or it can be blown for example by the reducing gas into contact with the work. If the steel work is progressing through the annealing chamber, the gas can bring the oxide against the work.

FERROUS METAL WORK The invention is applicable to two types of materials, one a low carbon steel which is of such low carbon content that it may be considered to be decarburized since this is the normal way that it is produced. In this case the carbon content is between 0.001 and 0.01%, the maximum carbon content usually being 0.005%, the balance being essentially the usual metalloids such as manganese, phosphorus, sulphur and iron.

The widest application of the invention, however, is to low carbon steel having a range of carbon content between 0.001 and 0.20% and stabilized by the presence of one of the metals titanium, columbium, vanadium or tungsten or a combination thereof. The balance of this steel is essentially the usual metalloids and iron.

These steels are not essentially alloy steels, but they may have alloy other than carbon and stabilizer totalling 5% or less by weight.

A typical steel of this kind, stabilized by titanium, is used in numerous experiments referred to below and has an analysis as follows:

Percent Carbon 0.05 Titanium 0.30 to 0.40 Manganese 0.30 Phosphorus 0.01 Sulphur 0.02 Copper 0.04 Aluminum 0.02 Silicon 0.03

The stabilized steels as conventionally manufactured have a lower limit of the stabilizing element content at the stoichiometric ratio and range in ratio of the weight of the stabilizing element to the weight of the carbon as follows:

Titanium Between 4 and 12 to 1. Columbium Between 8 and 20 to 1. Vanadium Between 4 and 12 to 1. Tungsten Between 15 and 50 to 1.

The reason for the broad spread in the case of tungsten is that tungsten forms two different carbides.

As explained later in the present specification, however, there will in some cases be advantages in using lower ratios of stabilizer to carbon and these should be included.

EFFECT OF STABILIZER RATIO I have discovered that in the case of the low carbon stabilized steels, there is another way to create a favorable surface condition for formation of smooth chromized layers with deposition rather than interchange. This technique can be used instead of or in addition to annealing.

Some of the stabilized low carbon steel specimens produce smooth deposition chromized layers without an nealing, while others if not annealed in the manner explained above produce bright, rough interchange chromized layers. By carefully determining the ratio of stabilizer to carbon in the particular steel, I find that a high ratio of stabilizer to carbon favors the formation of bright, rough interchange chromized layers, while a low ratio of stabilizer to carbon favors the formation of smooth, dull deposition chromized cases. This seems to confirm the idea that if there is a great content of stabilizer the carbon is not free to produce carbide at the surface which can promote the formation of a smooth, dull deposition chromized case.

Furthermore, it appears that use of stabilizer ratios to carbon which are less than the stoichiometric theoretical, definitely favors the formation of smooth, dull deposition chromized cases.

In the instance of titanium stabilized steels, in order to have a favorable result in promoting a smooth, dull deposition chromized case the ratio of titanium to carbon should be between 2.5 and 5.75 to 1. The most eifective ratio is between 3 and 5.5 to l.

With other stabilizers the ratios should be as follows:

The chromizing may be pack chromizing, in which the ferrochrome or metallic chromium or other source of chromium will be adjacent to or in contact with the ferrous metal work. The procedure employed may conform to Baldi and Nice U.S. patent application Serial No. 488,711, filed Sept. 2, 1965, for chromizing and Priming, now U.S. Patent No. 3,375,128; Samuel U.S. Patent 2,899,322, granted Aug. 11, 1959, for Chromizing Method and Composition; Samuel and Bell U.S. Patent 2,921,877, granted Jan. 19, 1960, for Process of chromizing Air Hardening Tool Steel; Samuel U.S. Patent 2,851,375, granted Sept. 9, 1958, for Ductile chromizing; Samuel U.S. Patent 2,811,460, granted Oct. 29, 1957, for Process of Chromizing; Samuel U.S. Patent 2,825,658, granted Mar. 4, 1958, for Method of chromizing; and Samuel U.S. Patent 2,811,466, granted Oct. 29, 1957, for Process of Chromizing.

When pack chromizing is referred to in the experiments, the following technique has been used for the temperature and time and with the particular specimens referred to in the particular experiment:

A inch diameter Inconel retort 21 inches high was used. A 1 inch bed of porous ferrochrome containing 68 percent chromium and 32 percent iron by weight which in turn contained in its pores 1 percent by weight of chromous chloride was placed on the bottom of the retort. Five grams of aluminum chloride was spread on top of the 1 inch bed of primed ferrochrome. Next the specimens to be chromized were packed in the rest of the retort in contact with the ferrochrome primed with the 1 percent chromous chloride. The specimens were well surrounded by the ferrochrome and they were not permitted to contact each other so as to prevent shielding. The top of the retort containing a bleeder tube was sealed to the body by a rubber gasket. The head was cooled by flowing water through it. The retort but not the head was next placed in a gas fired furnace and the temperature increased up to the chromizing temperature specified. This temperature is usually between 1700 F. and 1900 F. During the heat-up the aluminum chloride forced out of the retort any existing moisture or air. The evolved gases were readily observed by placing the bleeder tube in a container of water. After the operating temperature was attained and all of the contaminates such as oxygen, air, and moisture were out of the retort, a slight vacuum was encountered as indicated by a slight withdrawal of water (about 1 to 2 inches) in the glass bleeder tube. At this point, the retort was pinched off by clamping a rubber tube at the exit of the retort. In this manner the chromizing react-ion took place under a slight vacuum. After holding a temperature for the specified time period (normally 2 to 10 hours) the furnace was turned off and the retort permitted to cool under the slight vacuum. After the internal contents of the retort had attained room temperature, the retort was opened and the parts unloaded and washed for inspection.

The invention is also applicable to gas chromizing, of the type disclosed in Samuel and Bell U.S. Patent 3,222,- 212, granted Dec. 7, 1965, for Process of chromizing, in which a carrier gas such as hydrogen and a halogencontaining gas such as hydrogen bromide, hydrogen chloride, bromine or chlorine is circulated in a furnace to pass the gas under the action of a pump or fan continuously through a source of chromium to form chromous halide and then in contact with the work such as an open coil of steel to be chromized to form a chromized layer.

When gas chromizing is referred to in later experiments the particular technique was as follows, applying it for the time and at the temperature and on the particular work referred to in the particular experiment: five baskets three feet in diameter and 8 inches high were stacked on top of one another. A perforated sheet consisting of a multitude of A inch diameter holes was supported by a collar at the bottom of each basket. A 32 Tyler mesh sieve was placed on top of the perforated sheet. A 1 inch bed of porous ferrochrome containing 68% chromium and 32% iron by weight was then placed on top of the screen. Note that this ferrochrome was not primed with any halide. A 32 Tyler mesh screen was then placed on top of the bed of ferrochrome. The parts of specimens to be processed Were placed in the 4 lower baskets on top of the screen and not touching the ferrochrome. An Inconel retort was placed over the baskets and sealed to the base by a rubber gasket-flange assembly. The base was water cooled. A 1 inch tube extended through the base into the retort so that various atmosphere gases and halides could be injected at this point. Another 1 inch tube was placed at from the inlet tube. This tube acted as the outlet and extended from within the retort through the base to the outside. A fan was located at the base and its purpose was to pull gases entering in the inlet tube around the outside of the baskets and through the inside of the baskets so as to have a continuous recirculation during the processing cycle. The processing cycle consists of purging out the retort with nitrogen at a flow of 350 c.f.h. After one hour of purging, the furnace was fired up (gas fired furnace) and the nitrogen purge was continued until a temperature of approximately 900 F. was attained inside the retort. At this temperature a flow of 350 c.f.h. of hydrogen was injected into the retort and the nitrogen flow stopped. After the chromizing temperature was reached (1700 to 1900 F.), the hydrogen flow was lowered to about 10 c.f.h. Twenty c.f.h. of argon and 4 c.f.h. of hydrogen bromide were also injected into the retort along with the hydrogen. The hydrogen bromide first contacted the bed of ferrochrome and reacted with it to form chromous bromide. The chromous bromide then reacted with the steel specimens to carry out the chromizing reaction. The reaction products then contacted the bed of ferrochrome in the second basket to again form chromous bromide which in turn reacted with the steel specimens to perfect the chromizing reaction. The gases were recirculated around the retort and thereby maintaining a good chrome potential through the chromizing reaction. An amount of gas mixture was constantly being ejected through the exit pipe in order to maintain a loW dew point in the system and to permit continuous generation of chromous bromide by the introduction of hydrogen bromide at the inlet tube. The chromizing cycle can be anywhere from two to twenty-four hours to provide the case depth required. At the end of the chromizing cycle, the hydrogen bromide and argon flow were stopped and the hydrogen flow increased to 350 c.f.h to purge out the retort. When the temperature attained about 900 F., 350 c.f.h. of nitrogen were injected into retort and the hydrogen flow stopped. The flow of nitrogen was continued until the internal temperature of the retort was below 200 F. The retort was then removed and the parts unloaded from the baskets, washed, and inspected.

THEORY In the case of the annealing technique followed by either pack chromizing or gaseous chromizing, it would seem that several different effects are taking place during the annealing:

1) The steel is to a considerable extent decarburized by the action of the hydrogen or other reducing gas.

(2) A small but appreciable quantity of the metal is reduced from the oxide in contact with the work and is deposited on the surface of the work. This is very strange because the free energy of formation of the oxide is greater than that of the carbide, but the fact of reduction 7 is clearly established. It seems that the metal oxide may initially function as a catalyst to promote reduction before it participates in the reaction.

(3) This deposited metal, which may in a particular case be titanium, forms surface carbide, say titanium carbide, which has a substantial negative free energy of formation and will tend to be stable.

(4) In the latter chromizing this metal carbide tends to reduce the etching or attack by the halogen on the surface of the work and prevent the formation of iron halide which would otherwise take place and would encourage an interchange reaction.

(5) The titanium carbide or other carbide of substantial negative free energy of formation at the surface does not readily react with the reducing gas to permit decarburization and therefore it tends to hold carbon which would otherwise be removed by decarburization, creating a carbon-rich area at the surface.

(6) From this carbon-rich area at the surface carbon is available to diffuse into the case, making the case harder than it otherwise would be and preventing sticking of the source of chromium to the surface in pack chromizing.

(7) Since more of the reaction conveys chromium to the work and less of the reaction conveys iron halide away from the work, there is a thicker case deposited in a given time, and there is less contamination of the ferrochrome by iron. Since there is little etching and specially less preferential etching, there is greater smoothness of the chromized surface.

These theoretical considerations, while not essential to the disclosure, seem to be factual in view of the increased weight of the specimen after annealing, and the buildup of carbon in the case by this procedure.

Where the stabilizer ratio to carbon alone is the controlling factor, as in titanium stabilized steel without annealing, it would appear that the ratios of free energy of various compounds may be controlling. For example, let us consider the free energy of formation of various titanium compounds at room temperature and at 1,800 F.

TiC

Case Core grain structure depth, Gain or loss in Weight Identification before chromizing Surface after chromizing mlls after chromizing Reaction mechanism Untreated 3748 Elongated Rough and brig g -l qit Etc 'ng plus lnterchange. Untreated 488772. d0 0 gr-lsqin-.. Do. Annealed 3748.... Equiaxed Smooth and dull. -1 gnsq. ft.--. Deposition. Annealed 488772 do do 3 gL/SQ. it Do.

creased the conversion to titanium tetrachloride is less feasible. This is borne out by Example 14, in which one lot of low carbon titanium bearing steel was chromized in three different conditions, with a scaled or oxidized surface, a pickled surface and a sand blasting surface. The most uniform deposit was obtained on the scaled surface, which of course had at the surface the highest amount of titanium dioxide.

Example 1 Samples of titanium stabilized steel having an analysis as set forth above taken from two lots (3748 and 48872) which gave rough chromized surfaces in gas chromizing as above explained were obtained in the form delivered by the steel mill and treated as follows:

(1) Degreased with a solvent.

(2) Placed in a retort having a semi-tight lid packed all around in contact with the steel with powdered alumina which had previously been calcined at a temperature of about 1900 F. The retort was placed in an annealing furnace with hydrogen flowing through the retort and and out the opening in the lid and displacing all air from the retort, and the retort and contents were heated to 1,800 F. and held at this temperature in contact with the flowing hydrogen for five hours. The hydrogen had a purity of 99.9% and it had a dew point rating of about minus 100 F.

After completion of the hydrogen annealing as above described, the retort and its contents still in contact with the hydrogen was cooled to room temperature, and opened and the steel specimens were removed.

The specimens as annealed above in hydrogen in contact with alumina were then chromized in contact with a pack of porous ferrochrome which had bee impregnated with anhydrous magnesium chloride according to the procedure in my copending application, Serial No. 488,711, filed Sept. 2, 1965, for Chromizing and Priming, now US. Patent No. 3,375,128. In a sealed retort, in which the air was excluded by the specimens as treated above and other specimens from the same steel lots which had merely been degreased were heated to 1,775 F. and then held at this temperature in contact with the impregnated ferrochrome for six hours and then cooled in the retort while it remained in the furnace.

The results are summarized in the following table:

It will be noted that the free energy of formation of titanium dioxide and titanium tetrachloride is very high. Since titanium tetrachloride is very volatile and has a high vapor pressure, in a chloride atmosphere during chromizing it is very likely to form and remove titanium from the metal interface if titanium remains there as metallic titanium. Accordingly, this might lead to etching and formation of rough surface in chromizing a titanium hearing steel that has a large amount of titanium, forming a rough surface. Lower titanium contents would, therefore, be more favorable.

Titanium carbide has a lower free energy of formation than the oxide or the chloride and is apt to be less stable. The higher the titanium content with a fixed carbon content, the more susceptible the base metal is to the interchange or interchange and etching reaction. The lower the titanium content with the fixed carbon content, the less susceptible it is to the above type reactions and the greater it leans toward a deposition reaction. On the other hand, if the amount of titanium dioxide present were in- In the case of the annealed specimens of both lots, it was found that the primed ferrochrome was easily and readily removed from the surface of the specimens after chromizing. In the case of the untreated specimens it was found that the primed ferrochrome sintered and stuck to the specimens after chromizing and a great deal of labor was required to remove the ferrochrome from the specimens. Even so the resulting case was rough after the primed ferrochrome had been removed.

Based upon this experiment, it appears that hydrogen annealing in contact with alumina prior to chromizing produced the following advantages:

(1) The surface of the chromized work was very smooth.

(2) In a given time and at a given chromizing temperature, the thickness of the chromized case was about 15- 25% greater in the instance of the specimens which had first been annealed in hydrogen in contact with alumina.

(3) Where the specimens had not been annealed in hydrogen in contact with alumina, etching occurred which not only impaired the surface of the specimens, but deposited a great deal more iron on the ferrochrome and thus more rapidly deteriorated the source of chromium. In the case of the chromized specimens which had first been annealed in hydrogen in contact with alumina, a deposition reaction occurred without etching, thus tending to extend the life of the ferrochrome as well as improve the surface appearance.

Example 2 Samples of low carbon plain carbon decarburized steel having a carbon content of 0.005% were degreased.

Samples of low carbon titanium stabilized steel, Lot 488772, as received from the steel mill were vapor degreased.

Some of the samples were chromized in contact with primed ferrochrome according to the procedure set forth above, and others of the specimens were first annealed in a pack in contact with previously calcined alumina in a stream of circulating hydrogen in a retort having a loose lid at 1,800" P. for five hours, and then chromized. The hydrogen quality was as above defined and the circulation of hydrogen was as above set forth.

In pack chromizing the previously treated and previously untreated specimens were held in contact with the primed ferrochrome at 1,775 F. for five hours in a closed retort protected from the air. Others of the previously treated and previously untreated specimens were gas chromized according to the procedure set forth above at 1,775 F. for five and one-half hours.

The results of the two sets of experiments were as follows:

hydrogen and hydrogen bromide for five hours and then cooled to room temperature in the furnace. The parts were removed from the retorts and given a nitric acid support test, which demonstrated no appreciable chromizing had taken place. It is, however, possible that a minute film of chromium might have diffused into the surfaces, but if so, it was not sufficient to give corrosion resistance under the nitric acid support test, either because of lack of thickness or lack of continuity or both. Some of the bolts from each lot were then contact chromized at 1775 F. for five hours. The results observed were as follows: I

The bolts which had been annealed in contact with alumina and not hydrogen had smooth chromized surfaces.

The bolts which had been annealed in hydrogen only without contact with alumina had smooth chromized surfaces.

Other bolts which had not undergone any annealing but were chromized in the as-received condition had rough chromized surfaces.

This experiment indicates that under some conditions, annealing other than in contact with an oxide such as alumina can produce a smooth surface after chromizing. One explanation may be that a slight diffused chromium coating in the annealing accounts for this effect.

The indications are that a carbide formed on the surface due to diffusion of carbon from the core into the surface and ingress into the retort of hydrogen and chromium containing gases (from circulation of the hydrogen bromide through a source of chromium) and that this resulted in formation of a chromium carbide surface due to the reduction of the chromium halide by the hydrogen. In

Contact Chromizing Gas Chromizing Percent sur- Gain or loss in face chromi- Gain or loss in Case Surface after weight after um (X-ray Surface after weight after depth Identification chromizing chromizing fluorescence) chromizing chromlzing (mils) Untreated titanium stabilized N on-uniform, with 3.0 gr./sq. ft 37.6 Slightly rough 3.6 gr./sq. ft 1. 1

steel. local roughness. throughout. Annealed titanium stabilized Smooth throughout. +8.4 grJsq. ft 48. 1 Smooth surface +2.8 gr./sq. ft 1. 4

ste Untreated low carbon decar- Slight to moderate 1.1 grJsq. ft Shghfly ough 7.99 grJsq. ft

burized steel. roughness throughthroughout.

ou Annealed low carbon decarbu- Smooth throughout.-. +8.1 grJsq. ft mooth throughout..- +1.3 gin/sq. ft

rized steel.

Example 3 Some titanium stabilized low carbon steel specimens and decarburized low carbon steel specimens were annealed in a retort with a semi-tight cover in contact with previously calcined alumina in a circulating hydrogen atmosphere and other specimens from the same lots were annealed under the same conditions in contact with hydrogen except that no alumina was used.

All of the specimens were then contact chromized as in Example 2. Those specimens of titanium stabilized steel which were annealed in hydrogen in contact with alumina after chromizing give a smooth surface with a case of 1.9 mil thickness.

Those specimens of titanium stabilized steel which were annealed in hydrogen without any alumina in contact with them gave surfaces which were in some places rough and some places smooth, the case being 1.6 mils thick.

Specimens of low carbon titanium stabilized steel which had simply been degreased and were not annealed at all were chromized in the same batches and they gave rough surfaces having a case thickness of 1.5 mils.

Similar results were obtained for low carbon decarburized steel.

Example 4 Low carbon titanium stabilized steel bolts were in some cases placed in powdered alumina in a retort with a semi-tight lid, and in other cases placed in an empty retort initially containing air and having a semi-tight lid.

Both retorts were placed in a gas chromizing furnace according to the Samuel and Bell patent above referred to and heated to 1775" F. in a chromizing atmosphere of other words, instead of the hydrogen reducing the oxide of alumina or chromic oxide in the pack technique, the same thing occurred in gaseous chromizing. It will, of course, be understood however that the ingress of these gases into the retorts was slight because of the semi-tight lid, as otherwise chromizing would have taken place instead of annealing during the gaseous treatment.

Example 5 Low carbon titanium stabilized steel as above described in the form of sheet panels was solvent degreased, packed in a retort in powdered chromic oxide (Cr O the retort having a semi-tight lid, and annealed in a furnace at 1,775 F. for three hours under circulating hydrogen. The samples were then cooled in the furnace under hydrogen and removed from the retort and separated from the chromic oxide. The panels as treated above together with degreased untreated low carbon titanium stabilized steel panels from the same lot were contact chromized under the procedure above described at 1,775 F. for six hours. The results were as follows:

Case

depth in Identification Surface appearance mils Annealed in chromic oxide Smooth dull surface 1. 5 Untreated Rough bright surface 1. 2

1 1 Example 6 greased low carbon titanium stabilized steel from the same lot were annealed in hydrogen in contact with alumina for various times and at various temperatures, the specimens being weighed before and afterward and a determination made of the Weight change. The results were as follows:

Weight Tempergain in ature, F. Hours mg./sq. it.

1, 670 3 8 to 44. 1, 775 3 96 to 112. 1, 875 3 170 to 307. 1, 775 6 180.

It is thus evident that a slight but substantial gain in weight of the speciments results from annealing in hydrogen in contact with alumina. This is surprising in view of thermodynamic data which would lead one to doubt that aluminum could be reduced from alumina under these conditions.

Example 7 Attempts were made to determine the nature of any addition to the low carbon titanium stabilized steel panels by annealing in hydrogen in contact with aluminum oxide and in contact with chromic oxide. Untreated low carbon titanium stabilized steel panels, and specimens annealed in hydrogen in contact with calcined alumina and specimens annealed in hydrogen a contact with chromic oxide were subjected to X-ray fluorescence to determine the content of aluminum and of chromium as the case may be.

Percent Percent aluminum chromium Appearance of Identification by weight by weight surface Untreated 0. 057 0.001 Steel-like. Annealed in hydrogen in 0.155 Aluminum-like.

alumina. Annealed in hydrogen 0. 042 Bluish.

in chromic oxide.

The liberated metal M forms a carbide on the surface of the specimen.

Example 8 Corrosion tests were made of degreased low carbon titanium steel specimens untreated, annealed in hydrogen as previously described in contact with alumina and annealed in hydrogen as previously described in contact with chromic oxide. Exposure was made outdoors in June in a residential area for one week, which included two days of rain. The specimens on inspection showed the following:

Surface condition in per- Identification: centage of surface rusted Annealed in hydrogen in alumina Annealed in hydrogen in chromic oxide 50 Untreated 70 Annealed in hydrogen only 90 12 This experiment indicates that the thin diffused coatings of aluminum and chromium gave some degree of protection against rusting as compared with the untreated steel.

Example 9 Degreased panel specimens of low carbon titanium stabilized steel as above described were annealed in hydrogen in calcined alumina at 1,800 F. for five hours, as previously described. Half of each panel surface was immersed in concentrated hydrochloric acid to remove the surface metal. The panel was then rinsed, dried, buffed and reimmersed half way in concentrated hydrochloric acid and then again rinsed and dried.

The specimens thus prepared were then contact chromized according to the previous procedure at 1,775 F. for five hours. The specimens Were then' cooled to room temperature and examined. Those areas where the surface of the annealed panel was removed by hydrochloric acid showed a rough non-uniform chromized case. Those areas Where the panels had not been immersed in hydrochloric acid had a chromized case that was very smooth in appearance. This experiment confirms the fact that the annealing in hydrogen in alumina produces a surface effect which is beneficial to the smoothness of the subsequently chromized case.

Example 10 A151 1070 steel was annealed in hydrogen in alumina according to the previous processing procedure at a temperature of 1,800 F. for five hours. Annealed samples and unannealed samples were compared as to carbon content:

Percent Unannealed 0.79'5 Annealed 0.345

This experiment demonstrates that the annealing in hydrogen in alumina is very effective to decarburize the steel, so that evidently decarburizing is one of the effects.

Example 11 Chromized surface layers were stripped from the panels of the previous examples by dissolving the core in 30% by volume boiling nitric acid. The carbon contents of the chromized cases were then dedetermined as follows: smooth, dull cases stripped from annealed specimens, 0.25% carbon; rough, bright cases stripped from untreated specimens, 0.05%. This indicates that a change in the distribution of carbon in the core has occurred during annealing. Some of the carbon has diffused from the core to the surface during annealing and as aluminum of chromium was deposited on the surface of the annealed specimen, aluminum carbide or chromium carbide for-med.

During the subsequent chromizing, it is evident that this carbon enters the chromized layer as chromium carbide.

This carbide in the chromized layer, if it represents chromium carbide or less stable metal carbide than chromium carbide, will under some conditions adversely affect corrosion resistance, so that chromized layers containing it would be recommended more highly from the standpoint of heat resistance than corrosion resistance as such. More stable carbides, such as titanium carbide, calcium carbide and the like, do not have this disadvantage from the standpoint of corrosion resistance.

Annealing may be conducted in a reducing gas such as hydrogen, or cracked ammonia or a mixture of hydrogen, nitrogen and carbon monoxide, in contact with an Aluminum Tantalum Boron Thorium Calcium Titanium Columbium (Niobium) Tungsten Chromium Uranium Molybdenum Vanadium Silicon Zirconium Strontium Example 12 Two lots of low carbon titanium-bearing steel were studied, Lot 45 944-2 and Lot 485 88-2. The analyses obtained from the steel manufacturer, which are believed to be ladle analyses, and therefore subject to modification during subsequent processing, are as follows:

Percentage Element Lot 4-5944-2 Lot 4-8588-2 In order to check, an analysis of certain elements was made based on the actual samples used for tests, with the following results:

Percentage Element Lot 4-5944-2 Lot 4-8588-2 Ti 0. 30 0. 36 C 0. 060 0. 054 Mn 0. 33 0. 33

Panel samples of the two lots were solvent degreased and without any preliminary annealing were chromized using gas chromizing technique as above described at 1,825 F. for four hours and then furnace cooled. The following results were obtained:

Lot 4-5944-2 Lot 4-8588-2 Case depth (nitric acid etch), inch 0023 0017 Weight gain or loss, g./sq. it +9. 3 -4. 2

Lot 4-5944-2 gave a smooth, dull chromized layer, and the mechanism of chromizing was deposition and Lot 4-85882 gave a bright, rough chromized layer, and the mechanism of chromizing was etching and interchange.

Based upon the more accurate chemical analysis made, the ratio of titanium to carbon for the two lots was as follows:

Lot 4-5944-2 5.1/1 Lot 485882 6.7/1

Example 13 The chromized specimens obtained in Example 12 were ground to expose a bare edge, and the steel core was dissolved completely in a boiling solution of 30% by volume of nitric acid. The chromized cases were then analyzed with the following result:

From the previous tests it is apparent that in the specimens which gave the smooth, dull chromized cases, the titanium to carbon ratio was lower in both the core and in the case while in the specimens which gave a rough, bright chromized case the titanium to carbon ratio was substantially higher in both the core and the case.

Further experiments indicate that the ratio of stabilizer to carbon in a stabilized steel has a powerful effect on the question of whether the steel will form a smooth or a rough chromized layer. With titanium as the stabilizer, the most powerful influence is exerted to form a smooth chromized layer by a titanium-to-carbon ratio of between 2.5 and 5.5 to 1, and less effect but still powerful effect is exerted by the ratio between 5.5 and 5.75 to 1. The best ratio is between 3 and 5.5 to 1.

Other stabilizers instead of titanium should be used in the low ratios to carbon as above set forth in order to get benefit in promotion of smooth deposition-type chromizing.

Example 14 A single lot of low carbon titanium bearing steel of a typical analysis as above set forth was received with a scaled or oxidized surface. After solvent degreasing, three sets of specimens were prepared for chromizing as follows:

( 1) Untreated (2) The surface was pickled with hydrochloric acid until the scale was removed.

(3) The surface was sand blasted until the scale was removed.

All three specimens were chromized in contact with a primed pack of ferrochrome as above set forth at 1,825 F. for four hours; The results were as follows:

(1) The specimen with the untreated surface had a dull and smooth chromized layer free from etching.

(2) The specimen with the pickled surface had a bright and rough chromized surface.

(3) The specimen with the sand blasted surface had a bright and rough chromized surface.

Both of the latter surfaces showed etching.

Example 15 Specimens which had been solvent degreased from low carbon titanium stabilized steel of Lots 459442 and 4-8588-2 were both annealed in hydrogen in contact with powdered titanium dioxide at 1,775 F. for four hours. The specimens were cooled in the furnace in contact with hydrogen and then removed and chromized using the pack technique as above set forth.

Lot 4-5944-2 showed the same results as previously set forth in Example 12, with a dull, smooth chromized layer and a gain in weight indicating deposition.

Lot 4-8588-2 on the other hand now gave a smooth, dull chromized surface with greater case depth and a gain in weight, contrary to the previous behavior in Example 12.

This shows that annealing in hydrogen and in titanium dioxide corrected the tendency of Lot 4-8588-2 to produce rough chromizing.

It would seem that a slight reduction of titanium dioxide powder has taken place producing titanium which has diffused into the surface of the core metal and reacted with carbon to form titanium carbide, and that the titanium carbide has then functioned to produce the dull,

smooth chromized surface. The dullness can easily be corrected by burnishing.

Example 16 Corrosion tests were made of low carbon titanium stabilized steel which has been annealed in hydrogen and a compound such as chromic oxide, or titanium dioxide and then chromized to produce a smooth, dull chromized case. These tests indicate that as compared with smooth, dull chromized cases on low carbon titanium stabilized steel produced without pro-annealing and having a critical ratio of titanium to carbon within the range of 25:1 and 5.75:1, the corrosion resistance of the specimens which have been annealed prior to chromizing is not as good as the corrosion resistance of the specimens which have not.

Example 17 Low carbon titanium bearing steel panels were placed in alumina in a retort containing a semi-tight cover. The retort was heated to 1800 F. in hydrogen and held for 15 minutes. Afterwards it was immediately cooled to room temperature. The specimens were next chromized along with low carbon titanium bearing steel specimens from the same lot which had not been previously annealed, the chromizing being carried out in the contact process at 1775 F. for six hours. The specimens which had previously been annealed were examined in comparison with the specimens which had previously been unannealed, and it was found that the previously annealed specimens had slightly smoother surfaces.

FIGURE 1 illustrates in central vertical section an open coil annealing mechanism suitable for annealing according to the present invention, which embodies certain features present in Samuel and Bell US. Patent 3,222,212, granted December 7, 1965, for Process of Chromizing. The mechanism will only be described insofar as it differs from that in this patent.

An open coil 20 of steel sheet or strip is provided, wound with suitable open spacing between turns, preferably in the range of 0.13 to 0.22 inch. The coil rests on a base 21 supported by radial ribs 22 on a suitable refractory furnace base 23. The base 21 has a solid center 24 which prevents unintended short circuiting of gas and a grill 25 on which the open coil rests on edge in vertical position as shown in FIGURE 1. Below the grill 25 the base has downwardly and inwardly directing bafiles 26 to funnel circulating gas into a center opening 27 which is in line and immediately adjoining rotor 28 of a centrifugal blower or fan turning on shaft 30 which is suitably sealed against the supporting base 23 to prevent leakage.

Surrounding the entire coil and base 21 and the fan is a bell closure 31 resting on the base 23 and suitably sealed when in assembled position. Inlet pipe 32 introduces gas, suitably hydrogen, into the retort space 33 inside the bell, and outlet pipe 34 draws ofi spent gas from the retort space.

In accordance with the present invention a quantity of granular oxide of the type above referred to is provided at 35, conveniently simply by pouring on top of the open coil before placing the bell to close the chamber, and the oxide is so finely divided preferably through 100 mesh and most desirably through 325 mesh per linear inch that it becomes gasborne and circulates like a dust cloud under the action of the pump or blower through the open coil in contact with the steel sheet or strip and in endless circulation. Very effective annealing according to the invention is obtained by this means.

Surrounding the bell 31 is placed a removable bell furnace 36 having electric heating elements 37 which when in place is capable of raising the entire work and the retort in which it is contained, closed from the air, to a suitable annealing temperature in the range from 1,500 to 2,000 F.

In the preferred embodiment of the furnace of FIG- URE 1 the open coil from the previous heat is removed,

removing the furnace 36 and the bell 31. Then the open coil for annealing in the next heat is introduced and the oxide in powdered form is placed on it, after which the retort bell 31 and the bell furnace are applied, the air is swept out of the retort suitably by nitrogen. Before reaching an elevated temperature hydrogen is introduced to fill the retort 33. The fan blower or pump 28 is then rotated in order to circulate the gas and the cloud of oxide particles in contact with the work for a time of at least one hour at temperature.

Since hydrogen is very light, it is preferable to include a dense gas in the gas mixture. Argon is a desirable gas to include with hydrogen or a mixture of hydrogen with a small amount of carbon dioxide or carbon dioxide and nitrogen. A desirable gas composition by volume is 50 percent hydrogen and 50 percent argon.

In case powder pack annealing is desired to be employed, the spaces between the open coils can simply be filled by sifting oxide particles into the space, and the blower need not be operated, simply maintaining static conditions and passing gas through the retort. As an alternative the coil for powder pack annealing can be coated with oxide particles, and in this case the turns can be close together if desired although they are preferably spaced in order to aid in the reduction reaction.

The annealing can be carried out continuously as shown in FIGURE 2.

In this form, work such as a strip or sheet 20' is unwound from one roll 40 onto another roll 41. The work. passes into an annealing furnace 42 through a series of slits or openings 43 sealed in any desired manner. A vestibule chamber 44 is provided at the entering end having an inlet gas port 45 and an outlet gas port 46 to remove air, and a vestibule chamber 44 is provided at the outlet and having an inlet gas pipe 45' and an outlet gas opening 46 to exclude air. A suitable protecting gas, which may be hydrogen, can be introduced in the vestibules and burned at the slits 43 adjoining the atmosphere.

The work 20' then passes into an annealing chamber 47 ,heated, for example, by electric heating elements 48 to a suitable temperature, for example, 1,500 to 2,000 F. A mixture of particles of suitable oxide 50, for example, alumina, is introduced from a suitable dry feeder 51 into a stream of gas, for example, hydrogen passing through pipe 52 into the annealing chamber and spent gas and oxide particles are removed through a pipe 53.

In operation the work passes through the annealing furnace and through the annealing chamber and in the annealing chamber comes in contact with gas-blown particles of the selected oxide as above mentioned in the presence of a reducing gas at a temperature of 1,500 to 2,000 F. In this case annealing can be accomplished in a time of 10 or preferably 15 minutes.

In view of my invention and disclosure, variations and modifications to meet individual whim or particular need will doubtless become evident to others skilled in the art to obtain all or part of the benefits of my invention without copying the process, apparatus and product shown.

Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. A process of producing a chromized layer on a ferrous metal, which comprises providing a wrought ferrous metal of the class consisting of steel of carbon content between 0.001 and 0.005% and stabilized steel of carbon content between 0.001 and 0.20% and having a stabilizing element of the class listed below in a ratio to carbon as listed below:

Titanium Between 4 and 12 to 1 Columbium Between 8 and 20 to 1 Vanadium Between 4 and 12 to 1 Tungsten Between 15 and 50 to 1 heating said metal to a temperature of 1,500 to 2,000 F. in contact with a granular oxide of an element of the class consisting of aluminum, boron, calcium, columbium,

17 chromium, molybdenum, silicon, strontium, tantalum, titanium, thorium, tungsten, uranium, vanadium, and zirconium in a reducing atmosphere for a time of at least ten minutes, separating said ferrous metal from said oxide and said reducing atmosphere and then heating said ferrous metal to a temperature between 1,550 and 2,000 F. for a time of at least one hour in an atmosphere protected from the air and containing a halogen chromizing catalyst in the presence of a source of chromium to deposit chromium on the ferrous metal, whereby a preferential tendency to make a smooth chromized layer is obtained.

2. A process of claim 1, in which the reducing gas is a hydrogen-containing gas.

3. A process of claim 1, in which the oxide is alumina.

4. A process of claim 1, in which the oxide is titania.

5. A process of claim 1, in which the oxide is chromic oxide.

6. A process of claim 1, which comprises circulating said oxide in the atmosphere in contact with the ferrous metal.

7. A process of claim 1, which comprises during the chromizing maintaining a granular source of chromium in contact with the ferrous metal being chromized.

8. A process of claim 1, which comprises during the chromizing circulating a halogen-containing gas through a granular source of chromium and then in contact with the ferrous metal.

9. A process of claim 1, in which the ratio of the element of the class to carbon is as follows:

Titanium Between 2.5 and 5.75 to 1 Columbium Between 4.9 and 11.2 to 1 Vanadium Between 2.5 and 5.75 to 1 Tungsten Between 9.1 and 20.8 to 1.

10. A process of claim 9, in which the ratio of the element of the class to carbon is as follows;

18 Titanium Between 3 and 5.5 to 1 Columbium Between 5.8 and 10.7 to 1 Vanadium Between 3 and 5.5 to.1 Tungsten Between 10.9 and 21.1 to 1.

11. A process of claim 9, in which the reducing gas is a hydrogen-containing gas.

12. A process of claim 9, in which the oxide is alumina.

13. A process of claim 9, in which the oxide is titania.

14. A process of claim 9, in which the oxide is chromic oxide. 5 15. A process of claim 10, which comprises blowing the oxide against the ferrous metal.

16. A process of claim 9, in which the steel is in contact with a source of chromium during chromizing.

17. A process of claim 9, which comprises circulating the halogen-containing gas through a source of chromium and in contact with the steel during chromizing.

References Cited UNITED STATES PATENTS 1,853,369 4/1932 Marshall 117-22 2,032,694 3/1936 Gertler 117-16 2,141,640 12/1938 Cooper 117-22 2,157,594 5/1939 Cooper 117-22 2,255,482 9/1941 Daeves et a1. 117-22 2,899,332 8/1959 Samuel 117-22 2,935,420 5/1960 Linden 117-22 X 3,028,261 4/1962 Wachtell et al 117-22 X 3,178,308 4/1965 Oxley et a1. 117-107.2 X 3,183,113 5/1965 Gemmer 117-21 3,208,870 9/1965 Criss 117-118 WILLIAM D. MARTIN, Primary Examiner.

PAUL ATTAGUILE, Assistant Examiner.

US. Cl. X.R. 117-50, 71, 118

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