Chromizing coating

2,885,301

Patented May 5, 1959

United States Patent Office

2,885,301

CHROMIZING COATING

George A. Satouel, White Plains, N.Y., assignor to Metal Diffusions, Inc., Philadelphia, Pa., a corporation of New Jersey

No Drawing. Application August 2,1956 Serial No. 601,606

Claims. (Cl. 117—22)

The present invention relates to chromizing metal parts; 15 particularly iron and steel.

A purpose of the invention is to permit chromizing in . a furnace in which oxidizing and combustion gases are present, without the need for an inert or reducing atmosphere furnace or an hermetically sealed retort. 20

A further purpose is to provide glass forming ingredients in the chromizing composition which form a protective skin and exclude oxygen partial pressure, thus making it possible to chromize in a fuel fired furnace, an electric furnace or other suitable furnace without 25 atmospheric control.

A further purpose is to provide a glass forming chromizing skin for coating which is rendered friable and disintegrates on cooling.

A further purpose is to prevent escape of the chromiz- 3θ ing catalyst during chromizing.

, A further purpose is to permit chromizing in a continuous furnace without an atmospheric control.

A further purpose is to permit chromizing powdered iron compacts without atmospheric control. 35

Further purposes appear in the specification and in the claims.

In the prior art, chromizing has almost invariably been carried out in a retort or vessel containing an inert or¹ reducing atmosphere protective from oxygen. This ⁴⁰constitutes a serious limitation, as it necessitates iise of very expensive equipment, and also in many cases requires packing of the articles in chromizing position in the retort.

Also, because of the use of such equipment, the time 45 of heating and the total elapsed time of chromizing has been very long.

It has not been possible to chromize in ordinary fuel fired or electric furnaces, roller hearth, chain hearth, or continuous furnaces, not provided with atmospheric 50 control.

In accordance with the present invention, it is possible to chromize in any of a wide variety of fuel fired, .· electric .dr. other suitable furnaces, without the need for exclusion of oxygen, and without requiring inert or re- ⁵⁵ducing atmospheres. Furthermore the parts to be chromized can be exposed to the interior of the furnace without packing chromizing compounds around them, heating rates are greatly increased, and the total time of the chromizing cycle is greatly reduced. ⁶⁰

According to the invention, therefore, chromizing can be carried out in many variable furnaces which are for the first time made suitable for chromizing. This is true not only with respect to components and parts made from iron, low carbon steel, intermediate carbon steel or even high carbon steel of both plain carbon and alloy grades, and also with respect to powdered iron compacts including those which contain copper of the order of 5 to 10 percent, and which are to be chromized or chromized and sintered.

In accordance with the invention file parts to be chromized are coated with the glass forming material which contains a chromizing catalyst, an inert bodying material and a source of chromium. Unlike the practice carried out in prior box chromizing, the chromizing catalyst is no longer a high volatile disassociating compound such as ammonium chloride, ammonium fluoride or the like, but is a relatively uhvolatile material at chromizing temperature, preferably a chromium halide, most desirably chromium fluoride but less desirable chromium chloride, bromide or iodide. The particular halide used may be either the chromous or chromic. The bromide and iodide are less desirable because they are volatile.

Tbe glass forming ingredients will suitably be metal halides such as barium chloride with desirable addition of sodium chloride to lower the melting point, or they may be powdered glass, with or without materials lowering the melting point such as boric acid or borax alone, sodium oxide or other alkali metal oxide alone, or boric acid or borax with alkali metal oxide.

The inert bodying material may be powdered kaolin; raw or calcine, powdered alumina, raw or calcined, or other suitable refractory.

The source of chromium will preferably be ferrochrome (preferably the 65 percent chromium grade, low carbon) but can equally well be powdered chromium metal.

All of the solid ingredients are preferably reduced to a powder finer than 100 mesh.

Example I

Steel parts include AISI 1010 steel sheet, hacksaw blades,^ drill rods (AISI 1090), were dipped in a slurry consisting of the following ingredients:

Grams

Barium chloride —___________—— ------300

Sodium chloride__________._____________ 75

Chromic flupride____________,____ 25

Uncalcmed kaolin powder------------------ 35

Ferrochrome powder ____________________— 100

This material was mixed with 200 grams of water. The fluoride used was CrF₃-3.5H₂O.

The parts were placed wet in a gas fired furnace at 1950° F. The coating dried immediately and formed a glaze or skin, which was relative impervious to the chromizing vapor, and substantially chromizing was obtained in five minutes, while the chromizing was very pronounced in twenty minutes. The chromium diffusion on high and medium carbon steel parts was 0.0005 inch in five minutes and 0.001 inch in twenty minutes.

After chromizing the parts were withdrawn from the furnace and allowed to cool, and the residual coating was wiped off. Tapping of the parts followed by washing in water will also remove the residual powder.

The composition above may vary widely, good results being obtained with a composition as follows:

Percent

Barium chloride_________________________45 to70

Sodium chloride_________________________11 to20

Chromium fluoride_______________________ 2 to10

Inert bodying material____________________ 2 to15

Source of chromium_____________________ 5 to25

Experiments indicate that chromizing as above is faster than that obtained in pack chromizing techniques even if allowance be made for the more rapid heating.

2,886,301

Example II

Chromizing according to the present invention may be carried out using the following composition:

Grams

Powdered glass 120

Boric acid________ 40

Sodium carbonate 40

Chromic fluoride______________,____ 25

Raw kaolin powder 40

Metallic chromium powder __ 65

This mixture is coated on the parts and they are placed wet or after drying as desired in a furnace at 1900° F.

An impervious glass-like Skin forms a chromizing case very rapidly as above.

The composition may vary widely, the preferred range being:

Percent

Powdered glass_______________;___________ 30 to 60

Boric acid______________________________ 5 to 15

Sodium carbonate___________________;____ 5 to 15

Chromic fluoride________________________ 2 to 25

Raw powdered kaolin ____________________ 5 to 30

Chrome metal powder_____________________ 5 to 25

When reference is made herein to boric acid, it will be understood that borax and sodium borate can equally be used. The preferred powdered glass is glass of conventional window or jar glass composition, though special glasses can be used as long as the melting point is brought within the operating range.

The technique of Example I or II may be applied with success to the chromizing and sintering of green powder iron compacts, with or without the presence of for example, 5 or 10 percent of copper.

This offers the advantage of providing sintering of such compacts without the need of atmospheric control and also protecting the surface by chromizing. It also enables the use of continuous furnace without vestibules in chromizing.

It will be evident of course that the slurry can be applied in any suitable way as by dipping, spraying, brushing, roller coating, or otherwise.

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 and product shown, and I, therefore, claim all such insofar as they fall within the reasonable spirit and scope of my claims.