US2823150A

US2823150A - Method of descaling metals with molten alkali metal hydroxide baths and compositionstherefor

Info

Publication number: US2823150A
Application number: US626844A
Authority: US
Inventors: John A Henricks
Original assignee: Devex Corp
Current assignee: Devex Corp
Priority date: 1956-12-07
Filing date: 1956-12-07
Publication date: 1958-02-11
Anticipated expiration: 1975-02-11

Description

United States Patent John A. Henricks, Logansport, Ind., assignor to Devex Corporation, Cleveland, Ohio, a corporation of Ohio No Drawing. Application December 7, 1956 Serial No. 626,844

6 Claims. (Cl. 1486.11)

This invention relates to a process for descaling ferrous metals, particularly stainless steels, and modifying the oxide on their surfaces so that they may be drawn and otherwise worked.

This application is -a continuation-in-part of my copending application, Serial No. 274,839, filed March 4, 1952, now abandoned, which in turn is a continuation-in-part of my application, Serial No. 193,290, filed October 31, 1950, now reissued Patent No. 24,017, and formerly U. S. Patent 2,588,234.

The bulk of the scale collected on the surface of the stainless steel stock from hot rolling or annealing ordinarily can be removed by immersing it in a fused caustic bath activated by sodium hydride or a strong oxidizing agent such as sodium nitrate. While such baths remove scale from the residual surfaces of the metals, they are not well adapted to drawing or forming without further treatment. While lubricants may adhere to the residual surfaces left by such baths, the severity of subsequent drawing operations is necessarily limited unless additional surface treatment is provided such as forming an integral oxalate coating on the work. Oxalate coatings, however, are expensive and difiicult to control.

It is an object of the present invention to provide a bath that can descale efficiently and can also supply an integral coating for all ferrous metals comprising an iron sulfide coating.

It is also an object to provide a method for treating scaled red-hot annealed ferrous metals so that they can be descaled and provided with an integral sulfide coating in the same bath.

It is a further object to provide acceleration of the subsequent acid pickling bath by the accelerating action of the iron sulfide formed in the fused caustic bath.

The present invention comprises a substantially anhydrous caustic bath which is activated with preferably about 1% to by weight of lump or solid sulfur so that the bath will effectively descale ferrous metal objects and also provide them with an integral iron sulfide coating. This integral coating, being fusible, acts as an excellent lubricant as well as a superior prime coating for subsequent application of extreme pressure lubricants.

The preferred percentage by weight of elemental sulfur to be dissolved in an anhydrous bath of sodium hydroxide or the like is about 1% to 5% by weight of the caustic bath. Benefits of the present invention may be obtained with as little as .5% by weight, and also amounts of elemental sulfur greater than 5% by weight may be used but with little increase in descaling action or sulfide coating action.

The elemental sulfur that reduces the iron oxide scale is predominantly oxidized to the polythionic sulfide and other higher valence forms so that the total active sulfur content generally remains below 5% by weight. It is advantageous to have the elemental sulfur form polythionates rather than sodium sulfides because the sulfides tend to hydrolyze to H 8 which is very toxic. When more "ice than 20% to 25% sulfur is used, hydrogen sulfide may be evolved so that there is danger of toxicity, or fire and explosion due to increased heat of solution and the increased reaction between the sulfur and caustic.

The sulfur acts as a reducing agent for the iron oxide scale formed on metal such as stainless steel. Sulfur thus functions as a good descaling agent and also reacts with sodium hydroxide to formsodium sulfide, sodium thiosulfate and other polythionic sulfides so as to react with iron and form an integral iron sulfide coating. The temperature of the bath may be from about 600 F. to 1200" F. in order to obtain the best descaling and sulfurizing action.

The preferred method of handling metals which are particularly difiicult to descale, such as stainless steels, is to dip the stainless steel work in an anhydrous caustic bath so as to cover it with a relatively thick layer of at least several mils thickness of caustic material. Thereafter the work is quickly quenched in a water quench tank to provide a clean and pickled surface by removing any residual scale from the surface thereof. The steps of immersing in the caustic solution and quenching in water are then preferably repeated in order to provide a fusible iron sulfide coating on the surface without trapping or retaining abrasive particles in the sulfide layer.

The following example illustrates my invention: j

EXAMPLE 1 Fused caustic sulfurize' I prefer to use a novel bath of fused sodium hydroxide in which from 1% to 5% lump or solid sulfur is dissolved as the activator. This novel bath will both descale and sulfurize. The work attains a temperature of 800 F. to 1000 F. and when it is removed from the bath it is filmed with a relatively thick layer of fused sodium hydroxide. The work is quickly quenched in water with almost explosive violence accompanied by a copious evolution of steam. This violent reaction frees the work surface of a large bulk of the adhering layer of scale. The action is partially chemical in'nature by which the lower oxides at the scale-to-metal interface are dissolved as sodium ferrite or sodium chromate, but it appears to be largely mechanical and brought about by a violent release of steamwhen the fused sodium hydroxide is quenched in water so that the scale is burst or cracked from the work surface.

After this initial hot caustic dip and quench, the residual scale is removed in a hydrochloric or salt-sulfuric acid pickle. In order to obtain a scale free and sulfurized surface, the fused caustic dip and quench has to be repeated after the work has been acid pickled so that no abrasive particles contaminate the sulfurized layer. Since the quench bath contains sodium sulfide it could even be used as a sulfurizing bath for red hot annealed stainless steel to combine the quenching and sulfurizing steps into one operation.

A suitable sulfurizing quench bath for use in connection with the caustic bath of Example 1 can be made from the following formulation:

This is made up from 2 to 6 ounces per gallon of solution. Upon removal from the caustic bath, the work is quenched in this bath and simultaneously provided with a sulfide coating, the thickness of the coating depending 7 upon the time which it is left in the bath.

The sodium hydroxide of the fused caustic bath of Example 1 may be substituted for by other alkali metal hydroxides such as potassium hydroxide, mixtures of sodium hydroxide and potassium hydroxide, and mixtures of the above hydroxides with other corresponding alkali metal carbonates. Generally by using any one of the above mix tures, it is possible to obtain a bath with a lower melting point than would be obtained by using one component alone such as sodium hydroxide. A sulfurizing quench bath such as shown in Example 2, which augments the sulfurizing action of the caustic bath and Water rinse, may preferably be formed from a dry composition comprising about 10 to 20 parts of sodium hydroxide, to 45 parts by Weight of elemental sulfur, about 5 to 15 parts of starch and about 30 to 50 parts by weight of trisodium phosphate.

The iron sulfide and other sulfur compounds formed by the reaction of iron and sulfur in the final caustic bath greatly accelerates chemical solution of the ferrous metal in subsequent acid pickling operations by acting as a depolarizer. Thus, the time required to pickle stainless and high alloy steels can be greatly decreased.

Although several embodiments of the invention have been herein shown and described, it will be understood that in accordance with the provisions of the patent statutes, numerous modifications of the process shown may be resorted to without departing from the spirit of the invention.

What I claim is:

1. A method of descaling ferrous metal by holding the scaled metal in a bath of fused sodium hydroxide activated by 1% to 5% elemental sulfur and then quenching the work in water and subsequently pickling 013? the scale residues in an acid bath.

2. A method of descaling ferrous metal and coating said metal with iron sulfide comprising the steps of immersing a scaled metal object in a substantially anhydrous bath of a molten alkali metal hydroxide containing about 0.5 to 25% by weight of sulfur and the oxidation products thereof so as to coat said metal object with a relatively thick film of bath liquid, quenching said metal object rapidly in a tank of water, pickling off the scale residues in an acid bath, and thereafter repeating said immersing and quenching steps to provide an integral iron sulfide coating on said metal object.

3. A method of descaling ferrous metal and coating said metal with iron sulfide comprising the steps of immersing scaled metal in a substantially anhydrous bath of a molten alkali metal hydroxide containing about 1% to 5% by weight of sulfur and the oxidation products thereof so as to coat said metal with bath liquid, quenching said metal rapidly in a tank of water, pickling off the scale residues in an acid bath, and thereafter repeating said immersing and quenching steps to provide an integral iron sulfide coating on said metal.

4. A method of descaling ferrous metal objects and coating said metal objects with iron sulfide comprising the steps of immersing scaled metal objects in a substantially anhydrous bath of a molten alkali metal hydroxide activated by 1% to 5% by Weight of elemental lump sulfur so as to coat said metal objects with bath liquid, quenching said metal objects rapidly into a tank of water, and pickling off the scale residues in an acid bath.

5. A method of decaling annealed stainless steel and providing an integral iron sulfide coating thereon comprising the steps of immersing said steel in a substantially anhydrous fused alkali metal hydroxide bath containing about 1% to 5% by weight of sulfur and the oxidation products thereof and thereafter quenching the steel in a water bath.

6. A method of descaling annealed stainless steel and providing an integral iron sulfide coating thereon comprising the steps of immersing said steel in a substantially anhydrous fused sodium hydroxide bath containing 1% to 5% by weight of sulfur and the oxidation products thereof, and thereafter quenching the steel in an aqueous quench bath containing about 2 to 6 ounces per gallon of bath solution of a dry composition comprising about 10 to 20 parts by Weight of sodium hydroxide, 15 to 45 parts by weight of sulfur, about 5 to 15 parts of starch, and about 30 to 50 parts by weight of trisodium phosphate.

References Cited in the file of this patent UNITED STATES PATENTS 40,904 Brandeis Dec. 15, 1863 339,168 Hall et a1 Apr. 6, 1886 1,572,848 Porter et al Feb. 9, 1926 2,067,530 Ihrig Jan. 12, 1937 2,120,276 Grant June 14, 1938 2,223,037 Ihrig Nov. 26, 1940 2,299,139 Grafton Oct. 20, 1942 2,343,569 Neely et al Mar. 7, 1944 2,350,491 Butler et al June 6, 1944 2,357,342 Montgomery Sept. 5, 1944 2,440,743 Gary May 4, 1948 2,470,062 Whitbeck Oct. 8, 1948 2,489,309 Mills et al Nov. 29, 1949 2,530,837 Orozco et a1 Nov. 21, 1950 2,530,838 Orozco Nov. 21, 1950 2,588,234 Henricks Mar. 4, 1952 2,609,594 Whitbeck Sept. 9,1952 2,664,399 Kluender Dec. 29, 1953 2,697,072 Roden Dec. 14, 1954 2,717,221 Christner Sept. 6, 1955

Claims

1. A METHO OF DESCALING FERROUS METAL BY HOLDING THE SCALED METAL IN A BATH OF FUSED SODIUM HYDROXIDE ACTIVATED BY 1% TO 5% ELEMENTAL SULFUR AND THEN QUENCHING THE WORK IN WATER AND SUBSEQUENTLY PICKLING OFF THE THE SCALE RESIDUES IN AN ACID BATH.