US2673166A - Chromium steel, process, and articles - Google Patents

Description

Patented Mar. 23, 1954 CHROMIUM STEEL, PROCESS, AND ARTICLES Maurice E. Oarruthers,

signer to Armco Steel Ohio, a corporation of Middletown, Ohio, asorporation, Ohio Middletown,

No Drawing. Application November 4, 1948,

Serial No. 58,359

12 Claims.

My invention. relates to improvements in chromium steel and to processes for obtaining such improvements therein, as well as to articles made from the improved alloy steel. g V

The alloy steels with which my invention is most useful are those containing substantially to of chromium, and preferably substantiallybetween 2 3 and 27% of chromium. Nickel, if present at all, occurs in small amounts generally not over .50%. The steels may contain up to 35% carbon, up to 1.0% manganese, up to .04% phosphorus, up to .03% sulphur and up to 1.5% silicon, the balance being substantially all iron. Other elements may occur as residuals,- but they are not important in the present invention. In general, the type of steel with which my invention is concerned is that commercially designated as A. I. S. I. Type 446, which type of steel usually contains between 23 0 and 27% of chromium.

Type 446 stainless steels are used for the most part in applications where resistance to destructive scaling in both oxidizing and reducing atmospheres at relatively high temperatures (up to 2000 F.) is important. Furnace parts, annealing boxes, exhaust manifolds, combustion chambers and liners are typical articles made from such steel. The alloy is also useful in apparatus and equipment for handling nitric acid, and it possesses excellent resistance to many other corrosive solutions.

Type 446 stainless steels have also been used with great advantage in applications where glass is to be sealed to metal, for example, in the fabrication of television tubes where the glass face of the tube is joined to a cone-like metal body formed of the alloy. There are also uses in the manufacture of radio tubes of a type in Which metal base pins or contractor prongs pass through a glass bulb, and in certain types of chemical and scientific apparatus. The coeificient of expansion of Type 446 steel is almost identical with that of glass, and this property makes possible the joining of the metal and the glass in the fabrication of articles. A further advantage of the alloy in this connection is that it acquires under oxidizing conditions a tightly adhering scale which has the property of" being readily wet by molten glass and of increasing the bond between the metal and the glass.

A disadvantage of the alloy steels of the type referred to is that the metal can not be readily cold worked; andthis disadvantage has greatly limited the scope and variety of-cold fabricating operationswhich'-may--be-performed-on it, and hence its utility in" various important fields. Hitherto Type 446 stainless steels have produced such large numbers of rejections when subjected to cold forming operations such as folding, flanging, deep drawing or spinning, that the cost of articles so produced has been excessive, while certain metal shapes could not economically be produced at all without impractical, expensive and repeated annealing operations.

Again Type 446 stainless steels have a marked tendency to become brittle if they are cooled slowly through a temperature range of approximately 1100 to 700 F. Another type of brittlement, due to grain coarsening, commonly occurs also if the metal is held for prolonged periods above 1650 F. Accordingly, the most useful annealing practice for preparing the metal for cold working, prior to my invention, has been found to be to heat the metal to temperatures between 1450 and 1650 F., usually 1450 to 1550 F., f01- lowed by an air cool or water quench in the case of bars and wires, or in the case of sheet stock, to heat the metal to temperatures between substantially 1500 and 1600 F., followed by furnace cooling to about 1200 F. and air cooling to room temperature. Sheet stock has also been box cooled from the annealing temperature and a wide variety of cooling practices have been attempted with only slight variation in result. These annealing and. cooling methods, while satisfactory for mild or limited cold working, have not produced suflicient workability for severe bending, or for deep drawing, hanging or related operations. The annealed metal, for the most part, split, cracked, tore, exhibited seams, or sufiered other defects rendering it unsuitable for articlesrequiring severe cold work in their manufacture. The need for a technique to render this metal cold workable, is evident.

- Accordingly, it is an object of my invention to provide a new process for treating chromium steels of the type referred to, so as to render them workable and amenable to severe cold forming operations.

It is a further object of my invention to produce a new chromium steel that may be subjected to cold forming operations, such as bending; drawing, spinning, folding, crimping, twistingffianging and the like without danger of the metal splitting, cracking or tearing.

- Another object of the invention is to provide a method for making chromium steels cold formable with a minimum of increased cost of manufacture, and to provide a chromium steel adapted to" many commercial usesnot heretofore possible withchromium-steels of the type described.----- Qtherobjects manufacture of of my inven'tion concern the particular articles from Type l l l l 446 steels, such as bodies for television tubes, combustion chambers for oil furnaces, and the like. These and other objects of my invention which will be set forth hereinafter or will be apparent to one skilled in the art upon readin these specifications, I accomplish in that material, by those procedures, and in those fabricated articles of which I shall now describe certain exemplary embodiments.

Briefly, in the practice of my invention, 1 have discovered that if sheets of chromium steel of the type referred to above, after hot rolling, are given a preliminary heat treatment between substantially 1850 F. and 2050" F. for approximately 2 to 60 minutes depending upon the temperature used and the thickness of the stock being treated, but preferably at 1950 F. for about 15 minutes, followed by air cooling, and if the sheets are subsequently annealed within the usual temperature ranges of between 1450 F. and 1650 F., approx- Treatment 2 000 F. for 2 minutes plus box anneal 1,450" F. for 4 hours.

imately (preferably at around 1550 F.), followed by furnace cooling to 1200 F. and a rapid cooling to below 700 F. as by an air cool or a quench, the sheets will be much more amenable to severe cold forming operations than sheets of the same material treated by prior art practices. The same improved result will be obtained if the box anneal is concluded by a furnace cool, and there is added a brief reheating to around 1450" F. followed by a rapid cool to below 700 F. in single strand or sheet form.

Through the use of the procedure of my invention, cold forming is possible to a degree never before achieved with stainless steels containing substantially 20 to of chromium. My treated metal may readily be spun, pressed, deep drawn, stamped, folded, bent or otherwise cold shaped without difficulty. Various articles can be fabricated from my improved steel which were not commercially practical hitherto with steels of similar composition. It will be understood that my novel high-temperature heat treatment step for stainless steel of this composition is not analogous to a normalizing treatment of plain carbon steel. In plain carbonsteel and in lower chromium stainless steels a grain refinement can be accomplished by heating above the critical temperature, where an allotropic crystal transformation occurs and the'gralns are transformed to face-centered cubic austenite. Stainless steel of this composition, although ferritic at room temperature, does not transform to austenite at these elevated temperatures, so the action must be due to some other cause. The change produced by my high-temperature treatment is not visible under the microscope, and I am not aware of its true nature, although I have presented the theory that it is due to an altered form of the carbides present.

As a specific but non-limiting illustration of the working of the invention, a 10 ton heat of Type 446 stainless steel containing:

\ W l M-l l was hot worked to a thickness of .125 in the form of cross rolled sheets. The hot rolled sheets were then heat treated at 2000 F. for two minutes and allowed to cool in air. They were then box annealed between 14:50 F. to 1650 F. for four hours and allowed to cool in the box. The temperature drop during box cooling approximated about 25 to 50 per hour. When cooled, the annealed sheets were reheated to 1450" F., air cooled singly, cleaned and pickled and were leveled.

For purposes of comparison a group of sheets from the same hot rolled stock were given a straight box anneal of 1450 F. for 4 hours and were box cooled and then reheated and cooled singly as described above. The preliminary heat treatment, however, was omitted. These sheets were also cleaned and leveled.

Mechanical property tests of the two lots of sheets were made with the following results:

- 4 .27 Yield Tensile Elon ation Hardness ig i of Sti ength, Strength, Percent Rockwell e p. s. i. p. s. i. i112 B {longitudinal. 47, 600 76, 100 24. 5 86. 0 transverse.... 50, 700 80, 000 22. 5 86.0 longitudinal... 69, 700 88, 700 21. 5 94. 5 transverse 76, 000 92, 300 22. 0 94. 0

The improvement in properties as related to workability which are imparted by the preliminary heat treatment of this invention is immediately apparent.

The sheets were then stamped into fiat discs measuring .125" thick by 18" in diameter.

The blanks were then placed in a conventional spinning lathe and were spun to a cone intended for television tubes measuring 10" in length by 15" at the large end. 1 flanges were turned at the large end to a angle to the axis of the cone. The small end remained closed and retained its original thickness; the opposite flanged end was reduced to approximately .050 at the completion of the spinning operation. 'During the spinning of these blanks no defects of significance were encountered and they were formed without difliculty.

The ease with which these cones were shaped contrasts markedly with the dificulties encountered during the spinning of conventionally annealed sheet stock, as exemplified by the control samples. This material broke at the point of contact with the spinning tool in many cases after being deflected as little asl from the flat condition. The lack of workability in this case was So great as to render the production of cones impractical.

It will be evident that my invention produces a metal peculiarly adapted for special purposesin this instance, the fabrication of a television tube in which sealing of glass to metal is important in combination with equal or nearly equal thermal expansion of the two substances. Without the treatment of this invention the cost of fabricating the tube would be greatly increased; or the labor of shaping and completing the article would become vastly more complicated.

In a second non-limiting example of the practice of the invention a heat containing:

G M11 P Ni sisted of a hollow cylinder of 24 gauge material measuring approximately 6" in diameter by 18" in length. The design required an internal flange on one end and an external flange on the other. The finished shape was created by first flanging the ends of the sheet blank 90 in opposite directicns and then passing the flanged blank through a three roll bender to form a cylinder.

Hot rolled and conventionally annealed Type 446 material (annealed at 1500 to 1600 F., furnace cooled to 1200 F. and air cooled) did not prove successful. While some lots shaped without defect, many failed. The flanges split, showed seams or other imperfections making the cylinders unfit for use.

In contrast, sheet material supplied from this heat which was given a preliminary heat treatment in packs of 4 sheets for minutes at 1950 F. with an air cool, followed by a 1550 F. anneal for 1 hour and. air cooled, in accordance with this invention, was fabricated as described without difficulty. The fianging and bending operations were performed with case; no seams or splits developed. The number of rejected pieces was negligible. Equal performance was demonstrated whether the sheets were sheared with or across the direction of rolling.

Mechanical tests performed before flanging and bending, on the sheets processed in accordance with this invention yielded the following results:

These two examples are typical of fabricating problems solved by my invention. In view of the new increased workability of Type 446 steel (as a result of my invention), the variety and scope Of fabricated articles has increased; more complex shapes may now be manufactured and the excellent qualities of this hitherto olifiicultly worked steel are made available, to meet the needs of the more common use of high temperature industrial equipment, chemical apparatus, intricate furnaces and hearing devices, scientific apparatus, complex electrical machinery and the like where scale resistance and/or good glass-metal adherence is of importance.

Modifications may be made in my invention without departing from the spirit of it. Having thus described my invention what I claim as new and desire to secure by Letters Patent is:

1. A process for treating a scale resistant, ferritic, non-transforming chromium steel containing substantially 20% to chromium, to render it amenable to severe cold-forming operations which comprises, after hot rolling, first giving said steel a preliminary heat treatment at 1800 to 2100 F. for two to sixty minutes and allowing the steel to cool, and then heating said steel at 1450 to 1650 F. and allowing it to cool rapidly through the range between 1100 F. and 700 F.

2. A process of treating ferritic, non-trans forming chromium stainless steel so as to make it amenable to severe cold-forming operations which comprises the steps of giving the steel a preliminary heat treatment for from two to sixty minutes at 1850 to 2050 F. after hot rolling and following this by a second heat treatment nese, substantially 04% at 1450 to 1650 R, followed by a rapid cool through the range between 1100 F. and 700 F.

3. The process of claim 2 in which the steel is air cooled after the preliminary heat treatment.

4. The process of claim 3, in which the steel to be processed contains 20 to 35% chromium and not over substantially 50% nickel.

5. A process of treating hot rolled, scale resistant, ferritic, non-transforming, steel having a chromium content of substantially 23 to 27% which comprises the successive steps of heating the steel to between substantially 1B50 and 2050" F. for from two to sixty minutes, cooling the steel, heating the steel to between substantially 14:50 and 1650 F., and again cooling the steel.

6. The process of claim 5, in which the preliminary heat treatment is conducted at about 1950' F. for about fifteen minutes.

7. The process of claim 5 in which the preliminary heat treatment is conducted :at about 1950 F. for about fifteen minutes, and in which the final heat treatment is conducted at about 1550 F. for about one hour.

8. The process of claim 7 in which the steel is air cooled after the preliminary heat treatment, in which the second heat treatment is a box anneal, and in which the steel is cooled in the box, and then reheated and air cooled in single pieces to relieve brittleness.

9. The process of claim '7 in which the steel to be processed contains not more than substantially .35% carbon, substantially 1.0% mangaphosphorus, substantially .03% sulphur, substantially 1.5% silicon, and substantially .5% nickel, and in which the chromium content is substantially 20% to 35%, the balance being substantially all iron.

10. A process of treating a scale resistant, ferritic, non-transforming chromium steel so as to make it amenable to severe cold-forming operations, which comprises the steps of giving said steel an open anneal at 1800 to 2100 F. for two to sixty minutes, followed by at least one box anneal between 1450 and 1650 F., and in which the final cooling of the steel from 1100 to 700 F. is a rapid cool of the character of an air cool or quench.

11. The process of claim 10 in which the steel is cooled from box anneal temperature to about 1200 F., and in which the steel is then removed from the box and permitted to cool in single thickness condition through the range from 1100 to 700 F.

12. The process of claim 10 in which the steel is cooled slowly in the box, and in which the steel is then given a brief open reheating to about 1450 F. and is then permitted to cool in single thickness condition through the range from 1100 to 700 F.

MAURICE E. CARRUTHERS.

References Cited in the file of this patent Stainless Iron and Steel, Monypenny, Chapman & Hall, London, 1931, page 76.

Transaction of American Society for Metals, published by the Institute, Cleveland, Ohio, vol. '23, 1935, pages 968-987. 1

The Journal of the Iron and Steel Institute, published by the Institute, London, 1939, volume CXXXIX, No. 1, 1939, pages 99-135.

The Alloys of Iron and Chromium, Kinzel 8: Franks, McGraW-Hill Book 00., New York, vol. 2, 1940, page 182.

Claims (1)

1. A PROCESS FOR TREATING A SCALE RESISTANT, FERRITIC, NON-TRANSFORMING CHROMIUM STEEL CONTAINING SUBSTANTIALLY 20% TO 35% CHROMIUM, TO RENDER IT AMENABLE TO SEVERE COLD-FORMING OPERATIONS WHICH COMPRISES, AFTER HOT ROLLING, FIRST GIVING SAID STEEL A PRELIMINARY HEAT TREATMENT AT 1800 TO 2100* F. FOR TWO TO SIXTY MINUTES AND ALLOWING THE STEEL TO COOL, AND THEN HEATING SAID STEEL AT 1450 TO 1650* F. AND ALLOWING IT TO COOL RAPIDLY THROUGH THE RANGE BETWEEN 1100* F. AND 700* F.