US1918731A - Working austrnitic steels - Google Patents

Description

July 18, 1933. 'RHABORN 1,918,731

WORKING AUSTENITIC STEELS Fi'led Feb. 17, 1931 Patented July 18, 1933 UNITED STATES PATENT? OFFICE ROBERT E. ABORN, OF EAST ORANGE, NEW JERSEY, ASSII IGNOR TU UNITEO STATES STEEL CORPORATION, OF NEW YORK, N. Y., A CORPORATION 01' NEW JERSEY woaxme aus'rnm'rrc s'rnrms Application filed February 17, 1931. Serial No. 516,473.

This invention relates to an improved method of treating austenitic metals and is particularly applicable to the working or fabricating of austenitic alloys having corrosion resisting properties.

It has been found that a'ustenitic alloys harden during cold deformation at an unusually rapid rate when the cold work or deformation is applied at, below, and even slightly above, room temperature. The hardening is entirely inordinate and is the cause of the unique properties of the so-called Hadfield manganese austenitic steel. In the instance of the austenitic nickel rustless irons and stainless steels of approximately 18 per cent chromium, 8 per cent nickel and 0.10 per cent carbon the soft recrystallized metal may show a Brinell hardness of 86 and then after working to reduce its thickness by per cent the hardness may reach 400 Brinell, the deformation havingbeen applied at room temperature. On the other hand, nonaustenitic metals harden. much less, as for example, a very low carbon annealed steel would before'work show a Brinell hardness of about 103 and after similar cold work at room temperature develop about 160 Brinell hardness.

The unique property of rapid cold work hardening in the case of, austenitic steel is commonly explained by the factthat austenite is not only deformed by the cold work but is also transformed to a different crystalline form, the so-called ferrite or martensitic condition wherein the alpha form of iron is the basis of the crystal, instead ofthe gamma form, which is the basis in the austenitic metal. At any rate, regardless of the explanation, the transformed metal of austenitic alloys resulting from cold work shows an undesirable brittleness and severe loss of duetility and twist which normal metals do not exhibit. Furthermore, due to the transformation only moderate work is possible before cracks appear unless frequent annealings are employed. The present invention has for a feature thereof an improved process of cold working austenitic alloys'in such a manner that the transformation of austenite t0 the aforementioned undesirable modification is very greatly reduced, and in some instances is substantially eliminated. It has been found that thisfeature can be carried out by cold working (working below the recrystallizing temperature) the austenitic alloys at, or above, about 200 degrees centigrade.

Evidence of the advantages and desirabilit of the improved process as contrasted with the present day process of cold working the material at, below, or even slightly above room temperature, is afforded by testing the two materials for Brinell hardness and also submitting them to a bend test. Instead of developing about 400 Brinell hardness the alloy treated at or above 250 degrees centigrade shows only about 260 and is vastly more ductile.

In the drawing, Figures 1 and 2 indicate bend tests of material treated first in accordance with the present invention, and second, in accordance with methods now in practice.

As disclosed in Figure 1 of the drawing, a bend test of a small strip showed that the metal worked in accordance with the improved method developed no cracks when ent about a radius less than its own thickness, whereas, the ordinarily worked material broke off before bending about a larger radius.

A quantitative measure of the efliciency of the improved method resides in magnetic measurements which show an entirely different process occurring in the crystalline nature of the metals when worked by the contrasting methods. The same sample of metal worked by the ordinary cold method develops sufiicient transformed metal (the non-magnetlc form changes to the magnetic form) to produce, for example, 930 magnetic units, wh1le the material worked at 300 degrees centlgrade showed only about 10 units. Completely decisive X-ray tests showed correspondingly great differences as to transformatmn.

It may be remarked incidentally that the energy involved in applying equal deformation to the metal is several times greater at room temperature thanat, for example, 300 degrees centigrade.

It is to be appreciated that the term work- 100 ing is used herein in its general sense to ture range being in excess of about 100 C. include working, fabricating and generally but below the temperature of recrystallizadeforming metals. Likewise, it is to be untion. derstood that the applicant is to be permitted 2. The method of working austenitic chroa certain latitude in respect to the range mium-nickel steels which comprises cold through which he carries out the improved workin the same within atemperature range process. Although for purposes of descripadapted to substantially inhibit allotroplc tion the range can perhaps best be defined as crystal transformation therein, said temperabelow the recrystallizing temperature and ture range ap roximating 200 to 400 C. above about 200 degrees centigrade, it is 3. The method of working austenitic low pointed out that the treatment contemplated carbon chromium, nickel iron alloys which by the invention consists in deliberately or incomprises cold working the same within a tentionally heating the metal before or during temperature range adapted to inhibit allo the cold working of it. If the metal is heated tropic crystal transformation, said temperaduring working then to come within the scope ture being in excess of about 100 C. but beof the invention such heating must be deliblow the temperature of recrystallization. erate and not as a result of the frictional en- 4. The method of working austenitic steel gagement of the metal and tools. Although alloys comprised of about 18% chromium, it is stated above that for purposes of definiabout 8% nickel, about .10% carbon balance ,tion the range can be defined as below the mainly iron, which comprises cold working recrystallizing temperature and above about the same within a temperature range adapted 200 degrees centigrade, it should be explained to inhibit allotropic crystal transformation, that working the metal at temperatures as said temperature range approximating 200 low as 100 degrees centigrade increases subto 400 C. stantially the desirable qualities contemplat- 5. As an article of manufacture, a substaned by the invention. tially non-magnetic cold worked austenitic Having thus described my invention, what chromium-nickel steel alloy.

claim as new and desire to secure by Letters 6. As an article of manufacture, a substan- Patent of the United States is: tially non-magnetic cold worked austenitic 1. The method of working austenitic chrosteel alloy, said alloy being comprised of mium-nickel steels which comprises cold about 18% chromium, about 8% nickel, about workin the same within a temperature ran e .10% carbon and balance mainly iron. adapte to substantially inhibit allotropic crystal transformation therein, said tempera- ROBERT H. ABORN.

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