US2273731A - Stainless steel containing bismuth - Google Patents

Description

Patented Feb", 1942 Columbus, Ohio, assignors to Alloy Castin Institute, of New York, an unincorporated association of New York no Drawing. Application .Ianuary 17, 1940, Serial No. 314.244

6 Claims. (Cl. 75-126) This invention relates to stainless irons and steels containing bismuth and more particularly to austenitic stainless steels containing a small but eflective quantity of bismuth for the purpose of improving the machinability oi the stainless steels and their resistance to corrosion.

We have found that the machiuability of stain- I less steels can be materially increased by the inrelativelysmall but effective proportion of bismuth. I

It is a further important object of this invention to provide a stainless steel of improved resistance to corrosion.

. Other important objects of this invention will bccomeapparent from the following description and appended claims.

Our invention is applicable senerally'to any of the so-called "stainless steels," either the straight chromium or the chromium-nickel steels. We prefer, however, to employ the austenitic stainless steels, containing both chromium and nickel. Chromium may be as high as 30% and is usually present in amounts of 12% or more. The stainless steels may be either in the cast or wrought condition;

The amount of bismuth that may be added to a stainless steel to improve its macl'iinability lies,

in general, between the limits of 0.02 and 1.0%, preferably within the narrower limits of from 0.1 to 0.6%. The bismuth can be added in any suitable manner, but most advantageously may be incorporated into the stainless steel just prior to pouring, or while in the ladle, since bismuth has a lower boiling point than the melting point of stainless steels and therefore tends to volatilize out of the molten metal, probably in the form of bismuth trioxide.

In order to incorporate as much as 0.6% of bismuth in the stainless steel, it may be necessary to add a much larger proportionate amount of bismuth in the first instance, owing to loss by volatilization. The percentages of bismuth in the stainless steel, as given herein, are those that maybe found by analysis of the stainless steel. Also. while it is believed that bismuth is retained in the stainless steel largely in its elemental form, the term bismuth" refers to the element itself, regardless of the form in which it may be present in the stainless steel.

. Heats of stainless steels with and without bismuth additions have been melted and subjected to machlnability tests. The tests were made by sawing and drilling the steels under rigorously controlled conditions in automatic machines. The results are expressed as a "saw index" and a drill index." An index of 200, for example, indicates that the material machines twice as easily in that particular type of machining operation, as one whose index is 100. The remarkable and-heretofore unrecognized effect of small amounts of bismuth canbe seen' in the data for 5 six series of stainless and manganese steels of I further been substantiated by machine shop difierent types shown in Table I. The efiect has lathe observations.

ticeable under corrosive conditions that gives rise to a pitting type of attack. This has been demonstrated byqualitative tests of a large number of these steels in an aqueous solution containing ferric chloride and sodium chlo- A ride. A 19% chromium, 9% nickel alloy is severely pitted in this solution in 2 hours.

TABLE I Composition I Saw Drill C M11 S1 S P Cr N1 B1 index index SERIES 1 SERIES 11 0 11 0. 82 0. 0. 02 0. 01 25. 0 12. 0 0. 00 100 0.11 0.02 0.95 0.02 0.01 25.0 12.0 0.11 112 132 0. 11 0. s2 0. 95 0. 02 0. 01 25. 0 12. 0 0. 13 133 147 0. l3 0. 82 1. 15 0. 02 0. 01 25. 8. 12. 2 0. 25 142 0. 12 0. 88 1. 10 0. 02 o. 01 20. 0 12. 0 0. 37 205 SERIES III SERIES IV SERIES v 0. 00 0. 50 0. 30 12. 0 0. 0 0. 00 100 0. 00 0.50 0. so 12. 0 0.0 0.10 SERIES v1 1.10 14.0 0.40 0.02 0.02 0.0 100 1.10 14. 0 0. 40 0. 02 0. 02 o. 20 1, 100 I 1 Maximum.

It will be noted that the tests of Series VI re- 50 TABLE 11 late to a manganese steel. Any manganese steel, that is, a steel containing between-about 11 and c Weight loss in 15% of manganese, exhibits a markedly improved ompw g 9 i /gig machinability upon the addition thereto of a 501011011, 00:11:00- small percentage of bismuth. 55 and agitated.

c M 1 P 1: Ni Bi 7211 Tests of samples of snmlar steels with and n s r 1 02 without bismuth additions indicate increased corrosion resistance when small amounts of bismuth 0.11 0.82 0.95 0.02 0.01 25.0 12.0 0.00 372.0 are present. The improvement is particularly no- The benefits resulting from the addition of bismuth to stainless steels are accompanied by a negligible change in the tensile properties of the steel. This is exemplified in the tensile data of Table 111.

TABLE III Composition Alloy N0.

CMnSi s P 01 N131 Tensile prop rties (average) rm Yield m Reduc- Alloy No. tio f $0.29}, Tensile tioni: -tionin limit d orma.) 21h. area Rm. p.11. P. .0. Percent Per cent 1 23,000 30,000 10,200 0202. 00.0 2 22,000 Y 30,200 10,000 00.0 12.3 a 10,500 00,400 09,000 01.0 14.0

It will, of course, be understood that various details of composition may be varied through a wide range without departing from the principles-of this invention and it is, therefore, not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claims.

1 We claim as our invention:

1. A ferrous ,alloy containing from 0.02 to 1.0-

per cent bismuth, an amount of alloying material suflicient-to render the structure of the alloy austenitic and selected from the group consisting of nickel, nickel and chromium, and manganese, and the remainder substantially all iron,

said bismuth content effectively improving machinability oi the alloy. 2. An alloy having improved maehinabllity con taining 12 to 30 per cent chromium. 0.02 to 1.0 per cent bismuth and the remainder substantially all iron.

3; An alloy having improved machinability containing 12 to 30 per cent chromium, 8 to 12.6 per cent nickel, 0.02 to 1.0 per cent bismuth, and the remainder substantially all iron.

4. An austenitic steel alloy havingimproved um, and manganese, and containing from 0.02

to. 1.0fper cent bismuth, said alloy having improved machinability due to the said presence of bismuth therein. I I

ROBERT S. PEOPLES.

HENRY A. HOLDEN PRAY.