US3811875A

US3811875A - Free machining austenitic stainless steel alloy

Info

Publication number: US3811875A
Application number: US00220108A
Authority: US
Inventors: K Goda; G Aulenbach
Original assignee: Carpenter Technology Corp
Current assignee: Carpenter Technology Corp
Priority date: 1969-09-04
Filing date: 1972-01-24
Publication date: 1974-05-21
Anticipated expiration: 1991-05-21

Abstract

Austenitic stainless steel alloy containing about 10 percent to 26 percent chromium, 4 to 46 percent nickel, 0.4 to 15 percent manganese, 0.015 to 0.75 percent sulfur and/or selenium, 0.5 to 10 percent copper, 0.25 to 2 percent aluminum, 0.001 to 0.1 percent tellurium, and the balance being iron, optional elements and incidental impurities. Austenitic compositions are provided having improved free-machining properties or an improved combination of free-machining and corrosion-resistance properties in selected media.

Description

United States Patent [191 Goda, Jr. et a1. 7 i

1 a 1451 May21, 1974 1 FREE .MACHINING AUSTENITIC STAINLESS STEEL ALLOY [75] Inventors: Kermit J. Goda, Jr., Leesport;

. Grant M. Aulenbach, Mohnton,

both of Pa. [73] Assignee: Carpenter Technology Corporation,

. Reading, Pa.

[22] Filed: Jan. 24, 1972 21 Appl. N0.: 220,108

Related US. Application Data [63] Continuation-impart of Ser. No. 855,416, Sept. 4,

1969. Pat. NO. 3,645,722.

UNITEDQSTATES PATENTS .R23,685 7/1953 Clarke ..75/1 2s1 3,152,934, 10/1964 Lula ..75/124 1127.226 1/1970 MOSkOWitZ 75/128 P 2,661,279 12/1953 Wilcox 75/128 P 3,362,813 l/l968 ZiOlkOWSki 75/124 I 1,961,777 6/1934 Palmer 75/128 P 2,697,035 12/1954 Clarke 75/128 P 3,151,978 7 10/1964 .Perry 75/12'4 3,340,046 9/1967 Dulis 75/128 P 10/1968 Myers 75/124 Primary 'Examirier-Hyland Bizot Attorney, Agent, or Firm-Edgar N. Jay

{57 ABSTRACT Austenitic stainless steel alloy containing about 10 percent to 26 percent chromium, 4 to 46 percent nickel, 0.4 to 15 percent manganese, 0.015 to 0.75

. percent sulfur and/or selenium, 0.5 to 10 percent cop- 12 Claims, No Drawings FREE MACHINING AUSTENITIC STAINLESS STEEL ALLOY This application is a continuation-in-part of our copending application filed Sept. 4, 1969, Ser. No.

r 855,416 and now US. Pat. No. 3,645,722, granted to modify standard grades by means of free-machining additives for use where the accompanying reduction in corrosion resistance couldbe tolerated. For example, A.l.S.l. type 303 and type 303(Se) are illustrative of grades which contain respectively substantial amounts of sulfur andselenium, that is more than about 0.15 percent and usually 0.3 percent or more, for improved machinability. r

' .A number of other elements have also been used in combination with or in place of the elements-sulfur and selenium, but the results left much to be desired, usually because the increase in the cost of the alloy and/or the detrimental effect on other properties such as corrosion resistance were not sufficiently offset by the gain in free-machining properties. Our present invention stems from our discovery that when a relatively small amount of tellurium is present together with the elements manganese, copper and aluminum, they work together with sulfur or. selenium or both of them to impart a unique degree of free machinability to austenitic chromium-nickel stainless steel alloys without reducing the corrosion resistance to the extent expected from previous experience with the sulfur or selenium additions required to attain a like degree of machinability.

lt is therefore a principal object of the present inven. tion'to provide an austenitic stainless steel characterized by an enhanced degree of free machinability compared to currently available corresponding grades of austenitic free-machining stainless steel and having at least about as good corrosion resistance.

By providing an austenitic chromium-nickel stainless steel-alloycontaining the elements indicated in the fol-' lowing broad. range, much-of the foregoing objects are achieved while the prefered range ensures attainment of the foregoing as well as additional objects, both ranges being given in approximate weight percent:

Broad Preferred Range Range Carbon up to 0.25 up to 0.2 Chromium 10-26 lO-ZO Nickel 4-46 I 4-35 Manganese 0.4-J5 0.4-2 (Sulfur and/or 0.0l5-0.75 0.02-0.35 Selenium) Copper 0.5-l0 0.75-4 Aluminum 0.25-2' 0.5-l .25 Tellurium 0.00l-0.l 0.0 l-0.06

In addition, there can be included varying amounts of other elements, as for example: up to about 3 percent silicon but preferably no more than about 1 percent, up to about 0.5 percent preferably no more than 0.035 percent phosphorus, up to about 3.5 percent molybde-v num, or tungsten can replace all or part of the molybdenum in the ratio of about 2 m 1, up to about 0.01 percent boron, columbium up to about 10 times the percent'carbon plus nitrogen but no more than about 2 percent, titanium in an amount up to about equal to six times the percent carbon plus nitrogen but no more than about 1.2 percent, and up to about 0.35 percent nitrogen. Except for incidental impurities, the balance of the alloy is iron.

The elements sulfur and/or selenium together with controlled amounts of manganese, copper, aluminum and tellurium work together in our composition to provide an unexpected degree of free machinability and corrosion resistance. While sulfur in amounts ranging I from about 0.015 to 0.75 percent can be present, above about 0.5 percent increasing difficulty ,may be encountered in both hot and cold working the composition. Therefore, when necessary to minimize working difficulties, we limit sulfur to no more'than about 0.4 percent and preferably to no more than about 0.35 percent. Selenium on a one-for-one basis can be substituted for all or part of the sulfur in our composition as is indicated in the foregoing tabulation where the ranges'stated are to be read as the broad and preferred amounts of the combined content of both sulfur and selenium. The elements sulfur and selenium, individually or together, are not equivalent to'and cannot be substituted for the element tellurium in'our composition.

For its effect in contributing to free machinability, manganese preferably in an amount from about 0.4 to 2 percent is included in our composition, while best results can be obtained with about 1.4 to 1.6 percent. As is well known, manganese can be used in addition to or in place of part of the nickel content in establishing the austenitic balance in austenitic stainless steel alloys, and up to about 15 percent manganese can be used for that. purpose inour alloy. Having in mind that freemachining stainless steel alloys are not intended for use where they would be exposed to very corrosive media, the .best combination of corrosion resistance and free machinability is obtained with about 0.4 percent to the latter are present). The passivation treatment serves to remove the manganese sulfides (or selenides) from the surface of the treated part. Of course, if the passivated surface from which the manganese sulfides have been removed wears away in use, then the under lying material becomes exposed and is subject to attack.

Copper is present in solid solution in our composition and primarily appears to enhance the corrosion resistance of our alloy. The effect of copper on free machinability seems to result from lowering the work hardening rate. For this purpose we preferablyuse about 0.75 to 4 percent copper. Below about 0.5 percent, there is presence of an effective amount of tellurium. Because .aluminum is a powerful ferrite former, its use must be carefully controlled in compositions like the present which are to be entirely or primarily austenitic. Broadly, from about 0.25 to 2 percent aluminum can be present in our composition. Larger amounts of aluminum make it increasingly difficult to preserve the austenitic balance of our composition. We prefer to use aluminum in an amount ranging from about 0.50 to l.25 percent. v

In the absence of at least a small but effective amount of tellurium, the outstanding free machinability of our composition cannot be attained. We preferably use about 0.01 to 0.06 percent tellurium although as little as 0.005 percent or even 0.001 percent can have an effect and as much as 0.1 percent tellurium can be used to some advantage. In spite of the fact that tellurium in the presence of sulfur but without the necessary amounts of manganese, aluminum and copper does not improve the free machinability of austenitic stainless steel and appears to detract from it, we have found that with the aluminum, manganese and copper present in our composition together with tellurium, there is an unexpected synergistic effect upon the free machinability of our composition as measured by the drill test.

We do not have a complete explanation for or completely understand the phenomenon, but based on our instances with some improvement in machinability. On the other hand, when the previously acceptable sulfur level is maintained, the addition of the elements manganese, aluminum, copper and tellurium, all within the 5 stated ranges, makes possible significantly improved free machinability.

The examples having the analyses in approximate weight percent as shown in Table I serve to illustrate our invention.

Ingots were cast of each of the foregoing examples,

hot worked and shaped to form test pieces which were annealed before testing. Our composition is wholly or predominantly austenitic both at room temperature and at the usual hot working temperature range for 5 such alloys, that is from a furnace temperature of about 2,100 F. Preferably, ingots are first subjected to some light hot working to break up the as-cast structure. Then after surface preparation, e.g., grinding to re-' move surface checks, hot working is completed in the usual way from a furnace temperature of about 2,100

F. In the case of more difficult to work ingots, somewhat better results can be attained by saddening at a higher temperature, up to about 2,300 F.

Examples l-6 were annealed at about 1,950 F for one hour followed by cooling in air. The hardness of the examples in their annealed'condition was measured on the Rockwell B Scale, and the results are recorded at the bottom of Table l.

The machinability of the specimens of each of the examples was determined as the average depth of penetration in thousandths of an inch into the specimens under carefully controlled conditions. While there is no 4 generally accepted standard for measuring machinability, the free machining values were obtained by meaobservations, it appears that the tellurium forms telluring the epth f Penetration into the specimens by um-rich compounds which we thus far have identified as tellurides that are attached to the sulfides. The more tellurium present, the more the sulfides are surrounded by the tellurium-rich compounds.

By the addition of the elements manganese, aluminum, copper and tellurium in accordance with our invention, the sulfur content of a given alloy ,can be reduced so as to obtain better corrosion resistance with little or no loss in free machinability or even in some a quarter-inch drill in a time interval of 15 seconds with the drill rotatingat or very close to 670 rpm. under constant torque. Before the start of each drilling operation, the drill mounted in a conventional drill press was 40 brought against the surface of the specimen where it 7, TABLE 1 EXAMPLE NO. 1 3 4 5 6 c .120 091* 120* .131 .095 .095 Mn 1.72 1.63 1.68 1.68 1.67 1.68 Si .45 .51* .39 .44 44* P .005 .012 005* .012 .012 012* s .31 .27 .31 .27 .27 .24 c: 17.26 16.63 I716 16.82 17.14 17.14 Ni 10.00 10.06 1000* 10.08 9.81 9.81 Mo .01 .01 .01* .01 01* .01- Al .84 .50 .67 .41 .92 .92 Cu .85 1.47 1.68 .79 .74 1.49 Te .001 .004 .016 .04 .04 .04 N .037 .046 .037 .032 .042 .042 Rockwell B 79.5 78 I 81 86 84.5

Drill Test" 547 For economy. the heats were split to permit variation in the elements Mn. S. Al. Cu and Te. The asterisk indicates analyses of elements obtained by measurement from other portions of the same heat to avoid unnecessary analysis.

"In thousandths of an inch.

. rium-is not added in accordance with our invention. On

For comparison, the following alloys were prepared, (H)! formed into specimens and tested as was described in connection with Examples l-6. Nitrogen do. 0.35

TABLE 11 T A B c .D

c .101 .107 .108" .115- Mn .07 1.47 1.76 1.69 st .24 .31 43* .38

P .007 .007 006* .006 s .37 .42 .32 .31 c: 16.95 17.02 17.19* 17.09 Ni 9.76 9.79 979* 9.79 MO .02 .02 02* .01 Al .0l .0l .36 .92 c .05 .06 .76 1.54 Te .0002 .0002 .0002 .o01 N, .043 .045 .036 .038 Rockwell B 88.5 88 85 81.5 D1111 Test 338 429 423 443 The asterisk indicates analyses of elements obtained the same heat to avoid-unnecessary, analysis. "In thousandlhs of an inch.

Alloy A shows the effect of sulfur in such a'chromium-nickel austenitic stainless steel with low manganese and without an addition of aluminum, copper and tellurium. With a hardness of R 88.5 in the annealed condition, the average drill test penetration of Alloy A was found to be .338 inch. Alloy B'primarily demonstrates the effect of adding manganese in increasing the average drill test penetration to .429 inch, but the increase in sulfur from .37 to 42 percent also contributed its relatively small effect. Alloys C and D demonstrate that the addition of the elements aluminum and copper does not provide a synergistic effect with manganese and could result in a reduction in free machinability if telluthe other hand, Example I, which has virtually the same analysis as Alloy D except for the small addition of tellurium, gave an' average drill test penetration of .547 inch. See also Example 2 which, with only .27 percent sulfur and .004 percent tellurium, gave an average drill test penetration of .455 inch. The as-tested hardness of the specimens of Examples 1 and 2, and Alloy D ranged from R ,79.5 to R ,8l.5 and thus'were not considered to differ significantly.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such tenns and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

1. An austenitic stainless steel alloyhaving good free machinability in its annealed condition consisting essentiallyin weight percent of about:

Chromium 10-26 Nickel 4-46 Sulfur plus Selenium 0.0l50.75

Copper 0.5-l0 Aluminum 0.25-2

Tellurium 0001-0. 1

Manganese 0.4-15

Carbon 4 4 up to about 0.25

Silicon 1 do. 3

Phosphorus do. 0.5 Molybdenum do. 3.5 5

by measurement from other portions of in which an equivalent amount of up to about 7 percent tungsten in the ratio of about 2/] can replace all or part of the molybdenum, and the balance essentially ir'on.

2. The austenitic stainlesssteel alloy as set forth in claim 1 containing about 0.02-0.5 percent sulfur plus selenium, about 0.4-2 percent manganese, about 0.75-4 percent copper, about 05-125 percent aluminum, and about 0.005-006 percent tellurium.

3. The austenitic stainless steel alloy as set forth in claim 2 containing about 1.4-1.6 percent manganese.

4. The austenitic stainless steel alloy as set forth in claim 1 containing about 0.4-2 percent manganese.

5. The austenitic stainless steel alloy as set forth in claim 4 containing about 0.5-4 percent copper.

6. The austenitic stainless steel alloy as set forth in claim 5 containing about 10-20 percent chromium, and about 4-35 percent nickel.

7. The austenitic stainless steel alloy as set forth in claim 6 containing no more than about 1 percent silicon, and no more than about 0.035 percent phosphorus.

8. The austenitic stainless steel alloy as set forth in claim 2 containing about Weight Percent Chromium l0-20 Nickel 4-35 Carbon up to about 0.2 Silicon do. I Phosphorus do. 0.035 Columbium do. I l0 C+N) Titanium do. 6 C+N) 9. "11..'zntesitizstani s 844511575.4 1.51. 1;.

claim 11 containing about 0.4-0.7 percent manganese. l

Claims

2. The austenitic stainless steel alloy as set forth in claim 1 containing about 0.02-0.5 percent sulfur plus selenium, about 0.4-2 percent manganese, about 0.75-4 percent copper, about 0.5-1.25 percent aluminum, and about 0.005-0.06 percent tellurium.
3. The austenitic stainless steel alloy as set forth in claim 2 containing about 1.4-1.6 percent manganese.
4. The austenitic stainless steel alloy as set forth in claim 1 containing about 0.4-2 percent manganese.
5. The austenitic stainless steel alloy as set forth in claim 4 containing about 0.5-4 percent copper.
6. The austenitic stainless steel alloy as set forth in claim 5 containing about 10-20 percent chromium, and about 4-35 percent nickel.
7. The austenitic stainless steel alloy as set forth in claim 6 containing no more than about 1 percent silicon, and no more than about 0.035 percent phosphorus.
8. The austenitic stainless steel alloy as set forth in claim 2 containing about
9. The austenitic stainless steel alloy as set forth in claim 8 containing about 1.4-1.6 percent manganese.
10. The austenitic stainless steel alloy as set forth in claim 8 containing about 0.02-0.35 percent sulfur plus selenium.
11. The austenitic stainless steel alloy as set forth in claim 10 containing about 0.01-0.06 percent tellurium.
12. The austenitic stainless steel alloy as set forth in claim 11 containing about 0.4-0.7 percent manganese.