Free-machining austenitic stainless steel

Details Free-machining austenitic stainless steel Free-machining austenitic stainless steel

1996-12-17

1998-11-17

Yee, Deborah

Alloys or metallic compositions

Ferrous

Nine percent or more chromium containing

420 49, C22C 3842

Patent

active

058371907

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

In general, stainless steels are more difficult to machine than carbon and low-alloy steels because stainless steels have high strength and work-hardening rates compared to the carbon and low alloy steels. Consequently, it is necessary to use higher powered machines and lower machining speeds for machining the known stainless steels than for machining carbon and low-alloy steels. In addition, the useful life of a machining tool is often shortened when working with the known stainless steels.
In order to overcome the difficulties in machining the known stainless steels, some grades of stainless steels have been modified by the addition of elements such as sulfur, selenium, phosphorus, or lead. For example, AISI Type 303 stainless steel is a resulfurized, austenitic stainless steel having the following composition in weight percent:


______________________________________ wt. % ______________________________________ C 0.15 max Mn 2.00 max Si 1.00 max P 0.20 max S 0.15 min Cr 17.0-19.0 Ni 8.0-10.0 Fe Balance ______________________________________ require good machinability and non-magnetic behavior, in combination with good corrosion resistance. However, a need has arisen for an austenitic stainless steel having significantly better machinability than Type 303 stainless steel, particularly under production-type machining operations such as on an automatic screw machine.
U.S. Pat. No. 4,784,828 (Eckenrod et al.) relates to a resulfurized Cr--Ni austenitic stainless steel in which the total amount of carbon plus nitrogen is restricted to 0.065 w/o max. The data presented in the patent appears to show that the alloy provides improved machinability in short term laboratory tests because of the restricted amount of carbon and nitrogen. However, it has been discovered that the alloy disclosed in the '828 patent has less than desirable machinability under production-type machining conditions such as are encountered on an automatic screw machine. Furthermore, an austenitic stainless steel in which the carbon and nitrogen are reduced as taught in the '828 patent, provides an undesirably high magnetic permeability, in the cold drawn condition.
Given the foregoing, it would be highly desirable to have an austenitic stainless steel that provides a better combination of magnetic permeability and machinability than is provided by the known austenitic stainless steels.


SUMMARY OF THE INVENTION

The problems associated with the known austenitic stainless steel alloys are solved to a large degree by an alloy in accordance with the present invention. The alloy according to the present invention is an austenitic stainless steel alloy that provides improved machinability compared to AISI Type 303 alloy while maintaining a low magnetic permeability, especially in the cold worked condition.
The broad, intermediate, and preferred compositional ranges of the austenitic stainless steel of the present invention are as follows, in weight percent:


______________________________________ Broad Intermediate Preferred A Preferred B ______________________________________ C 0.035 max 0.030 max 0.025 max 0.01 max Mn 1.0-2.0 1.0-2.0 1.0-2.0 1.0-2.0 Si 1.0 max 0.5 max 0.5 max 0.5 max P 0.2 max 0.1 max 0.1 max 0.1 max S 0.15-0.45 0.15-0.45 0.25-0.45 0.25-0.45 Cr 16.0-20.0 17.0-19.0 17.0-19.0 17.0-19.0 Ni 9.2-12.0 9.2-11.0 9.2-10.0 9.5-12.0 Mo 1.5 max 0.75 max 0.75 max 0.75 max Cu 0.8-2.0 0.8-2.0 0.8-1.0 0.5-2.0 N 0.035 max 0.030 max 0.025 max 0.035 max Se 0.1 max 0.05 max 0.05 max 0.05 max ______________________________________
The balance of the alloy is essentially iron except for the usual impurities found in commercial grades of such steels and minor amounts of additional elements which may vary from a few thousandths of a percent up to larger amounts that do not objectionably detract from the desired combination of properties provided by this alloy.
The foregoing tabulation is provided as a convenient summary and is not intended thereby

REFERENCES:
patent: 3902898 (1975-09-01), Denhard, Jr. et al.
patent: 4613367 (1986-09-01), Eckenrod et al.
patent: 4933142 (1990-06-01), Haswell, Jr. et al.
patent: 4994122 (1991-02-01), DeBold et al.
patent: 5482674 (1996-01-01), Kosa et al.
"Residual and Minor Elements in Stainless Steels", Handbook of Stainless Steels, (1977), pp. 14-2, 14-3, 14-6, 14-7.
"Material Specifications: Type 303 Hot Rolled Annealed Rod", Illini Wire Mill, Inc., Rev, #1 (Oct. 10, 1991).
G.O. Rhodes, J.J. Eckenrod and K.E. Pinnow, "A New High Manganese Free-Machining Austenitic Stainless Steel", Proceedings of a Conference on Manganese Containing Stainless Steels held in conjunction with ASM's Materials Week '87, 10-15 Oct. 1987, pp. 53-59.

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