US3010823A - Easily machinable, non-magnetic, manganese steel - Google Patents

Description

United States Patent 3,010,823 EASILY MACHINABLE, NON-MAGNETIC, MANGANESE STEEL Howard S. Avery and Henry J. Chapin, Mahwah, N.J., assignors to American Brake Shoe Company, New York, N.Y., a corporation of Delaware No Drawing. Filed Aug. 7, 1959, Ser. No. 832,163

4 Claims. (Cl. 75123) The properties of standard non-magnetic austenitic manganese steel with manganese of 11 to 14% and containing 1.0 to 1.4% carbon, are well known. Manganese at these levels contributes an essential austenitic stabilizing effect, and virtually unequalled toughness and work-hardening characteristics can be achieved by an austenitizing heat treatment at 1800-1900 F. followed by a Water quench. In industrial applications where impact resistance and resistance to wear are required manganese steelis virtually unequalled.

However, manganese steel is virtually unmachinable, and surface finishing to a limited extent is restricted to grinding and slow cutting speeds with expensive tools and heavy equipment where possible at all.

Austenitic stainless steels are also strong and tough and can be made non-magnetic with suitable composition control. The so-called 19Cr-9Ni (wrought 304 and cast OFS stainless grades) is usually the most economical for non-magnetic conditions, but the cost of the required alloying material is considerably greater than for austenitic manganese steel, and additionally the alloying materials are considered strategic. While the 19Cr-9Ni stainless steel grades are not particularly easy to machine, they can he lathe turned, drilled, tapped and threaded, much more easily than the standard grade of manganese steel, and for this reason, in spite of cost and strategic-alloying conditions, have usually been selected in preference to manganese steel for ship-board installations where nonmagnetic properties are essential in castings that need to have at least elementary and feasible machining operations performed thereon.

There is, therefore, an unfulfilled need for an easily machinable non-magnetic alloy of low cost and nonstrategic alloy content, and which can be strong and tough, and the object of the present invention is to enable this to be achieved in an austenitic manganese steel of novel composition.

A fully austenitic 13% manganese steel is virtually nonmagnetic with a permeability at H=24 of about 1.03 or less, but on the other hand manganese steel work hardens so easily that even the best cutting tools with ordinary equipment ordinarily dulls very rapidly when machining its attempted. Manganese steel is, therefore, generally typed as nnmachinable; however, it can be cut to a limited extent with continually resharpened cemented carbide and so-called cobalt high-speed tools, but even in this instance heavy, rigid equipment continually in good condition is required coupled with a surface speed not exceeding 30-40 surface feet per minute (s.f.m.). It can be ground, and this is the standard surface finish treatment used as a substitute for what ma-' chining operations are equivalent in result thereto. Drilling and tapping large diameter holes is possible with special equipment, but such machining operations are practically impossible for small holes such as one-quarter inch in standard manganese steel; and While small diameter holes are sometimes diflicult to drill with a Wrought 304 grade stainless this can be done as a practical matter.

It has been found in accordance with the present invention that manganese steel can be made commercially machinable by conjointly using exceptionally high manganese contents of the order of 20%, lowering carbon appreciably below the standard minimum of 1.0%, and adding bismuth in small amounts. Even more superior results are obtained by the further addition of sulfur. Advantageously, the alloy is heat treated and quenched in the usual fashion to assure an austenitic structure of non-magnetic character where this is important as in shipboard applications, and this will of course provide the usual exceptional levels of toughness and work hardening. Furnace gases, however, may induce a thin magnetic skin. In situations requiring a high degree of nonmagnetic character, this skin should and can be easily ground ofi or otherwise removed. However, a further unexpected result under the present invention is that non-magnetic character can be achieved in the as-cast alloy as will be shown.

In applications where exceptional levels of toughness are not essential, the usual heat treatment need not be resorted to since machinability and non-magnetic character are attained with as-cast alloys under the present invention. Moreover, it has been found that alloys of the present invention containing bismuthare also nonmagnetic in the as-cast condition after cold working.

Preferably, the manganese content of the present alloy is held between about 19% and 20%, carbon between about 0.4% to 0.6% and recoverable bismuth in the alloy about 0.2% to 0.4%. Sulfur, and nickel plus copper or either one alone, maybe used in small amounts for reasons to be explained, but these are not essential for the ultimate desired machinability. Based on extrapolations of tests and properties, and experiences, the alloy of the present invention falls within the following scope at least:

EXAMPLE 1 Ingredient: Percent by weight Carbon 0.3-0.8

Manganese 17-22 Bismuth 0.1-1.0 Nickel 0-3 Copper 0-3 Sulfur 0-0.15 Iron Balance 7 1 Except for contaminants and impurities and the usual refining additives such as silicon and deoxidation agents.

are classified as strategic. This type of stainless steel was used as the basis for comparative data on machinability in the present instance, and as will be set forth below the alloy of the present invention is equal to or superior thereto both in machinability and mechanical properties. Briefly, it was found that a manganese steel analyzing essentially about 20% manganese, 0.6% carbon, and 0.3% bismuth can be lathe turned at 265 surface feet per minute (s.f.m.), threaded, drilled and tapped. Force requirements and friction in machining are well below those of standard Hadfield manganese steel. After toughening by a water quench from about 1900 F., the alloy has Izod impact resistance of 55 foot pounds or above, yield strength from 35,000 to 45,000 p.s.i., tensile strength above 87,000 p.s.i. and elongation 40% or more. 7

In arriving at the ultimate discovery of machinability and adequate mechanical properties attained by raising manganese, lowering carbon and adding bismuth, numerous heats'were run for testing various addition agents suspected as possibly contributing to the achievement of a machinable manganese steel. These included lead, selenium, zirconium and silver, but each of these was found objectionable or ineffective in standard manganese steels at about 13% manganese, 1% carbon levels. A

. bismuth addition to a standard manganese steel improved machinability but only at slow speeds of the lathe, and the over-all results for such bismuth addition, particularly for fast lathe speeds, were not acceptable. By lowering carbon to sub-standard levels and raising manganese above standard levels, a bismuth addition resulted in machinability superior to the stainless grade and friction substantially lower than the stainless grades. Low friction is' desirable, of course, and in this instance is a measureof the resistance to rubbing between the chip produced from the part by machining and the face of the tool. Friction consumes energy and induces heating and therefore undue wear of the tool.

In obtaining the machinability. data, a commercial lathe was equipped with :a dynamometer to measure the cutting force components. Cuts were performed with a commercial sintered carbide cut-ting tool, and a fresh, unused cutting edge insert was used for each test run. All inserts had the same cutting edge geometry (60 triangular) and were of course supported uniformly in the lathe throughout the tests. Test bars were cast from the various heats, :and to assure that. any decarburized surface skins did not enter as an unknown variable, each test bar was finished to a one-inch diameter.

In other words, the machinability data were obtained under circumstances where the difference in results would be accounted for only by the alloy variations.

The character of the chip produced by the cut important since it may be of such nature as to interfere With the cutting area. Such undesirable chips are'represented by. continuous chips. that curve back on themselves without any definite coil pattern, or' in an open helical flat-backed coil, and: these chips were. observed with standard manganese steel (:heat 2072) and the st-ain-- less gradesstudied (heats 304 and 261). 'Machinable manganese steel alloys of the present invention, however, displayed goodchip character, varying from small discontinuo'us curls to tight short spiral cones which do not interfere with or obscure the cutting area and which are ordinarily encountered in conventional easily machinable alloys. 7

.Table I immediately following lists the various heats 4 Table I COMPARATIVE MACHINABILITY AND FRICTION RATINGS Balance 0, Mn, Si, sub- Heat perperper- Others, percent stan- No. cent cent cent 1 trally STEEL TOUGHENED BY WATER QUENCH Machining 0.1 Machining 0.1 out at 39 s.i.rn. cut at 265 s.f.tu. He at Type HF VF F0 HF ,VF F0 304 1 Stainless 2 155 260 0. 86 125 210 0. d 220 0. 67 135 280 0. 70 155 295 0. 76 155 370 0. 62 85 270 0. 49 145 370 0. 59 275 0. 60 180 395 0. 67 145 295 0. 72 125 325 0. 58 145 285 0. 74 125 330 0. 57 130 285 0. 55 150 345 0. 64 130 285 0. 67 65 290 0. 37 280 0. 61 310 0. 58 275 0. 67 190 380 0. 73 125 280 0. 66 145 350 0. 61 140 285 0. 71 125 275 O. 66 486- 20% Mn+S 95 250 0. 57 416- 20% D/IH-l-Bl 42 230 0. 33 40 215 0. 33 003..- 20% MI1+Bi 35 225 0. 29 75 225 0. 51 007-. Bi+S+Ni+Gu 30 0.37 35 175 0. 34

, 1 Mill annealed and cold finished. 2 Rough. 3 Welds to cutting tool.

STEEL AS CAST VF: Vertical Machining 0.1 Out at 265 s.f.m.

Heat

N 0. Type Hori- Vertical Friction zontal force, coeffiforce, lbs. clent lbs.

. Table '11 immediately following sets" forth mechanical 'properties of'the present alloys together with magnetic permeability data at H .24, and like properties for rat 5 standard manganese steel and the stainless grades are included for comparison yield and tensile data are in p.s.i. with yield at 0.2% ofiset.

Table II MACHINABLE MANGANESE STEELS Chemical analysis of heats Heat No 0, per- Mn, per- Si, per- Ni, per Cu, per- Bi, per- S. percent cent cent cent cent cent cent Properties of machinable manganese steel alloys Heat- Ultimate Elong. Red. Hard- Mag. Izod Heat No treat- Yield tensile in2, area, ness, perm. impact ment percent percent BHN Standard 13% manganese and stainless grades for comparison Heat- Ultimate Elong. Red. Hard- Mag. Izod Heat No. treat- Yield tensile in 2", area, ness. perm. impact ment percent percent BEN Same magnetic permeability after cold working. NoTn.-AC:As cast. WQ: Standard treatment 1900 F. for 2 hours followed by water quench. MC=M111 certified annealed and cold finished.v

Based on the foregoing, the two alloys ultimately NOTOHED BAR IMPACT RESISTANCE determined as preferred under the present invention are as follows: Y All 0y type A B Std. EXAMPLE 2 Izod impact-It. lbs 7548 -105 Type 0,7 Mn, Si, 1? 5,7 Ni 7 Cu,7 Bi,7'

" 0 MAGNETIC PERMEABILITY n=24 A 0.55 20.0 0.6 0.05 0.02 0.3 Alloy type A B Std. B 0.40 20.0 0.6 0.05 0.09 2.2 2 0 0.35 55 As-east 1. 003 1. 003 l Maidmurn. 'lt ugghfineg bydwater quene 1. 883 1. 882

or ar ene 1. '1. These upon further testing are found to have the following properties, yield being at 0.2% offset:

Table III TENSILE PROPERTIES AT ROOM TEMPERATURE Yield Ult. Elong Red. Hard- Heat treat- Alloy strength, tens. in 2, area, ness, ment, F.

type p.s.i. strength, percent percent BHN hrs. quench S1551i 13% 52, 000 120, 000 40 35 200 1,900-2-water.

40, 000 93, 000 .g E 9 DD 50, 000 67, 000 6. 0 6 187 As cast FORCE REQUIREMENTS FOR SINGLE POINT LATHE Type B alloy is slightly more expensive thantype A, but the machinability characteristics examined show 75 superiority over type A, and it appears that use of the type B alloy will result in longer tool life. Both of the above alloys have tensile properties superior to those of mild steel and are adequately strong and tough for most engineering applications, and magnetic permeability is quite low either in the toughened condition (austenitize plus water quench) or in the as-cast condition with or without Work hardening. Moreover, while the stand-.

ard manganese steel toughening treatment is recommended, strength and ductility in light sections as-cast are adequate for many useful applications not requiring superior toughness and hence the standard toughening treatment is not essential.

One-quarter inch diameter holes can be drilled and' tapped in any of the low-carbon, high-manganese, bismuth-containing alloys of the present invention using ordinary high speed steel tools and standard machine shop equipment.

It appears that at least 1.0% bismuth should be added to the melt to achieve the desired recoverable amount of about 0.3% bismuth in the ultimate alloyanalysis However, in our alloys the bismuth analyzed in the effective alloy can be as low as 0.1%, the carbon can be as low as 0.3% or as high as 0.8% (without nickel or copper) and the manganese should not be less than about 17% in achieving an austenitic manganese steel which is The type B alloy is the easiest to machine of all those investigated. a a

The alloy can be made by melting techniques that are standard. A suitable mixture of iron or steel, and

. ferro-manganese or metallic manganese, is melted down,

a suitable deoxidizer such as ferrosilicon is added (the .source of silicon) and finally metallic bismuth or a suitable alloy or compound thereof is added in the furnace or ladle. 'In otherwords, alloys of the present inven- .tion are produced under standard manganese steel melt techniques, and the objects of the present invention are ,achieved .by fundamental alterations in carbon and manganese contents plus the bismuth addition in the furnace or ladle. The alloy is thenpoured into castings, ingots or whatever is needed for the next stage of manufacture.

It will be manifest from the foregoing, and in particular from the numerous heats investigated,,that a great deal ofextensive research was involved indeveloping the ultimate analyses .that gave the improved machinability results, such culminating in the series of heats 416, 003 and 007, and these lead to the ultimate development of the type A and type B alloys set forth above.

It will therefore be evident that while We disclose and claim specific novel alloys that render manganese steel fully and acceptably commercially machinable, for the first time to the best of our knowledge, variations therefrom but within the realm of what We designate abnormally low carbon levels below the 1.0% standard level and abnormally high levels of manganme above the 13% standard level, together with effective amount of bismuth, are of course possible in achieving specific machinability characteristics or a balance between machinability and desirable mechanical properties under and in accordance with our invention.

We claim:

1. An easily machinableaustenitic manganese steel alloy which is substantially non-magnetic both as-cast or when cold worked, said alloy containing carbon in effective amounts between about 0.3 and 0.8 percent, manganese about 17 to 22 percent, and bismuth about 0.1 to 0.5 percent.

2. An alloy according to claim '1 containing about 4 percent of an austenitic stabilizing metal selectedfrom the group consisting of nickeljand copper.

3. An alloy according to claim 1 containing about 0.4 to 0.6 percent carbon, about 19 to20 percent manganese, sulfur up to about 0.09 percent, and characterized by a friction coefficient of not more than about 0.48 and an elongation of not less than about 13 percent for the as cast alloy and a friction coeiiicient of not more than about 0.57 and an elongation of not less than about 39 percent when the alloy is heat-treated at about 1900" F. for about two hours followed by a quench.

4. An alloy according to claim 3 and containing about 2 percent each of nickel and copper.

References Cited in the file of this patent OTHER REFERENCES Riedrich: Stahl and Eisen, vol. 60, No. 37, September 12, 1940, pages 815-818. Published by Verlag Stahleisen rn.b.H., Dusseldorf, Germany.

Claims (2)

1. AN EASILY MACHINABLE AUSTENTIC MANGANESE STEEL AL

1. AN EASILY MACHINABLE AUSTENITIC MANGANESE STEEL ALLOY WHICH IS SUBSTANTIALLY NON-MAGNETIC BOTH AS-CAST OR WHEN COLD WORKED, SAID ALLOY CONTAINING CARBON IN EFFECTIVE AMOUNTS BETWEEN ABOUT 0.3 AND 0.8 PERCENT, MANGANESE ABOUT 17 TO 22 PERCENT, AND BISMUTH ABOUT 0.1 TO 0.5 PERCENT.