US2438267A - Graphitic steel - Google Patents

Description

Patented Mar. 23, 1948 'niiirto stares Harem ori ice qaaimncernpn FrederickR. Bonte, Canton,

Ohio, assignor to The Timken; Roller Bearing CompanyrGanton; Ohio, acorporation o f Qhio v No'Dravving. Application 'March 23, 1942,

This invention relates to graphitic steels. I

Certain types of graphitic steels are disclosedin Patent No. 2,087,764, granted July 20, 1937,0n-an application filed by me.

improvements in used especially Widely. One of these-generally contains about 1.5 per cent of carbon and about 1 percent of silicon, While the other is-of similar composition but contains: additionally *a small amount of molybdenum: I Another type of graphitic steel is disclosed and claimed in my copending application Serial-No.

347,767, filed July 26,1940, no-w'Patent'No.

2,283,664, dated May 19, 1942.: This latter type,

certain unusual properties; contains invite-preferred embodiment about 1.5 percent of-ca-rbon, about 0.4 per cent of manganese, about 0.5 per Two types of-the steels disclosed and claimed in that patent havebeen v Which possesses the-properties commonr'to-agraphitic steels but Which additionally: possesses cent of molybdenum, about 0.65- per cent of-silicon, and about 2.8 per cent of tungsten.-

The graphitic steels described in the foregoing patent and application are characterized generally by containing'free graphite and-by the ability treatment. They possess high resistance to abrasion, metal pick-up and scoring, andth'ey are classed as being free machining. Such properties coupled ,With other desirable physical to develop high hardness upon appropriate heat 5 properties have caused the graphitic ste'elstc'i be used extensively for the making of dies of many kinds, punches, spinning, slitting'and forming rolls, breaches, and other tools used for'related purposes, especially Where resistance/to fatigue and abrasion is requisite.

amounts, but their use siderably.

The graphitic steels made heretofore are hardis being extendedlconened by quenching into oil, Water or brine," and i none has possessed the capability of airhardening. The necessity for quenching into a liquid is undesirable in some instances, as in the case of parts of non-uniformsection, or partsof relatively great length and small section, because "the parts may become distorted as a result ofstrains set up in the quenching operation. Lik'evvi'se, quenching in liquid coolant requires" special equipment, and some plants are not supplied with be d ficult or undesirable to install: such equipment. v v Ma phitic air hardening te l are-avail,

able but many of them must be hardened atvery Not only have those steels been used satisfactorily and in large such equipment, or for various reasonsit" may fullypresent; Y

b It is among the objects of this invention to pro- 1 Serial No. 435842W 4Claims. (01.17542 v must! operate at: temperatures- Whichare relatively high, depending ;-upon--thealloy being cast,

and the operating temperatures may be so high a in some instances that the beneficial properties of-the previously used=graphitic steels-are not vide graphitic'steels and'artieles made therefrom in. Which-"theeproperties generally possessed-by such steels arecombined 'with air hardening ability; which are -of relatively simple and inexpensive composition and do not require the use of large amounts of alloying elements, whichin shapes of relatively great length. or relatively small. section, Orboth, can behardenedawithout 30f serious distortion; and- Which' can be hardenedtto provide .austen-iticstructures; that are adapted for use attemperatures above those at which the previously-known .gr-aphitio steels :losehardness eri usly h In accordance with this inventionagraphitic steelnarticles are made,- from steels containing iromfa'bout 1 to Z-per' cent ofcarbonabout-c 0-25 to :1;0 percent of chromium, about;0.75zto-1.5 per cent ofmanganese about 0.25 to 1.0 .per cent-oi 'molybdenum,' about 1.5 to 2.5 percent of nickel,

and about.-0..75 :to" 1.25 percent oisilicon. Within such: ranges it ise-preferred for many purposes to form the =xarti'cles from :st'eels containing iabout 1.5'per1-ent*bf."carbon,-abouti0.5 per cent-of chromium;- about- 1.25' per Teen-11* of-manganese,

about"""0.5 "per cent of molybdenum; about 1.75 per cent of nickel, andraboutlzll' percent of silicon Neither the phosphorousvnor the'sulfur content should exc'eedabout 05025 per cent.

The-remainder of the steels is iron-together'with impurities in the iamounts customarily encountered m such 'steelsg but it Willibe 'understood'that other 'alloying elements may be present provided slowly, to 1380 .the preferred composition .able heat treatment, structure possessing other desirable qualities 75 which reason the remainder of the steels may be said to be effectively iron inasmuch as such additional alloying elements do not alter the essential character of products made in accordance with the invention. The foregoing elements are balanced in accordance with customary graphitic steel practice.

The content of manganese is substantially above that ordinarily used in the previously known'graphitic steels, and it contributes to the air hardening properties of these steels. Manganese in such amounts and chromium tend to repress graphitization, by forming stable "carbides. In order to provide graphitic carbon in the structure, therefore, the silicon is somewhat higher than in the usual graphitic steels, and W graphitization is assisted present. Thus the presence of a functionally active amount of graphitic carbon in the structure of the products is assured while producing the other desirable properties that characterize this invention.

by the nickel which is The steels provided by this invention may be made in accordance with procedures standard in the art for the production of graphitic steels. Preferably they are made in an electric furnace following standard killed steel practice, the various alloying elements being introduced suitably in the form of ferro-alloys, or otherwise as desired.

The ingots are working in a manner treated prior and during hot understood in the graphitic steel art, i. e., so that the carbon will remain substantially entirely in the combined form during hot working. Generally speaking, the ingots should be hot Worked, as by rolling o'r forging, at a temperature not over about 1950 F. or 2000 F. to produce shapes of desired form and size for conversion into final products. In case it is necessary to forge the hot rolled material to form dies or other tools the hot rolled shapes should be heated slowly to 2000 F. and held until they have been heated through, after which they are forged to shape with care taken that the temperature during forging does not fall below about The shaped articles are then graphitized by subjecting them to normalizing and annealing treatments. heating above the critical range, suitably at about 1700 F., to cause decomposition and diffusion of carbides. They are then cooled, in accordance with ordinary normalizing practice, by being removed from the furnace and cooled in 'air.

"Thereafter the articles are annealed by reheating them into or above the critical range, say by. heating to 1450" F. and holding at that temperature for four hours. The articles are advantageously at about 20 F. per hour, F. where they are held for four hours and then cooled at the same rate to about 1250 F. Thereafter they are cooled slowly to about 700 F., after which they are removed from the I'furnace and allowed to air cool; In this manner the carbides are partially decomposed with production of:

graphitic carbon and with spheroidization of residual carbides.

Treated in the manner just described, steels of given hereinabove will contain about 0.5 per cent of free graphite, the remainder of the carbon being in combined form. The graphitic carbon confers surface lubricating qualities desirable in articles provided by the invention, while the carbides provide, upon suith ehw ar r sistan nd a then cooled To this end they are normalized by 7 After being graphitized the articles are machined or otherwise finished to shape and size, after which they are subjected to a hardening treatment in which they are heated above the critical range and air cooled. tures can be produced, depending upon the conditions of the hardening treatment.

As indicated above, austenitic structures are produced by hardening heat treatment of articles made in accordance with the invention. For many purposes it is now preferred to conduct such hardening heat treatment to produce an austenitic-martensitic structure containing about 10 per cent of retained austenite, and it is particularly desirable to use a double hardening procedure. With sections up to'about 2 inches thick, it suffices for such purposes usually to pack the articles in a suitable compound, heat to 1625 F., and cool in air, and then to reharden at1550 F. With sections from about 2 inches to 4 inches thick, the first heating should be about 1650 F., while with sections over 4 inches the first hardening should be at about F.; in both cases rehardening is effected at 1550 F., at which temperature the articles are held until heated throughout, the exact time depending, of course, upon their mass. On the first hardening there will be. retained a substantial proportion of austenite. The rehardening step is desirable because it gives increased wearing properties resultant from production of martensite by conversion of some of the austenite. Hardnesses of 60 to 65 R0. are regularly obtainable through'hardening in the manner described. One advantage of these steels is that at higher hardnesses they contain more graphitic carbonthan the other graphitic steels. Their remarkable properties are due in part to uniform dispersion of the carbides and the graphite throughout the structure, both being finely divided.

The austenitic structure which distinguishes these steels from other graphitic steels has various advantages. Particularly, the products are adequately hard "while being ductile and consequently resistant to fracture. A further consequence is that articles made in accordance with the invention can be operated at higher tempera-. tures without objectionable loss of hardness, thus Widening substantially the field of utility of the graphitic steels. As an example, the steels of this invention are particularly suited for die casting dies, for which use the hardness is retained to substantially higher temperatures than in the previously used non-graphitic steels. It is suitable also for use as dies for hot sizing shell blanks. Articles made as described may be flame hardened to produce true austenitic surfaces which are more expensive but have necessarily had to be used heretofore where air hardening ability was necessary.

' As exemplifying the' benefits which fio'w from A variety of structhe invention, a die for drawing switch boxes was made from the preferred embodiment, and it was double hardened as described above. This die produced 34,000 pieces before regrinding was necessary. Previously there had been used regularly for this work a high carbon, high chromium die steel which cost the user 18 cents per pound more than the steel used in the practice of this invention, and which presumably Was the best material that the user had found for this purpose, but the die life averaged only 1000 to 1500 pieces. In consequence of this experience the user bought a substantial number of dies according to this invention.

A particular advantage is that through the practice of the invention it is possible to efiect hardening without distortion tools of variablesection or of considerable length and small section, Whose quenching in oil or water is hazardous. Thus the steels may be heat treated Without distortion and at points where liquid quenching equipment is lacking.

One of the remarkable properties of these steels is that they are capable of hardening in oil or water, so that they may be quenched in such coolants if that is desirable for any reason. It may be noted that it is rather unusual to find steels which are air hardening and are likewise capable of oil or water quenching. By quenching in liquid media the structures may be varied considerably, and. more so by tempering the quenched articles. Thus they may comprise from to 90 per cent of austenite and the balance martensite, or full martensite and other non-austenitic structures. Hardnesses of 60 to 65 R0. in the hardened state can be had thus, and by tempering the hardness is, of course, reduced, e. g. 40 to 50 Re.

The steels likewise possess satisfactory mechanical properties. For example, in the annealed state the preferred composition has an ultimate strength of 135,000 p. s. i., a yield point of 77,500 p. s. i., 17 per cent elongation in 2 inches, 34 per cent reduction of area, and a Brinell hardness of 262.

According to the provisions of the patent statutes, I have explained the principle and method of practicing my invention and have described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described.

I claim:

1. As a new article of manufacture, a. hot worked and heat treated alloy steel article formed from air hardening steel containing about 1 to 2 per cent of carbon, about 0.25 to 1 per cent of chromium, about 0.75 to 1.5 per cent of manganese, about 0.25 to 1 per cent of molybdenum, about 1.5 to 2.5 per cent of nickel, 0.75 to 1.25 per cent of silicon, and the remainder effectively iron, and characterized by an austenitic-martensitic structure containing graphitic carbon and spheroidized carbides dispersed finely through the structure, and by high resistance to wear.

2. As a new article of manufacture, a hot worked and heat treated alloy steel article formed from air hardening steel containing about 1.5 per cent of carbon, about 0.5 per cent of chr0- mium, about 1.25 per cent of manganese, about 0.5 per cent of molybdenum, about 1.75 per cent of nickel, and about 1.0 per cent of silicon, and the remainder effectively iron, and characterized by an austenitic-martensitic structure containing graphitic carbon and spheroidized carbides dispersed finely through the structure, and by high resistance to wear.

3. Alloy steel containin about 1 to 2 per cent of carbon, about 0.25 to 1 per cent of chromium, about 0.75 to 1.5 per cent of manganese, about 0.25 to 1 per cent of molybdenum, about 1.5 to 2.5 per cent of nickel, about 0.75 to 1.25 per cent of silicon, and the remainder effectively iron, and characterized by capability of being hot worked and of producing upon heat treatment austeniticmartensitic structures containing graphitic carbon and spheroidized carbides dispersed finely through the structure.

4. Alloy steel containing about 1.5 per cent of carbon, about 0.5 per cent of chromium, about 1.25 per cent of manganese, about 0.5 per cent of molybdenum, about 1.75 per cent of nickel, about 1.0 per cent of silicon, and the remainder effectively iron, and characterized by capability of being hot worked and of producing upon heat treatment austenitic-martensitic structures containing graphitic carbon and spheroidized carbides dispersed finely through the structure.

FREDERICK R. BONTE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Monypenny: Stainless Iron and Steel, 1931, pp. 139445.

Sherry: Steel Treating Practice, 1929; pp. 133- 135.

American Society for Metals, Metals Handbook, 1939, Pp. 574, 575, 614, and 618.

Symposium on Pearlitic Malleable Cast Iron, published by American Society for Testing Materials, Philadelphia, Pa., 1936, pages 1, 6, 7, 13-16.

Bonte: Graphitic Steels, published in Steel, vol. 109, Nov. 24, 1941, pp. 80, 82, 96 and 100.