US2763545A - Tool steels - Google Patents

Description

Sept. 18, 1956 W. R, BREELER TOOL STEELS Filed June 10, 1955 INVENTOR Wa/Ter R. Breeler ATTO TOOL STEELS Walter Robert Breeler, Fredonia, N. Y., assignor to Allegheny Ludlum Steel Corporation, Brackenridge, Pa., a corporation of Pennsylvania Application June 10, 1955, Serial No. 514,437

4 Claims. (Cl. 75-126) This invention relates to tool steels, and in particular to graphitic tool steels.

Graphitic tool steels have been known heretofore and have been widely used in the industry. In the known graphitic tool steels the prior art has taught that the amount of carbon present within such steels in the form of graphite should be at least one tenth of the total carbon content, but is usually in the range of from one fifth to one third of the total carbon, in order to obtain good machinability characteristics. However, with the use of such graphite contents, very poor surface characteristics are imparted to the steel when it is machined. Attempts have been made to improve the surface quality by sacrificing the graphite content, but have resulted in a production having poor machinability characteristics.

An object of this invention is to provide a graphitic carbon tool steel having good machinability characteristics and good surface finish.

Another object of this invention is to provide a graphitic carbon tool steel containing from about 0.10% to about 0.50% graphitic carbon and from about 0.05 to about 0.25% sulfur which will have an excellent surface finish, together with good machinability characteristics equivalent to those in a non-sulfur bearing tool steel containing at least 0.60% graphitic carbon.

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing in which:

. Figure l is a photomicrograph taken at a magnification of 500 times of a graphitic tool steel embodying the teaching of .this invention; and,

Fig. 2 is a photomicrograph taken at a magnification of 500 times of a grapbitic tool steel produced in accordance with a prior art.

In practicing this invention, a combination of elements is employed which will provide a graphitic tool steel satisfactory for use in the form of dies, punches and other tools. The tool steel of this invention contains graphitic carbon and has the characteristic of having high hardness, deep hardenability, resistance to abrasion and excellent machinability characteristics, together with good surface finish characteristics.

In its broader aspects the tool steel of this invention comprises about 0.5% to 3.0% carbon, about 0.1% to 3.0% manganese, about 0.5% to 3.0% silicon, about 0.05 to 0.50% chromium, about 0.05 to 0.25% sulfur, about 0.005% to 0.05% phosphorous and the balance iron with incidental impurities. The steel may also contain up to 3.0% of metal selected from the group molybdenum, tungsten and vanadium with detrimentally alfecting the characteristics thereof. Excellent results have been obtained where the steel contained up to 1% molybdenum as a preferred amount of this group of carbide formers. When heat treated as described hereinafter the tool steel of this invention contains a graphitic carbon content of from 0.10% to 0.50%.

Each of the elements performs a specific function in the tool steel of this invention. A portion of the carbon 5 Fe 5 Graphite 5 ice within the range given combines with the chromium and iron present and with a portion of the molybdenum, tungsten and/or vanadium if such elements are present, to form the respective carbides of these elements. These carbides provide the hardness, toughness, strength and abrasion resistance needed within this tool steel. The remainder of the carbon is present in the tool steel in the form of graphite which, when finely dispersed throughout the tool steel as referred to hereinafter, greatly contributes to the internal damping capacity and the shock resistance of the tool steel when employed in its intended use. Manganese, an austenite-forming element, is used to impart hot workability to the tool steel. Manganese also contributes substantially to the hardenability of .this alloy, making possible a deeper hardening of the tool steel of this invention with a given quenching medium than is possible with other similar alloys. Silicon functions primarily to induce the graphitization of part of the carbon when the chromium is maintained in the range given. The chromium forms chromium carbides and contributes to hardenability but is utilized primarily as a control on the amount of graphite formed, the graphite content being substantially indirectly proportional to the chromium content so that as the chromium content increases from 0.05% to 0.50% the graphite content decreases from 0.50% to 0.10%.

Molybdenum, tungsten and vanadium when present singly or in combination in amounts up to 3.0%, form hard alloy carbides to impart wear resistance and contribute to the hardenability of the resultant alloy. Sulfur, while imparting better machinability, cooperates with the graphite thereby producing the desirable properties of both good machinability and excellent surface finish. Further, the chromium and sulfur of this steel combine to form a uniform dispersion of graphite particles in the required amounts.

Reference may be had to Table I which contains the general range and the optimum range of the tool steel of this invention and alloy A which is within the general range and alloy B which is a graphitic tool steel known in the art. It will be appreciated that where the balance is reported as iron, it is to be understood that such balance includes the incidental impurities which are present and normally found in a heat of tool steel.

Table 1 Element General Optimum Range Alloy Alloy Range A B *Molybdenum, tungsten and/or vanadium up to 3.0%.

The alloy of this invention may be produced in any of the well known manners for making tool steels, for example, by electric furnace arc melting of said steels. Predetermined quantities of scrap and/or hot metal are placed within the electric furnace together with sufficient alloying elements in order to produce the desired analysis. This practice is common to the art and therefore will not be described in detail. The molten metal is cast into ingots which are thereafter forged at a temperature of 1950 F. to 2050 F. While forging is preferred, it will be appreciated that hot pressing or any other operation usually used in the fabrication of tool steels from ingot to bar or other similar shapes may be employed. The bar or forged stock is normalized at about 1550 F. and

Patented Sept. 18, 1956.

thereafter air cooled. This treatment is followed by an s annealing treatment at a temperature of 1450 F. to 1500 F., the purpose of which is to soften the steel and to promote the graphitization of a portion of the carbon to form from 0.10% to 0.50% graphite. Since the steel is in a soft condition and the graphite is present, the steel may be machined to any desired shape. I

When the steel has been machined to its finished shape, it may be hardened by a simple hardening and drawing operation. A preferred hardening treatment consists of heating the steel to 'a temperature in the range between 1400 F. and 1650 F. for from /2 to 1% hours, quenching the heated steel in oil, water or air depending upon the analysis and the physical properties required, and thereafter subjecting the steel to drawing'at a temperature in the range between 300 F. and 1000" F, depending upon the hardness desired. Reference may be to Table II illustrating the wide range of hardness that may be imparted to alloy A of Table I by oil quenching and tempering through the range of 300 F. to 1000 F.

Table II Hardening Temperatures Shepherd Fracture Eating 9% 9% 9% 9 8% Initial Hardness as Quenohad 62 65 65 64. 5 63. 5

Drawing Temperature Re Re Re Re Re 300 F. 61 54 64. 5 64. 63. 5 400 F (i 62 62. 62 G2 500 F 59. 5 61 61. 5 62 0 5 600 F. 58 60V 60. 5 61 61. 5 700 F 57 58. 5 59 59. 5 00 800 F 53 54. 5 55'. 5 55 56 900 F 49. 5 51 52 52 53 1000 F 45. 5 48 i8. 5 49 50 From Table II it can be seen that there is a wide range of temperature over which the tool steel of this invention can be heat treated in order to obtain any desired hardness. This alloy possesses a fine grain size, is tough, and lacks brittleness as is evidenced by the Shepherd fracture ratings. The toughness. and lack of brittleness is primarily attributed to. the low graphite content of the tool steel. As the chromium content is increased, and where metal from the group molybdenum, tungsten and vanadium is present, more carbides are formed which increase the abrasion resistance of the tool steel but results in less carbon being present to form graphite. The machinability characteristics of this tool steel however are not impaired by the low graphite content since the sulfur content of the steel cooperates with such graphite to give the steel excellent machinability characteristics while effecting a substantial increase in the quality of the finished surface. Also, with chromium carbides and one or more of molybdenum tungsten or vanadium carbides present, the tool steel may be made deeper hardening than tool steels which do not contain such elements. 7

The foremost characteristics of the tool steel of this invention are its good machinability, its improved. surface qualities and finely distributed sulfur and graphite. particles making possible even the most difficult machining operations. This can be demonstrated by taking. substantially similar compositions with the exception of sulfur and graphite contents, or with the exception of sulfur, and performing identical machining tests to obtain a. quantitative measurement of these characteristics. In

all cases the tool steel of this. invention machines better F and has a better surface finish than similar compositions; which. do not contain the sulfur or sulfur and graphite content; For example, where one inch round: pieces of alloy A and alloy B listed in Table I were; machined in. the same manner, that is, identical roughing cuts. and

finishing cuts were taken on each with the tool bit being resharpened after each cut, alloy A machined better and had a substantially better surface finish than alloy B.

From the foregoing it is apparent that the sulfur and low graphite contents of the alloy of this invention cooperate in the steel to give outstanding machinability and surface finish characteristics thereto while improving the wear resistance thereof. To obtain such outstanding characteristics the graphite content is limited to between 0.10% and 0.50% where the sulfur is maintained at between 0 .05 and 0.25%. The distribution of the graphite in the steel is changed by maintaining a low graphite content and the sulfur content as described. Thus alloy B of Table I, which has a high graphite content of 0.56%, when hardened, is characterized by a dull appearing coarse fracture whereas. alloy A of this invention, which has a low graphite content of .27% when fractured, is found to have a fine grain structure. This is evidenced by the Shepherd fracture rating which is recorded in Table II.

Reference may be had to Figs. 1 and 2 which are photomicrographs of the microstructure of alloys A and B; respectively. The basic structure of alloy A and alloy B is the same in both cases, that is, there is a matrix of ferrite 10 having spheroidal carbides 12 contained therein. However, there is a significant difference in Fig. 1 as compared to Fig. 2 which accounts for the excellent machinability and superior surface qualities in alloy A as compared to alloy B. In Fig. 1 the graphite 14 is very fine and substantially evenly distributed throughout the microstructure of alloy A. However, in Fig. 2, the higher graphite content of alloy 13 has resulted in large globules 16 of the graphite which show a somewhat erratic distribution and which, when the steel is machined, results in a poor surface finish. On the other hand, the fine graphite 14 and sulfides 1'8 of the alloy of Fig. 1 cooperate to provide good machinability and superior surface qualities by providing for a shorter chip and less surface irregularities than would be provided with the large globules 16 of graphite alone as found in the alloy of Fig. 2. In all cases it has been found that the presence of sulfur and particularly in amounts of 0.07% to 0.14% cooperates with a graphite content of only 0.20% to 0.35% to impart to the alloy of this invention machinability characteristics equivalent to or better than the machinability charac-- teristics of a similar tool steel having a graphite content of from 0.50% to 0.60%. Further, the surface finish ofv the tool steel. of this invention is far superior to. that obtained on such high graphitic tool steels. In effect, I have. found that about 0.10% sulfur is the equivalent of about .25% graphite in its effect on machinability and cooperateswith the finely distributed graphite to give improved surface finish qualities to the steel.

There are no special skills required to produce the tool steel of this invention, nor any particular apparatus re.- quired in its fabrication to tools. The heat treatments are simple, do not require extended holdings for long periods of time at elevated temperatures, nor are excessive temperatures required. The tool steel possesses excellent machinability, together with good surface finish, and is quite flexible in the degree to which it can be hardened. The alloying content. of the tool steel is substantially low and entirely devoid of strategic alloying elements.

I claim: a

1. A. graphitic tool steel comprising. aboutv 0.50%. to 3.0% carbon, about 0.10% to 3.0% manganese, about- 0.50% to 3.0% silicon, about 0.05% to 0.50% chromium, up to 3.0% of metal selected from the group consisting. of molybdenum, tungsten and'vanadium, about 0.05% to 0.25% sulfur, not more than 0.05% .maximumz. phosphorus, and the remainder iron, a portion of the: carbon being present inthe form of graphitiocarbon inanamount; of albout 0.10% to 0.50% dispersed finely throughout: the stee 2. A graphitic tool steel consisting: of. about 1.0% to 1.5% carbon, about 0.70% to 1.0% manganese, about 1.0% to 1.50% silicon, about 0.10% to 0.40% chromium, up to 3.0% of metal selected from the group consisting of molybdenum, tungsten and vanadium, about 0.07% to 0.14% sulfur, about 0.03% maximum phosphorus, and the remainder iron, a portion of the carbon being present in the form of graphitic carbon in an amount of about 0.20% to 0.35% dispersed finely throughout the steel.

3. A graphitic tool steel consisting of about 1.3% to 1.50% carbon, about 0.7% to 1.0% manganese, about 1.0% to 1.5% silicon, about 0.10% to 0.40% chromium, up to 1.0% molybdenum, about 0.07% to 0.14% sulfur, not more than 0.03% maximum phosphorus, and the remainder iron, a portion of the carbon being present in the form of graphitie carbon in an amount of about 6 0.20% to 0.35 finely dispersed throughout the steel to impart an excellent surface finish thereto, the sulfur content cooperating with the graphitic carbon to give the steel machinability characteristics equivalent to those of a nonsulfur bearing graphitized steel containing from 0.50% to 0.60% graphitic carbon.

4. A graphitic tool steel consisting of about 1.37% carbon, about 0.94% manganese, about 1.42% silicon, about 0.30% chromium, about 0.26% molybdenum, about 0.105% sulfur, not more than 0.03% phosphorus, and the remainder iron, a portion of the carbon being present in the form of graphitic carbon in an amount of about 0.27% dispersed finely throughout the steel.

No references cited.

Claims (1)

1. A GRAPHIC TOOL STEEL COMPRISING ABOUT 0.50% TO 3.0% CARBON, ABOUT 0.10% TO 3.0% MANGANESE, ABOUT 0.50% TO 3.0% SILICON, ABOUT 0.05% TO 0.50% CHROMIUM, UP TO 3.0% OF METAL SELECTED FROM THE GROUP CONSISTING OF MOLYBDENUM, TUNGSTEN AND VANADIUM, ABOUT 0.05% TO 0.25% SULFUR, NOT MORE THAN 0.05% MAXIMUM PHOSPHORUS, AND THE REMAINDER IRON, A PORTION OF THE CARBON BEING PRESENT IN THE FORM OF GRAPHITIC CARBON IN AN AMOUNT

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