US3231433A

US3231433A - Manufacture of metal band saws and the like

Info

Publication number: US3231433A
Application number: US622169A
Authority: US
Inventors: Laurance F Van Mater; Kenneth E Campbell
Original assignee: Simonds Saw and Steel Co
Current assignee: Simonds Saw and Steel Co
Priority date: 1956-11-14
Filing date: 1956-11-14
Publication date: 1966-01-25
Anticipated expiration: 1983-01-25

Description

Jan. 25, 1966 L. F. VAN MATER ETAL 3,

MANUFACTURE OF METAL BAND SAWS AND THE LIKE Filed Nov. 14, 1956 3 Sheets-Sheet 1 E g-i.

HIGH SPE E D 1 E STEEL EIJBFLV I l MELT REDUCING CARBON --1 N1. l-- FORM HOT ROLL CONTENT TO NOT EXCEEDING I N MELT l BILLET TO STRIP ABOUT 0.55% L g l 3 4 PREPARE THE COLD FINISHED STRIP PREPARE STRIP FOR FOR AND GARBURIZE AND SELECTED EDGE ZONE;

coNTRoL COOL FOR COLD ROLL T0 GUAGE PART'AL ANNEAL FORM CUTTING EDGE CLEAN 8.9. BY TOOTHING AND NON-CARBURIZED SETTING SAME, LOCATING AREA TooTH GULLETS BELOW GARBURIZED zoNE HEAT TREAT FOR HARDENING IN SINGLE CONTINUOUS AND ONE-TEMPERATURE OPERATION TEMPER FORM CUTTING EDGE STEPS I THROUGH 5 AS FOR STEP 8 OF FIGJ, STEPS 6 7 As IN FIG.|. CORRELATING TOOTHED 0R 9 AND "0 T (WITH OR WITHOUT OTHER EDGE DEPTH To THAT 8 sTEP 2) PREDETERMINED FOR A SUBSEQUENT CARBURIZATION Jan. 25, 1966 1.. F. VAN MATER ETAL. 3 L

MANUFACTURE OF METAL BAND SAWS AND THE LIKE Filed Nov. 14, 1956 5 Sheets-Sheet 2 Jan. 25, 1966 L. F. VAN MATER ETAL 3,231,433

MANUFACTURE OF METAL BAND SAWS AND THE LIKE 3 Sheets-Sheet 5 Filed Nov. 14, 1956 ZOmm 0 00 mm Om mi 0% 4w UTE. 2O ZOmm 0 how um I 8) SSBNCIHVH l'zwerziows:

Laurence F Vanilizier,

383MB: beZZ,

' QLMJM United States Patent 3,231,433 MANUFACTURE OF METAL BAND SAWS AND THE LlKE Laurance F. Van Mater, and Kenneth E. Campbell, Lockport, N.Y., assignors to Simonds Saw and Steel Company, Fitchburg, Mass, a corporation of Massachusetts Filed Nov. 14, 1956, Ser. No. 622,169 1 Claim. (Cl. 14816.5)

This invention concerns the manufacture of tools comprising flexible metal strips having cutting edges at one or both longitudinal margins, and of alloy materials and blanks for such tools. It includes the provision of improved high speed tool steels and unitary metal strips formed thereof and adapted to incorporate one or more edge zones differentiated as to hardness from the remainder or body zone of the strip. The invention further comprises simplified methods for the manufacture of edgetype flexible tools and blanks therefor, especially such as hand saws, hack saws, belt knives and the like.

In the accompanying drawings illustrative and explanatory of the invention:

FIG. 1 is a flow chart of steps in one process or method of the invention;

FIG. 2 is a similar chart of a modified method;

FIG. 3 is a side view of a portion of a metal band saw embodying the invention;

FIG. 3A comprises photomicrographs showing the plural zone composition such as that of FIG. 3 of a tool or of a blank thereof in accordance with the invention; viewing the same edgewise transversely of the zoning; and

FIG. 4 is a graph of certain carbon to hardness relations.

This invention embraces and turns upon our discovery that various types of high speed tool steels if melted with markedly low carbon ranges result in distinctive new high speed tool steels, herein termed carburizing grades.

Table A appearing later herein sets out certain standard high speed tool steels, by way of identifying those found suited for a calculated and controlled lowering of carbon content in the melt, producing the carburizing grades here concerned. These will be designated by the same type numbering, thereby indicating the same alloy ingredients and proportions other than the carbon, plus the term carburizing grade to indicate the lowered carbon range in the melt. They are characterized by having an affinity for carbon such that strips thereof possess capacity for the novel edge carburizing to be described. By reason of said capacity strips of our carburizing grades of high speed tool steels hot rolled from billets and cold rolled to gauge are edge carburizable for production of thincutting-edge tools of the flexible type in a single continuous one-temperature heat hardening operation.

While other uses for the low-carbon or carburizing grades of high speed tool steels are within the scope of the invention they are especially suited for the production of the mentioned thin-cutting-edge flexible type tools. These have two main requirements as to physical properties: (a) a sufficiently hard toothed or other edge for eflicient cutting, and (b) a relatively soft backing or carrying body having appropriate fatigue life to withstand continued flexing about pulleys as in band saw machines. Hence at least two longitudinal portions or zones of different hardness are essential, one comprising the teeth or cutting edge and another the backing.

For illustration, the invention is more particularly described with reference to the manufacture of high speed metal and other band saws, and strips and blanks suitable therefor. It will be understood as applying also to other flexible-type thin cutting edge tools, including belt 3,231,433 Patented Jan. 25, 1966 knives, hack saws and the like. In the term metal band saws we include those primarily for cutting metal and also other equivalent flexible thin edge-type cutting tools for which a hardness of the toothed or other cutting edge is desired comparable to that for metal band saws, i.e. of the order of hardness number of about 60 to 65 Rockwell C scale and upwards.

Under present manufacture so far as we are aware high speed metal band saws employ the standard M-2 type of high speed steel. Such steel has proved satisfactory for some cutting tools but is found subject to disadvantages for metal band saws.

The latter require fabrication of a relatively thin strip, in the order of about .035 in. thickness for an average width of such saw. Other saws, to which this invention also applies where edge-hardness is desired of the order here concerned, may have an average thickness up to about .065 in. for those up to 2 in. widths, and up to about 0.125 in. thickness for wider saws.

In cold rolling the conventional high speed tool steel, such as that of the M2 type, down to a strip of approximately the thickness mentioned, the inherent ductility is low. Thus the percentage reduction between anneals is low, and is accompanied by severe edge checking. This has heretofore required slitting of the strip at each anneal to eliminate the defective edges. The metal trimmed off is scrapped as useless. Consequently such rolling procedure is expensive and the cost of the finished strip is correspondingly high. Under the present invention we produce a strip suitable for high speed steel metal band saws and the like but which can be cold rolled easily and economically on conventional cold rolling mills.

Another disadvantage in making metal band saws from high speed steel strip such as the standard M-2 type arises in connection with hardening of the cutting edge, for which high temperatures are needed, of at least about 2150 to 2225 F. for said M2 type. metal band saw manufacture the requisite longitudinal zones distinctly differing in hardness, as previously re ferred to, are obtained with M2 type strips only by separate and successive heating operations. In these the entire band-saw-forming strip is first heat treated to a Rockwell C hardness of about 45. Then the longitudinal series of teeth is heated to a temperature of approximately 2200 F. as by means of a flame treatment. The heat treatment at the lower temperature causes partial hardening in the backing zone and the subsequent treatment of the tooth zone at the higher temperature develops full hardness in the teeth.

But in the treatment of the teeth at the higher approximately 2200 F. it is difficult to control the temperature over the entire tooth and particularly in the vicinity of the points of the teeth and the bottoms of the gullets. Resultantly the teeth are frequently under-hardened or over-heated, both of which conditions objectionably re duce the cutting efficiency of the saw. Somewhat similar two-stage heat treatment has been used with some success for hardening teeth on band saws of plain carbon steel. But the hardening temperatures there needed are much lower, in the range of 1400 to 1600 F. as contrasted with the approximate 2200 F. and upwards required for high speed tool steels with which the present invention is concerned.

Other proposals have been made for producing high speed tool steel saw blade blanks of a duplex or composite structure presenting zones differing in hardness, as

for example in Replogle Patent 2,683,923. But such have involved the provision of separate strips or zones of different steels which are then joined longitudinally as by welding. The difliculties and added operations required for such process are obviated under the present invention.

Under present By contrast with the foregoing, the present invention employs principles new to this thin edge-type cutting tool art, whereby a unitary strip of stock in the high speed tool steel category is produced, as distinguished from plural piece strips such as those of Replog'le, and which is heat treatable for hardening in a single continuous one-temperature operation. Under the invention a high speed steel strip, sheet, plate or bar is produced which has at least two zones of diiferent carbon content and which under heat treatment in a single hardening operation affords with proper tempering a full-hard working zone or area and a softer back-up zone or area. And in so doing, the final hardness for the working or cutting zone after the single heat treatment is controlled by predeterminedly lowering the carbon content of the high speed tool steel at the time of melt.

Our carburizing grades of high speed tool steels as already explained comprise distinctive low-carbon melts otherwise constituted as set out in the following Table A. These include both molybdenum base and tungsten base types, as indicated by the letters M and T respectively in the type designations. The principles of the invention are believed to apply also to the molybdenum-cobalt and the tungsten-cobalt types involving the addition of cobalt to the compositions such as those of Table A.

The compositions in Table A are in the nature of averages for the several types represented and are not for the purpose of specific limitation. The type numbering and approximate compositions are those generally accepted in the steel industry, as appearing for example in The Iron Age for March 8, 1956, at pages 179 to 182. Similar high speed tool steel alloy compositions appear with other numerical designations in Metals Handbook of American Society for Metals, 1948 Edition reprint of 1956, as at pages 656 and 664, under the classification V. Additional minor substances such as manganese and silicon are omitted in the following exemplary Table A, as in the Iron Age table.

As stated, the resultant carburizing grades may be as in Table A save as to carbon content. However, we have found that the general purposes of the invention are further promoted by the addition of up to about 1.0% nickel. The effect of carbon on the heat-treated hardness on one example of the steels (M-2 type) with and without the addition of approximately 1% nickel is graphed in FIG. 4. Similar comparative results obtain for nickel additions from about 0.50% to about 2.0%. The percentages herein are of the total at the melt, by weight.

From Table A it is seen that the standard high speed tool steels there analyzed have a carbon content averaging about 0.85% and being in every case at least 0.70%. By contrast our high speed tool steels, carburizing grade, are of a carbon content lowered in the melt to a range from not less than 0.25% to not more than 0.55%.

From the foregoing it is apparent that our present invention comprises the production of certain new steel alloys, herein designated high speed tool steels, carburizing grades, e.g. M-2 carburizing grade, M-lO carburizing grade, T-l carburizing grade, and the others as exemplified in Table A.

The chart of FIG. 4 shows the characterizing effect of the low carbon range on the hardness after heat treatment, for our carburizing grades of the high speed steels. Using the M-2 type, carburizing grade, the results are plotted over our low-carbon range from 0.25 to 0.55%, both Without nickel (the lower curve) and also with the addition of approximately 1.0% nickel (the upper curve). It is evident from the upper curve that a significantly higher range of hardness for our carburizing grades of the high speed steels results with the addition of nickel; also that there is a flattening of the without nickel curve toward the upper portion of the carbon range, i.e. the hardness increase in the presence of nickel is greater in the lower portion and decreases toward the upper portion of our carbon range. In other words, the heattreated hardness varies with the carbon, and the nickel additive varies the rate of such variation.

FIG. 4 also illustrates graphically the low carbon range in the melt, for our carburizing grades, as compared with the normal or higher carbon-ranges of at least 0.70% and upwards for ordinary high speed steels (Table A), with which approximately full hardness is obtained on the parent metal under standard heat treatment. Thus for the carburizing grade of an M-2 type, and similarly for carburizing grades of the other types exemplified in Table A, the carbon in the steel as melted is held to a low range between a minimum of about 0.30% and a maximum of about 0.55 As indicated by the dotted portion at the lower ends of the curves of FIG. 4 the carbon may in some instances be as low as about 0.25 but in general it is found desirable to maintain a minimum of approximately 0.30% carbon in the melt to avoid possible formation of intermetallic compounds.

The preceding paragraph includes specific reference to the M-2 type steel of the FIG. 4 example. Under the established standard, high-speed tool steel of the M-2 type has 0.80% carbon in the mix. Obviously for the others of the Table A types which standardly have more than 0.80% carbon the reduced carbon content for such other types under the invention may be proportionately higher than for our M-2 low carbon carburizing grade so long as it satisfies the stated object of the invention, namely to give the desired controlled less-than-full heattreated hardness. Thus for example with types M-3 and T-3 which standardly have 1.0% and 1.05% carbon, respectively, Table A, the proportionate reduction of carbon in the melt under our invention provides correspondingly more carbon than for the M-2 type. It is further evident from the low carbon carburizing grade highspeed tool steel of the M-2 type of the FIG. 4 example that for a desired preselected heat-treated hardness, as for instance 45 Rockwell C, the appropriate percentage of carbon in the mix is lower in the presence of 1% nickel, namely about 0.34% carbon, than for the same mix without nickel, namely about 0.42% carbon. In this respect FIG. 4 is exemplary and the same principle as to inclusion or omission of nickel applies with respect to the other steel types of Table A. And in any instance in accordance with the invention there is a characterizing reduction of the carbon content in the melt, such as to afford the controlled less-than-full hardness as desired for a non-carburized portion of the resultant strip after heat treating.

Further in accordance with the invention, we utilize said steel compositions to produce a carburizing grade of high speed steel strip, sheet, plate or bar which upon preferential edge carburizing presents at least two zones of different carbon content. Such preferentially carburized blank, with a cutting edge formed either before or after the carburizing, possesses the novel capacity of being heat treatable with a single hardening operation, followed by proper tempering, to provide in the desired thin-edge flexible-type tool a full-hard working area and a soft or less hard backing area.

In the backing area or zone, in the resultant tool, the

final and desirably less-than-full hardness after the one-.

temperature heat treatment is determined by lowering and controlling the carbon content of the steel at the melt stage, as explained.

As to the working area or zone for the tool, that which is to present the toothed or other cutting edge of a flexible strip tool, such as for a metal band saw, full hardness is obtained by the selective carburization of that area, to be further described. Moreover the carbon content can be controlled in the carburized area, by determined control of temperature and carbon potential, to afford a definite carbon range for the given application.

Thus it is found possible by varying the carbon content of the parent metal, within the low range indicated, and of the preferentially carburized zone, to produce the desired combination of hardness and other properties for different areas or zones of a given product. In practice the carbon content in the carburized zone, i.e. after the preferential carburizing, has been run up to in excess of 1.0% and up to 1.50% or more in some steels without the formation of visible austenite during subsequent heat treating procedures.

By way of illustration the process of the invention as applied to the manufacture of flexible-type edge tools, FIG. 1 presents a flow sheet of steps from preparation of the steel through formation of the blank to the tool product. Where the description may refer to a particular tool of the class concerned, or to some one of the carburizing grade high speed steels, the same will be understood as typical rather than as limiting thereto; the same applies also to content percentages, which may be within the ranges as already stated. In FIG. 1 the steps are indicated generally; in the following example they are set out with respect to a high speed metal band saw utilizing an M-2 type steel carburizing grade for a saw size of l in. width x .035 in. thickness.

EXAMPLE 1 Melt heat of steel by conventional procedure to nominal analysis of M2 type high speed with the addition of approximately 1% nickel and with the carbon content approximately 0.35%; FIG. 1,

steps

1 and 2.

Form billet and reduce to hot rolled strip of convenient gauge and width by conventional procedures. For the analysis given above such a hot rolled strip might run 6 to 7 in. wide and 0.125 in. thick; FIG. 1,

steps

3 and 4.

Then prepare the strip for cold rolling by any suitable combination of annealing and cleaning operations in common usage, and reduce the strip stock to the final gauge of 0.035 in. by a series of cold rolling and annealing operations. With the analysis as given above cold reduction in the neighborhood of 50% between anneals are perfectly practical. After reaching finish gauge or at a convenient heavier gauge the stock is slit into strip 1 in. wide. These strips 0.035 in. x 1 in. x random length are the raw stock for the manufacture of metal band saws; FIG. 1, step 5.

As to the following operation, comprehended in step 6 of FIG. 1, there are a number of variations in procedure available for effecting selective edge carburization; the following represents our present preference, as best facilitating the commercial manufacture: first clean and then copper plate the cold rolled strip to a thickness sufficient to prevent carbon penetration (approximately 0.001 in.) during the carburizing operation. Then remove the copper from one edge of the strip by filing, grinding, or other suitable means. Place the strip, preferably coiled, in a carburizing furnace so that all edges free of copper are exposed to the carburizing medium. This medium may be solid or gaseous but for convenience of control we prefer the gaseous such as produced in a Leeds and Northrup Homocarb Furnace with Microcarb control. Any method of carburization in which the temperature and carbon potential of the medium are under reasonable control, however, is deemed satisfactory. Our steels of the various high speed types, carburizing grade, and particularly those containing molybdenum have a much greater aflinity for carbon than do the plain carbon and low alloy steels. Thus, they are easier to carburize but require reasonable care in controlling the carburized carbon content. The desired carbon of the carburized area will vary With the application and with the type of high speed steel but for metal band saw of the norminal M-2 type we prefer to hold the carburized zone in the range of 0.80 to 1.00%. This does not preclude the use of higher or lower carbon contents for special application. After the carburizing cycle is complete, it is preferred to slow cool the charge in order to obtain a partial anneal of the stock; FIG. 1, step 6.

The coils are then cleaned of copper by deplating, acid pickle or other suitable means. The strips are then ready for toothing along the carburized edge. The method of toothing and the depth of penetration of carburization are so controlled that only areas of the teeth above the gullet are carburized, thereby avoiding embrittlement of the gullet after heat treatment.

After the saw has been toothed and the teeth set it is ready for heat treatment; see FIG. 1, step 9. The hardening operation is carried out in a single continuous one-temperature operation, at a conventional temperature for M-2 type high speed steelnamely 2170 to 2250 F. depending upon the equipment used. Either salt bath or furnace may be employed, with the full width of the band reaching temperature. We prefer to harden the band saw in continuous strip rather than in a cut or in subdivided lengths. The lower carbon backing is sufficiently ductile for the band to be coiled directly from the hardening operation, and preferably it is so coiled. The coil is then tempered (single or double) at approximately 1050 F. The tempering operation may of course also be done in continuous strip form but for simplicity of equipment we prefer to handle coils in batch lots.

The analysis of the steel and the treatment described above produces a band saw with the teeth having a hardness of 64 to 66 Rockwell C and the balance of the blade a Rockwell C hardness of approximately 45. Thus the teeth are file hard, by which is understood anything over 60 Rockwell C. These are standard hardness values such as sought for high speed metal band saw heretofore produced by multiple heat-treating and other difficult and relatively complex procedures. However, under our process the hardness of the backing may be raised or lowered to meet other conditions merely by raising or lowering the carbon content of the original melt within the range herein described, all other processing and heat treatment remaining the same as above stated.

Modifications of the process will occur to those skilled in the art.

EXAMPLE '2 For instance, as represented in FIG. 2 flow chart, teeth may be cut in the band before carburizing. In such practice all of the band except the teeth are blocked off by copper or other suitable coating before carburizing.

While found generally helpful in the control of hardening in the low-carbon ranges of our carburizing grades of the steels, apparently by reducing the amount of free ferrite in the hardened structure, the nickel may within the invention be omitted from the melt. Such omission, however, increases the ditficulty of.controlling the heat treated hardness of the backing and reduces the percentage of reduction between anneals in cold rolling. The percentage of nickel content may be varied. 0.50% Ni is effective; anything substantially below 0.50% nickel may be considered alloy pick-up from scrap, furnace lining, etc. Increasing nickel content increases the hardenability of the alloy at the same carbon content but also increases the difiiculty in annealing the stock. Experiments indicate that a content of appreciably over 2.00% nickel is not practical.

FIG. 3 shows a metal band saw embodying the invention, and as produced under the process thereof. The saw is indicated generally at 10, the view including several typical teeth 11 with the bottoms of the gullets indicated at 12. What we have herein referred to as the working area or zone, that which presents the toothed or other cutting edge for the thin flexible-type tool, is approximately demarked by the dot and dash lines and the associated reference letter a. As apparent from comparison with the photomicrographic views of FIG. 3A this working zone a comprises an area of full hardness extending in from the edge of the blank or stock and over a major portion of the cutting edge of the tool, e.g. of the teeth 11 of FIG. 3, and then merging into a transition area at or near the region of the gullet bottoms 12.

The adjoined structurally integral backing zone, of the lesser hardness after the one-temperature heat treatment uniformly accorded to the entire strip, is designated by the reference character b. It extends from the inner margin of the working zone 0, preferably somewhat above the toothed gullet bottoms as in the instance of the illustrative metal band saw, so that the gullet bottoms 12 are in the backing zone b, and continues across to the other edge of the tool opposite the toothed or cutting edge. It will be understood that the invention is equally applicable to the manufacture of double-edged tools, by providing working zones such as a of FIGS. 3 and 3A at both edges of the stock or blank.

As stated, the photomicrographs in FIGS. 3A represent the resultant structure in a typical instance utilizing our high-speed steel, carburizing grade. The particular example illustrated is an M-2 type of such carburizing grade high speed steel, looking edgewise at a blank or a tool thereof, after the initial and strip preparatory steps 1 to of FIGS. 1 and 2, and after copper plating and carburizing in from the edge and thereafter heat treating for hardening in a single continuous one-temperature operation, in this case in salt at 2170 F and double-tempered at 1050 F. The picture at the top in FIG. 3A, viewing the sheet turned lengthwise, is taken at 75 magnifications. The three aligned pictures at the bottom in FIG. 3A are taken at the much higher magnification of 1500X. These portray characteristic points of the areas designated 1, 2 and 3 of the top picture of FIG. 3A, as indicated by the connecting lead lines and the corresponding legends Carburized area 1, Transition area 2 and Uncarbonized area 3 below the bottom pictures of FIG. 3A.

As related to the metal band saw of FIG. 3 the 75X enlarged view of FIG. 3A corresponds for example to that within the small dotted circle upon FIG. 3, where the arrow indicates the edgewise manner of viewing. Thus in the cross-zone direction, transversely of the metal band saw 10, the FIG. 3A upper picture includes the entirety of the working zone a, both the full hard and the transition areas thereof, and an integrally joined portion of the relatively soft backing zone b, having the flexibility as requisite for the particular application. The black margins at three sides of the top picture of FIG. 3A may be regarded as photographic background, being actually the Bakelite mounting material used for specimen positioning. The distinctive full hardened structure of the working zone a, as contrasted with that of the relatively softer but similarly heat treated backing zone b, is readily apparent in the 75 X picture in FIG. 3A while the consistency thereof as to carbon content is further apparent in the higher 1500 magnifications at the bottom in FIG. 3A.

From the foregoing it is apparent that our invention comprises the novel low-carbon carburizing grades of high speed steels, and strips and blanks and flexible-type edge-cutting tools formed thereof, together with the manufacturing processes as illustrated and described. An important characterizing feature of the invention is the production of a high speed tool steel strip, sheet, plate or bar presenting at least two zones of different carbon content such that the article when heat treated in a single hardening operation followed by appropriate tempering has a full hard working area and a softer backing area.

Such articles of the invention are plural zonar and differentiated as to final hardness of the zones but are structurally unitary.

The final lesser hardness of the backing area such as b is predeterminedly regulated by controlling the carbon content of the carburizing grade high speed steel at the time of melting, preferably as aided by the addition of nickel in the order of 0.50% to not over about 2.0%, as described with reference to FIG. 4 and as indicated by the dash-line optional step 2 of FIGS. 1 and 2. FIG. 4 also indicates that for an M-2 type of our carburizing grade of high speed steel it is desirable to maintain a minimum of approximately 0.30% carbon in the steel as melted, for avoidance of formation of intermetallic compounds, but that acceptable results in some instances have been had with as low as 0.25% carbon as indicated by the dotted portions at the lower ends of the graph curves.

Another advantage from the incorporation of nickel in the stated proportion in conjunction with our novel lowcarbon high-speed tool steel mixes for edge-type tools as here concerned is that it is found to retard the occurrence of appreciable quantities of ferrite. Since ferrite generally acts against obtaining maxirnum heat-treating hardness and may appear in appreciable quantity with reduction of carbon content in the mix, the nickel by restricting the appearance of ferrite enables a greater reduction in carbon content for our controlled low-carbon carburizing grade of highspeed tool steel consistent with attaining the objective leSs-than-full hardness after heat treating; in other words, the nickel serves the additional function of reducing that objectionable ferrite occurrence which the lowered carbon content of our invention might otherwise cause.

The full hardness of the working area such as zone a of FIGS. 3 and 3A is bad by selective edgewise carburization of this area, preferably by the closely controllable procedure as represented by the Leeds and Northrup Homocarb furnace with micro-carbon control, and as contrasted with procedures of the solid carburizer type or those with certain furnaces using endothermic gases. These latter, while available for practice of the invention have proven less satisfactory by reason of less close control.

Thus under the invention the hardness of each of the plural zones is predeterminedly controllable, and the hardness ratio between them also is controllable. Accordingly the invention makes possible, for example, the controlling of the ratio of back hardness to tooth hardness in a metal band saw. Hence under the adjustable ratio and good control of tooth hardening it is not necessary to have the back less hard than customary under the more complex and laborious plural-step heat treating and other procedures heretofore in use for that class of flexible tools.

The various attendant advantages under the invention are evident from the foregoing. Outstanding among them are that the lower carbon carburizing grade raw stock can be more readily hot and cold rolled. With addition of nickel and proper adjustments of carbon, cold reductions of 50% and over are practical between anneals, whereas conventional types of high speed steel possess only very limited cold workability and this is generally accompanied by edge checking with resultant high scrap losses. Final heat treated hardness of uncarburized areas can be adjusted to suit the particular application by varying the carbon content of the steel during melting. Full hardness of working edge (carburize'tl area) and desired hardness of backing (uncarburized area) can be accomplished by a single hardening operation. Present types of high speed steel are limited in their maximum carbon content largely by their hot workability. By carburization of the finished raw stock, the carbon content of the steel may be materially increased above these limits where desirable. In certain applications such as high speed metal band saws it is necessary to weld the ends of the band together after the saw is finished. The conventional type of high speed steel produces a hard brittle weld which requires extreme care in annealing to obtain reasonable ductility. In the lower-carbon backing of a preferentially carburized band of our carburizing grade high speed steel, with its reduced maximum hardness, welding technique is materially simplified.

Our invention both as to the methods and products is not limited to the exemplary embodiments or steps herein illustrated or described, and we set forth its scope in our following claim.

We claim:

In the manufacture of heat-hardened metal-cutting bandsaws, hack saws or the like having a toothed edge portion of a hardness in excess of 60 Rockwell C merging into a body portion of uniformly greater flexibility and toughness than said edge portion and of a uniformly less hardness than that of said edge portion, that method of improving uniformity in the extent of edgewise penetration of hardness in excess of said uniform lower body portion hardness which comprises increasing the carbon content of an edge portion only of a flat homogeneous alloy steel strip, having a carbon content of from about 0.30 to about 0.55% but otherwise having an alloy content corresponding to that of a high speed tool steel, into the range of, substantially uniformly through the thickness of the strip, from 0.70% to 1.50% while retaining the lower carbon content through the body portion of the strip, by submitting said strip to an edgewise carburization at said edge portion, slow cooling the carburized strip, forming teeth at said edge portion to a depth locating the tooth roots and gullet bottoms below the locus of carburization extending in from the outer terminus of said edge portion, and thereafter heat treating for hardness the entire toothed strip uniformly in a single heat treatment at a temperature in the high range of about 2150 F. to about 2250 F. and then References Cited by the Examiner UNITED STATES PATENTS 774,959 11/ 1904 Tresidder 148-12.1 907,167 12/ 1908 Neill. 1,537,381 5/ 1925 Strauss. 2,239,058 4/ 1941 Schlunipf -126 2,278,315 3/1942 Houdremont 75126 2,371,600 3/1945 Bartholomew 148-12.1 X 2,401,818 6/ 1946 Eckel 148'2 2,768,915 10/ 1956 Nachman 148-2 2,786,788 3/1957 Anderson 14821.55

OTHER REFERENCES Materials & Methods, June 1948, pp. 68-71.

Tool Steels, by Gill et al., published by the ASM, 1944, pages 494-501.

Tool Steels, page 415; edited by Gill et al.; published in 1944 by The American Society for Metals, Cleveland, Ohio.

DAVID L. RECK, Primary Examiner.

CLYDE C. LE ROY, RAY K. WINDHAM, NATHAN MARMELSTEIN, MARCUS U. LYONS, Examiners.