US2291943A - Process for producing high speed cutting tools - Google Patents

Description

Patented Aug; 4, 1942, c

" UNITED STATES PATEN Tet-m I raocass roalraonuoma' HIGH-slump v CUTTING rooLs j Lewis SQBei-gen, Chappaqua, and Janet Z. Briggs, j

New York, N." Iqassignors to Crucible. Steel 2 Company of America, New York,,N. Y., a cor- 'zporation of New Jersey Serial No. 389,884 I NoJDrawing. Application April'23, 1941,

[1 Claim. (c1. 1148 -21) This invention pertains 'to' improvementsfiin relatively low alloy, tungsten-and molybdenumcontaining, high speed cutting tools, characterized by high resistance to decarburization when subjectedto hardening and tempering treatments in the absence of a protective coating, such-as borax, etc., or a protective. iumace atmosphere,

'such as nitrogen, etc., and to methods of so hardening and tempering such tools as to inhibit or prevent decarburization.

This application is a continuation in part of application Serial No. 301,776, filed October 28, 1939.

"Of "the many molybdenum containing, speed analyses heretofore proposed: forv cutting tools, all of those which show performance ratingscomparable to the high: tungsten types-and hence are on a competitivebasis therewith-jas regards this factor,- are subject-to serious surface decarburization, inthe absence 'ofaspecial and expensive precautions, during the requisite heat treatmentsincident 'to conversion of vtheflsteel into finished tools ready for -use.'--, This decarbu'riiation; which results from exposure-tothe An object of the invention is to provide a cutting tool characterized as aforesaid and having cutting and other desirable properties of a high speed cutting tool which are commensurate of those of the vhigh alloy, high tungsten types containing little or no molybdenum.

' Based on long experience, cutting tools of the latter type, such as those made of the so-called "18-4 -1 and 18-4-2 steels, etc., containing about 18% tungsten, about 4% chromium and 1 to 2% or so of vanadium, have'assumed a commanding position in the high speed field ..due to combining such desirable qualities in service as high cutting performance, toughness,

long life, etc., with such other desirable quali-v ties in f abrication, 'as relative ease of heat treatment, resistancev to decarburization, etc. Such relatively expensivebecause of their high content ofjalloys, particularly of tungsten, which is not only'rather costly, butsometimes, as in case ofwar, difiicult or impossible to secure in commercial quantities; f

Accordingly, numerous proposals have been .made .tosubs'titute'the most closely related element, namely molybdenum, in'whole or in part for tungsten in high speed steels, for cutting tools, and more recently in conjunction theregwith to reduce the total alloy content. But tools -of such steels have'univers ally proved defective iin comparision with the molybdenum-free,-high tungsten types either as regards performance in service or as regards 'difliculties in fabrication 01 both, in consequence of which the high tungsten types have maintained their osition despite'their higher cost.

atmosphere'when usual heat treating procedures areemployed, may be overcome by excluding the atmosphere from" contact with the. metalwhile at elevated temperaturesi as for example ;by, a coating of 'borax or by heat treatin'g in' a, salt bath or in a controlled atmospherefurnace,,etc.

But the "added expense and delay: ofv such pro- 'cedures more thanofi'set the advantage in lower first cost which such-steels possess over the high tungsten types. --*Decarburization, on theother "hand; is productive of a sofi; outer-skin; on the metal which mustbe'cutor groundaway at I great expense -'before an acceptable cutting tool "is obtained. Moreover, i'nzsomerinstancesythis cannot be done owing to vthecomplicated shape of the finishedto'oLp 1 30 I tools have, however, the disadvantage of being Now we have discovered, in accordance, with a fundamental concept-"of our: invention; that toolsmade of molybdenurn'containing high speed steels can be made inherently stable and complet'ely resistant to decarburization during-normalh'eat treatment, provided the aggregatecons tent of tungsten and vanadium present in the steel is maintained sufiiciently high'in relation to the aggregate content of chromium and molyb- 4 denurn. And wehaveffurther determined that it this relationship is maintained for steel containmg chromium, tungsten, molybdenum and vanadium within the limits hereinafter specified, that i not only 'is the resulting product as easy to fabricateinto tfools asarethe high't'ungsten s'tand- 1.. ards, such as18 4;1, etc,'but' -that the perform- @ance of suchtools inservice is in general equally good or b etter More specifically awe have determined: that :substantially complete resistance to decarbur ization is imparted to" the tools'during normal]? heat treating practices,' if the ratio of tun gsten;g-,

or in a hot salt or lead bath, etc. The subsequent tempering is usually carried out by re- 7 I and vanadium to that of chromium and molybdenum is in excess of 0.77; that reasonably good stability is obained if this ratio lies within the range of about 0.55 to 0.77; and that the steel becomes increasingly susceptible to decarburization as the ratio is progressively decreased below 0.55. This information may be summarized as follows, designating the ratio in question as the stability ratio or more briefly S. R.:

(a) over 0.77: steel completely stable (b) =0.55 to 0.77: steel fairly stable under 0.55: steel unstable heating up to about 950 to 1150 F. for about one to three hours.

The performance of the tools in service, i. e. their cutting ability, etc., follows closely the ratios 5 above stated as applied particularl to analyses within the following limits:

In the above formulation, the percentages of W,

V, Cr and Mo refer to the percentages by weight of the total composition of the steel in which these constituents are present.

The relationships above summarized, which are based on actual tests, appear to rest on sound theoretical considerations. Tungsten-free molybdenumhigh speed steels are notoriously susceptible to decarburization when heat treated. This is due to the fact that the molybdenum-and chromium carbides present are attacked by oxygen at relatively low temperatures, whereas the tungsten and vanadium carbides are stable at much higher temperatures. Based on these considerations, it may be inferred that molybdenumand chromium containing high speed steels may be stabilized by additions of tungsten and vanadium, suiiicient to stabilize the carbides and prevent their attackby the atmosphere during heat treatment. Since other considerations than decarburization place rather definite restrictions on the vanadium content, the balance required to stabilize the steel may be effected by additions of tungsten.

As our tests have shown, it is not necessary that all of the carbon in the steel be present as tungsten and vanadium carbides, but only that a certain proportion be present in this form. Based on the test results presented below in Table II, it is found that if over 62% of the total carbon in the steel is present as carbides of tungsten and vanadium, that the steel will be stable; and that if this percentage lies between about 50 and 62%, the steel will not be completely resistant to decarburization under all conditions, but will be inmost cases. on the other hand, steel for which the percentage is below about 50%, will be subject to decarburization in all heat treatments unless the surface is protected against atmospheric attack. H

By normal heat treatment for tools in accordance with our invention, is meant hardening and tempering treatments in which the tools are first heated above the critical,v and at about 2100 to 2350 F., preferably at about 2175 to 2275 F., for an interval sumcient to austenitizeand to produce sumcient carbide solution to give cutting performances comparable to those of the high tungsten steels, such as 18-4-1. Ordinarily, a relatively short time of heating at the aforesaid temperatures will suflice, i. e. a matter of minutes, for example about one to five minutes in the usual case, although heating at ,such temperatures for periods as high as one or two hours is permissible in some instances, following which the steel is rapidly cooled or quenched to harden, for example by air cooling: or quenchingin oil,

steels for which the stability ratio is above 0.77 show up in lathe tool tests from approximately equal to as much as 40% better than the 18-4-1 steel, depending on the analysis; that tools made 20"of 'the'steelsfor"whichthis' ratio is within the range of 0.55 to 0.77 are in general somewhat below the 18-4-1 in this respect; and that the lathe tool rating is considerably lower for steels having a stability ratio below 0.55, as evidenced by the data presentedbelow in Table II for steels of the analyses given in Table I. Within the above limits, particularly good steels are found within the following preferred limits for the al- ,loying constituents:

Tungsten About 3.5

- g Per cent Carbon About 0.8 to 1.0 Chromium "About 3.0 to 4.5 Molybdenum "About 2.5 to 6.0 to 7.0 Vanadium About 1.0 to 3.0

.' Table 1 Analysis Steel Mo. W

Table II Decarburization after thte abravemen lone Stab lity n orm 81 heat Lathe tool Steel treatment mung W+% V 18-4-1- Mo 18-4-1-100 Measured File depth of test 1. 28 None A 1. 17 None A 1. 13 None A 1. 12 None A i. 06 None A l. 05 None A 1. 05 None A 93 1.00 None A 99 0. 99 None A 110 O. 94 None A 116 0. 91 None A 100 0.91 None A 100 0.89 N one A 0. 88 None A 91 0. 85 None A 139 0. 84 None A 133 0. 83 None 13 139 0.82 None A 111 0. 82 None B 0. 82 None A 0. 78 None B 116 0. 77 None B 0. 76 None 105 0. 76 None A 126 0. 72 None B 0. 71 None A 73 0. 70 None A 78 0. 70 None B 82 0.65 None B 0. 65 G 78 0. 64 B 20 0. 57 B 0. 55 B 0. 52 C 0. 51 C 0. 46 B 0. 43 D 0. 37 0 0. 36 C O. 35 0. 27 0. 27 0. l9 0. 14

In the lathe tool ratings of the above Table II, the value of the tool is measured by the elapsed time of cutting before failure under conditions where speed, depth of cut and rate of feed are maintained constant. For comparative purposes, cutting tests were made on a heat treated chromium-nickel steel log having a Brinell hardness of 220-240, employing cutting tools made of each of the steels for which lathe tool ratings are given in Table II,'and also with tools made of the 18-4-1 steel. The ratings given in the last column of Table II are expressed on a comparative basis in terms of a rating of 100' for the 18-4-1 steel. The file test ratings given in Table II were of the usual type, in'which the rating A represents steels on which the file made no impress when drawn across them; rating B is accorded to steels on which the file had an extremely slight drag; rating C those for which the file had a definite bite into the steel; and rating D, steels for which the file had apronounced bite representing a soft outer skin.

In general, the balance of the alloy, aside from those above specified in the proportions stated, will consist substantially of iron except for the usual tolerances for impurities and such elements unavoidably present in small amounts, as manganese, silicon, etc. Silicon in excess of about 0.5% is found to be detrimental to the cutting properties of the steel, and hence is preferably held under this figure.

The alloy is preferably made by melting the constituents in the usual manner'in the electric or crucible furnace, casting into ingots, and hot working the ingots by forging, rolling, etc. The steel is then made into cutting tools in the usual way, and the tools heat treated in the manner above described.

We claim:

A process for minimizing surface decarburiz'ation of molybdenum high speed steel during heating for hot working, hardening and the like, which comprises: forming the steel of an alloy containing about 0.7 to under 1.25% carbon, about 3 to 8% chromium, about 2.5 to 7% molybdenum, about 3 to 10% tungsten, about 1 to 6% vanadium, under-0.5% silicon, and the balance substantially all iron, in which the ratio of tungsten plus vanadium to molybdenum plus chromium exceeds 0.77, and subjecting the steel to ele-' vated temperature for hot working, hardening and related treatments, in the absence of special precautions including the use of protective fluxes,

controlled furnace atmospheres "and the like, for

protecting the steel against surface decarburization. LEWIS S. BERGEN.

JANET z. BRIGGS.