sign in sign up
<<
Home , Alloy steel, Tool bit, Tool steel

Copyright © 1989 ASM International® ASM Handbook, Volume 16: All rights reserved. ASM Handbook Committee, p 51-59 www.asminternational.org

High.Speed

Alan M. Bayer and Bruce A. Becherer, Teledyne Vasco

HIGH-SPEED TOOL STEELS and their Table 1 Requirements for Table 2 Significant dates in the requirements are defined by The American high-speed tool steels per ASTM development of high-speed tool Society for Testing and Materials in Speci- A 600 steels fication A600-79 as follows: High-speed tool Requirement Standard Intermediate Date Development steels are so named primarily because of 1903 ...... 0.70% C, 14% W, 4% Cr prototype of their ability to machine materials at high Chemical requirements modem high-speed tool steels speeds. They are complex -base Minimum content by major 1904 ...... 0.30% V addition alloys of , , , mo- elements 1906 ...... Introduction of electric furnace lybdenum, or , or combinations Carbon ...... 0.65 0.70 melting thereof, and in some cases substantial Chromium ...... 3.50 3.25 1910 ...... Introduction of first 18-4-1 Vanadium ...... 0.80 0.80 composition (AISI Tl) amounts of . The carbon and alloy Tungsten + 1.8% ... 11.75 6.50 1912 ...... 3 to 5% Co addition for improved hot contents are balanced at levels to give high Minimum total alloy content based hardness attainable hardening response, high wear on tungsten equivalents (t/3 Cr 1923 ...... 12% Co addition for increased cutting resistance, high resistance to the softening + 6.2 V + W+ 1.8 Mo) speeds Grades containing less than 5% 1939 ...... Introduction of high-carbon effect of heat, and good toughness for effec- cobalt ...... 22.50 13.00 high-vanadium super high-speed tive use in industrial cutting operations. Grades containing 5% or more tool steels (M4 and Tl5) Commercial practice has developed two cobalt ...... 21.00 12.00 1940-1952 ...... Increasing substitution of molybdenum for tungsten groups of cutting materials: Hardening response requirements 1953 ...... Introduction of sulfurized • The recognized standard high-speed tool Ability to be austenitized, and free-machining high-speed tool tempered at a temperature not 1961 ...... Introduction of high-, which serves almost all applica- less than 510 °C (950 °F) with a high-cobalt super hard high-speed tions under mild to severe -cutting fine-grain structure (Snyder-Graff tool steels (M40 series) conditions grain size 8 min) to ...... 63 HRC 62 HRC 1970 ...... Introduction of powdered metal • A smaller group of intermediate steels, high-speed tool steels 1973 ...... Addition of higher /nitrogen which are satisfactory for limited applica- content to M-7 to increase hardness tions under mild to moderate metal-cut- their primary alloying elements and an M 1980 ...... Development of cobalt-free super ting conditions high-speed tool steels for those steels that have molybdenum ad- 1982 ...... Introduction of aluminum-modified The minimum requirements that must be ditions as one of their primary alloying high-speed tool steels for cutting met to be classed as a standard high-speed elements. In addition, there is a number that , and those for an intermediate follows either the M or the T. Thus, there high-speed tool steel, are listed in Table I. are high-speed tool steels designated M l, To be acceptable for either group, an alloy M2, M41, T l, T I5, and so on. That number is increased, the carbon content is usually must meet all of the requirements shown for does not have any special significance other increased (Ref l). that group. than to distinguish one from another. For The tungsten type T l does not contain A chronology of some of the significant example, M I does not mean that it is more molybdenum or cobalt. Cobalt-base tung- developments in high-speed tool steels is highly alloyed than M2 or has greater hard- sten types range from T4 through T l5 and given in Table 2. The research work in 1903 enability or poorer wear resistance, and so contain various amounts of cobalt. on a 14% tungsten alloy led to the develop- on. It merely separates the types and at- Molybdenum types MI through MI0 (ex- ment of the first high-speed tool steel, tempts to simplify selection for the user. cept M6) contain no cobalt, but most con- which is now designated Tl. Table 3 lists the nominal analyses of the tain some tungsten. The cobalt-base, mo- common M and T types. lybdenum-tungsten, premium types are M and T Classification generally classified in the M30 and M40 Effect of Alloying Elements series. Super high-speed steels normally There are presently more than 40 individ- range from M40 upward; they are capable of ual classifications of high-speed tool steels, The T series contains 12 to 20% tungsten, being heat treated to high hardnesses. according to the American Iron and Steel with chromium, vanadium, and cobalt as The M series steels generally have higher Institute (AISI). When these are compound- the other major alloying elements. The M abrasion resistance than the T series steels ed by the number of domestic manufactur- series contains approximately 3.5 to 10% and less distortion in heat treatment; also, ers, the total number of individual steels in molybdenum, with chromium, vanadium, they are less expensive (Ref 2). Tools made the high-speed tool steel category exceeds tungsten, and cobalt as the other alloying of high-speed tool steel can also be coated 150. The AISI established a classification elements. All types, whether molybdenum with nitride, titanium , and system for the high-speed tool steels many or tungsten, contain about 4% chromium; numerous other coatings by physical vapor years ago. That system consists of a T for the carbon and vanadium contents vary. As deposition technique for improved perfor- those steels that have tungsten as one of a general rule, when the vanadium content mance and increased tool life. 52 / Materials

Table 3 Composition of high-speed tool steels Molybdenum forms the same double car-

AIS! type UNS designation C Si Cr V W Mo Co bide with iron and carbon as tungsten does but has half the atomic weight of tungsten. Molybdenum high-speed tool steels As a consequence, molybdenum can be

MI ...... T11301 0.83 0.35 3.75 1.18 1.75 8.70 • • • substituted for tungsten on the basis of M2 approximately one part of molybdenum, by Regular C ...... T11302 0.83 0.33 4.13 1.98 6.13 5.00 • • • weight, for two parts of tungsten. High C ...... • • • !.00 0.33 4.13 1.98 6.13 5.00 • • • The melting point of the molybdenum M3 Class I ...... TII313 1.05 0.33 4.13 2.50 5.88 5.63 .. • steels is somewhat lower than that of the Class 2 ...... T11323 1.20 0.33 4.13 3.00 5.88 5.63 • • • tungsten grades, and thus they require a M4 ...... T11304 1.33 0.33 4.25 4.13 5.88 4.88 • • • lower hardening temperature and have a M6 ...... T11306 0.80 0.33 4.13 1.50 4.25 5.00 12.00 narrower hardening range. The M-type M7 ...... T11307 1.01 0.38 3.75 2.00 1.75 8.70 . • • MI0 high-speed tool steels are tougher than the Regular C ...... TII310 0.89 0.33 4.13 2.00 • • • 8.13 • • • T-type high-speed tool steels, but the hot High C ...... • • • 1.00 0.33 4.13 2.00 • • • 8.13 • • • hardness is slightly lower. Compensation MI5 ...... T11315 1.50 0.33 4.00 5.00 6.50 3.50 5.00 for this reduced hot hardness is partially M30 ...... T11330 0.80 0.33 4.00 1.25 2.00 8.00 5.00 M33 ...... T11333 0.89 0.33 3.75 1.18 1.70 9.50 8.25 accomplished by the addition of tungsten M34 ...... T11334 0.89 0.33 3.75 2.10 1.75 8.48 8.25 and, to a lesser extent, vanadium to the M35 ...... T11335 0.80 0.33 4.00 2.00 6.00 5.00 5.00 plain molybdenum grades. This is one im- M36 ...... T11336 0.85 0.33 4.13 2.00 6.t)0 5.00 8.25 portant reason for the popularity of the M41 ...... T11341 1.10 0.33 4.13 2.00 6.63 3.75 8.25 M42 ...... T11342 I. 10 0.40 3.88 I. 15 1.50 9.50 8.25 tungsten-molybdenum grades (like M2, M3, M46 ...... T11346 1.26 0.53 3.95 3.15 2.05 8.25 8.30 M4): they afford good hot hardness, which M48 ...... T11348 1.50 0.33 3.88 3.00 10.00 5.13 9.00 is so desirable in high-speed tool steels. M50(a) ...... T11350 0.80 0.40 4.13 1.00 • • • 4.25 • • • Vanadium was first added to high-speed M52(a) ...... T11352 0.90 0.40 4.00 1.93 1.25 4.45 •. • M62 ...... T11362 1.30 0.28 3.88 2.00 6.25 10.50 • • • tool steels as a scavenger to remove slag impurities and to reduce nitrogen levels in the Tungsten high-speed tool steels melting operation, but it was soon found that TI ...... T12001 0.73 0.30 4.13 I. 10 18.00 ...... the element materially increased the cutting T4 ...... T!2004 0.75 0.30 4.13 1.00 18.25 0.70 5.00 T5 ...... TI2005 0.80 0.30 4.38 2.10 18.25 0.88 8.25 efficiency of tools. The addition of vanadium T6 ...... TI2006 0.80 0.30 4.38 1.80 19.75 0.70 12.00 promotes the formation of very hard, stable T8 ...... TI2008 0.80 0.30 4.13 2.10 14.00 0.70 5.00 , which significantly increase wear TI5 ...... TI2015 1.55 0.28 4.38 4.88 12.38 1.00 5.00 resistance and, to a lesser extent, hot hard- (a) Intermediate high-speed tool steel ness. An increase in vanadium, when proper- ly balanced by carbon additions, has relative- ly little effect on the toughness. For this Various elements are added to M and T . Generally, manganese is not reason, vanadium-beating grades are a very series high-speed tool steels to impart certain high in concentration in high-speed tool good choice when very fast cutting operations properties to the tool steels. These elements steels. This is because of its marked effect are demanded, as in finishing cuts, or when and their effects are discussed below. in increasing brittleness and the danger of the surface of the material is hard and scaly. Carbon is by far the most important of cracking upon . The special characteristics of the high-speed the elements and is very closely controlled. has no effect on any of the tool steels that are due to high vanadium While the carbon content of any one high- desired properties of high-speed tool steels, additions have given rise to several specially speed tool steel is usually fixed within nar- but because of its well-known effect in developed steels for very severe service re- row limits, variations within these limits can causing cold shortness, or room-tempera- quiting high toughness as well as exceptional cause important changes in the mechanical ture brittleness, the concentration of phos- hot hardness and wear resistance. The T I5, properties and the cutting ability. As the phorus is kept to a minimum. M4, and MI5 grades are in this category; their carbon concentration is increased, the Chromium is always present in high- vanadium content is 4.88, 4.13, and 5.0(0, working hardness also rises; the elevated speed tool steels in amounts ranging from 3 respectively. temperature hardness is higher; and the to 5% and is mainly responsible for the Cobalt. The main effect of cobalt in high- number of hard, stable, complex carbides . Generally, the addition is 4% speed tool steel is to increase the hot hard- increases. The latter contribute much to the because it appears that this concentration ness and thus to increase the cutting effi- wear resistance and other properties of the gives the best compromise between hard- ciency when high tool temperatures are high-speed tool steels. ness and toughness. In addition, chromium attained during the cutting operation. Co- Silicon. The influence of silicon on high- reduces oxidation and scaling during heat balt raises the heat-treating temperatures speed tool steel, up to about 1.0(O, is slight. treatment. because it elevates the melting point. Hard- Increasing the silicon content from 0.15 to Tungsten. In the high-speed tool steels, ening temperatures for cobalt high-speed 0.45% gives a slight increase in maximum tungsten is of vital importance. It is found in tool steels can be 14 to 28 °C (25 to 50 °F) attainable tempered hardness and has some all T-type steels and in all but two of the higher than would be normal for similar influence on carbide morphology, although M-type steels. The complex carbide of iron, grades without cobalt. Cobalt additions there seems to be a concurrent slight decrease tungsten, and carbon that is found in high- slightly increase the brittleness of high- in toughness. Some manufacturers produce at speed tool steels is very hard and signifi- speed tool steels. least one grade with silicon up to 0.65%, but cantly contributes to wear resistance. Tung- Cobalt steels are especially effective on this level of silicon content requires a lower sten improves hot hardness, causes rough or hogging cuts, but they are not maximum austenitizing temperature than secondary hardening, and imparts marked usually suited to finishing cuts that do not does a lower silicon level in the same grade, if resistance to . When the tungsten involve high temperatures. Cobalt types overheating is to be avoided. In general, how- concentration is lowered in high-speed tool usually perform quite well when cutting ever, the silicon content is kept below 0.45% steels, molybdenum is usually added to materials that have discontinuous chips on most grades. make up for its loss. such as or nonferrous . High-Speed Tool Steels / 53

The necessity of using deep cuts and fast hardened high-speed tool steel. By partially metal, or various other types of tests to speeds or of cutting hard and scaly mate- dissolving during heat treatment, these car- indicate a relative rating rials justifies the use of cobalt high-speed bides provide the matrix of the steel with • Toughness: Ability to absorb (impact) tool steels. the necessary alloy and carbon content for energy Sulfur, in normal concentrations of 0.03% hardenability, hot hardness, and resistance or less, has no effect on the properties of to tempering. The relative importance of these proper- high-speed tool steels. However, sulfur is While all high-speed tool steels have many ties varies with every application. High added to certain high-speed tool steels to similar mechanical and physical characteris- machining speeds require a composition contribute free-machining qualities, as it tics, the properties may vary widely due to with a high initial hardness and a maximum does in low-alloy steels. The amount of changes in chemical composition. Basically, resistance to softening at high tempera- free-machining high-speed tool steels is a the most important property of a high-speed tures. Certain materials may abrade the small but significant percentage of the total tool steel is its cutting ability. Cutting ability cutting edge of the tool excessively; hence, consumption of high-speed tool steels. One depends on a combination of properties, the the wear resistance of the tool material may of the major areas for free-machining high- four most important of these being: well be more important than its resistance speed tool steels is in larger-diameter tools to high cutting temperatures. such as hobs, broaches, and so on. • Hardness: Resistance to penetration by Hardness is necessary for cutting harder Sulfur forms complex sulfides, containing diamond-hard indenter, measured at materials and generally gives increased tool chromium, vanadium, and manganese, room temperature life, but it must be balanced against the which are distributed throughout the steel • Hot hardness: The ability to retain high toughness required for the application. as stringer-type inclusions. The stringers hardness at elevated temperatures The desired combination of properties in interrupt the steel structure and act as • Wear resistance: Resistance to abrasion, a high-speed tool steel may be obtained, notches, which aid the metal-removing ac- often measured by grindability, metal-to- first, by selection of the proper grade and, tion of a cutting tool when machining the high-speed steel, because the resulting chip is discontinuous, a characteristic of free- machining steels. Very high sulfur additions Temperature,°F (up to 0.30%) are made to some powder 0 200 400 600 800 1000 1200 1400 1600 (P/M) high-speed tool steels for 1000 69 improved machinability/grindability by globular sulfides rather than string- ers (see the article "P/M High-Speed Tool 900 67 Steels" in this Volume). Nitrogen is generally present in air- melted high-speed tool steel in amounts varying from approximately 0.02 to 0.03%. 800 64 The nitrogen content of some high-speed tool steels is deliberately increased to about - 0.04 to 0.05%, and this addition, when com- bined with higher than usual amounts of 700 60 silicon, results in a slight increase of maxi- mum attainable tempered hardness and some change of carbide morphology. 600 55

Noncobalt- ":~i!iiiill/ili~~ O 0ase*pe \ or" Properties of "1- High.Speed Tool Steels 500 49 u~ t- High-speed tool steels, regardless of "El whether they are an AISI M-type or T-type, "r have a rather striking similarity in their 400 physical makeup:

• They all possess a high-alloy content • They usually contain sufficient carbon to 300 30 permit hardening to 64 HRC • They harden so deeply that almost any section encountered commercially will have a uniform hardness from center to 200 surface • They are all hardened at high tempera- tures, and their rate of transformation is 100 such that small sections can be cooled in still air and be near maximum hardness

All high-speed tool steels possess excess 0 carbide particles, which in the annealed -18 93 204 316 427 538 649 760 871 state contain a high proportion of the alloy- Temperature, oC ing elements. These carbide particles con- Comparison of the hot hardness of cobalt-base (M33, M36, M4, and T15) type versus noncobalt-base tribute materially to the wear resistance of Fig. 1 (M1, M2, MA, M7, and T1)type high-speed tool steels 54 / Cutting Tool Materials

22 1 1 Hot Hardness. A related and important AISI Austenitizing component of cutting ability is hot hardness. 20 type temperature, ° C It is simply the ability to retain hardness at T15 1218 elevated temperatures. This property is im- M4 1210 f portant because room temperature hardness M3 1204 ._ values are not the same values that exist at T1 1279 J M10 1204 the elevated temperature produced by friction M2 1210 J between the tool and . 16 M1 1190 M7 1190 Hot hardness values of some repre- M42 1176 sentative grades are plotted in Fig. 1. It is 14 noteworthy that the cobalt-base types as a 7...... ~M2 group exhibit higher hot hardness than non- a~ 12 cobalt-base types. For a comparison of the -~~- / M7 hot hardness of other metals, alloys, car- bides, and ceramics to that of high-speed ~ ~o tool steels, see Fig. 1 in the article "Cast Cobalt Alloys" in this Volume. Wear Resistance. The third component of cutting ability is resistance to wear. Wear M3 resistance of high-speed tool steels is affect- ed by the matrix hardness and composition, by precipitated MeC and MC carbides re- 4 L I sponsible for secondary hardness, by the volume of excess alloy carbides, and by the nature of these excess carbides. In practi- ..._.~..._._._...~._._...... _~.~...,-~ T15 cally any given high-speed tool steels, wear resistance strongly depends on hardness of 0 68 66 64 62 60 58 56 54 the steel, and higher hardness, however achieved, is an aim when highly Hardness, HRC cutting conditions are encountered (Fig. 2). Fig. 2 Effect of hardness on wear rate for high-speed tool steels, each having been double tempered to the For the ultimate in wear resistance, car- indicated hardness bon content has been increased simultane- ously with vanadium content, to permit the introduction of a greater quantity of total carbide and a greater percentage of ex- tremely hard vanadium carbide in high- speed tool steel. Examples of this effect are given when discussing the effect of vanadi- um on the properties of high-speed tool steels. Steels TI5, M3 (class 2), M4, and 12 "1-- M I5 are in this category, and all have extremely high wear resistance. Laboratory tests for wear resistance are c ~0 diverse, making comparisons between dif- ferent procedures difficult. Therefore, pro- duction tests on actual tools are used to a ~ 8 ~ great extent. However, laboratory tests can produce valuable data on the relative wear resistance for these steels (Fig. 3). The data -~ 6 p ...... rr given in Fig. 3 were generated by measuring i the volume loss of a high-speed tool steel sample against the volume loss of a known 4 vitrified abrasive wheel after a predeter- mined grinding procedure. Toughness. The fourth component of cut- 2 ting ability mentioned above is toughness, I : F] ~ which is defined as a combination of two factors: 0 !ii! T15 M15 M4 M3 T5 M42 T1 M7 M10 M2 M1 • The ability to deform before breaking (ductility) Fig. 3 Comparison of relative abrasion resistance at typical working hardness for high-speed tool steels • The ability to resist permanent deforma- tion (elastic strength) second, by the proper heat treatment, two tance check of a heat-treated tool. All high- If either of these factors is to be used to equally important decisions. speed tool steels can be hardened to room describe toughness (a practice not to be Hardness. Hardness is the most com- temperature hardness of 64 HRC, while the condoned), the second appears more prac- monly stipulated requirement of a high- M40 series, some of the M30 series, and T 15 tical for high-steel tool steel because rarely speed tool steel and is used as an accep- can reach nearly 69 HRC. are large degrees of flow or deformation High-Speed Tool Steels / 55

60 Heat Treatment of \ "C High.Speed Tool Steels ..... 50 Proper heat treatment is as critical to the success of the cutting tool as material selec- . tion itself. Often the highest-quality steel made into the most precise tools does not 54 40 "fi perform because of improper heat treatment. O c- The object of the or harden-

O , ing operation is to transform a fully an- E 41 a0 ~ nealed high-speed tool steel consisting

O mainly of ferrite (iron) and alloy carbides M7 ~, ._ M42 into a hardened and tempered martensitic O o t- structure having carbides that provide the o "~ 27 2o ~t- cutting tool properties (see Fig. 6 and 7). X O c- T15 The heat treatment process can be divid-

r- ed into four primary areas, preheating, aus- tenitizing, quenching, and tempering. Fig- ure 8 outlines graphically these four heat treatment steps. Preheating. From a metallurgical stand- point, preheating plays no part in the hard- 0 0 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 ening reaction; however, it performs three important functions. The first of these is to Hardness, HRC reduce thermal shock, which always results Fig, 4 Plot of impact toughness versus hardness for high-speed tool steels when a cold tool is placed into a warm or hot furnace. Minimizing thermal shock re- duces the danger of excessive distortion or 50 cracking. It also relieves some of the stress- es developed during machining and/or form- 68I ing, although conventional stress relieving is more effective.

54 40 ¢-~ The second major benefit of preheating is to increase equipment productivity by de- creasing the amount of time required in the ¢- ¢- high-heat furnace. Thirdly, if the high-heat t-- o 41 furnace is not neutral to the surface of the tool 30 ::3 0 or part, preheating will reduce the amount of ._> 4-, carburization and decarburization that would

"O result if no preheat were employed. In com- O mercial salt bath hardening, a two-step pre- ~ 27 2o ~ N heat is typically used for high-speed tool e- steels. The fh'st preheat is carried out be- c'- 0 C O tween 650 and 760 °C (1200 and 1400 °F); the C second preheat cycle is carried out between 14 - -10 ~ 815 and 900 °C (1500 and 1650 °F). In atmo- sphere or vacuum heat treating, the furnace is usually heated slowly to a single preheat of 790 to 845 °C (1450 to 1550 °F). Preheat duration is of little importance as long as the M2 M1 M4 M3 T1 M7 M10 M42 T5 T15 M15 part is heated throughout its cross section. Austenitizing (hardening) is the second Fig. 5 Relative toughness of high-speed tool steels at typical working hardness step of the heat treatment operation. Aus- tenitizing is a time/temperature dependent permissible with fine-edge tools. The first, greater value when toughness is considered reaction. High-speed tool steels depend however, cannot be ignored, as frequently than when hardness is in question. Labora- upon the dissolving of various complex al- the stress applied to a tool (through over- tory tests for the measurement of toughness loy carbides during austenitizing to develop loads, shock, notches, and sharp corners) of hardened high-speed tool steel include their properties. These alloy carbides do not exceeds the elastic strength. bend, unnotched or C-notch impact, static dissolve to any appreciable extent unless Toughness tests on high-steel tool steel torsion, and torsion impact tests. Figures 4 the steel is heated to a temperature within are usually conducted at room temperature. and 5 compare relative unnotched impact 28 to 56 °C (50 to 100 °F) of their melting Tool failures that occur from spalling of the values for representative high-speed tool point. This temperature is dependent upon tool edge generally occur during the initial steels. Modest improvements in toughness the particular high-speed tool steel being contact of the tool with the work, and once (within a grade) can be made by lowering treated and is in the range of 1150 to 1290 °C the tool becomes heated, its performance in the tempered hardness. Lower austenitizing (2100 to 2350 °F). The generally recom- this respect is much superior. Therefore, temperatures enhance the toughness for a mended hold time for high-speed tool steel room temperature tests are perhaps of given hardness and grade. is approximately 2 to 6 min, depending upon 56 / Cutting Tool Materials

nching_ Critical temperature _ transformation

-,u~"'- 6 Microstructure of fully annealed high-speed tool steel consisting of ferrite (iron) and alloy Preheating ~ ', i / carbides. 1000 x L~ ,~',.' First )-~, ,'~".~.!;~-~Second. ,l"-'2:additional~'-". \ ,,7..,,.'.~,'~.tem.per:~'.,.~.,,:: ,temper.;.-~,. /~.~-;;~ tempers ?;'/_~,

Time

Fig. 8 Time versus temperature plot illustrating sequences required to properly heat treat high-speed tool steels

~iiiiiiiiii¸~!!I iii~ ~ Tempering. Following austenitizing and ing step to fresh . Some precipita- .....!~ i~ ¸ i~ ~ quenching, the steel is in a highly stressed tion of complex carbide also occurs, further state and therefore is very susceptible to enhancing secondary hardness. It is this pro- cracking. Tempering (or drawing) increases cess of transforming retained and :!! the toughness of the steel and also provides tempering of newly formed martensite that secondary hardness, as illustrated by the dictates a multiple tempering procedure. peak on the fight of the tempering curve in High-speed tool steels require 2 to 4 tempers Fig. lO. Tempering involves reheating the at a soak time of 2 to 4 h each. As with steel to an intermediate temperature range austenitizing temperatures and quenching (always below the critical transformation rates, the number of tempers is dictated by temperature), soaking, and air cooling. the specific grade. High-speed tool steels "'lll~l~° 7 Microstructure of hardened, tempered high- Tempering serves to stress relieve and to speed tool steel having martensitic structure should be multiple tempered at 540 °C (1000 with carbides. 1000 × transform retained austenite from the quench- °F) minimum for most grades. It is essential to

815 high-speed tool steel type, tool configura- 1 1500 tion, and cross-sectional size. 5HRC Lowering the hardening temperature (un- +C derhardening) generally improves the impact 705 1 1300 toughness while lowering the hot hardness. Raising the hardening temperature increases heat-treated room-temperature hardness and 595 A+F+C 1100 also increases the hot hardness. %%4"~ ( formation) Quenching. The quenching or cooling of \ O 11 the workpiece from the austenitizing tem- \ \ o o 480 9OO ~- perature is designed to transform the austen- ite that forms at the high temperature to a t13 hard martensitic structure. The rate of cool- c~ \ \ E 370 700 E ing, which must be controlled, is dictated by I-- the analysis of the particular steel. Some- / times high-speed steels are two-step / A+F+C quenched, initially in a molten salt bath 260 ( formation) . 500 maintained at approximately 540 to 595 °C (1000 to 1100 °F) or an oil quench, followed Beginning of martensite N,-.... 15% \ by air cooling to near ambient temperature. 150 300 The least drastic form of quenching is cool- 50% ing in air, although only in the smaller and/ 90% or thinner cross sections would high-speed 40 100 tool steels air quench rapidly enough to 1 60 60 60 transform the majority of the structure into the desirable martensitic condition. The austenite-martensite transformation is ex- Time emplified in Fig. 9 illustrating a time-tem- Fi g. 9 Time-temperature-transformation diagram for M2 high-speed tool steel that was annealed prior to quenching. perature-transformation curve. Austenitizing temperature was 1230 °C (2250 °F), and critical temperature was 830 °C (1530 °F). High-Speed Tool Steels / 57

Tempering temperature, °F atmosphere at approximately 540 °C (1000 Table 4. The increased production obtained 300 450 600 750 900 1050 1200 °F). The black oxide surface has little or no with a coated tool justifies the application of 70 effect on hardness, but serves as a partial the coating despite the resulting 20 to 30% 0n" 65 I barrier to galling of similar ferrous metals. increase in the base price of the tool (Ref 3). T ~~~ The surface texture also permits retention Coated tools can meet close-tolerance ~- 60 - t of lubricant. requirements and significantly improve the ,-° 55 Low side side of -~ Nitride Finish. is a method of machining of carbon and alloy steels, stain- of curve c u rye introducing nitrogen to the surface of high- less steels (especially the 300 series, where -r 50 speed tool steels at a typical temperature of galling can be a problem), and aluminum 480 to 595 °C (900 to 1100 °F) and is accom- alloys (especially aircraft grades). Coated 150 230 315 400 480 565 650 plished either by the dissociation of ammo- high-speed tool steels are less of a factor in Tempering temperature, °C nia gas, exposure to sodium cyanide salt the machining of certain titanium alloys and mixtures, or bombardment with nitrogen some high- alloys because of chemical -,t~t:"". I 0 Tempering curve for M2 high-speed tool steel. To optimize the transformation of ions in order to liberate nascent nitrogen, reactions between the coatings and the retained austenite to fresh martensite during the temper- which combines with the steel to form a workpiece materials (Ref 3). ing sequence, the high (right) side of the secondary hard iron nitride. Nitriding improves wear hardness peak curve is preferred, and the low (left) side resistance of high-speed steel, at the ex- should be avoided. High.Speed Tool Steel pense of notch toughness. Applications Coated High-Speed Tool Steels. The favor the right (high) side of the secondary addition of wear-resistant coatings to high- High-speed tool steels are used for most hardness peak of the tempering curve in order speed tool steel cutting tools lagged behind of the common types of cutting tools includ- to optimize the above-described transforma- the coming of carbide inserts by approximate- ing single-point tools, , , tions. ly 10 years until the development of the taps, cutters, end mills, hobs, , Subzero treatments are sometimes used low-temperature physical vapor deposition and broaches. in conjunction with tempering in order to (PVD) process, an innovation, which is much continue the transformation of austenite to more suitable for coming high-speed tool Single-Point Cutting Tools martensite. Numerous tests have been run steels than is the older chemical vapor depo- The simplest cutting tools are single-point on the effect of cold treatments, and the sition (CVD) process, and which also elimi- cutting tools, which are often referred to as findings generally prove that cold treat- nates the need for subsequent heat treatment tool bits, lathe tools, cutoff tools, or inserts. ments used after quenching and first temper (Ref 3). As described in Ref 4, titanium nitride They have only one cutting surface or edge in enhance the transformation to martensite, is the most commonly used and most durable contact with the work material at any given in much the same way that multiple temper- coating available, although substitutes such time. Such tools are used for , thread- ing causes transformation. Cold treatments as other nitrides (hafnium nitride and zirconi- ing, , planing, or shaping, and most are administered to high-speed tool steels im- um nitride) and carbides (, mounted in a toolholder that is made of some mediately after quenching can result in carbide, and hafnium carbide) are type of tough . The performance of cracking or distortion because the accom- being developed. These other comings are such tools is dependent on the tool material as panying size change is not accommodated expected to equal or surpass the desired prop- well as factors such as the material being cut, by the newly formed, brittle martensite. It is erties of titanium nitrides in future years. The the , the , and generally accepted that subzero treatments hard thin (2 to 5 Ixm, or 80 to 200 ixin. thick) fixturing. Following is a discussion of materi- are not necessary if the steel is properly deposit of high-density titanium nitride, al characteristics and recommendations for hardened and tempered. which has 2500 HV hardness and imparts a the most popular lathe tools. characteristic gold color to high-speed tool M l, M2, and T I are suitable for all- Surface Treatments steels, provides excellent wear resistance, purpose tool bits. They offer excellent minimizes heat buildup, and prevents strength and toughness and are suitable for Tools made of high-speed tool steel are of the workpiece material, while improving both roughing and finishing and can be used available with either a bright, black oxide or the surface finish of high-speed tool steels for machining wrought steel, cast steel, cast nitride finish or they can be coated with (Ref 5). iron, brass, bronze, , aluminum, and titanium nitride and other coatings using a The initial use, in 1980, of titanium nitride so on (see the Section "Machining of Spe- vapor disposition process that greatly in- coatings was to coat gear cutting tools. Sub- cific Metals and Alloys" in this Volume). creases tool life. sequent applications include the coming of These are good economical grades for gen- Bright Finish. Most tools are finished both single-point and multipoint tools such as eral shop purposes. with a ground or mechanically polished lathe tools, drills, reamers, taps, milling cut- M3 class 2 and M4 high-speed tool steels surface that would be categorized as a ters, end mills, and broaches (Ref 3). Today, have high-carbon and high-vanadium con- bright finish. Bright finished tools are often titanium nitride coated hobs and tents. The wear resistance is several times preferred to tools with an oxide finish for dominate high-production applications in the that of standard high-speed steels. These machining nonferrous work material. The automotive industry to such an extent that bits are hard and tough, withstanding inter- smooth or bright finish tends to resist gall- 80% of such tools use this coming. mittent cuts even under heavy feeds. They ing, a type of welding or buildup associated As described in Ref 6, significant cost are useful for general applications and espe- with many nonferrous alloys. However, savings are possible because the titanium cially recommended for cast steels, cast work materials of ferrous alloys tend to nitride coating improves tool life up to 400% iron, plastics, brass, and heat-treated steels. adhere to similar, iron-base tools having a and increases feed and speed rates by 30%. On tool bit applications where failure oc- bright finish. This buildup on the cutting This is primarily attributable to the in- curs from rapid wearing of the cutting edge, edges to increased frictional heat, creased lubricity of the coating because its M3 class 2 and M4 will be found to surpass poor surface finish, and increased load at coefficient of friction is one-third that of the the performance of regular tool bits. the cutting edge. bare metal surface of a tool. T4, T5, and T8 combine wear resistance Black Oxide Finish. This characteristic Examples of increased tool life obtained resulting from the higher carbon and vana- black finish is typically applied to drills and when using coated versus uncoated single- dium contents together with a higher hot other cutting tools by oxidizing in a steam point and multipoint cutting tools are listed in hardness, resulting from a cobalt content. 58 / Cutting Tool Materials

Table 4 Increased tool life attained with coated cutting tools cutting tools because many drills are not used for production applications. Also, the cost of I Cutting tool . I machined before resharpening coating (predominantly with titanium nitride) High-speed Type tool steel, AISI type Coating Workplecematerial Uncoated Coated is prohibitive because it represents a higher percentage of the total tool cost. M7 ...... 1022 steel, 35 HRC 325 1 200 End mill M7 ...... TiN 6061-T6 aluminum 166 1 500 Drills coated with titanium nitride reduce alloy cutting forces (thrust and torque) and im- End mill M3 ...... TiN 7075T aluminum 9 53 prove the surface finishes to the point that alloy they eliminate the need for prior core - Gear hob M2 ...... TiN 8620 steel 40 80 Broach insert M3 ...... TiN Type 303 stainless 100 000 300 000 ing and/or subsequent reaming. Coated steel drills have been found especially suitable Broach M2 ...... TiN 48% nickel alloy 200 3 400 for cutting highly abrasive materials, hard Broach M2 ...... TiN Type 410 stainless 10 000-12 000 31 000 nonferrous alloys, and diffficult-to-machine steel Pipe tap M2 ...... TiN 3 000 9 000 materials such as heat-resistant alloys. Tap M2 ...... TiN 1050 steel, 30-33 60-70 750--800 These tools are not recommended for drill- HRC ing titanium alloys because of possible Form tool TI5 ...... TiC 1045 steel 5 000 23 000 chemical bonding of the coating to the Form tool TI5 ...... TiN Type 303 stainless 1 840 5 890 steel workpiece material. When drilling gummy Cutoff tool M2 ...... TiC-TiN Low-carbon steel 150 1 000 materials (1018 and 1020 steels, for exam- Drill M7 ...... TiN Low-carbon steel 1 000 4 000 ple) with coated tools, it may be necessary Drill M7 ...... TiN Titanium alloy 662 9 86 to provide for chipbreaking capabilities in layered with D6AC tool steel, the tool design (Ref 3). 48-50 HRC End mills are produced in a variety of Source: Ref 3 sizes and designs, usually with two, four, or six cutting edges on the periphery. This shank-type is typically made from general-purpose high-speed tool steels Because of the good resistance to abrasion brass, aluminum, and plastics. Tool bits of M l, M2, M7, and M l0. For workpieces and high hot hardness, these steels should T I 5 can cut ordinary materials at speeds 15 to made from hardened materials (over 300 be applied to the cutting of hard, scaly, or 100% higher than average. HB), a grade such as Tl5, M42, or M33 is gritty materials. They are well adapted for Often an engineer will specify a grade that more effective. Increased cutting speeds making hogging cuts, for the cutting of hard is not necessary for a given application. For can be used with these cobalt-containing materials, and for the cutting of materials example, selecting M42 for a general appli- high-speed tool steels because of their im- that throw a discontinuous chip, such as cation that could be satisfied with M2 does proved hot hardness. cast iron and nonferrous materials. The high not always prove to be beneficial. The logic One manufacturer realized a fourfold in- degree of hot hardness permits T4, T5, and is that the tool can be run faster and there- crease in the tool life of end mill wear lands T8 to cut at greater speeds and feeds than fore generate a higher production rate. when he switched to a titanium-nitride coat- most high-speed tool steels. They are much What happens many times is that the M42 ed tool (Fig. l l). Titanium nitride coated more widely used for single-point cutting will chip because of its lower toughness end mills also outperform uncoated solid tools, such as lathe, , and level, whereas the M2 will not. carbide tools. When machining valves made tools, than for multiple-edge tools. from type 304 , a switch from Superhard tool bits made from the M40 Multipoint Cutting Tools solid carbide end mills to titanium nitride series offer the highest hardness available Applications of high-speed tool steels for coated end mills resulted in a fivefold in- for high-speed tool steels. The M40 steels other cutting tool applications such as drills, crease in tool life, that is, 150 parts com- are economical cobalt alloys that can be end mills, reamers, taps, threading dies, pared to 30 finished with the carbide tools treated to reach a hardness as high as 69 milling cutters, circular saws, broaches, and (Ref 3). Furthermore, the cost of the coated HRC. Tool bits made from them are easy to hobs are based on the same parameters of high-speed steel end mills was only one- grind and offer top efficiency on the diffi- hot hardness, wear resistance, toughness, sixth that of the carbide tools. Both types of cult-to-machine space-age materials (tita- and economics of manufacture. Some of the 19 mm (3/4 in.) fluted end mills were used to nium and nickel-base alloys, for example) cutting tools that require extensive a 1.6 mm (!/16 in.) deep slot at a and heat-treated high-strength steels requir- have been produced of P/M high-speed tool speed of 300 rev/min and a feed of 51 mm/ ing high hot hardness. steels (see the article "P/M High-Speed rain (2 in./min). T15 tool bits are made from a steel capable Tool Steels" in this Volume). Reamers are designed to remove only of being treated to a high hardness, with General-purpose drills, other than those small amounts of metal and therefore re- outstanding hot hardness and wear resis- made from low-alloy steels for low produc- quire very little flute depth for the removal tance. The exceptional wear resistance ofT l 5 tion on wood or soft materials, are made from of chips. For this reason, reamers are de- has made it the most popular high-speed tool high-speed tool steels, typically M l, M2, M7, signed as rigid tools, requiting less tough- steel for lathe tools. It has higher hardness and M I0. For lower cost hardware quality ness from the high-speed tool steel than a than most other steels, and wear resistance drills, intermediate high-speed tool steels M50 deeply fluted drill. The general-purpose surpassing that of all other conventional high- and M52 are sometimes used although they grades M l, M2, M7, M l0, and T I are typi- speed tool steels as well as certain cast cutting cannot be expected to perform as well as cally used at maximum hardness levels. For tool materials. It has ample toughness for standard high-speed tool steels in production applications requiring greater wear resis- most types of cutting tool applications, and work. For high hot hardness required in the tance, grades such as M3, M4, and Tl5 are will withstand intermittent cuts. These bits drilling of the more difficult-to-machine alloys appropriate. are especially adapted for machining materi- such as nickel-base or titanium product, M42, Milling Cutters. The size, style, config- als of high-tensile strength such as heat- M33, or TI5 are used. uration, complexity, and capacity of milling treated steels and for resisting abrasion en- High-speed tool steel twist drills are not cutters is almost limitless. There are stag- countered with hard cast iron, cast steel, currently being coated as extensively as gear gered-tooth and straight-tooth, form-re- High-Speed Tool Steels / 59

E 0.050 "- Sizes range from 0.076 mm (0.003 in.) thick they possess the lowest hot hardness and E 1.0 x~\ \x~//-~ 7/,//~ 7/, F/z/, I O. 030 -- by 13 mm (1/2 in.) outside diameter to more wear resistance of all the high-speed tool -E 0.50 X 0.020 x~ than 6.4 mm (I/4 in.) thick by 203 mm (8 in.) steels. The addition of vanadium offers the t- a) 0.40 "~,'~,\\,1,'/ //,'1/ //, "//,///11 0.015 ,-- outside diameter. Used for cutting, slitting, advantage of greater wear resistance and t- ~ O and slotting, saws are available with straight- o 0.25 0.010 ~= hot hardness, and steels with intermediate "O tooth, staggered-tooth, and side-tooth config- vanadium contents are suited for fine and E 0.005 o. 1o I~N~I///~//~//]Y//~//y//J I 0.003 urations and are made from alloys similar to roughing cuts on both hard and soft materi- 0.075 ~. ~. 0 10 20 30 40 50 60 70 80 g0 100 those used for milling cutters. Again, M2 als. The 5% V steel (TI5) is especially Number of parts milled high-speed tool steel is the general-purpose suited for cutting hard metals and alloys or material, but, because of the typical high-strength steels, and is particularly suit- Fig 1 1 We.~ Io.as developed with uncoated and thinness of these products, toughness is opti- able for the machining of aluminum, stain- • titanium nitride coated end mills show a 4:1 increase in tool life with coated tools. The cross- mized with lower hardness. There are rela- less steels, austenitic alloys, and refractory hatched area at left (extending from 0 to 20 parts) tively few saws that are made from M3 or M4 metals. Wrought high-vanadium high-speed indicates the number of pieces produced by uncoated end high-speed tool steel because generally T I5 tool steels are more difficult to grind than mill after 0.25 mm (0.010 in.) wear land on the tool; the and M42 are the two alternative materials to their P/M product counterparts. The addi- crosshatched area at right represents quantity produced by titanium nitride coated end mill after 0.25 mm (0.010 the standard M2 steel. M42 is often used to tion of cobalt in various amounts allows still in.) wear land on tool. Source: Ref 3 machine stainless steels, aluminum, and brass higher hot hardness, the degree of hot hard- because it increases saw production life and ness being proportional to the cobalt con- ~ieved and formed milling cutters with sizes can be run at considerably higher speeds. T I5 tent. Although cobalt steels are more brittle that range from 51 to 305 mm (2 to 12 in.) is used for very specialized applications. than the noncobalt types, they give better and are used to machine slots, grooves, Saws made of high-speed tool steel are used performance on hard, scaly materials that racks, sprockets, gears, splines, and so on. to cut, slit, and slot everything from steel, are machined with deep cuts at high speeds. They cut a wide variety of materials, includ- aluminum, brass, pipe, and titanium to gold High-speed tool steels have continued to be ing plastics, aluminum, steel, cast iron, su- jewelry, fish, frozen foods, plastics, rubber, of importance in industrial commerce for 70 peralloys, titanium, and structures. and paper. to 80 years despite the inroads made by The general-purpose high-speed tool steel Broaches. M2 high-speed tool steel is the competitive cutting tool materials such as cast used for more than 70% of milling cutter most frequently used material for broaches. cobalt alloys, cemented carbides, ceramics, applications is M2, usually the free-machin- This includes the large or circular broaches and cermets. The superior toughness of high- ing type. It has a good balance of wear that are made in large quantities as well as speed tool steels guarantees its niche in the resistance, hot hardness, toughness, and the smaller keyway and shape broaches. cutting tool materials marketplace. strength and works well on carbon, alloy, Sometimes the higher-carbon material is and stainless steels, aluminum, cast iron, used, but generally free-machining M2 is ACKNOWLEDGMENTS and some plastics (generally any material used because it results in a better surface The authors wish to express their thanks that is under 30 HRC in hardness). When finish. P/M products are very popular for to the following individuals for their useful higher hardness materials or more wear- broaches in both M2 as well as M3 class 2 contributions to this article: Gene Bistrich, resistant materials need to be milled, M3 or and M4 when they are used to improve wear National Broach and Machine Division, M4 are selected. The higher carbon and resistance. M4 is probably the second most Lear Siegler, Inc.; George Kiss, The Cleve- vanadium content in those materials im- widely used material for this application. land Twist Drill Company; Jim Kucynski, proves wear resistance and allows for the M42 and TI5 are often used for difficult-to- The Weldon Tool Company; Ron Oakes, machining of materials greater than 35 HRC machine materials such as the nickel-base Niagara Cutter Inc.; Robert Phillips, in hardness. For workpiece hardness levels alloys and other aerospace-type alloys. Pfauter Maag; Ira Redinger, Thurston Man- above that and as high as 50 HRC, either A high-nickel (48%) alloy magnet manufac- ufacturing Company; and Scott Schneier, M42 with its high hardness and high hot turer using a 3.2 x 13 x 305 mm (I/a x V2 × 12 Regal-Beloit Corporation. hardness properties or T I5 with its high in.) flat broach made of M2 increased tool life wear resistance and high hardness charac- from 200 pieces to 3400 pieces when a titani- teristics are desirable. The powder metal- um nitride coating was added, and also ob- lurgy grades in M4 and T15 are increasing in tained a smoother surface finish. Replacing REFERENCES popularity for milling cutters because of the flat broach with an uncoated 11.99 mm 1. Machining, Vol 1, Tool and Manufac- their ease of grinding and regrinding. (0.472 in.) diam by 660 mm (26 in.) long round turing Engineers Handbook, Society of Hobs are a type of milling cutter that oper- broach increased the production to about Manufacturing Engineers, 1983, p 3-6 ates by cutting a repeated form about a cen- 7000 pieces, and coating the round broach 2. S. Kalpakjian, Manufacturing Processes ter, such as gear teeth. The hob cuts by with titanium nitride further increased the for Engineering Materials, Addison- meshing and rotating about the workpiece, magnet production to about 19 000 pieces Wesley, 1984, p 524 forming a helical pattern. This type of metal (Ref 3). Thus, going from an uncoated flat 3. C. Wick, HSS Cutting Tools Gain a cutting creates less force at the cutting edge broach to a coated round broach increased Productivity Edge, Manufacturing Engi- (less chip load on the teeth) than do ordinary production by a factor of 95. neering, May 1987, p 38 milling cutters. Accordingly, less toughness 4. W.D. Sproul, Turning Tests of High and edge strength is required of hob materi- Factors in Selecting Rate Reactively Sputter-Coated T-15 als; wear is more commonly a mode of fail- High.Speed Tool Steels HSS Inserts, Surf. Coat. Tech., Vol 33, ure. Most hobs are made from a high-carbon 1987, p 133 version of M2, although normal carbon levels No one composition of high-speed tool 5. TiN Coatings Continue to Revolutionize are also used. M2 with a sulfur addition or steel can meet all cutting tool requirements. the Metalworking Industry, Machining P/M product for improved machinability and The general-purpose molybdenum steels Source Book, ASM INTERNATIONAL, surface finish is often used for hobs. such as M l, M2, and M7 and tungsten steel 1988, p 98 Saws are quite similar to milling cutters in T I are more commonly used than other 6. How Cutting Tools Can Help You Make style and application, but they are usually high-speed tool steels. They have the high- a QuiCk Buck, Machining Source Book, thinner and tend to be smaller in diameter. est toughness and good cutting ability, but ASM INTERNATIONAL, 1988, p 20