High Speed Steels
High Speed Steels
High speed steels forms a special class of highly alloyed tool steels, combining properties such as high hot hardness and high wear resistance. These are so named mainly because of their ability to machine materials at high cutting speeds. These steels have been widely adopted as the most basic material for cutting tools in mechanical processing since it was first developed. At present, high speed steel cutting tools still occupy the leading position of the tool market. These steels are primarily used for the manufacture of cutting tools such as taps, dies, tool bits, drill bits, milling cutter, reamer, broaches, and long run punches and dies etc.
High speed steels are used not only for cutting tools but also various forming tools which need higher wear resistance and toughness. Along with conventional type steels, there are some grades made by powder metallurgy (P/M) process which have outstanding wear resistance and toughness because of higher alloy content and uniform fine microstructure.
High speed steels are tool steels with high carbon and high alloying elements. They are complex iron-base alloys of carbon (C), chromium (Cr), vanadium (V), molybdenum (Mo) or tungsten (W), or combinations thereof, and in some cases substantial amounts of cobalt (Co). The content of carbon is normally in the range of 0.7 % to 1.6 %, and the sum of the percentage of the major alloy elements, such as tungsten, molybdenum, chromium, vanadium and cobalt is between 10 % and 40 %. Another characteristic which is different from other types of steel is that the content of chromium is fixed at around 4 %. The carbon and alloy contents in high speed steels are balanced at levels to give high attainable hardening response, high wear resistance, high resistance to the softening effect of heat, and good toughness for effective use in industrial cutting operations.
The three main classes of high speed steels are (i) tungsten high speed steel, (ii) molybdenum high speed steel and (iii) tungsten-molybdenum high speed steels. The other classes of high sped steels can be made by addition of cobalt, and increasing the carbon and vanadium content to the three main classes. High speed steels with carbon content higher than 1.25 % and vanadium content higher than 2 % can be separately grouped by commercial designation ‘super high speed steels’. Industrial practice has developed two groups of cutting materials namely (i) the recognized standard high speed steels, which serves almost all applications under mild to severe metal cutting conditions, and (ii) a smaller group of intermediate steels, which are suitable for limited applications under mild to moderate metal cutting conditions.
High speed steels have certain characteristics, such as (i) they are harder than the materials of work pieces, not only at room temperature but also at high temperature during machining process, (ii) they have ductility which is large enough to withstand shock loads occurring during the process of machining with termination / interruption or when cutting work pieces which have hard spots, (iii) they have the thermal shock load which is needed when significant temperature changes occur regularly, (iv) they have low adhesive properties to reduce the affinity of work pieces against the tools thus reducing the wear rate, and (v) they have low solubility of elements or materials of the tools which is needed to minimize the wear rate due to diffusion mechanism.
The minimum requirements which are to be met to be classed as standard high speed steel and those for intermediate high speed steel as given in ASTM A 600 are shown in Tab 1. An alloy to be acceptable for either group is needed to meet all of the requirements shown for that group in the Tab 1.
|Tab 1 Requirements for high speed steels as per ASTM A 600|
|Minimum alloy content by major elements|
|Tungsten + 1.8% molybdenum||11.75%||6.50%|
|Minimum total alloy content based on tungsten equivalents (1/3 %Cr + 6.2 %V + %W+ 1.8 %Mo)|
|Grades containing less than 5% cobalt||22.50%||13.00%|
|Grades containing 5% or more cobalt||21.00%||12.00%|
|Hardening response requirements|
|Ability to be austenitized, and tempered at a temperature not less than 510 deg C with a fine-grain structure (Snyder-Graff grain size 8 min) to hardness||63 HRC||62 HRC|
There are presently more than 40 individual classifications of high speed steels, according to the American Iron and Steel Institute (AISI). When these are compounded by the number of other producers, the total number of individual steels in the high speed steels category exceeds 150. The AISI established a classification system for the high speed steels several years ago. This system consists of a T for those steels which have tungsten as one of their primary alloying element and an M for those steels which have molybdenum additions as one of their primary alloying element. In addition, there is a number which follows either the M or the T. Hence, there are high speed steels designated T1, T3, T15, M1, M2, M41, and so on. The number does not have any special significance other than to differentiate between one steel and other steel. For example, M1 does not mean that it is more highly alloyed than M2 or has a higher hardenability or poorer wear resistance, and so on. It only separates the types and attempts to simplify selection for the user.
High speed steels are high carbon alloy steels which contain large quantities of alloy carbides. Carbide plays an important role on the quality and performance of the high speed steels. Carbide is a double-edged sword for high speed steels since it ensures high hardness, wear resistance, and red hardness with the reasonable heat treatment, but also at the same time can be an important source of various quality problems. Poor heat treatment leads to overheating and over burning which affect the tool life. There are several kinds and forms of carbides in the high speed steels, and the number, type, distribution, shape and size of carbide can affect the performance and quality of the high speed steels.
In order to prolong the life of high speed steel tools, it is necessary to completely know the carbides in steel and understand the principles of its generation and variation in detail for the purpose of improving the tool life. It is necessary to control the degree of heterogeneity of carbides, to improve the morphology and distribution of carbides by heat treatment, and pay attention to the solubility of carbides, and to examine the changes taking place in the tempered carbides.
High speed steels are versatile materials whose properties can be adjusted through heat treatment. They are complex steels because of the complex relationship between composition, microstructure, and performance, and the difficult production process consisting of smelting, pouring, forging, rolling, plastic forming and heat treatment. Heat treatment of the high speed steels has several characteristics which are described later.
Effect of alloying elements
The T series of the high speed steels contains 12 % to 20 % of tungsten, with chromium, vanadium, and cobalt as the other major alloying elements. The M series of the high speed steels contains around 3.5 % to 10 % of molybdenum, with chromium, vanadium, tungsten, and cobalt as the other alloying elements. All types of the high speed steels, whether tungsten or molybdenum, contain around 4 % chromium with a varying contents of carbon and vanadium. As a general rule, when the vanadium content is increased, the carbon content is also normally increased).
The tungsten type T1 does not contain molybdenum or cobalt. Cobalt-base tungsten types range from T4 through T15 and contain various amounts of cobalt. Molybdenum types M1 through M10 (except M6) contain no cobalt, but most contain some tungsten. The cobalt-base, molybdenum-tungsten, premium types are normally classified in the M30 and M40 series. Super high speed steels normally range from M40 upward. These steels are capable of being heat treated to high hardnesses.
The M series of the high speed steels normally have higher abrasion resistance than the T series of the high speed steels and less distortion in heat treatment. Also, these steels are less costly. Tools made from high speed steels can also be coated with titanium nitride, titanium carbide, and various other coatings by physical vapour deposition technique for improved performance and increased tool life. A variety of elements are added to T series and M series of the high speed steels to impart certain properties to the tool steels. These elements and their effects are described below.
Carbon – It is by far the most important of the elements and is very closely controlled. While the carbon content of any one high speed steel is normally fixed within narrow limits, variations within these limits can cause important changes in the mechanical properties and the cutting ability the steel. As the carbon content is increased, the working hardness also increases with the high temperature hardness are higher and the number of hard, stable, complex carbides increases. The latter contribute much to the wear resistance and other properties of the high speed steels.=
Silicon – The influence of silicon on the high speed steels, upto around 1 % is small. Increasing the silicon content from 0.15 % to 0.45 % gives a small increase in the maximum attainable tempered hardness and has some influence on the carbide morphology, although there seems to be a simultaneous slight decrease in toughness. Some manufacturers produce at least one grade with silicon upto 0.65 %, but this level of silicon content needs a lower maximum austenitizing temperature than does a lower silicon level in the same grade, if overheating is to be avoided. However, silicon content is normally kept below 0.45 % in the majority of the grades.
Manganese – Manganese content is normally not high in the high speed steels. This is because of its marked effect in increasing brittleness and the danger of cracking upon quenching.
Phosphorus – Phosphorus has no effect on any of the desired properties of high speed steels, but because of its well known effect in causing cold shortness, or room temperature brittleness, the content of phosphorus is kept to a minimum.
Chromium – Chromium is always present in high speed steels in amounts ranging from 3 % to 5 % and is mainly responsible for the hardenability. Normally, the addition is 4 % since it appears that this content gives the best compromise between hardness and toughness. In addition, chromium reduces oxidation and scaling during heat treatment.
Tungsten – In the high speed steels, tungsten is very important. It is present in all the T-type of the high speed steels and in all but two of the M-type of the high speed steels. The complex carbide of iron, tungsten, and carbon which is present in the high speed steels is very hard and contributes to the wear resistance considerably. Tungsten improves the hot hardness, causes secondary hardening, and imparts marked resistance to tempering. When the tungsten content is lowered in high speed steels, molybdenum is normally added to make up for the loss.
Molybdenum – Molybdenum forms the same double carbide with iron and carbon as tungsten does but has half the atomic weight of tungsten. As a result, molybdenum can be substituted for tungsten on the basis of around one part of molybdenum, by weight, for two parts of tungsten. The melting point of the molybdenum steel grades is somewhat lower than that of the tungsten steel grades, and thus they need a lower hardening temperature and have a narrower hardening range.
The M-type high speed steels are tougher than the T type high-speed steels, but the hot hardness is slightly lower. Compensation for this reduced hot hardness is partially achieved by the addition of tungsten and, to a lesser extent, vanadium to the plain molybdenum grades. This is one important reason for the popularity of the tungsten-molybdenum steel grades (such as M2, M3, and M4). These steel grades afford good hot hardness, which is needed in the high speed steels.
Vanadium – Vanadium was first added to the high speed steels as a scavenger to remove slag impurities and to reduce nitrogen levels in the melting operation, but it was soon found that the element materially increased the cutting efficiency of the tools. The addition of vanadium promotes the formation of very hard, stable carbides, which considerably increase wear resistance and, to a lesser extent, hot hardness. An increase in vanadium, when properly balanced by carbon additions, has relatively little effect on the toughness. For this reason, vanadium bearing steel grades are a very good choice when very fast cutting operations are needed, as in finishing cuts, or when the surface of the material is hard and scaly.
The special characteristics of the high speed tool steels which are due to high vanadium additions have given rise to several specially developed steel grades for very severe service needing high toughness as well as exceptional hot hardness and wear resistance. The T15, M4, and M15 steel grades are in this category and their vanadium content is 4.88 % %, 4.13 %, and 5 % respectively.
Cobalt – The main effect of cobalt in high speed steels is to increase the hot hardness and hence to increase the cutting efficiency when high tool temperatures are attained during the cutting operation. Cobalt raises the heat-treating temperatures since it increases the melting point. Hardening temperatures for cobalt high speed steels can be 14 deg C to 28 deg C higher than the normal for similar grades without cobalt. Cobalt additions slightly increase the brittleness of high speed steels.
Cobalt high speed steels are especially effective on rough or hogging cuts, but they are not normally suited to finishing cuts which do not involve high temperatures. Cobalt high speed steels normally perform quite well when cutting materials which have discontinuous chips such as cast iron or nonferrous metals. The necessity of using deep cuts and fast speeds or of cutting hard and scaly materials justifies the use of cobalt high speed steels.
Sulphur – Sulphur, in normal concentrations of 0.03 % or less, has no effect on the properties of high speed steels. However, sulphur is added to certain high speed steels to contribute to the free machining qualities, as it does in low alloy steels. The quantity of free machining high speed steel tool is small but considerable percentage of the total consumption of high speed steels. One of the major areas for the free machining high speed steels is in larger diameter tools such as hobs, broaches, and so on.
Sulphur forms complex sulphides, containing chromium, vanadium, and manganese which are distributed throughout the steel as stringer type inclusions. The stringers interrupt the steel structure and act as notches, which aid the metal removing action of a cutting tool when machining the high speed steels, since the resulting chip is discontinuous, a characteristic of free machining steels. Very high sulphur additions (upto 0.30 %) are made to some powder metallurgy high speed steels for improved machinability / grindability by forming globular sulphides rather than stringers.
Nitrogen – Nitrogen is normally present in air melted high speed steels in quantities varying from around 0.02 % to 0.03 %. The nitrogen content of some high speed steels is intentionally increased to around 0.04 % to 0.05 %, and this addition, when combined with higher than normal quantities of silicon, results in a slight increase in the maximum attainable tempered hardness and some change of carbide morphology.
Properties of high speed steels
High speed steels, regardless of whether they are an AISI T type or AISI M type, have a rather striking similarity in their physical make up since, (i) they all possess a high alloy content, (ii) they normally contain sufficient carbon to permit hardening to 64 HRC, (iii) they harden so deeply that almost any section encountered commercially has a uniform hardness from centre to surface, and (iv) they are all hardened at high temperatures, and their rate of transformation is such that small sections can be cooled in still air and have near maximum hardness.
All high speed steels have excess carbide particles, which in the annealed state contain a high proportion of the alloying elements. These carbide particles contribute materially to the wear resistance of hardened high speed steels. By partially dissolving during heat treatment, these carbides provide the matrix of the steel with the necessary alloy and carbon content for hardenability, hot hardness, and resistance to tempering.
While all high speed steels have several similar mechanical and physical characteristics, the properties can vary widely due to the changes in chemical composition. Basically, the most important property of a high speed steel is its cutting ability. Cutting ability depends on a combination of properties, the four most important of these being (i) hardness which is the resistance to penetration by diamond hard indenter and measured at room temperature, (ii) hot hardness which is the ability to retain high hardness at high temperatures, (iii) wear resistance which is the resistance to abrasion, frequently measured by grindability, metal-to-metal, or various other types of tests to indicate a relative rating, and (iv) toughness which is the ability to absorb impact energy.
The relative importance of these properties varies with every application. High machining speeds need a composition with a high initial hardness and a maximum resistance to softening at high temperatures. Certain materials can abrade the cutting edge of the tool excessively. Hence, the wear resistance of the tool material can well be more important than its resistance to high cutting temperatures.
Hardness is necessary for cutting harder materials and normally gives increased tool life, but it is to be balanced against the toughness needed for the application. The required combination of properties in high speed steel s can be achieved, first, by selection of the proper grade and, second, by the proper heat treatment. Both are equally important decisions.
Hardness – Hardness is normally the most stipulated requirement of the high speed steels and is used as an acceptance check of a heat treated tool. All high speed steels can be hardened to room temperature hardness of 64 HRC, while the M40 series, some of the M30 series, and T15 can reach nearly 69 HRC.
Hot Hardness – A related and important component of cutting ability is hot hardness. It is simply the ability to retain hardness at high temperatures. This property is important since the room temperature hardness values are not the same values which exist at the high temperature produced by friction between the tool and work piece. Fig 1 shows hot hardness values of some representative grades of the high speed steels. It is worth mentioning that the cobalt-base steel types as a group show higher hot hardness than non-cobalt base types.
Fig 1 Comparison of hot hardness of high speed steels
Wear resistance – The third component of cutting ability is resistance to wear. Wear resistance of high speed steels is affected (i) by the matrix hardness and composition, (ii) by precipitated MeC and MC carbides responsible for secondary hardness, (iii) by the volume of excess alloy carbides, and (iv) by the nature of these excess carbides. In practice, the wear resistance of any given high speed steels strongly depends on the hardness of the steel, and achieved higher hardness is an aim when highly abrasive cutting conditions are encountered. For the ultimate in wear resistance, carbon content is to be increased simultaneously with vanadium content, to permit the introduction of a larger quantity of total carbide and a higher percentage of extremely hard vanadium carbide in high speed steels. Fig 2 shows effect of hardness on the wear rate of the high speed steels.
Fig 2 Effect of hardness on the wear rate of high speed steels
Toughness – The fourth component of cutting ability is toughness which is defined as a combination of two factors namely (i) the ability to deform before breaking (ductility), and (ii) the ability to resist permanent deformation (elastic strength). If either of these factors is to be used to describe toughness, the second appears more practical for the high speed steels since rarely are large degrees of flow or deformation permissible with fine-edge tools. The first, however, cannot be ignored, as frequently the stress applied to a tool (through overloads, shock, notches, and sharp corners) exceeds the elastic strength.
Toughness tests on high speed steel are normally conducted at room temperature. Tool failures which occur from spalling of the tool edge normally occur during the initial contact of the tool with the work piece, and once the tool becomes heated, its performance in this respect is much superior. Hence, room temperature tests are perhaps of greater value when toughness is considered than when hardness is in question. Laboratory tests for the measurement of toughness of the hardened high speed steels include bend, unnotched or C-notch impact, static torsion, and torsion impact tests. Modest improvements in toughness (within a grade) can be made by lowering the tempered hardness. Lower austenitizing temperatures enhance the toughness for a given hardness and grade. Fig 3 shows relationship between impact toughness and hardness for the representative high speed steel grades.
Fig 3 Relationship between impact toughness and hardness
Heat treatment of high speed steels
Proper heat treatment is as critical to the success of the cutting tool as material selection itself. Frequently the highest quality steel made into the most precise tools does not perform because of improper heat treatment. The objective of the heat treatment or hardening process is to transform fully annealed high speed steels consisting mainly of ferrite (iron) and alloy carbides into a hardened and tempered martensitic structure having carbides which provide the cutting tool properties (Fig 4).
Fig 4 Change of microstructure during heat treatment of high speed steel
The heat treatment process can be divided into four process steps namely (i) preheating, (ii) austenitizing, (iii) quenching, and (iv) tempering. Fig 5 outlines graphically these four heat treatment steps.
Fig 5 Graphical representation of steps for heat treatment of high speed steels
Preheating – Preheating plays no part in the hardening reaction from a metallurgical viewpoint. However, preheating performs three important functions. The first of these is to reduce thermal shock, which always results when a cold tool is placed into a warm or hot furnace. Minimizing of the thermal shock reduces the danger of excessive distortion or cracking. It also relieves some of the stresses developed during machining and / or forming, although conventional stress relieving is more effective.
The second major advantage of preheating is to improve the equipment productivity by decreasing the amount of time needed in the high heat furnace. The third function of preheating is that if the high heat furnace environment is not neutral to the surface of the tool or part, then the preheating reduces the quantity of carburization and decarburization which normally results if no preheating is used. In commercial salt bath hardening, a two step preheating is typically used for high speed steels. The first preheating step is carried out between 650 deg C and 760 deg C. The second preheating cycle is carried out between 815 deg C and 900 deg C. In atmosphere or vacuum heat treating, the furnace is normally heated slowly to a single preheating of 790 deg C to 845 deg C. Preheating duration is of little importance as long as the part is heated throughout its cross section.
Austenitizing (hardening) – It is the second step of the heat treatment process. Austenitizing is a time / temperature dependent reaction. High speed steels depend upon the dissolving of different complex alloy carbides during austenitizing to develop their properties. These alloy carbides do not dissolve to any appreciable extent unless the steel is heated to a temperature within 28 deg C to 56 deg C of their melting point. This temperature is dependent upon the particular high speed steel being treated and is in the range of 1,150 deg C to 1,290 deg C. The normal recommended holding time for high speed steels is around 2 minutes to 6 minutes, depending upon the high speed steel type, tool configuration, and cross-sectional size.
Decreasing the hardening temperature (under-hardening) normally improves the impact toughness while lowering the hot hardness. Increasing the hardening temperature increases heat-treated room temperature hardness and also increases the hot hardness.
Quenching – The quenching or cooling of the work piece from the austenitizing temperature is designed to transform the austenite which forms at the high temperature to a hard martensitic structure. The rate of cooling, which is to be controlled, is dictated by the analysis of the particular steel. Sometimes high speed steels are quenched in two steps. Initially in a molten salt bath kept at around 540 deg C to 595 deg C or an oil quench, followed by the air cooling to near ambient temperature.
The least drastic form of quenching is cooling in air, although only in the smaller and / or thinner cross sections. High speed steels air quench rapidly enough to transform the majority of the structure into the desirable martensitic structure.
Tempering – Following austenitizing and quenching, the steel is in a highly stressed state and hence is very susceptible to cracking. Tempering (or drawing) increases the toughness of the steel and also provides secondary hardness. Tempering involves reheating the steel to an intermediate temperature range (always below the critical transformation temperature), soaking, and air cooling.
Tempering serves to stress relieve and to transform retained austenite from the quenching step to fresh martensite. Some precipitation of complex carbide also occurs, further enhancing secondary hardness. It is this process of transforming retained austenite and tempering of newly formed martensite which dictates a multiple tempering procedure.
High speed steels need 2 tempers to 4 tempers at a soak time of 2 hours to 4 hours each. As with the austenitizing temperatures and quenching rates, the number of tempers is dictated by the specific grade. High speed steels are to be multiple tempered at 540 deg C minimum for the majority of the grades. It is necessary to favour the high side of the secondary hardness peak of the tempering curve in order to optimize the above described transformations.
Sub-zero treatments are sometimes used in conjunction with tempering in order to continue the transformation of austenite to martensite. Several tests have been made to know the effect of cold treatments, and the findings normally prove that cold treatments used after quenching and first temper enhance the transformation to martensite, in much the same way which the multiple tempering causes transformation. Cold treatments given to the high speed steels immediately after quenching can result in cracking or distortion since the accompanying size change is not accommodated by the newly formed, brittle martensite. It is normally accepted that sub-zero treatments are not necessary if the steel is properly hardened and tempered.
Vacuum heat treatment with gas quenching has now become the leading process to harden high speed steels. It has the advantage over the traditional salt bath treatment that it is environmentally friendly, less costly to operate, and easier to control. There are different designs of the furnaces, however, typical features for modern single chamber furnaces are that heating upto around 800 deg C is made by gas (normally nitrogen) convection and then by radiation in vacuum upto the austenitizing temperature. Quenching is then made by gas (normally nitrogen) with vacuum at high gas velocity. The temperature cycle can be programmed and is controlled by a furnace thermocouple (for the heat supply) and charge thermocouple(s) to follow the actual temperature.
Tools made of high speed steels are available with either a bright finish, black oxide finish, or nitride finish or they can be coated with titanium nitride and other coatings using a vapour disposition process. This increases tool the life considerably.
Bright finish – The majority of tools are finished with a ground or mechanically polished surface which is normally categorized as a bright finish. Bright finished tools are frequently preferred to tools with an oxide finish for machining non-ferrous materials. The smooth or bright finish tends to resist galling, a type of welding or build-up associated with several non-ferrous materials. However, ferrous materials tend to adhere to similar, iron-base tools having a bright finish. This build-up on the cutting edges leads to increased frictional heat, poor surface finish, and increased load at the cutting edge.
Black oxide finish – This characteristic black oxide finish is typically applied to drills and other cutting tools by oxidizing in a steam atmosphere at around 540 deg C. The black oxide surface has little or no effect on hardness, but serves as a partial barrier to galling of similar ferrous metals. The surface texture also permits retention of the lubricant.
Nitride finish – Nitriding is a method of introducing nitrogen to the surface of high speed steels at a typical temperature of 480 deg C to 595 deg C and is carried out either by the dissociation of ammonia gas, or exposure to sodium cyanide salt mixtures, or bombardment with nitrogen ions in order to liberate nascent nitrogen, which combines with the steel to form a hard iron nitride. Nitriding improves wear resistance of high speed steels, at the cost of the notch toughness.
Coated high speed steels – The addition of wear-resistant coatings to high speed steel cutting tools lagged behind the coming of carbide inserts by around 10 years until the development of the low temperature physical vapour deposition process which is much more suitable for the coating of the high speed steels than is the older chemical vapour deposition process, and which also eliminates the need for subsequent heat treatment.
Titanium nitride is the most commonly used material which forms most durable coating, although substitutes such as other nitrides (hafnium nitride and zirconium nitride) and carbides (titanium carbide, zirconium carbide, and hafnium carbide) are being developed. These other coatings are expected to equal or surpass the desired properties of titanium nitrides in the coming years. The hard thin (2 micrometers to 5 micrometers thick) deposit of high density titanium nitride, which has 2500 HV hardness and imparts a characteristic gold colour to high speed steels, provides excellent wear resistance, minimizes heat build up, and prevents welding of the work piece material, while improving the surface finish of the high speed steels.
The initial use, in 1980, of titanium nitride coatings was to coat gear cutting tools. Later applications include the coming of both single-point and multi-point tools such as lathe tools, drills, reamers, taps, milling cutters, end mills, and broaches. Today, titanium nitride coated hobs and shapers dominate high production applications in the automotive industry to such an extent that 80 % of such tools use this coating.
Considerable cost savings are possible since the titanium nitride coating improves tool life upto 400 % and increases feed and speed rates by 30 %. This is primarily attributable to the increased lubricity of the coating since its coefficient of friction is one-third that of the bare metal surface of a tool. The increased production achieved with a coated tool justifies the application of the coating despite the resulting 20 % to 30 % increase in the base price of the tool.
Coated tools can meet close-tolerance requirements and considerably improve the machining of carbon and alloy steels, stainless steels (especially the 300 series, where galling can be a problem), and aluminum alloys (especially aircraft grades). Coated high speed steels are less of a factor in the machining of certain titanium alloys and some high nickel alloys because of chemical reactions between the coatings and the work piece materials.
Applications of high speed steels
High speed steels are used for most of the common types of cutting tools including single-point lathe tools, drills, reamers, taps, milling cutters, end mills, hobs, saws, and broaches.
Single-point cutting tools – The simplest cutting tools are single-point cutting tools, which are frequently referred to as tool bits, lathe tools, cut off tools, or inserts. They have only one cutting surface or edge in contact with the work material at any given time. Such tools are used for turning, threading, boring, planing, or shaping, and the majority are mounted in a tool holder which is made of some type of tough alloy steel. The performance of such tools is dependent on the tool material as well as various factors such as the material being cut, the speeds and feeds, the cutting fluid, and fixturing. The material characteristics and recommendations for the majority of the popular lathe tools are described below.
M1, M2, and T1 are suitable for all purpose tool bits. They offer excellent strength and toughness and are suitable for both roughing and finishing and can be used for machining wrought steel, cast steel, cast iron, brass, bronze, copper, and aluminum etc. These are good economical grades for general shop purposes.
M3 class 2 and M4 high speed steels have high carbon and high vanadium contents. The wear resistance is several times that of standard high speed steels. These bits are hard and tough, withstanding intermittent cuts even under heavy feeds. They are useful for general applications and especially recommended for cast steels, cast iron, plastics, brass, and heat treated steels.
On tool bit applications where failure occurs from rapid wearing of the cutting edge, M3 class 2 and M4 are found to surpass the performance of regular tool bits. T4, T5, and T8 combine wear resistance resulting from the higher carbon and vanadium contents together with a higher hot hardness, resulting from cobalt content. Because of the good resistance to abrasion and high hot hardness, these steels are to be applied to the cutting of hard, scaly, or gritty materials. They are well adapted for making hogging cuts, for the cutting of hard materials, and for the cutting of materials which throw a discontinuous chip, such as cast iron and nonferrous materials. The high degree of hot hardness allows T4, T5, and T8 grades to cut at higher speeds and feeds than the majority of the high speed steels. These grades are much more widely used for single-point cutting tools, such as lathe, shaper, and planer tools, than for multiple-edge tools.
Super hard tool bits made from the M40 series present the highest hardness available for high speed steels. The M40 steels are the economical cobalt alloys which can be heat treated to reach hardness as high as 69 HRC. Tool bits made from these steels are easy to grind and present top efficiency on the difficult-to-machine space-age materials (such as titanium and nickel base alloys) and heat treated high strength steels needing high hot hardness.
T15 tool bits are made from steel capable of being treated to a high hardness along with outstanding hot hardness and wear resistance. The exceptional wear resistance of T15 steel has made it the very popular high speed steel for lathe tools. It has higher hardness than the majority of the other steels, and has wear resistance surpassing that of all other conventional high speed steels as well as certain cast cutting tool materials. It has sufficient toughness for most types of cutting tool applications, and withstands intermittent cuts. These bits are especially adapted for machining materials of high tensile strength such as heat treated steels and for resisting abrasion encountered with hard cast iron, cast steel, brass, aluminum, and plastics. Tool bits of T15 can cut ordinary materials at speeds 15 % to 100 % higher than average.
Frequently people specify a steel which is not necessary for a given application. As an example, selecting M42 for a general application which can be satisfied with M2 does not always prove to be beneficial. The reason is that the tool can be run faster and hence generate a higher production rate. Many times it happens that the M42 grade chips because of its lower toughness level, whereas the M2 grade does not chip.
Multi-point cutting tools – Applications of high speed steels for other cutting tool applications such as drills, end mills, reamers, taps, threading dies, milling cutters, circular saws, broaches, and hobs are based on the same parameters of hot hardness, wear resistance, toughness, and economics of production. Some of the cutting tools which need extensive grinding have been produced of P/M high speed steels.
General purpose drills, other than those made from low alloy steels for low production on wood or soft materials, are made from high speed steels, typically from M1, M2, M7, and M10 grades. For lower cost hardware quality drills, intermediate high speed steels M50 and M52 are sometimes used although they cannot be expected to perform as well as standard high speed steels in the production work. For high hot hardness needed for the drilling of the more difficult-to-machine alloys such as nickel base or titanium base product, M42, M33, or T15 grades are used. High speed steel twist drills are not at present being coated as extensively as gear cutting tools since several drills are not used for production applications. Also, the cost of coating (mainly with titanium nitride) is prohibitive since it represents a higher percentage of the total tool cost.
Drills coated with titanium nitride reduce cutting forces (thrust and torque) and improve the surface finishes to the point that they eliminate the need for prior core drilling and / or subsequent reaming. Coated drills have been found especially suitable for cutting highly abrasive materials, hard non-ferrous alloys, and difficult-to-machine materials such as heat-resistant alloys. These tools are not recommended for drilling titanium alloys because of possible chemical bonding of the coating to the work piece material. When drilling gummy materials (e.g. 1018 and 1020 steel grades) with coated tools, it can be necessary to provide for chip breaking capabilities in the tool design.
End mills are produced in a variety of sizes and designs, normally with two, four, or six cutting edges on the periphery. This shank type milling cutter is typically made from general purpose high speed steels M1, M2, M7, and M10. For work pieces made from hardened materials (more than 300 HB hardness), a grade for example T15, M42, or M33 is more effective. Increased cutting speeds can be used with these cobalt containing high speed steels because of their improved hot hardness.
One producer has achieved a four-fold increase in the tool life of end mill wear lands when there has been a switch to a titanium nitride coated tool. Titanium nitride coated end mills also outperform uncoated solid carbide tools. When machining valves made from type 304 stainless steel, a switch from solid carbide end mills to titanium nitride coated end mills has resulted in a five-fold increase in tool life, that is, 150 parts in comparison to 30 finished with the carbide tools. Also, the cost of the coated high speed steel end mills has been only one sixth that of the carbide tools. Both types of 19 mm fluted end mills have been used to machine a 1.6 mm deep slot at a speed of 300 revolutions per minute and a feed of 51 mm per minute.
Reamers are designed to remove only small amounts of metal and hence need very little flute depth for the removal of chips. For this reason, reamers are designed as rigid tools, needing less toughness from the high speed steel than a deeply fluted drill. The general purpose high speed steel grades M1, M2, M7, M10, and T1 are typically used at maximum hardness levels. For applications needing higher wear resistance, high speed steel grades such as M3, M4, and T15 are suitable.
In case of milling cutters, the size, style, configuration, complexity, and capacity is almost limitless. There are staggered tooth and straight tooth, form relieved and formed milling cutters with sizes which range from 51 mm to 305 mm and are used to machine slots, grooves, racks, sprockets, gears, and splines etc. They cut a wide variety of materials, including plastics, aluminum, steel, cast iron, super alloys, titanium, and graphite structures.
The general purpose high speed steel used for more than 70 % of milling cutter applications is M2 grade, normally the free machining type. It has a good balance of wear resistance, hot hardness, toughness, and strength and works well on carbon, alloy, and stainless steels, aluminum, cast iron, and some plastics (normally any material which is under 30 HRC in hardness). When higher hardness materials or more wear resistant materials need to be milled, M3 or M4 grades are used. The higher carbon and vanadium content in these grades improves wear resistance and allows for the machining of materials with hardness higher than 35 HRC. For work piece hardness levels above that and as high as 50 HRC, either M42 grade with its high hardness and high hot hardness properties or T15 grade with its high wear resistance and high hardness characteristics are desirable. The powder metallurgy grades in M4 and T15 are increasing in their popularity for milling cutters because of their ease of grinding and regrinding.
Hobs are a type of milling cutter which operates by cutting a repeated form about a centre, such as gear teeth. The hob cuts by meshing and rotating around the work piece, forming a helical pattern. This type of metal cutting creates less force at the cutting edge (less chip load on the teeth) than do ordinary milling cutters. Accordingly, less toughness and edge strength is needed of hob materials where wear is more commonly a mode of failure. A majority of hobs are made from a high carbon version of M2 grade, although normal carbon levels are also used. M2 grade with a sulphur addition or P/M product for improved machinability and surface finish is frequently used for hobs.
Saws made of high speed tool steel are used to cut, slit, and slot everything from steel, aluminum, brass, pipe, and titanium to gold jewellery, fish, frozen foods, plastics, rubber, and paper. Saws are very similar to milling cutters in style and application, but they are normally thinner and tend to be smaller in diameter. Sizes range from 0.076 mm thick by 13 mm outside diameter to more than 6.4 mm thick by 203 mm outside diameter.
Used for cutting, slitting, and slotting, saws are available with straight tooth, staggered tooth, and side tooth configurations and are made from alloys similar to those used for milling cutters. Again, M2 grade high speed steel is the general purpose saw material, but, because of the typical thinness of these products, toughness is optimized with lower hardness. There are relatively few saws which are made from M3 or M4 grades high speed steels because normally T15 and M42 grades are the two alternative materials to the standard M2 grade steel. M42 grade is frequently used to machine stainless steels, aluminum, and brass since it increases saw production life and can be run at considerably higher speeds. T15 grade is used for very specialized applications.
In case of broaches, M2 grade high speed steel is the most frequently used material. This includes the large or circular broaches which are made in large quantities as well as the smaller keyway and shape broaches. Sometimes the higher carbon material is used, but normally free-machining M2 grade is used because it results in a better surface finish. P/M products are very popular for broaches in both M2 grade and M3 class 2 and M4 grades when they are used to improve wear resistance. M4 grade is probably the second most widely used material for this application. M42 and T15 grades are frequently used for difficult-to-machine materials such as the nickel base alloys and other aerospace type alloys.
A high nickel (48 %) alloy magnet producer using a 3.2 mm x 13 x 305 mm flat broach made of M2 grade increased tool life from 200 pieces to 3400 pieces when a titanium nitride coating was added, and also achieved a smoother surface finish. Replacing the flat broach with an uncoated 12 mm diameter by 660 mm long round broach increased the production to around 7000 pieces, and coating the round broach with titanium nitride further increased the magnet production to around 19,000 pieces. Hence, going from an uncoated flat broach to a coated round broach increased production by a factor of 95.
Factors in selecting high speed steels
No single composition of high speed steel can meet all cutting tool requirements. The general purpose molybdenum high speed steels such as M1, M2, and M7 grades and tungsten high speed steel T1 grade are normally used than other high speed steels. They have the highest toughness and good cutting ability, but they possess the lowest hot hardness and wear resistance of all the high speed steels.
The addition of vanadium provides the advantage of higher wear resistance and hot hardness, and steels with intermediate vanadium contents are suited for fine and roughing cuts on both hard and soft materials. The 5 % vanadium high speed steel (T15 grade) is particularly suited for cutting hard metals and alloys or high strength steels, and is mainly suitable for the machining of aluminum, stainless steels, austenitic alloys, and refractory metals. Wrought high vanadium high speed steels are more difficult to grind than their P/M product counterparts.
The addition of cobalt in various amounts allows still higher hot hardness, the degree of hot hardness being proportional to the cobalt content. Although cobalt steels are more brittle than the non cobalt types, they give better performance on hard, scaly materials which are machined with deep cuts at high speeds.
High speed steels have continued to be of importance in industry for around 80 years despite the inroads made by competitive cutting tool materials such as cast cobalt alloys, cemented carbides, ceramics, and cermets. The superior toughness of high speed steels ensures its place in the cutting tool materials market place.