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A New Guide for Selecting Ferrous Alloys, Tungsten Carbides and Ceramics for Tooling

 

October, 1999


 

By Robert E. Carnes, Robert Powell and Jack A. Brothers
Carpenter Technology Corporation
Reading, PA
USA

The universal desire of metalworkers to increase parts productivity, reduce costs, improve quality and compete more effectively is making it imperative to use the best possible tooling.

Some of the newer difficult-to-machine metallics and non-metallics are pushing many shops to take another look at the material used for their tools and dies. From a tooling standpoint, shops planning for the future are also addressing the growing trend to dry machining which better meets environmental requirements, improves working conditions and eliminates the costly separation of coolant from chips.

The need for tooling that will yield optimum results even seems to have tempered the thinking of many who had been most concerned with the initial cost of the tooling material or the tool. More than ever, manufacturers are focusing on the more important down-stream benefits in terms of more and better parts, longer tool life, better machine performance and less downtime. Nothing has changed the fact that good tooling costs only a small fraction of the value it can deliver.

The tooling industry can select from a wide range of tool materials that have been developed over the years to meet various fabrication requirements. Most commonly used for tooling applications today are ferrous-based alloys, cemented tungsten carbides and ceramics.

With tooling materials increasing in number, and application requirements becoming more demanding, it's not easy for tool engineers, designers and specifiers to select the best tooling material for a given job. The best choice, of course, is that which allows parts to be produced at the lowest possible unit cost.

To help in the selection process, Carpenter has developed a special chart (Fig. 1) that plots the relative wear resistance and toughness of representative tool steels, carbides and ceramics. The chart, first of its type in the tooling industry, positions 21 different tooling materials, allowing the reader to tell at a glance approximately what combination of wear resistance/hardness and toughness is likely to serve his/her purpose. Final choice may be made after checking the more specific properties offered by the narrowed-down candidate material(s).

TA00004image001 

The materials located on the chart, and discussed later in this article, consist of four conventional tool steels, one ultra-high strength tooling material, five powder metallurgy (PM) high speed steels, two PM cold work tool steels, seven carbides with three different types of grain structure, and two ceramics.

Wear resistance and toughness were selected (and measured) as the primary selection criteria because most tool failures are caused by lack of these key properties, ie: poor wear, chipping, micro- chipping and breakage. Hardness and, for selected applications, hot hardness are factored into the wear resistance of some materials.

Location of the materials in the chart assumes that they are in the proper condition (i.e. heat treated hardness) for use and are being used in an appropriate application. For instance, although MMA11 in the chart appears to be "better" than T15, it has no red hardness; thus it cannot be used in high speed applications such as form tools, which become hot in service.

Before selecting the most appropriate tool material for a given application, it is important that the individual understands the environment in which the tool is to be used. For instance, if the tool must make interrupted cuts, impact properties will be important.

The material being machined also has to be considered in the selection process. If it's carbon steel, a conventional tool steel will work fine. If the work piece is made of a nickel- or cobalt-base superalloy, a tungsten carbide may be a better choice.

Conventional Tool Steels

Conventional cast/wrought tool steels are still widely used for tools and dies in general purpose machining and forming. These grades , quenched either by water, oil or air .are iron-based and alloyed with carbon, chromium and small amounts of other elements.

These mainstay alloys are customarily melted in an electric arc furnace (sometimes in controlled atmosphere), refined to produce final chemistry and eliminate undesirable elements, and poured into molds to solidify into ingots. The ingots are often remelted via ESR for improved structure, then hot rolled or forged into mill forms such as bar, plate and sheet. These forms are then cut to length for tool manufacture using fabrication operations such as turning, milling, boring, drilling, etc.

The wear and toughness properties used to position these and all other ferrous alloys on the selection chart were obtained by strict adherence to ASTM test procedures. Dry sand rubber wheel (ASTM G65 Method A) or separate cross cylinder (ASTM G83) tests were used to measure wear resistance. Charpy or Izod impact tests, per ASTM E23, were used to determine impact resistance.

AISI Type A2 may be the most widely used of the conventional tool steels. This is an air hardening tool steel that can be hardened throughout, even in heavy sections. It has been used in applications in which the sections are very large, and in applications requiring superior dimensional stability after hardening. Typical applications include blanking dies, thread roller dies, punches, trimming dies and forming dies.

AISI Type D2 is an air hardening, high-carbon, high-chromium tool steel with more wear resistance than A2, but slightly less toughness. The alloy's high chromium content gives it mild corrosion resisting properties. It has been used for blanking dies, forming dies, coining dies, extrusion dies and drawing dies.

AISI S7 is a general purpose air hardening tool steel with high impact and shock resistance, along with good resistance to softening at moderately high temperatures. This combination of properties makes the alloy suitable for many hot-work and cold-work applications. S7, slightly tougher than A2, has been used for applications such as chisels, punches and lathe centers.

AISI Type H13 is the toughest of the conventional tool steels. It is a 5% chromium, hot work tool steel designed for applications requiring extreme toughness combined with good red hardness. H13 will provide an extra margin of safety in tools subject to hammer blows, and tools containing deep recesses and sharp corners. Although the alloy has been designed primarily as a hot work tool steel, it has found many applications in cold work tools where extra toughness is required at the sacrifice of some wear resistance. It has been used in such hot working tools as aluminum extrusion dies, die casting dies, forging dies and hot piercing punches.

Ultra-High Strength Steel

AerMet®-for-Tooling alloy, positioned at the bottom right of the chart, is in a league of its own. This alloy, originally developed for aerospace applications, has been used for a variety of tools that require high strength and hardness, along with exceptional fracture toughness and ductility. It reaches an ultimate tensile strength of 2069 MPa (285 ksi) and a fracture toughness of 110 MPa sqrt (M) (100 ksi sqrt (in.)).

AerMet alloy, toughest of all the tooling materials on the chart, may be considered as a substitute for conventional tool steels to prevent the premature cracking or breaking of tools under heavy load or impact, or to solve those problems when they do occur. It is air-hardenable, and virtually free of distortion when heat treated. This is a bonus for the tool engineer who must make tools with complex shapes and/or critical size tolerances.

The alloy has been particularly useful in applications where tool failure could be very costly, if not catastrophic. Due to its great compressive strength, AerMet alloy has been used for auxiliary tooling such as the stem and ram on extrusion presses. It also has been used for ring-type tool holders subjected to millions of lbs. per sq. in., and for a variety of heavy duty punches, blanking dies, crimping dies and embossing dies.

AerMet alloy costs more than conventional tool steels because it is more highly alloyed and more difficult to process.

Powder Metal Steels

Powder metal tool steels and high speed steels are iron based, like conventional tool steels, but are more heavily alloyed with additions of cobalt, tungsten, vanadium and molybdenum. The additional alloying elements and more extensive processing increase the cost, but also improve performance.

PM steels are nitrogen gas atomized to form a powder which is blended, poured into a canister, hot isostatic pressed to produce a 100% dense billet and subsequently processed by state-of-the-art specialty steelmaking methods including hot rolling and rotary forging.

Millforms such as wire, bar, flat, plate and sheet processed in this manner have refined microstructure – smaller, more uniformly distributed carbide particles and finer grain size. The refined microstructure and lack of segregation result from the significantly higher quench rate attained in the as-atomized metal powder particles, compared with the slow cooling rate that occurs in conventionally cast ingots.

The benefits of PM technology and its clean microstructure are plentiful. For the tool maker, PM steels are more forgiving than standard tool steels. They respond to heat treatment more predictably. They are more dimensionally stable when making a tool, and exhibit less out-of-round distortion after heat treatment. Machinability is improved in the annealed condition.

The tool user can expect increased productivity, lower-cost production, less downtime, improved product uniformity, longer tool service life and more consistent behavior. Tools are easier to grind and sharpen in the hardened and tempered condition, with no loss in abrasion resistance of the finished tool. Wear resistance and cutting performance are both improved.

Cold Work PM Steels

A11, shown in a blue oval on the chart, is Carpenter's Micro-Melt® A11 cold work tool steel (AISI A11). This is a high-vanadium powder tool steel that possesses wear resistance superior to most other tool steels, including the high speed steels, along with good strength and toughness characteristics. It can be considered for applications such as: punches, dies for blanking, forming rolls and dies, cold heading, slitter knives and shears.

A11-LVC, with slightly lower vanadium content, is another Micro-Melt cold work steel made by the powder metallurgy process. This grade possesses wear resistance superior to many other tool steels along with good strength and toughness characteristics. A11-LVC alloy can be considered for applications similar to those identified for A11, but more particularly those where more toughness is required, and less wear resistance is needed.
 

High Speed PM Steels

M3T2 is powder high speed AISI M-3 Class 2 alloy with the highest toughness properties of all the PM high speed steels. This is a tungsten-molybdenum alloy possessing the superior wear resistance required for difficult cutting operations. M3T2 (Micro-Melt M3 Class 2) has the lowest alloy content of any of the powder high speed steels, yet exhibits better wear resistance than any of the conventional tool steels.

M4 alloy (AISI Type M4) is a molybdenum-tungsten-bearing powder high speed tool steel containing high carbon and vanadium. It has very high wear resistance coupled with high strength. This Micro-Melt grade is one of the most popular PM high speed steels, more highly alloyed than M3T2 alloy and more resistant to wear. It has been used most frequently for broaches, hobs and form tools.

HS30 alloy is an 8% cobalt, high hardenability tungsten-molybdenum alloyed high speed steel with excellent hot hardness combined with good wear resistance and toughness. It has more wear resistance than M4 alloy, but not as much toughness. This grade is recommended for cutting tools used with difficult-to-machine materials and high cutting speeds. It has been used for applications such as milling cutters, end mills, gear cutting tools and broaches.

T15 alloy (AISI Type T15) is a high-carbon tungsten-cobalt-vanadium high speed powder metal tool steel possessing excellent abrasion resistance and hot hardness. This alloy, one of the most popular powder high speed steels, offers more wear resistance than HS30 alloy. It has been used for applications such as form tools, broaches, blanking dies and punches.

Maxamet® alloy, the most highly alloyed of the PM steels currently produced by Carpenter, is a premium high speed steel that bridges the hardness gap between high speed steel and cemented carbide. It offers higher wear resistance and hot hardness than current powder high speed steels, along with substantially greater toughness and better machinability than carbide.

This new Micro-Melt alloy has room temperature hardness of HRC 70, which is higher than the maximum hardness achieved by conventional powder high speed steels. This approaches carbide hardness of HRC 75 and above, as converted from the HRA scale. Maxamet alloy may be considered by shops that need to produce parts faster in order to stay competitive. The alloy has been formulated to approximate the performance of carbide, but at lower cost.

Maxamet is a candidate alloy for applications such as hobbing tools, form tools, broaches, shave cutters, end mills, drills and a variety of wear components currently made from either conventional high speed steel or carbide.
 

Tungsten Carbides

The use of cemented tungsten carbides for tooling has grown vigorously in recent years, primarily because of their superior wear resistance and hardness. As with most engineered materials, selection of the most suitable grade for an application boils down to an educated compromise. The materials specifier must decide what properties are needed, then make the tradeoffs that best fulfill those requirements.

Carbides are available in preforms specified by dimensional tolerances. They come in rectangular and square blanks, rounds, rod and tubing in size ranges best defined by the manufacturer. Blanks also can be custom made to drawings.

Properties determining the relative positioning of the six carbides on the chart were obtained by means of standard industry test procedures. Wear resistance, correlating directly with hardness, was measured in accordance with ANSI/ASTM B611-76. In this test, an abrasive rubber wheel charged with aluminum oxide crystals was applied to a test specimen. After running a given length of time, weight loss and dimensional change were measured.

ANSI/ASTM B406-76 was used to measure toughness in terms of transverse rupture strength (TRS). TRS values were obtained by placing a rectangular carbide bar across two carbide cylinders, then exerting compressive load on the top bar until shear break.

The properties of a tungsten carbide are determined by the relationship between carbide grain size and percentage of binder which "cements" the carbide grains together. The smaller the carbide grain size, the higher the hardness and abrasion resistance; but the lower the shock resistance. The larger the carbide grain size, the greater the toughness and shock resistance but the lower the hardness.

As color coded on the chart, three categories of carbide grades have been used for tools and dies. One is the conventional grade with a carbide grain size of 1 to 6 microns. The second has a submicron 0.7 micron average grain size. The third classification is an ultrafine submicron 0.5 micron carbide grain size. All three use cobalt for the binder, although nickel or nickel-chrome may be used in corrosive or non-magnetic environments.

The conventional carbide grades have been used typically where light, medium or heavy shock loads are encountered. The submicron carbide grades have been used for the same types of application, but more particularly when a fine, keen cutting edge is also required. The ultrafine submicron grades have been used for the same range of shock loads but, in addition, they provide an unusually high fracture toughness for a given hardness. This combination of properties is advantageous where thin, sharp sections may be subjected to moderate shock loads.

While tungsten carbides are most often selected for their wear resistance, they also offer toolmakers the highest compressive strength of any material available. That makes them a good choice for tooling that is under high compressive load or for the nib inside a steel die used for cold heading. When service under high compressive load is required, carbides can last from 10 to 100 times longer than most steels, depending on the application.

Cemented tungsten carbides have higher wear properties than steel, but still lower than ceramics. The higher cost of carbides can be justified when they wear 10 times or longer than steel. Indeed, most switches to carbides have improved wear life 20 to 50 times over steel, and occasionally up to 100 times.

All of the carbide grades shown on the Fig. 1 chart have higher hot hardness than tool steels, as shown by the curves in Fig. 2. Note that the hot hardness of high speed steels declines dramatically at about 1400°F (404°C), showing a Rockwell "A" hardness of about 64. At that same temperature, tungsten carbides retain a Rockwell "A" hardness of about 77.

Carbides also have a higher heat variance than steel. That is, carbides can operate up to 1850°F before thermal plastic deformation begins. Deformation of most tool steels starts at around 1400°F. On the other hand, the heat variance of ceramics is higher than that of the carbides and steels.

TA00004image003
 

Carbide Grades

C2, represented by a green oval in the chart, is a conventional carbide grade that is most commonly used for tooling that requires better wear resistance than that provided by PM steels. It has 2 micron grain size and uses 6% cobalt for the binder. This grade has 92.0 Rockwell A hardness with a TRS value of 250,000 psi and a density which is 2½ times that of steel. It has, however, only about half the toughness of PM steels.

This C2 grade has been used for applications requiring resistance to light-to-medium shock loads, as well as wear resistance. The material has been used, for example, for form rolls that crush specific shapes into abrasive grinding wheels used in the automotive, aerospace and steel industry, and for poppet valve seats for airless paint spray compressors. These seats have lasted 25 times longer than the previously used steel seats.

The C2 grade symbolized by the yellow oval is a submicron carbide with 0.7 micron grain size bonded by 10% cobalt. It has the same hardness as the C2 with 2 micron grain size, but higher impact resistance and a TRS of 340,000 psi. This grade, with a C10 TRS rating, has been used for shear knives in cutting applications in the paper converting and non-woven industry, for circular slitters used to cut audio, video and memory tape, and for a variety of punches and dies.

C11, another of the four conventional carbide grades, is a general purpose die and wear grade with medium to heavy shock resistance. It has 2 micron grain size, is bonded by 12% cobalt has a Rockwell A hardness of 89.6, and exhibits a TRS of 475,000 psi. C11 has been used in applications such as tube draw mandrels, and burnishing rolls for railroad journals.

C12, the third conventional carbide grade, has 2 micron particulate size and a binder of 16% cobalt. This material has 87.7 Rockwell A hardness and a TRS of 535,000 psi, highest of the grades in this classification. C12 has been used for heavy shock loads such as those encountered in blanking punches and dies, and mill work rolls.

C4 is one of the two ultrafine submicron grades with 0.5 micron grain size and an 8% cobalt binder. It offers 94.4 Rockwell A hardness, a TRS of 320,000 psi and the highest wear resistance of all the carbides. It has been used in applications requiring superior resistance to wear and abrasion, such as air jet dies, nozzles for water jet abrasive cutting, and nozzles in general.

C10 is an ultrafine submicron grade with 0.5 micron grain size bonded by 15% cobalt. It has a Rockwell A hardness of 91.5, and a TRS of 570,000 psi, the highest in this classification. This grade has been used most often where medium-to-heavy shock loads are encountered, and where keen and exceptionally strong cutting edges are required. When this grade was used for shear blades to cut photographic film, it lasted four times longer than conventional carbide grades. When used for stick mold dies in compacting very abrasive silicon carbide crystals, the C10 grade lasted five times longer than conventional carbides.

The C14 grade has been used most often for heavy shock load, high impact applications such as cold heading dies for the automotive and aerospace industries, and for heavy duty blanking dies. It is the fourth conventional carbide grade, also with a medium size grain of 2 to 3 microns. C14, with a 25% cobalt binder, has Rockwell A hardness of 84.5 and a TRS of 520,000 psi. This grade is considered the toughest of all the carbides, with the greatest impact resistance, but the lowest wear life.
 

Ceramics

The use of ceramics for tooling components is increasing steadily, with choice depending on anticipated production cost benefits, productivity gains and quality improvements. Toughened zirconia ceramic (ZrO2) has been used most commonly for non-impact tooling applications such as drawing, roll forming, extrusion and crimping. Alumina ceramic (Al203) shown in upper left of Fig. 1, has been used to a lesser degree for special cutting tools, and some form and press tool applications.

Compared with carbides and the ferrous tooling materials, toughened zirconia ceramics generally have provided longer life, reduced wear rates (permitting better dimensional control of product), and improved surface finish. Shops concerned with pollution and disposal of used cutting fluids also appear to like ceramics because they often can be run lubricant-free.

Ceramic tools are produced by compacting ceramic powders using such methods as uniaxial pressing, isostatic pressing or injection molding. The resulting compacted forms may be machined in the "green state" before sintering at high temperatures. The sintered parts, at full density and hardness, can have special features, tolerances and surface finishes added, usually by diamond grinding.

In applications such as extrusion dies or drawing dies, the ceramic die insert is press- or shrink-fit into a metal housing to withstand the hoop or tensile stresses applied. This is the same process as that employed with carbide dies.

Rolls may be made entirely from zirconia ceramic, depending on the size required; but doing so could be expensive. In the case of large rolls, it is more economical and common to insert a ceramic tire inside a tool steel hub or housing to offset tensile forces. The ceramic is needed only for the tool surface that is subject to wear. Ceramics may provide best performance, in fact in a tooling package that includes tool steels and/or carbides. The trick is to determine what combination will work best, based on the operation conditions involved.

Zirconia ceramic can be considered when (a) die, roll or tooling life is low or (b) excessive wear results in poor product quality or (c) tooling corrosion occurs or (d) galling is a problem or (e) electrical conductivity, such as that required in weld rolls, is a problem.

Typically, zirconia ceramic has high toughness roughly similar to cast iron, high flexural strength twice that of mild steel and a high hardness of 75 Rockwell C. It is an electrical insulator and is chemically inert to most reagents. In addition, it has good abrasion resistance and is non galling when mated with most metals.

Ceramic Grades

Carpenter's zirconia ceramic is a transformation-toughened partially stabilized zirconia (PSZ) which, with its 3% magnesia content, imparts resistance to crack propagation. It has been made under a patented process, bridging the transition from a hard and brittle ceramic to a tough, fracture- and crack-resistant ceramic. See Fig. 3 for key properties.

Zirconia ceramic has been highly effective in the production of tooling for medium-to-low impact applications, but is not considered appropriate for metal cutting. It has been effective in a variety of can tooling applications, most notably seaming rolls. Other applications include: copper and brass extrusion dies; rolls and guides for steel and aluminum tubing; inserts for turkshead rolls for tube production; tooling for dry cell battery production, etc.

Fig. 3 - Properties of Zirconia Ceramics

Property  

Value 

Toughness 

Flexural strength 

Coefficient of thermal exp. 

Density 

Hardness 

Tensile strength 

Compressive strength 

Electrical resistivity (20°C) 

8-12 MPa* sqrt (M) 

800 MPa 

10.x 2 10-6/°C 

5.74 g/cm3 

1120 kg/mm2 (75 HRC) 

450 MPa 

1990 MPa 

>108 

In general, zirconia ceramic tooling costs 3 to 10 times as much as traditional tool steel tooling, 2 to 5 times the cost of PM tooling and about twice the cost of tungsten carbides. The size and complexity of the component often influence this factor.

As with any tooling material, the higher cost of ceramic tooling must be justified in terms of longer life versus traditional materials, increased productivity (process does not have to be stopped to clean or polish the tooling or adjust the machines) or improvement in product quality (fewer scratches, less surface damage, better dimensional conformance, etc.)

Two typical applications support the good judgment in using zirconia ceramic for tooling. Seaming rolls of this material for making and closing steel food cans have lasted 5 to 10 times longer than conventional steel and stainless steel rolls. The ceramic rolls significantly reduced the need for machine adjustment, produced scratch-free can seams and eliminated tinplate and lacquer pick-up.

In another case, zirconia ceramic weld rolls and rotary seam guide rolls outlasted steel and carbide rolls 3 to 10 times. In addition, the non-conductive ceramic rolls eliminated arcing between the RF welder and roll, thus preventing surface damage to the steel tube.

Alumina ceramic traditionally has been used for wear applications when high hardness is needed, along with corrosion resistance.

Although it has been used less frequently than zirconia ceramic in applications such as rolls and dies, alumina has been an important material for cutting tools. Such tooling may be made from high purity alumina or from any of several alumina alloys and composites that are alloyed with TiC, SiC, ZrO2, Cr203 and silicon carbide whiskers.

Unfortunately for the materials specifier, much of the hard data on ceramic tooling is empirical, and not collated in a manner that allows one to make an easy and well informed selection. The causes of wear in ceramics are not the same as for metals; hence, test procedures differ, and data are not directly comparable.

There are no industry standards for ceramics, such as those in the metals industry. The properties of materials reported by different suppliers are measured by a variety of methods. As a result, field trials are ultimately required to confirm performance. Also, it is important to understand that similar ceramic products can vary greatly from one supplier to another. When a successful ceramic tooling application has been developed, build on it. If the materials specifier must look for alternatives, s/he should do so very carefully.

Summary

Choosing the right material for tooling is seldom an easy task, particularly when one considers the impressive advances made in the machines using the tooling and the associated need for higher standards of performance. Proper tool material selection is critical to meeting the productivity, quality and cost goals of metalworking companies everywhere.

In an attempt to make the selection process easier, this article describes some of the most common ferrous tooling alloys, tungsten carbides and ceramics that might be considered. It does not and cannot include every grade or every other material that has some tooling application.

When narrowing the choice, it is generally advisable to consult with the tooling material supplier. Too much is at stake to be unsure of the choice contemplated. In the case of ceramic tooling, with few industry standards and no uniformity of properties from one producer to the next, consultation can be of especially great value.