TOOL STEEL

To develop their best properties, tool steels are always heat treated. Because the parts may distort during heat treatment, precision parts should be semi finished, heat-treated, then finished. Severe distortion is most likely to occur during liquid quenching, so an alloy should be selected that provides the needed mechanical properties with the least severe quench (Table T.6).

Steels are used primarily for cutters in machining, shearing, sawing, punching, and trimming operations, and for dies, punches, and molds in cold- and hot-forming operations. Some are also occasionally used for nontool applications. Tool steels are primarily ingot-cast wrought products, although some are now also powder-metal products. Regarding powder-metal products, there are two kinds: (1) mill products, mainly bar, produced by consolidating powder into "ingot" and reducing the ingot by conventional thermo mechanical wrought techniques, and (2) end products tools, produced directly from powder by pressing and sintering techniques. There are seven major families of tool steels as classified by the American Iron and Steel Institute: (1) high-speed tool steels, (2) hot-work tool steels, (3) cold-work tool steels, (4) shock-resisting tool steels, (5) mold steels, (6) special-purpose tool steels, and (7) water-hardening tool steels.

High-Speed Tool Steels

These steels are subdivided into three principal groups or types: the molybdenum-type, designated M1 to M46; the tungsten-type (T1 to T15); and the intermediate molybdenum-type (M50 to M52). Virtually all M-types, which contain 3.75 to 9.5% molybdenum, also contain 1.5 to 6.75% tungsten, 3.75 to 4.25% chromium, 1 to 3.2% vanadium, and 0.85 to 1.3% carbon. M33 to M46 also contain 5 to 8.25% cobalt, and M6, 12% cobalt. The T-types, which are molybdenum-free, contain 12 to 18% tungsten, 4 to 4.5% chromium, 1 to 5% vanadium, and 0.75 to 1.5% carbon. Except for T1, which is cobalt-free, they also contain 5 to 12% cobalt.

Both M50 and M52 contain 4% molybdenum and 4% chromium; the former also contain 0.85% carbon and 1% vanadium, the latter 0.9% carbon, 1.25% tungsten, and 2% vanadium.

TABLE T.6

General Characteristics of Tool Steels

The molybdenum types are now by far the most widely used, and many of the T-types have M-type counterparts. All of the high-speed tool steels are similar in many respects. They all can be hardened to at least Rockwell C63, have fine grain size, and deep-hardening characteristics. Their most important feature is hot hardness: they all can retain a hardness of Rockwell C52 or more at 538°C. The M-types, as a group, are somewhat tougher than the T-type at equivalent hardness but otherwise mechanical properties of the two types are similar. Cobalt improves hot hardness, but at the expense of toughness. Wear resistance increases with increasing carbon and vanadium contents. The M-types have a greater tendency to decarburization and, thus, are more sensitive to heat treatment, especially austenitizing. Many of the T-types, however, are also sensitive in this respect, and they are hardened at somewhat higher temperatures. The single T-type that stands out today is T-15, which is rated as the best of all high-speed tool steels from the standpoint of hot hardness and wear resistance. Typical applications for both the M-type and T-type include lathe tools, end mills, broaches, chasers, hobs, milling cutters, planar tools, punches, drills, reamers, routers, taps, and saws. The intermediate M-types are used for what somewhat similar cutting tools but, because of their lower alloy content, are limited to less-severe operating conditions.

Hot-Work Tool Steels

These steels are subdivided into three principal groups: (1) the chromium type (H10 to H19), (2) the tungsten type (H21 to H26), and (3) the molybdenum type (H42). All are medium-carbon (0.35 to 0.60%) grades. The chromium types contain 3.25 to 5.00% chromium and other carbide-forming elements, some of which, such as tungsten and molybdenum, also impart hot strength, and vanadium, which increases high-temperature wear resistance. The tungsten types, with 9 to 18% tungsten, also contain chromium, usually 2 to 4%, although H23 contains 12% of each element. Tungsten hot-work tool steels with higher contents of alloying elements are more heat resistant at elevated temperatures than H11 and H13 chromium hot-work steels but the higher percentage also tends to make them more brittle in heat treating.

The one molybdenum type, H42, contains slightly more tungsten (6%) than molybdenum (5%), and 4% chromium and 2% vanadium. These alloying elements (chromium, molybdenum, tungsten, and vanadium) make the steel more resistant to heat checking than tungsten hot-work steels. Also, their lower carbon content in relation to high-speed tool steels gives them a higher degree of toughness.

Typical applications include dies for forging, die casting, extrusion, heading, trim, piercing and punching, and shear blades.

Cold-Work Tool Steels

There are also three major groups of cold-work tool steels: (1) high carbon (1.5 to 2.35%); high chromium (12), which are designated D2 to D7; (2) medium alloy air-hardening (A2 to A10), which may contain 0.5 to 2.25% carbon, 0 to 5.25% chromium, 1 to 1.5% molybdenum, 0 to 4.75% vanadium, 0 to 1.25% tungsten, and, in some cases, nickel, manganese or silicon, or nickel and manganese; and (3) oil-hardening types (O1 to O7). They are used mainly for cold-working operations, such as stamping dies, draw dies, and other forming tools as well as for shear blades, burnishing tools, and coining tools.

Shock-Resistant Tool Steels

These steels (S1 to S7) are, as a class, the toughest, although some chromium-type hot-work grades, such as H10 to H13, are somewhat better in this respect. The S-types are medium-carbon (0.45 to 0.55%) steels containing only 2.50% tungsten and 1.50% chromium (S1), only 3.25% chromium and 1.40% molybdenum (S7), or other combinations of elements, such as molybdenum and silicon, manganese and silicon, or molybdenum, manganese, and silicon. Typical uses include chisels, knockout pins, screwdriver blades, shear blades, punches, and riveting tools.

Mold Steels

There are three principal mold steels: (1) P6, containing 0.10% carbon, 3.5% nickel, and 1.5% chromium; P20, 0.35% carbon, 1.7% chromium, and 0.40% molybdenum; and P21, 0.20% carbon, 4% nickel, and 1.2% aluminum. P6 is basically a carburizing steel produced to tool-steel quality. It is intended for hubbing — producing die cavities by pressing with a male plug — then carburizing, hardening, and tempering. P20 and P21 are deep-hardening steels and may be supplied in hardened condition. P21 may be carburized and hardened after machining. These steels are tough but low in wear resistance and moderate in hot hardness; P21 is best in this respect. All three are oil-hardening steels and they are used mainly for injection and compression molds for forming plastics, but they also have been used for die-casting dies.

Special-Purpose Tool Steels

These steels include L2, containing 0.50 to 1.10% carbon, 1.00% chromium, and 0.20% vanadium; and L6, having 0.70% carbon, 1.5% nickel, 0.75% chromium, and, sometimes, 0.25% molybdenum. L2 is usually hardened by water quenching and L6, which is deeper hardening, by quenching in oil. They are relatively tough and easy to machine and are used for brake-forming dies, arbors, punches, taps, wrenches, and drills.

Water-Hardening Tool Steels

The water-hardening tool steels include W1, which contains 0.60 to 1.40% carbon and no alloying elements; W2, with the same carbon range and 0.25% vanadium; and W5, having 1.10% carbon and 0.50% chromium. All are shallow-hardening and the least qualified of tool steels in terms of hot hardness. However, they can be surface-hardened to high hardness and, thus can provide high resistance to surface wear. They are the most readily machined tool steels. Applications include blanking dies, cold-striking dies, files, drills, countersinks, taps, reamers, and jewelry dies.

Coatings

To prolong tool life, tool-steel end products, such as mills, hobs, drills, reamers, punches, and dies, can be nitrided or coated in several ways. Oxide coatings, imparted by heating to about 566°C in a steam atmosphere or by immersion in aqueous solutions of sodium hydroxide and sodium nitrite at 140°C, are not as effective as traditional nitriding, but do reduce friction and adhesion between the work-piece and tool. The thickness of the coating developed in the salt bath is typically less than 0.005 mm, and its nongalling tendency is especially useful for operations in which failure occurs this way. Hard-chromium plating to a thickness of 0.0025 to 0.0127 mm provides a hardness of DPH 950 to 1050 and is more effective than oxide coating, but the plate is brittle and, thus, not advisable for tools subject to shock loads. Its toughness may be improved somewhat without substantially reducing wear resistance by tempering at temperatures below 260°C, but higher tempering temperatures impair hardness, thus wear resistance, appreciably. An antiseize iron sulfide coating can be applied electrolytically at 191°C using a bath of sodium and potassium thiocyanate. Because of the low temperature, the tools can be coated in the fully hardened and tempered condition without affecting hardness. Tungsten carbide is another effective coating. One technique, called Rocklinizing, deposits 0.0025 to 0.0203 mm of the carbide using a vibrating arcing electrode of the material in a hand-held gun. Titanium carbide and titanium nitride are the latest coatings. The nitride, typically 0.008 mm thick, has stirred the greatest interest, although the carbide may have advantages for press tools subject to high pressure. In just the past few years, all sorts of tools, primarily cutters but also dies, have been titanium nitride-coated, which imparts a gold- or brasslike look. The coating can be applied by chemical vapor deposition (CVD) at 954 to 1066°C or by physical vapor deposition (PVD) at 482°C or less. Thus, the PVD process has an advantage in that the temperature involved may be within or below the tempering temperature of the tool steels so that the coating can be applied to fully hardened and tempered tools. Also, the risk of distortion during coating is less.

Another method being used to prolong tool life is to subject the tools to a temperature of -196°C for about 30 h. The cryogenic treatment, which has been called Perm-O-Bond and Cryo-Tech, is said to rid the steel of any retained austenite — thus the improved tool life.

Properties Toughness

Toughness in tool steels is best defined as the ability of a material to absorb energy without fracturing rather than the ability to deform plastically without breaking. Thus, a high elastic limit is required for best performance since large degrees of flow or deformation are rarely permissible in fine tools or dies. Hardness of a tool has considerable bearing on the toughness because the elastic limit increases with an increase in hardness. However, at very high hardness levels, increased notch sensitivity and brittleness are limiting factors.

In general, lower carbon tool steels are tougher than higher carbon tool steels. However, shallow hardening carbon (W-1) or carbon-vanadium (W-2) tool steels with a hard case and soft core will have good toughness regardless of carbon content. The higher alloy steels will range between good and poor toughness depending upon hardness and alloy content.

Abrasion Resistance

Some tool steels exhibit better resistance to abrasion than others. Attempts to measure absolute abrasion resistance are not always consistent, but in general, abrasion resistance increases as the carbon and alloy contents increase. Carbon is an influential factor. Additions of certain alloying elements (chromium, tungsten, molybdenum, and vanadium) balanced with carbon have a marked effect on increasing the abrasion resistance by forming extremely hard carbides.

Hardness

Maximum attainable hardness is primarily dependent upon the carbon content, except possibly in the more highly alloyed tool steels. Tool steels are generally used somewhat below maximum hardness except for deep-drawing dies, forming dies, cutting tools, etc. Battering or impact tools are put in service at moderate hardness levels for improved toughness.

Hot Hardness

The ability to retain hardness with increasing temperature is defined as hot hardness or red hardness. This characteristic is important in steels used for hot-working dies. Generally, as the alloy content of the steel is increased (particularly in chromium, tungsten, cobalt, molybdenum, and vanadium, which form stable carbides), the resistance to softening at elevated temperatures is improved. High-alloy tool steels with a properly balanced composition will retain high hardness up to 593°C. In the absence of other data, hardness after high-temperature tempering will indicate the hot hardness of a particular alloy.

Heat Treatment Hardenability

Carbon tool steels are classified as shallow hardening, i.e., when quenched in water from the hardening (austenitizing) temperature, they form a hardened case and a soft core. Increasing the alloy content increases the hardenability or depth of hardening of the case. A small increase in alloy content will result in a steel that will harden through the cross-sections when quenched in oil. If the increase in alloy content is great enough, the steels will harden throughout when quenched in still air. For large tool or die sections, a high-alloy tool steel should be selected if strength is to be developed throughout the section in the finished part.

For carbon tool steels that are very shallow in hardening characteristics, the P/F test, Disc test, and PV test are methods for rating this characteristic. Oil-hardening tool steels of medium-alloy content are generally rated for hardenability by the Jominy End Quench test.

Dimensional Changes during Heat Treatment

Carbon tool steels are apt to distort because of the severity of the water quench required. In general, water-hardening steels distort more than oil hardening, and oil hardening distort more than air-hardening steels. Thus, if a tool or die is to be machined very close to final size before heat treatment and little or no grinding is to be performed after treatment, an air-hardening tool steel would be the proper selection.

Resistance to Decarburization

During heat treatment, steels containing large amounts of silicon, molybdenum, and cobalt tend to lose carbon from the surface more rapidly than steels containing other alloying elements. Steels with extremely high carbon content are also susceptible to rapid decarburization. Extra precaution should be employed to provide a neutral atmosphere when heat treating these steels. Otherwise, danger of cracking during hardening will be present. Also, it would be necessary to allow a liberal grinding allowance for cleanup after heat treatment.

Machinability

Since most tool steels, even in the annealed state, contain wear-resistant carbides, they are generally more difficult to machine than the open-hearth grades or low-alloy steels. In general, the machinability tends to decrease with increasing alloying content. Microstructure also has a marked effect on machinability. For best machinability, a spheroidal microstructure is preferred over pearlitic.

The addition of small amounts of lead or sulfur to the steels to improve machinability has gained considerable acceptance in the tool steel industry. These free machining steels not only machine more easily but give a better surface finish than the regular grades. However, some caution is advised in applications involving transverse loading since lead or sulfur additions actually add longitudinal inclusions in the steel.

Available Forms

Tool steels are available in billets, bars, rods, sheets, and coil. Special shapes can be furnished upon request. Generally, the material is furnished in the soft (or annealed) condition to facilitate machining. However, certain applications require that the steel be cold-drawn or prehardened to a specified hardness.

A word of caution: Mill decarburization is generally present on all steel except that guaranteed by the producer to be decarburization-free. It is important that all decarburized areas be removed prior to heat treating or the tool or die may crack during hardening.