This is out of my book, Building Superior Brazed Tools
Hope it helps.
One thing.
Steel typically takes a sharper edge than carbide but the new micrograin carbides have narrowed the gap considerably.
Tom
Tool Steel There are basically two kinds depending on how it is made. These are ingot cast and powder metal. Tool steels can be hardened to at least Rockwell C63 and will retain Rockwell C52 at 1,000 F. T-15 is generally considered to be best in the widest number of applications
Seven major kinds of tool steel High speed Hot work Cold work Shock resisting Mold steels Special purpose Water hardening
Historical progression
Historically the progression went somewhat as follows =B7Steel =B7Tool Steel (High speed steel) =B7 Cutting alloys - Co-Cr-W-Fe-Si-C (Haynes alloys, Stellite=AE, Talonite=AE) =B7 Tungsten carbides (mostly tungsten carbides) =B7 Cermets & Ceramics =B7 Cubic Boron Nitride =B7 Diamond
The major focus has always been machining steel. The desire has been to do as much work as fast as possible. As you work faster in machining you generate more heat. I attended a lecture where the following values were given. The point was to show how the development of newer materials effected machining operations. The example is based on a certain amount of work taking 100 minute using tool steel. The same amount of work can be done more rapidly using other materials. This example is certainly interesting but is very narrow and overlooks a huge range of variables. A big part of the difference was feed and speeds. A bigger part must have been changing the tools as they wore out from heat, wear, corrosion, etc.
Comparative times to cut steel including tool changes and tool servicing =B7 Tool Steel - 100 minutes =B7 Cutting alloys - 50 minutes =B7 Tungsten carbides 15 minutes =B7 Cermets & Ceramics - 5 minutes =B7 Diamond - 1 minute
Run times - in typical wood sawing applications Steel 2 - 4 hours Talonite=AE (Stellite=AE) 4 - 12 hours Tungsten carbide 8 - 40 hours Cermets 8 - 120 hours
Knoop Hardness Ratings Diamond 6,000 - 6,500 Silicon tungsten carbide (solid) 2,130 - 2,140 Aluminum oxide (corundum) 1,635 - 1,680 Tungsten carbide (Co binder) 1,000 - 1,500 Hardened steel 400 - 800
8=2E Tool Steels
Tool steels are iron-based alloys that are melted and processed to develop characteristics useful in the working and shaping of other metals. Many tool steels are also suitable for machinery components and structural applications in which particularly stringent requirements must be met. Such parts include high-temperature springs, ultrahigh-strength fasteners, special-purpose valves, and bearings of various types for elevated-temperature service.
Tools are typically subjected to extremely high loads that are applied rapidly. The tools must withstand these loads without breaking and without undergoing excessive wear or deformation. No single tool material combines maximum wear resistance, toughness, and resistance to softening at elevated temperatures. Consequently, the selection of the proper tool material for a given application often requires a trade-off to achieve the optimal combination of properties.
Most tool steels are wrought, but some are cast and some are made via powder metallurgy. This article describes the effects of alloying elements, discusses the categories of tool steels, and describes advanced powder metallurgy tool steels and nickel-base tool alloys.
Effects of Alloying Elements
Metals producers customarily melt tool steel alloys in an electric arc furnace. The alloys are refined to produce final chemistries and eliminate deleterious elements, and then poured into molds to solidify into ingots. The ingots are often electroslag remelted (ESR) to improve the microstructure prior to hot rolling or forging into mill forms such as bar, plate, and sheet. Producers then cut the forms to length for fabrication into tools for turning, milling, boring, and drilling.
Conventional cast/wrought steels are preferred for tools and dies in general-purpose machining and forming. These grades are alloyed with carbon, chromium, and small amounts of other elements.
Carbon Carbon is the element most critical to tool properties. In the metal's matrix, carbon and iron form the hard martensite phase (distorted body centered cubic) when heat treated. Carbon also combines with iron, chromium, vanadium, molybdenum, and tungsten to form very hard carbide particles that contribute to wear resistance.
Manganese Manganese additions impart a deeper hardening depth (greater hardenability). Increasing manganese also helps reduce the hardening temperature, and can reduce heat-treating distortion.
Silicon Silicon helps improve impact resistance and, like manganese, provides greater depth of hardening.
Chromium Chromium combines with carbon to form chromium carbides, which enhance wear resistance. Chromium also promotes hardenability, toughness, and scaling resistance.
Tungsten Tungsten adds wear resistance because it combines with carbon to form very hard carbides. It also adds red hardness and promotes secondary hardening.
Molybdenum Molybdenum behaves much like tungsten in that it promotes good hardenability and adds red hardness. Its carbides add wear resistance.
Vanadium Vanadium forms very hard primary carbides that contribute to high wear resistance. Vanadium also adds red hardness and promotes a fine-grain microstructure.
Cobalt Cobalt plays a very important role because of its contribution to red hardness. Normally, this element, like tungsten, is added only to high-speed tool steels.
Sulfur Sulfur additions in very small amounts impart machinability to tool steels. Sulfur is tied up by manganese to form manganese sulfides that act as chip breakers.
Cold-work tool steels Cold-work tool steels do not have the alloy content necessary to make them resistant to softening at elevated temperatures. Therefore, they are restricted to applications that do not involve prolonged or repeated heating above 205 to 260=B0C (400 to 500=B0F). For cold-work applications, they may be air- or oil-quenched during hardening. Because they do not require a severe water quench, such alloys have less heat treat distortion and cracking than do the water-hardening alloys.
Cold-work tool steels share similar characteristics, such as chemistry. Common cold-work alloys include the oil-hardening types, such as 02; the air-hardening grades such as A2 and A6; and the high-chromium, high-carbon, air- or oil-hardened alloys, such as D2.
This group includes carbon steels, shock-resisting grades, and air-hardening and oil-hardening grades.
Carbon-steel tool alloys (group W) Carbon-steel tool alloys (group W) contain carbon as the principal alloying element in amounts ranging from 0.60 to 1.4%. They have very little alloy content, and must be | water quenched to achieve proper I hardness. Also known as water-hardening grades, they have a very hard surface or case, and a softer core. They are suitable for cold heading, striking, coining,) taps, reamers, and similar parts. Group W alloys are not as popular today as in the past.
Shock-resisting grades (group S) Shock-resisting grades (group S) have high toughness for applications such as chisels and punches. Carbon content is about 0.5% for all group S steels, and this results in a combination of high strength, high toughness, and low to medium wear resistance. Group S steels vary in hardenability from shallow hardening (S2) to deep hardening (S7). Type S2 is normally water quenched; types Sl, S5, and S6 are oil quenched. Type S7 can be air hardened, but atmosphere control is important, as S7 is susceptible to decarburization.
Air-hardening, medium-alloy grades (group A) Air-hardening, medium-alloy grades (group A) contain enough alloying elements to achieve full hardness in sections up to about 100 mm (4 in.) diameter when air cooled. Because they are air-hardening alloys, Group A tool steels exhibit minimum distortion and the lowest tendency to crack during hardening. Manganese, chromium, and molybdenum are the principal alloying elements. Types A2, A3, and A7, A8, and A9 contain
5% chromium. Typical applications include shear knives, punches, forming dies, and coining dies.
High-carbon, high-chromium grades (group D) High-carbon, high-chromium grades (group D) contain 1.5 to 2.35% carbon and 12% chromium. All group D tool steels (except D3) are air-hardening and reach full hardness when cooled in still air. Type D3 is usually quenched in oil, making it more susceptible to cracking. Group D steels have high resistance to softening and excellent resistance to wear. Typical applications include forming dies, rolls, burnishing tools, and slitter knives.
Molybdenum high-speed (group M) Molybdenum high-speed (group M) steels contain molybdenum, tungsten, chromium, vanadium, cobalt, and carbon as principal alloying elements. Type M2 contains 4.5 to 5.5% molybdenum, and 5.5 to 6.75% tungsten. Type M7 contains 8.2 to 9.2% molybdenum, and only 1.4 to 2.1% tungsten.
Group M Group M steels were primarily developed for the manufacture of cutting tools. Their most important property is the ability to retain high hardness at elevated temperatures. These materials are heavily alloyed, and their composition provides high strength as well as excellent wear resistance. For this reason, they are often chosen for cold-work applications such as punches and dies.
Tungsten high-speed steels (group T) also contain molybdenum, tungsten, chromium, vanadium, cobalt, and carbon as principal alloying elements. However, T2 contains only 1% max of molybdenum, and 17.5 to 19% tungsten. T8 contains 0.4 to 1% molybdenum and 13.25 to 14.75% tungsten. Group T steels are characterized by high red hardness and wear resistance, as well as deep hardening. They are primarily chosen for cutting tools such as bits, drills, reamers, taps, broaches, and high-temperature structural parts.
Because a combination of factors can contribute to cold-work tooling failure, alloy selection can be challenging. The best choices for cold-work tooling applications typically result from the right compromise between wear resistance and toughness. In selecting cold-work tool steels, the user must first accurately predict the service conditions. For example, breaks or chips in the tool indicate insufficient toughness, suggesting replacement by a shock-resistant tool steel such as S7. Excessive abrasion or galling indicates a need for a wear-resistant alloy such as D2 or M2.
Hot-Work Tooling Alloys
Hot-work steels (group H) were developed to withstand the combination of heat, pressure, and abrasion associated with punching, shearing, or forming at high temperatures. Group H steels have carbon contents ranging from 0.35 to 0.45%. Contents of chromium, tungsten, molybdenum, and vanadium range from 6 to 25%. Common hot-work alloys include Hll, H13, and H19. These alloys have good resistance to heat softening because of their chromium contents of 3 to 5.5%.
Important characteristics of hot-work alloys include hardness at the tool's elevated working temperature, temper resistance, and impact toughness.
The hot hardness of a low-alloy steel diminishes rapidly upon heating to 230=B0C (450=B0F). However, chromium die steels such as H13 or H19 are relatively unaffected until heated to over 425=B0C (SOOT).
Temper resistance reflects an alloy's ability to retain its hardness after lengthy exposure to elevated temperature. When subjected to thermal fatigue, it is critical that these hot-work tooling alloys exhibit resistance to heat checking. High toughness and high hardness have been found to prevent these surface cracks.
Powder Metallurgy Tool Alloys
Powder metallurgy cold-work tool steels offer superior wear resistance compared with conventional tool steel grades, and have good strength and toughness characteristics. Upgrading to powder cold-work steels should be considered for applications such as cold extrusion, cold extrusion barrels, cold extrusion liners, cold heading, compacting tools, dies for blanking, forming rolls and dies, nozzles, pelletizer blades, piercing dies, plastic injection molds, punches, shears, slitter knives, steel mill rolls, and woodworking tools.
To make powder metal alloys, molten metals containing the required alloying elements are nitrogen-gas-atomized to produce powder particles.
The high quench rate of the gas-atomized metal powder particles allows for finer grain size; smaller, more uniformly distributed carbide particles; and reduced or eliminated segregation.
These powders are blended and poured into a canister then hot isostatically pressed to produce 100% dense billets. The billets are further processed by state-of-the-art specialty steelmaking methods. The more refined and homogeneous microstructure of wrought powder metal mill forms such as bar, wire, flats, and plate offer significant benefits over conventional cast/wrought stock