Argentum Solutions, Inc.
Sterling guidance on corrosion and materials degradation
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David C. Silverman |
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General Classification A tool steel is any steel used to make tools for cutting, forming, or other types of metal-working applications. Though the earliest tool steels were plain carbon steels, the modern tool steels contain alloying elements to provide the properties required for their numerous applications. The resulting alloys can contain relatively large amounts of tungsten, vanadium, molybdenum, manganese, and chromium. Carbon, silicon, and cobalt can also be added. The large number of such steels is a result of the fact that no single tool material combines maximum wear resistance, toughness, and resistance to softening at elevated temperatures. Low-alloy tool steels will harden deeper and faster than plain carbon tool steels. The medium-alloy tool steels will form hard, wear-resistant carbides and are generally used for cutting tools especially those taking finishing cuts. The high-speed tool steels are those that possess large amounts of carbides creating wear resistance. They resist cracking, distortion, and warping. If the carbon content is low, the steels have good impact resistance. If the carbon content is high, the steels have high abrasion resistance. The mechanical properties and machinability of an alloy can be altered considerably merely by changing heat treatment parameters of time and temperature because of their effect on microstructure. The American Iron and Steel Institute (AISI) has developed the classification system most often used. It is usually written as "LX" where "L" is the letter "W", "M", "T", "H", "A", "D", "O", "S", "L", or "P" and "X" is one or two digits. This classification is by composition. This specification is used in this tutorial to differentiate among the alloys. The Society of Automotive Engineers (SAE) uses the same specification though not all tool steels are classified in this designation system. UNS numbers for tool steel are written as a letter followed by five digits,"TXXXXX", in which the first letter is always "T". This designation uniquely specifies each of the alloys. Tool steels are produced to various standards including those of the American Society for Testing and Materials (ASTM). These standards tend to group several AISI classifications into one standard that set out mechanical and metallurgical specifications to which the alloys are produced. Water-hardening steels, the "W" group, are used in such tools as striking, coining and embossing, wood-working, and hard metal cutting. They usually have shallow hardening meaning that they have a hard case surrounding a strong, tough, and resilient core. High speed steels are tool materials developed largely for use in high speed cutting-tool applications. Two classifications of steels would be used in this application, (1) the "M" group which are those steels having a significant amount of molybdenum and (2) the "T" group which are those steels having a significant amount of tungsten. Many manufacturing operations involve punching, shearing, or forming of metals at high temperature. Hot work tool steels, the "H" group, would be used in these applications. These alloys contain a number of constituents and are sub-classified according to those constituents. Cold work tool steels are used in applications which do not require prolonged or repeated heating. Three categories of cold work steel could be used in this type of application, (1) the "A" group which are air-hardening steels, (2) the "D" group which are high carbon, high chromium steels, and (3) the "O" group which are oil-hardening steels. Shock-resisting steels, the "S" group, are used for chisels, rivet sets, punches, and other applications requiring high toughness and resistance to shock loading. Low alloy, special service steels, the "L" group, contain relatively small amounts of alloying elements. These alloys tend to be used machine parts which require good strength and toughness. Mold steels, the "P" group, have relatively low resistance to softening at elevated temperatures. They are used in die-casting dies, in molds for injection, or in compression molds for plastics. General Machining Characteristics The term machinability rating when used with tool steels refers to the ease with which an alloy may be machined relative to the water-hardening "W" group. The water-hardening steels are assigned the value of 100 and the machinability of all other tool steels are related to that value. This rating is used to relate machining speeds between alloys for comparable operations. In this case, the number is the percent of the average machining speed for W group steels required to perform the same operations of turning, boring, drilling, slab milling, and end milling on other tool steels. The lower the number the more difficult the alloy is to machine relative to W tool steels. When discussing machinability of a number of alloys, the B-1112 resulfurized carbon steel or the UNS G12120 (resulfurized and rephosphorized) carbon steel are usually referred to as being 100% machinable (100 on that scale). The relationship between these two scales is that B-1112 or UNS G12120 offers about 75% better machinability (75% faster rate for a given machining operation) than the "W" group. Thus, a 50 on the tool steel scale is about a 25 to 30 on the B1112 scale (approximately 50/1.75). All machinabilities discussed in this bulletin are based on the "W" steels having a machinability rating of 100. Machinability depends on the point in the heat treating cycle at which the alloy is machined. There can be a fairly wide variation in hardness for a given alloy because of heat treatment. This variation in hardness can result in a variation in machinability for the same alloy. Machinability depends on the carbon and alloy content. The high carbon and, in some case, high alloy content tend to make these alloys more difficult to machine than plain carbon steels. Alloys containing less than about 0.75 wt% carbon tend to have poorer surface finish and cause shorter tool life. The carbide spheroids are more likely to become large and widely dispersed in large areas of ferrite. Machinability tends to worsen with increasing alloy content because alloy carbides tend to be more abrasive than iron carbide. Most tool steels become easier to machine after they have been annealed to a state in which the carbide particles are small and spheroidal in a matrix of ferrite. Decreasing machinability translates to greater power and rigidity being required in the machine tool. All tool steels must be heat treated to impart final properties. Normally this treatment has two main steps. The first step is an annealing step in which the alloy is taken to a high temperature under controlled conditions to create the crystal structure corresponding to austenite (austenitizing). The alloy is placed in a relatively soft condition as compared to that after quenching. The hardness can range from values of 100 to 150 (Brinell hardness) for some of the P series mold steels to values of 250 to 300 (Brinell hardness) for some of the M and T series high-speed tool steels. This state is the one in which the tool steels are generally received from the supplier and, as mentioned above, the state in which the alloy is normally machined. If, however, the steels are subjected to hot or cold forming, often they must be fully annealed again before subsequent heat treating operations. Machinability as measured by speed in turning for a given depth of cut tends to decrease as hardness increases in this state. The annealed tool steel is hardened in a second step in which the sample is quenched or cooled under controlled conditions in water, brine, oil, salt, inert gas, or air depending on composition and part thickness. The cooling is rapid to obtain maximum hardness. This quenching may be followed by a tempering step. The final hardness can be as high as Rockwell C 65 to C70 but can also be as low as Rockwell C30 to C35. In addition, sometimes the hardening is not completely through the part but only nearer to the surface. At other times, the complete part is hardened. Machining after quench hardening can be extremely difficult. For example, the acceptable speed in turning a quenched tool steel may be only 20% to 50% that for the same material in the annealed state. Following is a table that provides an overview of the general machinability of tool steels. The percentages are a measure of the relative times to perform a given operation. The percentages were determined by comparing the average recommended cutting speeds for turning, boring, drilling, slab milling, and end milling of these steels.
Tool steels are often subjected to surface grinding. The wide differences in composition among tool steels give rise to wide variations in grinding characteristics. A "grindability index" has been established as a measure of the ease of removing stock by grinding as a measure of wheel wear. Numerically, it is equal to the volume of work removed per volume of wheel wear for a given set of conditions. This index only refers to gross removal of material, not to the susceptibility of the material to cracking during or after grinding or to the ease of obtaining good surface finish. Grindability tends to decrease as hardness increases but the variation is complicated by composition, especially carbides. Surface roughness tends to decrease as hardness increases, all other factors held constant. Classification of steels as low, medium, or high in grindability is usually adequate for practical purposes. Those alloys with greater amounts of vanadium above about 3 wt% have low grindability. In fact, all alloys with greater than about 1 wt% vanadium are fairly difficult to grind because of the formation of vanadium carbide. The following Table shows the relative grindability of tool steels.
Individual Tool Steel Classifications Water Hardening Tool Steels (W Group)The water hardening tool steels are designated by a W followed by a single digit in the AISI classification. These tool steels are considered the plain carbon tool steels that have carbon content in the range of 0.8 to 1.5 wt% with small amounts of chromium and vanadium added to provide toughness. The Society of Automotive Engineers defines four grades of these type of steels:
Type W tool steels are the standard against which machining of tool steels is compared. They are the easiest to machine (a 100% rating) but, as a group, they are only about half as machinable as a free cutting steel, e.g. B1112. They are usually machined in the annealed state in which the hardness is about 150 to 200 on the Brinell scale. These values are about the softest of the annealed tool steels. These alloys tend to be in the general classification of high grindability. They can be susceptible to tempering if overheated during grinding. High Speed Steels (M and T groups) Members of these classes of tool steel are specified by the letters "M" or "T" followed by one or two digits. The M group contains high concentrations of molybdenum. The T group contains high concentrations of tungsten. Both groups also tend to contain chromium, vanadium, and, in some cases, cobalt. Members of these classes are the steels that deep harden (harden far below the surface), retain that hardness at elevated temperatures, and have a high resistance to wear and abrasion. They are used for lathe centers, blanking dies, hot forming dies, lathe cutting tools, drills, taps, and other high speed tooling. Most of the M class alloy compositions fall in the range of 4 to 9 wt% molybdenum, 4 wt% chromium, 1.5 to 6 wt% tungsten, about 2 wt% vanadium, and about 0.7 to 1.5 wt% carbon. In the case of the T class alloys, levels of tungsten range between 17 and 20 wt% in place of the molybdenum which is usually present at under 1 wt%. The mechanical properties of these two classes are similar. Hardness depends on tempering temperature. A hardness of 60 to 70 on the Rockwell C scale can be obtained, the exact value also being a function of the alloy content. The machinability ranges from 25 to 30 for M15 and T15 tool steels to 40% to 50% for M2 and T1 tool steels. Heat treatment can have an effect. For example, quenched T1 might be ordinarily machined at about 50% of the rate of the W group. Annealed T1 might be machined at about 70% of the rate of the W group, about 40% of the rate of B1112 steel. Machinability decreases as hardness and abrasiveness increase. The high-carbon, high-vanadium types like M4, M15, T9, and T15 are examples that follow this type of adverse machinability behavior. Machining in the annealed state (Brinell hardness of 200 to 250) tends to be easier than after the alloy is quenched (Rockwell C hardness of 40 to above 70). The problem is that a wide variation in hardness can exist for a given tool steel depending on annealing conditions. This variation means that two blocks of the same tool steel could machine differently only because of differences in the heat treatment. The lower alloy members tend to be classified as medium in grindability. The more highly alloyed types, especially those with 3 wt% vanadium or greater (e.g. M3(class 2), M4, and T15) tend to be low in grindability. There is great variation in grindability for each type as a function of hardness. Hot Work Steels (H group) Members of this class of steels are designated by the letter "H" followed by two digits. This class is sub-divided further into three classes:
The chromium hot work steels have a chromium content of about 3 to 5 wt% and 1 wt% silicon. Some contain molybdenum ranging between 1 and 3 wt%. Some contain tungsten, either at the 1 to 2 wt% level or at the 4 to 5 wt% level and all contain small amounts of vanadium. They have good resistance to softening under heat because of the chromium and the ability to form alloy carbides. They are all deep hardening. The tungsten hot work steels have large amounts of tungsten ranging from a low of 8 to 10 wt% (H21) to a high of 17 to 19 wt% (H26). They do not contain molybdenum but do contain chromium at levels comparable to the H10 to H19 series except for H23 which contains 11 to 13 wt% chromium. All contain small amounts of vanadium. The high alloy content of these steels makes them more resistant to elevated temperature than the chromium hot work steels but they also have increased brittleness. Their properties are similar to high speed steels. The molybdenum hot work steels have molybdenum replacing the tungsten. At this time, there is only one member of this class H42 which contains 4.5 to 5.5 wt% molybdenum, 5.5 to 7 wt% tungsten, and 3.75 to 4.5 wt% chromium with smaller amounts of vanadium (~2 wt%) and no silicon. The alloy is similar to the tungsten hot work steels. When used for tools, the "H" steels are machined in the annealed condition with machinability ratings between those of "A" and "D" steels. In the annealed state, the alloys can have a Brinell hardness of around 200 which is much softer than the hardness of the 35 to 60 Rockwell C range found after quenching. Some of the alloys, like H11, containing slightly higher carbon levels have been used for aerospace applications. Such parts made from these alloys are often machined in the quenched and tempered condition which is more difficult. Sometimes small quantities of sulfur are added to the steels to make them easier to machine because of their tendency to be very hard after quenching. If the alloy contains additional sulfur for machinability, it would be denoted as “sulfurized HX” (for example, sulfurized H10). The alloys containing higher levels of tungsten tend to be more difficult to machine than the others. All of the hot worked tool steels are generally classified as medium in grindability with some variation caused by degree of alloying. Those with higher alloy content tend to be harder to grind than those with lower content. These alloys may require more frequent dressing of the wheel. Cold Work Steels (A, D, and O groups) Members of these classes are designated as "A", "D", or "O" followed by one or two digits. The steels are classified as follows:
The air-hardening cold work tool steels contain about 4.75 to 5.5 wt% chromium, carbon levels as low as 0.5 wt% (A8) to as high as about 2.5 wt% (A7), sometimes vanadium and tungsten, and about 1 to 2 wt% molybdenum. The complex chromium or chromium-vanadium carbides enhance the wear resistance provided by the martensitic matrix. These alloys function well under abrasive conditions. The alloys are used for gages and precision measuring tools, brick molds, and ceramic molds. The high carbon, high chromium cold work steels contain about 11 to 14 wt% chromium, 1 to 2.5 wt% carbon, about 1 wt% molybdenum (except for D3) and small amounts of vanadium or tungsten (except for D7). Alloy D3 contains about 1 wt% tungsten in place of molybdenum. Alloy D7 contains about 4 wt% vanadium. After hardening, the alloys have high resistance to softening and wear. All of these steels contain massive carbides that make them susceptible to edge brittleness. Typical applications are long-run dies for blanking, forming, thread-rolling, and deep drawing; gages; and burnishing tools. The oil-hardening cold work steels have a lower alloy content than those above, about 1 to 1.5 wt% carbon and small amounts of chromium and tungsten or molybdenum. The carbon and alloy contents are high enough so that small to moderate sections of the steel can attain full hardness with an oil quench after annealing. As a whole, after hardening the alloys have high resistance to wear but low resistance to softening. Typical applications include dies and punches for blanking, trimming, drawing, flanging, and forming. The relatively high carbon and alloy content in types A and D tool steels promote the formation of large alloy-carbide particles making low hardness difficult to achieve even in the annealed state. The hardness of annealed types A and D tool steels range between 200 and 270 (Brinell) while the hardness of annealed type O tool steels range between 180 and 210. Type O tool steels are easier to machine than either of the other two groups. As a group, these steels tend to be machined in the annealed state because of its softer condition before quenching. Both the carbides and hardness work against the ease of machining these alloys. The machinability of these steels can be rated in the following order from easiest to machine (A2) to hardest to machine (D7): A2>A6>A4>A5>D1>D2>D5>D6>D3>D4>A7>D7. The list is not all-inclusive. In general, the D steels are more susceptible to grinding damage than are the A steels. With the exception of A7 and D7, the cold work tool steels in the A and D class are listed as medium in grindability. Variation in chromium content can affect grindability but the vanadium level is not high enough to affect grindability. The A7 and D7 alloys with greater than 4 wt% vanadium are classified as low in grindability. Shock Resisting Steels (S group) The shock resisting steels are designated by the letter "S" followed by a single digit. No sub-groups exist. The alloys tend to contain low levels (up to several percent) of the elements manganese, silicon, chromium, and molybdenum. Small amounts of vanadium are added. The alloys contain about 0.5 wt% carbon. This combination of alloying elements creates a combination of high strength, high toughness, and low to medium wear resistance. The alloys vary in hardenability from shallow hardening (softer interior) (S2) to deep hardening (similar hardness throughout) (S7), hardness being controlled by composition. The alloys are used for chisels, rivet sets, punches, driver bits, and applications requiring a combination of toughness and resistance to shock loading. Sometimes, they appear in structural applications. As with the other tool steels, most machining is done using annealed material. The hardness of annealed S group tool steels is usually between those of the O and W group. Most machining of S type steels is done under conditions similar to those used for O steels. The best rule of thumb is to begin machining with conditions closer to the W steels and then adjust downward as needed. These alloys tend to be in the general classification of high grindability. They can be susceptible to tempering if overheated during grinding. Low Alloy Special Purpose Steels (L group) The low alloy special purpose tool steels are designated by the letter "L" followed by a single digit as there are only two remaining representatives of this class L2 and L6. The alloys contain less than 1 wt% silicon and small amounts of chromium, molybdenum, and vanadium. Type L2 is available in several ranges of carbon between 0.45 and 1 wt% in increments of 0.1 wt%. Type L6 has about 0.7 wt% carbon. This type also contains about 1.5 wt% nickel for toughness. These alloys are generally used for machine parts such as cams, chucks, collets, and other types of applications requiring good strength and toughness. These steels are preferably machined in the (spherodized) annealed state where they have about 65% to 75% of the machinability as W1 tool steel. Since the carbon content must be specified in L2, it must be known when assessing machinability characteristics for this alloy type. Not all annealed L2 material machines the same because the alloy can have lamellar pearlite along with the spherodized structure. This combination would make the surface rougher and shorten tool life. The microstructure of L2 must be checked if questions arise about the surface roughness and shorter tool life when machining this alloy. These alloys tend to be in the general classification of high grindability. They can be susceptible to tempering if overheated during grinding. Mold Steels (P group) Mold steels are designated by the letter "P" followed by one or two digits (P1-P6, P21, and P22). These alloys contain chromium and nickel as the main alloying elements. The chromium content ranges from a low of 0.2 to 0.3 wt% in P21 steel to 4.0 to 5.25 wt% in P4 steel. The nickel content ranges from not specified in P4 and P20 steels to 3.9 to 4.25 wt% in P21 steel. Molybdenum is present in P2, P4, and P20 tool steel at levels of 1 wt% or less. Aluminum and vanadium are present in P21 tool steel. Significant differences in properties are found among these alloys. Types P2 and P6 have low hardness and low resistance to work hardening in the annealed state. After forming an impression, the mold is carburized, hardened, and tempered to hardness of about 58 on the Rockwell C scale. Types P4 and P6 are deep hardening. Types P20 and P21 normally are supplied already heat treated to a hardness of 30 to 36 on the Rockwell C scale. Type P21 is an aluminum-containing precipitation hardening steel. All of these alloys are used almost exclusively in low temperature die-casting dies and in molds for injection or compression. Dies are often made from Type P mold steels by pressing a hard master form into the die to produce the die impression. This procedure is acceptable because many of the alloys are soft. Type P1 can have a Brinell hardness of 80 to 100 in the annealed state but this alloy is rarely encountered today. Both hardness and alloy content increase together so that the other alloys tend to have higher hardness ranging from about Brinell 100 to 120 for Types P2-P6 to 180 or so for Types P20 and P21. The softness of Types P1 to P6 means that these alloys tend to have poor machinability. Chips tend to be gummy and cause a built-up-edge on cutting edges and drill margins. Dies must be hardened before use which tends to cause distortion. Few turning operations are encountered. Grinding is usually done on the hardened and tempered alloy and rarely on the softer annealed version. Care must be exercised to avoid grinding burn, overheating, and heavy grinding of sharp internal angles. The grinding characteristics are similar to 1 wt% carbon, water hardening tool steels. The higher chromium content of Type P4 makes it more difficult to grind. Special Purpose Tool Steels (F Group) Special purpose tool steels are designated by the letter "F" followed by a "1", "2", or "3". These alloys are also known as carbon-tungsten special purpose tool steels. They have 1 wt% (F1) or 1.25 wt% (F2, F3) carbon, 1.25 wt% (F1) or 3.5 wt% (F2, F3) tungsten, and, for Type F3, 0.75 wt% chromium. These steels are used mainly in applications requiring wear resistance such as cold drawing dies. Machinability varies among these alloys. Type F1 in the annealed state is about 75% as machinable as Type W1 with the same amount of carbon (1 wt%). Types F2 and F3 with higher hardness in the annealed state are about 60% as machinable as Type W1. These alloys are classified as having medium grindability. When hardened, they can reach Rockwell C 65 to 70. Grinding tends to be similar to that for the high speed steels (Types M and T). | |||||||||||||||||||||||||||||||||||||||||||||
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David C. Silverman, Ph.D. - Primary Consultant |
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