Tool steels are used for working, cutting, and forming metal components, moulding plastics, and casting dies for metals with lower melting points than steel. Accordingly, tool steels need high hardness and strength combined with good toughness over a broad temperature range.
The microstructure of all tool steels is based on a martensitic matrix. Molybdenum additions in tool steels increase both their hardness and wear resistance. By reducing the critical cooling rate for martensite transformation, molybdenum promotes the formation of an optimal martensitic matrix, even in massive and intricate moulds that cannot be cooled rapidly without distorting or cracking. Molybdenum also acts in conjunction with elements like chromium to produce substantial volumes of extremely hard and abrasion resistant carbides. Increasing physical demands on tool steels result in an increasing molybdenum content. Depending on their application, tool steels are classified into:
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Cold-work tool steels are tool steels used for forming materials at room temperature or at slightly raised temperatures (~ 200°C). Specifically, tools for blanking metallic and non-metallic materials, including cold-forming tools, are manufactured from these steels.
Fundamentally, cold-work tool steels are high carbon steels (0.5-1.5%). The water-quenched W-grades are essentially high carbon plain carbon-manganese steels. Steel grades of the O series (oil-hardening), the A series (air-hardening), and the D series (high carbon-chromium) contain additional alloying elements that provide high hardenability and wear resistance as well as average toughness and heat softening resistance.
The four major alloying elements in such tool steels are tungsten, chromium, vanadium, and molybdenum. These alloys increase the steels' hardenability and thus require a less severe quenching process with a lower risk of quench cracking and distortion. All four elements are strong carbide formers, also providing secondary hardening and tempering resistance.
Hot-work tool steels are tool steels used for the shaping of metals at elevated temperatures. Their principal areas of application include pressure die casting moulds, extrusion press tools for processing light alloys, and bosses and hammers for forging machines. The stresses encountered here are cyclical, often with abrupt temperature changes and recurring mechanical stresses at high temperatures. Hot-work steels must constantly endure tool temperatures above 200°C during use. To achieve optimum performance, hot-work tool steels require the following properties:
Cycle times applied in plastic injection moulding, pressure die casting or press hardening (hot stamping) can be reduced considerably by increasing the tool steel’s thermal conductivity, which significantly raises productivity. Heat conductivity is influenced by several material parameters such as microstructure, defects, and alloying elements.
Armco iron is nearly pure iron with a low defect density and high heat conductivity in the order of 70-80 W/mK. Compared to Armco iron, traditional hot-work steel such as H13 (1.) has much lower heat conductivity in the range of only 20-30 W/mK. This reduced thermal conductivity is due to high lattice distortion and defect density of the (tempered) martensitic microstructure as well as to a substantial content of alloying elements. All these characteristics interact with phonons, electrons, and magnons as the “vehicles” of heat transport.
Since all hot-work steels have a defect-rich martensitic microstructure, the difference in optimizing heat conductivity lies in the alloying composition. When in solid solution, alloying elements can cause local lattice distortion (size misfit vs. iron), modify the electronic structure, and/or have influence on magnetism. Generally, heat conductivity is reduced as the alloy content increases. Looking at individual elements in a solute state, nickel, chromium, and silicon were found to negatively influence heat conductivity. The effects of vanadium and molybdenum appear less detrimental. After tempering, the amount of solute vanadium, chromium, and molybdenum decrease by carbide precipitation, which diminishes their negative effect on heat conductivity.
Effect of alloying element on properties of hot-work steel Property Si Mn Cr Mo Ni V Wear resistance - - + ++ - ++ Hardenability + + ++ ++ + + Toughness - ± - + + + Thermal stability + ± + ++ + ++ Thermal conductivity -- - -- ± - ±Tools for processing plastics are mainly stressed by pressure and wear. According to the type of plastic, corrosive conditions can prevail in addition to stresses. The type of plastic and processing method define the key requirements in addition to those generally valid to hot-work steels:
When tool steels contain a combination of more than 7% molybdenum, tungsten, and vanadium, and more than 0.60% carbon, they are referred to as high-speed steels. This term describes their ability to cut metals at “high speeds”. Until the s, T-1 with 18% tungsten was the preferred machining steel. The development of controlled atmosphere heat treating furnaces then made it practical and cost effective to substitute part or all the tungsten with molybdenum.
Tool steels refer to a variety of carbon and alloy steels that are well-suited and widely used to make tools primarily used for perforating and fabrication. Tool steels are made to a number of grades for different forming and fabrication applications. The most common scale used to identify various grades of steel is the AISI-SAE scale.
In addition, each grade of tool steel has heat treatment guidelines that must be followed to achieve optimum results. (The heat treating processes for stamping applications are different from those used for cutting tools.)
Let's examine tool steel types (characteristics and features) and the heat treatment processes and options.
Tool steels are very different from steels used in consumer goods. They are made on a smaller scale with stringent quality requirements, and are designed to perform in specific applications, such as machining or perforating.
Different applications are made possible by adding a particular alloy along with the appropriate amount of carbon. The alloy combines with the carbon to enhance the steel's wear, strength, or toughness characteristics. These alloys also contribute to the steel's ability to resist thermal and mechanical stresses.
The chart shows some of the commonly used tool steels and their alloy content.
Each alloy element shown in the chart below contributes to a specific characteristic in the finished steel. It can also create an undesirable side effect, particularly when used in excessive amounts. In addition, alloys can react with each other–either enhancing or detracting from the desired results.
Typical Composition
SteelAISIJISDINCMnSiCrWMoVH13H13SKD 611..400.401.005.251.351.00S7S7*1..500.750.253.251.40A2A2SKD 121..000.750.305.001.000.25PM 1V***0.550.400.504.502.152.751.00D2D2SKD 111..500.300..000.750.90PM 3V***0.800.301.007.501.302.75M2M2SKH 511..850.280.304.156.155.001.85PM M4 (PS4)M4SKH 54*1.420.300.254.005.505.254.00PM 9V***1.900.500.905.251.309.10PM 10V (PS)A11**2.450.500.905.251.309.75PM 15V***3.400.500.905.251..50Note: The steels shown above are a representative sampling of commonly used steels and their alloy content.
*No designation
Toughness of tool steel is defined as the relative resistance to breakage, chipping, or cracking under impact or stress. Using toughness as the only criterion for selecting a tool steel, H13 or S7 (shown in the chart above) would be the obvious choice. However, all desired characteristics–and the needs of the job–must be considered when making your selection.
Tool steel toughness tends to decrease as the alloy content increases. Toughness is also affected by the manufacturing process of the steel. The PM (particle metallurgy) production process can enhance the toughness of the steel grade due to the uniformity of its microstructure.
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Hardness also affects toughness. Any given grade of tool steel will have greater toughness at a lower hardness. The lower hardness, however, could have a negative effect on other characteristics necessary to achieve acceptable tool life.
Wear resistance is the ability of the tool steel to resist being abraded or eroded by contact with the work material, other tools, or outside influences such as scale, grit, etc. There are two types of wear damage in tool steels–abrasive and adhesive. Abrasive wear involves erosion or breaking down the cutting edge. Adhesive wear is experienced when the work piece material adheres to the punch point, reducing the coefficient of friction, which increases the perforating pressure.
Increased alloy content typically means increased wear resistance because more carbides are present in the steel, as illustrated in the chart.
Carbides are hard particles that provide wear resistance. The size and dispersion of the majority of carbides are formed when alloys, such as vanadium, tungsten, molybdenum, and chromium combine with carbon as the molten steel begins to solidify.
Greater amounts of carbide improve wear resistance, but reduce toughness.
Compressive strength is a little known and often overlooked characteristic of tool steels. It is a measurement of the maximum load an item can withstand before deforming or before a catastrophic failure occurs.
Two factors affect compressive strength. They are alloy content and tool steel hardness.
Alloy elements such as Molybdenum and Tungsten contribute to compressive strength. Higher hardness also improves compressive strength.
Dayton's in-house metallurgy lab is designed to develop new products and to test and analyze the quality and viability of materials used in the manufacture of Dayton products. Laboratory services include: hardness testing; metallography (e.g., coating thickness); and failure analysis.
Equipment includes a high-resolution scanning electron microscope used to evaluate metal structures and a full complement of high-tech equipment used for specimen preparation, routine testing, microscopy, heat treatment evaluation, and failure analysis.
Dayton's metallurgy lab utilizes leading-edge equipment, employs professional, experienced metallurgists; and is the first full-service laboratory of its kind in the industry.
Heat treatment involves a number of processes that are used to alter the physical and mechanical properties of the tool steel. Heat treatment–which includes both the heating and cooling of the material–is an efficient method for manipulating the properties of the steel to achieve the desired results.
A vacuum furnace is used to heat the metals to very high temperatures and allow high consistency and low contamination in the process. Each grade of tool steel has specific heat treating guidelines that must be followed to acquire optimum results for a given application. Unlike cutting tools, the nature of the stamping operation places a high demand on toughness. Thus, a specific steel grade used as a tool steel for stamping must be heat treated differently than one used in a cutting tool.
Tool steel heat treatment processes include: material segregation; fixturing; pre-heating; soaking; quenching; and tempering. The following procedures are general guidelines for tool steel heat treatment. Certain steels require different timing, preheating and soaking temperatures, and number of tempers, e.g., M2, PM-M4, & CPM-10V.
Segregation by size is extremely important because different individual sizes require different rates in preheat, soak, and quench. Fixturing ensures even support and uniform exposure during heating and cooling.
During pre-heating, both cold-work & high speed tool steels are evenly heated to prevent distortion and cracking. Soaking (austenitizing) is done for a specific time to force some of the alloy elements into the matrix of the steel.
Quenching is the sudden cooling of the parts from the austenitizing temperature through the martensite transfer range. The steel is transformed from austenite to martensite, resulting in hardened parts.
Untempered martensitic steel is very hard, but too brittle for most applications. Tempering is heating the steel to a lower-than-critical temperature to improve toughness. Tool steels are typically tempered at temperatures between 400° - °F.
Cryogenics is a process that aids in transformation of austenite to martensite, ensuring greater hardness results and reduced internal stresses. This process takes place at temperatures between -150° and -310°F and will vary in duration, depending on the size of the parts.
Dayton maintains a state-of-the-art heat treatment facility, including support equipment and systems monitored by our in-house metallurgist.
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