Alloy steel used for tool making is well-suited for producing tools such as hand tools and machine dies. The hardness, abrasion resistance, and ability to maintain shape at high temperatures are key characteristics of this material. Heat-treated tool steel is often used because it has a higher hardness.
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Low-alloy steel is commonly known as "Alloy steel" in actuality, whereas High-alloy steel is "Tool steel." The term tool steel stems from this material group mainly used to make cutting, pressing, extruding, and other tools.
Due to added chemical qualities like vanadium, certain grades have increased corrosion resistance. In addition, the manganese concentration of some grades is limited to reduce the risk of cracking during water hardening. Other classes provide alternatives to water for hardening the material, such as oil.
Their hardness, resistance to wear and deformation, and ability to maintain a cutting edge at high temperatures all contribute to their applicability. Tool steels are categorized into numerous main classes, with some of them subdivided further based on alloy composition, hardenability, or mechanical characteristics.
Water-Hardening Tool Steels (Carbon Tool Steels)
These are classified as Type W by AISI, and their usable qualities are exclusively determined by carbon content. Because these steels come in shallow, medium, and deep hardening varieties, the alloy chosen is determined by the cross-section of the item and the desired surface and core hardnesses.
Steels Resistant To Shock (Type S)
They're sturdy and durable, but they're not as wear-resistant as other tool steels. These steels can withstand both one-time and recurring loads. Pneumatic tooling components, chisels, punches, shear blades, bolts, and springs exposed to mild heat in service are examples of applications.
Tool Steels for Cold Work
Oil and air-hardening are two examples. Varieties O, A, and D are more expensive than water-hardening types, but they can be quenched more easily. Type O steels are oil hardening, whereas Type A and D steels are air-hardening (with the least severe quench) and are best suited for machine ways, brick mold liners, and fuel injector nozzles.
Thin parts or components with extreme variations in cross-section parts that are prone to cracking or distorting during hardening - are designated for air-hardening types. These steels have a high surface hardness when hardened; nonetheless, these steels should not be specified for use at high temperatures.
Hot-Work Steels (Type H)
These serve nicely at high temperatures. The tungsten and molybdenum high-alloy hot-work steels are heat and abrasion-resistant. Although these alloys do not soften at these high temperatures, they should be warmed before and cooled gently after service to avoid breaking.
The chromium grades of hot-work steels are less costly than the tungsten and molybdenum grades. One of the chrome grades, H11, is used widely for airplane parts such as principal cargo-support lugs, catapult hooks, airframe structures, and elevon hinges. Grade H13, identical to H11, is typically more easily accessible from vendors.
High-Speed Tool Steels (Tungsten & Molybdenum Alloy)
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These produce good cutting tools because they resist softening and maintain a sharp cutting edge at high service temperatures. This trait is also dubbed "red hardness." These deep-hardening alloys are utilized for sustained, high-load circumstances rather than shock stresses. Typical applications include pump vanes and pieces for heavy-duty strapping machines.
Mold Steels of Type P
These steels are specially intended for plastic-molding and zinc die-casting dies. Nontooling components are rarely made from these steels.
Special-Purpose Tool Steels
Other grades include low-cost, Type L, and low-alloy steels, commonly requested for machine components when wear resistance combined with toughness is necessary. Carbon-tungsten alloys (Type F) are wear-resistant and shallow hardening. However, they are not appropriate for high temperature or shock use.
Avoiding thermal damage
As heat treatability of high alloy tool steels is a quality criterion, thermal influence during cutting has to be avoided in order to ensure a true representation of the actual microstructure. When cutting larger sections, this preparation step has to be carried out with great care.
Fig. 2: Thermal damage due to faulty cutting conditions
Preserving carbides and inclusions
The main difficulty during grinding and polishing of high alloy tool steels is ensuring that carbides and non-metallic inclusions are retained. In cold working tool steels, the primary carbides are very large and fracture easily during grinding. In fully annealed conditions, secondary carbides are very fine and can easily be pulled out from the softer matrix.
Fig. 3: Fractured primary carbides (Mag: 200x)
Large volume processing of high alloy tool steels
For quality control teams working within high alloy tool steel production, processing large sample volumes requires a very efficient organization of the workflow, automatic equipment and standard procedures.
Table 1: Preparation method for high alloy tool steel on large automatic equipment.
DiaPro diamond suspensions can be substituted with DP-Diamond suspension P as follows: For FG with 9 μm, DP 2 with 1 μm used with DP-Blue/Green lubricant.
Table 2: Preparation method for high alloy tool steel on table-top semi-automatic equipment.
DiaPro diamond suspensions can be substituted with DP-Diamond suspension P as follows: For FG with 9 μm, DP 1 with 3 μm, DP 2 with 1 μm used with DP-Blue/Green lubricant.
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High alloy tool steel samples are usually initially examined unetched to identify inclusions and carbide size and formation. To reveal the microstructure, various concentrations of nital or picral are used.
For example, to show the carbide distribution in cold work steel, a 10% nital ensures the matrix is dark and the white primary carbides stand out. For fine globular pearlite, a brief submersion into picric acid followed by 2% nital gives a good contrast and avoids staining.
Nital etching solution:
100 ml ethanol
2-10 ml nitric acid (Caution: Do not exceed 10% of the solution as it becomes explosive!)
Picral etching solution:
100 ml ethanol
1-5 ml hydrochloric acid
1-4 g picric acid
Fig 5: Cold work tool steel etched with 10% nital, primary carbides stand out white
Fig. 6: Hot work tool steel etched with picral and nital, globular pearlite (Mag: 500x)
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