TOOL STEEL GENERAL INFO
A tool must often be harder, stronger, and more wear-resistant than the material it processes. Therefore, the materials used for tool making must, with a few exceptions, be as hard and strong as possible in accordance with the conditions of their application, but with sufficient ductility. Especially in tools that separate, shape, and reform, or are subjected to impact or shock, extremely high hardness, good wear resistance, and safety against breakage at the highest achievable hardness with high ductility are desired.
The most important parameter in characterizing the operating properties of a tool is the hardness, usually determined by Rockwell or Vickers methods. When an impression of the indenter on the surface is not desired, hardness can also be measured by the rebound method. For very hard and brittle materials, the Knoop hardness test can also be used. Although the elastic limit, yield strength or 0.2% proof stress, tensile strength, elongation at break, and reduction of area determined by the tensile test can be taken as criteria for evaluating the strength and deformability in tool materials, since very little plastic deformation occurs until fracture in tool materials, these are not sufficient to define the material. Better evaluation of mechanical properties can be made by the 0.1% bending limit determined by the static bending test, bending strength, and determination of elastic and plastic deformation work. Torsion and impact torsion tests can also be used to test tool materials in accordance with practical conditions. Although tool life is determined by separating it from the danger of breakage, determining quantitative values for ductility and fracture resistance remains problematic.
Until now, ductility is mostly determined on the basis of the work spent until fracture, in connection with the yield strength and bending fracture strength. Accordingly, qualitative ductility levels can be classified as follows:
- Brittle: Low strength, low plastic deformation work
- Ductile soft: Low strength, high plastic deformation work
- Ductile hard: High strength, high plastic deformation work
The determination of the ductility state of high-hardness materials has mostly been done by impact bending and static bending tests on unnotched and notched specimens. However, with the development of fracture mechanics, new possibilities for characterizing ductility based on fracture toughness (KIC) are also provided for relatively brittle tool materials. Thus, defining this material characteristic as resistance against unstable crack propagation can also be linked to wear conditions.
The capacity and life of cutting tools are determined primarily by the wear phenomenon at the contact point between the tool and the material. Since temperatures up to over 1000 °C can arise during machining, thermal stresses as well as mechanical stresses affect the wear mechanism. Thus, microscopic adhesion wear and crack formation events occur as a result of the decrease in thermal strength and adhesion developing with the tearing off of cutting particles.
Cutting edge wear seen in the form of rounding of the cutting edges is mostly seen in unalloyed and low-alloy steels. The formation of the wear land, whose width is defined as wear mark "B", is characterized as clearance face wear. Rarely, wear called rake face wear is observed on the rake face. This form of wear is observed in the form of crater wear, especially in tools made of high-speed steels and hard metals. In addition to cutting, small flat craters form, deepening with the continuation of cutting time and shifting along the cutting edge, rapidly leading to destruction and blunting.
As the wear parameter, the crater depth on the rake face and the distance of the crater axis to the cutting edge are important. To determine this, a machinability test is required. For this purpose, a wear characteristic test is carried out, where the wear mark width B is determined depending on the cutting time or cutting path. The acceptable wear land width depends on the material, type of tool, and economic perspective. At the very beginning, by directly measuring wear during the operation process, optimization of the machining method can be performed.
MELTING AND FORMING
Tool steels are produced in principle as high-grade steel, and mostly in basic arc furnaces. The most important approach for good quality is the use of clean scrap containing small amounts of Cr, Ni, and Cu. High-quality tool steel production, in which melting is performed under vacuum in an electron-beam multi-chamber furnace (EBM) and as electroslag remelting (ESR), is also becoming widespread. While in the EBM method the improvement of quality is achieved by solidification at low pressure and in a water-cooled crystallizer, in the ESR method it is reached by refining through a reactive slag through which the steel drops. The structure of ultra-pure (UP) steels produced in this way is free of cavities, blowholes, pores, and shrinkage, and they show better chemical homogeneity due to very little segregation tendency. The good core properties obtained in this way are especially beneficial for large-sized tools. Another advantage of ultra-pure steels is the significantly reduced gas content. Thus, in the EBM method, the amount of oxygen is reduced to about 70% and the amount of nitrogen to 30% to 50%. Along with the reduction of the amount of sulfur, the amount of non-metallic inclusions is greatly reduced and a significant improvement in microscopic cleanliness is achieved. In addition, when remelting under vacuum, easily volatile elements such as Pb, Bi, Sb, and As, which reduce hot workability and hot ductility, can be completely removed from the steel. Polishability (production of high-polymer materials and cold rolling tools), improved wear resistance (increased hot wear resistance), and increased hot ductility (reduced risk of burning and cracking) are of great importance in the manufacture of tools from ultra-pure steels. The life of tools manufactured from ultra-pure steels is 20% to 100% higher than steels produced in the conventional manner, depending on the tool type and operating conditions.
With a further development of the heating system, the appearance of crystal segregations has also been prevented and isotropic properties have been given to the steels. As a result of providing a single-phase structure, the ductility rising in the direction perpendicular to the rolling direction improves the rupture strength and reduces notch sensitivity in multi-axial stress states. Cold work and hot work tool steels produced according to this method have two to three times higher service life.
After the ingots are cast, they are processed again by rolling or forging. Since a cementite network can form in high-carbon steels and black fracture can occur at low temperatures, care must be taken to maintain the hot forming temperature exactly.
Materials produced by steel casting or precision casting are only limitedly preferred as they can be economical for tools produced in large quantities. In the case of using steel casting, the hot strength and wear resistance of the tool increase and better isotropy is provided in mechanical properties.
EFFECTS OF ALLOYING ELEMENTS
By adding alloying elements, the properties of tool steels can be changed manifold. The alloying elements, which are either dissolved in the iron lattice or added for special carbide formation, improve hardenability, tempering resistance, hardness, strength, ductility, and wear resistance to different extents. The special effect of each of the important alloying elements in tool steels is summarized as follows:
- Carbon: With rapid quench hardening, it is possible to achieve a hardening depth of 1 to 4 mm in unalloyed steels. Above 1% carbon, the maximum hardness that can be achieved is constant, but wear resistance increases gradually with increasing carbide content.
- Manganese: Due to lowering the transformation rate, it increases hardenability and thus provides hardenability in larger sections. However, it also causes grain coarsening and leads to temper embrittlement. It has a cold hardening tendency to increase wear resistance under impact and compressive stresses.
- Silicon: It increases resistance to oxidation, but at the same time the tendency for decarburization also increases. Due to raising the elastic limit, silicon alloy steels are used for tools with good spring properties. In hot work tool steels, the sticking tendency is reduced with 1% Si.
- Chromium: It lowers the critical cooling rate and thus increases hardenability. Since it forms special carbides, it increases wear resistance and cold resistance. It is one of the most important alloying elements in tool steels.
- Tungsten: It acts as a grain refiner, reduces sensitivity to overheating, and forms special hard carbides that improve wear resistance, hot strength, and tempering resistance. The downside is that it reduces thermal conductivity and, in connection with this, increases the tendency for crack formation during heat treatment.
- Molybdenum: It prevents temper embrittlement and increases hardness, wear resistance, and tempering resistance as a strong carbide former.
- Vanadium: As a result of forming difficult-to-dissolve carbides, it prevents grain growth at high austenitizing temperatures and increases wear resistance. Therefore, at high vanadium contents, the polishability of the tool deteriorates.
- Cobalt: It increases the solubility of carbide-forming elements in austenite and also raises hot strength, hot hardness, tempering resistance, and thermal conductivity.
- Nickel: It improves hardening depth and refines the grain. The addition of nickel is of particular importance in increasing ductility in tools operating with impact and shock loads.
From tools that shape or reform material, a good resistance to abrasive wear is expected alongside high strength with sufficient ductility. When hot work tool steels are used for high-temperature forming, they must withstand both mechanical and thermal stresses. In addition to good hot hardness and hot strength, they must have a sufficient level of thermal stability, expressed as resistance to scaling due to oxidation and sensitivity to burning and heat cracking. If the tool is subjected to excessive temperature variations in a periodic workflow, such as a die casting mold or forging die, heat cracks can form. The tool surface in direct contact with the heated material heats up suddenly and expands within less than a second. Compressive stresses are formed due to the lesser expansion of the cooler inner layers of the tool material, and tensile stresses are formed in the subsequent cooling, creating the opposite situation.