Josef Filípek

Notch aspects of RSP steel microstructure

ČERNÝ, M., FILÍPEK, J., MAZAL, P., VARNERD.: Notch aspects of RSP steel microstructure. Acta univ. agric. et silvic. Mendel. Brun., 2012, LX, No. 5, pp. 49–60 For a rather long time, basic research projects have been focused on examinations of mechanical properties for Rapid Solidifi cation Powder (RSP) steels. These state-of-art steels are commonly known as “powdered steels“. In fact, they combine distinctive attributes of conventional steel alloys with unusual resistance of construction material manufactured by so called “pseudo-powdered” metallurgy. Choice of suitable materials for experimental verifi cation was carried out based on characteristic application of so called “modern steel”. First, groups of stainless and tool steel types (steel grades ČSN 17 and 19) were selected. These provided representative specimens for the actual comparison experiment. For stainless steel type, two steel types were chosen: hardenable X47Cr14 (ČSN 17 029) stainless steel and non-hardenable X2CrNiMo18-14-3 (ČSN 17 350) steel. They are suitable e.g. for surgical tools and replacements (respectively). For tooling materials, C80U (ČSN 19 152) carbon steel and American D2 highly-alloyed steel (ČSN “equivalent” being 19 572 steel) were chosen for the project. Finally, the M390 Böhler steel was chosen as representative of powdered (atomized) steels. The goal of this paper is to discuss structural aspects of modern stainless and tool steel types and to compare them against the steel made by the RSP method. Based on the paper ́s results, impact of powdered steel structural characteristics on the resistance to crack initiation shall be evaluated. stainless steel, tool steel, powdered RSP steel, metallography, fracture behavior Metallic materials have always played a signifi cant role in history of mankind. Some metals (Au, Ag, Cu, Sn, Pb, Fe, and Hg) have been utilized for more than 2000 years. Iron processing ranked among the most important discoveries of the ancient world. Archeological period called “Iron Age” began in the Central Europe approximately in the seventh century BC (Ptáček, 2002). With gradual development of metallurgy, producers have reached limits in terms of material properties. This applies to cast, formed and thermally processed steels. Nevertheless the development has been going on even in this traditional area of metal processing industry. Curiously enough, in some procedures steel is not manufactured by die casting anymore. Some sort of “metal mass” is produced that is then turned into homogenous parts. Examples of such sophisticated mass include state-of-art powders with precise composition properties. The powder is manufactured using physical-mechanical, chemical, electrochemical, and physicalchemical methods (Kraus 2011). Resulting powders are processed using hot or cold compaction. In this phase, pressurized conditions may be used. Then, sintering without or with the liquid phase is performed. Hot forming of these stocks and their thermal processing provide way of reaching unique properties that can never be achieved by the means of other technological procedures (Černý, 2012; Hosford et al., 2007; Franta, 2012). Manufacturing of machine parts made out of sintered steels began in the fi rst half of the twentieth century. This production was economically favorable in case of parts with simple shaped and/ or less stressed parts in mass production. Today powdered steels are extensively used e.g. for punches, metal splitting blades and other tools. All these parts must provide extreme durability and 50 M. Černý, J. Filípek, P. Mazal, D. Varner fracture resistance. With standard steels, fracture strength is reduced structurally e.g. by notch eff ect of coarse carbide particles. These particles represent stress concentrators and are active during loading of the material. Minimal sizes and round shapes of carbides in powdered steels signifi cantly increase values of critical fracture stress. Stressdeveloped pre-nucleation microcrack in the carbide fragmentation probably does not reach critical size for crack initiation resulting in grain destruction (Nauka o materiálu, 2012, Halbych, 2009). No wonder the RSP steels exhibit up to double strength values in comparison with standard steels. It is worth noting that RSP steels represent highly-desired combination of “confl icting” iron alloy properties: toughness and hardness. MATERIALS AND METHODS Total of fi ve steel samples were subject to metallographic analysis. They represent stainless and tool steel materials (Tab. I). Steel X2CrNiMo18-14-3 (ČSN 17 350) – chromium-nickel-molybdenum austenitic weldable steel with very low amount of carbon and resistant to intergranular corrosion. Suitable for food processing industry and medical science applications. This steel can be surgically inserted into bones. The sample was taken from a common hip joint implant. Steel X47Cr14 (ČSN 17 029) – chrome martensitic stainless steel. This steel is suitable for applications where hardness and wear-resistance are required. This includes kitchen knives, scissors, surgical instruments, gauges, and so on. The steel is not suitable for welding. The sample was taken from forged medical scalpel (both blade and handle). Steel C80U (ČSN 19 152) – carbon-rich hardenable tool steel. Applications include forming/cutting tools and hand-operated tools. Steel X165CrMoV12 (D2, ČSN 19 572) – American steel with increased corrosion resistance. Applications include cutting tools, cold cutting tools, crushing tools and milling tools. M390 Böhler steel – powdered steel that can be used to manufacture dies for plastics processing, pressure casting of metals/alloys and so on. The samples were obtained by MTH® metallographic saw with of Al2O3 cutting disk (Halbich 2009, 2011). To observe and display the structure of the material, the following equipment was used: Neophot 32 light microscope for lowzoom images; SEM (Scanning Electron Microscope) for high-zoom imaging and spectral analysis.