• Hryha, Eduard
• Dudrova, Eva

Abstract

Among steel-making techniques Powder Metallurgy (PM) concept utilizes unique production cycle, consisting of powder compaction and sintering steps that give high productivity with low energy consumption and high material utilization. Due to the presence of residual porosity, mechanical properties of PM components are inferior in comparison with structural components produced by other technologies. Improvement of mechanical properties at the same level of porosity can be achieved primarily by adding variety of alloying elements. Therefore modern PM technology for production of high-performance PM parts for highly stressed steel components for automotive industry, for example, rely on techniques of utilization of different alloying elements additionally to adjustment of technological process depending on alloying system used. When talking about high-strength low-alloyed structural steels, the most common alloying elements, additionally to carbon, added in order to increase mechanical performance, are chromium, manganese, silicon and some other strong carbide and carbonitride-forming elements (V, Nb, Ti etc.). In comparison with classical steelmaking practice, alloying of PM steels is much more complicated as additionally to influence of alloying elements type and content on microstructure, mechanical properties, hardenability etc., number of additional aspects influencing powder production and further component processing has to be considered. Traditionally, PM high-strength steels are alloyed with Cu, Ni, and Mo. This results in a considerable difference in price of material between conventional and PM steels, used for the same high-load application, as the price of currently employed PM alloying elements like Mo and Ni is dozens of times higher in comparison with that of Cr or Mn. This situation creates a strong economical stimulation to introduce cheaper and more efficient alloying elements to improve the competitiveness of PM structural parts. So, why the potential of most common for conventional metallurgy alloying elements as Cr, Mn and Si is not utilized in PM? First and basic question that arise is how to introduce these elements in PM – as admixed elemental powder (or master-alloy) or by prealloying of the base steel powder. Chromium prealloyed steels are already successful introduced on the PM market. However due to peculiar properties of manganese (oxygen affinity, high vapour pressure, ferrite strengthening etc.) attempts to develop Mn sintered steels are still ongoing. Issue of appropriate alloying mode, that is the starting point of manganese introduction in PM, is the basic question that has to be answered at the beginning and is the basic topic of this chapter. The easiest way to introduce manganese is by admixing of ferromanganese powder that is cheap and widely available on the market in different grades. This approach was firstly proposes around 30 years ago and have been scrutinized thoroughly from different perspectives (Šalak, 1980; Cias et al., 1999; Šalak et al., 2001; Dudrova et al., 2004; Danninger et al., 2005; Cias&Wronski, 2008, Hryha, 2007). The first thing that has to be considered when dealing with admixed with manganese systems is high affinity of manganese to oxygen, implying possibility of considerable oxidation during component processing due to high activity of manganese in admixed elemental powder. However, the possibility of sintering of admixed with manganese powders was assumed due to so-called ‘‘self-cleaning’’ effect, discovered by Šalak (Šalak, 1980). This effect utilizes unique property of manganese to sublimate at relatively low temperature and created during heating stage manganese vapour protect specimen from further oxidation. Another advantage of admixed manganese systems is manganese homogenization in Fe–Mn powder compacts involving Mn-gaseous phase during the heating stage. Second assumption deals with alloying by different master-alloys that firstly allowed a successful introduction of high oxygen affinity elements in PM industrial production (Zapf et al., 1975; Schlieper & Thummler, 1979; Hoffmann & Dalal, 1979). First developed master-alloys containing manganese–chromium–molybdenum (MCM) and manganese–vanadium–molybdenum (MVM) had a wide range of mechanical properties depending on alloying content, sintered density and processing conditions. Nevertheless, these master-alloys faced with many problems during application (oxides formation during manufacturing process, high hardness of the particles that lead to intensive wear of compacting tools etc.) and fully disappears from manufacturing and research areas. Recent development of Fe–Cr–Mn–Mo–C master-alloys was much more successful and show promising properties for their future industrial utilization (Bei(...)

Topics

• density
• impedance spectroscopy
• molybdenum
• Carbon
• chromium
• phase
• Oxygen
• strength
• carbide
• hardness
• Silicon
• forming
• porosity
• homogenization
• Manganese
• sintering
• vanadium
• structural steel