Oxide Transformation in Cr-Mn-Prealloyed Sintered Steels: Thermodynamic and Kinetic Aspects
- PDF / 4,425,674 Bytes
- 12 Pages / 593.972 x 792 pts Page_size
- 70 Downloads / 173 Views
E introduction of cheap, effective, and environmentally friendly alloying elements, typically used in wrought steels, such as chromium, manganese, and silicon, has been the main purpose of the numerous attempts of the metallurgist in powder metallurgy (PM) for the last three decades. Price fluctuations of the commonly used alloying elements in PM such as Ni, Mo, and Cu, associated with health and recyclability issues connected to these alloying elements, additionally reinforce research in the area of prealloyed with Cr, Mn, and Si PM steels. There are two substantial obstacles that hinder wide utilization of the Cr, Mn, and Si in PM steels: (1) negative effect on the powder compressibility due to ferrite strengthening by the mentioned alloying elements and (2) high oxygen affinity meaning risk of the oxidation during the powder manufacturing, handling, and further consolidation. Previous research[1] shows that in the case of low-alloyed (200) statistically confirms this observation. With further temperature increasing, inclusions take a spherical shape and slightly decrease in size. The most significant drop in the amount of oxide inclusions was observed between 1273 K and 1393 K (1000 C and 1120 C) where pores become fully oxide free. An odd change in the inclusions’ appearance inside developing inter-particle necks is observed above ~1373 K (1100 C)—significantly lower number of inclusions are
observed, but they are characterized by a larger size and complex elongated shape. The amount is significantly larger in compacts processed in nitrogen atmosphere and they can be seen even after sintering for 30 minutes at 1393 K (1120 C), see Figure 2, but less frequently due to their reduction during sintering. A closer look at such inclusions at high magnification reveals their inhomogeneous structure consisting of a ‘‘fine crystalline’’ phase, enclosed in some ‘‘liquid’’ phase, see Figure 2. A fine crystalline phase is more pronounced closer to the pores (neck surface) and is rich in Cr and Mn with the same ratio of 2:1, see Figure 2. At the same time, inclusions with higher amount of ‘‘liquid’’ phase are located mostly in the center of the neck and are characterized by high silicon content with trace amount of Mn and Cr, see Figure 2. Sintering at 1473 K (1200 C) leads to considerable reduction of oxides (and hence inclusions amount) and only very rarely can residues be observed. Complex inclusions (as, e.g., presented in Figure 2) cannot be observed any longer; however, close observation of the inter-particle necks allows to find their residues in some sites, see Figure 3. As it is confirmed by high-resolution imaging, only a fine crystalline phase remained, that has now, additionally to Cr and Mn with ratio 2:1, become enriched in silicon, see Figure 3 (Sp.2). ‘‘Liquid phase’’-like inclusions are not typical of high-temperature sintered compacts, but seldom observed ones are also rich in oxygen, silicon, and iron with only a trace amount of other elements, see Figure 3 (Sp.1). Such complex-shaped inclusions are not typical for
Data Loading...
