The Role of Oxygen Transfer in Sintering of Low Alloy Steel Powder Compacts: A Review of the “Internal Getter” Effect

Application of thermal analysis techniques to study the oxidation/reduction phenomena during sintering of steels containing oxygen-sensitive alloying elements

“…As observed in Fig. 3 Oxidation/reduction phenomena on steel powder mixes containing additions of different oxidationsensitive elements Thermal analysis techniques have been proved to be very efficient tools for studying the oxidation/reduction phenomena taking place when sintering steel powders [1,2,9,16,17]. Thermogravimetry curve and m28 (CO) signal-which is in all experiments considerably more intense than m44 (CO 2 )-are shown in Fig.…”

“…Thus, the oxygen is simply transferredthrough the gaseous species CO and CO 2 -from the easily reducible Fe oxide covering the Fe base powders to the oxygen-sensitive alloying particles. This phenomenon has been identified as an ''internal-gettering'' effect caused by the presence of elements with a high oxygen affinity and has been described in detail in [9,23].…”

“…Thermogravimetry analysis coupled with mass spectrometry has proved to be the most effective method for analyzing the oxidation/reduction reactions at different stages of sintering [1][2][3]. Identification of critical temperature ranges with these techniques has provided the tools for an optimum design of the sintering cycles for conventional steels and even for Cr-prealloyed grades [1,[7][8][9].…”

Effects of H2 Atmospheres on Sintering of Low Alloy Steels Containing Oxygen-Sensitive Masteralloys

“…However, as is well known from previous studies on Cr-prealloyed powders, 2,3 when elements with high oxygen affinity are introduced by prealloying, an internal getter effect takes place in this case, not through the atmosphere but within the powder particle itself. 8 As a consequence, the iron surface oxides are transformed into more stable oxides, and the reduction processes are shifted to higher temperatures. In Cr-prealloyed powders, the reduction is typically shifted to temperatures around 1100°C.…”

“…[1][2][3][4][5][6][7] It is well known that, when such alloying elements are introduced by admixing-either as elemental particles or as masteralloys (MA)-a significant amount of oxygen is introduced through the base iron powder itself and then transferred to the alloying particles as a consequence of the so-called ''internal getter effect''. [8][9][10][11][12][13] One possible way to avoid oxygen transference from the iron base powder to the oxygen sensitive particles is to reduce the iron oxides with H 2 during the early stages of the sintering cycle ($400°C) when the reactivity of the alloy elements is still low. 2,13 However, in studies of sintering cycles carried out in H 2 , the formation of methane at intermediate temperatures ($700-900°C) has been observed, and it seems to be related to an enhanced homogeneous decarburization as compared to samples sintered in inert atmospheres.…”