Dissolution of carbon in Cr-prealloyed PM steels: effect of carbon source

Abstract

Modern water-atomised steel powder grades are characterised by the presence of two types of surface oxides: a thin iron oxide layer, covering more than 90% of the powder surface, and more thermodynamically stable particulate oxides. The development of inter-particle necks and carbon dissolution in the iron matrix both require efficient removal of the iron oxide layer. Hence, carbon reactivity strongly affects the surface oxide reduction that determines inter-particle neck development and carbon dissolution, and so microstructure development. An analysis is presented of the effect of three carbon sourcessynthetic graphite, natural graphite and carbon black -on microstructure and inter-particle neck development in Cralloyed PM steels. Metallographic and fractographic studies indicate that the most significant property of the carbon sources affecting reactivity is the carbon powder size. Carbon black shows the highest reactivity at elevated temperatures but is fully inert at temperatures below 900uC.

Effect of Active Components of Sintering Atmosphere on Reduction/Oxidation Processes During Sintering of Cr‐Alloyed PM Steels

“…Another important feature of the observed mass gain is that it is almost independent on graphite presence—only slightly lower mass gain was observed in case of compacts admixed with graphite, see Table . This indicates that the utilized graphite grade is inactive until ~1000°C, as was confirmed by thermal analysis, metallographic, and fractographic studies. Such mass gain in this temperature range can be connected to two processes—oxidation and/or carburization.…”

“…Reduction by solid carbon (graphite)—direct carbothermal reduction described by Eq. —is possible only in the places of direct contacts between the surface oxide and carbon source powder and is dependent on the carbon source properties . This reaction is thermodynamically possible above Boudouard equilibrium (> ~720°C), however, experimentally is observed above ~900°C according to the results obtained applying a number of different gas analysis techniques …”

“…(2)-is possible only in the places of direct contacts between the surface oxide and carbon source powder and is dependent on the carbon source properties. [3][4][5][6] This reaction is thermodynamically possible above Boudouard equilibrium (>~720°C), however, experimentally is observed above~900°C according to the results obtained applying a number of different gas analysis techniques. 13,[15][16][17][18] Reduction in surface oxides by gaseous reducing agents is of paramount significance due to the high kinetics of the reduction reaction.…”

Effect of Processing Route on the Microstructure and Mechanical Properties of Hot Work Tool Steel

“…An EDS analysis (Table 3) shows the chemical composition of both areas, with different C, Mn and V contents. This was caused by decarburization of the samples, which created a border region lacking Mn and C. This is associated to a carbothermal reduction phenomena of surface oxides 24 , as observed by Danninger et al 25 in Cr pre-alloyed steels, where deoxidation takes place leading to a mass loss and formation of carbon-monoxide and carbondioxide (CO/CO 2 ). Note that this carbon loss would cause the dashed line indicated in the phase diagram of the Figure 9 to move left, first increasing the temperature at which liquid phase happens, but for higher carbon losses, decreases it (the phase diagram was constructed based on the alloy composition as function of the carbon content).…”

Reactivity of Carbon Based Materials for Powder Metallurgy Parts and Hard Metal Powders Manufacturing

“…However, there are only a few studies about the mechanisms of its dissolution and reactivity in dependence on the carbon sources with solid metals/oxides [2][3][4] . In particular, there is little knowledge about the connected interactions between solid carbon, the atmosphere and solid metal/oxides.…”