Abstract
For the consolidation of steel parts manufactured by powder metallurgy (PM) techniques, removal of the surface oxides covering metallic powder particles is a necessary prerequisite. In PM steels with conventional compositions, reduction of the oxides is easily achieved in traditional sintering furnaces. However, processing steels containing alloying elements with a high oxygen affinity represents a big challenge that requires a deeper understanding of the chemical processes occurring during sintering. In the present work, thermogravimetry analysis coupled with mass spectrometry is used to describe the oxidation/reduction phenomena that take place when sintering steel powders and how these processes are modified by the addition of admixed particles containing oxygen-sensitive elements. Carbothermal reduction processes are studied using pure oxides (Fe2O3, MnO2, Cr2O3 and SiO2) as well as water-atomized Fe powders mixed with small amounts—4 mass/%—of Cr, Mn and Si powders or Fe–Mn–Si–(Cr) master alloy powders. The results show that there is an oxygen transfer from the base iron particles to the oxidation-sensitive elements—“internal getter effect”—taking place mostly through the gas phase. Different alloying elements (Cr, Mn, Si) show different temperature ranges of susceptibility to oxidation. Combination of these oxygen-sensitive alloying elements in the form of a master alloy powder reduces their sensitivity to oxidation. Also, the use of master alloys promotes the concentration of the oxides on the surface of the alloying particles and not in the grain boundaries of the surrounding iron particles—as occurs when using Mn carriers—which should have a beneficial impact on the final mechanical performance.
Highlights
Sintering of steels containing oxidation-sensitive elements such as Cr, Mn and Si is an important challenge for the powder metallurgy (PM) industry
In order to set the basis for analyzing the carbothermal reduction of the oxides covering metallic powder particles, carbothermal reduction of different powdered oxides was studied using differential thermal analysis/thermogravimetry (DTA/TG) coupled with mass spectrometry (MS)
Analysis of the reduction processes on metallic powders suggests an enhancement of the carbothermal reduction processes due to the fact that the oxides are located on the surface of metallic powders, being able to diffuse into the base metal
Summary
Sintering of steels containing oxidation-sensitive elements such as Cr, Mn and Si is an important challenge for the powder metallurgy (PM) industry. It should be considered that thermodynamic stability of carbon monoxide increases with temperature (presents a positive slope in the Ellingham diagram), and from the thermodynamics point of view, the direct mechanism— Eq 1—should be much more enhanced at high temperatures than the indirect one—Eq 2 Both reduction mechanisms entail carbon losses that need to be at least controlled in order to maintain the desired final carbon content in the steel. Thermogravimetry analysis coupled with mass spectrometry is used to assess the effect of admixing small amounts of oxidation-sensitive alloying elements on the reduction/oxidation reactions occurring during sintering. These samples were sintered for 1 h in Ar at 900 °C in a lab furnace AHT Silitstabofen This temperature was chosen after analyzing the results from thermal analyses, and it is a critical temperature at which all the alloying powders studied presented a tendency to oxidation. For the sake of simplicity, the results from EDS analyses have been represented as numerical values; this information can only be qualitatively interpreted, as quantitative evaluation is not possible
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