Transient liquid phase sintering was examined for Fe-Ti powder mixtures. The experimental plan included the effects of several processing variables, such as green density, particle size, composition, heating rate, sintering temperature, and sintering time. During heating, pores form at the Ti particle sites. At the first eutectic temperature (1085 °C), liquid spreading is inhibited by a surrounding intermetallic envelope, leading to swelling. At the second eutectic temperature (1289 °C), the liquid penetrates along the iron grain boundaries and provides densification. The amount of densification depends on the amount of liquid formed at the second eutectic temperature and its duration as determined by the titanium content and heating rate.

Processing of iron-titanium powder mixtures by transient liquid phase sintering

Influence of heat treatment on the high temperature oxidation mechanisms of an Fe–TiCN cermet

Oxygen-sensitive alloying elements such as Mn, Si, and Cr have a high potential for improving the properties of low alloyed sintered steels while reducing the alloying cost. However, it is necessary to find a way for avoiding, or at least minimizing, the oxidation of these elements especially during the early stages of the sintering cycle. In this study Mn, Si, and Cr were introduced in the form of a master alloy powder designed to be mixed with the iron base powder and provide the final composition of the steel during the sintering process. The reduction/oxidation phenomena taking place during the heating stage were studied by thermogravimetry, dilatometry, and mass spectroscopy, using either reducing (H2) or inert (Ar) atmospheres. The results show how the difference in chemical activity between base iron powder and master alloy causes the so called “internal-getter” effect, by which the reduction of less stable iron oxides leads to oxidation of the elements with higher affinity for oxygen. This effect can be somehow minimized when sintering in H2, since the iron oxides are reduced at lower temperatures at which the reactivity of the elements in the master alloy is lower. However, H2 concentration in the processing atmosphere needs to be carefully adapted to the specific composition of the materials being processed in order to minimize decarburization by methane formation during sintering.

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New Alloying Systems for Sintered Steels: Critical Aspects of Sintering Behavior

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.

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

Surface phenomena during the early stages of sintering in steels modified with Fe–Mn–Si–C master alloys

A simple method of producing alloy steels by mixing, pressing, and sintering iron powder with carbides of transition metals is described. The carbides must have a high carbon content and be soluble in iron. These characteristics are found in the carbides of chromium, tungsten, molybdenum, and vanadium. Tensile strengths of ∼800 N/mm2 have been achieved in annealed specimens having carbide additions of 1–12 wt.-%, depending on the carbide. The additions produce steels with partly air-hardening properties.The effects of concentration and particle size of the carbides, sintered density, sintering conditions, and cooling rate on mechanical properties have been determined. The alloys are not very sensitive to the sintering atmosphere. The effect of heat-treatment on the mechanical properties of Fe-Cr3C2 alloys is reported. A tensile strength >1300 N/mm2 was observed with 4 wt.-% Cr3C2. Dilatometric measurements were conducted with different carbide concentrations and heating conditions to study the sin...

L. Albano-Müller

Papers

High-strength sintered iron-base alloys by using transition metal carbides*