Raquel Oro

Lean alloys in PM: from design to sintering performance

Abstract Low alloy sintered steels with an optimised content of alloying elements require the use of promising candidates such as Mn and Si, which can provide improved properties with minimum contents at a lower and less volatile price. The introduction of these alloying elements in the form of a master alloy powder prevents, to some extent, the oxidation and allows a proper ‘tailoring’ of its composition to accomplish particular goals. In this sense, low melting point alloys are especially interesting since they provide the formation of a liquid phase that enhances sintering and promotes the homogeneous distribution of alloying elements within the compact. In this work, the product developing process of master alloys containing Fe–Mn–Si is described from the theoretical design to the sintering performance. The effects of the liquid phase (produced by the added master alloys) are studied by differential thermal analysis and dilatometry. Moreover, to depict the behaviour of the liquid phase during heating, interrupted sintering experiments under high cooling rates were carried out. The results reported allow us to conclude that diffusion of carbon seems to be beneficial for lowering the melting temperature of the alloying particles, and the fact that the master alloys studied can dissolve part of the iron base particles has been shown to be beneficial for wetting.

Study of high temperature wetting and infiltration for optimising liquid phase sintering in low alloy steels

Abstract Liquid phase sintering is commonly used in powder metallurgy to improve physical properties through densification enhancement. With the aim of combining the advantages of liquid phase sintering and the use of promising alloying elements such as Mn and Si, liquid promoters with complex compositions were designed to provide a low melting point to form a liquid phase below the common sintering temperatures. The properties of these liquid phases were characterised in terms of contact angle, spreading evolution and infiltration. Using a Krüss drop shape analysis system, both wetting angle experiments and infiltration experiments were performed by changing the substrate characteristics from sintered to green iron specimens respectively. The discussion is based on the different features found for these liquids compared with copper, which is a well known liquid phase former used for improving the properties of low alloy steels. Simulations of the thermodynamic and kinetic processes taking place were performed by combining ThermoCalc and DICTRA software analysis.

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

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.

Design of Low-Melting Point Compositions Suitable for Transient Liquid Phase Sintering of PM Steels Based on a Thermodynamic and Kinetic Study

The possibility of tailoring the characteristics of a liquid metal is an important asset in a wide number of processing techniques. For most of these processes, the nature and degree of the interaction between liquid and solid phases are usually a focus of interest since they determine liquid properties such as wettability and infiltration capacity. Particularly, within the powder metallurgy (PM) technology, it is considered one of the key aspects to obtain high performance steels through liquid phase sintering. In this work, it is proved how thermodynamic and kinetics software is a powerful tool to study the liquid/solid interactions. The assessment of different liquid phase promoters for transient liquid phase sintering is addressed through the use of ThermoCalc and DICTRA calculations. Besides melting temperatures, particular attention is given to the solubility phenomena between the phases and the kinetics of these processes. Experimental validation of thermodynamic results is carried out by wetting and infiltration experiments at high temperatures. Compositions presenting different liquid/solid solubility are evaluated and directly correlated to the behavior of the liquid during a real sintering process. Therefore, this work opens the possibility to optimize liquid phase compositions and predict the liquid behavior from the design step, which is considered of high technological value for the PM industry.

From Sintered Iron to High Performance PM Steels

Since low alloyed sintered steels were introduced in the market of the structural parts, we have followed the evolution of a material with poor mechanical properties and any uniformity (in the sense of reproducibility) to materials that today are produced with high reliability and performance. The working efficiency could be equivalent in many cases with the best wrought steel, and maintaining a good margin in terms of cost and competitively. In this paper a complete review of the topic is accomplished, from the early times when the first parts were made by plain iron or iron-carbon, going through the different alloying systems: Fe-Cu, Fe-P, Fe-Cu-Ni-Mo, and more recently Fe-Cr-Mo and Fe-Mn. The development in processing routes has been considered too. The main milestones in the field of new alloying systems have been: 1) the introduction of Cu in 60-70’s, 2) the new complex systems with Cu-Ni-Mo in the 80’s and 3) the introduction of alloying elements with high oxygen affinity (in the late 90’s). Regarding the milestones in processing could be considered: 1) the development of new mixing procedures, 2) the warm compaction and high velocity compaction, 3) the improvements in sintering control and high temperature sintering. Several decades of research and innovation, acting on the processing system (mixing, pressing, sintering, post-sintering operations,…) and on the alloying system (from the earliest times with plain iron to complex systems used today), has allowed us to have a highly competitive materials, in terms of performance, and processes in terms of cost. The future is still open to new developments.