Christian Gierl-Mayer

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.

Particle rearrangement during liquid phase sintering of Cu-20Zn and Cu-10Sn-10Pb prepared from prealloyed powder

Abstract It would be useful to be able to produce brass and bronze components made from prealloyed powders by supersolidus liquid phase sintering. The microstructures obtained in such alloys are sensitive to constituent alloying elements and small change in sintering temperature. Although the formation of liquid during sintering is potentially attractive for densification, the effects of gravity on the liquid phase can result in graded densification. Evaporation of alloying elements and their solubility in the base metal also affect the extent to which heterogeneous cross-sections are obtained. The aim of the present study was to examine the effect of alloying and sintering temperature on the mode of particle rearrangement, and consequently on graded densification, by microstructural and fractographic analysis. Comparing the fracture morphology from top to bottom of the fracture surface is also helpful in developing a model to describe the phenomena during sintering of similar alloys.

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

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.

Tailoring master alloys for liquid phase sintering: Effect of introducing oxidation-sensitive elements

The use of low melting point master alloy (MA) powders contributes beneficially to sintering by increasing the distribution rate of alloying elements, enhancing homogenisation and sometimes also promoting densification. However, working with liquid phases poses important challenges like maintaining a proper dimensional control and minimising the effect of secondary porosity on the final performance of the steel. In this work, three different MA systems are compared: a low dissolutive Cu-based MA, and two systems with a higher degree of iron dissolution but different content in oxidation-sensitive elements. The combination of wetting experiments, step sintering tests and dilatometry studies show how the evolution of the microstructure, dimensional stability and overall densification are strongly affected by the characteristics of the liquid MA and in particular by its ability to dissolve the iron base particles, and by the amount of oxidation-sensitive elements present in the composition of the MA powder.

On the Densification and Elastic Modulus of Sintered Cr-Mo Steels

The densification and elastic properties of sintered Cr-Mo prealloyed steels of varying porosity were investigated as a function of Cr concentration, compacting pressure, and sintering temperature. Experiments were designed using the response surface methodology in order to model and evaluate the response of densification, porosity, static Young’s modulus, and dynamic Young’s modulus to the manufacturing parameters. Analysis of variance was used to validate the adequacy of the proposed models. The obtained mathematical models are useful not only for predicting the densification and elastic properties with higher accuracy but also for selecting optimum manufacturing parameters to achieve the desired properties.

Macroscopic illustration of Zn evaporation during liquid phase sintering of Cu–28Zn prepared from prealloyed powder

Abstract The formation of a liquid phase during sintering of prealloyed Cu–28Zn powder is potentially attractive for densification, but the effect of gravity results in graded densification which tends to result in heterogeneous cross-sections. The formation of a necklace structure along grain boundaries due to the accumulation of liquid phase with high Zn content, especially at higher temperatures, is also of interest. Evaporation of zinc is another essential feature to consider, since loss of Zn during sintering can influence strongly the mechanical properties of brass products. A macroscopic visualisation of Zn evaporation has been achieved using a copper substrate placed within the gas stream near the sample. FEGSEM observation and XRD analysis of the deposited white mass revealed the formation of nanocrystalline ZnO as a consequence of Zn evaporation. It is proposed that this method could usefully show the evaporation of other alloying elements during sintering of similar alloys.

Chemical reactions during sintering of PM steel compacts as a function of the alloying route

ABSTRACT Due to their large specific surface, powders and powder compacts typically contain significant amounts of oxygen which has to be removed in the early stages of sintering. In the present study, it is shown that the homogeneity/heterogeneity of the oxygen affinity within the compact, which depends on the alloying route chosen, strongly affects the de-oxidation process. For several types of alloy steels, it is shown that systems alloyed through the mixing or master alloy route and containing elements with widely varying oxygen affinity exhibit oxygen transfer reactions through the internal gas phase. For pre-alloyed grades, in which the oxygen affinity is homogeneous, i.e. consistent between the powder particles, transfer reactions occur rather within the metallic particles themselves. In any case, the sintering temperature should be selected such as to grant reduction also of the most stable oxides contained. Special block from the conference RoPM2017 guest edited by Ionel Chicinas, Technical University, Cluj-Napoca.

Master alloys in powder metallurgy: the challenge of exploring new alloying compositions

ABSTRACT Sintering of steels containing oxidation-sensitive elements such as Cr, Mn and Si is a big challenge for the powder metallurgy (PM) industry but also a chance that could open the door to a new variety of compositions, properties and prices. However, even when very small amounts of these elements are mixed with an iron base powder, the chemical reactions taking place during sintering can be changed significantly. Application of high purity sintering atmospheres is not sufficient to avoid the formation of stable oxides on the surface of the alloying particles, since the source of oxygen can be – and in general is – the base powder itself. This is because the gaseous reaction products from the reduction of the oxides covering the base powder particles act as oxidising agents for elements with higher oxygen affinity. In this study, thermoanalytical techniques have been applied in a systematic study about the influence of different alloying additions either as elemental powders (Cr, Mn, Si) or as master alloys. While Si shows to be relatively inert up to 900°C, Cr and especially Mn present high tendency to act as ‘oxygen-getters’ already at 400–500°C. When these elements are added in the form of a master alloy, their reactivity decreases, alleviating considerably the gettering effect. Moreover, reduction of the iron oxide layers with H2 at 400°C shows positive results, however, from 400°C Mn shows an important tendency to oxidation by reaction with the sintering atmosphere. This paper is part of a special issue on the Advances in Materials and Processing Technologies (AMPT) 2015 and has subsequently been revised and extended before publication in Powder Metallurgy.

Processing of a new high entropy alloy: AlCrFeMoNiTi

ABSTRACT This work, a new composition of high-entropy alloys (HEAs) was designed. The composition was carefully tailored with the aim to obtain a solid solution with a BCC crystalline structure to be an alternative binder in cermets. Thus, the composition of the HEA has been designed taking into account various criteria which has fulfilled a metallic binder of a Ti(C,N) cermet:(1) high hardness and oxidation resistance and (2) good wetting behaviour with Ti(C,N) particles because the processing of cermets is by LPS. The design of the alloy has been performed using theoretical calculations of physicochemical properties of the elements involved and the calculation of phase diagram by Thermocalc. The designed alloy has been processed by casting and powder metallurgy (PM) to study the influence of the processing route on the phases formed and on the resulting properties. The powders were produced by gas atomisation and then consolidated by hot pressing. Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden.

Magnetic measurement of retained austenite in sintered steels – benefits and limitations

ABSTRACT Measuring the magnetic saturation to quantify the retained austenite is a common method, but it is based on a relative calculation. Thus the reference, which ideally should be the same material but without retained austenite, is of extreme importance. This work focuses especially on obtaining the saturation of the reference material by using different models and applying them for determining the retained austenite content of sintered alloy steels. To verify these calculations, additional measurements of the phase fractions by X-ray diffraction were done. As a result, the contents of retained austenite vary significantly when calculating them from the saturation data using different models. Furthermore, the agreement of the results of the magnetic method and those of the X-ray measurement is not quite satisfactory. Especially when using simple approximations (reference = magnetic saturation of plain iron) the differences are very pronounced. However, it is shown that using the presented models results in markedly better agreement between the results of both methods (magnetic and X-ray diffraction) than using plain iron as a reference.