Eduard Hryha

Influence of powder properties on compressibility of prealloyed atomised powders

Abstract The compressibility of metal powders depends on many factors, including morphological and mechanical properties of particles. Alloying elements increasing ferrite solid solution strengthening can influence the compressibility of prealloyed steel powders. The contribution deals with compressibility of Fe–Mn–Cr–[Mo–Ni] prealloyed and premixed powders with total alloying ∼2·0%. Quantification of powders compaction was performed using equation P=P 0exp (−Kp11); proposed by Parilak et al. (1994), where P is the porosity at pressure p; P 0 is apparent porosity; K and n are parameters related to morphology and plasticity of powder particles. The equation enables to study the compressibility in relation to geometry and mechanical properties of powder particles, through development of K and n parameters on pressing pressure p. The tested powders exhibited low varieties in the density at pressing pressures higher than ∼400 MPa, but some differences were identified during the first stage of compaction, at pressing pressures up to ∼200 MPa.

Carbon control in PM sintering: industrial applications and experience

Abstract The challenges in controlling carbon potential during sintering of steel powder have been discussed in many experimental and theoretical studies. The main issues lie within the complex thermodynamics and kinetics of processing atmosphere chemistry in continuous sintering furnaces. Although many models have been proposed to address the problem, these have rarely come to reality and entered industry practice. The purpose of this article is to summarise these discussions and investigate the interaction of the atmosphere constituents with the sintered compact within a sintering furnace. An important aim is to provide the PM industry with a fresh understanding of furnace operations and to provide recommendations to improve the control of furnace conditions. A case study is given of an existing furnace installation using Sinterflex technology which allows continuous monitoring and/or control of the furnace atmosphere. The reduction of oxides and carbon potentials to optimise the production parameters is described.

The Sintering Behaviour of Fe-Mn-C Powder System, Correlation between Thermodynamics and Sintering Process, Mn Distribution and Microstructure

To study of the sintering behaviour of the Fe-0.8Mn-0.5C powder system the cylindrical specimens with a density of ~7.0 g/cc were sintered in container at the temperature of 11200C for 30 min in a gas mixture of 7%H2/93%N2 with the inlet dew point of -600C. The composition (CO/CO2- content) and the dew point of the flowing and “container micro-climate” atmospheres during the whole sintering cycle were monitored. It was shown, that carbothermical reduction and formation esp. CO/CO2 occurs in two different temperature ranges. Three peaks of dew point profile also can be distinguished during sintering cycle. Following sintering the changes of ferromanganese particles, Mn-content distribution and microstructures around the Mn-source were micro-analytical evaluated at cross-section of specimens using the SEM with EDX microanalyses. The results showed that manganese travels through porous iron matrix up to ~60 μm. The type of local microstructure constituents is determined by the local Mn- and C contents.

The Sintering Behaviour of Fe-Mn-C Powder System, Correlation between Thermodynamics and Sintering Process, Manganese Distribution and Microstructure Composition, Effect of Alloying Mode

Among steel-making techniques Powder Metallurgy (PM) concept utilizes unique production cycle, consisting of powder compaction and sintering steps that give high productivity with low energy consumption and high material utilization. Due to the presence of residual porosity, mechanical properties of PM components are inferior in comparison with structural components produced by other technologies. Improvement of mechanical properties at the same level of porosity can be achieved primarily by adding variety of alloying elements. Therefore modern PM technology for production of high-performance PM parts for highly stressed steel components for automotive industry, for example, rely on techniques of utilization of different alloying elements additionally to adjustment of technological process depending on alloying system used. When talking about high-strength low-alloyed structural steels, the most common alloying elements, additionally to carbon, added in order to increase mechanical performance, are chromium, manganese, silicon and some other strong carbide and carbonitride-forming elements (V, Nb, Ti etc.). In comparison with classical steelmaking practice, alloying of PM steels is much more complicated as additionally to influence of alloying elements type and content on microstructure, mechanical properties, hardenability etc., number of additional aspects influencing powder production and further component processing has to be considered. Traditionally, PM high-strength steels are alloyed with Cu, Ni, and Mo. This results in a considerable difference in price of material between conventional and PM steels, used for the same high-load application, as the price of currently employed PM alloying elements like Mo and Ni is dozens of times higher in comparison with that of Cr or Mn. This situation creates a strong economical stimulation to introduce cheaper and more efficient alloying elements to improve the competitiveness of PM structural parts. So, why the potential of most common for conventional metallurgy alloying elements as Cr, Mn and Si is not utilized in PM? First and basic question that arise is how to introduce these elements in PM – as admixed elemental powder (or master-alloy) or by prealloying of the base steel powder. Chromium prealloyed steels are already successful introduced on the PM market. However due to peculiar properties of manganese (oxygen affinity, high vapour pressure, ferrite strengthening etc.) attempts to develop Mn sintered steels are still ongoing. Issue of appropriate alloying mode, that is the starting point of manganese introduction in PM, is the basic question that has to be answered at the beginning and is the basic topic of this chapter. The easiest way to introduce manganese is by admixing of ferromanganese powder that is cheap and widely available on the market in different grades. This approach was firstly proposes around 30 years ago and have been scrutinized thoroughly from different perspectives (Salak, 1980; Cias et al., 1999; Salak et al., 2001; Dudrova et al., 2004; Danninger et al., 2005; CiasW Schlieper & Thummler, 1979; Hoffmann & Dalal, 1979). First developed master-alloys containing manganese–chromium–molybdenum (MCM) and manganese–vanadium–molybdenum (MVM) had a wide range of mechanical properties depending on alloying content, sintered density and processing conditions. Nevertheless, these master-alloys faced with many problems during application (oxides formation during manufacturing process, high hardness of the particles that lead to intensive wear of compacting tools etc.) and fully disappears from manufacturing and research areas. Recent development of Fe–Cr–Mn–Mo–C master-alloys was much more successful and show promising properties for their future industrial utilization (Beiss, 2006; Sainz et al., 2006). High affinity of manganese for oxygen and Mn loss by sublimation can be minimized by lowering the manganese activity than can be done by Mn introduction in pre-alloyed state. However powder alloying by manganese faces some difficulties starting from powder production, handling and following compaction and sintering steps. This is connected with manganese selective oxidation on the powder surface during atomization and further annealing depending on processing conditions during powder production(Hryha et al., 2009-b; Hryha et al., 2010-a). A further negative impact of manganese utilization in pre-alloyed state is the expected lower compressibility of such pre-alloyed powders due to ferrite solid solution strengthening by manganese. This chapter is focused on the influence of alloying mode, utilizing premix systems with different ferromanganese grades and high-purity electrolytic manganese as well as fully prealloying of water atomized powder. While respecting all the benefits and problems with sintered steels alloyed with manganese some basic directions have been chosen — theoretical evaluations of required sintering atmosphere composition for preventing of manganese alloyed steels from oxidation during every stage of sintering, analyzes of sintering cycle coupling with simultaneous atmosphere monitoring and further analysis of sintered specimens using number of advanced spectroscopy and thermoanalytic techniques. Various phenomena, connected with manganese evaporation and reduction/oxidation behaviour of manganese alloyed sintered steels were theoretically evaluated and tested experimentally applying interrupted sintering experiments, when specimens where sampled at different stages of the sintering cycle for extensive study by HR SEM+EDX, XPS, TG+MS etc. Thermodynamic calculations enabled to determine a required sintering atmosphere composition (maximal permitted partial pressures of active gases CO/CO2/H2O) for preventing of Mn alloyed steels prepared by different alloying mode from oxidation during every stage of sintering. The results were verified by continual monitoring of CO/CO2/H2O profiles in sintering atmosphere and further analysis of sintered specimens.

Electron Beam Melting Manufacturing Technology for Individually Manufactured Jaw Prosthesis: A Case Report

In the field of maxillofacial reconstruction, additive manufacturing technologies, specifically electron beam melting (EBM), offer clinicians the potential for patient-customized design of jaw prostheses, which match both load-bearing and esthetic demands. The technique allows an innovative, functional design, combining integrated porous regions for bone ingrowth and secondary biological fixation with solid load-bearing regions ensuring the biomechanical performance. A patient-specific mandibular prosthesis manufactured using EBM was successfully used to reconstruct a patients mandibular defect after en bloc resection. Over a 9-month follow-up period, the patient had no complications. A short operating time, good esthetic outcome, and high level of patient satisfaction as measured by quality-of-life questionnaires-the European Organisation for Research and Treatment of Cancer QLQ-C30 (30-item quality-of-life core questionnaire) and H&N35 (head and neck cancer module)-were reported for this case. Individually planned and designed EBM-produced prostheses may be suggested as a possible future alternative to fibular grafts or other reconstructive methods. However, the role of porosity, the role of geometry, and the optimal combination of solid and porous parts, as well as surface properties in relation to soft tissues, should be carefully evaluated in long-term clinical trials.

Parameters Controlling the Oxide Reduction during Sintering of Chromium Prealloyed Steel

Temperature intervals of oxide reduction processes during sintering of the Fe-3%Cr-0.5% Mo prealloyed powder using continuous monitoring of processing-exhaust gas composition (CO, CO2, and H2O) were identified and interpreted in relation to density (6.5-7.4 g/cm(3)), sintering temperature (1120 and 1200 degrees C), heating and cooling rates (10 and 50 degrees C/min), carbon addition (0.5/0.6/0.8%), type (10% H-2-N-2, N-2), and purity (5.0 and 6.0) of the sintering atmosphere. The progress in reduction processes was evaluated by oxygen and carbon contents in sintered material and fracture strength values as well. Higher sintering temperature (1200 degrees C) and density <7.0 g/cm(3) resulted in a relative decrease of oxygen content by more than 80%. The deterioration of microclimate purity of inner microvolumes of compacts shifted the thermodynamic equilibrium towards oxidation. It resulted in a closing of residual oxides inside interparticle necks. The reducing ability of the N-2 atmosphere can be improved by sintering in a graphite container. High density of 7.4 g/cm(3) achieved by double pressing indicated a negative effect on reduction processes due to restricted replenishment of the microclimate atmosphere with the processing gas. In terms of strength properties, carbon content should not be higher than similar to 0.45%.

Effectiveness of reducing agents during sintering of Cr-prealloyed PM steels

Abstract Development of strong inter-particle necks requires successful removal of surface oxides, present on the powder particles, during the initial stages of sintering. In the case of water-atomised powder prealloyed with chromium, the surface oxide consists mainly of an iron oxide layer with some more stable fine particulate oxides. The formation of sufficiently strong inter-particle necks requires as a minimum full removal of the iron surface oxide layer. This can be achieved by gaseous reducing agents (e.g. H2, CO or a mixture of both) or by carbon, typically admixed in the form of graphite. The reducing power of various sintering atmospheres (active gas content ≤10 vol.-%) and their combined effect with graphite has been investigated by a thermal analysis technique. Results indicate that a combination of a dry hydrogen-containing atmosphere and fine graphite allows successful sintering of chromium alloyed PM steels.

Improvement of Mechanical Properties of Fe-Cr-Mo-[Cu-Ni]-C Sintered Sintered Steels by Sinter Hardening

The effect of high temperature sintering and high cooling rate on shifting the microstructural composition to the favourably of martensite-bainite structures and thus effective improvement of mechanical properties of sintered steels based on Astaloy CrL powder with an addition of 1 and 2% Cu or 50% Distaloy AB powder and 0.65% C was investigated. All the systems were processed by both sinter-hardening and conventional sintering. The vacuum sintering at high-temperature of 1240 0C and at common temperature of 1180 0C were integrated with high (6 0C/s), medium (3 0C/s) and slow (0.1 0C/s) cooling rates; conventional sintering at 1180 0C with cooling rate of ~0.17 0C/s was carried out in a N2+10%H2 atmosphere. In dependence on chemical composition, the yield and tensile strengths of 890-1150 MPa and 913-1230 MPa respectively and impact energy of 10-15 J were achieved by sinter-hardening. The yield and tensile strengths are approximately double than those resulting from conventional sintering.

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 sources – synthetic graphite, natural graphite and carbon black – on microstructure and inter-particle neck development in Cr-alloyed 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 900°C.

Surface chemistry of the titanium powder studied by XPS using internal standard reference

ABSTRACT Surface chemistry of the titanium powder has particularly growing interest due to the increasing application of titanium components prepared by powder metallurgy, in particular metal injection moulding and additive manufacturing. Due to the high chemical activity, number of titanium oxides, calcium and complex Ca–Ti–oxides can be expected on the component/medical implant surface, depending on powder and component manufacturing and post-treatment, but are very difficult to analyse due to the lack of the experimental data and analysis methodology. Therefore, a methodology for the analysis of the surface chemistry of the Ti-powder by XPS utilising internal standard reference was developed. The obtained methodology was used for the surface analysis of titanium powder and identification of its surface oxide composition. The results show that the powder surface is covered by TiO2 layer in the form of rutile with a thickness of 4.4 nm. Carbon and nitrogen impurities were also found present on the powder surface. GRAPHICAL ABSTRACT