Eva Dudrová

Industrial processing, microstructures and mechanical properties of Fe–(2–4)Mn (–0.85Mo)–(0.3–0.7)C sintered steels

Abstract The potential of PM Mn steels has been established in laboratory experiments. This paper deals with sintering of Fe–(2–4)Mn–(0.3/0.7)C, also with 0˙85%Mo addition, in an industrial pusher furnace at 1180°C in an atmosphere of 25% hydrogen plus 75% nitrogen, obtained from a cryogenic liquid, giving an inlet dew-point of −55 °C. Tensile, bend (including fatigue) and miniature Charpy specimens were sintered in flowing gases and in semiclosed containers with a getter of ferromanganese, carbon and alumina. The quenched and tem- pered state was investigated, as was sinter hardening (cooling rate of 55 K min −1), simulated for comparison with slow cooling at 10 K min −1. As there was no forma tion of oxide networks at the combination of sintering temperature and dewpoint, in accordance with the Ellingham–Richardson diagram for Mn oxidation/reduction, the use of semiclosed containers was superfluous. The quenched and tempered specimens were brittle. Sinter hardening lead to an improvement in mechanical properties. The reproducibility of tensile and TRS data was high for the sintered materials, characterised by Weibull moduli m of 12–41. All the alloy microstructures were complex and heterogeneous, consisting of, depending on the local manganese and carbon contents, the diffusive and non-diffusive transformation products (pearlite, bainite, martensite) and additionally ferrite and retained austenite. The highest mechanical properties in the entire range of compositions investigated in the furnace cooled state: yield, tensile and bend strengths of 499, 637 and 1280 MPa, respectively, with impact energy of 18 J, and tensile and bend strains of 1˙17 and 1.57%, were achieved for the Fe–2Mn–0.85Mo–0.5C alloy, marginally superior to Fe–2Mn–0.7C. For the sinter hardened Fe–4Mn–0.3C alloy yield, tensile and bend strengths were 570, 664 and 1263 MPa, respectively, at an acceptable impact energy of 14 J, with tensile and bend strains of 0.52% and 1.8%. Many of the results compare favourably with the requirements of MPIF standard 35. Mn is a more effective strengthening agent than either Ni or Cu, or their combination, though generally at reduced plasticity.

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.

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.

Fracture micromechanics of static subcritical growth and coalescence of microcracks in sintered Fe–1·5Cr–0·2Mo–0·7C steel

Abstract Nucleation of microcracks, their growth and coalescence are analysed in powder metallurgy (PM). Fe–1·5Cr–0·2Mo–0·7C steel by fractography allied to surface replica microscopy – at several stress levels as the maximum tensile stress in three-point bend specimens was raised to 99·6% of the transverse rupture strength TRS of 1397 MPa. The fatigue limit in this material is ∼240 MPa, at which stress level no microcracks were detected in static loading. Numerous microcracks, ranging in size from <5 to ∼20 μm, however, were nucleated above ∼800 MPa, i.e. beyond the yield strength of ∼620 MPa. With increasing stress, some microcracks became dormant, whilst others grew subcritically, stress step-wise, to some 400 μm. Of particular importance are observations of the coalescence of two and three of such microcrack systems to produce a critical, propagating crack. The then estimated stress intensity factor K a, could reach K 1C, independently estimated to be ∼36 MPa m1/2. Microcrack coalescence was associated with easy paths for crack growth, principally prior particle boundaries linking pores. Ways of making subcritical crack growth more difficult and hence improving both static and dynamic mechanical properties, are considered.

A review of failure of sintered steels: fractography of static and dynamic crack nucleation, coalescence, growth and propagation

The failure of sintered steels differs from the behaviour of wrought steels because of factors such as porosity, remnants of previous particle surfaces and generally more complex microstructures. All these factors influence initiation, growth and propagation of microcracks when the sintered microstructure is mechanically loaded. Fracture paths and fracture resistance are shown to be related to details of the microstructures comprising ferrite, austenite, bainite, martensite, pores and weak interfaces. All these have characteristic fracture resistance properties resulting in, frequently combinations of, dimple rupture, cleavage, intergranular and interparticle failure micromechanisms. Results are presented of systematic studies, enabling identification of relevant stresses, in static and dynamic three-point loading, as the cracking process progresses. In static loading, microcracking has been detected in some steels below the macroscopic yield stress and in the first 100 cycles in fatigue. Microcracks nucleate, grow and coalesce, in a step-wise manner, before achieving a catastrophic size – for which conventional fracture mechanics holds. Thus, application of Paris-type analysis to Stage II fatigue is therefore inappropriate. The review focuses on failure micromechanisms and interpretation of fracture surface composition of sintered steels, particularly of those based on Distaloy AE and Astaloy CrL powders. The relationships between microstructure and mechanical properties are discussed.

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%.

Microstructure evolution in Fe–Mn–C during step sintering

Abstract Mn was introduced, with graphite, as fine ferromanganese particles to form compacts of Fe–3%Mn–0·5%C. Each was sintered in dry 25%H2 + 75%N2 for 3 min at 770, 1040, 1080, 1170 and 1220°C respectively, held for 3 min and quenched. The surface of a successively heated specimen was similarly examined. Specimens were sectioned and, especially the reacting ferromanganese particles and adjoining regions, investigated using light and scanning electron microscopy and EDX. Development of microstructure and microcompositions during sintering was related, from 740°C, to diffusion and condensation of Mn vapour on iron particle surfaces and subsequent chemical reactions. Above 1080°C microstructures included features resulting from a transient liquid phase, in accord with a ∼45 wt%Mn ternary eutectic calculated by ThermoCalc. The thicknesses of the highly Mn enriched regions were substantially higher than those resulting from bulk Mn diffusion in the Fe lattice; our interpretation invokes predominant operation of another type of mechanism: diffusion induced grain boundary migration.

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.

Wear Characteristics of Vacuum Sintered Steels

The present paper is focused on the wear characteristic of vacuum sintered Cr-Mo-[Mn]-[Cu] steels. The effect of chemical composition and the processing conditions in a vacuum furnace were evaluated. In such furnaces the cooling rate is generally determined by the pressure of the gas (N2) introduced into the chamber, the average cooling rates were calculated in the range of 1240°C to 400°C. The wear characteristics were analyzed as function of the processing and microstructures of the tested alloys through pin on disk test. Sintering of specimens in vacuum together with rapid cooling resulted in the formation of dominant martensitic microstructures with some small bainitic areas. The effect of both surface hardness and microstructure on the wear behaviour of the investigated steels shows the relation between the hardness and the wear rate. The influence of processing condition on the amount of martensite is also presented.