Surface composition of the steel powders pre-alloyed with manganese

This study examines the sintering behavior and properties of Fe-0.8Mn-0.5C manganese powder metallurgy steels. The study focuses on the influence of mode of alloying—admixing using either high-purity electrolytic manganese or medium carbon ferromanganese as well as the fully prealloying of water-atomized powder. Three main aspects were studied during the whole sintering process—microstructure development, interparticle necks evolution, and changes in the behavior of manganese carrier particles during both heating and sintering stages. The prealloyed powder shows considerable improvement in carbon homogenization and interparticle neck development in comparison with admixed materials. The first indication of pearlite for the fully prealloyed material was registered at ~1013 K (740 °C) in comparison with ~1098 K (825 °C) in the case of the admixed systems. The negative effect of the oxidized residuals of manganese carrier particles and high microstructure inhomogeneity, which is a characteristic feature of admixed systems, is reflected in the lower values of the mechanical properties. The worst results in this respect were obtained for the system admixed with electrolytic manganese because of more intensive manganese sublimation and resulting oxidation at lower temperatures. According to the results of X-ray photoelectron spectroscopy and high-resolution scanning electron microscopy and energy dispersive X-ray analyses, the observed high brittleness of admixed materials is connected with intergranular decohesion failure associated with manganese oxide formation on the grain boundaries.

Critical Aspects of Alloying of Sintered Steels with Manganese

Multiaxial fatigue life assessment of sintered porous iron under proportional and non-proportional loadings

Stainless steel 316L MIM components can be made from either prealloyed powders or from master alloys blended with carbonyl iron powder. In this study these two techniques were compared using prealloyed and master alloyed gas atomised powders of − 16 μm and − 22 μm sizes. Four different compounds were prepared, characterised and injection moulded into tensile bars. The bars were compared for green strength, green defects, sintered strength and microstructure. The green components are stronger when carbonyl iron powder is used with the gas atomised master alloy. This material also seems to be less susceptible to moulding defects. The sintering strength of the material produced using the pre-alloyed powder was higher than the master alloyed prepared material. Little difference in mechanical properties existed between the materials fabricated from gas atomised prealloyed − 16 μm and the − 22 μm powders. Also, the viscosity of the mixtures was higher for the − 16 μm material and the master alloy mixtur...

Mechanical properties of metal injection moulded 316l stainless steel using both prealloy and master alloy techniques

Damage evolution in sinter-hardening powder-metallurgy steels during tensile and fatigue loading

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 pr...

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

Detailed quantitative micrographic data are presented for Stages I and II of a Powder Metallurgy Fe-1.5Cr-0.2Mo-0.7C steel specimen fatigued in bending with R = -1 at 24 Hz and a stress amplitude of 312 MPa. The fatigue limit was ∼240 MPa, at which stress level no microcracks were detected in static loading. Testing was interrupted at 100 cycles and at further 29 intervals until failure after 49 900 cycles. For each arrest, surface replicas were made in the two regions where maximum stress was applied. Microcracks could nucleate below 100 cycles, when their sizes ranged from <5 to ∼20 μm. Fractographic examination identified the failure-originating site, which was then associated with the crack system observed on the 'last' pre-failure micrograph. Detailed examination of the eventual failure region showed nucleation, at various cycle intervals, of 18 microcracks, their subcritical growths, arrests and coalescences with continuing cycling to form a critical crack 2.25 mm deep. Stepwise microcrack growth was probably rapid - to the next arrest or coalescence. For each (micro)crack size stress intensity factors, K a s, were estimated and, at the end of Stage II, for the coalesced crack, K a reached K 1C , independently estimated to be ∼36 MPa m 1/2 .

Microcrack nucleation, growth, coalescence and propagation in the fatigue failure of a powder metallurgy steel

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...

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

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...

Microstructure evolution in Fe–Mn–C during step sintering

In sintered steels there is evidence for static, as well as dynamic, loading that microcracks are nucleated, grow and coalesce, before attaining catastrophic size (Stage III in fatigue) for which fracture mechanics holds. The mechanisms are step-wise, thus Paris type analysis, applied e.g. to artificial cracks, does not apply. These processes were studied by combining fractography with surface replication of the most highly stressed region of specimens undergoing three-point bending, as progressively the tensile stress or the number of fatigue cycles was increased. Typically 10-20 microcracks were nucleated, some were arrested, others grew and coalesced until the final coalescence resulted in the catastrophic crack. Assuming semi-elliptical shape for the surface microcracks, the local stress intensity factor, Ka was calculated for each microcrack using Irwin’s formula. Initial values of Ka were 1-4 MPa.m 1/2 and the eventual values correspond well to K1C, independently determined.

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M. Kabátová

Papers

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

Microcrack nucleation, growth, coalescence and propagation in the fatigue failure of a powder metallurgy steel

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

Microstructure evolution in Fe–Mn–C during step sintering

Static and dynamic fracture micromechanics of sintered steels: a review