Margita Kabátová

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%.

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.

Preparation and Compaction Behaviour of Poly(methyl methacrylate) Coated Iron Microparticles

The p oly(methyl methacrylate) (PMMA) coatings onto surface of iron particles were electrochemically prepared and the efiect on both surface structure and internal structure of the resulted material after compaction was carried out. The electrochemical polymerization treatment was performed in a ∞uidized bed electrolyzer using sulphuric acid solution containing potassium persulphate and methyl methacrylate (MMA). The surface topography and the microstructure of the samples were observed by scanning electron microscopy (SEM) and optical microscopy (OM). It was found that the PMMA layer coated onto iron particles results in improvement of their compressibility compared with uncoated powders, and classical lubricants are not necessary for compacting particles coated.

Mixed and Vacuum/Pressure Impregnated Fe/SiO2/Shellac Composites

Fe/SiO2/shellac composites based on iron powder consisting of irregularly and/or spherical shaped particles coated with SiO2 layer or with admixed SiO2 powder were prepared either by mixing the Fe-SiO2 powder with shellac or by vacuum/pressure impregnation (VPI) of sintered Fe-SiO2 compacts with shellac dissolved in ethanol. The effect of iron particle shape, density of compacts and method of shellac application on electrical resistivity and magnetic coercivity was evaluated and correlated with microstructure and features of insulating layer. For VPI composites based on spherical particles a uniformly thick insulating layer was created. This resulted in the electrical resistivity and coercivity of 780-1120 μΩ.m and 177-290 A/m. For composites based on irregularly shaped particles the shellac admixing is more preferable.

The Influence of Preparation Methods on Magnetic Properties of Fe/SiO_2 Soft Magnetic Composites

The Influence of Preparation Methods on Magnetic Properties of Fe/SiO2 Soft Magnetic Composites J. Füzerováa,∗, J. Füzer, P. Kollár, M. Kabátová and E. Dudrová Department of Applied Mathematics and Informatics, Faculty of Mechanical Engineering, Technical University Košice, Letná 1, 042 00 Košice, Slovakia Institute of Physics, Faculty of Sciences, P.J. Šafárik University, Park Angelinum 9, 041 54 Košice, Slovakia Institute of Materials Research, Slovak Academy of Sciences, Watsonova 47, 040 01 Košice, Slovak Republic

Dry sliding wear behaviour of low alloyed sintered steels in relation to microstructure and fracture behaviour

The contribution deals with dry sliding wear behaviour of different Fe–Cr–Mo–[Mn]–[Cu] sintered powder metallurgy systems using a pin on disc test. The effect of chemical composition and two different processing conditions, industrial atmosphere at 1453 K (1180°C) and vacuum atmosphere at 1513 K (1240°C) related to a new proposed type of sinterhardening process, was investigated. The wear phenomena and the wear track characteristics were analysed in relation to processing and microstructures of the tested alloys through light and scanning electron microscopy. A delamination and ploughing wear features were observed for all the sintered steels, and sintering in industrial atmosphere led to an oxidation wear features, too. The results showed positive effect of sinterhardening process via suitable modification of microstructure, and this affected the prior particle boundaries on improving the wear resistance of the tested systems.

Oxide Reduction Processes, Microstructure and Properties of Sintered Chromium Pre-Alloyed Steel

The reduction processes during the sintering of Fe-3Cr-0.5Mo+0.5C pre-alloyed powder using continuous monitoring of exhaust gas composition (CO, CO2, H2O) have been identified and interpreted in relation to the density (6.5-7.4 g/cm3), sintering temperature (1120 and 1200°C), heating and cooling rates (10 and 50°C/min) and type of sintering atmosphere (10%H2-N2, N2), respectively. The progress in reduction processes was evaluated by the change in C and O contents, and fracture strength values as well. The results were compared with metallographic study of microstructure and fractographic observations of fracture surfaces. Higher sintering temperature (1200°C) and low density (<7.0 g/cm3) resulted in a relative decrease in oxygen content by more than 80%. Higher cooling rate (50°C/min) eliminates re-oxidation during cooling. High density of 7.4 g/cm3,achievedby double pressing, indicated to have a negative effect on reduction processes due to restricted replenishment of the “microclimate” atmosphere with the processing gas. Higher O2 content causes weakening of interfaces with residual oxides.

Microstructure of Composites Based on Phosphated Iron Powder

The commercial carbonyl iron powder coated with iron phosphate (20 wt.%) was dried (60°C for 2 h in air), calcinated at 400°C for 3 h in air, compacted at 600 MPa into cylindrical samples and subsequently sintered at 820, 900 and 1110°C for 30 min in N2-10%H2 atmosphere. By means of EDX and XRD analyses the phase composition of the coating and sintered microstructure was studied. Microstructure resulting from sintering at 820 and 900°C was formed by initial iron particles surrounded with the crystalline FePO4 and α-Fe2O3 phases. Due to liquid phase sintering at 1110°C a mixed microstructure containing spheroidized α-Fe phase surrounded by solidified liquid phase consisting of iron oxides and phosphorous compounds has been formed. In order to prepare a network composite microstructure the compacts based on spherical iron particles size of 100-160 µm coated with 2 wt.% of iron phosphate were dried, calcined at 400°C, compacted and liquid phase sintered at 980°C.