Stephen C. Mitchell

Microstructure and mechanical properties of sintered (2–4)Mn–(0·6–0·8)C steels

AbstractMechanical properties of 2–4% manganese PM steels were determined in tension and in bending following laboratory sintering in dry, hydrogen rich atmospheres. Youngs modulus determined by an extensometric technique was about 115 GPa; when measured by an ultrasonic method it was about 153 GPa, in accordance with the‘law of mixtures’. The microstructures, significantly devoid of oxide networks, were predominantly pearlitic, but frequently with variability for specimens similarly processed, resulting in appreciable variations in the stresses for macroscopic yielding and fracture. The majority of the experiments were conducted on 3 and 4Mn–0·6C alloys and for these R0·1 was in the range 275–500 MPa, tensile strength (TS) 300–600 MPa, and (apparent) transverse rupture strength (TRS) 640–1260 MPa. Statistical techniques were employed to analyse the data. When careful control of processing was maintained, the Weibull modulus was highest, at about 17, for TS of furnace cooled specimens, and lowest, about ...

Sintering, microstructure, and mechanical properties of PM manganese–molybdenum steels

Abstract The effects of 0·5 wt-%Mo addition on the processing, microstructure, and strength of PM Fe–3·5Mn–0·7C steel are described. Water atomised and sponge irons, Astaloy 1·5Mo, milled ferromanganese, and graphite were the starting powders. During sintering in 75H2 /25N2 or pure hydrogen the dewpoint was controlled and monitored; in particular the effects of improving it from -35 to -60°C were investigated. Faster heating rates (≥20 K min-1), sufficient gas flowrates, milling the ferro alloy under nitrogen, a low dewpoint (<-60°C), and a getter powder can all contribute to the reduction or prevention of oxidation of the manganese, in particular formation of oxide networks in the sintered steels. For 600 MPa compaction pressure densities up to 7·1 g cm-3 were obtained; these were not significantly affected by sintering at temperatures up to 1180°C. The sintered microstructures were sensitively dependent on the cooling rate. Irrespective of the presence of Mo, slow furnace cooling at ∼4 K min-1 resulted in mainly pearlitic structures with some ferrite and coarse bainite, whereas fast cooling at ∼40 K min-1 produced martensite and some retained austenite, very fine pearlite, bainite, and some ferrite. Youngs modulus, determined by tensile and ultrasonic tests, was in the range 110–155 GPa. Sintering with -60°C dewpoint resulted in tensile and transverse rupture strengths of420 and 860 MPa for the Mn steel, rising to 530 and1130 MPa as a result of the Mo addition. This contrasts with strength decreases observed when processing included use of high oxygen containing ferromanganese and sintering with -35°C dewpoint.

Corrosion of mild steel and 316L austenitic stainless steel with different surface roughness in sodium chloride saline solutions

The corrosion behaviour of mild steel and 316L austenitic stainless steel was investigated in saline solution containing 1 and 3%NaCl. Specimens with surface roughness of 200, 600 grit emery paper and 1μm diamond paste were investigated. The anodic polarization measurement technique was performed at a scan rate of 1mV/s for a fixed period of 1 hour. The experimental results revealed that chloride ions have a significant effect on the corrosion behaviour of both steels as expected. As the surface roughness of 316L stainless steel increased, the breakdown potential (Ebreak), the free corrosion potential (Ecorr) and the width of passivity decreased, hence the corrosion rate increased. However, in the case of mild steel specimens, improving surface finish lead to shifts in the corrosion potential to more noble states and increased the corrosion rate. Metallographic examination of corroded specimens after electrochemical corrosion tests confirmed that the breakdown of the passive region was due to pitting corrosion.

Liquid phase sintering, heat treatment and properties of ultrahigh carbon steels

Abstract Thermo-Calc modelling was employed to predict liquid phase amounts for Fe–0·85Mo–(0·4–0·6)Si–(1·2–1·4)C in the temperature range of 1285–1300°C and such powder mixes were pressed and liquid phase sintered. In high C steels, carbide networks form at the prior particle boundaries, leading to brittleness, unless the steel is heat treated. To assist the break-up of these continuous carbide networks, 0·4–0·6% silicon, in the form of silicon carbide, was added. After solution of processing problems associated with the formation of CO gas in the early part of the sintering cycle, and hence large porosity, densities in excess of 7·75 g cc−1 were attained. A spheroidising treatment resulted in microstructures having the potential of producing components, which are both tough and suitable for sizing to improve dimensional tolerance. Yield strengths up to 410 MPa, fracture strengths up to 950 MPa and strains up to 16% were attained.

Influence of maleic anhydride compatibiliser on properties of polpropylene/multiwalled carbon nanotube composites

Abstract The effect of maleic anhydride compatibilser on the mechanical, rheological and thermal properties of polypropylene–carbon nanotube composites is reported. A commercial grade of polypropylene copolymer was melt compounded with acid purified multiwalled carbon nanotubes at loadings up to 12 wt-%, with and without the addition of a maleic anhydride grafted polypropylene. Tensile and flexural moduli were found to increase with filler content at all nanotube loadings and these properties were enhanced by addition of maleic anhydride. Ultimate tensile strength increased with nanotube loadings up to 8 wt-% and then decreased; maleic anhydride had a negative effect on tensile strength most likely resulting from nanotube agglomerations. Rheological and morphological characterisations confirmed that dispersion was relatively poor in both systems suggesting that maleic anhydride improved interfacial bonding without significantly improving dispersion, for the materials and compounding conditions examined in this work.