Search

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.

Powder metallurgy (PM) parts acceptance is determined by compositional and processing parameters and their controls. Statistical procedures are used for assessment; normal distribution and 'six sigma' appear to predominate. For fatigue of metallic materials and strengths of ceramics, fibres and composites, however, Weibull probability of survival analyses are widely used. The original analysis considers a threshold stress at which the probability of failure is zero. This stress is frequently taken to be zero, simplifying the analysis to two parameters. The yield stress has been suggested for the threshold stress, probably not sufficiently conservative for less ductile PM materials. A new three-parameter Weibull analysis, in which it is taken to be the fracture strength minus six standard deviations, is presented. Powder metallurgy manganese steels are under commercial consideration and this approach is applied to 12 variants of such laboratory processed specimens. It is compared with the two-param...

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...

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

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...

Tensile properties of powder metallurgy 3% manganese-0·8% carbon (content of green compact) steels were determined following laboratory sintering in (nearly) full, semiclosed containers with no getter powders in dry, 0-100% hydrogen-nitrogen atmospheres. Manganese was mixed with the NC 100·24 sponge iron powder as low carbon ferromanganese and carbon as a graphite addition. Dogbone compacts were pressed at 660 MPa, the sintering temperatures were 1120 and 1250°C and cooling rates ~65 K min- 1. In specimens sintered in nitrogen containing atmospheres at 1120°C, final carbon content was ~0·7% and for those processed at 1250°C ~0·6%. Sintering in dry hydrogen resulted in lower carbon and oxygen contents. Independent of the H2/N2 ratio in the furnace atmosphere, however, all the specimens were ductile and exhibited similar strengths. Yield strengths R 0·2 were in the range: 426-464 MPa, tensile strengths Rm were 724-780 MPa and strains to failure were 1·6-2·0% after sintering at 1250°C. The 1120°C si...

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 (

Mechanical properties of 2–4% manganese PM steels were determined in tension and in bending following laboratory sintering in dry, hydrogen rich atmospheres. Young's 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 ...

Calculated multicomponent phase diagrams were used to identify high speed steel (HSS) type alloys having the potential to exhibit enhanced sinter ability. The requirement was for an extensive austenite + carbide + liquid phase field. Of the six tungsten and molybdenum based systems studied, Fe–14Mo–C + 4Cr–8Co systems were potentially the most promising. Appropriate compositions were water atomised and additional alloys prepared by blending annealed powders with graphite powders. Powders were compacted to green densities of about 70% theoretical and then vacuum sintered. Sinterability was assessed in terms of sintered densities and microstructures. Alloys containing Fe–13Mo–1·3C, Fe–14Mo–4Cr–1·3C, and Fe–14Mo– 8Co–4Cr–1·4C were sintered to full density at temperatures as low as 1170°C, 70–150 K lower than for existing HSSs. Sintering windows were 20– 30 K, a significant improvement on existing HSSs. As sintered microstructures consisted of angular M6 C carbides dispersed in martensitic matrixes, w...

The increased market for high speed steels (HSSs) is increasingly associated with their use as wear resistant materials. The tribological properties can be enhanced by incorporating into the HSS further ceramic particles to increase wear resistance and solid lubricants to minimise friction, either at the surface, or by powder metallurgy into the steel matrix, already containing some 15% of 'natural', predominately MC and M6C, carbides. The difficulty of relating engineering performance of a component in reciprocating motion to a simple tribological test, such as pin on disc, is well recognised. The present paper will present a methodology of fretting testing, where the eventual failure is comparable to service damage of autoengine valve train parts, for which modified HSSs are prospective materials. Wear induced by fretting and microcracking of the surface and bulk material were studied in M3 class 2 and T42 HSSs in a newly developed fretting machine. The HSS plane specimens underwent reciprocati...

An attempt was made to achieve, by direct vacuum sintering of water atomised T15 powder, the microstructure and optimum transverse rupture strength (TRS), ∼3200 MN m−2, characteristic of the containerlessly hipped undersintered counterpart. The processing variables were: compacting pressure 400–800 MPa, sintering temperature 1260–1290°C, and sintering time 5–60 min. Ground, ground and polished, and hipped and unhipped specimens were tested, and the failure process investigated by metallographic and fractographic techniques. The highest strengths were associated with the absence of porosity and grain boundary films, which are responsible for cracking of oversintered specimens, and uniform distribution of fine (

The cold pressing and vacuum sintering characteristics of two batches of BTl high speed steel water atomized powder were investigated. The powder particle sizes, shapes, and microstructures before and after annealing were compared. Relationships were established between compacting pressure and green density, die fill, and linear shrinkage after sintering. Carbon was mixed with both batches before sirttering, 0·3% to batch 1 and 0'15%' to batch 2. The sintered densities of both batches with and without added carbon, are reported for a'range of sintering temperatures between 1230 and 1340°C together with typical sintered microstructures and grain and carbide size 'measurements. Carbon, oxygen, and nitrogen contents were estimated at major steps in the process. The differences found between the sintering characteristics of two batches of the same grade of high speed steel having only slight differences in alloy content were larger than expected. The addition of carbon, however, was found to promote sintering in several ways as well as bringing the final composition up to the specified value: the residual porosity in both batches was eliminated, the temperature range extended over which maximum density was achieved, and the microstructure at the optimum sintering temperature improved. PM/0328

A comparison was made of the secondary hardening characteristics of a conventionally wrought and two batches of directly sintered BT1 high speed steel. Minor differences in hardness between the types were observed after austenitizing between 1050 and 1280°C and tempering from 400 to 600°C. The recommended hardening temperature for BT1 was considered too high in at least two types by as much as 30 K. The wrought material had the finest microstructure throughout, e.g. 8 μm grain diameter for 1200°C austenitization treatment. Considerable grain refinement, however, was achieved in both directly sintered products by a post-sinter transformation anneal. The grain size after quenching was 2–3 times smaller than for the as-sintered, e.g. 15 and 19 μm compared to 46 and 36 μm respectively for the same austenitization temperature. PM/0331