EVALUATION OF IRON POWDERS FOR SINTERED STRUCTURAL COMPONENTS

Abstract

A new method for evaluation of iron powders is suggested. Ultimate tensile strength is chosen as a base parameter, and the relations between this property and compacting pressure and raw material cost, respectively, are shown. For this purpose it has been necessary to deduce two supplementary parameters, Relative Pressure Response (P_r) and Relative Raw Material Requirement (M_r), which are functions of compacting pressure and ultimate tensile strength, and of compacting pressure and density, respectively.

It is shown that the importance of compressibility of iron powders is overrated in current opinion and, consequently, that it is misleading to judge the overall merits of an iron powder according to its compressibility.

Raw material costs of sintered steels are lower, if sponge-iron powders are used instead of atomized powders, even if the price of all iron powders were equal. This tendency is more strongly emphasized at low densities, where the sponge-iron powder with the lowest apparent density value is preferable. The differences are beginning to lessen and disappear gradually at densities approaching or exceeding 7·0–7·2 g/cm³ (for single-pressed and single-sintered materials).

Alloy composition has a stronger influence on raw-material costs than the choice of iron-powder grades. Close and reliable control of carbon contents and avoidance of oxidation of manganese is essential for lowering of costs in the PM structural-component manufacturing industry.     

STUDIES ON THE ACTIVATION-SINTERING OF IRON POWDER

Abstract

A study of the sintering behaviour of iron compacts containing additions of tin up to 1 wt.-% has been made. A tensile strength of 234 MN/m² (34 x 10³ lbf/in²) has been achieved with an optimum tin addition of 0·5 wt.-%, sintering being carried out for 10 min at 1100°C (1373 K) in a reactive halide atmosphere. Combination of the two ‘activating’ techniques (addition of tin and sintering in a reactive atmosphere) permits current properties to be attained at considerably lower sintering temperatures or sintered densities, and is much more effective than when they are applied individually. A tensile strength of 165·3 MN/m² (24 x 10³lbf/in²), achieved by sintering at 1200°C (1473 K) for 10 min with an addition of 0·5 wt.-% tin can be obtained by reactive-sintering the same composition at 900°C (1173 K) for 10 min. Alternatively, the density of the part can be reduced from 6·7 to 6·2 g/cm³ with no loss of strength or elongation. Tin in excess of 0·5 wt.-% causes deterioration in properties under the sintering conditions studied and a reason for this is cited. The improvements in properties are lost also if admixed lubricant is used in the compactionprocess.     

THE EFFECT OF PHOSPHORUS ADDITIONS ON THE TENSILE,FATIGUE, AND IMPACT STRENGTH OF SINTERED STEELS BASED ON SPONGE IRON POWDER AND HIGH-PURITY ATOMIZED IRON POWDER

Abstract

The mechanisms operating during the sintering of iron-phosphorus PM alloys are discussed, as well as the factors contributing to the unique combination of strength, ductility, and toughness that is characteristic of these materials. Alloying methods are reviewed with special reference to powder compressibility, tool wear during compaction, and homogenization during sintering. The preferred production method is to add phosphorus in the form of a fine Fe₃P powder to iron powder. The mechanical properties of a number of sintered steels made with and without Fe₃P additions to sponge iron or to high-purity atomized iron powders are reported. Use of atomized powder makes it possible to reach extremely high density by single pressing and the resulting phosphorus-containing sintered steels have very high ductility and impact strength. The fatigue strength is related linearly to the tensile strength, with a correlation coefficient of 0·91. It is concluded that structural factors other than those that control ductility and toughness are responsible for the fatigue resistance of sintered steels.     

THE HEAT -TREATMENT AND MICROSTRUCTURE OF A SINTERED NICKEL-MOLYBDENUM STEEL

Abstract

The effects of post-sintering heat-treatment on the properties and microstructure of a 5Ni–0·5Mo–0·5C sintered steel have been studied in detail. It has been demonstrated that by using a relatively high tempering temperature after oil-quenching, UTS and elongation can be simultaneously raised above their as-sintered values, while impact-resistance is at least maintained. By tempering at 650–675°C a UTS of 900 N/mm² can thus be combined with 4% tensile elongation and with a Charpy unnotched impact value of 26 J. Properties close to these can be achieved even if oil-quenching is replaced by cooling in a furnace cold zone. Alternatively, by quenching and then tempering at 200°C the UTS can be raised to ～1400 N/mm² with 1 ½ elongation. The constituents of the microstructures, which are generally not homogeneous, have been identified for each heat treatment condition. It is concluded that for this steel, which is predominantly martensitic on quenching, the properties are controlled by the tempering of the martensite, and the heterogeneity generally does not have any pronounced effect on properties. Studies of the effect of raw material variables have indicated: (a) that iron powder compressibility and even the final density of the steel are not a safe guide to properties after high-temperature sintering; (b) that costs may be reduced without detriment to properties by using ferromolybdenum powder instead of elemental molybdenum; (c) that the use of ferronickel powder or codecomposed iron/nickel gives results inferior to those achieved with elemental carbonyl nickel powder.     

Influence of Powder Type and Density on Pore Closure and Surface Hardness Changes Resulting from Steam Treatment of Sintered Iron

Abstract

Two types of Höganäs iron powder – sponge, and atomized with very high compressibility (ASC), after compaction to densities of 6·0, 6·4, and 6·8 Mg m^?3 and sintering under standard conditions were subjected to steam oxidation at 450, 525, and 600°C. The progress of oxidation was studied by measurement of weight gain and hardness. X-ray methods were used to determine the type of oxide present after treatment. During steam oxidation the type of powder has an important influence on the extent of pore closure and on the morphology of the oxide produced. The kinetics of oxidation were always faster for sponge iron than for atomized iron and there was a corresponding increase in the rate of pore closure and in surface hardness. For effective sealing of surface pores components should be of high density and be steam treated at 600°C but for attainment of maximum hardness components should be of low density and be steam treated at 525°C.     

Effect of boron on vacuum carburising depth of iron powder metallurgy compacts

Abstract

The work was aimed at determining the effect of boron on vacuum carburising of iron compacts with density over 7·2 g cm^–3. An attempt was made to determine the effectiveness of boron on carbon diffusion rate into the material of compacts with no additional effect of interconnected porosity. Vacuum carburising of compacts made of iron powder with an addition of boron was carried out at 1050°C in a laboratory vacuum furnace.

The effect of boron content within 0·005 to 0·02% on the vacuum carburising depth was analysed. It was found that the boron addition up to 0·01% increased the carburising depth by ～0% in comparison with the compacts of pure iron.     

HIGH-VELOCITY EXTRUSION FORGING OF BILLETS COMPACTED FROM IRON AND STEEL POWDERS

Abstract

Billets 25 mm in dia. and weighing ～135 g have been compacted from sponge-iron powder using a single-end-pressing technique. A range of densities from 5·6 to 7·2 g/cm³ was produced. The billets were then hot extrusion forged, at high speed, into tensile specimens of gauge-dia. 10 mm, gauge-length 28 mm, and head dimensions of 13 mm dia. × 12·5 mm long. About half the billets were pre-sintered before heating to forging temperature, while others were hot-forged without a pre-sintering stage. The tensile specimens were then tested and selected ones examined metallographically.

The work was extended to investigate the extrusion forging of alloy steel powder.     

THE INFLUENCE OF POROSITY AND CARBON CONTENT ON THE FRACTURE TOUGHNESS OF SOME SINTERED STEELS

Abstract

Fracture toughness (K_IC) and mechanical properties at room temperature (RT) and at 200 K were evaluated for a range of sintered steels. In a series of quenched and tempered samples prepared from a lowalloy, atomized powder (Höganäs ATST-A) with 0·45 wt.-%C and density varying between 6·7 and 7·8 g/cm³ (powder forged), the K_IC varied between 28 and 80 MN/m^3/2 at RT and between 34 and 58 MN/m^3/2 at 200 K. There was a correlation between the K_IC and the yield strength of the porous materials, due to the fact that fracture of the specimen is effected by plastic instability on a micro scale. Adding carbon (0–0·84%C) and copper (2%) to a sponge-iron powder and sintering to a final density of 6·8 g/cm³ gave a material with a fracture toughness of ～34 MN/m^3/2 when the C content was >0·6%. At lower C contents the toughness was considerably increased, but it was not possible to obtain a valid. K_IC determination. In this investigation a new type of fracture-toughness specimen was used, the RCT specimen, diameter 75 mm and thickness 29 mm; by using this type as opposed to the ordinary CT variant, specimens ～50% larger could be used.     

THE CORRELATION BETWEEN RAW MATERIALS,PREPARATION CONDITIONS,AND PROPERTIES OF SINTERED IRON

Abstract

Three plain iron powders of different types (sponge-iron, atomized and electrolytic iron powder) were studied with respect to their sintering behaviour and to the influence of manufacturing parameters—i.e., compacting pressure, sintering temperature, and sintering atmosphere—on the microstructure and the properties of sintered compacts. The changes of length, electric conductivity, and strength during sintering are explained in physical and chemical terms. Technical sintering diagrams are presented. The influence of sintering atmospheres on the mechanical properties of sintered compacts is shown for the three types of powder. The correlation between pore structure and strength is discussed; analytical relationships are developed which are in agreement with the experimental results.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

FRICTION COEFFICIENTS BETWEEN IRON POWDER COMPACTS AND DIE WALL DURING EJECTION USING VARIOUS ADMIXED ZINC STEARATE LUBRICANTS

Abstract

Laboratory compaction and ejection studies have been made using a reduced iron powder mixed with a number of zinc stearates having median particle sizes between 4 and 22μm. Comparable experiments were carried out on a fully instrumented production press, which was operated at compacting pressures between 300 and 500 MN/m² to produce compacts with true densities ranging from 5·90 to 6·70 g/cm³. Determination of ejection forces by the two methods enabled calculations of the coefficients of friction between compact and die wall to be made for mixtures containing 0·5–2·0 wt.% zinc stearate. These showed that the behaviour during compaction and ejection was comparable on both laboratory and production scales and gave very similar results. An interpretation of the results is given and values of coefficients of friction are presented which show that these are dependent on the type of zinc stearate used.     

THE POWDER METALLURGY OF RUTHENIUM

Abstract

An investigation of the powder metallurgy of ruthenium is described, from the reduction of ammonium ruthenium chloride to the working of sintered compacts. The powder properties measured were specific surface area, by a simplified BET method, and tap density. The dependence of these properties on the conditions of reduction has been determined. The surface area of powders varies from 1 to 10 m²/g in the temperature-of-reduction range 700-350°C. The tap density is also variable (1–3 g/c.c.) and is generally related to the surface area. The effects of compacting pressure and temperature on sintering are described, the progress of sintering being observed by measurements of the “open” and “closed” porosity present in samples. Compact densities up to 95% of theoretical can be obtained by sintering at 1500°C. The selection of powder properties and compacting pressures to be used in the production, by vacuum sintering at 1500°C, of high-density compacts for working, is governed by the necessity to maintain open porosity during the heating cycle up to at least 1200°C, as considerable gas evolution occurs at this temperature; at the same time it is essential that good densification shall have occurred even at this stage. These conditions can be met by using powder with a surface area of 2–5 m²/g and compacting pressures in the range 0·5–25 tons/in ².

Observations on the hot working of sintered compacts indicate that ease of working is related to the surface area of the powder.     

OPTIMIZATION OF PROPERTIES OF POWDERED IRON AND STAINLESS-STEEL PARTS BY STATISTICAL DESIGN AND COMPUTER ANALYSIS

Abstract

A statistically designed experiment was formulated to study the effect of several major powder variables on the strength properties of porous iron and stainless-steel parts. The resulting data were analysed by means of a suitable computer programme to develop individual response equations relating the chosen dependent variables with selected independent variables. Computer analysis of the data and the optimization techniques adopted led to an improvement of ～50% in the strength of sintered parts by comparison with those made by conventional processes. A certain set of powder properties and process variables resulted in a tensile strength of 170 MN/m², 11.5% elongation, and very low dimensional change in a sintered iron sample with 25% porosity. In a 316L stainless-steel part with 25% porosity, a tensile strength of 435 MN/m², 0.2% yield strength of 269 MN/m², and 12.6% elongation were reached–values far above those that can be obtained without the benefit of statistical design.     

On suitability of novel fluidised bed technique for separation of metallic powders during commercial powder metallurgical processing

Abstract

Experiments have been performed to test the efficiency with which a novel fluidised bed technique could separate different metallic powders in terms of size and density. The overall aim was to assess the potential of this technique for the commercial separation of defective powder fractions from mechanically alloyed (MA), iron based powders. Separation in terms of size was readily achieved, with the largest powder particles sinking to the bottom of the fluidised bed. In a simulated commercial process, density separation of defective Fe₃Al powder could not be demonstrated as any differences in density were overshadowed by size and morphology differences. However, from a batch of iron based powder (ρ=8·02 g cm^?3), seeded with six other metallic powders, aluminium powder (ρ=2·70 g cm^?3) segregated strongly to the top of the bed from where samples containing 93 vol.-%Al were taken. The process is thought to be sensitive to differences in density of a factor of 1.1–3.     

Effect of Compacting Pressure and 0·5%Mo on Properties of Sintered Iron Powder and Manganese Steels

Abstract

An investigation has been carried out on the effect of compacting pressure, in the range 150–600 MPa, and of the addition of 0·5%Mo on the properties of sintered Hametag iron powder and manganese steels. Higher sintering activity compared with standard iron resulted in higher density and higher mechanical properties in the presence of manganese vapour. The addition of 0·5%Mo in the form of ferromolyb-denum caused an increase in density and strength properties in the Fe–C and Fe–Mn–C steels. PM/0157     

THE INFLUENCE OF PRESSURE,TEMPERATURE, AND TIME ON THE HOT PRESSING OF COMMERCIAL BERYLLIUM POWDER

Abstract

For a particular batch of Brush Super-Pure – 200-mesh beryllium powder the hot-pressability (defined as the length of a standard compact of < 98% theoretical density), increased: (1) continuously with pressure over the range 0–1·25 ton/in² for compacts pressed for 1 h at 1100°C;(2) with temperature from 1000°C to a maximum at 1150°C when pressed for 1 h under 1 ton/in²; and (3) to a lesser extent with time over the range 10–120 min when pressed at 1050°C under 0·25 ton/in² and at 1100°C under 1·0 ton/in². Differences in hot-pressability between various batches of the same powder were small compared with the effects of temperature and pressure.

Compaction during hot pressing occurs in two stages: first, collapse of the powder column causing bulk powder flow; followed, secondly, by sintering of particles forced into close contact. The latter is accompanied by a diminution in both the number of pores and their average size and is associated with grain growth, particularly above 1100°C; after pressing at 1200°C any remaining porosity assumes a thermally stable, spherical configuration.     

THE PRESSING AND SINTERING PROPERTIES OF IRON POWDERS

Abstract

Some twenty different iron powders are available in Europe for the manufacture of sintered bearings and structural parts. These powders can be grouped under four headings: reduced, atomized, comminuted, and electrolytic. Details are given of the experience gained in testing these types of powder by the methods in common use in the metal-powder industry.

The influence of the data thus obtained on the processing, in particular the pressing and sintering conditions, of iron powders is discussed in detail. Consideration is given to the way in which the properties of the powders affect their sinterability, the pressing operation and tool design, and also the physical characteristics of the finished product.     

THE PROPERTIES OF IRON CONTAINING A FINE OXIDE DISPERSION

Abstract

Gel precipitation has been used to produce fine dispersions of thoria in iron. The process involves coprecipitation of hydroxides followed by a hydrogen reduction. Iron containing up to 7·65 vol.-% thoria was prepared with particle sizes in two ranges averaging 6 and 110 nm in diameter. The dispersion, stable up to at least 1300°C, raised the tensile and yield strengths by 80% compared with the iron made in the same way while retaining an acceptable level of ductility at ambient temperature and below. Impact testing showed that the presence of thoria did not have an adverse effect on the ductile-brittle transition temperature for thoria contents up to 4·55 wt.-%. It was not possible to estimate the strengthening due to the dispersions from the models of Orowan bowing of dislocations between particles because of the presence of two size ranges of particles in these alloys. The thoria dispersions did not raise the primary recrystallization temperature of the iron but were extremely effective in restricting secondary recrystallization. Grain sizes of <25μm were obtained after 15h at temperatures up to 1400°C. The thoria-containing alloys did not work harden more rapidly than the iron and with thoria contents up to 3 wt.-% could be readily cold worked.     

THE PRODUCTION OF GRAIN-ORIENTED 50 : 50 NICKEL–IRON MAGNETIC STRIP BY COLD ROLLING FROM SINTERED COMPACTS

Abstract

50 : 50 nickel–iron strip was cold rolled from sintered compacts to thicknesses of 0·0015 and 0·004 in., with final reductions of 92–99%. After annealing at 1050°–1200° C., the cube texture developed, thus giving a material with a rectangular hysteresis loop when magnetized in either the rolling or the transverse direction.

The alloys with the best magnetic properties contained 48–50% nickel. In this range of composition the remanence ratio and coercivity varied from 0·91 to 0·94 and 0·11 to 0·19 respectively, depending upon the processing schedule used.

With the recommended processing schedule, values for the remanence ratio and coercivity of 0·92–0·94 and 0·11–0·13 respectively were obtained.

Unlike the texture of strip rolled from a conventionally cast ingot, the development of cube texture in strip rolled from a sintered compact is not critically dependent on the temperature of the final anneal.     

THE EFFECT OF MINOR ADDITIONS OF SULPHUR ON PORE CLOSURE AND CASE-HARDENABILITY OF SINTERED IRON

Abstract

Interconnected porosity in sintered iron structural parts can have a detrimental effect on case-hardening by permitting penetration of the carburizing gases to the interior of the compact. Experiments have shown that small additions of sulphur to the iron powder may provide a means of effecting the desired pore closure, though this method has still to be proved applicable on an industrial scale.