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.     

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.     

Water based processing of iron powder utilising starch consolidation

Abstract

The effects of water based shaping, by means of starch consolidation (SC), of an iron powder system regarding oxygen/carbon content and sintering performance were evaluated. Specifically, the influence of the drying conditions and the use of two different thickeners, xanthan gum and cellulose ether, were studied. The results showed that cellulose ether gave lower sintered density than xanthan gum, mainly because of less favourable rheological impact and air/gas entrapment at mould filling and consolidation. Due to less oxidation at drying and less removal of carbon at sintering, freeze dried specimens sintered to a higher density than room temperature air dried ones. The degree of oxidation and removal of carbon also influenced the as sintered microstructure. Ferrite grains surrounded by iron phosphide were found in both air dried and freeze dried specimens. However, the higher carbon content in freeze dried specimens also resulted in a significant amount of iron carbide grains (inclusions), which can be a potential strength limiting factor.     

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.     

Behaviour of metal powders during cold and warm compaction

Abstract

An instrumented die was used to investigate the behaviour of metal powders during cold (ambienttemperature) and warm (up to 140^°C) compaction. This instrument enables simultaneousmeasurement of density, die wall friction coefficient, the triaxial stresses acting on the powderduring the course of compaction and ejection pressure. Commercial iron, titanium, aluminium,316L stainless steel (SS) and aluminium–silicon powders were employed for investigation. Theresults demonstrated the advantages of powder preheating on the compaction behaviour of metalpowders concerning green density, dimensional changes, frictional behaviour, ejectioncharacteristics and compactibility. However, the outlines also determined that the response ofthe non-ferrous powders to powder preheating is somehow different from those of the ferrouspowders. In this context, the behaviour of prealloy aluminium–silicon powders during compactionwas found of particular interest, as their compactibility is strongly affected by powder preheating,whereas the dimensional changes after ejection decrease considerably. This article presents theeffect of cold and warm compaction on the consolidation and ejection characteristics of ferrousand non-ferrous metal powders. The influence of compaction condition (pressure andtemperature) with considering of the powder characteristics and densification mechanisms areunderlined.     

Characterization,Physical and Mechanical Behavior of Sintered Atomized Iron–Zinc Stearate Composite

Powder metallurgy is an effective method to process the iron component in near net shape. In this paper, the influence of particle size, lubricant and compaction load on the physical and mechanical properties of the sintered iron–zinc stearate composite sample has been investigated. Atomized iron powders of particle size 100–200 and 200–300 mesh with zinc stearate 2.5, 5.0 and 7.5 wt% were used for preparing the samples. Green samples were prepared by cold compaction at various loads of 200, 180 and 160 KN and the sintering is done at 500 °C. The physical properties and the mechanical properties such as density, hardness and compression strength have been measured for the sintered samples. Scanning electron microscope was used to characterize the atomized iron powders and the sintered samples. It were confirmed that the shape of atomized iron powder particles were irregular, dendritic and acicular. Energy dispersive spectroscopy was used to identify the elemental compositions of powders and the sintered samples. Taguchi (L₁₈) method was effectively used to develop the regression model and describe the contribution of the input parameters in compressive strength, density, and hardness. The highest effect on density was powder particle size followed by compaction load and lubricant wt%. The Hardness value increased with increase in the powder particle size and compaction load and decreases with increase in the lubricant wt%. The compression strength increased with increase of compaction load and decrease of lubricant wt% and particle size.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

Powder metallurgy materials in hot forming

Abstract

The purpose of the present paper is to determine the apparent yield stress of powder metallurgy (PM) materials at high temperatures. A brief introduction concerning the theory of yielding of PM materials is included. The models of loading functions for porous materials are recalled. The experiments have been undertaken by the author to identify the parameters of PM materials in hot forming. Two materials are considered: pure iron and aluminium powders.     

Effect of compaction pressure and powder grade on microstructure and hardness of steam oxidised sintered iron

Abstract

Steam oxidation has proven to be an effective process to improve the properties of sintered iron components. The oxide formed on the surface and in the interconnected porosity strongly influences both the tribological and mechanical properties of these materials, for example through the extent of pore closure and the nature and morphology of the oxide produced. In this paper, the influences of compaction pressure and powder size on the microstructure, oxide content, hardness, and surface topography of steam treated sintered iron are analysed. Specimens prepared from atomised iron powders of different sizes (<65, 65–90, 90–125, and >125 µm) were compacted at four different pressures (300, 400, 500, and 600 MPa), sintered for 30 min at 1120°C and then subjected to a continuous steam treatment at 540°C for 2 h. A clear influence of the processing parameters on porosity was highlighted. Low porosity was always associated with high compaction pressure and greater powder size. Pore size was affected in the same way by compaction pressure, even though the effect of powder size acted in the opposite sense. Changes in compaction pressure and powder size had no significant effect on pore shape. Decreasing powder size always led to high hardness. The effect of compaction pressure on hardness is clear evidence of a compromise between porosity and blockage of the pore network by oxide. Samples produced with smaller powder sizes showed a continuous decrease in hardness as the compaction pressure increased, although for the large powder size there was a slight increase to a constant value of ultimate hardness. For the intermediate powder size a maximum hardness was obtained as the compaction pressure increased. X-ray diffraction showed that the oxide layer is composed of magnetite and haematite.     

The history of Distaloy

Abstract

The low alloy steel powder Distaloy, is today widely used in applications demanding high strength and wear resistance. Its basic properties and composition were designed half a century ago in the USA. The advantage lay in the fact that it was a partial prealloy, i.e. the alloying elements – copper, nickel and molybdenum – were bonded in particulate form to the basic iron particles, thus avoiding impairment of the compressibility. By balancing the contents of nickel and copper it was possible to minimize dimensional change on sintering. Bonding the alloy particles to the iron particles minimised segregation and also contributed to dimensional stability. Carbon was added conventionally as fine graphite. However, the new powder, marketed as Ancoloy, did not take off in North America, due to the lack of suitable applications, the cost of the alloying elements and – above all – the poor compressibility and high oxygen content of the iron powder then available. The high, variable oxygen content made it impossible to control the carbon content with the precision necessary to achieve the desired strength and hardness.

In the 1960s, demand for high strength precision parts emerged in the European car industry, initially at Citroen, which pioneered increased use of PM parts in European cars. The component was (and still is) the synchronising hub used in manual transmissions. Höganäs had in the mid-1960s developed a sponge iron powder with much higher compressibility, and this was taken as a raw material for an improved grade, later to be called Distaloy SA. This new powder had improved compressibility and very low oxygen and carbon contents, which made it possible to make the high strength precision parts that the car industry required. Distaloy was immediately accepted and used, first in the French car industry, then elsewhere in Europe and subsequently also in Japan. Some years later, when high compressibility atomised powder became available, the same basic technique was applied to these, to produce the Distaloy A grades, which now are most popular. Höganäs continues to improve and refine the production techniques and to come up with compositions for new applications.

The properties and the metallurgy of Distaloy-based materials have been thoroughly studied by metallurgists at Höganäs and at PM laboratories throughout the world, and new results are still being reported with respect to both applications and fundamental properties. A parameter of great relevance is of course the cost of raw materials and much effort is going into finding more cost effective ways of achieving the desired results.     

Mechanical properties of green compacts. II. Effect of powder relative bulk density on the strength of compacts with different forming temperature conditions

Mechanisms of strength for green compacts made from powders of iron, nickel and its alloys, copper, tin, and zinc are analyzed. The strength of green compacts prepared from metal powders of medium fineness with a relative bulk density (RBD) from 0.119 to 0.568 by two-way compaction in rigid dies with homologous temperatures from 0.15 to 0.59 (pressure from 200 to 800 MPa, powder deformation rate 10^?2–10^?3 m/sec) is studied. Compact strength is determined by diametric compression of cylindrical compacts. The dependence of strength on compact porosity is studied by the Bal’shin equation. The possibility is demonstrated of using this relationship in order to describe hot compaction and formally describe cold compaction of powders with RBD up to 0.40. The effect of homologous temperature and powder RBD on compact strength is determined. The homologous temperature for transition from warm to hot compaction and the effect of compact density (degree of deformation) on this temperature is studied. It is shown that linear approximation is possible for the dependence of compact strength on powder RBD according to the equation σ _f.c = 87–217?RBD.     

Production of titanium grade 4 components by powder injection moulding of titanium hydride

Abstract

Recent developments are presented on powder injection moulding of titanium from metal hydride powders and binders composed of polyethylene, paraffin wax and stearic acid. The feasibility of using this route to process fit for purpose, complex parts is assessed. Titanium hydride offers a low cost solution compared with pure titanium powders. Feedstocks for powder injection moulding were prepared in a sigma mixer. Tensile test specimens and demonstration parts were injection moulded. Solvent debinding in heptane was followed by thermal debinding and dehydrogenation under argon. Titanium parts were sintered at 1200°C under argon. Sintered parts exhibit a linear shrinkage of about 20%, good shape preservation and reproducibility. The yield strength (519 MPa), ultimate tensile strength (666 MPa), elongation to fracture (15%) and interstitial content measured by quantitative analysis meet the requirements for titanium grade 4.     

Powder reaction mechanism in fabrication of high silicon iron alloy

Abstract

In the present paper, the reaction mechanism of silicon and iron powders under different sintering conditions during the fabrication of high silicon iron sheet (～6·5 mass-%Si) is clarified. It is indicated that the phases, Fe₃Si (Si) and FeSi, play an important role in the reaction between iron and silicon powders. Two temperature regions of the powder reaction are very important for producing commercial high silicon iron sheets: the temperature region of ～1000°C in which the ductile composite structure can be produced, and the temperature region of ～1200°C in which the density and homogeneity can be improved.     

Optimising grey iron powder compacts

Abstract

The grey iron microstructure Fe–2C–2Si powder based compact is tailored by different kinds of in situ and post sintering processing. This has been achieved by combining thermodynamic and kinetics modelling of microstructure development with sintering and controlled heat treatment experiments of tensile test specimens die compacted at 600 MPa. Applying optimised sintering conditions led to a grey iron like microstructure with 95% relative sintered density. Sinter hardening the compacts led to 500 MPa in yield strength and 600 MPa in ultimate tensile strength in combination with ductile fracture. Quenched and tempered condition showed the same strength values, but combined with brittle fracture due to martensitic structure. Pore rounding and partial pore filling by graphite were obtained by austenising isothermal hold during the cooling of the sintering cycle.     

Improving iron compact green strength using powder surface modification

Abstract

In powder metallurgy, green strength has important consequences for part production rates and product end quality. Mechanical interlocking and interparticle cold welding are the main mechanisms responsible for green strength. These mechanisms are affected by compaction pressure, temperature, amount of lubricant and additives admixed to the powder, and surface characteristics of the powder. The present paper describes the effect of iron powder surface modification on the green strength of compacted specimens. The green properties of compacts fabricated from iron powder treated with diluted sulphuric acid and coated with copper by a non-catalytic displacement plating method are presented. The results indicate that surface modifications strongly influence the green strength of the compacts.     

PLASTIC DEFORMATION IN METAL POWDER COMPACTION

Abstract

Some current opinions on the role of plastic deformation in powder compaction are reviewed. The results of mechanical testing, metallographic examination, and X-ray-diffraction analyses of some atomized iron powder compacts are presented, together with those of metallographic examination of compacted spherical high-temperature, high-strength alloy powders. Extensive plastic deformation occurs even during the first stage of compaction but this is not the only cause of consolidation. A sequence of compacting mechanisms is described for the iron powder. It is suggested that the transition from Stage 1 to Stage 2 of compaction corresponds to the change from local to homogeneous plastic flow.     

粉末冶金工艺对纯铁软磁材料性能的影响

利用粉末冶金技术制备纯铁软磁材料,在不同温度和压力下将不同粒径铁粉压制成生坯,并在保护气氛下进行烧结。结果表明:不同粒径铁粉混合有助于压坯密度的增加,适宜的压制温度可以有效地促进粉末流动,避免大尺寸孔洞的形成,优化组织。140℃、800 MPa温压条件下雾化铁粉压坯密度最高可达7.35 g·cm-3。对比常温压制,温压压坯烧结后孔洞分布均匀。烧结体密度随温度的升高而上升,雾化铁粉压坯在1250℃烧结后密度最高可达7.47 g·cm-3。在一定范围内,软磁材料磁性能与密度成正比,混粉压制试样的密度接近理论值,但在混合铁粉中,较细的铁粉夹杂于粗粉中,阻碍磁畴壁移动,造成饱和磁化强度（M_s）偏小、矫顽力（H_c）偏大的现象,M_s为205.51 emu·g-1,Hc为7.9780 Oe。     

Influence of Lubricants on Dimensional Changes and Mechanical Properties of Sintered Ferrous Compacts

Abstract

The influence of admixed zinc stéarate on the shrinkage of uniaxially pressed iron powder compacts has been studied. For pressing conditions which caused inhibition of compaction the removal of the stéarate during sintering produced an increase in shrinkage parallel to the pressing axis and in direct proportion to lubricant content. Additions of stearic acid (varying particle size), zinc stearate, lithium stearate, stearamide, and Cosmic 64 wax were used to investigate the influence of lubricant on mechanical properties of green and sintered iron powder compacts. Green strength was reduced relative to unlubricated material only by lubricants whose physical and chemical properties enabled them to produce and maintain extensive interparticle films during pressing. Vapour from the rapid initial decomposition of lubricants which reduced green strength could have a deleterious physical influence on the tensile strength of dewaxed or sintered Fe compacts. Decomposing lubricants also produced undesirable chemical effects. These arose from reactions between lubricant decomposition products and the matrix or by these products interfering with reactions between matrix and sintering atmosphere.     

Influence of sulphur addition by cored wire injection on machinability of grey iron

Abstract

Sulphur powder was added to grey iron melts using cored wire injection technology to improve machinability. The influence of different injection speeds into the ladle on sulphur recovery efficiency was studied, followed by an evaluation of the microstructure, mechanical properties and machinability of the cast iron. The peak sulphur recovery efficiency is 82·5%. Machinability increases with sulphur content.     

PM processing of elemental and prealloyed 6061 aluminium alloy with and without common lubricants and sintering aids

Abstract

A comparison has been made between compaction, sintering, microstructural and mechanical properties of the 6061 aluminium alloy prepared via premixed elemental (EL) and prealloyed (PA) powders (as received and degassed) with and without additions of sintering aids and various solid and/or liquid lubricants. Both EL and PA powders were cold pressed at different pressures, ranging from 250 to 770 MPa, and sintered under vacuum in the range 580–640^°C for 30–120 min. and then under pure nitrogen atmosphere for comparison. Vacuum degassing of the PA powder provided better compressibility and thus higher green densities than those for the as received PA or the premixed EL powder compacts pressed at compaction pressures ≥340 MPa. Near full sintered densities of , ～98%TD were obtained for both EL and PA 6061 Al alloys. Degassed PA Al with 0·6 wt-% paraffin wax (PW) or with only 0·12 wt-%Pb addition as sintering aid and no lubricant, and premixed EL with only 0·12 wt-%Pb addition and no lubricant gave the best optimum properties. It became apparent that additions of some solid lubricants such as lithium stearate (LS) and acrawax to both the premixed EL and PA powders provided reasonable green densities, but had deleterious effect on sintered densities and microstructures, particularly under vacuum sintering. Heating data curves during the sintering cycle, revealed formation of both transient and persistent liquid phases for the EL and mainly supersolidus liquid phase sintering (SLPS) mechanism for the PA. Tensile properties of the degassed, vacuum or nitrogen sintered PA Al alloy in T6 condition were higher than those of the equivalent alloy prepared by EL mixing with the former giving a tensile strength of 330 MPa and 6–8% elongation to failure, which are similar to those of the commercial (wrought) 6061 Al alloys.