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.     

HIGH-STRENGTH SINTERED IRON-BASE ALLOYS BY USING TRANSITION METAL CARBIDES

Abstract

A simple method of producing alloy steels by mixing, pressing, and sintering iron powder with carbides of transition metals is described. The carbides must have a high carbon content and be soluble in iron. These characteristics are found in the carbides of chromium, tungsten, molybdenum, and vanadium. Tensile strengths of ～800 N/mm² have been achieved in annealed specimens having carbide additions of 1–12 wt.-%, depending on the carbide. The additions produce steels with partly air-hardening properties.

The effects of concentration and particle size of the carbides, sintered density, sintering conditions, and cooling rate on mechanical properties have been determined. The alloys are not very sensitive to the sintering atmosphere. The effect of heat-treatment on the mechanical properties of Fe-Cr₃C₂ alloys is reported. A tensile strength >1300 N/mm² was observed with 4 wt.-% Cr₃C₂. Dilatometric measurements were conducted with different carbide concentrations and heating conditions to study the sintering process. The important process of homogenization was investigated by electron microprobe analysis.     

Effect of Fe addition on sintering behaviour of titanium powder

Abstract

The effect of iron on the sintering behaviour of titanium powder was investigated from two aspects: (1) diffusional homogenisation of iron; (2) densification of Ti-5Fe alloy. Under the present process conditions (heating rate of 5 K min^-1 and iron content 5 wt-%), iron dissolved into the titanium matrix thoroughly before the first eutectic temperature; potential liquid phase did not appear. The addition of iron enhances the sinterability of titanium alloys because the mobility of titanium atoms is accelerated by the rapid diffusion of iron. Most sintering shrinkage is achieved during the heating stage from 950 to 1250°C. Based on the diffusion creep mechanism of Nabarro-Hering, the result can be explained as a combination of the diffusion coefficient D and inherent local sintering stress σ, and the dissolution of iron in titanium is expected to reduce the creep strength of the Ti matrix at high temperatures due to its very fast diffusion rate. The effect of iron on the microstructure of Ti-5Fe alloy is also discussed. The formation of a Widmanstättenlike microstructure in Ti-5Fe alloy can be attributed to a β stabilising effect and a high diffusion rate of iron during furnace cooling.     

Effectiveness of reducing agents during sintering of Cr-prealloyed PM steels

Abstract

Development of strong inter-particle necks requires successful removal of surface oxides, present on the powder particles, during the initial stages of sintering. In the case of water-atomised powder prealloyed with chromium, the surface oxide consists mainly of an iron oxide layer with some more stable fine particulate oxides. The formation of sufficiently strong inter-particle necks requires as a minimum full removal of the iron surface oxide layer. This can be achieved by gaseous reducing agents (e.g. H₂, CO or a mixture of both) or by carbon, typically admixed in the form of graphite. The reducing power of various sintering atmospheres (active gas content ≤10 vol.-%) and their combined effect with graphite has been investigated by a thermal analysis technique. Results indicate that a combination of a dry hydrogen-containing atmosphere and fine graphite allows successful sintering of chromium alloyed PM steels.     

Recovery of metallic iron from high phosphorus oolitic hematite by carbothermic reduction and magnetic separation

Abstract

This study aims to provide theoretical and technical basis for economical and rational use of high phosphorus oolitic hematite. Following physical, chemical and microscopic characterisation of high phosphorus oolitic hematite ore the feasibility of separation of phosphorus and metallic iron by reduction roasting and magnetic separation process were investigated. The results indicate that such a process is a feasible and efficient method for iron and phosphorus separation of high phosphorus oolitic hematite. The recovery of metallic iron and dephosphorisation rate is relatively low without additives but is significantly improved by appropriate CaO and Na₂CO₃ addition. With 8%CaO and 3%Na₂CO₃ the recovery of metallic iron and dephosphorisation rate reach 95.1 and 94.0% respectively.     

THE DEVELOPMENT OF VACUUM-SINTERED STEELS

Abstract

The work described is concerned with the development of vacuum-sintered ferrous materials that can be used for high-strength structural parts. The reasons for choosing this production route are given, and the development of the materials is traced from the simple iron–carbon alloys to the more complex iron–carbon–nickel–chromium alloys. The metallography of the various series of alloys is included and the mechanical properties are fully reported. Tensile strengths > 70 tons in ^?2 were obtained after heat-treatment. An indication of the dimensional changes that took place during sintering is also given.     

THE PREPARATION AND SOME PROPERTIES OF CHROMIC OXIDE-CHROMIUM CERMETS MADE BY PARTIAL REDUCTION OF THE OXIDE

Abstract

Compacts of chromic oxide/carbon mixtures have been sintered in vacuum to produce oxide-metal cermets. The effect of carbon addition, type of carbon, purity of oxide, compacting pressure, and sintering temperature on the green and sintered density has been studied, and this has been supplemented by tests of hardness and compressive strength.

Considerable densification can be attained by a small addition of carbon to the powder mixture, and this is accompanied by an increase in compressive strength to 20 tons/in², compared with 3 tons/in² for the pure oxide sintered to the same temperature.     

METHODS OF EXTENDING THE APPLICABILITY OF SINTERED STEELS

Abstract

It is a common opinion among users of structural parts that applications for sintered steels are limited to those where requirements for strength are low to moderate. Furthermore, sintered steels of moderate strength are thought to be very brittle. It is the object of this paper to draw attention to significant improvements which have been achieved in the last few years. These are basically a result of powder developments which are based partly on traditional alloying additions, such as Cu, Ni, Mo, and C, and partly on unique combinations of iron powders and phosphorus or on combinations of iron powders, phosphorus, carbon, and/or copper. Unusually favourable combinations of strength and ductility can be achieved with diffusion-alloys based on iron and phosphorus. Components of high-duty sintered steels capable of replacing components of conventional wrought steels can be produced from partially prealloyed combinations of iron, copper, nickel, molybdenum, and carbon. For many applications these materials can also be an alternative to powder-forged steels. All the above powder combinations show consistent and low dimensional changes during sintering so that close tolerances of intricately shaped components can be maintained. Material and processing costs are such that the improved properties can be achieved economically.     

SINTERED IRON-BASE ALLOYS WITH HIGH STRENGTH USING SELECTED NITRIDES AND SILICIDES

Abstract

The techniques used in alloying in iron powder metallurgy have been extended by employing special compounds. The introduction of the alloying elements in this form and the decomposition of selected nitrides and silicides are described. Elements that oxidize readily at high temperatures (e.g. Cr, Si) can be added in a relatively pure and homogeneous state. These elements stabilize the α phase and thus improve the sintering behaviour.

The paper deals mainly with the preparation of binary Fe-Cr, Fe-Si, and also ternary Fe-Cr-Si alloys obtained by ‘in situ’ decomposition of Si₃N₄, Cr₂N, and CrSi₂ in an iron matrix (WP-150).

The study covers the properties of the powders and their mixtures, the pressing and sintering conditions, the sintering behaviour in the range 1000–1300°C with varying alloying additions, for different sintering times and atmospheres. The tensile strengths observed are ～525 N/mm² at a densityof 6·7 g/cm³, with ～3% elongation at fracture. With respect to the low density and the carbon free state of the alloys, the strength values may be considered as rather high. A study of the homogenization process is being carried out.     

Properties of Anisotropic Nd(Fe,Co)B Type Sintered Magnets Produced by Powder Injection Moulding

Abstract

Anisotropic Nd(Fe,Co)B type sintered permanent magnets were fabricated by powder injection moulding (PIM) using paraffin wax as a binder. A conventional process was also used in order to compare the resultant properties with those of sintered magnets produced by PIM. Magnetic properties, microstructure, and constituents were investigated by dc fluxmeter, scanning electron microscopy, X-ray diffractometry, wavelength dispersive X-ray analysis, and ir absorption analysis. Effects of particle alignment of the sintered magnets on the magnetic properties and anisometric linear shrinkage ratios were studied. Particle alignment of sintered magnets produced by PIM was 4% lower than that of the conventional process. The PIM sintered magnet exhibited a maximum energy product of 232 kJ m^?3. The residual carbon after debinding affected the magnetic properties and sintering characteristics by contaminating the liquid phase during PIM sintering. PM/0782     

Sintering of modified M35MHV HSS under diferent nitrogen pressures

Abstract

The sintering behaviour of high carbon–high vanadium water atomised M35MHV HSS (1·8 wt-%C, 4·2 wt-%V) is analysed as a function of the nitrogen pressure in the sintering atmosphere. Uniaxially pressed compacts were sintered to full density (≥ 98%TD) under different N₂ atmospheres in a range of pressures from vacuum to 8 bar. It is observed that the optimum sintering temperature (OST) depends on the absorbed nitrogen and is as low as 1050°C when the nitrogen content in the steel is 1·2 wt-%. The absorbed nitrogen affects not only the OST but also the matrix and carbides composition and the phases present after sintering. Compared with other powders processed under the same conditions, it is shown that the amount of absorbed nitrogen depends not only on the nitrogen partial pressure in the sintering atmosphere but also on the amount of vanadium and carbon and even on the heating rate. Hardness, fracture toughness, and fracture strength values are reported after heat treatment.     

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.     

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.     

Liquid phase sintering of ferrous powder by carbon and phosphorus control

Abstract

Powder mixtures composed of liquid forming master alloy powder and coarse iron powder were sintered to near full density by having a high amount (20 wt-%) of liquid phase during sintering. This was made possible by the use of the Fe-P-C system with or without Cu. Without post-sintering treatment, a brittle microstructure was obtained. By means of altered C and P control and decarburisation heat treatment of the as sintered material, the final non-brittle microstructure was achieved. Using the open porosity and liquid phase as a diffusion path, rapid decarburisation is created and the local combination of carbon and phosphorus in the microstructure is avoided. In this way, iron phosphide is not formed on grain and/or particle boundaries. Presence of pores is confirmed to be beneficial for grain growth control.     

Reactions between ferrous powder compacts and atmospheres during sintering – an overview

ABSTRACT

This overview paper describes the interaction of powder metallurgical iron-base alloys with the atmosphere during sintering. The methods of thermal analysis serve to clarify the processes that take place especially during the heating stage of the sintering cycle. After a discussion of the physical and chemical fundamentals of the sintering process, the methods of thermal analysis are explained. The differences between plain iron and alloyed systems are discussed in detail. Classical PM low alloy steels with alloying elements, such as Cu, Ni and Mo, react in a similar way as unalloyed carbon steels. The situation changes dramatically, when oxygen sensitive elements as chromium, manganese and even more silicon come into play. The removal of the surface oxygen is much more crucial, and there are several competing reactions, which have to be considered when these systems should be sintered in industrial scale to reach the desired mechanical and dimensional properties.     

Heat-Treatment Response and Cutting Properties of Sintered Type M2 High-Speed Steel

Abstract

The difficulties in the production of steel parts, with a uniform carbon content from the surface to the centre and a constant carbon content from one part to another, are well known. Recent progress on sintering atmospheres with regard to carbon content is assessed and found lacking, especially when compared with the current demands of the automotive industry. The author investigates the operating conditions in a furnace with a good production rate and submits possible explanations for this, as well as suggesting a means by which the sintering atmosphere may be controlled. PM/0142     

Rapid sintering of iron powders under action of electric field

Abstract

A new rapid sintering technique for iron powders compacted under the action of an electric field with high current density has been advanced. The results show that the sintering densification of iron powder could be finished in less than 6 min at a temperature of 800^u C reached at a heating rate of 600 K s^?1, and the relative density of the sintered compact was over 95%. Moreover, the sintering densification was almost finished in the heating stage of the compact.     

Dilatometric analysis of thermal debinding of injection moulded iron compacts

Abstract

The less than desired tolerance control of powder injection moulded compacts is a result of inconsistent dimensional changes in the compacts accumulated during moulding, debinding, and sintering. This study investigated the in situ length changes and their causes during thermal debinding on compacts which have been solvent debound. The dilatometric analysis showed that the specimen shrank in the early stage between 250 and 370°C, not because of sintering, but through the loss of N, C, and O in the carbonyl iron powder. At temperatures between 370 and 450°C, the specimen expanded owing to the carburisation of the iron powder. The length change was also influenced by the heating rate, debinding atmosphere, and the amount of the backbone binder. These dilatometric results are helpful in establishing the guidelines in designing binder compositions and debinding schedules.     

JPMA development awards

Abstract

The challenges in controlling carbon potential during sintering of steel powder have been discussed in many experimental and theoretical studies. The main issues lie within the complex thermodynamics and kinetics of processing atmosphere chemistry in continuous sintering furnaces. Although many models have been proposed to address the problem, these have rarely come to reality and entered industry practice. The purpose of this article is to summarise these discussions and investigate the interaction of the atmosphere constituents with the sintered compact within a sintering furnace. An important aim is to provide the PM industry with a fresh understanding of furnace operations and to provide recommendations to improve the control of furnace conditions. A case study is given of an existing furnace installation using Sinterflex technology which allows continuous monitoring and/or control of the furnace atmosphere. The reduction of oxides and carbon potentials to optimise the production parameters is described.