Enhanced sintering and mechanical properties of 316L stainless steel with silicon additions as sintering aid

Abstract

Compaction, effect of ball milling, vaccum sintering, microstructures, volume shrinkage, interconnected porosity, thermal reactions and mechanical properties of 316L stainless steel with and without additions of elemental silicon have been investigated. It was found that the silicon addition enhanced the sintering process by providing a series of liquid phase reactions with the base powder which took place at temperatures below their melting points and the normal solidus range for stainless steels. Differential thermal analysis confirmed formation of liquid phases at three different temperatures which are believed to be responsible for the enhanced sintering process.The first two appeared at ~1060 and 1155°C by two exothermic peaks and the third one at ~1190°C by an endothermic peak. The ball milling operation provided higher green and sintered densities resulting in better mechanical properties due to less agglomorations with finer and much more uniform particle size distribution. Sintered densities of up to 7·44 g cm^-3 with tensile strength of 482 MPa, hardness value of 153 HV10 and 15% elongation were obtained with ball milled plus 3 wt-%Si addition. Low levels of interconnected porosities (~4%) were recorded within the temperature range 1250-1300°C suggesting the possibility of good corrosion resistance.

The sintered microstructures consisted of ferrite and austenite (duplex structure), complex silicide and eutectic phases within grains and at grain boundaries, pools of liquid (rich in Si) and some medium and small pores preventing full density to be achieved despite the liquid phase formation.     

Production of Al–Cu and Al–Cu–Si Alloys by PM Methods

Abstract

The possibilities of the production of aluminium-base copper and/or silicon alloys by conventional powder compaction and sintering methods have been studied. The effects of various lubricants, pressing, and sintering conditions on the behaviour of Al–Cu and Al–Cu–Si alloys were evaluated systematically. The role of copper and silicon additions during compaction and sintering and their advantages or disadvantages are discussed. All alloys underwent large dimensional changes (sudden swelling followed by rapid contraction) during sintering at temperatures greater than Al–Cu eutectic temperature and it is suggested that a process of particle rearrangement is largely responsible for this behaviour. The mechanical properties of the alloys were highly dependent on the sintering temperature. PM/0215     

烧结双相不锈钢的相结构和强韧性

研究了粉末烧结双相不锈钢及其时效后的相结构和强韧性。结果表明,在316L粉末中添加4%(wt)St,用粉末烧结法可以获得奥氏体加18%(Vol)铁素体的双相不锈钢。该双相不锈钢在800℃时效时,沿奥氏体和铁素体界面析出条状σ相,使双相不锈钢的硬度增加,塑性下降。     

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.     

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.     

Some Factors Influencing the Sintering Behaviour of Austenitic Stainless Steel Powders

Abstract

Densification studies of sintered stainless steel powder blends have shown that the introduction of delta-ferrite into the micro structure increases substantially the sintering rate of the compact. Model experiments, in which spherical particles of a 316 powder were sintered on several different base plates including iron, 18Cr–12Ni stainless steel, iron–12% nickel, have been carried out to provide more detailed information. Quantitative observations have been made on the rate of neck growth and on the compositional gradients which develop between particle and plate. It has been found that the sintering rates were very sensitive to the composition of the base plate and to the presence of austenite/ferrite interfaces. Micro-examination and the use of electron probe microanalysis indicate that interdiffusion of elements between particles and plates is by both surface and volume diffusion.     

Microstructural changes in Mo steels during sintering and effect on electrical conductivity

Abstract

An electrical conductivity measurement method was used for studying the sintering mechanism and microstructural changes of low alloyed PM Mo steels in a temperature range between 600-1300°C. The influences of alloying method (elemental or prealloyed), Mo content (1·5 and 3·5 wt-%), and sintering temperature were investigated. The results show that the effects of, for example, formation of Mo carbide(s), ferrite-austenite phase transformation, as well as liquid phase formation during heating of the steel compacts can be detected by the technique cited. Mo dissolution during sintering of compacts from mixed powders results in a decrease of the conductivity with increasing sintering temperature while compacts from Fe-Mo prealloyed powders exhibit the standard behaviour of higher conductivity after sintering at higher temperature. Moreover, the relationship between Mo dissolution, formation of sintered contacts, and mechanical properties was demonstrated to assess the viability of the conductivity measurement method for studying the sintering behaviour of PM materials and its influence on physical and mechanical properties. An approach was also demonstrated for relating the conductivity to the microstructural parameters, e.g. total porosity and contiguity between solid phase, that would be useful for predicting relative changes in mechanical properties dependent on porosity and pore morphology.     

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.     

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.     

Diary

Abstract

The addition of amorphous Fe–Si–B particles to Fe powder increases the shrinkage of sintered components resulting in higher densification rates. Consequently, several research groups worldwide have studied the properties of such systems in an attempt to produce superior structural alloys. In the present work, Fe₇₅Si₁₀B₁₅ ribbons obtained by melt spinning were milled in a high energy Spex mill for times varying from 2 to 32 h. The resulting powders were characterised by differential thermal analysis and X-ray diffraction. The results showed that the amorphous characteristics of the ribbons persisted after the milling process. Next, samples consisting of a mixture of Fe powder and 4 wt-% milled amorphous phase were uniaxially pressed and sintered following a series of thermal cycles. High temperature microstructures were obtained for compacts subjected to rapid cooling from the sintering temperature. The results of scanning electron microscopy and energy dispersive spectroscopy revealed substantial precipitation of fine Fe₂B particles before α → γ allotropic transformation. In addition, an oxide phase was observed in the interface between Fe and the additive particles. Preliminary analysis suggested that the oxide particles can be easily reduced by adding small amounts of carbon to the system. PM/0765     

On phosphorus as additive in iron based soft PM magnets

Abstract

The effect of a phosphorus addition to iron powder for production of soft magnetic materials is investigated. The phase diagrams calculated by ThermoCalc¹ and experiments show the beneficial effect of the phosphorus addition especially when sintering in a carburising atmosphere. The carbon uptake is greatly reduced and is explained by the presence of ferrite in the material during sintering. The phosphorus addition also reduces the magnetic aging both in samples sintered in a carbon containing atmosphere but also in a hydrogen atmosphere. No magnetic aging from precipitation of iron phosphide is seen.     

Effect of MnS powder additions and vibratory ball peening on corrosion resistance of sintered 316LSC stainless steels

Abstract

As sintered parts are to be machined after sintering, the MnS powder is usually added to improve the machinability. Vibratory ball peening is used for deburring and improving the surface finish of sintered components in local PM industries. The effect of the MnS powder content and vibratory ball peening on the corrosion resistance of the sintered 316LSC alloys was investigated. Experimental results show that the addition of MnS powder slightly decreases the sintered density. The weight loss rate of the sintered specimens immersed in the 10%FeCl₃ corrosion test solution increases slightly with increas- ing MnS content, but decreases with increasing sintering temperature. Vibratory ball peening effec- tively decreases the weight loss rate of the sintered stainless steels. The chromium atoms actively migrate across the phase boundary and diffuse into the MnS particles during sintering. This intensive chromium diffusion affects the corrosion performance of the sintered alloys with MnS added. The surface morphology of the as sintered and the ball peened specimens before and after the corrosion test were studied with a SEM.     

The founding of EPMA

Abstract

Liquid phase sintering is commonly used in powder metallurgy to improve physical properties through densification enhancement. With the aim of combining the advantages of liquid phase sintering and the use of promising alloying elements such as Mn and Si, liquid promoters with complex compositions were designed to provide a low melting point to form a liquid phase below the common sintering temperatures. The properties of these liquid phases were characterised in terms of contact angle, spreading evolution and infiltration. Using a Krüss drop shape analysis system, both wetting angle experiments and infiltration experiments were performed by changing the substrate characteristics from sintered to green iron specimens respectively. The discussion is based on the different features found for these liquids compared with copper, which is a well known liquid phase former used for improving the properties of low alloy steels. Simulations of the thermodynamic and kinetic processes taking place were performed by combining ThermoCalc and DICTRA software analysis.     

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.     

Microstructure evolution and densification behaviour of powder metallurgy Al–Cu–Mg–Si alloy

ABSTRACT

An Al–Cu–Mg–Si alloy was prepared by conventional press-sintering powder metallurgy using elemental Al powder. The phase transformation process of Al–Mg, Al–Si alloy and Cu during the sintering process was investigated in details. It was found that a series of phase transitions take place in the alloy to disrupt the oxide film of Al particle and enhance the densification process. The relative density of the sintered samples reached 98%. A new Al–Mg–Cu–O compound was found at the grain boundaries except the MgAl₂O₄ phase, it is speculated that the disruption of the oxide film was also associated with the other alloy compositions except for Mg. Furthermore, no detectable AlN compound was found at the grain boundary region although sintering with flowing nitrogen atmosphere, which is benefit from the high density of the green compact and the excellent wettability between the liquid phase and the aluminium.     

Effect of nickel boride and boron additions on sintering characteristics of injection moulded 316L powder using water soluble binder system

Abstract

A new metal injection moulding system for 316L stainless steels has successfully been derived and tested. A mixture of small water atomised powder (average size 15 µm), larger gas atomised powder (average size 75 µm), and sintering additives has been coupled with a new water soluble binder system for economical powder injection moulding. The details for each process step and the effect of sintering additives are described. The binder system consists of poly (2-ethyl-2-oxazoline) as the leachable polymer, polyethylene as the backbone, and stearic acid as a surfactant and plasticiser. This binder system provides satisfactory mixture stability, excellent mouldability, and reasonably fast water leaching and thermal debinding rates. The optimum powder/binder compositions were determined using torque and capillary rheometry. Densification was by persistent liquid phase sintering through additives, such as nickel boride and boron. This 316L powder system was sintered to 7·9 g cm ^-3 (98·75% of theoretical) at 1285°C using nickel boride addition and at 1245°using boron addition. nickel boride additions are particularly effective at increasing the tensile strength and ductility. In contrast, the boron additions only increase the tensile strength and decrease ductility. Based on microstructure evaluations, this effect is traced to a continuous boride phase on the grain boundaries of the boron doped samples and a discontinuous boride phase on the grain boundaries of the nickel boride samples.     

Nitrogen Ceramics: Liquid Phase Sintering

Abstract

The role of a minor silicate eutectic liquid phase as a transport medium in sintering hot–pressed silicon nitride (β Si₃N₄) ceramics was identified in the 1970s. A similar mechanism is applicable to hot–pressed Si–Al–O–N ceramic alloys which offer an advantage in control of the final liquid volume and hence in superior high temperature mechanical properties. By increasing the liquid volume it is possible to densify ceramic alloys without application of pressure at the sintering temperature and hence to fabricate components of complex shape. The Lucas Syalon ceramics typify the new range of pressureless–sintered ceramics based on the β Si₃N₄ structure. They are fabricated from the ultrafine compound powders α Si₃N₄, SiO₂, Al₂O₃, Y₂O₃, and a polytypoid phase (a substitute for A1N). The ceramics consist of submicrometre solid solution crystals of general composition Si_3?xAl_xO_xN_4?x(x < 1) within a minor matrix phase which may be either a glassy Y–Si–Al oxynitride or be crystallized to form yttrogarnet. Analysis of matrix glass compositions shows them to be residues of liquids near to a ternary eutectic in the Y₂O₃–SiO₂–Al₂O₃ system which is well below the sintering temperature of ～ 1800°C. Sintering models, based on particle rearrangement due to dissolution of the major α Si₃N₄ component in the eutectic liquid and its reprecipitation as a β Si₃N₄ solid solution, are discussed. Properties and current applications of Syalon ceramics are surveyed briefly. PM/0266     

Influencing mechanism of Al2O3 on sintered liquid phase of iron ore fines based on thermal and kinetic analysis

The sintering behaviour of ore fines under different Al₂O₃ content and different alkalinity was studied by DSC–TG method. The reaction kinetics of the sintering process was calculated. Simultaneously, the experimental products were analysed by microscopic analysis. The effects of different content of Al₂O₃ and alkalinity on the sintered liquid phase were investigated. The results show that the increase of Al₂O₃’s content promoted the formation of calcium ferrite and the increase of liquid phase's formation in a certain range. When the content of Al₂O₃ is 2.5%, that is, A/S?=?0.42, the heat flux is the largest, correspondingly, the maximum amount of liquid phase is formed under this condition. The liquid phase's formation during the reaction under high alkalinity conditions is greater than that under low alkalinity conditions. The influencing mechanism of Al₂O₃ and alkalinity on the liquid phase of sintering is verified by the kinetic analysis.     

Microstructural development during liquid phase sintering of Fe and Fe–Mo alloys containing elemental boron additions

Abstract

The sintering behaviour of Fe and Fe–Mo prealloyed powder compacts containing from 0·5 to 3·5 wt-%Mo and fixed boron additions has been studied with special emphasis on the microstructural development, the formation of the liquid phase and the liquid phase sintering mechanisms involved during the densification process. The basic phenomena involving the formation of a liquid phase and the temperature at which the liquid is generated is strongly influenced by the Mo/B ratio in the initial powder mixture. The effect produced by Mo and its concentration, both, on the final microstructure and on the behaviour of boron prior to, during and after the formation of the liquid phase, was studied under both the optical and the scanning electron microscope. For this purpose interrupted sintering experiments followed by water quenching from specific temperatures and times within the sintering cycle have been carried out. The study shows that the formation of a liquid phase is preceded by noticeable enhancement of solid state sintering at intermediate temperatures. This is accompanied by boron diffusion into the metallic particles, generating inter- and intragranular precipitates in amounts dependent on the Mo concentration. At a later stage boron is found to be preferentially located at the boundaries as the formation of a continuous Fe/Mo/B liquid phase with excellent wetting characteristics proceeds thus producing densification by pore filling and shape accommodation. Final densities up to 7·82 g cm^?3 were obtained for these alloys.     

Precipitation in CF-8M duplex stainless steel welds

Welds of CF-8M, a cast 316-type stainless steel which normally solidifies as primary delta-ferrite, were induced to solidify as primary austenite by the addition of nitrogen to the shielding gas used during gas tungsten arc welding. Those welds which experienced a shift in solidification mode formed eutectic ferrite during the terminal transient stage of solidification. Primary delta-ferrite and eutectic ferrite are differentiated by their location in the dendritic microstructure. The shape of the ferrite/austenite interface tends to be rounded for primary delta-ferrite and more angular for eutectic ferrite. Elemental profiles were plotted from STEM/EDS measurements across the two types of ferrite, and showed differences between the composition of the austenite immediately adjacent to the primary delta-ferrite, as opposed to the eutectic ferrite. In addition, while the primary delta-ferrite/austenite interfaces are largely devoid of precipitation, the eutectic ferrite/austenite interfaces are densely covered with small precipitates ofx-phase. The mean stoichiometry of this phase has been calculated from STEM/EDS data on extraction replicas, and approximates Fe₅₀Cr₃₂Mo₁₃Ni₅. Intragranular inclusions were also examined and found to be complex, with most of them containing varying quantities of Mn, Si, and S.