Influence of die wall lubrication on tensile properties of high temperature sintered and sinterhardened low alloy steel

ABSTRACT

The influence of die wall lubrication on the green strength and the tensile properties of a 1.5%Mo and 0.5%C steel, both as sintered and sinterhardened at 1250°C was investigated.

Samples compacted using die wall lubrication had green strengths more than 100% higher than samples compacted at the same pressure using bulk lubrication, and a sintered density up to 98% of the pore-free ones may be achieved, compared to a maximum of 95% with bulk lubrication.

Pore morphology and the matrix microstructure and microhardness were not affected.

The tensile properties of both sintered and sinterhardened materials were much better for die wall lubrication than for bulk lubrication. Tensile strength increased up to 20%, tensile elongation up to 60%. The effectiveness of the system that delivers the lubricant on the die wall surface every stroke was verified successfully in the production of tensile specimens that do not have an axisymmetric geometry.     

Tribological property of in situ directly nitrided and sintered Al/AlN composite

Abstract

An in situ directly nitrided and sintered Al/AlN composite was developed using a chemical reaction between aluminium and nitrogen gas at temperatures below 823 K. It is clear from SEM and TEM observation that the composite has good bonding of the in situ formed AlN to the aluminium matrix. The composite shows an unusually low friction coefficient (0·004–0·008) under oil lubrication, and also has excellent wear resistance and antiseizure properties when compared with hard anodised aluminium alloys. AlN particles protruding from the matrix create oil microgrooves and which work as oil pits to enhance the formability of oil films. Therefore, it is easy to form hydrodynamic lubrication with continuous oil films on a sliding surface.     

FACTORS CONTROLLING THE SHAPE REGULARITY OF SINTERED U02 PELLETS

Abstract

As explained mathematically, the magnitude of the ‘diabolo’ effect in sintered nuClear-fuel pellets is a function of the slope and position of the sinterability curve and the density gradient existing in the green pellets. If sinterability is representedby a straight line passing through the origin, the diabolo effect is negligible; this condition can be fulfilled when using a deactivated powder.

In any other case, the density gradient must be kept as low as possible to produce sintered pellets of regular shape. This is discussed as a function of powder roughness, lubricant quality, lubrication technique, and compaction characteristics.

When the sinterability curve cannot be adapted and powder quality and lubrication conditions are fixed, the possibility still exists of minimizing irregularities in sintered diameter by using a single-action press and compacting the powder in an adapted conical die with a moving lower punch.     

DETERMINATION OF HOOP STRESSES INDUCED IN A CYLINDRICAL STEEL DIE BY COMPACTING METAL POWDERS

Abstract

Because of the lack of stress data, steel dies for compacting metal powders were designed in the past on the assumption of hydraulic pressure transmission by powders.

True die stresses existing in a hardened steel die during the compacting of various lubricated and non-lubricated powders have been measured by mounting strain-gauges on the periphery of the die at numerous points along its length.

Values of the hoop stress varied from approximately 0·05 to 0·40 of the calculated and measured hydraulic fluid stress. For a particular metal powder, the stress was a function of the average particle size, the powder height, and the amount of lubrication.     

Mould lubrication and friction behaviour with hydraulic oscillators in slab continuous casting

Abstract

It is very important to obtain reliable lubrication from casting powder both at the meniscus and in the gap between strand and mould as it affects slab surface quality and caster productivity. With knowledge of mould friction, a quantitative insight into the behaviour of powder during caster operation is possible. In the present research, the friction was studied based on a slab continuous caster equipped with hydraulic oscillators. The effects of mould oscillation and the abrupt change of casting speed on mould friction force were evaluated, and the characteristics of lubrication behaviour in a casting sequence were investigated. In particular, a comparison between the mould friction force between sinusoidal oscillation mode and non-sinusoidal oscillation mode was made. Finally, the characteristics of friction before a breakout are discussed. The experimental and analytical results may contribute to the development of mould friction online measurement and more clearly learn the lubrication behaviour in different conditions.     

THE EFFECT OF LUBRICATION ON THE PRESSING OF METAL POWDER COMPACTS

Abstract

Admixture of lubricant with metal powder can assist or retard densification according to the applied pressure and the lubricant content. Lubricant exuded on to the die wall under pressure seems to be the main lubricating factor. The change-over or transition pressure from lubrication to inhibition varies in a Gaussian manner with lubricant content. Molecular films of lubricant are sufficient, but for reduction in ejection pressure a minimum content of 0.2% lubricant is necessary. Die-wall lubrication is far more useful and effective than admixed lubricant. Simultaneous die lubrication and admixture is of no value.     

Break temperatures of mould fluxes and their relevance to continuous casting

Abstract

The break temperatures of mould fluxes are important since they help to control the horizontal heat transfer and lubrication between the steel shell and the mould, and consequently affect occurrences of longitudinal cracking and sticker breakout in continuous casting. Break temperatures T _br have been determined for both steady state and dynamic measurement of the viscosity, and equations relating T _br to chemical composition have been obtained for both cases. It has been found that T _br can be affected by (i) cooling rates and (ii) fluorine losses during the measurements.     

Relationship between physical structure and machinability of green compacts

Abstract

The dependence of green machinability on compact density and strength was investigated for room temperature and warm compacted steel powder compacts containing two different types of lubricant. Brazilian disc compression tests were employed to determine green strength, while machinability was assessed in terms of response to drilling.

For the room temperature compacted materials, it was found that high compact densities and strength were not, in most cases, associated with improvements in machinability. Furthermore, it was shown that lubrication (both type and quantity) and compaction pressure plays a critical role in determining the level of breakouts observed. In contrast, the use of warm compaction, in conjunction with specially designed lubricants, has been shown to be a suitable method of producing high density, high strength compacts while retaining good green machining characteristics. Mechanisms responsible for the observed behaviours of both the room temperature and warm compacted specimens have been forwarded in the present paper.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 2 Quality issues

Abstract

With many billet producers adopting mould powder lubrication, there is a need to clarify the gains in quality that can be achieved with this practice. Over the past three decades considerable research has been conducted to establish the relationship between mould behaviour and defect formation for billets continuously cast with oil lubrication, but little has been done to compare oil cast billets with powder cast billets. In this study, conducted at a Canadian minimill, four faces of a copper mould were instrumented with thermocouples and mould temperatures and billet quality were monitored with mould powder lubrication during casting of 208 × 208 mm billets. In the first part of this two part series (in Ironmaking & Steelmaking No. 1 2000), the results of the mould heat transfer analysis and the influence of variables were presented, together with a comparison between oil and powder lubrication. In the present paper, Part 2, billet quality is examined in detail. The difference in turbulence at the meniscus between oil and powder lubrication is established, and the need to tune mould level sensors when switching to mould powders is demonstrated. Previous work has shown that mould level fluctuations have a strong influence on defects such as offsquareness and transverse depressions, both of which are markedly reduced when casting with mould powders. The inherent stability of the meniscus is improved when employing mould powder lubrication and a submerged entry nozzle. Furthermore, the significant reduction in mould heat transfer at the meniscus, when mould powders are employed, particularly for medium carbon steels has been shown to correlate well with the observed reduction in offsquareness. The paper also elucidates the reasons for the reduction, and in most cases, elimination of transverse depressions in B–Ti grades when casting with mould powders. The mechanism of longitudinal depression formation and subsurface cracking observed in many of the powder cast, medium carbon billets has also been established.     

Simulation of heat transfer and strain behaviour in bloom casting mould

Abstract

An optimum casting model was developed to simulate the effect of mould flux on bloom heat transfer and strain behaviour based on a 3D MiLE method, and the influence of casting speed and superheat on bloom heat transfer and lubrication were also investigated. The simulation results showed that solidified shell thickness growth conforms to a Square Root Law, and that the model predicted results are basically in agreement with previous data in the literature, and provide confidence in model. The bloom temperature distribution range in the corner area is smaller than that in the mid-face, and the corner regions form a high cracking risk zone. The hot tearing indicator and effective stress in the corner area are significantly greater than that in the mid-face, so the corner area is the dangerous zone of cracking; The mould flux lubrication in the bloom mid-face is better than for the bloom corner region, due to a higher shell temperature and a fluid slag; The increasing of casting speed can delay air gap formation of the bloom corner area, improving the lubrication conditions, but when the casting speed is changed, it is also necessary for the mould flux viscosity and crystallization temperature be changed also. Increasing the superheat has little influence on the completely solidified distance of liquid flux in the bloom corner area.     

Optimization of Mold Oscillation for High Speed Casting — New Criteria for Mold Oscillation

Abstract

Consistent and uniform lubrication in the mold during casting is an issue especially with high speed casting. However, criteria of mold oscillation related to lubrication, has not been clarified for critical conditions such as thin slab casting. This paper discusses the mechanism of powder penetration into the meniscus, in terms of negative strip area ratio and positive strip time, including the case of non-sinusoidal oscillation which is effective for better mold lubrication. In this context, a lubrication index is proposed as a new criteria which can be used to estimate mold powder consumption rate. © 2000 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.

Résumé

La lubrification consistante et uniforme du moule lors de la coulée est un sujet important, spécialement pour la coulée à haute vitesse. Cependant, les critéres d'oscillation du moule reliés à la lubrification n'ont pas été clarifiés pour des conditions critiques telles que la oulée de brame mince. Ce document discute du mécanisme de pénétration de poudre dans le ménisque, en termes de rapport négatif de surface d'écoulement et de durée positive d'écoulement, incluant le cas d'oscillation non sinusoïdale, plus efficace pour la lubrification du moule. Dans ce contexte, on propose un index de lubrification comme nouveau critére pouvant être utilisé dans l'estimation du taux de consommation de poudre du moule. © 2000 Canadian Institute of Mining and Metallurgy. Published by Elsevier Science Ltd. All rights reserved.     

Effect of Zinc Stearate on Apparent Density,Mixing, and Compaction/Ejection of Iron Powder Compacts

Abstract

The behaviour of zinc stéarate (0–4 wt-%) during mixing with iron powder has been studied by monitoring changes of apparent density and comparison with powder prepared by depositing zinc stéarate from solution onto the iron particles. Apparent density was increased by the development of thin surface films at low lubricant contents but was decreased by the formation of thick layers and free lubricant at high contents. Pressure/density data were obtained from floating-die compaction of cylindrical iron powder compacts containing deposited zinc stéarate with and without die-wall lubrication to determine the relative significance of interparticle friction, particle/die-wall friction, and compaction inhibition. An optimum lubricant addition minimized interparticle friction during compaction. This was greater than the optimum observed for loose packing owing to the more arduous conditions prevalent in compaction. Compaction pressure losses associated with interparticle friction were lower than those caused by die-wall friction. The latter was only minimized by lubricant additions which also caused undesirable compaction inhibition. The forces required to overcome particle/die-wall friction were similar for both compaction and ejection.     

Mould heat transfer and continuously cast billet quality with mould flux lubrication Part 1 Mould heat transfer

Abstract

With the drive to cast higher quality, many minimills are adopting mould powder as a lubricant for the continous casting of steel billets. Over the past three decades considerable experience has been accumulated on the relationship between mould behaviour and billet quality for oil lubrication, but comparatively few studies have been conducted for mould powder lubrication. This study, conducted at a Canadian minimill, involved instrumenting four faces of a copper mould with thermocouples and monitoring mould temperatures during casting of 208 × 208 mm billets with mould flux lubrication. Billet samples were also taken to coincide with periods of measurements. Mould temperatures were monitored for two different mould powder compositions, for different mould oscillation frequencies, two mould cooling water velocities, and a range of steel compositions. An inverse heat conduction model was developed to calculate mould heat transfer from the measured temperatures. In this paper, which is the first part of a two part series, details of the inverse heat conduction model and mould heat transfer data are presented. The results obtained for mould flux lubrication have been compared with those for mould heat transfer for oil lubrication. For peritectic steels, with carbon content in the range 0·12–0·14%, it was found that lubricant type has little influence on the measured mould heat flux distribution at the centreline of a face. The peak mould heat flux was found to be approximately 2500 kW m^-2 . In contrast, for medium carbon steels, mould heat transfer with mould powder was significantly lower than when oil was employed as a lubricant. For instance, at the meniscus, the peak heat flux with mould powder was approximately 2500 kW m^-2 , which was half that recorded with oil as a lubricant. The influence of oscillation frequency, mould cooling water velocity, and mould powder type on mould heat flux has also been presented.     

IMPORTANCE OF STEARATES IN THE FABRICATION OF UO2 AND (U,Pu)O2 PELLETS

Abstract

Eighteen qualities of metallic soaps have been studied, either as lubricants or as additives for the fabrication of UO₂ and (U,Pu)O₂ pellets. Compounds of the same family, such as zinc stearates, differ in their effectiveness as lubricants. This property is strongly dependent on the particle size of the powder and its purity. The behaviour of the dry lubricants when used as admixtures in UO₂ and also when employed for the direct lubrication of the pelletizing tools is discussed.

The efficiency of metallic soaps as additives to lower the density of the sintered pellets is related to the nature of the metallic ion. Thus, zinc stearate has little influence on the fired density of UO₂, whereas barium stearate and other barium compounds are very effective. Barium inhibits sintering, but zinc, which is eliminated at low temperature, does not diminish the sinterability of the oxide powders. In the latter case, the decrease in density is caused by thermal decomposition of the organic molecule.     

Experimental research on effect of control model on lubrication for slab continuous casting mould

Abstract

In casting the control model relation between casting speed and frequency plays an important role in the optimisation of process parameters and the improvement of lubrication condition. The mould friction between mould and strand can directly describe the powder lubrication situation in a mould. In the present research, based on the slab continuous caster equipped with hydraulic oscillators, the transient friction forces were obtained by the differences between the monitored driving forces of casting and the calculated driving forces of no casting at same oscillation parameters. According to the trends of friction force with different control models, the characteristics and disadvantages of the traditional positive control model, and the improved positive control model and the inverse control model are discussed. This experimental study contributes to studying the frictional behaviour in mould under different control models, as well as selecting reasonable control model for high speed continuous casing.     

ASPECTS OF THE VOLUME PRODUCTION OF GREEN POROUS BEARING COMPACTS

Abstract

The history and method of manufacture of self-lubricating porous metal bearings are briefly outlined. The importance of achieving appropriate quality at each stage of manufacture, and particularly during compacting, is emphasized and the qualities desired in a green bearing compact are detailed. Various factors relating to the economic large-scale manufacture of green bearing compacts are then considered. The virtues of elemental powders and the influence of lubrication upon powder mix properties and pressing and ejection of compacts are discussed. The special powder flow problems associated with long, thin-walled bearings and the advantage, in this respect, of a moving core rod are described. The particular difficulties encountered in compacting such bearings are stated and special consideration is given to the ejection problems involved. It is concluded that limitations of bearing length and wall section must be accepted.     

Effect of Ni addition and cryogenic hardening on the mechanical and tribological properties of self-lubricating steels produced by MIM

ABSTRACT

This paper addresses the relation between mechanical properties and tribological behaviour of self-lubricating steels produced by in-situ dissociation of SiC. Literature shows that an increase in the mechanical strength of these materials lowers their friction coefficient and wear rates. These works have studied steels with mechanical strength up to 800?MPa, but it is unclear if this trend continues with further increments of mechanical strengths. To tackle this question, self-lubricating steels with Ni and Mo were sintered, half of the samples with 8 wt-% Ni were treated cryogenically hardened. Results show that this trend does not escalate with mechanical strength, furthermore, when ductility is low, graphite reservoirs are not easily accessed, and lubrication is incomplete. Also, plastic deformation allows to better distribute the load of the counter body at the surface, which reduces the wear rates of the specimens and the counter bodies.     

Rationale on composition of slags in oxygen steelmaking processes

Abstract

A study of plant analytical data on slags for the BOS and OBM processes revealed consistent interrelations between the concentrations of FeO, CaO, and SiO₂ in the slag at the end of the oxygen blow. Over a wide composition range, the amounts of CaO and SiO₂ decrease with increasing FeO, slag basicity increases with increasing FeO, and the amount of MgO decreases with increasing slag basicity. At all levels of lime and silica contents, the amount of FeO in the slag varies by 5–6% between the low and high contents. Consequently, the slag/ metal distribution ratios (%P)/[%P] and (%S)/[%S] as functions of basic and/or acidic oxide contents are within a composition range bordered by the equilibrium curves for the low and high FeO contents.     

Transmission electron microscopy sample preparation technique for sintered alloys

Abstract

Powder metallurgy alloys are typically inhomogeneous with a significant amount of porosity. This complicates conventional transmission electron microscopy sample preparation. However, the use of focused ion beam milling allows site specific transmission electron microscopy samples to be prepared in a short amount of time. This paper presents a method that can be used to produce transmission electron microscopy samples from an Al-Cu-Mg PM alloy.     

Thin slab casting with liquid core reduction

Abstract

A liquid core reduction method has been developed for the flexible production of slabs of extensive thickness. In this method, achievement of the target slab thickness and the prevention of internal cracking are of prime importance. Using an experimental continuous caster, a series of tests of thin slab casting with liquid core reduction was conducted. The caster had three segments (1–3) for reduction at exit from the bending unit. The slabs were reduced in each segment with tapered alignment of the rollers over a 1 m length. The amount of reduction in each of the segments was changed in several patterns. The slabs were able to be reduced by 40–25 mm from 100 or 90 mm thickness without internal cracking. The variation in the amount of reduction in each segment was assessed by solidification analysis and strain analysis during liquid core reduction. In reduction along a tapered alignment, the incremental strain produced by the reduction can be comparatively low. The optimum reduction pattern to obtain the target slab thickness and to minimise tensile strain caused by the reduction has been established.