POWDER MIXTURE FREE OF SEGREGATION

Abstract: Iron based powder mixtures, which except iron or steel powder and one or more alloying elements in powder form also contain an addition of up to 0.5 % of talloil to prevent segregation/dusting.

Description: TITLE OF INVENTION POWDER MIXTURE FREE OF SEGREGATION TECHNICAL FIELD The present invention relates to iron based powder mi with low risk of segregation and/or dusting. According to t invention it is now possible to produce mechanical mixes of iron or steel powders and alloying powders with low risk of segregation and dusting without deteriorating the character istic physical properties of the mixture. In powder metallurgical manufacturing of various type components iron or steel powders are often used together wi one or more alloying elements such as copper or nickel in o to reach mechanical properties which cannot be obtained whe using plain iron or steel powders. Nowadays powders for these purposes are in general pr pared in two ways, viz. either as powder mixtures or as ful prealloyed powders. Powder mixtures are prepared by mixing iron or steel powder with powder containing the desired all ing element or elements, either in the elementary form or as master alloys. The fully prealloyed steel powders are manuf ured e.g. by atomizing a steel melt containing the desired alloying elements to a powder. One of the drawbacks of powder mixtures is related to fact that such powders consist of particles which often diff considerably in size, shape and density, and which are not mechanically interconnected. This means that such a powder m ture is susceptible to segregation during its transport and handling. This segregation leads to varying composition of t green compacts manufactured from the powder, and thus to var dimensional changes during the sintering operation and to va mechanical properties in the as-sintered pτoduct. Another drawback of powder mixtures is their tendency dust especially if the alloying element is present in the fo of very small particles. This can lead to difficult environ- mental problems when the powder mixture is handled. In the case of fully prealloyed powders there is no ri of segregation as every powder particle has the same composi ion, also the risk of dusting is redu • having small particle size is included. However, the preallo powder has another great drawback, viz. its low compressibil which is a result of the solid solution hardening effect wh the alloying elements have on each powder particle. High co pressibility is essential when high density is a prerequisi for reaching high mechanical properties. The compressibility of a powder mixture is on the oth hand substantially the same as the compressibility of the i powder included therein. This fact together with the flexib lity as regards the alloying composition have made powder m tures the most commonly used raw material inthe.production low alloy sintered steels. In such powder mixtures the plai iron powder is used as a base powder. BACKGROUND ART The Swedish patent application No. 76122T7-5 describe method to produce an iron powder containing copper, which h a low risk of segregation and dusting at the same time as t powder properties are maintained. According to this method the powder is produced by an annealing treatment of a mixtu of iron and copper powder, at which a so-called partially diffusion alloy between iron and copper is obtained. As certain alloying elements, such as e.g. phosphorus the form of a ferrophosphorus powder and carbon in the form graphite powder, cannot be sufficiently diffusion alloyed an iron or steel powder without deteriorating the compressi lity, there is a risk that mixtures in which these alloying elements are used are prone to segregation and/or dusting. The Swedish patent application No. 8001764-3 describe method to prevent segregation and/or dusting by adding to a powder mixture up to 1% of a sticky binder, which does not change its sticky properties with time at normal temperatur However, it has been found that when binding agents of this nature are added to a dry powder mix the amount, which can added, is controlled by the characteristic flowability of t powder mix. The amount which can be used with regard to the flow results in a decreased bonding effect between the iron particles and the particles of the alloying elements, why a optimal powder mix from segregation/dusting point of view cannot be obtained. DISCLOSURE OF INVENTION The aim of the present invention is therefore to provi powder mixtures on iron powder base, in which the risk of se gregation and dusting is very low at the same time as the physical powder characteristics are maintained. According to the invention this aim is fulfilled by ad ing during the mechanical mixing operation a binding agent b means of which the alloying particles are attached to the ir or steel powder particles. According to the invention it is proposed to use a bin ing agent with good wetting properties in liquid state, whic after the admixing is transformed into solid state when expo to the oxygen in the air. By using such a binder a dry powde mix with good flow properties can be obtained. Furthermore, the binding agent should have such proper ties that is can be burned off without any problems at a sui able temperature, e.g. during the sintering of the component made of the powder mixture. As the binder should be active in the powder mixture u after the compaction it is not allowed to affect the charact ristic physical powder properties of the mixture such as app rent density, flow, compressibility and green strength. To fulfil the above mentioned demands it is preferred add up to 0.5 , preferably 0.10 - 0.30 % , of the binding ag Here and in the following ■■ ." is referred to as percent by weight. According to the invention the binding agent that is p ferred is talloil. According to the invention an iron based powder is mix with one or more alloying elements, such as graphite of phos phorus, in powder form together with a lubricant for some mi tes in order to obtain some homogenization of the mixture. A total content of up to 0.5 , preferably 0.10 - 0.30 , of t binder is then added and the mixing operation is carried out for a period of time sufficient to obtain a homogeneous OMP IP mixture. If desired, a lubricant might be added during the ing operation to facilitate the pressing of the powder in a tool at the final use. EXAMPLES In the following the invention is exemplified and in connection therewith the experiments which have been made w powder according to the invention are described together wi the surprising results which the experiments have given. Example 1 A number of powder mixtures consisting of 98.2 % spon iron powder with a maximum particle size of 175 μ , 1.0 % g phite powder with a mean particle size of about 5.0 μm, 0.8 of zincstearate and different additions of polyethyleneglyc 400 and talloil up to 0.5 I respectively were prepared. The mixtures were analyzed with regard to both the bonding eff between the iron particles and the alloying particles and t flowability of the powder. The bonding effect was determine by blowing a certain amount of air through the powder mixtu and then determine the loss of graphite. The results obtain are shown in Fig. 1 (bonding effect) and Fig. 2 (flowabilit of the powder) . From the results it can be seen that when talloil is as a binder a more or less segregation-free powder mixture iron powder and graphite powder with maintained or improved flow properties can be manufactured. The amount added shoul be in the range of 0.10 - 0.30 % when iron powder with a particle size mainly below 175 μm is used. When polyethyleneglycol 400 is added to such type of powder the amount, which can be permitted to maintain the characteristic flowability of the powder mixture, is not bi enough to result in a completely satisfying binding effect segregation point of view. When working with powder mixtures, whose particle siz mainly below 175 μm, it is therefore not satisfying to use this type of binder. However, a binder according to the pre invention makes it possible to use iron-graphite-mixtures, which are free of segregation. Exa ple 2 Two powder mixtures 1 and 2 with a composition as show in the table below were prepared: Mix 1: 98.8 % iron powder with a particle size mainly below 147 μm, 1.2 % graphite with a particle size below 45 μm. Mix 2: 98.8 iron powder with a particle size mainly below 147 μm, 1.2 % graphite with a particle size below 45 μm, 0.1 %- talloil To both mixes 0.8 % zincstearate was admixed as lubricant. A fullscale " production test was carried out at a manu¬ facturer of sintered components, where 10,000 parts of each 1 and 2 were compacted and sintered under normal conditions. The component in question was included in the normal product run of the manufacturer who normally manufactured it from ma rial according to mixture 1. The compacts of the two mixes w sintered at the same time at 1115 C in a mesh belt furnace i endothermic atmosphere. After sintering a sufficient number of components from statistical point of view was sampled and the carbon content of these parts were measured. For mixture 1 carbon contents between 0.97 % and 1.11 % were obtained, while the same numb for mixture 2 were 1.07 % and 1.10 % , i.e. the carbon conten range for the material corresponding to mix 1 was 0.14 % and 0.03 ' for the material made of mix 2. These results are sho in Fig. 3. The above results clearly show that the variation in carbon content within a production series is substantilly le when the components have been manufactured from mixture 2 th when the components have been manufactured from mixture 1. OMPI

Claims: CLAIMS 1. Iron based powder mixture, characterized in that it besides iron or steel powder and one or more alloying ele ments in powder form also contains an addition of up to 0.5 preferably 0.10 - 0.30 % , of talloil to prevent segregation and/or dusting. 2. Iron based powder mixture according to claim 1, characterized in that the powder mixture contains up to 2.0 of graphite powder. 3. Iron b.ased powder mixture according to claim 1, characterized in that the mixture contains up to 1.5 % of phosphorus added in the form of a ferrophosphorus powder wit a particle size of maximum 44 μm and a phosphorus content of 14 - 18 .. 4. Iron based powder mixture according to claim 3, characterized in that the mixture contains up to 2.0 % of graphite powder. ΓURI OMP

Inventors: ENGSTROEM ULF FREDRIK IVAR, ENGSTROEM, ULF, FREDRIK, IVAR
Applicants: HOEGANAES AB
Application year: 1984
Main classification: B22F1
IPC codes: B22F1, B22F3, C22C33