COMPACTING AND SINTERING STEEL POWDER

Abstract: The invention concerns a method of getting shaped elements by pressing, sintering and cold-compacting of powder mixtures of steel with alloy additions, being used for production of machine parts, e.g. gears, bearings and others of high surface pressure strength and wear resistance. The method is characterized in that a preformed semi-final product is treated with presintering, preferably in dissociated-ammonia atmosphere at temperature 700 - 750 DEG C, preferably 720 - 730 DEG C during 20 - 40 min, preferably 30 min, and first compacting under pressure 700 - 800 MPa, preferably 750 MPa, and then second compacting with simultaneous calibration under pressure 900 - 1000 MPa, preferably 950 MPa, then is treated with final sintering at temperature 1100 - 1200 DEG C, preferably 1120 - 1150 DEG C, during 40 - 50 min, preferably 45 min.

Description: C OM P A C TI N G A ND SINTERING STEEL POW DER The invention concerns a method of getting shaped elements by pressing, sintering and cold-compacting of powder mixtures of steel with alloy additions, being used for production of machine parts, e.g. gears, bearings and others of high wear resistance and surface pressures. In well-known solutions, constructional parts from powder mixtures with alloy additions are made by pressing and sintering at temperature 1 100 - 1 150 °C in a protective atmosphere. Products obtained by this method are characterized by high porosity up to 12% of volume and low compression strength. In other method, a product from powder alloy steel is twice pressed and twice sintered at temperatures 800 °C and 1 150 °C. In this solution, obtained products also do not get required high strength parameters. There is also known a method of getting parts from powder mixtures of constructional steel with alloy additions, consisting in pressing, presintering at temperature 800 - 850 °C, strain of presintered semi-finished product and final sintering at temperature 1 120 - 1 180 °C in a protective atmosphere. Parts obtained by this method have insufficient strength parameters, in particular low bending strength. Moreover there is known a method of getting parts from metal powders with alloy additions, where pressing, presintering at temperature 650 °C, second pressing and second sintering at temperature 1150 °C are carried out. Parts obtained by this method are characterized by low strength parameters, in particular low compressibility of ring products at radial action of a force. Presented above methods of getting products of steel powder mixtures with alloy additions do not precise pressure values that are to act onto shaped part, as well as they do not specify exact time during which sintering should be performed, giving a very wide temperature range. Compacting of powders in a closed pressing die under pressure is performed as a result of mutual shift and plastic strain of material particles. Their mutual wedging at some stage stops the mutual shift of particles. Further increasing of product densities under influence of press pressure force can take place thanks to deformation of particular particles. Intensive cold work of being pressed particles and further increasing pressing force are used to overcome outside friction forces at contact: powder material and a pressing tool. The result is to get high porosity after single pressing within limits from 15% to 20%. Further increasing of product density can be obtained thanks to increasing of compact plasticity, which occurs during presintering at temperatures equal to annealing temperature of common metal. Process of presintering at annealing temperature increases powder material plasticity making possible to increase product density through its plastic strain. Plastic forming of a presintered semi-final product, like for solid material, follows forming and shift of dislocation through crystal lattice along definite surfaces and dislocation forces. Plasticity of a sintered product, particularly density of a final product increases if, during first sintering, there is elimination of forming of factors inhibiting motion of dislocation shift, such as forming: a hard solution, fine-grained particles, secondary phases, etc. That is why the first sintering should be conducted in conditions of recrystallization annealing at temperature A c ι. Structure of die stampings from constructional steel powders is a mixture of separate components (if a matrix is iron powder with graphite additions) or an alloy ferrite (if a matrix is iron powder with alloy additions). Optimum temperature of presintering should be from 720 °C to 730 °C. At this temperature, solubility of carbon in α-iron is about 0.03%, and after sintering in state of recrystallization annealing of a product, is characterized by low hardness coefficients as a result of lack of such consolidation factors as secondary phases, particle dispersability, etc. It is well-known that diffusion motion of iron atoms in carbon steels in state before transformation (in constructional steels, with admixture of Cr, Mo, Ni, Cu) at 720 °C - 730 °C is approximately of the same value as in a compound of γ-iron at 1 100 °C - 1200 °C. Incorrect, diffusion mobility of iron atoms in the range of temperatures 720 °C - 730 °C leads, at material sintering, to formation of powder interaction focuses, the concentration of which is determined by sintering time. The powder interaction focus may be generally defined by positions of atoms belonging to both powder particles, separated with interparticle boundaries - contact points. The powder interaction focus, defined by above definitions, is convergent with „space contact lattice", and its growth takes place not only as a result of shift of „space contact lattice" along boundary surfaces but also by approaching parameters of „space contact lattice" to matrix crystal lattice. If a solution of carbon and alloy additions in iron at 720 °C - 730 °C does not take place, then parameters of „space contact lattice" can be changed in wide range by changing time of sintering at above temperatures. After determined time of sintering, interparticle contact points make a system of glide planes, which is used as a system of glide planes inside grains and particles at ambient (room) temperature. Gliding along interparticle connections takes place under outside presssure, what has an essential importance to the process of plastic strain. Similar model can be called a model for „super dislocation" because it is based on an effective gliding along particle boundaries as a result of particular shift kind of „space contact lattice". This leads to additional, significant increase of sintered semi-final product palsticity, because it is known that metal material plasticity increases significantly if an additional mechanism of plastic strain, besides gliding at dislocation shift, is introduced into a process of plastic metal flow. Taking into account above-mentioned conditions, a method being a subject of the invention has been developed. The essence of the invention, concerning a method of getting shaped elements by pressing, sintering and compacting of powder mixtures of steel with alloy additions, consists in that a preformed element is treated with presintering. preferably in dissociated-ammonia atmosphere at temperature 700 - 750 °C, preferably 720 - 730 °C, during 20 - 40 min, preferably 30 min, and first compacting under pressure 700 - 800 MPa, preferably 750 MPa, and then second compacting with simultaneous calibration under pressure 900 - 1000 MPa, preferably 950 MPa, then final sintering at temperature 1100 - 1200 °C, preferably 1 120 - 1 150 °C, during 40 - 50 min, preferably 45 min. Thanks to use of the method according to the invention, the following technical-and-operational effects were gained: - high wear, crushing, shear and compressive strength of got product - high yield point, - density of got product above 7.6 g/cm 3 , - low material flow coefficient, in the range 10 "2 - 10 "3 mm, - decrease of production power consumption, - possible use for products of optional shapes, optional application, especially for products of required high strength as gears, bearings, etc. Subject of the invention is shown in the following exemplary embodiments: Example I Preformed semi-final product from powder mixtures of steel with alloy additions of brass, nickel, copper, and others is treated with presintering in dissociated-ammonia atmosphere at temperature 725±5 °C during 30 min, and treated with compacting under pressure 750 MPa. Then it is treated with second compacting under pressure 950 MPa and calibration, then final sintering at temperature 1 130 °C during 45 min. Example II Preformed semi-final product from powder mixtures of steel with alloy additions of brass, nickel, copper, and others is treated with presintering in dissociated-ammonia atmosphere at temperature 740±5 °C during 25 min, and first compacting under pressure 780 MPa. So prepared element is treated with second compacting under pressure 980 MPa and calibration, then final sintering at temperature 1 180 °C during 40 min. Example III Preformed semi-final product from powder mixtures of steel with alloy additions of brass, nickel, copper, and others is treated with presintering in dissociated-ammonia atmosphere at temperature 710±5 °C during 35 min, and first compacting under pressure 710 MPa, then it is treated with second compacting under pressure 910 MPa and calibration, then final sintering at temperature 1 100 °C during 50 min. In the enclosed table, the results of strength tests of parts made from powder mixtures of steel with alloy additions, made with the method according to the invention at different temperatures of the first sintering compared with standard sample made with hitherto method.

Claims: Claim A method of getting shaped elements by sintering and pressing, of powder mixtures of steel with alloy additions, characterized in that a preformed semi-final product is treated with presintering, preferably in dissociated-ammonia atmosphere at temperature 700 - 750 °C, preferably 720 - 730 °C during 20 - 40 min, preferably 30 min, and first compacting under pressure 700 - 800 MPa, preferably 750 MPa, and then second compacting with simultaneous calibration under pressure 900 - 1000 MPa, preferably 950 MPa, then treated with final sintering at temperature 1 100 - 1200 °C, preferably 1120 - 1150 °C, during 40 - 50 min, preferably 45 min.

Inventors: KUCZMA JURIJ JAKOWLEWICZ, LESZCZYNNSKI WOLF MICHAJLOWICZ, SEWASTIANOW EWGIENIJ STEPANOWI, STOJANOW ALEKSANDR ANATOLEWICZ, WISNIEWSKA-WEINERT HANNA, KUCZMA, JURIJ, JAKOWLEWICZ, LESZCZYN&NACUTE, SEWASTIANOW, EWGIENIJ, STEPANOWICZ, STOJANOW, ALEKSANDR, ANATOLEWICZ, WI&SACUTE
Applicants: INST OBROBKI PLASTYCZNEJ, INSTYTUT OBROBKI PLASTYCZNEJ
Application year: 2000
Main classification: C22C33
IPC codes: B22F3, C22C33