WO2017200405A1

WO2017200405A1 - Method of manufacturing sintered elements having matrix of iron or iron-alloy

Info

Publication number: WO2017200405A1
Application number: PCT/PL2017/000049
Authority: WO
Inventors: Mateusz SKAŁOŃ; Marek NYKIEL; Jan KAZIOR
Original assignee: Politechnika Krakowska im. Tadeusza Kościuszki
Priority date: 2016-05-16
Filing date: 2017-05-15
Publication date: 2017-11-23
Also published as: WO2017200405A4; PL232783B1; PL417218A1

Abstract

A method of manufacturing sintered elements having matrix of iron or iron-alloy in which compacts formed from a homogenous mixture containing iron powder or iron- alloy powder, additives or master alloys, lubricants and additives inducing liquid- phase are heated in a protective atmosphere, sintered isothermally and cooled. The mixture intended for making the sintered elements contains 0.05 to 1.00% by weight boron in active form and at least one of nitrides, excluding boron nitride, selected from the nitrides having their melting points or decomposition temperatures higher than the sintering temperature, in particular Si, Cr, Ti, Al, Mo, Mn, V nitrides. The amounts of the nitrides and boron in the mixture are determined in such a manner that the molar ratio of nitrogen to boron atoms is between 0.05 and 2.15, preferably 0.1 to 1.4.

Description

TITLE: Method of manufacturing sintered elements having matrix of iron or iron-alloy

TECHNICAL FIELD:

The present invention relates to a method of manufacturing sintered elements having a matrix of iron or iron-alloy, including stainless steels, which enables to obtain elements characterized by low porosity, relatively high mechanical strength, including impact strength, and at the same time free from sintering defects, such as shape or dimension deformations.

BACKGROUND ART:

In industry, especially automotive and metal industries, there is a big and still growing interest in the use of sintered elements made of iron powders or iron-alloy powders.

In individual industrial applications such characteristics as shape and dimensional-accuracy, as well as high relative density of sinters are basic requirements for this type of elements.

It is commonly known, that low porosity of sintered elements increases their mechanical properties, which enables to reduce the weight of a particular element and thus the weight of a vehicle or a machine being made. Since pores are easy to penetrate for corrosive agents, decreasing of porosity results in increasing of corrosion resistance. The intent to decrease the porosity of sintered machine parts is of particular importance for mechanically loaded elements because the decreasing of porosity means the increasing of mechanical properties.

In commonly used industrial methods of the production of the sintered iron-based elements, the materials for manufacturing sinters are the mixtures of iron powders or iron-alloys powders, alloying additives or master alloys and lubricants, mainly waxes.

A typical, known for a number of years, industrial process for manufacturing sintered elements consists in compressing the powder mixture, i.e. iron powder or iron- alloy powder, alloying additives or master alloys and lubricants and then putting the resulting compacts (profiles) into a furnace in order to sinter them in a protective gaseous atmosphere. This known process is described in many references, for example in W. Schatt, KP Wieters, "Powder Metallurgy - Processing and Materials", EPMA 1997, ISBN 1 899072 05 5, which contains a full description of the powder metallurgy technology, and takes into account both materials used in the production and secondary processes, as well as devices for the production of sinters.

In the case of serial production continuous furnaces (belt, roller) are used for sintering the compacts of metal powder. The compacts are moved sequentially through a heating zone, in which the lubricants are eliminated (evaporated), an isothermal sintering zone and a cooling zone. If high purity of the protective atmosphere is required, chamber furnaces are used. The chamber furnaces are used for sintering, for instance, stainless steel green compacts. Detailed indications on the conditions of carrying out this process are contained in professional literature, including the monograph referred above.

According to well-known and commonly used industrial methods, in order to make a sinter having an iron matrix or an iron-alloy matrix and characterized by high dimensional accuracy and at the same time high relative density, iron-alloy powders or mixtures of elements are used. Said mixtures are pressed in such a manner that the obtained compacts have the highest possible density available under given conditions, which depends on the stock of pressing tools. However, since the sintering process takes place in the presence of a solid phase, the high density coefficient cannot be obtained, which means that the porosity of the obtained sintered element is only slightly lower than the porosity of the compact.

The above-presented, conventional method enables to obtain iron-based sinters or iron-alloys based sinters, typically having the porosity of 7-12%, which is insufficient for a number of industrial applications, especially when high strength of elements or low open porosity, in order to increase corrosion resistance, is required. If high relative density of the sinter is required, two alternative processes, in practice, are applied:

- pressing and sintering followed by re-pressing and re-sintering (double sintering process);

- sintering in the presence of a liquid phase.

The first of the aforementioned processes leads to obtain the sinters having high density, but it considerably raises the cost of production and also requires an extensive machine stock.

In the second of these processes, the presence of the liquid phase during the sintering process results, in a large number of cases, in deformation of the shape, which can be reduced by using two -stage sintering, but it significantly complicates the whole process and increases energy consumption.

It is understood for those skilled in the art that the principal task of said liquid phase is to condense the sintered material by giving occasion to re-arranging both powder particles and grains and to intensifying the transport of the matter by activating new diffusion paths, i.e., by liquid.

It is also well known from professional and patent literature that carbon, copper, phosphorus and boron are often used as the additives (sintering activators) to induce the formation of the liquid phase in the sintering process of iron powders and iron-alloys powders.

For example, the description of the patent CA 1041795 discloses the method of manufacturing sintered elements made of austenitic stainless steel and having relative density of at least 95%. The method consists in pressing and sintering of the powder whose composition corresponds to the composition of said steel, moreover, the powder contains 0.1 to 1% of boron as the additive inducing the liquid-phase formation during the sintering process.

The description of the US Patent No. 3,980,444 discloses that in order to reduce the porosity and to improve the strength of the sinter made of the stainless steel, 0.1 to 1% of boron is introduced into the powder to be sintered. The sintering process is carried out in the presence of the liquid phase induced by the addition of boron. This method enables to obtain the sintered elements having at least 95% relative density, however, it is recommended to double the pressing and the sintering to achieve required shape and required dimensional tolerances of the sintered elements.

In the publication of KS Narasimhan, FJ Semel "Sintering of powder premixes", Paper No. 2007-01-0145 Hoeganoes Corporation, (2007); (full text available at: https://www.reseai-chgate.net/publication/237732467_ SINTERING_ OF_ _POWDER_ PREMIXES_ -_ A- BRIEF_ OVERVIEW) the use of carbon, copper, phosphorus and boron as the additives which induce the formation of liquids in iron- alloy sinters and their effects on the properties of the obtained sinters has been described. The publication takes up the technological problems which occur during the process of sintering in the presence of the liquid phase. The publication states expressly the unsuitableness or limited usefulness of boron as the additive inducing the liquid-phase formation in the sintering process due to its low solubility in iron. It enables to obtain the sinters having the density of approximately 90%, but, however, the impact strength of 7 J/cm². The publication suggests the use as the sintering activators boron-free mixtures, such as Fe-Cu-C, Fe-Ni-C, Fe-P and Fe-Cu-P-C systems. In addition, the possibility of using boron in the form of known master alloys (e.g. FeB) was mentioned. The publication recommends double pressing and double sintering in order to guarantee required dimensional tolerances and strength properties.

In the monograph by J. Kazior, "Boron in sintered austenitic stainless steels", Cracow University of Technology, ed. Cracow University of Technology (2004) ISBN: 82-7242-305-9 - the principles of selection of the activators, such as boron, carbon, copper and phosphorus, for the sintering of steel and iron-alloys powders are described. The effectiveness of these activators, including elemental boron, was compared to the sintering process carried out in the presence of the liquid phase, as well as the effect of these activators on the properties of the sinters made of stainless steel- based powders. It has been pointed out that the amount of liquid-inducing activator introduced into the powder must be as low as possible in order to condense the profile and at the same time not to cause dimensional distortion. An excess of liquid-inducing additive is disadvantageous, because after exceeding a certain fraction of the volume of the liquid in the sinter, said sinter loses its shape. In addition, it has been pointed out that in the case of sintering in the continuous presence of the liquid phase, the liquid generally solidifies in the form of precipitates located on grain boundaries. The precipitates isolate neighboring grains from each other and reduce the contact surface of the grains, and as the consequence they decrease the mechanical properties of the sinter, mainly impact strength, despite the reduction of its porosity.

The requirements for low porosity, relatively high mechanical strength, high impact strength and dimensional and shape accuracy of the sintered elements, as well as objectively existing restrictions concerning metallurgy of iron powders and iron-alloy powders sintered in the presence of the liquid phase, significantly limit the choice of the sintering activators. In practice, such elements as phosphorus, carbon or copper are often chosen, but boron is rarely used as the sintering activator due to its lack of solubility in iron and iron-alloys. It precipitates at the grain boundaries in the form of continuous grid.

The boron precipitates, which surround grain boundaries, reduce significantly the mechanical properties of the sintered elements and the coagulation of the liquid causes dimensional distortions of the sinter, which disqualifies the sinter despite its very high relative density which can reach up to 100%.

The above mentioned disadvantages, associated with the use of boron as the sintering activator, can be eliminated by a selective-sintering laser process, as disclosed in the U.S. Patent Application 2009208361. However, this method is useless in commonly used industrial methods in which the compact, intended to be sintered throughout its volume in a single process, is introduced into a furnace.

Although this waste-free technology requires considerably lower pressures and significantly lower sintering temperatures in comparison with conventional technology, the production of sinters based on iron and iron-alloys with presence of the liquid phase is still limited due to the difficulties mentioned above and still unsolved technical problems.

The situation described above results, for the most part, from the liquid's tendency to deform the sintered profile because of its uneven distribution throughout the sinter's volume. Such deformations occur unpredictably. They are unrepeatable and they render the element useless, since the shape deviations can reach up to several millimeters in extreme cases (for relatively large elements). Overcoming the technological difficulties in the production of the sinters in the liquid-phase sintering process is primarily connected to the selection of suitable additives inducing the formation of the liquid, and furthermore to the accuracy in making the powder mixture, homogenization of the mixture, forming the compacts and the use of sinter furnaces equipped with precise temperature control systems.

It is obvious and understandable that the addition of liquid-inducing additives results in an increase in the price of the powder mixture, but this is to a large extent compensated by reducing the sintering temperature, which may be lower from about 100°C to 300°C in comparison with the temperature required for the process without the liquid phase. In addition, the costs of the production decrease because the sintering time is reduced in comparison with the process without the liquid phase. Moreover, the possibility of achieving the high level of density of the obtained sinters in relation to the density of the compact enables to use lower pressure than in conventional sintering process without the presence of the liquid phase.

The aim of the present invention is to provide a simple method for solving the technical problem concerning the production of sintered elements having an iron or iron-alloy matrix and characterized by low porosity, relatively high mechanical properties, including impact strength, and dimensional accuracy, sintered throughout the whole volume of a compact in a one-stage process.

SUMMARY OF THE INVENTION

It has appeared, that relative density and dimensional variations of the sintered compact having iron or iron-alloy matrix can be easily controlled by introducing into the mixture the combination of liquid-inducing additives and subsequently forcing an disappearance of the liquid phase, as a consequence of its reaction with other components of the mixture.

Surprisingly, it has been found that the introduction to the mixture made of iron powder and/or iron-alloy powder additives that generate the liquid phase which subsequently, when the sinter has already been condensed, systematically disappears, enables to obtain the sinters having high relative density, high impact resistance and required dimensional and shape tolerances.

According to the nature of the invention, a method of manufacturing sintered elements having matrix of iron or iron-alloy in which a homogenous mixture intended for forming compacts contains iron powder or iron-alloy powder, additives or master alloys, lubricants, liquid-phase inducing additives and said compacts are heated in a protective atmosphere, sintered isothermally and cooled. Said mixture intended for making sintered elements contains 0.05 to 1.00% by weight boron in an active form and at least one of nitrides, excluding boron nitride, selected from nitrides having a melting point or decomposition temperature higher than the sintering temperature, in particular Si, Cr, Ti, Al, Mo, Mn, V nitrides, and the amounts of the nitrides and boron in the mixture are determined in such a manner that the molar ratio of nitrogen to boron atoms is between 0.05 and 2.15, preferably 0.1 to 1.4.

Boron is introduced into the mixture in the active form, which in the present invention means an elementary form, a form of alloy or a form of chemical compound. This active form of boron is capable of generating the liquid phase as the result of eutectic reaction with the matrix or it is capable of self-melting and forming a boron- containing liquid phase even before the compact reaches the sintering temperature.

The mixture intended to be sintered contains boron in active form and nitrides of alloying elements, which are nitrogen-carriers. Boron presented in the liquid phase reacts with nitrogen and forms "in situ" boron nitride (BN) during the sintering process. The precipitates of boron nitride appeal- at the place of reaction i.e. phase boundary between the eutectic-liquid and the nitride. The elements released in this process, in particular Si, Cr, Ti, Al, Mo, Mn, V, whose solubility in iron is greater than the solubility of boron, dissolve in the matrix. The consequence of this reaction is the reduction in the total volume of the solidified eutectic network covering grain boundaries, which in turn increases the contact surfaces of adjacent grains of the matrix, and in the consequence increases dimensional accuracy and mechanical properties of the sinter.

In the process according to the invention the heating of the compacts is carried out at a rate of not more than 30°C/min., preferably not more than 10°C/min., and the isothermal sintering is carried out in the gaseous protective atmosphere at a temperature of 1100-1300°C for up to 60 rnin., preferably for 10 - 40 min. The method according to the invention solves the technical problem described above and the application of the method does not require modifications of the existing technological process or replacement of the stock of technological tools.

The invention creates the possibility of producing sinters having low porosity and high mechanical parameters, free from sintering defects such as dimensional and shape deformations, as well as surface tarnish.

BEST MODE FOR CARRYING OUT THE INVENTION Although for those skilled in the present art is known, for example from Li

Sun , Yong-Ha Kim , Dave (Dae-Wook) Kim and Patrick Kwon , "Densification and Properties of 420 Stainless Steel Produced by Three-Dimensional Printing With

Addition of Si3N4 Powder " Journal of Manufacturing Science and Engineering , 131 (6), (2009) doi: 10.1115 / 1.4000335, that the introduction of a nitride powder into the steel intended for sintering process results in improvement of mechanical properties of the sintered material, but it does not increase the density of the sintered elements, and therefore, it does not eliminate the technical problem solved by the present invention.

Therefore, the fact that in order to completely solve the technical problem mentioned above it is necessary to generate an interaction between the nitrides and the liquid-phase which contains boron, and said interaction, carried out in the sintering temperature, leads to "in situ" synthesis of boron nitride and simultaneously releases the elements forming the previous nitrides and the released elements incorporate into the matrix, is an unexpected effect of the invention and is new and surprising information in the field of metallurgy of iron and iron-alloys powders.

The practical realization of the invention is described in the following examples.

The examples concern sintered cylindrical elements (cylindrical samples) made of the alloy whose chemical composition corresponds to conventional austenitic stainless steel AISI 316L alloys (or: X2CrNiMo17-12) and alloys which correspond to conventional steels Astaloy CrM and Distaloy SE.

In the first group of examples the mixture of the powder of said AISI 316L stainless steel was used. In total, the powder contained, apart from iron and unavoidable impurities, (in% of weight): 0.03% C, 1.0% Si, 2.0% Mn, 13.0% Ni, 0.05% P, 0.015% S, 18.4% Cr, 2.5% Mo, 0.11% N.

In the next groups of examples, in the case of the powder of Astaloy CrM steel, the mixture contained, besides iron and unavoidable impurities (in % of weight): 2.96% Cr, 0.49% Mo and 0.48% C, and in the case of powder of Distaloy SE steel, the mixture contained besides iron and unavoidable impurities (in% of weight): 4% Ni, 1.5% Cu and 0.5% Mo.

Cylindrical compacts having dimensions of Φ 20 x 5 mm were prepared from the mixtures of the powders mentioned above, boron and nitrides. Said cylindrical compacts were prepared using the single-pressing method at pressure of 600 MPa.

In particular embodiments of the invention, into the powder of AISI 316L steel the addition of boron in amount of 0.4% by weight and silicon, chromium, titanium and aluminum nitrides in varying amounts were introduced. The second group of samples contained the powder of AISI 316L steel, the addition of boron in amount of 0.4% by weight, manganese, vanadium, and molybdenum nitrites in amounts which guarantee the molar ratio of N/B = 0.2.

The variable parameters in the first group of examples were the amounts of silicon, chromium, titanium and aluminum nitrides, which guaranteed the molar ratio of nitrogen to boron of 0.09 to 2.15, The reference samples in both groups of examples were free from nitrides (molar ratio N/B = 0).

The time-temperature profile, which represents the sintering process, was characterized by a heating rate of 10°C/min, a cooling rate of 20°C/min, and an isothermal stop at 1240°C for 30 min. The protective atmosphere for the sintering process was hydrogen.

The reference samples of AISI 316L steel were sintered and subjected to density and surface-flatness tests, the results of which are shown in Table 1.

The results of these examples should be compared with the reference sinter in which to the powder of AISI 316L steel boron in amount of 0.4% wt. and no nitrides were added. The obtained sinters had the density of 7.43 g/cm³ (relative density 95.30%) and the deviation from flatness was equal to 11.09 μm. The attached drawings show:

Fig.l - density changes and standard deviation of flatness for the sinter as the function of molar ratio N/B, for the example in which nitrogen was introduced as silicon nitride; Fig.2 - density changes and standard deviation of flatness as the function of molar ratio N/B, for the example in which nitrogen was introduced as titanium nitride;

Fig.3 - density changes and standard deviation of flatness as the function of molar ratio N/B, for the example in which the nitrogen was introduced as chromium nitride;

Fig.4 - density changes and standard deviation of flatness as the function of molar ratio N/B, for the example in which the nitrogen was introduced as aluminum nitride

In the next group of examples cylindrical samples were sintered under the conditions described above. The first sample was made of the powder of AISI 316L steel + 0.4 % wt. B + 1.648 % wt. Mo₂N, (MB = 0.2);

The second sample was made of the powder of AISI 316L steel + 0.4% wt. B + 0.772 % wt. Mn₃N_2, (N/B = 0.2);

The third sample was made of the powder of AISI 316L steel + 0.4 % wt. B + 0.26 % wt. VN, (N/B = 0.2);

The reference sample was made of the powder of AISI 316L steel + 0.4% wt. B.

The flatness of their bases was measured according to the measurement scheme shown in Fig.5.

The results of measurements of flatness of the samples are shown in Fig. 6, Fig. 7 and Fig. 8, where:

Fig.6 shows the flatness of the sinter made of AISI 316L + 0.4%wt. B + 1 ,648%wt. Mo₂N (N/B - 0.2) and the flatness of the reference sample;

Fig.7 shows the flatness of the sinter made of AISI 316L + 0.4%wt. B + 0.772%wt. Mn₃N₂ (N/B = 0.2) and the flatness of the reference sample;

Fig.8 shows the flatness of the sinter made of AISI 316L + 0.4%wt. B + 0.26% wt. VN (N/B = 0.2) and the flatness of the reference sample.

In the next group of examples cubic samples having dimensions of 5x12x30 mm were sintered under the conditions described above and then they were subjected to the tests of density, surface flatness and triple-bending strength. The samples were made of the powder of AISI 316L steel to which boron in quantities of 0.1% wt, 0.2% wt., 0.3 wt% and 0.4 wt% was added and silicon nitride S13N4 powder in various amounts was doped in order to obtain various N/B molar ratio nitride to boron.

The results are shown in Table 2.

In the next group of examples cylindrical samples of the powder of Astaloy CrM doped with various amounts of boron and nitride powders were sintered under the conditions described above, and constant N/B molar ratio for the individual nitrides was maintained. Furthermore, a reference sample made of the powder of Astaloy CrM doped with 0.4% wt. of boron was prepared and sintered under the same conditions.

The obtained sinters were subjected to density and flatness tests, the results of which are shown in Table 3.

In further set of examples, cylindrical samples of the powder of Distaloy SE doped with various amounts of boron and nitride powders were sintered under the conditions described above, and constant N/B molar ratio for the individual nitrides was maintained. Furthermore, a reference sample made of the powder of Distaloy SE doped with 0.4% wt. of boron was prepared and sintered under the same conditions.

The obtained sinters were subjected to density and surface flatness tests, the results of which are shown in Table 4.

Table 4

In further group of examples, cylindrical samples of the powder of AISI 316L steel doped with various amounts of boron and nitride powders were sintered under the conditions described above and constant N/B molar ratio for the individual nitrides was maintained. Furthermore, a reference sample made of the powder of AISI 316L steel doped with 0.4% wt. of boron was prepared and sintered under the same conditions. The obtained sinters were subjected to density and surface flatness tests, the results of which are shown in Table 5.

The analysis of the obtained results clearly indicates that the properties of the sinter are strongly influenced by the molar relationship between boron and nitrogen introduced in the form of nitride. It has also been found that the range of the preferred molar ratio of nitrogen to boron depends on the nitride in which nitrogen is introduced into the sinter.

Claims

Patent Claims

1. A method of manufacturing sintered elements having a matrix of iron or iron-alloy, in which a homogenous mixture intended for forming compacts contains iron powder or iron-alloy powder, additives or master alloys, lubricants, liquid-phase inducing additives and said compacts are heated in a protective atmosphere, sintered isothermally and cooled, characterized in that said mixture intended for making sintered elements contains 0.05 to 1.00% by weight boron in active form and at least one of nitrides, excluding boron nitride, selected from the nitrides having their melting points or decomposition temperatures higher than the sintering temperature, in particular Si, Cr, Ti, Al, Mo, Mn, V nitrides, and the amounts of the nitrides and boron in the mixture are determined in such a manner that the molar ratio of nitrogen to boron atoms is between 0.05 and 2.15, preferably 0.1 to 1.4.

2. The method according to claim 1, characterized in that boron is introduced into the mixture in an elementary form, or in a form of an alloy, or in a form of a chemical compound, and said forms of boron are capable of generating a liquid phase as a result of a eutectic reaction with the matrix or capable of self-melting and generating a boron- containing liquid phase even before the compact reaches the sintering temperature.

3. The method according to claim 1 or 2, characterized in that the heating of the compacts is carried out at a rate of not more than 30° C/min._; preferably not more than 10° C/min., and the isothermal sintering is carried out in the gaseous protective atmosphere at the temperature of 1100°C-1300°C for up to 60 min., preferably for 10 - 40 min.