EP0533812B1

EP0533812B1 - Iron-based powder, component produced therefrom, and method of producing the component

Info

Publication number: EP0533812B1
Application number: EP91911997A
Authority: EP
Inventors: Per Engdahl; Caroline Lindberg
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 1990-06-11
Filing date: 1991-06-07
Publication date: 1995-08-16
Anticipated expiration: 2011-06-07
Also published as: JPH05507967A; SE9002070D0; SE468583B; EP0533812A1; DE69112214D1; ATE126461T1; KR930700243A; WO1991019582A1; JP3280377B2; ES2075961T3; DE69112214T2; BR9106546A; KR100189234B1; SE9002070L; CA2084679C; CA2084679A1

Abstract

An iron-based powder for producing impact-resistant components by powder compacting and sintering contains, in addition to Fe, 0.3-0.7 % by weight of P, 0.3-3.5 % by weight of Mo, and not more than 2 % by weight of other alloying elements. A method of powder-metallurgically producing impact-resistant steel components comprises using an iron-based powder which, in addition to Fe, contains 0.3-0.7 % by weight of P, preferably 0.35-0.65 % by weight of P, 0.3-3.5 % by weight of Mo, preferably 0.5-2.5 % by weight of Mo, and not more than 2 % by weight, preferably not more than 1 % by weight, of other alloying elements; compacting the powder into the desired shape; and sintering the compact.

Description

The present invention relates to an iron-based powder for producing impact-resistant components by powder compacting and sintering.
The invention also concerns a powder-metallurgically produced component made from this powder. Finally, the invention bears upon a method of powder-metallurgically producing such a component.
The remaining porosity of sintered powder-metallurgical materials impairs the mechanical properties of the materials, as compared with completely dense materials. This is a result of the pores acting as stress concentrations, as well as reducing the effective volume under stress. Thus, strength, ductility, fatigue strength, macro-hardness etc. in iron-based powder-metallurgical materials decrease as the porosity increases. Impact energy is, however, the property the most adversely affected.
Despite their impaired impact energy, iron-based powder-metallurgical materials are, to a certain extent, used in components requiring high impact energy. Naturally, this necessitates high precision when manufacturing the components, the effect of the porosity on impact energy being well-known.
The impact energy of sintered steel may be increased by alloying with Ni, which augments the strength and ductility of the material and, furthermore, causes shrinkage of the material, i.e. a density increase. The effect of Ni-alloying is especially pronounced when the sintering is carried out at a high temperature, i.e. above 1150°C. Naturally, the high temperature results in a more active sintering and produces rounder pores than do low sintering temperatures. In addition, the rounder pores also increase the impact energy. Alternatively, a more active sintering can be achieved by adding P, which increases strength and ductility, as well as rounds off the pores even at lower sintering temperatures, i.e. below 1150°C.
To sum up, the impact energy of sintered materials can be increased by reducing the stress concentration effect of the pores. This can be achieved by liquid-phase sintering, high-temperature sintering, sintering of a ferritic material, double compacting, and by adding alloying elements having a shrinking effect.
In many cases, however, sufficient impact energy is only achieved with a combination of the above measures, which usually requires extensive and costly processing when using alloying systems known in the powder-metallurgical techniques of today.
DE-B-2321103 discloses an iron based powder containing 0.5% Mo, 2% Ni, 0.3 %P, rest Fe apart from impurities (mixture D) to produce a phosphorus containing powder which, gives as low tool wear as possible during compacting and small dimensional change during sintering.
The object of the present invention is, therefore, to provide an iron-based powder which simplifies the processing, yet yields sufficiently impact-resistant components by powder compacting and sintering.
It is further desired that simple powder compacting as well as sintering can be carried out in a belt furnace, i.e. at temperatures below about 1150°C.
This object is achieved by an iron-based powder which, in addition to Fe, contains Mo and P, and in which the content of other alloying elements is maintained on a low level as defined in claim 1. This material is, inter alia, characterised by the fact that sintering even below 1150°C results in an impact energy which is higher than that of today's powder-metallurgical materials sintered at higher temperatures. Further, the material has excellent compressibility and is capable of considerable shrinkage, giving a sintered material of high density. For one and the same density, the material of the invention further has a substantially higher impact energy than today's powder-metallurgical materials.
The amount of Mo in the material should be 0.3-3.5% by weight, preferably 0.5-2.5% by weight, and the amount of P should be 0.3-0.7% by weight, preferably 0.35-0.65% by weight, most preferably 0.4-0.6% by weight. In addition, C may be present in a maximum amount of 0.1% by weight, preferably 0.07% by weight. Further, the amount of other alloying elements should not exceed 1% by weight, and preferably not 0.5% by weight.
This powder can be produced by making a base powder of pure Fe, or Fe and Mo in solid solution. This can be produced either as a water-atomised powder or as a sponge powder. Suitably, the base powder is annealed in a reducing atmosphere to lower the content of impurities. Then, the powder is mixed with P, or Mo and P, and is compacted into the desired shape, whereupon sintering is carried out a temperature which advantageously is below 1150°C.
The invention is defined in claims 1, 8, 9 and 10. Preferred embodiments are shown in claims 2-7.

Example

A base powder of Fe containing 1.5% by weight of Mo was prepared by water-atomisation. Then, 0.5% by weight of P was added. Test pieces were produced by compacting at a pressure of 4-8 ton/cm². The test pieces were sintered at 1120°C for 30 min. The resulting densities and impact energies are apparent from the upper curve in Fig. 1, where the compacting pressure in ton/cm² is the parameter. For instance, an impact energy of 180 J and a density of 7.46 g/cm² were obtained at a compacting pressure of 8 ton/cm².
A test piece produced in the manner described above, but without Mo, had a much lower impact energy, as is apparent from the lower curve in Fig. 1.
At high-temperature sintering, the material shrinks more, which leads to higher density and, consequently, to higher impact energy. This is illustrated by the point A on the upper curve in Fig. 1, which was obtained at a compacting pressure of 6 ton/cm² and by sintering at 1250°C for 30 min.
It should be observed that the combined addition of P and Mo results in a higher sintered density than does a binary system of Fe and P, even if subjected to double compacting. For one and the same density, the material of the invention further gives a much higher impact energy, which in all probability should be attributed to a more active sintering and a positive interaction between Mo and P.
A powder according to the invention containing 1.5% by weight of Mo and varying amounts of P in the range of 0-0.8% by weight was produced. Test pieces were made by compacting at 589 MPa and sintering at 1120°C. The resulting impact energy in J is apparent from Fig. 2. As shown therein, a maximum value is achieved at 0.5% by weight of P; good values are obtained in the range of 0.3-0.7% by weight of P; even better values are obtained in the range of 0.35-0.65% by weight of P; and the best values are obtained in the range of 0.4-0.6% by weight of P.
Similarly, a powder containing 0.5% by weight of P and varying amounts of Mo in the range of 0-4% by weight was produced. Test pieces were produced by compacting at 589 MPa and sintering at 1120°C. The resulting impact energy values are apparent from Fig. 3. As shown therein, 0.3-3.5% by weight of Mo constitutes a useful range, whereas 0.5-2.5% by weight of Mo constitutes a preferred range.
Very likely, the results obtained are due to the fol-lowing. The addition of P entails that a liquid phase is obtained during sintering at a comparatively low temperature, resulting in a better distribution of P in the material. P diffuses into the iron particles, and, to some extent, austenite is transformed to ferrite, which facilitates the diffusion of Mo. Both P and Mo are ferrite stabilisers, and the transformation to ferrite increases the self-diffusion of Fe. This gives an active sintering, resulting in shrinkage and round pores.
Suitably, P is present in the form of a phosphor compound, preferably iron phosphide, e.g. Fe₃P.
The other alloying elements may be of a type not affecting the impact energy adversely, and common in powder metallurgy. As non-restrictive examples, mention may be made of Ni, W, Mn and Cr. Cu should not be used at all.

Claims

An iron-based powder for producing impact-resistant components by powder compacting and sintering, characterised by containing, in addition to Fe, 0.3-0.7% by weight of P, 0.3-3.5% by weight of Mo, not more than 0.1% by weight of C and not more than 1% by weight of other alloying elements.
The powder of claim 1, characterised in that the amount of Mo is 0.5-2.5% by weight.
The powder of claim 1 or 2, characterised in that the amount of P is 0.35-0.65% by weight.
The powder of claim 1 or 2, characterised in that the amount of P is 0.4-0.6% by weight.
The powder of any of the claims 1-4, characterised in that P is present in the form of iron phosphide, preferable Fe₃P.
The powder of any one of claims 1-5, characterised in that the amount of other alloying elements does not exceed 0.5% by weight.
The powder of any one of claims 1-6, characterised by not containing more than 0.07% by weight of C.
A method of powder-metallurgically producing impact-resistant steel components, characterised by using iron based powder which, in addition to Fe, contains 0.3-0.7% by weight of P, preferably 0.35-0.65% by weight of P, 0.3-3.5% by weight of Mo, preferably 0.5-2.5% by weight of Mo, not more than 0.1% by weight of C and not more than 1% by weight of other elements; compacting the powder into the desired shape and sintering the compact.
A powder-metallurgically produced component, characterised by containing, in addition to Fe, 0.3-0.7% by weight of P, 0.3-3.5% by weight of Mo, not more tha 0.1% by weight of C and not more than 1% by weight of other alloying elements.
Use of a powder for producing impact-resistant components by powder compacting and sintering, characterised in that the powder, in addition to Fe, comprises 0.3-0.7% by weight of P, 0.3-3.5% by weight of Mo, not more than 0.1% by wieght of C and not more than 1% by weight of other alloying elements.