EP0183666A1

EP0183666A1 - A manganese sulphide-containing iron powder mixture and a process for the production thereof

Info

Publication number: EP0183666A1
Application number: EP85850374A
Authority: EP
Inventors: Ulf Engström
Original assignee: Hoganas AB
Current assignee: Hoganas AB
Priority date: 1984-11-30
Filing date: 1985-11-21
Publication date: 1986-06-04
Also published as: SE445715B; ES8701562A1; JPS61147801A; ES549415A0; SE8406054D0; SE8406054L; JPH0645804B2

Abstract

The machinability of a sintered body produced by sintering from a pressed iron powder mixture can be increased substantially by utilising an iron powder mixture in which manganese sulphide is included in the form of a very fine powder having a particle size of less than 10 µm. The iron powder mixture is prepared by adding the manganese sulphide in the form of the said fine powder having a particle size of less than 10 µm and in an amount such that the finished mixture will contain 0.05-5% by weight of manganese sulphide, preferably 0.3-1.0% by weight of manganese sulphide.

Description

The present invention relates to a manganese sulphide-containing iron powder mixture for the production of sintered bodie The invention also relates to a process for producing such a manganese sulphide-containing iron powder mixture.
The invention aims at improving the workability of sintered steel without affecting the dimensional change during sintering or deteriorating the strength characteristics of the finished sintered bodies.
Although it is one of the main advantages of powder metallurgy that it is possible to produce sintered bodies with very narrow tolerances, products are being produced where the tolerance requirements are set so high that the sintered body must be subsequently worked. Alternatively, the finished product may have a design such that it cannot be compacted to final shape, and certain details must therefore be machined after sintering.
The machining of sintered bodies normally gives a rough surface, primarily because of the porosity of the sintered material, which means constant chipbreaking for the tool employed as well as jerky tool movements and increased tool wear.
In melt metallurgy, it has long been tried to improve the machinability of steel, and different additives, such as lead, copper, zinc and sulphur, have been tested. In most cases, however, improved machinability has at the same time resulted in inferior physical characteristics. According to U.S. patent specification 3,705,020, E.S. Nachtman, the additives previously employed in the production of steel presumably have acted as an inner lubricant or imparted to the cutting tool a coating that has reduced wear. The said U.S. patent specification 3,705,020 discloses a completely new method for improving the machinability of steel. By uniform distribution of fairly coarse inclusions (10-100 pm) of a hard and brittle material such as alumina or manganese sulphide, in the steel, an improved workability could be achieved. No explanation of this phenomenon is given in the said patent specification, but the result is in all probability due to the fact that the brittle inclusions easily split and thus make way for the cutting tool. In its enumeration of the conventional lubricants lead etc., this patent specification is back in prior art technique, with the sole difference that the lubricant is stored in depots.
For sintered materials, the techniques used in melt metallurgy have proved to be impracticable because the additives have given rise to excessive dimensional changes and inferior strength characteristics.
The present invention follows an entirely different path. Thus, it has surprisingly been found that manganese sulphide in sintered steel gives a result which is entirely opposite to what U.S. patent specification 3,705,020 has shown to be applicable in the field of melt metallurgy.
According to the Examples in U.S. patent specification 3,705,020, the best machinability is obtained with an admixture of 1-1.5% of manganese sulphide powder having a grain size of 30-40 µm. According to the present invention, the machinability of a sintered steel is vastly improved, without affecting the other characteristics, by admixture of a very fine manganese sulphide powder having a grain size of less than 10 µm. This must be regarded as highly surprising and remarkable in view of U.S. patent specification 3,705,020. The reason for this aberrant behaviour when using manganese sulphide powder in sintered steel has not been fully explained. One explanation may be the porous structure of the sintered steel. The manganese sulphide is entirely neutral during sintering and then is held only mechanically at the grain boundaries. Presumably, the manganese sulphide powder acts as an inner lubricant, but it is not inconceivable that it is given a function reminiscent of a ball bearing during machining.
The invention will be exemplified in more detail in the following Examples in which % is % by weight.
Different iron powder mixtures with the additives specified below were compacted at 6 tons/cm² and sintered at 1120°C for 30 minutes in an endothermic atmosphere consisting of a mixture of nitrogen gas, hydrogen gas and carbon monoxide gas. The machinability was determined by the drilling method, which involved drilling holes into the samples until the drill had been worn down. The number of drilling holes for each worn-down drill here is a quantity which indicates the workability. For all tests, the same type of drill was used, i.e. the drills were of the same quality and of the same dimensions.

Examples

Mixture A: An iron powder containing 1.75% Ni, 1.50% Cu, 0.50% Mo and 0.60% C, the balance being Fe.
Mixture B: Mixture A + 0.50% MnS having a particle size of between 10 µm and 100 µm.
Mixture C: Mixture A + 0.50% MnS having a particle size of less than 10 µm.
Mixture D: Mixture A + 0.15% S.
The characteristics of the mixtures will appear from the following Table in which ΔL/L represents relative dimensional change, σB represents tensile strength in N/mm , δ represents elongation in %, and HV5 is hardness according to Vicker.
As will be seen from the above Examples, also sulphur improved the workability considerably, but in this case ΔL/L is unacceptably high and the strength has been reduced.
A further number of powder mixtures on an iron base with different alloying additives, such as Cu, Ni, Mo, Mn, Cr, Si, P and C, were mixed with manganese sulphide in contents of up to 5% by weight and in fractions of, on the one hand, up to 10 µm and, on the other hand, in the range 10-100 µm, were compacted and sintered. All samples gave equivalent and unambiguous results.
For instance, with a mixture of atomised pure iron powder containing 0.45% P, eleven drill holes were obtained before the drill was worn down. With an addition of 0.5% manganese sulphide having a particle size of less than 10 µm, ninety drill holes could be made before the drill was worn down.
For a mixture of atomised pure iron powder with an addition of 0.45% P and 0.6% C only five drill holes could be made before the drill was worn down. With an addition of 0.3% MnS to the last-mentioned mixture, seventy-five drill holes could be made in the sintered body produced from this mixture, before the drill was worn down.
To sum up, the additive according to the present invention has been found to improve substantially the machinability of all tested material mixtures which are based on iron powder and which, without this additive, are difficult to machine. The invention thus is applicable to iron powder mixtures with up to about 7.5% Cu, up to about 10% Ni, up to about 1.5% Mo, up to about 2.5% Mn, up to about 6% Cr, up to about 5% Si, up to about 1.5% P and up to about 1.5% S. Though it should be noted that some of these additives per se improve the cuttability, the additive according to the present invention always brings an essential improvement of the machinability as regards mixtures for sintered bodies which are not easily worked.

Claims

1. A manganese sulphide-containing iron powder mixture for the production of sintered bodies, cha- racterised in that the manganese sulphide is included in the form of a very fine powder having a particle size of less than 10 µm.

2. A manganese sulphide-containing iron powder as claimed in claim 1, characterised in that the finished mixture contains 0.05-5% by weight of manganese sulphide, preferably 0.3-1.0% by weight.

3. A manganese sulphide-containing iron powder as claimed in claim 1 or 2, characteris- e d in that the finished powder mixture contains one or more of the elements copper, nickel and molybdenum.

4. A manganese sulphide-containing iron powder as claimed in any one of claims 1-3, charac- terised in that the finished powder mixture contains one or both of the elements manganese and chromium.

5. A manganese sulphide-containing iron powder as claimed in any one of claims 1-4, charac- terised in that the finished powder mixture contains one or more of the elements carbon, phosphorus and silicon.

6. A process of producing a manganese sulphide-containing iron powder mixture for the production of sintered bodies, characterised in that the manganese sulphide is added to the iron powder mixture in the form of a very fine powder having a particle size of less than 10 µm.

7. A process as claimed in claim 6, charac- terised in that the manganese sulphide is added to the iron powder mixture in such an amount that the finished mixture will contain 0.05-5% by weight of manganese sulphide, preferably 0.3-1.0% by weight of manganese sulphide.

8. A process as claimed in claim 6 or 7, characterised in that one or more of the elements copper, nickel and molybdenum are added to the iron powder mixture.

9. A process as claimed in any one of claims 6-8, characterised in that one or both of the elements manganese and chromium are added to the iron powder mixture.

10. A process as claimed in any one of claims 6-9, characterised in that one or more of the elements carbon, phosphorus and silicon are added to the iron powder mixture.