The present invention relates to a machinability improving and/or wear-resistance improving supplementary powder for an iron- or steel-based powder for use in powder-metallurgical production of blanks. The invention also relates to an iron- or steel-based powder containing such supplementary powder.

Powder-metallurgical production of blanks, such as constructional elements, is frequently effected in the following process steps. To a basic powder, usually an iron or steel powder, alloying elements, such as nickel, copper, molybdenum and carbon, in the form of a powder an also a lubricant are added. The powder mixture is compact ed in a pressing die. The raw blanks obtained in the compacting step are sintered so as to obtain their final properties in respect of strength etc.

One of the main advantages of powder-metallurgical production of blanks is that compacting and sintering mak it possible to produce the blanks in an almost final shape. However, there are cases where subsequent machinin is required. For example, reworking can be necessary owin to high tolerance requirements or because the finished product may have a design such that it cannot be compacte directly but must be machined after sintering. More precisely, geometries such as holes transversely of the compacting direction, undercuts and threads require subsequent machining.

With a continuous development of new sintered steels having higher strength and, thus, also higher hardness, machining has become one of the main problems in powder- metallurgical production of components, and in many cases machining is a limiting factor when considering whether powder-metallurgical production is the optimal manufactur¬ ing process for a component from an economic point of view. This means that there is a great need of new and

more efficient processes or additives for improving the machinability of sintered steel.

The initial admixture of alloying elements gives powder metallurgy a unique opportunity of also adding other substances which, for example, improve the machinability of the material. An example of this is shown in SE-C-8406054-0 which describes how the machinability of a sintered steel can be improved by admixture of MnS in a steel powder. The object of the present invention therefore is to provide a supplementary powder for an iron- or steel-based powder for use in powder-metallurgical production of blanks, said supplementary powder being adapted to improve the machinability and/or the wear-resistance of the blank produced.

It has quite surprisingly appeared that this object can be achieved by a supplementary powder which consists of MnS and Te or a Te-compound and/or Se or an Se-com¬ pound. This supplementary powder produces a substantially improved cuttability increasing effect as compared with a supplementary powder consisting of MnS only.

According to US-A-3,152,890, US-A-4,279,646 and US-A- 4,434,006 it is prior art in conventional pyrometallurgy to produce steel with enhanced cuttability, i.e. what is generally called free cutting steel, by alloying with Mn, S, Se and/or Te. In this context, however, the alloying process occurs when melting the steel. The purpose of adding e.g. Te has further been to modify the geometrical shape of the inclusions in the steel which are active in respect of the free cutting properties of the steel. In hotworking of the materials, these inclusions obtain an undesired shape which can be improved by adding Te and/or Se.

The inventive supplementary powder gives the blanks produced a superior machinability and also improves the wear-resistance of the blanks, without affecting other important properties of the material, such as dimensional

changes of the blanks during sintering and their mechani¬ cal properties. By machinability is meant all existing working methods, such as drilling, turning, milling, grinding etc. In view of the above prior art in pyro- metallurgy, the improved machinability obtained by the supplementary powder according to the invention is unexpected, since in powder metallurgy any deformation of inclusions occurs isostatiσally during the compacting operation, and therefore the length/width ratio is the same after compacting. The reason why the supplementary powder according to the present invention has an essen¬ tially greater machinability enhancing effect than a supplementary powder of MnS only must therefore be found in some other effect than the modified shape of the inclusions. Thus, it is possible that the supplementary powder according to the invention improves the lubricating effect in machining. This can also be a reason why the supplementary powder according to the invention has proved to be a most efficient means of improving the wear-resis- tance of powder-metallurgically produced elements.

The supplementary powder according to the invention can be produced in different ways. Firstly, Te or a Te- compound and/or Se or an Se-compound can be alloyed with MnS by fusion of Mn, S, Te or a Te-compound and/or Se or an Se-compound, whereupon grinding is performed to the desired particle size. Secondly, an intimate mixture of particles of MnS and Te or a Te-compound and/or Se or an Se-compound can be performed by e.g. agglomeration or mechanical alloying, whereby all or substantially all particles in the powder will contain MnS and Te and/or Se. Thirdly, the supplementary powder according to the inven¬ tion can be produced by mixing separate particles of MnS and Te or a Te-compound and/or Se or an Se-compound. In a supplementary powder containing MnS and Te according to the invention, the weight ratio between MnS and Te should suitably be in the range of 10-2000, preferably 50-1000.

In a supplementary powder containing MnS and Se according to the invention, the weight ratio between MnS and Se should suitably be in the range of 10-1000, prefer¬ ably 10-500. In a supplementary powder containing MnS, Te and Se according to the invention, the weight ratio between MnS and Te should suitably be in the range of 20-4000, prefer¬ ably 100-2000, and the weight ratio between MnS and Se in the range of 20-2000, preferably 20-1000. According to the invention, there is also provided an iron or steel powder for powder-metallurgical production of blanks having improved machinability and/or wear-resis¬ tance. This powder is characterised according to the in¬ vention in that it contains a supplementary powder consisting of MnS and Te or a Te-compound and/or Se or an Se-compound.

In case the supplementary powder consists of MnS and Te or a Te-compound, the amount of MnS in the iron or steel powder should, according to the invention, suitably be in the range of 0.2-3% by weight and the weight ratio between MnS and Te in the range of 10-2000, preferably 50-1000.

In case the supplementary powder consists of MnS and Se or an Se-compound, the amount of MnS in the iron or steel powder should suitably be in the range of 0.2-3% by weight, and the weight ratio between MnS and Se in the range of 10-1000, preferably 10-500.

In case the supplementary powder consists of MnS, Te or a Te-compound and Se or an Se-compound, the amount of MnS in the iron or steel powder should suitably be in the range of 0.2-3% by weight, and the weight ratio between MnS and Te in the range of 20-4000, preferably 100-2000, and the weight ratio between MnS and Se in the range of 20-2000, preferably 20-1000. The invention and embodiments thereof will be de¬ scribed in detail below by means of a number of Examples.

Example 1

Additives according to the invention, consisting of a mixture of MnS and Se-particles, were prepared, the weight ratios between MnS and Te being as follows. The particle size of the MnS and Te powders was less than 56 μm.

Additive Weight ratio

MnS/Te

A ∞ (i.e. only MnS) B 500

C 333

D 167

E 83

F 33 G 17

These additives were mixed with a commercially avail¬ able, partially prealloyed steel powder containing 4% Ni, 1.5% Cu, 0.5% Mo and 0.5% C, in amounts of 0.5%. All amounts are stated in percent by weight.

From these powder mixtures containing the different

2 additives, elements were made by compacting (6 tons/cm ) and sintering (1120°C for 30 minutes in an endothermic atmosphere) for evaluation of the mechanical properties and the machinability. The measured values of important material properties, such as compacted density (GD) sintered density (SD), dimensional change during sintering [ΔL/L _n ] hardness (HV10), tensile strength (Rm), elongation [A] and machining index, are shown in the Table below. Moreover, the machinability of the materials of elements produced as stated above was determined by means of a drilling test. The number of drilling holes for each worn- down drill is a quantity which indicates the machin¬ ability.

As appears from this Example, the additives contain¬ ing MnS and Te have a superior machinability enhancing effect as compared with an additive containing MnS only.

The additive according to the invention appears to have the desired machinability improving effect even at very small amounts of Te in relation to MnS. When the amount of Te is increased in relation to MnS, there is a gradual deterioration of mechanical properties of the powder-metallurgically produced blanks. When the weight ratio between MnS and Te is in the range of 200-1000, the other properties are affected but to a very small extent. With lower quotas between MnS and Te, the strength of the powder-metallurgically produced blanks is affected. This can in some cases be tolerated, when an extremely good machinability of the material is desired. The weigth ratio between MnS and Te should thus be in the range of 10-2000, preferably 50-1000.

Example 2

Additives according to the invention, consisting of a mixture of MnS and Se-particles, were prepared, the weight ratios between MnS and Se being as follows. The particle size of the MnS and Se powders was less than 56 μm.

Additive

A H I J K

These additives were mixed with a commercially avail¬ able, partially prealloyed steel powder containing 4% Ni, 1.5% Cu, 0.5% Mo and 0.5% C, in amounts of 0.5%.

From these powder mixtures containing the different

2 additives, elements were made by compacting (6 tons/cm ) and sintering (1120°C for 30 minutes in an endothermic atmosphere) for evaluation of the mechanical properties and the machinability. The measured values of important material properties, such as compacted density (GD) sin¬ tered density (SD), dimensional change during sintering [ΔL/L _Q ], hardness (HV10), tensile strength (Rm), elonga¬ tion [A] and machinability are shown in the Table below.

The additive according to the invention consisting of MnS combined with Se does not appear to have the desired machinability improving effect until at a slightly lower quota between MnS and Se. The weight ratio between MnS and Se should thus be in the range of 10-1000, preferably 10-500.

Example 3

An additive consisting of a mixture of the additives C and J according to Examples 1 and 2, was added to the same, partially prealloyed powder as in Example 1 in the following ratio: 0.25% by weight of C and 0.25% by weight of J.

Samples for testing mechanical properties and machin¬ ability were produced in the same manner as in Example 1. The following results were obtained.

Rm A Machinability (MPa) (%) (number of drill holes)

675 2.0 45

Example 4

This Example shows the surprising synergistic effect obtained by adding a combination of MnS and Te and/or Se.

The additives consisted of MnS, Te or Se particles having a particle size of less than 56 μm. These additives were mixed with a commercially available, partially pre¬ alloyed steel powder containing 4% Ni, 1.5% Cu, 0.5% Mo and 0.5% C, in amounts corresponding to the individial total amounts of MnS and, respectively, Te and Se in the additives A, C and J.

Samples were produced from these powder mixtures for testing the same properties as in Example 1. The result was as follows:

As can be concluded from this Example, adding only Te and/or Se in the amounts stated above insignificantly affects the machinability of sintered steel, which means that there is a very strong synergistic effect in combining MnS with Te and/or Se.

Example 5

The additives A and C were added to four different powder-metallurgical materials.

1. Fe - 4% Ni - 1.5% Cu - 0.5% Mo - 0.5% C

2. Fe - 5% Cu - 0.5% C

3. Fe - 8% Ni - 1% Mo - 0.5% C

4. 410 L (a stainless martensitic material).

This experiment was made in order to prove that the additive C according to the present invention has an effect on other types of powder-metallurgically produced materials.

Powder mixtures were prepared from a commercially available, partially prealloyed steel powder containing 4% Ni, 1.5% Mo and 0.5% C and different amounts of the additive C according to the invention.

This Example illustrates the improvement of the machinability, which is obtained when the amount of the additive C is increased in a powder-metallurgical mate¬ rial.

Powder mix¬ ture with additive

C C C C

Example 7

For the purpose of exemplifying the increased wear- resistance of a powder-metallurgically produced material when the additive according to the present invention has been used, the following test was performed.

The additives A and C according to Example 1 were mixed with a pure iron powder in an amount of 1% by weight. To this iron powder, also the following ingre¬ dients/additives were added: 2% Cu, 20% addition of hard phase in the form of a commercially available high-speed steel powder M2. Samples were produced for testing the wear-resistance according to the Ogushi-method.

Material Additive Amount Wear index

6 C 1 2.53xl0~ ¹⁰

The worn-off volume per force and distance is reduced to one third when the additive according to the invention is used instead of MnS only. It can be read from this Example that the additive C according to the invention has a very high effect on the wear-resistance of a powder- metallurgically produced material. Finally, it should be mentioned that by using Te and/or Se-compounds instead of elementary Te and/or Se, the liability of the supplementary powder to dust and/or its poisonousness can be reduced. Copper compounds containing Te and/or Se are an example of such useful compounds.