A powder and a process for the production of a sintered body, and a sintered body

The present invention relates to sintering of tool steels. More specifically, the invention relates to a powder for the production of a sintered body with high speed steel properties and having a density of at least 7.60 g/cm ³ , preferably at least 7.80 g/cm ³ , said powder comprising at least 75% by weight of a base material powder consisting of, apart from unavoidable impurities, a high speed steel. Tool steels comprise for example cold-work steel, hot-work steel, steel for chisels, hammers, cutting blades and other tools, and high speed steel. Tool steels are non-austenitic iron (Fe) alloys comprising one or more second elements in different amounts, which besides carbon (C) may include e.g. manganese (Mn), silicon (Si), molybdenum (Mo), tungsten (W), vanadium (V), cobalt (Co), and chromium (Cr). Phosphorous (P) is an unwanted component in tool steels since it provides poor mechanical properties.

Tool steels are typically manufactured by either conventional casting and forging or by powder metallurgy (PM) routes. Conventional casting results in non-uniform distribution of alloying elements, which is very difficult to eliminate during forging or rolling and this leads to low mechanical properties. Therefore, sintering of tool steel powders is widely employed as an alternative process.

Today, two different PM processing routes for the manufacture of HSS are mainly applied in the industry. One process produces raw materials by hot isostatic pressing of spherical atomised powders. This leads to raw materials with good mechanical properties but also a relatively high cost level - the other process produces final shaped products by pressing and sintering of water atomised powders, which is a more economical process route. The present invention relates to the latter pressing-and-sintering route.

In a typical sintering process of the prior art a tool steel powder is pressed to a body in a powder press. Then the body is led into a furnace at elevated temperature for a period of time. The elevated tern-

perature typically lies below the melting point of the individual elements included in the alloy. Being exposed to the elevated temperature the individual metal particles bond together to form a sintered body. As mentioned in the above the sintering procedure is advantageous over casting in that it typically proceeds at a lower temperature and as a consequence requires less expensive equipment. In many cases it is possible to achieve equal or better material properties of the tool through the sintering route.

It is a well-known procedure to impart specific material proper- ties on an alloy by adding other elements or adjusting the amounts of the elements contained in the alloy. With the PM processing route, this can be done by means of a mixing step, by which the desired amount of other elements can be added. Although it is possible to some degree to give an estimate of the resulting change of properties from an addition of an element to an alloy, the complex distribution of constituents in the alloy and the many variables involved often makes it very difficult, if not impossible, to predict the properties of an alloy comprising more than even a small number of different elements. In most cases the only option for predicting the outcome is to apply a rule of thumb gained from previous experience of the individual elements, extrapolating from previously achieved empirical results. The skilled person typically only arrives at good predictions when applying this method on alloy compositions very close to compositions already thoroughly tested. Typically, only effects from minor adjustments of the composition can be predicted. This is even more true when predicting the material properties of a sintered body. The reactions during the sintering process are very complex and typically very hard to predict.

P is considered to be an undesirable element in tool steels since it typically confers added brittleness to the alloy. For sintering purposes it has been known for several decades that enhanced density of a body sintered from a powder of materials falling outside the general technical field of tool steels, i.e. either austenitic steel or steel consisting almost entirely of Fe, can be achieved by adding a powder comprising Fe and P. A liquid phase of P is obtained for a brief period, during which the molten

constituents enter the pores of the steel base material, thereby rendering it more compact. See e.g. DE 2648262.

High speed steels (HSS) constitute a family of high alloy tool steels mainly used for cutting tools and wearing parts. The characteriz- ing feature of these alloys is the ability to retain a high level of hardness (compared to conventional tool steels) even when subjected to elevated temperatures resulting from cutting metals at high speeds (hence the name). Cutting tools made from HSS retain their shapes at edge temperatures of up to 500 to 600 ⁰ C or even higher, making HSS a valuable alloy in cutting parts used for example in sawing machines, servomotors and hoggers. A sintered body with high speed steel properties is a sintered body with the ability to retain its shape at temperatures of up to 500 to 600 ⁰ C or even higher.

HSS owe their particular properties to a significant composition. Besides a relatively high content of C, they typically comprise relatively large amounts of Mo, V, Cr, Co, and especially W, as well as possibly other metals to a minor degree. As the alloy carbides of HSS are relatively insoluble, a high hardening temperature has traditionally been applied to obtain the desired hardness of the resulting alloy. As is the case with other steel tools, HSS tools are typically manufactured by either conventional casting or by sintering. It is generally accepted that sintering of HSS must be carried out at temperatures of at least about 1300 ⁰ C in order to arrive at the desired material properties of the resultant sintered body (see for example Elsevier's Diction- ary, Stekhoven and VaIk, 1970, page 107). This entails high requirements to the temperature resistance of the furnace applied.

When a large batch of sintered bodies of less temperature- demanding steels are to be produced, the most economical way is typically the use of a belt furnace since the capacity of the furnace is deci- sive for the economic outcome. Belt furnaces, making use of a conveying system for continuously transporting bodies in and out of the furnace, constitute an effective tool for carrying out continuous sintering of the bodies. However, high processing temperatures, i.e. temperatures above approximately 1160 ⁰ C, entail high costs directed to the mainte-

nance of the furnace since a much more frequent replacement of heat sensitive parts, e.g. conveyor belts, is made necessary. More heat resistant alternatives do exist on the market, but they are generally considered too expensive to justify sintering of HSS in belt furnaces. Further, high operation temperatures generally entail the drawback of a large consumption of energy, leading to high expenses and corresponding environmental strain.

The object of the present invention is to lessen costs and environmental strain connected to sintering of HSS and at the same time to maintain good mechanical properties.

In order to meet this object, the invention provides in a first aspect a powder for the production of a sintered body with high speed steel (HSS) properties and having a density of at least 7.60 g/cm ³ , preferably at least 7.80 g/cm ³ , said powder comprising at least 75% by weight of a base material powder consisting of, apart from unavoidable impurities, a HSS, characterized in that said powder further comprises an added material powder in an amount different from 0% and of up to 25% by weight, said added material powder comprising at least 55% by weight of Fe and 5 to 20% by weight of P and possibly a balance of an optional material.

By sintering a body pressed from the powder according to the invention at a temperature of less than 1300 ⁰ C and as low as 1160 ⁰ C and even lower and at atmospheric pressure it is surprisingly possible to provide a sintered HSS body without compromising the material proper- ties compared to those achieved at higher temperatures in the prior art. The resultant sintered body has high speed steel (HSS) properties and a density of at least 7.60 g/cm ³ , preferably at least 7.80 g/cm ³ , which ensures the mechanical properties of the body. The possibility of sintering at lower temperatures opens up the possibility of continuous sintering in a belt furnace, with significantly lowered costs associated with both acquisition and operation of the furnace, and improved efficiency. The sintering of a body produced from the powder according to the invention requires less heat energy, leading to lower expenses and correspondingly lessening the environmental strain associated with the process. The

skilled person would expect the addition of P to a HSS powder to lead to higher brittleness values of the resultant sintered body and would therefore would not consider applying P to any tool steel, and even less to a HSS. Without the wish to be bound by any specific theory the applicant believes that these advantages are achieved because the P provides a liquid phase for a short time during sintering, this liquid phase being provided between the grains of the base powder. Subsequently, a part of the P of the powder is alloyed with the HSS, changing the molecular structure of the material. With this change of molecular structure the liquid phase shifts to a solid phase resulting in a solid phase sintering. This sequence of events is believed to be the cause of the surprisingly good mechanical properties of the resultant body sintered from the powder according to the invention. The P is in local structures diffused into the HSS and is thus concentrated in local structures distributed throughout the resultant body, said local structures comprising more than 2,5%, preferably more than 5% by weight of P. The local structures to a high degree fill out the pores of the body, resulting in a high density of more than 7.60 g/cm ³ or more than 7.80 g/cm ³ , and even up to more than 7.90 g/cm ³ . Since P normally provides unfavourable properties to tool steels, mechanically speaking, it is surprising that local structures of a high P content provide sufficient mechanical properties to the sintered body even when sintered a low temperature. We believe this is achieved because the structures in which P has diffused into HSS hold together larger structures of more pure HSS.

The content of P in the resultant sintered body should be kept below 1.5%, preferably below 1.3 % by weight; otherwise deformations of the sintered body might occur, making it harder to comply with strict form tolerances. In a specific embodiment said high speed steel of said base material powder comprises, apart from unavoidable impurities, (in weight percentages): from 0.6 to 2.3 C, from 0 to 0.4 Mn,

from 0 to 1 Si, from 0 to 1 Ni, from 0 to 12.5 Mo, from 1 to 22 W, from 1 to 12.5 V, from 0 to 12 Co, from 0 to 0.3 P, from 0 to 0.3 S, from 1 to 8 Cr, and Fe balance.

Sintered bodies present improved material properties if said high speed steel of said base material powder comprises, apart from unavoidable impurities, (in weight percentages): from 0.8 to 1.5, preferably approximately 1.25, C, from 0 to 0.4, preferably approximately 0, Mn, from 0 to 1, preferably approximately 0, Si, from 0 to 1, preferably approximately 0, Ni, from 3 to 12, preferably approximately 5, Mo, from 4 to 8, preferably approximately 6, W, from 2 to 12, preferably approximately 3, V, from 0 to 1, preferably approximately 0, Co, from 0 to 0.1, preferably approximately 0, P, from 0 to 0.3, preferably approximately 0, S, from 3 to 5, preferably approximately 4, Cr, and Fe balance.

Preferably, the high speed steel is of an AISI M-2 or AISI M-3 type.

In an embodiment the balance of said added material consists, apart from unavoidable impurities, entirely of Mo. A fast and easy sintering process is obtained when the majority of the Fe and P of said added material is bound in Fe ₃ P. Fe ₃ P is a cheap material, which readily may be mixed with the base material. Fe ₃ P improves the liquid phase sintering.

In another embodiment said added material comprises in weight

percentages 5 to 20 P, preferably 14 to 16 P, 0 to 40 Mo, preferably 0 to 30 Mo and most preferred 0 to 10 Mo, and the balance Fe, and possibly an optional material, preferably the balance is Fe alone. The Mo can be added in the form of pure Mo, e.g. with a particle size of 1,5 to 4,5 μm, or in the form of ferromolybdenum.

The addition of Mo results i.a. in a harder material with increased formation of carbide during the sintering. Since carbides possess good hardness properties, increased amounts of carbide during the sintering step improve the resistance to wear in the resultant sintered body. Alternatively, said added material, apart from unavoidable impurities, consists of P and Fe.

The added material might also contain small amounts of C. The particles of the powder preferably have an average diameter of maximum 150 μm, more preferably maximum 50 μm. Superior material properties are achieved when the powder consists, apart from unavoidable impurities, of 85 to 97% by weight of said base material and 3 to 15% by weight of said added material, preferably 93.5% by weight of said base material and 6.5% by weight of said added material. In a second aspect the invention provides a process for the production of a sintered body with HSS properties and having a density of at least 7.60 g/cm ³ , preferably at least 7.80 g/cm ³ , comprising the steps of: pressing a powder according to the above into a body; and sintering the body at a temperature of 1100 to 1250 ⁰ C, preferably 1140 to 1190 ⁰ C, most preferred approximately 1160 ⁰ C.

Keeping the temperature below 1190 ⁰ C, preferably below about 1160 ⁰ C allows for efficient use of a belt furnace as mentioned in the above, however sintering in a belt furnace is possible at least at temperatures of up to about 1250 ⁰ C. Sintering in a belt furnace is preferred because of the significant operation advantages connected thereto. Sintering below 1140 ⁰ C typically leads to a more significant deterioration of the material properties of the resultant sintered body, and in all cases the temperature should be kept above about 1100 ⁰ C. An optimal combination between good material properties and an efficient and low-cost

process is achieved at about 1160 ⁰ C. Following this, the sintering is preferably carried out in a belt furnace.

In a preferred embodiment of the process according to the invention pressing is carried out with a pressure of 400 to 1200 MPa, pref- erably 500 to 1000 MPa, more preferably 550 to 650 MPa, most preferred approximately 600 MPa. Generally, a higher pressure provides for better material properties (better density) of the resultant sintered body. However, a good compromise between costs and material properties of the resultant body is achieved with a pressure of 550 to 650 MPa. The sintering is preferably carried out for a period of 20 to 120 minutes, preferably 50 to 70 minutes, more preferably approximately 60 minutes.

Moreover, the sintering is preferably carried out in a reducing atmosphere, preferably hydrogen gas, and preferably with a low dew- point temperature, more preferred a dew-point temperature of no more than -20 ⁰ C. A low dew-point temperature entails a better reduction of oxides, which are unwanted in large quantities. Alternatively, the sintering could be carried out in for example cracked ammonia gas or a mixture of gases comprising hydrogen, nitrogen and carbon monoxide, pref- erably with a similar low dew-point temperature.

The process advantageously comprises the step of mixing the powder with a lubricant before the sintering step. This ensures the homogeneity of the powder, which is important to the material properties of the resultant sintered body. The process may also comprise a presin- tering step.

In a third aspect the invention provides a sintered body with high speed steel properties and having a density of at least 7.60 g/cm ³ , preferably at least 7.80 g/cm ³ , c h a r a c t e r i z e d by comprising local structures distributed throughout the body, said local structures comprising more than 1%, preferably more than 2.5%, most preferred more than 5%, by weight of P.. The remainder of the body may consist, apart from unavoidable impurities, of other structures consisting mainly or entirely of HSS (i.e. comprising no P), and preferably some structures comprising P diffused into HSS. The latter structures are preferably con-

centrated along the outer edges of the structures consisting mainly of HSS, enclosing said structures comprising P diffused into HSS.

In the drawings:

Fig. 1 is a view enlarged 200 times of an unetched micro struc- ture of a body sintered from a powder according to the invention.

Fig. 2 shows as an experimental result a curve of the hardness in relation to the P content of a sintered body.

Fig. 3 shows as an experimental result a curve of the density in relation to the P content of a sintered body. Fig. 4 shows as an experimental result a curve of the hardness in relation to the Mo content of a sintered body.

Fig. 5 shows as an experimental result a curve of the density in relation to the Mo content of a sintered body.

Examples

One of the most important and significant mechanical properties of a sintered body with HSS properties is the resistance to wear. High wear resistance requires a high density and high hardness. In the examples an indication of the wear resistance is therefore given as the hardness measured in HVO.3 and the measured density. The hardness values are mean values of a number of hardness tests carried out on each resultant body. The density should be above 7.60 g/cm ³ , preferably above 7.80 g/cm ³ , in order to ensure the mechanical properties of the sintered body. 17 experiments were carried out. A HSS base material in the form of an AISI M3/II HSS with the following composition was used in all 17 experiments:

C Mo W Cr V Fe 1.25% 5% 6% 4% 3% BaI.

The base material was mixed with varying amounts of an added material in the form of powdered FesP with varying amounts of Mo. In experiments no. 1 to 9 the amount of FesP was varied in relation to the

amount of the HSS, the total amount of Mo in the powder being kept approximately constant. In experiments no. 10 to 17 the amount of Mo was varied in relation to the amount of the HSS, the total amount of Fe ₃ P in the powder being kept approximately constant. C was kept at a constant of 1.25% per weight of the resultant body in all experiments. Then the resultant powder was mixed with a lubricant powder in the form of zinc stearate for a period of 10 to 20 min. in order to ensure homogeneity of the mixture. The mixed powder was pressed to a body in a powder press at 600 MPa. Subsequently, the body was sintered in a conventional belt furnace at 1160 ⁰ C in pure hydrogen gas in about 60 min. Finally, the resultant sintered body was cooled and hardness measurements were performed on it.

Experiment no. 1 was carried out on pure HSS for reference. In experiments no. 2 to 17 the amount of added material and the composi- tion of the added material were varied. The powders of experiments no. 10 and 11 were adjusted to lower the total Mo content of the powder. Since the applied HSS contains 5% M3/II, Mo cannot be removed entirely from the powder. In order to lower the content of Mo, Fe was added, lowering the amount of HSS in the powder and in the resultant body. The results from the nine experiments are shown below in Table 1.

Table 1. Composition and material properties of the sintered body of experiments 1 to 17.

As expected, in experiment no. 1 the hardness and the density are too low values (as a rule HSS has to be sintered at higher temperatures in order to achieve good mechanical properties, see above).

The P content of the body does not significantly affect the hard- ness of the sintered body, cf. Fig. 2. As may be seen in Fig. 3 the density of the sintered body is increased with increased amounts of P in the resultant body. However, this is only the case up to a certain point (below 1.3% by weight of P in the present case, presumably about 1.1% P) at which the sintered body lost its dimensional stability, see experiment no. 9 (total P content 1.3%): here, the sintered body lost its dimensional stability because of the high content of P, which resulted in too much melting phase. However, other compositions of the inventive powder would probably be able to obtain sufficient mechanical properties with high amounts of P, such compositions containing for example other types of HSS or smaller amounts of Mo. Net shape or near net shape was achieved in all experiments but no. 9.

In experiment no. 2 the total amount of P in the powder by weight was about 0.39%, which appears to be to little when no Mo was added because the density of the resultant sintered body was unsatisfac- tory. In experiment no. 3 the total amount of P in the powder by weight was about 0.40%, and the density was significantly higher, i.e. 7.60 g/cm ³ . This density touches the lower border of sintered bodies finding any practical use. However, densities as low as about 7.4% could theoretically be of value if applying a different HSS. Further, a comparison between experiments no. 1 to 3 with experiments no. 12 to 17 show that a large content of Mo results in a higher density, cf. Fig. 5; however, the hardness does not seem to be significantly affected, cf. Fig. 4. Additionally, it seems as if an amount of around 2% of added material of the total weight of the powder is the lower limit for producing bodies with satisfactory mechanical properties. The lower limit would vary if the conditions of the experiments were altered, e.g. if another type of HSS or another sintering temperature were used.

An amount of HSS of the powder lower than about 75% seems

to result in large areas of smaller hardness, these areas consisting mostly of Fe. The hardness (and also the density) of the powder of experiment no. 10 is therefore unsatisfactory. The same applies, in principle, to experiment no. 11, however the hardness tests showed smaller variations of the hardness throughout the body. Further, the density of the body of experiment no. 11 is quite high, meaning that such a composition of the powder according to the invention could be relevant when composing a low-cost sintered body (Fe is typically cheaper than HSS). The powder compositions of experiments no. 3 to 8 and 11 to 17 were all found to result in a satisfactory quality of the resultant sintered body. Fig. 1 is a view enlarged 200 times of an unetched micro- structure of the resultant body of experiment no. 7. As can be seen, the pores are few, round, closed and scattered uniformly throughout the material, which provides good mechanical properties to the body. Similar structures were found in all satisfactory powder compositions. At present the powders of experiments no. 6 and 7 are preferred because of their superior mechanical properties.