EP0227001B1 - Method for manufacturing tools - Google Patents

Abstract

To manufacture tools from medium- and high-alloy steels or stellites by superplastic precision-forming, a powder-metallurgically produced starting material of equiaxial structure with more than 30 % by volume of a carbidic and/or boridic precipitation phase of a particle size of 1 to 0.2 mu m is adjusted by thermomechanical processing (hot-forming) to a matrix grain size of 1 to 3 mu m and formed in the superplastic state. <IMAGE>

Description

The invention relates to a method for producing tools from alloyed steels or stellites by thermoforming.

Tool steels and stellites or hard metals are generally characterized by high levels of carbon, chromium, cobalt, molybdenum, vanadium and tungsten. Together with the corresponding carbides, these elements give the material the necessary strength, in particular wear resistance and hardness. However, this is usually at the expense of toughness and is associated with a corresponding increase in the resistance to deformation.

With high resistance to deformation, cold as well as conventional hot forming to create the finished contour are ruled out and, as a result, only undergo primary shaping Block or continuous casting and a subsequent rolling or forging, or a molding and powder pressing in question. However, as a rule, these processes require machining of the preformed part to the finished contour and finished size. However, this is particularly difficult with wear-resistant parts insofar as machining requires tools with a wear resistance that significantly exceeds the wear resistance of the part to be machined. In addition, machining is associated with a considerable loss of material. The processing costs are therefore considerable without always having a good surface finish.

There are also process-specific disadvantages such as the high energy costs of hot rolling and forging or the impairment of the surface quality due to intensive oxidation processes of the alloys. Another disadvantage is that, especially with regard to entangled finished molds, the fluidity is usually not sufficient for primary molding as well as for molding. This leads to blanks that differ significantly from the finished part and therefore require machining that leads to a considerable loss of material. The associated costs are very considerable due to the high content of expensive alloying agents in the materials concerned. In addition, there is the need for high deformation forces resulting from the high resistance to deformation, which entails correspondingly expensive forming units and high energy costs.

A method is known from US Pat. No. 3,951,697 is known in which steels with a very high carbon content are set after a heat treatment at at least 500 ° C by deformation on an equiaxed basic structure with a finely dispersed spherulitic cementite. This process is also suitable for the use of powders as the starting material. This is done in such a way that a starting powder with a carbon content of more than 1%, the carbon of which is predominantly in the form of spherulitic cementite, is mixed with an iron powder with a grain size of less than 10 μm, then compacted and then sintered at 600 to 700 ° C. The structure of the sintered compact mainly consists of a coaxial grain with an average grain size of less than 10 µm and an evenly distributed, mainly spherulitic cementite in the temperature range from 723 to 900 ° C. Apart from powder compaction, no deformation takes place in the known method. In addition, the process only affects iron-carbon materials with a carbon content of over 1.0% and otherwise only impurities such as 0.4 to 0.5% manganese and 0.1 to 0.2% silicon.

The invention has for its object to provide a method which avoids the aforementioned disadvantages and allows the production of finished parts from alloys, which due to their high deformation resistance normally cannot be deformed or at most can be deformed into a blank which requires machining.

The solution to this problem is that in a method of the type mentioned in the invention, a preform made of a high-alloy steel with an equiaxial structure and over 30 vol .-% carbidic and / or boridic elimination phase below A 1 with a cyclic alpha / gamma phase transformation or in the austenitic state thermoformed and adjusted to a matrix with a grain size of 1 to 3 µm and a separation phase with a grain size of 0.2 to 1.0 µm and finally superplastically shaped.

In an alternative solution to the above-mentioned task, on the other hand, a stellitic preform with an equiaxial structure and over 30 vol.% Carbidic and / or boridic precipitation phase is thermoformed at 700 to 1000.degree. C., thereby forming a matrix with a grain size of 1 to 3 µm and one Elimination phase set with a grain size of 0.2 to 1.0 µm and finally superplastically shaped.

The small grain size set in the process according to the invention ensures a low yield stress due to grain boundary sliding and thus reduces the required forming force and tool wear.

The method according to the invention therefore runs in two stages; The first stage of the process is to use the powder-metallurgically produced, due to the high cooling rate of, for example, 10⁴ to 10⁵ SchmelzK / s during melt atomization, already fine crystalline, preferably already equiaxial, multi-phase structure of the alloy powder both with regard to the matrix and also with regard to the carbidic and / or boridic precipitation phase to further refine the consolidated state and thereby a thermally stable microstructure during the subsequent thermomechanical processing as a result of hot forming in the second process stage, with a grain size of 1 to 3 μm or 0.2 to 1.0, which is preferably fine both for the matrix and for the precipitation phase µm.

The material structure in the first process stage can be conditioned by thermomechanical processing, which is the case with steel alloys in austenitic Condition, for example, begins at about 900 ^o C and the γ / α phase conversion in the range from 750 to 820 ^o C to a final rolling temperature of 650 ^o C goes through. During the thermoforming, for example rolling or forging, the material to be deformed cools down continuously and, in addition to the phase change, the carbides and / or borides are eliminated.

In a similar manner, the carbides and / or borides are eliminated in the hot forming of stellites in the temperature range from 1000 to 700 ^° C. during the shaping and the associated continuous cooling. In addition, during thermomechanical conditioning, there is a refinement of the matrix grain, which is then equiaxial at the latest, as well as a finer dispersion of the carbide and boride particles as a result of the favorable conditions for nucleation during the phase change. Both have an impact in the direction of higher material strength.

Furthermore, the conditioning of the starting material produced by powder metallurgy can also be carried out by isothermal shaping with the aim of recrystallizing the structure and setting a fine-grained structure as a prerequisite for the superplastic state. The isothermal deformation takes place at temperatures below the transformation temperature, for example at 450 ^° C., preferably with a low degree of deformation, for example with a cross-sectional decrease of about 10%, and should include a cyclic γ / α phase transformation which, owing to the different volumes of the γ and α phase to internal tensions and thus to internal internal tensions induced deformation of the matrix grain. This can be followed by a short primary recrystallization annealing, for example 20 to 60 seconds, to refine the matrix grain size of the hot isostatically pressed blank, which leads to a further grain refinement.

Overall, the aim of the conditioning of the starting material is to establish a structure which is equiaxial for superplastic shaping in the second process stage and which is characterized by a fine structure grain which favors the forming behavior. With decreasing grain size, the resistance to deformation is reduced and, at the same time, the rate of deformation can be increased.

ergibt, in der s die Fließspannung, K eine Materialkonstante und ė die Verformungs- bzw. Kriechgeschwindigkeit für Stahllegierungen von 0,4 bis 0,5 und für Stellite von 0,35 bis 0,4 ist. Daraus ergibt sich, daß die Formgebung sehr geringer Fließspannungen bzw. Umformungskräfte bedarf; da sie zudem bei verhältnismäßig niedrigen Temperaturen stattfindet, zeichnet sich das erfindungsgemäße Verfahren, insbesondere wenn das Konditionieren in der ersten Verfahrensstufe durch isothermes Verformen unterhalb der Umwandlungstemperatur stattfindet, durch geringe Kosten sowohl unter dem Aspekt des apparativen Aufwandes als auch hinsichtlich des Energieverbrauchs aus.In the second stage of the process, the formed material, which is adjusted to a specific multiphase structure, is shaped at a temperature in the order of 50 to 70% of the melting temperature of, for example, 650 to 780 ^o C, which allows high degrees of deformation at low flow stresses and therefore also manufacture Complicated finished parts made of alloys, the composition of which does not allow shaping by forming without the special pretreatment of the first stage of the method according to the invention. The forming speed is preferably 10⁻³ to 5.10⁻¹ s⁻¹. The expansion speed exponent m, as it results from the equation

$\text{s = K.}{\dot{\text{e}}}^{\text{m}}\text{,}$

results in s the yield stress, K a material constant and ė the rate of deformation or creep for Steel alloys from 0.4 to 0.5 and for stellites from 0.35 to 0.4. It follows that the shaping requires very low yield stresses or forming forces; since it also takes place at relatively low temperatures, the process according to the invention, particularly if the conditioning in the first process stage takes place by isothermal shaping below the transition temperature, is characterized by low costs both in terms of the outlay on equipment and in terms of energy consumption.

The forming temperature is below the temperature of the beginning secondary crystallization or grain coarsening, since each grain growth increases the resistance to deformation and therefore requires higher deformation forces.

The method according to the invention is particularly suitable for high-carbon cold work steels such as
X 178 Cr V 5 2 9
X 155 Cr VW Co 4 5 12 5
X 135 Cr VW Mo 4 4 6 4
X 220 Cr V 17 6
X 245 Cr V 5 10
These have carbon contents from 1.0 to 2.5% and high alloy contents of chromium, vanadium, tungsten, molybdenum and cobalt from 4 to 17%.

The following alloys are also suitable:
X 375 Cr Mo Fe 25 10 60
X 220 Cr W Co 30 12 56
X 120 Cr Mo Co 27 4 60
X 100 Cr W Co NB 15 15 52 3.

The Stellite are iron and cobalt-based stellite with high boron and carbon contents of 1 to 4%, and contents of the alloying elements chromium, molybdenum, tungsten, 15 to 30%, which can be transformed at a relatively low temperature of 650-720 ^o C.

The superplastic shape can be followed by coarse grain annealing in order to increase the creep resistance or heat resistance.

Fig. 1

die Seitenansicht einer Ronde zum Herstellen eines Rotationsmessers, teilweise im Schnitt und

Fig. 2

das aus der Ronde der Fig. 1 durch superplastische Formgebung hergestellte Rotationsmesser teilweise im Schnitt.

The invention is explained below with reference to an embodiment shown in the drawing. The drawing shows:

Fig. 1

the side view of a circular blank for producing a rotary knife, partly in section and

Fig. 2

the rotary knife manufactured from the blank of FIG. 1 by superplastic shaping, partly in section.

The round blank 1 shown in FIG. 1 consists of the high-strength cold work steel X 245 Cr V 5 10, which was produced by powder metallurgy by hot isostatic pressing and was set to a structure with a matrix grain size of 1 to 3 μm. It serves to manufacture the disk-shaped rotary knife shown in FIG. 2 with a cone angle α of 150 to 160 ^° , a thickness of 1.0 to 1.5 mm and an inner diameter of 50 mm and an outer diameter of 100 mm.

The circular blank 1 was produced by punching from a powder metallurgy and then rolled out at a temperature of 1150 to 1250 ^° C. to a thickness of 2.5 mm and measuring 100 × 200 × 8 mm. In order to create a sufficient material reserve for the formation of the cutting edges 2 of the rotary knife, the thickness of the board exceeded the finished thickness of the rotary knife by 1 mm.

ergibt, in der A_o die Kreisringfläche der Ronde 1,ΔA die Kegelmantelfläche A, verringert um die Fläche A_o des Schlitzprofils ε und ε̇ = 5 . 10⁻³ s⁻¹ ist.The blank 1 had a diameter of 95 mm and a thickness of 2.5 mm, it was heated after punching to a temperature of 760 ^o C and with the help of a conventional tool preheated to 350 ^o C from the upper and lower die with a Forming speed of 5.10⁻³ s⁻¹ in a pressing time of 25 s to the rotary knife shown in Figure 2, as can be seen from the equation

in which A _{o is} the circular ring area of the round blank 1, ΔA is the conical surface area A, reduced by the area A _{o of} the slot profile ε and ε̇ = 5. Is 10⁻³ s⁻¹.

The low forming temperature saves energy, ensures minimal scaling and prevents harmful grain growth. In addition, superplastic forming results in a higher density because pores and cracks weld, as well as higher strength and toughness. Because there is no machining, there are no fatigue cracks in the machining grooves, which increases the tool life by 25 to 30%.

Experimentally, in good agreement with the calculated value, a forming time of 25 s was found. If you add a delivery time for the tool of 35 s, this results in a production time of 60 s for each rotary knife, which is far less than the machining time for machining a blank.

The method according to the invention is suitable for producing cut bells and tools, shape cutting tools, knives, for example disc, filter and tobacco knives with a thickness of less than 3 mm, embossing dies, jam and pressure rings for extruders, sintering compression tools, extrusion tools and dies, molding tools for the wobble extrusion and multi-hole plates each made of cold work steels, for the production of profile milling cutters, form turning steels and profile countersunk heads from high speed steels as well as for the production of glass blow molding tools, profile rods, nozzles, impellers, turbine disks and valve seats made of stellites. It is characterized by low forming temperatures and a low power requirement. The finely dispersed, equiaxial and texture-free microstructure guarantees constant and reproducible mechanical properties, in particular high strength with excellent ductility and good fatigue behavior. The dimensional accuracy and surface quality are so good that reworking is not necessary. The surface roughness is usually less than 1 µm.

Claims (16)

1. A process for producing steel tools in which a powder metallurgically produced starting body is hot consolidated, characterised in that a starting body of a high alloy steel having an equiaxed structure and more than 30% by volume of precipitated carbidic and/or boridic phase is hot worked beneath A₁ with a cyclic alpha/gamma phase transformation or in the austenitic state to bring it to have a matrix with a grain size of 1 to 3 µm and a precipitated phase with a grain size of 0.2 to 1.0 µm and then finish formed superplastically.
2. A process for producing tools in which a powder metallurgically produced starting body is hot consolidated, characterised in that a stellitic starting body having an equiaxed structure and more than 30% by volume of precipitated carbidic and/or boridic phase is hot worked at 700 to 1000°C to give a matrix with a grain size of 1 to 3 µm and a precipitated phase with a grain size of 0.2 to 3 µm, and then finish formed superplastically.
3. A process according to claim 1 or claim 2, characterised in that a starting material with an equiaxed structure is superplastically formed to final size.
4. A process according to one or more of claims 1 to 3, characterised in that powder metallurgical tool steels and stellites are superplastically formed at temperatures of about 0.5 to 0.7 Tm and then continuously cooled.
5. A process according to claim 4, characterised by a working temperature of from 900° to 650 °C.
6. A process according to one of claims 1 to 3, characterised in that a stellitic starting material is hot worked in the course of continuous cooling from 1000° to 760°C.
7. A process according to one of claims 4 to 6, characterised in that the degree of deformation exceeds 30% and the elongation amounts to some hundreds percent.
8. A process according to one of claims 1 to 3, characterised in that powder metallurgically produced tool steels are isothermally and superplastically formed below their transformation temperature.
9. A process according to claim 8, characterised in that the degree of deformation is up to 800%.
10. A process according to claim 8 or claim 9, characterised by grain boundary slip and dynamic recrystallisation at 600 to 700 C.
11. A process according to one of claims 1 to 10, characterised by superplastic shaping at a temperature below the temperature of the secondary recrystallisation and grain growth.
12. A process according to one of claims 8 to 11, characterised by superplastic shaping of alloy steels at 650° to 780°C.
13. A process according to claim 11 or claim 12, characterized in that the rate of deformation ε̇ = 10⁻³ to 10⁻¹s⁻¹.
14. A process according to one or more of claims 11 to 13, characterised in that the elongation rate exponent m = 0.4 to 0.5.
15. A process according to one or more of claims 11 to 13, characterised in that for stellites the elongation rate exponent m = from 0.35 to 0.4.
16. A process according to one or more of claims 11 to 15, characterised in that the shaped article is subjected to grain coarsening annealing.

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