CH629845A5 - High-temperature lubricant - Google Patents

Description

The invention relates to a high-temperature lubricant for the hot forming of metallic materials based on boron compounds.

The invention further relates to the use of the lubricant.

High temperature lubricants are generally used in the hot forming of metals and alloys using metallic tools. Its task is to prevent the workpiece from seizing or welding with the tool under the prevailing pressure and temperature conditions. Graphite-containing lubricants are known, for example, from metal extrusion. At very high temperatures and long-lasting deformation processes, such as isothermal drop forging, such lubricants generally fail because completely anhydrous graphite has insufficient sliding properties. The isothermal high-temperature shaping processes workpiece and tool have practically the same temperature, the mutual tendency to seizure increases, and the lubricant to be used is therefore subject to far more stringent requirements than is the case with conventional extrusion, hot drawing and forging. That is why lubricants based on oils, soaps, graphite or molybdenum sulfide are excluded. Attempts have already been made to apply lubricant films to the tool surface or to that of the blank to be deformed using boron nitride, mixtures of the former with boric acid and graphite and with glass-like substances. These substances were mostly used as suspensions or pastes in organic solvents such as alcohol or toluene or as slurries in other liquids. The metal bodies are coated with the lubricant by immersion, brushing or spraying, which is followed by a drying process at room temperature or by baking at an elevated temperature. Such lubricants and processes are known from various publications (for example T. Wat-mough, “Development of isothermal forging of titanium centrifugai compressor impeller”, AMMRC Report CTR 73-19, IIT Research Institute Chicago, May 1973, p. 16; US Pat. Pat. 3 635 068; "Isothermal forging of precision metal powder components", report AD-780 044 published by NTIS, National Technical Information Service, US Department of Commerce, December 1973, pp. 27, 41, 43).

The known lubricants and lubricant combinations as well as the corresponding processes only meet the conditions imposed on them with regard to adhesive strength, temperature and pressure resistance, chemical inactivity towards the metal surfaces to be protected and lubricated as well as lubricity and optimal viscosity in a very inadequate manner. This is clear

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i.a. from the aforementioned publications (see above: NTIS report, pp. 43, 50, 54, 58; also DJ Abson, FJ Gurney, "Heated dies for forging and friction studies on a modified hydraulic forge press", Metals and Materials, December 1973, p. 539). In addition, the known methods of applying the lubricant film on the interested metal surface are cumbersome, complex and time-consuming and do not allow for economical production of the forgings. The active constituents of conventional lubricants suspended in liquids must be applied wet to the workpiece or tool, the latter usually being at room temperature or in any case at a temperature which is considerably below the working temperature. The liquid medium must be given the opportunity to dry or evaporate. On the other hand, if the lubricant, e.g. Boron nitride is applied in solid form by spraying a powder, so its adhesive strength on the metal surface leaves something to be desired. In addition, this method makes it difficult to produce dense, coherent coatings. Glass-like lubricants usually have too high a viscosity, particularly in the case of cooled tools, which make it more difficult to eject the workpiece from the tool (die, die) at the end of the shaping process. Some glasses also show signs of decomposition at high working temperatures, which greatly affects their lubricity. Another problem is the inadequate filling of critical work surfaces such as corners and edges with a small radius of curvature, so that there is a risk of seizure due to poor lubricant wetting.

The invention has for its object to provide a high-temperature lubricant which has good wettability and adhesion compared to the tool, closes the pores of the tool surface and forms a coherent, chemically stable at the prevailing working temperatures, irreplaceable film. The lubricant should also be able to be applied directly to the tool or workpiece surface brought to the working temperature, so that continuous operation is made possible. In particular, the lubricant is said to be suitable for long-term production processes for hot forming, such as isothermal forging, high temperatures and pressures being practically permanently tolerable, without interruption and intercooling, without loss of lubricity. After all, the lubricant should not undergo any chemical reactions with the tool or the workpiece.

This is achieved according to the invention in that, in the high-temperature lubricant defined at the outset, the components of the lubricant which are in powder form at normal temperature contain at least one further oxygen-containing boron compound in addition to boron nitride, and in that the lubricant also has a glass-like viscous constitution at operating temperature.

According to the invention, this high-temperature lubricant is used to produce coatings on the surfaces of metallic tools and / or workpieces in such a way that the components of the lubricant, which are in powder form at normal temperature, are sprayed or sprayed onto the surface heated to at least 500 ° C. of the tool and / or workpiece are applied, producing a coherent glass-like, viscous coating which adheres firmly to the surface and contains a liquid phase.

The guiding principle for the lubricant according to the invention and its use is to combine the good high-temperature lubricating properties of boron nitride with the good adhesive strength and wettability of viscous boron-containing compounds which are glassy at operating temperature in such a way that in a direct way, i.e. a coherent, lubricious layer is produced on the interested metal surface without additional process steps.

Further details of the invention result from the exemplary embodiments explained in more detail below in part by figures.

It shows:

1 shows a cross section through the surface layer of a high-temperature tool,

2 shows the schematic longitudinal section through a forging press and the application of the high-temperature lubricant film,

3 the state of the work equipment and the workpiece at the beginning of the forging process,

Fig. 4 shows the state of the work equipment and the workpiece at the end of the forging process and

Fig. 5 shows the final phase of the operation with the discharge of the finished product.

1 shows a cross section through the surface layer of a high-temperature tool, for example a forging die. 1 designates the material of the tool, which is preferably a molybdenum alloy (TZM) or a nickel-based alloy. The metallic tool surface 2 in question has the unevenness and pores 3 resulting from the mechanical processing. The surface layer containing the high-temperature lubricant is essentially composed of two phases. 4 represents the coherent, glass-like coating which adheres firmly to the metallic tool surface 2 and in its pores 3 and is viscous at working temperature. This coating can consist of boron oxide glass (B2O3), borosilicate glass or borax (Na2B407 ■ n H2O). This viscous phase can also be applied to the metallic tool surface 2 in the form of boric acid (H3BO3),

however, at the existing working temperatures of several 100 ° C there is generally a complete dehydration up to B2O3. In the case of borosilicate glass, the viscous phase preferably has the following composition:

8-60% SÌO2 2-14% Na20 max. 4% CaO 30-90% B2O3 max. 2% AI2O3.

The reference numeral 5 represents the boron nitride particles embedded in the viscous phase. The ratio of the boron nitride content to the viscous phase content can be 5 to 95 percent by weight to 95 to 5 percent by weight. A preferred mixture consists of approximately equal parts by weight of both phases, i.e. for example 50% BN and 50% B2O3.

The following exemplary embodiments are based on the isothermal forging of an object (e.g. compressor wheel) made of a titanium alloy (e.g. Ti-6 A1-4V). Of course, other materials to be deformed at high temperature, such as nickel and cobalt alloys and steels, can also be converted into the end product in one operation, preferably using the same process. The use of the lubricant and the application of the process is also not limited to isothermal forging, but can in any case

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extend where materials are to be deformed at elevated temperatures. This also applies to presses, extrusion, extrusion, hot pressing, hot drawing and hot pressing, as well as conventional forging.

example 1

2 shows the schematic longitudinal section through a forging press and the application of the high-temperature lubricant film. The force-generating and force-transmitting elements of the press, such as the press cylinder, press table, etc., are not shown in this figure. The parts of interest here are essentially the lower die part 7, the upper die part 8, the ejector 9 and the induction heating winding 14 for heating the tool and the workpiece. The entire device is located in a gas-tight protective gas vessel 11, into which a protective gas 10 (e.g. argon) is introduced during the workflow. The vessel 11, which preferably has a cylindrical shape, is provided with a lateral flange 12 with a vacuum feedthrough, in which a lubricant spray system 15 can be moved both in the longitudinal direction and rotatable about its own longitudinal axis (indicated by arrows). Due to high pressure, the lubricant powder 16 reaches the tool surface, where it forms a coherent high-temperature lubricant film 17. The workpiece blank 'S can be moved transversely to the longitudinal axis of the press by means of feed tongs 18 via the feed and discharge tube 13 which is also flanged to the vessel 11.

In the present example, the lower die part 7 and the upper die part 8 were heated to a temperature of at least 800 ° C. by means of the induction heating winding 14. The lubricant spraying system 15 was then run in between the die halves (7, 8) and the lubricant powder 16 consisting of 50 percent by weight boron nitride (BN) and 50% boric acid (H3BO3) was atomized uniformly on the metallic tool surfaces 2 (FIG. 1) applied so that a coherent, firmly adhering, viscous lubricant film 17 was formed. The protective gas 10, in this case argon, was used as the pressure-generating, atomizing medium. This ensures a constantly renewing atmosphere of protective gas 10 which prevents the oxidation of the tool and the workpiece. After the application of the lubricant was finished, a workpiece blank 6 made of a titanium alloy (Ti-6 A1-4V) was introduced into the vessel 11 via the feed and discharge pipe 13 by means of the feed tongs 18. The workpiece blank 6 can be at room temperature or preheated to a temperature below the forging temperature. Whether the preheating of the workpiece blank 6 is carried out or not is irrelevant to the process and is merely a question of economy.

Fig. 3 shows the state of the work equipment and the workpiece at the beginning of the forging process. The workpiece blank 6 is located between the approximate die halves (7, 8), while the lubricant spraying system 15 and the feed tongs 18 are located outside the profile of the forging die. The latter are not shown in this figure for the sake of clarity.

In this example, the workpiece blank 6 was centered between the lower die part 7 and the upper die part 8 and the workpiece and die halves were brought to a temperature of 900 to 950 ° C. The isothermal forging process was then carried out while keeping the temperature constant.

As an alternative method step, the workpiece blank 6 can also be coated with a lubricant before being introduced into the forging press. The same mixture of 50 percent by weight boron nitride and 50 percent by weight boric acid mentioned above can be used for this purpose, which is applied to the previously preheated workpiece blank 6 by spraying or spraying. However, a boron nitride layer can also be applied to the workpiece blank 6 at room temperature. Applying the high-temperature lubricant to both the tool and the blank has advantages, particularly when shaping complicated workpieces with deep grooves, narrow ribs and recessed corners, since this is the safest way to avoid seizing during the subsequent flow process under the press.

4 shows the state of the work equipment and the workpiece at the end of the forging process. Due to the upward movement of the press table, the two die halves (7, 8) have come to rest against each other and the former workpiece blank completely fills the cavity between the lower die part 7 and the upper die part 8. The forged workpiece 20 has reached its final shape.

Fig. 5 shows the final phase of the operation with the discharge of the finished product. The ejector 9 movably arranged in the upper die part 8 is in the lower end position and the forged workpiece 20 lies on the discharge device 19.

In the present example, the lower die part 7 resting on the press table (not shown) was first lowered vertically, so that the discharge device 19 could be pushed through the feed and discharge tube 13 between the separate die halves. The ejector 9 was then actuated and the forged workpiece 20 was ejected downward, so that it came to rest on the discharge device 19. Subsequently, the workpiece 20 was removed from the press by means of the latter via the feed and discharge tube 13 and a protective gas lock (not further specified) and conveyed outside. In this way, the protective gas atmosphere around the die halves (7,8) is preserved. After this final phase of the work process has ended, a new cycle can begin (see FIG. 2).

Example 2

A workpiece blank 6 consisting of a titanium alloy was forged isothermally in the manner specified in Example 1. Here, however, the high-temperature lubricant powder 16 consisted of a mixture of 70 percent by weight boron nitride (BN) and 30 percent by weight boric acid (H3 BO3). The higher boron nitride content reduces the adhesion of the lubricant layer, which is an advantage for certain workpiece shapes in which the finished workpiece is difficult to eject from the die.

Example 3

The die halves (7,8), preheated to at least 800 ° C, were first sprayed with pure boric acid powder (H3BO3) ) filled out and adhered well to the latter. Then additional boron nitride powder (BN) was sprayed onto the die halves provided with this B203 coating, the BN particles being wetted and held in place by the liquid B203 film. This method is particularly suitable for complicated die shapes with narrow, deep grooves and protruding edges and corners, as well as for multi-section dies and in all cases where die material is used

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surface and surface expect poor wettability.

Example 4

A coating of borosilicate glass was first produced on the surfaces of the preheated die halves (7, 8) in an analogous manner to that in Example 3, and the boron nitride powder was then sprayed onto it. The borosilicate had the following composition:

20% SÌO2 2% Na: 0 77% B2O3 1% AI2O3

Of course, a borosilicate glass of a different composition can also be selected, the latter being able to be adapted to the working conditions such as pressure, temperature, rate of deformation and type of material to be deformed. Preferred compositions of the borosilicate glasses for producing the viscous phase of the high-temperature lubricant film are the boron-rich mixtures, which are in the following range:

20-40% SiOz 70-80% B2O3 max. 4% Na20 max. 2% AI2O3

The aim should be a composition that has a dynamic toughness of approx. 104 poises in the temperature range of 700 to 1000 ° C.

The method is not limited to the examples mentioned above. In particular, the lubricant coating can be applied in stages to both the tool and the workpiece, in all cases by first forming a liquid, viscous film from a substance containing the boron in oxidic form, in which boron nitride particles are subsequently incorporated. This process can also be repeated, so that layers containing boron nitride or boron oxide, silicate etc. are alternately formed.

The high-temperature lubricant can generally be used advantageously in the production of a film with good sliding, lubricating and electrical insulating properties on the metallic surface of a tool or workpiece. This is especially true for all those cases where absolute water freedom with high working temperatures629845

temperatures to avoid undesirable chemical reactions with metallic materials.

The high-temperature lubricants according to the invention and the corresponding processes have given materials and methods which meet the mechanical-thermal requirements in the hot forming of metallic materials to a high degree even at very high working temperatures and long-lasting shaping processes. This applies particularly to deformation processes at constant temperature in the superplastic area of the material, such as isothermal forging. Thanks to the presence of a liquid phase, the lubricant is characterized by good wettability and adherence to the metallic surface and can be continuously integrated into the work process without intermediate cooling of the tool or workpiece. This allows uninterrupted economical production even at working temperatures in the range of 900 to 1200 ° C, which in particular affects the hot forming of titanium, nickel and iron alloys in a favorable, cost-saving manner.

List of designations of Figures 1-5

1 =

Tool material (TZM, Ni-based alloy)

2 =

Metallic tool surface

3 =

Pores in the tool surface

4 =

Glassy, viscous coating (B203 gas,

Borosilicate glass)

5 =

Embedded boron nitride particles

6 =

Blank workpiece

7 =

Lower part of the die

8 =

Die top

9 =

Ejector

10 =

Shielding gas

11 =

Inert gas container

12 =

flange

13 =

Feed and discharge pipe

14 =

Induction heating winding

15 =

Lubricant spraying system

16 =

Lubricant powder

17 =

High temperature lubricant film

18 =

Feeding tongs

19 =

Discharge device

20 =

Forged workpiece

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B

2 sheets of drawings

Claims (14)

629 845 2nd PATENT CLAIMS 1. High-temperature lubricant for the hot deformation of metallic materials based on boron compounds, characterized in that the components of the lubricant present at normal temperature in powder form contain, in addition to boron nitride, at least one further boron compound containing oxygen, and that the lubricant is also a glass-like one at operating temperature , has a viscous constitution. 2. High-temperature lubricant according to claim 1, characterized in that it consists of a mixture of boron nitride and boron oxide powder in a ratio of 5 wt .-% to 95 wt .-% o BN to 95 wt .-% to 5 wt .-% B2O3 exists. 3. High-temperature lubricant according to claim 1, characterized in that it consists of a mixture of boron nitride and boric acid powder in a ratio of 5 wt .-% to 95 wt .-% BN to 95 wt .-% to 5 wt .-% H3BO3 consists. 4. High-temperature lubricant according to claim I, characterized in that it consists of a mixture of boron nitride and borosilicate glass powder in a ratio of 5% by weight to 95% by weight BN to 95% by weight to 5% by weight. % Borosilicate. 5. High-temperature lubricant according to claim 4, characterized in that the borosilicate glass has the composition 8-60 wt% SÌO2 2-14 wt% NaiO max. 4% by weight CaO 30-90% by weight B2O3 max. 2 wt% Al2O3 Has. 6. High-temperature lubricant according to claim 1, characterized in that it consists of a mixture of boron nitride and borax powder in a ratio of 5% by weight to 95% by weight BN to 95% by weight to 5% by weight Na2B407 consists. 7. Use of the high-temperature lubricant according to claim 1 for the production of coatings on the surfaces of metallic tools and / or workpieces, characterized in that the components of the lubricant present at normal temperature in powder form are heated to at least 500 ° C. by spraying or spraying Surface (2) of the metallic tool (1, 7, 8) and / or workpiece (6) are applied, a coherent glass-like, viscous coating (4) containing a liquid phase adhering to the surface being produced. 8. Use according to claim 7, characterized in that the high-temperature lubricant is applied in two stages to the surface of the metallic tool and / or workpiece, in such a way that initially with the liquid phase-forming component by spraying / spraying on the heated surface a firmly adhering, coherent glass-like, viscous coating is produced, onto which boron nitride powder is subsequently sprayed. 9. Use according to claim 7, characterized in that the glass-like, viscous coating is produced essentially by boron oxide or by decomposing boric acid and converting the latter into boron oxide. 10. Use according to claim 7, characterized in that the glass-like, viscous coating is formed essentially by borosilicate, the melting temperature and viscosity at working temperature being adjusted by changing their composition to the working conditions such as pressure, temperature, rate of deformation and type of material to be deformed . 11. Use according to claim 7, characterized in that the glass-like, viscous coating is formed essentially by borax. 12. Use according to claim 7 for the production of a film with good sliding, lubricating and / or electrical insulating properties on the surface of a metallic tool and / or workpiece. 13. Use according to claim 12 in hot extrusion, hot extrusion, hot pressing, hot drawing, hot pressing and hot forging. 14. Use according to claim 12 in the isothermal forging of nickel, titanium or iron alloys in undivided or multiply divided die halves.