AU622458B2 - Extruder mandrel and process for its manufacture - Google Patents

Description

COMMONWEALTH OF AUSTRALIA Patent Act 1952 COMPLETE SPECIFICATION

(ORIGINAL)

Class Int. Class Application Number Lodged Complete Specification Lodged Accepted Published or 4 4 4 A

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Priority: 1 July 1988 Related Art Name of Applicant Address of Applicant Actual Inventor Address for Service BOHLER Ges.m.b.H Elisabethatr. 12, Austria A-1010 Wien, 9 4 or q 04 Bruno Hribernik, Johann Stamberger F.B. RICE CO., Patent Attorneys, 28A Montague Street, BALMAIN. 2041.

4c p .4 .4 *4 Complete Specification for the invention entitled: "Extruder mandrel and process for its manufacture" The following statement is a full description of this invention including the best method of performing it known to Us:- -2 2 I The invention refers to an extruder mandrel for the manufacturing of tubes or tube-shaped bodies at higher temperatures as well as to a process to manufacture extruder mandrels for the manufacturing of tubes.

As an example, extruder mandrels have a length of 1 m and a diameter from approx. 40 to 80 mm. For extruder mandrels of this type the material, from which these are manufactured, should have good wear resistant properties at higher temperatues and, inter alia, be resistant to creep and resistant to thermal shocks, so that generally hot-work steel is used. During the production the mandrels manufactured from hot-work steel are hardened at least once and cooled before their austenising temperature annealed and at least once, preferably twice, to achieve S.o 15 the relevant strength and working hardness.

So Such mandrels are exposed to extraordinarily great loads. The to-be-extruded articles have temperatures from 400 to 9000 C, when dealing with non-ferrous alloys, and from 900 to 12500 C when dealing, for example, with S 20 iron-based or nickel-based alloys. At the same time the extruder mandrels receive surface temperatures which may lie above the annealing temperature above 6000 C), due to which the hardness of the mandrel in the region near to the surface will be reduced. At a temperature of 6000 C the normal, at room temperature, hardness of 46 to 51 is reduced to <30 HRC. Thus the friction during the extrusion causes a considerable wear, so that, for ri example, after 20 to 50 extrusions the internal surface of the produced tube is defective or dirty or will not produce the correct dimension anymore.

To improve the tool life of such mandrels it has been tried to coat the surface of the extruder mandrel. For this purpose hard coatings TiN) have been applied with a thickness of 7 pm onto the material of the mandrel. However, this has not proved very effective, as LL4 i_ ii 3 the basic material below the coating became soft and the coating flaked off.

Further it has been tried to weld thick hard material layers of approx. 2 to 10 mm onto the mandrel body.

However, the welding process has affected the zone under the weldings to its detriment; as the alloy of the basic body melted and became brittle as well as it has shown a decrease in hardness due to the influence of the heat, the thick hard material layers chipped off. Moreover, the thickness of the layer was often uneven and the mandrel became bent at high temperatures.

fit C Hard material layers of medium thickness with high C 'cc carbide content, which have been applied with a thickness of approx. 0.2 to 2 mm onto the material of the mandrel 15 by, for example, plasma welding, have also chipped off as they have a thermal expansion coefficient different from that of the basic material. Furthermore, when applying this hard material the surface layer of the basic material '0 is heated to a high temperature, so that a decrease of the 20 hardness of same occurred and the mandrel usually became t 4 distorted during the application of the hard material layer.

An extruder mandrel manufactured from a hard alloy, e.g. from Co hard alloys, has relatively good hardness (33 25 HRC) at high temperatures, like 600° C, for example.

These extruder mandrels have, however, inadequate resistance to creep, low strength (approx. 5 Nm/cm as cast), so that they cannot be easily used for production I processes as the danger of breaking especially in case of cooled mandrels, due to their thermal shock and their strength is inadequate and damage to the machinery may occur.

Nickel-based hard alloys have low heat hardness at high temperatures, like >600° C, for example. In addition, they have low resistance to movement and slight -4strength. These hard alloys cannot be used for production either.

It would be advantageous is to produce an extruder mandrel and a method to manufacture extruder mandrels, which have high hardness, strength and wear resistance even at high surface temperatures above 6000 C and a correspondingly long tool life. Further, the danger of breakage should be minimal and the sensitivity of the .o mandrel to thermal shock should be correspondingly high 00 10 and independent from the various cooling methods of the ro o o mandrel, e.g. internal cooling or spraying of cooling 0 O water. In addition, no bending of the extruder mandrel *0 0*o should occur during temperature changes, as in this case o 00 the manufactured tubes will have different wall thicknesses.

According to the invention an extruder mandrel of the type mentioned in the introduction is characterised in that the extruder mandrel has a mandrel body provided with "o oan external coating, which external coating is connected mo o with the mandrel body by forming a metallic bond layer or is bonded with the mandrel body by a metallic bond.

o0d4 According to the invention a process of the type mentioned in the introduction is characterised in that the 0mandrel body is coated with an external coating by forming a metallic bond.

A preferred embodiment of an extruder mandrel is characterised by that, that the mandrel body is manufactured from a hot-work steel, e.g. DIN material No.

1.2343 or DIN material No. 1.6358, and the mandrel body is coated by forming a metallic bond with an external coating of nickel or cobalt based, preferably cobalt based, alloy, which can be precipitation hardened. An external coating from a cobalt-based alloy is preferred due to the great hardness of this alloy. This external coating is solidly q4; 5 bonded with the mandrel body by forming a metallic bond.

1i I This external coating surrounds the mandrel body at least on its side surfaces.

This external coating can be manufactured preferably by powder metallurgy, e.g. by sintering, injection moulding, hot isostatic pressing, forging, rolling, extruding, plating, explosion plating, etc. of alloy powders. After the treatment the extruder mandrel is heat treated, hardened and tempered, respectively, while the heat treatment is adjusted to suit both materials. At the same time an austenitic process, cooling and tempering of t the mandrel body takes place as well as a solution treatment and precipitation hardening of the external coating. By this process the high hardness and high .strength of the mandrel body is achieved, a particularly 15 hard and wear resistant layer is formed which, however, due to the metallic bond would stand up to the highest demands without flaking taking place or adversely affecting the adhesion to the mandrel body. The extruder mandrel offers high strength and resistance.to thermal •0 o20 shock, while the external coating has a corresponding high hardness and wear resistance and thereby advantageous properties for the use are achieved.

The thus produced extruder mandrel can have any shape; the mandrels may be circular, oval, rectangular, 25 square, polygonal or have any other cross-section.

Important is that the metallic bond layer or the metallic bond between the mandrel body and of the applied external coating should be homogeneous and free, as far as possible, of brittle separations which would diminish the adhesion between the mandrel body and the external coating. The advantageous properties of the extruder mandrel are due, inter alia, to that, that the possible brittle separations in the metallic bond layer do not impair the bond.

It is an advantage if the alloy used for the external i U 1 6 coating has a low or a lower coefficient of thermal conductivity and/or small transformation in comparison with the alloy used for the mandrel body. A low coefficient of thermal conductivity of the external coating is an advantage, as the high temperatures occurring in the external coating during the production of tubes are not or are not completely conveyed into the mandrel body, which in this manner does not lose it hardness, strength and its good properties due to too high 10 temperatures or by annealing. In addition, a low E coefficient of thermal conductivity and/or the lack of rr transformation of the external coating in case of additional precipitation hardening of the external coating of the extruder mandrel is an advantage. At the same time 15 the surface of the external coating of the extruder mandrel can be brought for a short period to precipitation temperature approx 8000 while the mandrel body remains below the annealing temperature. Furthermore, it is an advantage if the material of the external layer has j 20 a higher coefficient of thermal expansion than that of the Sq6 material of the mandrel body. In any case the external SC coating only should be heated up or a temperature transfer from the external coating to the mandrel body should be kept minimal, as a heating up of the mandrel body above 25 5500 C impairs its properties. The surface zone of the external coating is increased by induction, flame heating or the like, if necessary several times, to temperatures in the range from 650 to 8500 C, particularly from 740 to 8200 C, to achieve a precipitation hardening by heating the surface for short periods, however without the increase of the core temperature above 5500 C.

An extruder mandrel constructed in this manner has the advantage, that the surface material precipitation hardens further in service due to the friction and the contact with the hot to-be-extruded alloys. It has been -7shown that as a consequence considerable increase of the hardness of the external coating occurs without reaching a thermal fatique.

The manufacture by powder metallurgy of the external coating can take place especially in the course of hot working, e.g. hot forging, hot rolling etc., with a reduction of the cross-section of the mandrel body.

The thickness of the applied external coating is approx. 1.5 to 25 mm, preferably 4 to 10 mm. It should be borne in mind, that in case of too thin a coating temperatures too high may enter into the core material, t 4 which would consequently become soft. However, when the external coating is too thick, there is the danger of breakage. Further, care should be taken that the eccentricity of the surface coating be a maximum 2.5 mm, S particularly maximum 1 mm, otherwise the extruder mandrel may bend during heating up. It has been shown, however, that mandrels manufactured in accordance with the 0. invention can be easily kept within these tolerances.

Iv 20 Furthermore, it is an advantage if care is taken to form a D 0homogeneous external coating.

It came as a surprise to the expert, that in case of the metallic bond layer between the mandrel body and the external coating one does not deal with a brittle intermediate layer, but with a flexible layer, which compensates even for the various heat expansions of the mandrel body and the external layer. The effect of different thermal expansion coefficients of the materials of the material body and of the external coating could not be observed particularly in case of manufacturing a cylindrical bond or a mandrel with circular cross-section; a chipping off of the external coating or a formation of cracks in the external coating has not been observed.

The external coating may be applied to the material of the mandrel body by known methods of powder -8metallurgy. For example, the mandrel body (on the side and at its face) may be surrounded at a distance by a cylindrical body and the powder of the relevant alloy for the external coating may be brought into the intermediate space. In a following heat treatment operation with reduction of the cross-section the corresponding sintering and compacting of the powder situated between the mandrel body and the pipe takes place, while a homogeneous metallic bond is formed. After the corresponding completion the cylindrical body is removed by turning and the mandrel is subjected to annealing and precipitation hardening, respectively.

it Figs. 1 and 2 show a mandrel with an external coating tr 1 and a mandrel body 2, where the external coating 1 and 15 the mandrel body 2 form between themselves a metallic bonding layer 3 or a metallic bond, which affects the good adhesion of the external coating.

Fig. 2 shows an additional bore 4 for the internal e:ar cooling of the mandrel.

Fig. 3 shows an etch pattern of a mandrel-cylindrical body in cross-section, where the mandrel body 2 is made .o from an alloy according to DIN material No. 1.2344 and the external coating 1 consists of 25% Cr, 5% W, 1% Si, 1.2% C, the remainder Co and impurities determined by the 25 manufacture. The cylindrical body 5 serves to manufacture it according to the powder metallurgy and is removed by turning during the manufacture of the extruder mandrel.

In particular, precipitation hardenable cobalt- or nickel-based alloys like stellite are suitable as materials for the external coating and the mandrels.

When using the extruder mandrels considerably longer tool lives could be established when compared with conventional mandrels.

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Claims (12)

1. 2. An extruder mandrel according to claim i, wherein the metallic bond layer or the metallic bond is homogeneous and free, as far as possible, of brittle separations. S3. An extruder mandrel according to claim 1 or 2, wherein the mandrel body and/or the external coating is formed from precipitation hardenable Ni- or Co-based Ialloys.
2. 4. An extruder mandrel according to any one of claims 1 to 3, wherein the external coating is produced by powder metallurgy.
3. 5. An extruder mandrel according to anyone of claims 1 to 4, wherein the mandrel body is made of hot-work steel, 0 ,which is metallically bonded with the external coating, I the external coating being made from a Co-based alloy.
4. 6. An extruder mandrel according to anyone of claims 1 to 5, wherein the material used for the external coating has a low or a lower coefficient of heat conductivity than the material used for the mandrel body. S7. An extruder mandrel according to anyone of claims 1 to 6, wherein the external coating is produced by powder i metallurgy and is bonded metallically with the mandrel body by sintering, injection moulding, hot isostatic pressing, forging, rolling, extruding, plating, e.g explosion plating.
5. 8. An extruder mandrel according to anyone of claims 1 to 7, wherein the thickness of the external coating is to 70%, preferably 10 to 30% of the diameter of the mandrel body. i 10
6. 9. A process for producing extruder mandrels for tube manufacture, wherein the mandrel bodies of the mandrels are coated with an external coating by forming a metallic bond. A process according to claim 9, wherein to produce the metallic bond, the external coating is applied to the mandrel body by forging, rolling, pressing, sintering, plating or hot isostatic pressing or in the course of an extrusion.
7. 11. A process according to claim 9 or 10, wherein the production of the metallic bond takes place during a cross-section reduction at increased temperature. I 12. A process according to anyone of the claims 9 to 11, wherein the mandrel body and/or the external coating are manufactured from precipitation hardenable Ni- or Co-based alloys.
8. 13. A process according to anyone of the claims 9 to 12, wherein the external coating and/or the mandrel body are "I 2r manufactured by powder metallurgy from corresponding alloy powders.
9. 14. A process according to anyone of the claims 9 to 13, wherein the surface zone of the external coating applied to the mandrel body is subjected to at least one heat treatment of particularly short duration for hardening at 650 to 8500 C, preferably at 740 to 8200 C, which is adjusted so that the temperature of the mandrel body remains below 5500 C. A process according to claim 14, wherein the external coating is subjected to an inductive heating of short duration or to a flame heating of short duration.
10. 16. A process according to anyone of the claims 9 to wherein the mandrel body is forged from hot-work steel and is metallically bonded with an external coating produced by powder metallurgy from a precipitation hardenable S Co-based alloy.
11. 17. An extruder mandrel substantially as hereinbefore described with reference to the accompanying drawings.
12. 18. A process for producing extruder mandrels substantially as hereinbefore described with reference to the embodiments. Dated this 29th day of June 1989 BOHLER Ges.m.b.H. ill $Patent Attorneys for the Applicant t 4 4 ti .4