GB2181745A

GB2181745A - Hot-deformed powder metallurgy articles

Info

Publication number: GB2181745A
Application number: GB08620692A
Authority: GB
Inventors: Christer Aslund; Jean-Claude Prouheze
Original assignee: Avesta Nyby Powder AB
Current assignee: Avesta Nyby Powder AB
Priority date: 1985-08-28
Filing date: 1986-08-27
Publication date: 1987-04-29
Anticipated expiration: 2006-08-27
Also published as: IT8621534A1; JPS6296604A; DE3530741C1; IT1197122B; GB2181745B; GB8620692D0; SE8603563L; SE8603563D0; IT8621534A0

Abstract

Process for the production of powder-metallurgy articles, a powder of a metal and/or metal alloys being filled into a thin-walled cartridge or capsule or the like, whereafter the same is sealed and hot-isostatically pressed to produce a blank which is then hot-shaped, more particularly by forging or rolling, into the end product, is characterised in that to obviate cracking in the hot-shaped end product, the powder-filled capsule or cartridge is pre-pressed with an increase in temperature and pressure to predetermined values without the predetermined temperature and pressure values being maintained, and then hot-shaped into the end product. To this end, the blank was heated during hot deformation to an even higher temperature e.g. above 1200 DEG C. Hot-isostatic compression is so performed that the blank has a density of approximately 90 to 98%, more particularly approximately 95 to 98%, of the theoretical density. In an Example a blank was heated to 1100 DEG C while the pressure was increased to 700 bar. Without dwell- i.e.without long remaining at this temperature and pressure the blank was then heated in a conventional oven to a temperature of approximately 1200 DEG C, and then rolled in this state in a pilgrim state rolling mill into a tube.

Description

SPECIFICATION A processforthe production of powder-metallurgy articles Description The invention relates two a process for the production of powder-metal articles as set out in the premable of claim 1.

It is known and is increasingly occurring in practice for powder consisting of metal and/or metal alloys to be pressed isostatically to form compact blanks which are then extruded to a required final shape. To this end, metal and/or metal alloy powder is filled into thin-walled metal capsules or cartridges orthe like of the requi red shape, whereafter the capsule orthe like is sealed, then pressed isostatically either cold or hot. Preferably, the capsule is filled with simultaneous vibration to improve the uniformity of powder distribution in the capsule. The sealed capsule is then hot processed, for example, by extrusion, into bar or tube sections.

Conventionally, the blank is cooled afterthe hot-isostatic pre-compacting (DE-OS 2 552 285). Using the process described, endeavours have been made in hot-isostatic pressing to produce large-volume blanks which are given subsequent rolling orforging to produce corresponding rolled orforged products. To this end, the powder-fil led capsules were pressed insostatically at relatively high temperatures and pressures maintained for a relatively long time, for instance, 3 hours. This led to substantiallytotal compacting of the powder in the capsule. However, the proposed process is very expensive as regards technology of the process and of the apparatus. The high temperatures and pressures require correspondingly dimensioned equipment.Also, it was found that despite this expenditure a very high percentage of the blanks thus produced were rejected in the subsequent hot shaping. Many of the hot-processed end products had unsatisfactory cracks, more part- icularly when the starting material used was powder of metals and/or metal alloys which could be hotprocessed with difficulty or, conventionally, not at all. Consequently, despite the relatively long treatment times it is found to be very difficult to produce in the manner described blanks,for example blocks, which could be processed without cracking by heat shaping into end products such as bars or plates or sheets or the like.This is probably explicable to some extent by the long treatment times at high temperatures and relatively high pressures leading to segregations in the grains of powder, with the resu It of hot or working cracking. In endeavours to obviate these disadvantages,the hot-isostatic pressing was carried out at such high pressures and temperatures asto produce a powder or blank density of substantially 100%, correspond- ing to the theoretical density. However, even these steps did not always lead to the required result - i.e.,the production of crack-free hot-processed products.

It is the object of this invention to provide a process of the kind specified in which blanks can be processed into end products readily- i.e., without cracking - by heat shaping.

The operative features of claim 1 solve this problem in a surprisingly simple way.

The solution ofthe problem which the invention provides is a departure from the conventional procedure of producing blanks having substantially 100% density - i.e., the theoretical density. Consequently, in accord ance with the invention working can proceed at lower temperatures and pressures and more particularly without relatively long dwells at elevated temperature and elevated pressure. The process according to the invention enables powder of metals and/or metal alloys which can be hot processed with difficulty or, conventionally, not at all to be hot-shaped into crack-free end products. The omission of a long dwell time at elevated temperature and pressure helps to reduce process costs considerably as compared with the earlier procedure.According to the invention, a blank density of appreciably less than 100% - i.e., the theoretical density - preferably a maximum of approximately 90 to 98% ofthe theoretical density, is aimed for.

The following examples will make clearly apparent the advantages of the invention as compared with the earlier procedure.

Example 1 Powderoflnconel 625 alloy (the trade markofthe Inco company) was used to prepare a blank having the following dimensions: Outside diameter: 410 mm Inside diameter: 225 mm Length: 1600 mm.

Aiso, an initial blank having the following dimensions: Outside diameter; 460 mm Inside diameter: 260 mm Length: 1800 mm was used. The initial blank was heated to a temperature of 1 1 OO"C while pressure was increased up to 700 bar.

Without dwell - i.e., without long remaining at this temperature and pressure -the blankwas then heated in a conventional oven to a temperature of approximately 1 2000C, then rolled in this state in a pilgrim step rolling mill intoatube having the following final dimensions: Outside diameter: 250 mm Inside diameter: 210 mm.

The density of the hot-isostatically pre-pressed blankthus produced was approximately 96% of the theoretical density.

The rolling out of the blan kto form the end product caused no problems. The end productwas free from cracks.

Byway of contrast, an initial blank having the same dimensions and made of the same material was heated to a temperature of approximately 1200with an increase in the external pressureto approximately 1050 bar. This temperature and pressure were held for approximately 3 hours (holding or dwell time). The blank thus hot-isostatically compressed and an approximately 100% density. The blank was then rolled out to form a tube in a pilgrim step rolling mill in the same conditions as herein before set out. The finished tube had micro-cracks making it unserviceable.

Example 2 A blank made of Hastalloy B2 (trade mark of the Cubat Corporation) having the following dimensions: Length: 1000 mm Width: 715 mum Height: 50mm was heated to a temperature of 1 1 50"C with a rise in pressure to 650 bar. After the hot-isostatispre-pressing the density ofthe blankwas approximately 95% ofthe theoretical density. The blankwasthen heated to 1225"C in a conventional pusherfurnacevirtuallywithout any dwell atthetemperature and pressure men tioned,then hot rolled to form plates 22 mm thick. Therewere no problems in the rolling, which it was possible to carry outwithout reheating.The end product was free from cracks, more particularly edge cracks.

Asimilar blank made ofthe same powder material and having the same initial dimension was hot isostatically pressed at a pressure 105 bar and at a temperature of 1175 C, remaining at this temperature and pressure for approximately 3 hours. The blank was non-porous and had a density of 100%. The blank was then rolled outto form a plate in the same manner as just described. However, it could not be rolled directly to finished size and required reheatings to obviate edge cracking. Also, rolling pressure had to be reduced by approximately 20% in the first phases of rolling. The result was unsatisfactory as regards both the end product and the process.

Claims

1. A processforthe production of powder-metallurgy articles, a powder of a metal and/or metal alloys being filled into a thin-walled cartridge or capsule or the like,whereafterthe same is sealed and hot isostatically pressed to produce a blank which is then hot-shaped, more particularly by forging or rolling, into the end product, characterised in that to produce the blankthe powder-filled cartridge or capsule or the like is pre-pressed isostatically with an increase in temperature and pressure to predetermined values and, without the predetermined temperatures and pressures being maintained, the resulting blank is hot deformed into an end product of substantially theoretical density, more particularly by heating to a higher hot-working tem perature which towards the end ofthehot-deformation step may possibly be reduced to a temperature below the temperature of the isostatic pressing.

2. A process according to claim 1, characterised in that the powderfilled into the capsule is compressed in the hot-isostatic pre-pressing to a density of from approximately 90 to 98%, preferably 95 to 97%, of the theoretical density.

3. A process according to claim 1 or2, characterised in that powder of metals and/or metal alloys of metals which can be hot-worked with difficulty or conventionally not at all, more particularly powder of high-alloy steels such as Inconel 625 or Hastalloy B2, is used.

4. A process according to any of claims 1 - 3, characterised in that the capsule is hot-isostatically prepressed with an increase of the temperature to approximately 1000 to 1200 C, more particularly 1100 to 1 1 50"C, and with an increase of pressure to approximately 600 to 100 bar, more particularly 650 to 700 bar.

5. A process according to any of claims 1 - 4, characterised in that immediately after the hot isostatic pre-pressing the blank is forged or hot rolled, more particularly in a pilgrim step rolling mill.

6. A process according to any of claims 1 to 5, characterised in that the blank is heated for hot deformation to a temperature of above approximately 1 200"C.

7. A process according to claim 1 and substantially as herein described.

8. A powder metallurgy article produced by the process as claimed in any one ofthe preceding claims.