GB2270324A

GB2270324A - Treatment of y"- hardening nickel-based alloys

Info

Publication number: GB2270324A
Application number: GB9318307A
Authority: GB
Inventors: Helmut Huff; Horst Pillhoefer; Jurgen Wortmann
Original assignee: MTU Motoren und Turbinen Union Muenchen GmbH
Current assignee: MTU Aero Engines GmbH
Priority date: 1992-09-04
Filing date: 1993-09-03
Publication date: 1994-03-09
Anticipated expiration: 2013-09-03
Also published as: GB2270324B; FR2695408A1; FR2695408B1; DE4229599C1; GB9318307D0

Abstract

The invention relates to a process for the heat-treatment of * small Greek gamma *"-hardening nickel-based alloys the normal final machining step is carried out directly after the post solution treatment cooling step with subsequent aging. This has the advantage that the workpiece can be accurately machined but the machining is done before the maximum hardness is reached.

Description

2270324 TREATMENT OF yll-HARDENING NICKEL-BASED ALLOYS The invention

relates to a process for the heat treatment of y"-hardening nickel-based alloys. Known process of this nature involve solution treatment, a cooling step, aging and then a final machining or processing step.

The great strength and heat-stability of y"hardening nickel-based alloys is due principally to an age or precipitation hardening which is achieved by solution treatment (annealing), a cooling step and agehardening or final annealing. The metastable orthorhombic intermetallic y" phase of the composition Ni3Nb is thus formed. Although it brings the advantage of a high tensile strength of over 1400 MPa, this heat treatment has the disadvantage that the material shrinks between 0.02 to 0.14% and therefore components of very precise dimensions have to be machined subsequently. Subsequent machining processing after the heat treatment procedure is over has the disadvantage of high tool wear and a low cutting speed in the case of machining because of the induced hardness of over 40 HRC.

It is the aim of the invention to provide a process for treating articles made of nickel alloys as mentioned above in which the uneconomical cutting speeds and the high degree of tool wear are reduced, with the dimensional accuracy of the finished component being maintained. 30 This aim is achieved by carrying out the machining step directly after the cooling step, the final aging or annealing step being carried out subsequently. The advantage of this is that directly after the cooling step the component of V"-hardening nickel-based alloy has a substantially lower degree of hardness, depending on the cooling rate, than after aging, and hence both the cutting speeds and the tool service life can be increased when using the process of the invention.

it was also found that with increasing cooling rate the hardness directly after the cooling step decreases. Furthermore, each cooling rate can be associated not only a specific reduced degree of hardness but also a specific degree of shrinkage experienced by the component during aging. Therefore, in a preferred variant of the process the final processing step is carried out taking into account a degree of shrinkage in dependence on the cooling rate during the cooling step.

Taking into account the degree of shrinkage, precise final dimensions can be advantageously achieved with the processing step brought forward, with the resulting reduced degree of hardness. During the subsequent aging the high degree of hardness of over 40 HRC is again set up.

Preferably the solution annealing takes place at temperatures between 920 and 9800C. This narrow temperature range has the advantage that constant initial conditions for the subsequent cooling step are set up.

The cooling step preferably takes place at a cooling rate of 50C/min to 100C/min, the workpiece being cooled to room temperature so as to make the subsequent processing step possible. The advantage of this range of cooling rate is that the hardness can be set to between 85 and 50 of the final annealed hardness.

For the interposed final processing this range of cooling rate gives a precise degree of shrinkage of 0.03 to 0.09%, preferably 0.05 to 0.07, which is established during the subsequent hardening procedure.

For the aging or final annealing stage the workpiece is preferably heated from room temperature, or the temperature which is established during the processing step, to a holding temperature from 7000C to 7800C. The workpiece is then kept for 1 to 8 hours at the holding temperature and then cooled at a rate of from 0.5'C/min to 50C/min to less than 65WC. During the aging y" deposits are advantageously formed in the volume of the grain which increase the hardness of the workpiece.

The aging may also preferably take place in two stages. It is carried out at workpiece temperatures of 7200C to 7600C and then at 6200C to 6500C. During the aging procedure the 100 age hardness of over 40 HRC develops, accompanied by the high tensile strength of over 1400 MPa.

The following examples describe preferred methods of carrying out the process, by of illustration of the invention.

Example 1

A thin-walled turbine housing of annular segments of 1000 mm diameter with a dimensional tolerance of less than 0.2 mm with respect to circularity and diameter is to be made from a solid ring of a y" hardening nickelbased alloy of 19 Cr, 3% mo, 5.1% Nb and Ta, 1% Ti, 0.5 Al and the rest Ni. Rings of nickel-based alloy are used as the starting material.

First of all the rings are solution annealed at 9500C for 3 hours and then cooled at a cooling rate of 60C/min to room temperature. A hardness of 60 of the aged hardness is thus established and the rings are processed, e.g. by machining, to a final dimension to form annular segments taking into account a shrinkage of 0.08%. A cutting speed two-and-a-half times that of the conventional process is then used without leaving wear tracks on the processing tool. Then the annular segments of the turbine housing were aged at 740C for 4 hours and then at 63WC for 6 hours to establish the final dimension and the age hardness for the components.

Example 2

A blank of a rotor disc of an alloy, as in Example 1, of 19% Cr, 3% Mo, 5.1% Nb and Ta, 1% Ti, 0.5% Al and residue Ni is first of all solution-annealed at 9800C for 2 hours. Then the workpiece is cooled to room temperature at 200C/min. A hardness of 70% of the age hardness is established. Taking into account a shrinkage of 0.05% the blank for the rotor disc is processed to the final dimensions with a cutting speed double that of conventional processes. Then the rotor disc thus processed is heated to 7600C and is kept for 5 hours at this temperature as a holding temperature. Finally the rotor disc is cooled at 1C/min to 6200C and then exposed to room temperature.

Claims

CLAIMS:

1. Process for the production of objects from a nickel-based alloy which are hardened by I"-aging in which a solution treatment is followed by a cooling step, characterised in that machining takes place directly after the cooling from the solution treatment temperature, and age-hardening is carried out after the machining step.

2. Process according to claim 1, in which the cooling rate after the solution treatment is selected in accordance with the required degree of shrinkage.

3. Process according to claim 1 or 2, in which the alloy consists substantially of 19% Cr, 3% Mo, 5.1% Nb and Ta, 1% Ti, 0.5% A1 and the rest Ni.

4. Process according to any preceding claim, in which the solution treatment takes place at temperatures between 900C and 1050'C.

5. Process according to any preceding claim, in which the cooling rate is between 50C/min and 100C/min and the workpiece is cooled to room temperature.

6. Process according to any preceding claim, in which the aging of the workpiece involves heating from room temperature to a holding temperature from 7000C to 7800C, this temperature being maintained for 1 to 8 hours and the workpiece then being cooled at a rate of 0.5'C/min to 5C/min to below 6500C.

7. Process according to any preceding claim, in which the aging is carried out in two stages at workpiece temperatures first of 7000C to 7800C and then at 6200C to 6500C.

8. Process according to any preceding claim, in which the machining at 2 to 2.5 times the cutting speed of conventional processes.

9. Process according to any preceding claim, in which the machining step takes into account a degree of shrinkage from 0.03% to 0.09% preferably 0.05% to 0.07%.

10. A process substantially as described herein with reference to any of the Examples.