GB2424422A

GB2424422A - Precipitation hardening of a steel

Info

Publication number: GB2424422A
Application number: GB0505997A
Authority: GB
Inventors: Ian John Chilton; Bruce Wynn Roberts; Stuart Richard Holdsworth; Herbert Bartsch
Original assignee: Alstom Technology AG
Current assignee: General Electric Technology GmbH
Priority date: 2005-03-23
Filing date: 2005-03-23
Publication date: 2006-09-27
Also published as: GB0505997D0

Abstract

A component 1 made from a precipitation hardenable steel is heat treated by sequentially solution heat treating, intermediate heat treating, ageing, re-solution heat treating a region 2 of the component 1 (e.g by induction) and then re-ageing. The component 1 may be a forged steam turbine blade, with the re-solution heat treated region 2 being located on the leading edge 3 of the blade. The component 1 is preferably re-aged at a lower temperature than the initial age and the hardness of the re-aged region 2 is substantially maximised and consequently erosion resistance is enhanced.

Description

Local Enhancement of Erosion Resistance This invention relates to the

local enhancement of the erosion resistance of a component, in particular (but not solely) a steam turbine blade.

In a steam turbine, since the steam is very wet in the last stage region, water droplet erosion of the blades in this region can be a problem.

US-A-S 351 395 discloses a method in which a component constituting a steam turbine blade member consisting of a martensitic steel is cast, the component is subjected to solution annealing to make it suitable for welding, a metal alloy insert is tempered to a hardness less than that of the airnealed component, the insert is welded to the leading edge of the blade member, the component and insert are subjected to ageing heat treatment, the component and insert are machined, and then the insert is hardened through substantially its entire thickness.

The present invention provides a method of locally enhancing the erosion resistance of a component consisting of a precipitation hardening steel, the method comprising the sequential steps of: (a) applying solution heat treatment to the component; (b) applying intermediate heat treatment to the component; (c) applying ageing heat treatment to the component; (d) applying re-solution heat treatment to a region of the component; and (e) applying ageing heat treatment to the component such that the said region has a higher erosion resistance than the remainder of the component.

Preferred and optional features of the invention are set forth in the subsidiary claims.

The invention will be described further, by way of example only, with reference to the accompanying drawing, in which the sole figure is a view of a steam turbine blade.

The blade I illustrated is to be mounted in the last stage of a steam turbine. The blade I is forged from a low-carbon precipitation hardening (PH) steel. The region of the blade most susceptible to erosion in the region 2, in the leading edge portion 3 of the blade, adjacent to the end of the blade (tip 4) remote from the root 5.

The blade 1 may consist of a low-carbon steel containing Cr and Ni, having the designation 15-5PH. In particular, this steel may have the following composition: up to 0.07 wt.% C, up to 1.0 wt% Si, up to 1.0 wt.% Mn, 13.0-15.5 wt% Cr, 3.5-6. 00 wt.% Ni, 1.4-4.5 wt.% total Cu and Mo, the Mo being optional, and 0.15- 0.45 wt.% Nb, the balance being Fe and incidental impurities. Two examples of 15-5 PH steel are: (1) up to 0.07 wt.% C, up to 0.7 wt.% Si, up to 1.0 wt% Mn, 13.0-15.0 wt. % Cr, 5.0-6.0 wt.% Ni, 1.2-2.0 wt.% Mo, 1.4-2.1 wt.% Cu, and 0.1 5-0.30 wt.% Nb, the balance being Fe and incidental impurities; and (2) up to 0.07 wt.% C, up to 1.0 wt.% Si, up to 1.0 wt.% Mn, 14.0-15.5 wt. % Cr, 3.5-5.5 wt.% Ni, 2.5-4.5 wt.% Cu, and 0.15-0.45 wt.% Nb, the balance being Fe and incidental impurities.

For a steam turbine blade made of 15-5 PH type steel, a typical heat treatment procedure following blade forging would, for example, consist of the following sequence of treatments (the times and temperatures being approximate): solution treatment: hold for lh at 1000 C, then air cool; intermediate treatment: hold for 3h at 800 C, then air cool; ageing treatment: hold for 4h at 530 C.

Alternatively the blade 1 may be made of a low-carbon steel containing Cr, Ni, and Mo and having the designation PH 13-8 Mo. The composition of this steel is as follows: up to 0.05 wt.% C, up to 0.1 wt.% Si, up to 0.1 wt.% Mn, 12.25-13.25 wt.% Cr, 7.5-8.5 wt.% Ni, 2.0-2.5 wt.% Mo, and 0.90- 1.35 wt.% Al, the balance being Fe and incidental impurities.

For a 15-5 PH type steel a preferred process in accordance with the present invention starts with solution heat treatment of the forged blade 1, e.g. at a temperature of about 1000 C for about 1 hour. This may be followed by air cooling or quenching to ambient temperature prior to heating to an intermediate treatment temperature, e.g. about 800 C, at which the blade is held, e.g. for about 3 hours, before air cooling or quenching again to ambient temperature. Ageing heat treatment is then carried out at a temperature in the range from about 500 C to about 530 C, at which the blade is held, e.g. for about 4 hours, before air cooling to ambient temperature.

Subsequently the region 2 of the blade I is subjected to local resolution heat treatment involving localised heating, preferably induction heating. The region 2 is held at a sufficiently high temperature, for a suffici'nt time, to dissolve precipitates in this region. The temperature may be in the range from about 1080 C to about 1100 C and the time less than 1 minute, for example. The temperature chosen will depend on other parameters such as the leading edge thickness, the speed of the induction coil, and the amount of back-face air cooling, and the combination of parameters chosen will depend on the desired profile of the heat-affected microstructure.

After this re-solution heat treatment of the region 2, the blade 1 is subjected to an ageing heat treatment at a lower temperature than the previous ageing treatment. This causes re-ageing of the region 2 but has no significant effect on the already-aged material. The blade may be held at a temperature below 5 00 C, e.g. about 45 0 C, for a suitable time, e. g. about 2 hours. The temperature and time are chosen so as to maximise the hardness of the region 2 and thereby enhance its erosion resistance. Suitable temperatures and times can be found empirically.

For a PH 13-8 Mo steel a preferred process according to the present invention again starts with solution heat treatment of the forged blade 1, followed by ageing at a suitable temperature and for a suitable time to give the blade the required properties. The temperature may be greater than 530 C, for example in the range from about 540 C to about 560 C. Subsequently the region 2 of the blade is subjected to local re-solution heat treatment. The temperature may, for example, be in the range from about 960 to about 980 C. The blade is then re-aged at a lower temperature than the previous ageing treatment, which may he below 520 C, in particular about 500 C for example. Again the temperature and time to niaximisc the hardness (and therefore erosion resistance) of the region 2 can be determined empirically.

EXAMPLE

1) A blade I consisting of a steel having the composition 15-5 PH (1) mentioned above is forged.

2) The blade is subjected to: solution treatment: lh, 1000 C, air cool, then intermediate treatment: 3h, 800 C, air cool, then ageing treatment: 4h, 500-530 C.

3) The above-descnbed region 2 of the blade 1 is subjected to induction heating, with a peak temperature in the range 1080-1100 C, for less than 1 minute.

4) The blade 1 is re-heated and held at a temperature of 450 C for 2h.

The locally-performed re-solution treatment causes re-solution of original precipitates in the locally-heated zone. The locally solution-treated material responds to the low- temperature final ageing treatment to generate the peak hardness condition and provide a hard surface with good water droplet erosion resistance. The originally aged material is not influenced by the additional low-temperature ageing and retains its original mechanical properties.

Claims

CLAIMS: 1. A method of locally enhancing the erosion resistance of a

component consisting of a precipitation hardening steel, the method comprising the sequential steps of: (a) applying solution heat treatment to the component; (b) applying intermediate heat treatment to the component; (c) applying ageing heat treatment to the component; (d) applying re-solution heat treatment to a region of the component; and (e) applying ageing heat treatment to the component such that the said region has a higher erosion resistance than the remainder of the component.
2. A method as claimed in claim 1, in which step (d) comprises induction heating.
3. A method as claimed in claim I or 2, in which step (c) comprises holding the component at a first ageing temperature for a first ageing period, and step (e) comprises holding the component at a second ageing temperature, for a second ageing period, the second ageing temperature being lower than the first.
4. A method as claimed in claim 3, in which the second ageing period is shorter than the first.
5. A method as claimed in claim 3 or 4, in which the second ageing temperature and the second ageing period are such that the hardness of the said region is substantially maximised.
6. A method as claimed in any of claims I to 5, in which the precipitation hardening steel has the following composition: up to 0.07 wt.% C, up to 1.0 wt.% Si, up to 1.0 wt.% Mn, 13.0-15.5 wt.% Cr, 3.5-6.00 wt.% Ni, 1.4-4.5 wt.% total Cu and Mo, the Mo being optional, and 0.15-0. 45 wt.% Nb, the balance being Fe and incidental impuriti Cs.
7. A method as claimed in claim 6, in which the precipitation hardening steel has the following composition: up to 0.07 wt.% C, up to 0.7 wt.% Si, up to 1.0 wt% Mn, 13.0-15.0 wt.% Cr, 5.0-6.0 wt.% Ni, 1.2-2.0 wt.% Mo, 1.4-2.1 wt.% Cu, and 0.15-0.30 wt.% Nb, the balance being Fe and incidental impurities.
8. A method as claimed in claim 6, in which the precipitation hardening steel has the following composition: up to 0.07 wt.% C, up to 1.0 wt.% Si, up to 1.0 wt.% Mn, 14.0-15.5 wt.% Cr, 3.5-5.5 wt.% Ni, 2.5-4.5 wt.% Cu, and 0.15-0.45 wt.% Nb, the balance being Fe and incidental impurities.
9. A method as claimed in any of claims 6 to 8, when dependent on claim 3, in which the first ageing temperature is in the range from approximately 500 C to approximately 530 C and the second ageing temperature is approximately 450 C.
10. A method as claimed in any of claims 6 to 9, in which step (d) comprises heating the said region to a temperature in the range from approximately 1080 C to approximately 1100 C.
11. A method as claimed in any of claims 1 to 5, in which the precipitation hardening steel has the following composition: up to 0.05 wt.% C, up to 0.1 wt.% Si, up to 0.1 wt.% Mn, 12.25-13.25 wt.% Cr, 7.5-8. 5 wt.% Ni, 2.0-2.5 wt.% Mo, and 0.90-1.35 wt.% Al, the balance being Fe and incidental impurities.
12. A method as claimed in claim 11, when dependent on claim 3, in which the first ageing temperature is greater than 530 C and the second ageing temperature is less than 520 C.
13. A method as claimed in claim 12, in which the first ageing temperature is in the range from approximately 540 C to approximately 560 C.
14. A method as claimed in claim 12 or 13, in which the second ageing temperature is approximately 500 C.
15. A method as claimed in any of claims 11 to 14, in which step (d) comprises heating the said region to a temperature in the range from approximately 960 C to approximately 980 C.
16. A method as claimed in any of claims Ito 15, in which the component is formed by forging.
17. A method as claimed in any of claims Ito 16, in which the component is a steam turbine blade.
18. A method as claimed in claim 17, in which the said region is located in the leading edge portion of the blade.
19. A method as claimed in claim 17 or 18, in which the said region is located adjacent the end of the blade remote from the root of the blade.
20. A method of locally enhancing the erosion resistance of a component, substantially as described in the Example.