FIELD OF INVENTION
The present invention relates to a method of salvaging overdimensioned machine parts
(Mechanical components machined beyond allowable minimum material condition)
using HVOF coating adaptable to static components of power producing turbo-
machines.
BACKGROUND OF THE INVENTION
The axial flow turbo-machines like gas turbines and steam turbines comprise of two
basic components:
(1) Stator: A horizontally split casing, housing critical components like liner
bearings, thrust bearings, stator blades and their carriers. These casting are
generally made up of cast iron or low alloy steels. The above mentioned
components are assembled in tight fit groove that are machined in the castings.
(2) Rotor: A shaft, solid or built up on which vanes are mounted.
Sometimes due to erroneous machining of these grooves, the dimensions are
deviated. In these machines where clearance between matching parts are very less,
a deviation of even 30 to 40 microns can severely affect the performance of the
turbo-machine.
The deviated or overdimensioned grooves are restored by various methods like
• Material buildup by high heat input processes like welding, brazing etc.
• Over machining the deviated groove to fix a patch plate with a over
dimension and subsequently machining to required dimension.
Repair by high heat input process may cause damage to the base metal, deformation in
components etc. Fixing a patch plate to the damaged part is time consuming and
expensive process.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a method of salvaging the
overdimensioned machining parts using HVOF coating which permits restoration of
rejected machine parts in the form of desired condition.
Another object of the present invention is to propose a method of salvaging the
overdimensioned machining parts using HVOF coating which is relatively simple, fast
and reliable manner.
A further object of the present invention is to propose a method of salvaging the
overdimensioned machining parts using HVOF coatings which is economical and
reduces repair time.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.l - shows the elevational view of the over dimension mechanical part of bearing
groove where the liner bearing sits
Fig.2 - shows the elevational view of parts fixed a patch plate with the help of screw as
prior art
Fig.3 - shows a process chart according to the present invention
DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE
INVENTION
Fig.l shows the part of bearing groove where the liner bearing sits. This forms a critical
area in gas turbine applications, as a small deviation from design dimensions can result
in vibrations. The locations marked A and B were deviated from deign dimension by 50
microns during machining.
The bearing can be salvaged by depositing a coat of HVOF coating on the affected area.
The application of coating permits the restoration of the desired condition in a relatively
simple, fast and reliable manner specifically with respect to the characteristics of
material and the geometry, including the surface quality.
As shown in Fig.3 in step-1, the damaged area is inspected for dimensional deviation.
This area is shaped by machining or grinding. The amount and range of removal of
materials need to be judged.
From the standpoint of preventing decrease in strength and degradation in reliability of
component, it is desirable to reduce to-be removed area to a minimum necessary
range.
In step-2, the shaped surface is roughened by blasting. Care should be taken to mask
the surrounding surfaces so as to prevent damage. The roughening process enhance
the bond strength of coating, thereby enhancing the reliability of sprayed coating.
In step-3, HVOF coating is applied to the prepared surface. The high velocity spray
should be performed at angle between 45° to 90° to the surface of material. This
ensures close contact of the coating on the base material and to obtain coating with
less pores, high density and high inter-particle coupling. With this method of spray
coating thicknesses up to 5 mm can be obtained.
In the fourth step, the coating is finished by machining or grinding to the required
dimension.
The defect inspection for checking presence/absence of defect and dimensional and
quality inspection are necessary for providing a defect free product. Even a small defect
can facilitate peeling of coating or initiate crack growth and is a factor that degrades
the performance of repaired part.
WE CLAIM
1. A method of salvaging overdimensioned machined parts comprising the steps of:
- shaping of mis-machined/damaged area of the machined parts;
- roughening the surface of oversized machining area using a mask to prevent
damages;
- applying of HVOF coating on the roughened surface of oversized machined parts;
- machining or grinding the coating part to required dimension;
- checking the presence/absence of defect and dimension;
characterized in that the said HVOF is sprayed at a high velocity maintaining at an
angle 45° to 90° to the surface of oversized machine parts such that HVOF ensures
close contact with base material and filled up a coating layer of 5 mm without any
pores.

A method of salvaging overdimensioned machined parts comprising the steps of
shaping of mis-machined/damaged area of the machined parts and roughening the
surface of oversized machining area using a mask to prevent damages and applying of
HVOF coating on the roughened surface of oversized machined parts and machining or
grinding the coating part to required dimension and checking the presence/absence of
defect and dimension characterized in that the said HVOF is sprayed at a high velocity
maintaining at an angle 45° to 90° to the surface of oversized machine parts such that
HVOF ensures close contact with base material and filled up a coating layer of 5 mm
without any pores.