FIELD OF THE INVENTION
The invention generally relates to the technical field of robotic inlet edge
hardening of steam turbine LP stage blade to reduce water droplet erosion, by
using temperature controlled high power diode laser. More particularly, the
invention relates to a method consists of precise robotic programming for inlet
edge hardening of high rating LP stage steam turbine blades using temperature
controlled diode laser hardening process.
BACKGROUND OF THE INVENTION
It is known that the leading edges of the last stage blades of steam turbines are
subjected to water droplet induced erosion, which limits the useful life of the
blades. The pits and craters formed due to erosion often acts as stress
concentration points, initiating cracks and subsequent blade failure. Also the
efficiency of the turbine goes down due to surface roughness. Traditionally, the
problem of water droplet erosion has been mitigated by adopting processes such
as flame hardening, induction hardening and stelliting during blade manufacture.
These processes have their own disadvantages for example, distortion and
shorter life. Prior art describes the process of laser surface hardening using
carbon-di-oxide laser for combating water droplet erosion. The major advantages
of laser surface hardening of turbine blades are :
• Distortion of the finished component is negligible.
• The presence of surface compressive residual stresses after hardening
significantly reduces the susceptibility of the material to stress corrosion
cracking.
• The fatigue limit of the laser hardened zone is enhanced compared to
that of the base material.
• The higher hardness resulting from faster cooling results in better erosion
resistance of the material, thereby enhancing the life.
High Power CO2 laser is presently used for hardening application; however, the
major disadvantage of the CO2 laser is the application of black laser absorption
coating. The coating contaminates the hardened zone leading to non uniform
hardness. Further, CO2 laser cannot be handled by a robot. However, the
inventors noticed that the use of diode laser eliminates the need for black laser
absorption coating on steel sample, as the absorption of 950 nm wavelength
diode laser on steel surface is quite effective. Further, diode lasers are compact
in size and can be easily handled by a robot, resulting in greater operational
flexibility which is required during hardening of inlet edge of blade having
complex profile.
OBJECTS OF THE INVENTION
It is therefore, an object of the present invention is to propose a method for inlet
edge hardening of high rating LP stage steam turbine blades using temperature
controlled high power diode laser hardening process.
Another object of the present invention is to propose a method for inlet edge
hardening of high rating LP stage steam turbine blades using temperature
controlled high power diode laser hardening process, in which laser power/scan
speed is controlled by precise robotic programming to produce erosion resistant
hard layer.
A still another object of the present invention is to propose a method for inlet
edge hardening of high rating LP stage steam turbine blades using temperature
controlled high power diode laser hardening process in which the inlet edge laser
hardened zone of 30 mm width with corresponding length, can be achieved by
maintaining a constant height of the focused rectangular laser beam, beam angle
of 90° and scan speed of 1.5-2.00 mm through out the inlet edge scan area of a
complicated blade profile.
Yet another object of the present invention is to propose a method for inlet edge
hardening of high-rating LP stage steam turbine blades using temperature
controlled high power diode laser hardening process, which achieves a
compressive residual stresses after laser hardening.
A further object of the present invention is to propose a method for inlet edge
hardening of high-rating LP stage steam turbine blades using temperature
controlled high power diode laser hardening process, which ensures improved
fatigue properties of the steam turbine blades.
A still further object of the present invention is to propose a method for inlet
edge hardening of high-rating LP stage steam turbine blades using temperature
controlled high power diode laser hardening process, which ensures flaw-less
surface after laser hardening.
Yet further object of the present invention is to propose a method for inlet edge
hardening of high-rating LP stage steam turbine blades using temperature
controlled high power diode laser hardening process, which provides an
improved droplet erosion resistance of the treated turbine blade.
SUMMARY OF THE INVENTION
According to the invention, there is provided a method for inlet edge hardening
of LP stage high rating steam turbine blades in a temperature controlled high
power diode laser hardening process, comprising : controlling laser power/scan
speed to produce droplet erosion resistant hard layer by laser hardening at a
temperature in the range of 1400-1700°C and scan speed of 1.5-2.00 mm to
achieve hardness of about 540 HV upto a depth of about 1.4 mm.
Thus, a laser hardening process using temperature controlled high power diode
laser by controlling laser power/scan speed is provided according to the
invention for inlet edge hardening of LP stage high rating steam turbine blades.
The laser hardening is carried out using six-axis robot and two-axis turn-table.
An exclusive robot program is implemented to maintain the uniform laser focus
length, scan speed, beam orientation and beam path during entire profile of inlet
edge of LPST blade during laser hardening.
A very high hardness of around 540 HV upto a depth of about 1.4 mm has been
achieved in the laser hardened zone as against the base metal hardness of
around 310 HV. The residual stresses measured in the hardened zone and
transition zone between hardened and non-hardened zone are found to be
compressive in nature. The laser hardened zone is having improved fatigue
properties and found to be free from defects during magnetic particle testing and
thereby producing improved droplet erosion resistant hard layer.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 : Inlet Edge Hardening of Steam Turbine Blades
Figure 2 : Typical hardness variation
Figure 3 : Microstructure after laser hardening
Figure 4 : Residual stress measurements using Barkhausen Noise Method :
Figure 5 : Reverse Bending Fatigue test results :
Figure 6 : Magnetic Particle testing :
Figure 7 : Droplet Erosion test result
DETAIL DESCRIPTION OF THE INVENTION
In the process of hardening, according to the invention, the diode lasers with
short wavelength for superior absorption is used. Direct diode lasers are utilized
in a power range of up to 8,000 W. The lasers are equipped with pyrometers for
temperature feedback control, including known optics. This optics generates a
line or rectangular spot optimizable to specific application, and results in a
constant hardening depth across the entire width. The use of the pyrometer
enables precise control of the hardening temperature avoiding surface melting.
This completely eliminates the risk of surface melting, even near edges, avoiding
the costs for reworking the damaged parts. The combination of short
wavelength, superior absorption and adapted beam profile warrant highly
efficient processing. The photograph of the robotic laser system configured for
blade hardening is shown in Figure 1. The diode laser is housed in a box so as to
restrict exposure to environment. Also a compressor supplies high purity
pressurized air to prevent any exposure to outside dusty environment.
According to the invention, the laser hardening is carried out at a temperature in
the range of 1400-1700°C using two-colour pyrometers to achieve smooth
transition zone at the end of hardened zone. The temperature variation has been
kept to a minimum of ± 1% while the laser beam is traversing along the blade
profile. Micro hardness measurement across the laser hardened zone is carried
out using Vickers hardness tester at a load of 300 grams. Typical hardness
variation is shown in figure 2. A very high hardness of around 540 HV is achieved
in the laser hardened zone as against the base metal hardness of around 310
HV. A substantially higher depth of hardening of about 1.4 mm can be achieved.
Figure 3 shows the microstructure in the laser hardened zone and in the base.
Extremely fine martensitic structure is observed in the laser hardened zone as
compared to comparatively coarse martensitic structure in the base metal. No
cracks are observed.
The diode laser is mounted on a six-axis robot. The robot can be programmed
through a controller to move the laser beam onto the job to be treated at the
localized inlet zone. There is a two-axis turn-table with a chuck for mounting the
components and can be programmed as an integrated robot axis for blade
rotation. The laser power is controlled by a laser power controller which in turn is
integrated with the robot controller through 'profibus' communication so that
whole operation is controlled by robot controller.
According to the invention, the compressive residual stress is measured by using
Barkhausen Noise Method. The equipment used is STRESS SCAN 500C, Model: S
500C 1990 supplied by Stresstech OY, FINLAND. Residual stresses are of great
significance to design and life evaluation of components because of their effect
on parameters such as fatigue strength, safe load limits, stress corrosion
resistance and component warping. Residual stresses are stresses in a
component which is free of external mechanical stresses. Residual stress
measurements were carried out in the laser hardened zone on the blade profile.
The 30 mm width of laser hardened zone was divided into three segments
(10mm, 15mm, 20mm) and measurements were carried out along the length of
hardened zone at intervals of 15mm. Number of measurements were carried out
and the results are tabulated in table 1. Graphical presentation of the values is
shown in Figure 4. From the figure 4, it can be seen that the residual stresses
are compressive in nature. Measurements were also carried out in the transition
zone between hardened and non-hardened zone and was found to be
compressive in nature.
In order to test the fatigue properties developed by the inventive process, the
steam turbine blades were tested through reverse bending process. Accordingly,
reverse bending fatigue test specimens were fabricated and laser hardened at
different temperatures ranging from 1400 to 1700°C. Careful robot programming
has been carried out to harden the entire gauge length of the sample as well as
50% of the gauge length to generate hardened and non-hardened zones.
Fatigue testing at 40 Kg/mm2 load of non hardened samples failed around
0.04696 x 107 cycles. Fatigue tests were carried out till samples failed or crossed
107 whichever was earlier. Laser hardened samples did not fail upto 107 cycles
and the tests were discontinued. The results (Fig.5) clearly showed excellent
fatigue properties of laser hardening treatment. Fatigue testing was carried out
on a Schenk reverse bending machine.
In order to ensure a flawless hardened surface of the treated turbine blade,
magnetic particle testing was carried out. The laser hardened LP turbine blade
was subjected for Wet Fluorescent Magnetic Particle Testing (WFMPT). The blade
was magnetized using coiling technique. Magnetic particles suspended in
kerosene were sprayed on to the blade during magnetization. Blade was
inspected using UV lamp after magnetization and spraying magnetic particles.
More attention was paid at the laser hardened area. Photos were taken during
Wet Fluorescent Magnetic Particle Testing of blade. Figure 6 is showing the
magnetization and inspection of the blade. No defects/flaws were noticed during
Magnetic particle testing.
A droplet erosion resistance test was carried out by using accelerated high
impact droplet test facility simulating droplet erosion condition at low pressure
steam turbine blades. The test facility consists of a 700 mm diameter chamber
and a round stainless steel disc where the test samples are positioned. Samples,
40 mm in length and 12.7 mm in diameter are affixed on the periphery of the
disk. The disc is rotated at 4575 rpm to obtain the test sample tangential velocity
of 147.0 m/s. Two water jets impinge on the cylindrical test samples and cause
impingement erosion. As such, a relative velocity of, 147.6 m/s is obtained. A
precision balance (±0.1 mg) was used for measurement of mass loss after
testing. The test duration depending upon energy and mass fluxes was selected
in such a way to achieve steady state erosion in limited cycles. The extent of
erosion damage is calculated from the mass loss divided by the density of the
material. The droplet erosion test has been carried out on laser hardened and
untreated X 10CrNiMoV1222 steel. The droplet erosion test results of High Power
Diode Laser hardened samples along with untreated X10CrNiMoV1222 steel are
given in figure 7. It can be seen from figure 7 that excellent performance is
given by HPDL hardened X10CrNiMoV1222 steel for energy flux of 57.167 x 106
J/m2s.
WE CLAIM :
1. A method for inlet edge hardening of LP stage high rating steam turbine
blades in a temperature controlled high power diode laser hardening
process, comprising : controlling laser power/scan speed to produce
droplet erosion resistant hard layer by laser hardening at a temperature in
the range of 1400-1700°C and scan speed of 1.5-2.00 mm to achieve
hardness of about 540 HV upto a depth of about 1.4 mm.
2. The method as claimed in claim 1, wherein uniform focus distance, beam
angle and scan speed through out the scan area of the complicated profile
of a turbine blade inlet edge area is maintained during diode laser
hardening through precise robot programming.
3. The method as claimed in claim 1 or 2, wherein compressive residual
stresses, improved fatigue properties and flaw-less surface in the
hardened zone and transition zone between hardened and non-hardened
zone of the turbine blade is achieved.
5. A method for inlet edge hardening of LP stage high rating steam turbine
blades in a temperature controlled high power diode laser hardening
process as substantially described and illustrated herein with reference to
the accompanying drawings.

The invention relates to a method for inlet edge hardening of LP stage
high rating steam turbine blades in a temperature controlled high power
diode laser hardening process, comprising : controlling laser power/scan
speed to produce droplet erosion resistant hard layer by laser hardening
at a temperature in the range of 1400-1700°C and scan speed of 1.5-2.00
mm to achieve hardness of about 540 HV up to a depth of about 1.4 mm.