FIELD OF THE INVENTION
This invention generally relates to a method of increasing surface hardness and
case depth of precipitation hardened steel substrate by laser hardening followed
by heat treatment for application in steam turbine blading. More particularly, the
invention relates to a method of increasing surface hardness of steam turbine
blading steel by use of high power diode laser.
BACKGROUND OF THE INVENTION
It is known that hardening of steels by the carburizing method is both energy
and labor intensive, and is therefore quite expensive. In addition, the carburizing
technique is quite time-consuming and requires a large amount of equipment,
including a furnace, quenching dies must be custom made for each part being
manufactured, masking equipment, and regrinding equipment.
An alternative to carburizing is induction hardening, where the part to be
hardened is placed inside a coil through which a rapidly alternating current is
flowing. Heat is rapidly generated within localized portions of the part by
electromagnetic induction, which the depth of the case being controlled by the
frequency of the current in the coil. The part is then quenched, and induction
hardening thus also presents the problem of distortion in the part which will
subsequently require final regrinding operations. As such, induction hardening is
also expensive and time-consuming.
Due to their unique properties, industrial lasers have shown great promise in
selective rapid heating of a surface. The surface is generally prepared by
applying an absorptive coating to the surface to be heated, aiding in energy

transfer from the laser beam to heat energy within the part. By using a laser to
quickly heat a surface, conventional quenching by a gas or a liquid is
unnecessary since only the shallow surface area itself is heated. The part will
actually self-quench, due to the extremely high heat differential between the
shallow surface layer heated by the laser and the bulk of the part being
processed. This is in sharp contrast to carburizing or induction heating, where
the part must be heated in one operation, and then is required to be quickly
quenched by a gas or a liquid.
It has been heretofore known to utilize lasers in the heat treatment of metals
wherein, the temperature of the piece being treated is not raised above the
melting temperature of the material. Further, it is known to utilize laser energy to
create a thin molten condition on the surface of a part thereby changing the
crystalline structure of the metal to an amorphous structure. This process is
called laser glazing. One problem encountered in the heat treating application is
that high energy lasers must be employed.
US4533400 teaches a method and apparatus for laser hardening of steel. This
invention utilizes a beam alternating device which precisely splits a single laser
beam into two identical beams, and focuses and aims the beams at both flanks
of a single gear tooth. By hardening both flanks of a single gear tooth in one
operation, back tempering of the gear tooth is completely eliminated as a
problem. Surface heat absorption problems such as heat build-up in a portion of
the part are eliminated by hardening every third or so tooth in the gear, and by
making subsequent passes around the gear to harden all the gear teeth.
By utilizing a split beam, several advantages are obtained. First, the portions of
the beam can be more precisely timed at the gear tooth. Secondly, the portions

of the beam can approach the surface of the gear in a direction which allows for
the surface of the gear tooth to be more uniformly hardened.
Such a device and method has a number of striking advantages. First of all, a
large amount of the energy required in the carburizing operation is simply not
required in the laser hardening operation since only the surface to be hardened
is heated, and not the entire gear. Secondly, since only the surface to be
hardened is heated, there is virtually no distortion whatsoever present in the
laser hardening process, thus completely eliminating the need for regrinding to
correct distortion.
Finally, the process utilizing laser hardening is extremely, quick, and may be
performed in a single operation, thus tremendously reducing the amount of time
and labor required.
US4313771 discloses a method of heat treating the surface of carbon steel work
pieces by applying a black coating on the surface of the steel work piece that
absorbs at least about 65% of the light emitted by a CO2 laser, and directing a
CO2 laser beam at the surface of the blackened work piece for a time sufficient
to raise the temperature of the surface into the gamma (y) or austenitic phase.
According to this prior art, a carbon dioxide laser beam is scanned over the
surface of the steel work piece to be hardened. The work piece prior to laser
scanning is subjected to blackening at least in the areas to be hardened. While
the black coating must be present in the areas to be hardened, it is desirable and
preferable to treat the entire steel work piece with the blackening solution,
described herein below, for ease of operation. The steel work piece, in
accordance with this invention, can have any form including tubular products,

rods, shafts, bars, plates, wire, machined parts, castings, forgings and the like
having complicated shapes.
The steel employed in the method of hardening as described herein is medium
and high carbon steel having a carbon content from about 0.4 percent to
about 1.2 percent by weight carbon. This includes all such steels having this
carbon content, typical examples of which are those steels bearing AISI number
steels 1141,11L41, 1060, 1080, 1090, tool steels and the like.
European patent No: 93310206.3 dated 17th December 1993 by General Electric
Company relates to a "Process for producing turbine buckets with water droplet
erosion protection and related product". It basically consists of providing a
bimetallic structure which will satisfy all the requirements in highly stressed last
stage buckets for steam turbines. It allows for the attachment of an erosion
resistant insert with the blade material while it provides superior droplet erosion
resistance than the parent blade material after flame or induction hardening.
However, the condition of near optimum weldability with the blade material
restricts the use of this patent. Also, the attachment methods invariably lead to
some degradation in the weld HAZ leading to scrapping of costly finished
buckets.
Japanese patent No JP06173604A dated 21st June 1994 entitled "Water droplet
shock erosion preventive turbine blade and manufacture thereof" by Beanto
Burennaa, et. al. relates to the use of carbon dioxide laser on the steel blade
material having 0.1 and 0.2%C with an anti-reflective paint for enhancing laser
absorption for enhanced droplet impact and cavitation resistance. This is
however, not suitable for X5 steel which hardens due to precipitation of copper
and not by formation of hard martensite.

Patent No. CN101089201 (A) by Maocai Wang, etal. dated 19th December 2007
relates to Hardening technology for precipitation hardening stainless steel laser
surface. This invention relates to a metal surface hardening technique of 17-4 PH
steels or of similar composition. A laser beam is scanned on the surface of the
precipitation hardening stainless steel within a certain condition, making the
surface of said steel to smelt. Low temperature shield gas is used for being cover
onto the scanned area, for speeding up the cooling. After that, the steel is
subjected to heat treatment, so the hardness of the surface in a certain thickness
is harder than that of the main body of the steel.
European patent EP 1213 363 (Bl) dated 12th June 2002 by Brenner Berndt of
Fraunhofer Institute, Germany refers to wear improvement of boundary layers of
precipitation hardened materials without deterioration of mechanical properties
of the component. It makes use of the carbon dioxide laser to attain solutionizing
of the precipitate. The drawback of this process is the requirement of an
additional step of pre-coating the component with an absorbent layer prior to
laser treatment to enhance the absorption of the C02 laser.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a method of
increasing surface hardness of steam turbine blading steel by using high power
diode laser. Diode lasers operate at wavelengths 0.840 and 0.980 micron and
hence have vastly improved absorptivity on steel surfaces compared to the
conventional carbon dioxide lasers which operate in the wavelength of 10.6
micron.

Another object of the invention is to propose a method of increasing surface
hardness of steam turbine blading steel by using high power diode laser, which
interalia improve the droplet erosion properties of the steel.
A still another object of the invention is to propose a method of increasing
surface hardness of steam turbine blading steel by using high power diode laser,
in which the hardness of the steel surface only is improved while leaving the
properties of the bulk intact.
SUMMARY OF THE INVENTION
Accordingly, there is provided a method of increasing surface hardness of steam
turbine blading steel by high power diode laser hardening, comprising the steps
of providing an initial two stage heat treatment of an annealed and as produced
steel sample to achieve hardness of around 360 HV0.3; subjecting this sample a
laser treatment using a high power diode laser followed by post laser heat
treatment such that the hardness of the sample surface reaches higher than 450
Hv 0.3, with the surface hardness of the untreated part restricted to 330 Hv 0.3,
wherein the optical laser power and the scan speed applied being 1500-2000
watt and 120-130 mm/min respectively, and wherein the wavelength of the
diode laser is 0.840 and 0.980 micron.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 shows a graphical representation of hardness variation across the
laser hardened region to untreated region.
Figure 2 Shows cavitation data of the laser hardened region and untreated
region.

DETAIL DESCRIPTION OF THE INVENTION
Surface treatment
According to the invention, annealed and as-produced X5 PH steel used in higher
rating steam turbines was first given a two stage heat treatment to achieve an
initial hardness of around 360 Hv0.3. Subsequently, these samples were
subjected to laser treatment using a high powered diode laser (HPDL) having
beam wavelength of 0.840 and 0.980 micron. At this wavelength, the absorption
of the laser beam on steel is an order of magnitude higher than that of the C02
laser whose wavelength is 10.6 micron. Laser treatment was carried out at a
scan speed of 120-300 mm/min and at a temperature close to melting but not
actually causing the sample to melt. These process parameters were optimized
during development of the invention so as to obtain minimum increase in surface
roughness. The Laser beam temperature was controlled in a closed loop by a
pyrometer system and thus a uniform surface temperature was maintained
during the process. The laser power was observed to be varying between 1500W
- 2000 W. The laser beam has a rectangular shape of spot size 30 mm*3mm
and was scanned over the sample maintaining a uniform distance of 300 mm
from the surface using a robot mechanism. This self-quenched sample contains a
supersaturated solution of copper in the matrix and its hardness is around 330
Hv0.3. Subsequently, the laser treated sample was given a low temperature heat
treatment at 400 °C for duration of 6 hours to ensure that the fine precipitation
of copper from the supersaturated solution reaches an optimum size. The size of
the copper precipitates is critical for obtaining maximum hardness. The hardness
of the sample increased to a maximum value of over 450 Hv0.3 following the low
temperature heat treatment. This time and temperature of heat treatment was
optimized after several trials and feedback using hardness data.

WE CLAIM:
1. A method of increasing surface hardness of steam turbine blading steel by
high power diode laser hardening, comprising the steps of :-
providing a two stage heat treatment of an annealed and as produced
steel sample to achieve an initial hardness of around 360 HV0.3;
subjecting the treated sample a laser treatment using a high power diode
laser followed by post laser heat treatment such that the hardness of the
sample surface reaches higher than 450 Hv 0.3, with the surface hardness
of the untreated part restricted to 330 Hv 0.3;
wherein the optical laser power and the scan speed applied being 1500-
2000 watt and 120-130 mm/min respectively, and wherein the wavelength
of the diode laser is 0.840 and 0.980 micron.
2. The method as claimed in claim 1, wherein the cavitation erosion
resistance X5 17-4 PH steel by around 50% better than that of the
untreated steel.
3. The method as claimed in claim 1, wherein the steel sample is X5 17-4 PH
steel used for steam turbine blading.

ABSTRACT

The invention relates to a method of increasing surface hardness of steam
turbine blading steel by high power diode laser hardening, comprising the
steps of :- providing a two stage heat treatment of an annealed and as
produced steel sample to achieve an initial hardness of around 360 HV0.3;
subjecting the treated sample a laser treatment using a high power diode
laser followed by post laser heat treatment such that the hardness of the
sample surface reaches higher than 450 Hv 0.3, with the surface hardness
of the untreated part restricted to 330 Hv 0.3; wherein the optical laser
power and the scan speed applied being 1500-2000 watt and 120-130
mm/min respectively, and wherein the wavelength of the diode laser is
0.840 and 0.980 micron.