|Title of Invention||
METHOD OF INCREASING SURFACE HARDNESS AND CASE DEPTH OF A STEEL SUBSTRATE BY SURFACE TEXTURISATION AND HIGH POWER DIODE LASER HARDENING
|Abstract||A method of enhancing laser absorption on steel substrate thereby achieving better properties of hardened case, comprising the following steps; surface texturing of a steel sample, to achieved a roughness value ranging from 1 to 2 microns, solvent cleaning of the surface after texturing for laser treatment, treatment of textured steel surface with high power diode laser (HPDL), testing for erosion of steel surface, treated with high power diode laser,|
|Full Text||TITLE: "A METHOD OF ENHANCING LASER ABSORPTION ON STEEL
SUBSTRATE THEREBY ACHIEVING BETTER PROPERTIES OF HARDENED
FIELD OF THE INVENTION
The invention relates to the field of hardening of steel surface in general and to a
method of increasing surface hardness and case depth of a steel substrate by
surface texturisation followed by hardening through high power diode laser, in
BACKGROUND OF THE INVENTION
The process of laser interaction with the base material is strongly dependent on
the texture of the surface. When the laser light beam falls on the sample surface,
part of the light photons are absorbed by the initial layers and heat gets
adiabatically transferred to the bulk whereas the rest of the photons are reflected
from the surface. The temperature attained at the surface depends on the sweep
rate of the laser beam and the conductivity of the medium. Since the laser light
beam is very focused and intense, it causes the temperature to rise rapidly
beyond austenitic phase start temperature (Ac1), where the steel undergoes
phase change. As the beam moves further, the rapid heating is followed by rapid
cooling by means of extraction of heat into the bulk, thereby causing
microstructure changes in the heat affected layers of the substrate. The beam
absorption depends strongly on the surface texture. In order to enhance
absorption, reflectivity has to be minimized.
One method adopted to increase absorption using carbon dioxide lasers is
reported in US Patent 4313771. The work piece intended for laser hardening in
first treated with a solution to cause blackening of its surface and subsequently
subjected to a laser beam.
Another method adopted to increase absorption is reported in US Pater.t
4414038. This involves directing a laser beam at the surface to be treated, such
as a metallic surface which is likely to reflect a large percentage of its incident
energy at an oblique incidence. In case of steel, the absorption is markedly
increased at angles of incidence greater than 45° and just short of glancing
Laser hardening is typically carried out on as-machined samples that are cleaned
with a solvent to remove fine dust, grease, etc. prior to the process. The
deficiency of the present process is relatively poor absorption of the laser beam
due to higher reflectivity from the metallic surface resulting in more power
consumption, lower hardness, and lesser case depth.
OBJECTS OF THE INVENTION
To overcome the drawbacks of the prior art, the following innovative remedial
actions were undertaken to improve it;
The method of invention requires texturing by hard fine shots impinging on the
martensitic steel substrate under pressure to create controlled roughness close
to the desired values. The surface finish of sample was measured before and
Using low power the resultant laser hardened surface after application of the
inventive method of surface texturisation followed by hardening using high
power diode laser, has a higher hardness value with increased case depth in
comparison to the untreated laser hardened surface and is shown with graphs in
Fig. 3, where (A) represents before i.e., untreated laser hardened surface and
(B) represents after surface finish and treated with laser.
A surface treatment (described below) prior to laser treatment is found to be
extremely important to attain the eventual quality of the hardened surface. It
helps in increasing the case depth significantly as well as minimizes the problems
of lens damage due to fine smoke/dust that emanates from a rough surface. The
combination of the initial surface treatment and high power diode laser
hardening results in excellent hardness, enhanced case depths, excellent liquid
impingement erosion properties and increased efficiency of laser operation.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
The invention will now be described with the help of accompanying drawings
which depict an exemplary embodiment of invention. However, there can be
other embodiments all of which are deemed covered by the description.
Fig. 1: Shows surface texture of X20Cr13 steel sample before surface texturing
Fig. 2: Shows surface texture of X20Cr13 after surface texturing.
Fig. 3: A graph showing case depth Vs. hardness, before and after surface
DESCRIPTION OF THE INVENTION
The surface texturing step carried out prior to the laser hardening process so as
to attain a desired surface roughness is a novel concept and it results in
substantial improvement in properties. It is carried out to increase the absorption
of laser radiation more effectively on the substrate surface. The hardness gets
improved by around 25%, case depth becomes doubled and power consumption
of the laser is reduced by 5%.
This texturing was carried out by hard fine shots impinging on the martensitic
steel substrate at sufficient pressure to create controlled roughness close to the
desired values (Ra upto measured before and after texturing. These are given in Figures 1 and 2. From
the Figures, it can be seen that their finish appears ideal for trapping the laser
light and thus minimizing reflectivity losses.
High Power Diode Laser (HPDL) surface treatment
Droplet erosion samples of the material typically used in lower rating steam
turbines were fabricated along with a fixture to hold and rotate them while
carrying out HPDL surface treatment. Each sample was fixed in self-centred
three jaw chuck at one end and supported on a fixture on the other side. The
fixture has a rotating seal, so that the samples can rotate freely and air used for
cooling the samples does not leak. In order to make the surface free from dust
and oil, the samples were thoroughly cleaned using acetone before the start of
the experiment. Rapid cooling of the sample was carried out during HPDL
treatment by introducing compressed air having high volumetric flow rate
through the central hole.
The laser treatment was carried out on the sample surface using a Robotic High
Power Diode Laser System. Laser beam power was controlled in a closed loop by
a pyrometer system and a uniform surface temperature was maintained during
the process. Uniform compressed air cooling of the sample was maintained
throughout the HPDL treatment process.
Droplet erosion testing of HPDL surface treated samples
The test facility consists of a large diameter chamber and a round stainless steel
disc where the test samples are positioned on the periphery of the disk. The disc
is rotated at a high rpm to provide the test sample with tangential velocity of
~150 m/s. Two water jets impinge on the cylindrical test samples and cause
impingement erosion. The cylindrical specimens were selected because the
impingement erosion on actual steam turbine blades occurs at the leading edge,
which also has similar leading edge radius. A precision balance was used for
measuring the mass loss after testing. Depending upon energy and mass fluxes,
the test duration was selected such that steady state erosion is achieved in
limited cycles. The accuracy and ability to repeat of the test have been
established on Martensitic steel sample which is taken as a reference material.
The extent of erosion damage is calculated from the mass loss and density data
of the material.
The hardness of the surface textured and laser treated samples are found to be
much superior than that by conventional laser treated samples. The depth of the
hardened case is also much higher at over 1000 micron. The resultant laser
hardened surface has a higher hardness value (~650 HV 100) as compared to
the hardness achieved from untreated laser hardened surface (~500 HV100) at a
lower power density by upto 5%, for attaining the same surface temperature as
compared to an untreated laser hardened surface, due to enhance absorption
and reduced reflectivity
1. A method of enhancing laser absorption on steel substrate thereby
achieving better properties of hardened case, comprising the following
- surface texturing of a steel sample, to achieved a roughness value
ranging from 1 to 2 microns,
- solvent cleaning of the surface after texturing for laser treatment,
- treatment of textured steel surface with high power diode laser
- testing for erosion of steel surface, treated with high power diode
2. The method as claimed in claim 1, wherein the said steel sample is of
3. The method as claimed in claim 1, wherein the said surface texturing is
done by hard fine shots impinging under pressure on steel substrate to
create controlled roughness.
4. The method as claimed in claim 1, wherein the said cleaning of surface is
done with acetone prior to said laser treatment.
5. The method as claimed in claim 4, wherein the said laser treatment is
done on said textured steel surface with Robotic high power diode laser
6. The method as claimed in claim 4, wherein the said laser treatment is
carried out under control of a pyrometer system which controls the laser
beam power through a closed loop.
7. The method as claimed in claim 4, wherein the said laser treatment
requires rapid cooling to provide a uniform surface temperature, which is
maintained by introducing compressed air having high volumetric flow-
rate through a control hole of the rotating sample during the said process
of laser treatment.
8. The method as claimed in claim 1, wherein said testing of erosion is
conducted by applying a plurality of water jets on the test sample which is
positioned on a high speed rotating stainless steel disc having large
diameter, thereby causing a tangential velocity of 130-170 m/s. resulting
in an impingement erosion.
9. The method as claimed in claim 8, wherein the said testing gives a result
to the extent of erosion damage which is calculated from the mass loss
measured with a precision balance and density data of the material.
10.The method as claimed in claim 1, wherein the said treatment with high
power diode laser being carried out at a lower power density upto 5
percent, receives the same surface temperature as compared to an
untreated laser hardened surface.
11.The method as claimed in claim 1, wherein the said laser treatment gives
an improvement of hardness compared to untreated hardened surface, as
for example values of hardness from 500 HV 100 to 650 HV 100.
12. The method as claimed in claim 1, wherein the said laser treatment gives
a resultant of increased case depth, for example at over 1000 microns,
compared to untreated laser hardened surface.
13.The method as claimed in claim 1, wherein the said laser treatement
being carried out at a lower power, gives an increased life of the lens
compared to untreated laser hardened surface.
A method of enhancing laser absorption on steel substrate thereby achieving better properties of hardened case, comprising the following steps; surface texturing of a steel sample, to achieved a roughness value ranging from 1 to 2 microns, solvent cleaning of the surface after texturing for laser treatment, treatment of textured steel surface with high power diode laser (HPDL), testing for erosion of steel surface, treated with high power diode laser,
|Indian Patent Application Number||1318/KOL/2008|
|PG Journal Number||29/2013|
|Date of Filing||01-Aug-2008|
|Name of Patentee||BHARAT HEAVY ELECTRICALS LIMITED|
|Applicant Address||REGIONAL OPERATIONS DIVISION (ROD), PLOT NO : 9/1, DJ BLOCK 3RD FLOOR, KARUNAMOYEE, SALT LAKE CITY, KOLKATA - 700091, HAVING ITS REGISTERED OFFICE AT BHEL HOUSE, SIRI FORT, NEW DELHI – 110049|
|PCT International Classification Number||B23K26/00|
|PCT International Application Number||N/A|
|PCT International Filing date|