FIELD OF INVENTION
The present invention relates to determination of optimum process parameters
for surface hardening by laser beam of hot-rolled commercial grade low carbon
steel for producing automotive body-component. More particularly, the present
Invention relates to a process to produce hardened surface of hot-rolled steel by
phase transformation mechanism through optimizing laser processing
parameters.
BACKGROUND OF INVENTION
Noise reduction and less fuel consumption are the major issues for the
automotive industry. Surface hardening by different methods (for example,
flame, Induction heating etc.) are well established but have several demerits. The
surface hardening using laser treatment has attracted much attention during the
past two decades. A high power laser beam on a steel surface can be adapted as
the tool for the process of case hardening. Because of the high degree of
controllability of the heat source, the process lends Itself quite effectively to
automation schedules. Furthermore, the shallowness of typical laser hardened
zones minimizes distortion problems compared to the other case hardening
methods.
The technique [1,2] of surface hardening using laser beams have been
extensively and commercially for high carbon steels, whereas for low carbon
steel this technique is yet to be explored. The benefits attributed to the use of
lasers are that they provide localized heat input, negligible distortion, ability to
treat specific areas, access to confined areas and short cycle times.

Nd:YAG and CO2 systems have both been used for a number of years but under
a limited consideration such as capital cost, perceived reliability of equipment,
low area coverage rates and complexity of operation. However, this system when
used with laser source for the study of surface hardening, the known problems
associated with high reflectance are observed as reported by Selvan et al. [3],
Katsamas [4] and Putatunda et al. [5].
Ehlers et al. [2] used a 2kW, diode laser to harden a medium carbon steel to
achieve the case depths of up to 1mm at speeds of 400 mm/min, although no
hardness is values were reported. An even energy distribution within the spot, and
a shorter wavelength produced by the diode laser, attribute many beneficial
points due to the use of the diode laser beam for surface hardening, for
instance, increased process efficiency, lower reflectance compared to the other
available laser types [2].
The transformation hardening of hypo-eutectoid and hypereutectoid steel surface
was reported by Ashby [6], using continuous wave CO2 laser beam. They have
concluded that steels with a carbon level below 0.1%wt does not respond under
laser treatment method because of low volume of martensite and low carbon
content.
Most of the prior art experimentations were however carried out for high carbon
steel for laser hardening using different types of laser beam. For instance, the
transformation hardening of hypo-eutectoid and hypereutectoid steel surface was
reported by Ashby [6], using continuous wave CO2 laser beam. They have
concluded that steels with a carbon level below 0.1%wt does not respond under

laser treatment method because of low volume of martensite and low carbon
content.
The main drawback of the prior art is that laser beam hardening process have
only been used for high carbon steels, which has limited use in automotive
industry.
OBJECTS OF INVENTION
It is therefore an object of the invention to propose a process for surface
hardening of low-carbon steel by laser beam applicable to automatic component.
Another object of the invention is to propose a process for surface hardening of
low-carbon steel by laser beam applicable to automatic component, which is
enabled to adapt multiple process variables during laser treatment for example
beam energy and laser scanning speed.
A still another object of the invention is to propose a process for surface
hardening of low-carbon steel by laser beam applicable to automatic component,
which selects the process variables to achieve a surface temperature capable to
bring about a phase transformation of the initial microstructure of the steel.
A further object of the invention is to propose a process for surface hardening of
low-carbon steel by laser beam applicable to automatic component, in which
both the surface of the steel specimen is hardened and a sandwitch structure is
produced to achieve an improved noise reduction property.

A Still further object of the invention to propose a process for surface hardening
of low-carbon steel by laser beam applicable to automatic component, in which
the formed microstructure of the surface is characterized both by SEM and
optical microscope including identification of the formed phases by known XRD
process.
SUMMARY OF INVENTION
The present invention provides the optimum parameter for surface hardening by
laser beam of hot-rolled commercial grade low carbon steel for application on
automotive component. The surface of hot rolled (HR) LC steel is heat treated by
a laser beam heating method with different process variables for example, beam
energy and laser scanning speed. The effects of laser beam processing (LP) on
the microstructure, micro hardness, and residual stress of the low carbon steel
are recorded. Laser beam processing of the surface causes a complex phase
transformation with some grain refinement up to a certain depth. Surface
hardness is measured to draw a hardness profile along the thickness of steel,
which shows an increased value up to 50%- 60% for both Cold rolled steels and
Hot rolled steels.
Variables
The process variables for laser surface hardening have been identified as 1100W
of laser power and a scan speed of 10mm/s. These variables can be selected
with respect to the desired depth of hardened layer and hardness level.

The inventive process adapts multiple process variables during laser treatment of
low carbon steel blanks. The best hardness profile is Identified In term of phase
transformation along the thickness based on laser power, scan speed, chemistry
of steel and blank thickness. The invention can be used for low carbon steel even
for 0.04% of carbon content.
According to the invention, the best hardenable, formabale steels for the surface
treatment namely HR (0.08%C), is identified. The selected grade comprises Mn
contents (1.3%) with presence of other elements. The table 1 shows the typical
chemical composition of this grade of steel. Steel blanks in the form of 150mm X
150mm area of flat sheets with thickness of 1.3 - 3.3mm is considered. The
Initial microstructure of the steel contains mainly ferritic structure. A laser beam
on the steel surface is applied using several combination of laser process
variables to achieve a definite surface temperature for phase transformation. The
laser powers used is in the range of 900-1100W at 100W Intervals. These powers
are applied during laser treatment in the conjunction of surface speeds In the
range of 8 - 12mm/s, at 2 mm/s intervals. Laser optics are arranged such that
the impingement spot size on the material constitutes a rectangle in shape of
approximately 17mm x 2mm in area. The beam is moved beneath the Flat steel
specimens using an x-y table, the movement of occurring along the short axis of
the beam rectangle. Surface temperature are assessed from parameters based
on prior data so that melting is avoided. The desired microstructure Is checked
(martensite and ferrite) Including measurement of the increased hardness level
(320 Hv) and fraction f different phases. After the laser heat treatment and air
cooled to room temperature, the treated surfaces are sectioned perpendicular to

the surface at 90° to the direction of beam travel. Following the standard
metallographic preparation, hardness measurements are made using a standard
microhardness testing machine. Initial indentation is made as close to the
surface as practicable and subsequent measurements taken at 200Mm intervals
perpendicular to the measuring surface. The microstructure of each surface is
characterized by SEM as well as by an optical microscope. The newly formed
phases are Identified using standard XRD method and attributed by residual
stress. Both the surface of the steel blank is hardened and a sandwich structure
Is made to achieve improved noise reduction property.
According to the invention, low carbon steel of commercial grade HR can be
surface hardened using high power diode laser beam with a highest hardness
level of 320 Hv with respect to Its base hardness level 180 Hv upto a depth of
0.6mm. The depth of hardened layer can be further increased using higher level
of laser power of applying a suitable combination of laser power and scan speed.
The hardened depths for HR steel blanks had been measured upto 0.6mm, which
have been achieved at optimum process condition of a laser power: 1100W and
scan speed: 10mm/s.
Microstructure contains a combination of ferrite and martensite. This fraction of
martensite was enough to achieve 320Hv hardness level. These results are very
much unique for surface hardening of such low carbon (0.08%) steel.
A transition zone was also identified with fine ferritic structure, which has been
showing higher hardness than base structure.

A compressive residual stress (-500MPa) has been developed during hardening
process which adds more merits for the auto component development.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1 - Graphically shows a hardness profile of a laser treated HR steel
blank along the thickness at different laser power and at a constant
scan speed of 10mm/s.
Figure 2 - Graphically shows harness profile of a laser treated HR steel blank
along the thickness at different laser power and at a constant scan
speed of 12 mm/s.
Figure 3 - Graphically exhibits hardness profile of a laser treated HR steel
blank along the thickness at different laser power and at a constant
scan speed of 8mm/s.
Figure 4 - Shows hardness level at different positions of a measuring surface
at laser power of 1100W with variable scanning speed of the HR
steel blank.
Figure 5 - Shows a laser hardened area of a steel blank in full width.
Figure 6(a)- Shows a micrograph of a laser treated cross section area of a HR
steel blank.

Figure 6(b)- Shows a micrograph of a laser treated cross-section of the base
steel blank after treatment under the condition of laser power of
1100W and scan speed of 10mm/s.
Figure 7(a)- Shows a 2theta scan of a laser treated steel.
Figure 7(b)- Shows a measurement result on residual stress in a laser-treated
steel.
Figure 8 - Shows a sandwich structure formed by laser treatment of both side
of a steel blank.
DETAIL DESCRIPTION OF THE INVENTION
Table 1 shows the chemical composition of an exemplary steel blank selected for
laser beam treatment according to the invention :-

Table - 2 shows that a combination of laser scan speeds and laser power can
produce hardened layer up to a depth of at least 500-600µm in an industrial
grade hot-rolled low carbon steel. Under combinations of slow speed and high

power surface, a melting was observed. The hardness of the treated steel was
measured along the thickness to plot the hardness profile, as shown in Figure 1,
Figure 2, Figure 3.
The average hardness level at different position along the thickness direction
were measured and plotted as a function of the scan speed for a constant laser
power (Figure 4). The measurement results clearly shows that a combination of
laser power and scan speed where the hardness level was measured, produces a
highest level of hardness irrespective of thickness of the steel blank. The scan
speed has significant effect on the hardness of the laser treated surface. With a
comparatively higher speed, the hardness did not increase, although the highest
hardness was found for 10mm/s scan speed for the same grade and thickness of
steel. For instance, the maximum hardness was recorded for HR steel blank as
320HV, in contrast of their base level hardness recorded as 180Hv (Figure 4).
The maximum width of the laser hardened zone was measured in the range of
14-15mm with a maximum thickness of 600µm (Figure 5). The SEM micrograph
of the hardened zone as well as that at the base level is shown in Figures 6(a)
and 6(b). The microstructure predominantly being a combination of acicular
ferrite and martensite were identified using the standard XRD method (Figure
7a,7b). The laser treated zone was evident of different layer of microstructure in
the laser treated zone from the optical micrograph (Figure 5). This layer are
likely, impingement surface is the most hardened zone with a depth of 30-50µm,
whereas immediately next layer contains a combination of ferrite and martensite
and the heat affected zone contains a finer ferritic structure.

Table 2: List of parameters for laser treatment experiments

Table 3: Calculated residual stress from XRD measurement data.

The grain size reduces to approximately 1/3 of tine base level grain size. The
residual stress was also measured using standard XRD techniques, which shows
the development of compressive stress (table 3) in the range of -500-600 MPa.
The sandwich structure was produced by hardening the both sides of the steel
blank to confirm the symmetry of desired hardness level at both sides (Figure 8).
The results indicates that it is possible to produce a hardened layer on low
carbon steel using laser processing conditions upto a certain thickness level. In
addition, the range of conditions between not producing a hardened layer and
melting of the metal surface was relatively narrow, high powers and relatively
moderate processing speeds giving the greatest flexibility for laser treatment of
the surface. The hardening response of Hot rolled (HR) grade of steel was much
better as compared to the other low carbon Cold rolled (CRCA) in respect of the
successful range of processing conditions, ability to produce a hardened layer,
the depths achievable and higher peak hardness. Due to the improved
hardenability conferred upon the material by the presence of sufficient amount
of carbon (0.08%) which provides superior hardening response over a range of
laser powers and scan speed, the parameters as described hereinabove have
been possible to achieve.
References:
1. W.M. Steen, Laser Material Processing, Springer, London 1991.
2. B. Ehlers, et al., Proceedings of the ICALEO, Section G, 1998, pp. 75-84.
3. J. Selvan, et al., J. Mater. Process. Technol. 91 (1) (1999) 29-36.
4. A.I. Katsamas, Surf. Coat. Technol. 115 (2) (1999) 249-255.
5. S.K. Putatunda, et al., Surf. Eng. 13 (5) (1997) 407-414.
6. M.F. Ashby et al., Acta Metall. Vol. 32, No. 11.(1984), 1935-1948.

WE CLAIM
1. A process for surface hardening of low-carbon steel by using high power
diode laser beam adaptable to automotive components, the process
comprising the steps of:
- providing a hot rolled low carbon steel blank in the form of flat sheet, and
having chemical composition by weight percentage, c, Mn, S, P, Si, Al, V, Nb,
and Ti respectively of 0.08, 1.4, 0.05, 0.14, 0.05, 0.4, 0.01, 0.01, and 0.002;
- selecting several combination of laser process variables to produce a
temperature capable for phase transformation of the initial microstructure of
the steel sheet;
- optimizing the laser power including the surface speed of the laser beam for
different intervals;
- applying the selected laser power including the surface speed at
predetermined intervals on the surface of the steel sheet;
- providing laser optics associated to operate the laser beam such that the
impingement spot size on the sheet are of rectangular shape, including an x-y
table to enable the movement of the laser beam under the specimen along a
short axis of the beam rectangle;
- controlling the surface temperature to eliminate the possibility of melting of
the sheet based on on-line comparison with respect of pre-stored data;

- checking periodically the development of desired microstructure while
application of the laser beam including measuring the hardness level and
fraction of different phases; and
- air-cooling at room temperature the steel sheet after completion of laser
beam application.

2. The process as claimed in claim 1, wherein the formed microstructure of each
surface is characterized by SEM, and further by adapting an optical
microscope.
3. The process as claimed in claim 1, wherein the hardness measurement is
made by adapting a standard microhardness testing machine, and wherein
the initial indentation is made closest to the surface with subsequent
measurements taken at least at 200 µm intervals perpendicular to the
measurable surface.
4. The process as claimed in claim 1, wherein the newly formed phases are
identified by adapting standard XRD method and arrtributed by residual
stress.
5. The process as claimed in claim 1, wherein both the surface of the steel sheet
is hardened and a sandwitched structure is formed which achieves improved
noise reduction properties.

6. A process for surface hardening of low-carbon steel by using high power
diode laser beam adaptable to automotive components as substantially
described and illustrated herein with reference to the accompanying
drawings.

The invention relates to a process for surface hardening of low-carbon steel by
using high power diode laser beam adaptable to automotive components, the
process comprising the steps of providing a hot rolled low carbon steel blank in
the form of flat sheet, and having chemical composition by weight percentage, c,
Mn, S, P, Si, Al, V, Nb, and Ti respectively of 0.08, 1.4, 0.05, 0.14, 0.05, 0.4,
0.01, 0.01, and 0.002; selecting several combination of laser process variables to
produce a temperature capable for phase transformation of the initial
microstructure of the steel sheet; optimizing the laser power including the
surface speed of the laser beam for different intervals; applying the selected
laser power including the surface speed at predetermined intervals on the
surface of the steel sheet; providing laser optics associated to operate the laser
beam such that the impingement spot size on the sheet are of rectangular
shape, including an x-y table to enable the movement of the laser beam under
the specimen along a short axis of the beam rectangle; controlling the surface
temperature to eliminate the possibility of melting of the sheet based on on-line
comparison with respect of pre-stored data; checking periodically the
development of desired microstructure while application of the laser beam
including measuring the hardness level and fraction of different phases; and air-
cooling at room temperature the steel sheet after completion of laser beam
application.