FIELD OF THE INVENTION
The present invention relates to a process to improve tensile strength and
fatigue life of hot-rolled commercial grade low carbon steel sheet by laser
surface hardening for producing automotive body-component. More particularly,
the present invention relates to a process to produce hardened surface on hot-
rolled steel by phase transformation mechanism to increase the overall strength
and fatigue life of the steel.
BACKGROUND OF THE INVENTION
Surface hardening by different methods for example, flame, induction heating,
are known in the art, and have several demerits. The surface hardening of steel
using laser has attracted much attention during the past two decades High
power laser beam of specific size can be used as a potential case-hardening tool
due to various advantages like high degree of controllability, precision in
treatment areas, excellent amenability to automation, high processing speed.
Furthermore, a typical shallow laser hardened zone allows minimization of
distortion and elimination of post-treatment process requirements as compared
to other known case-hardening techniques.
Typically, when a laser beam of specific power density and spot size is scanned
on a steel surface, the surface temperature increases to the extent of
austenetization temperature, depending on the scanning speed, transformed to
austenite and upon self-quenching by conduction, results in martensite formation
beneath the steel surface. The extent of martensite formation in the
microstructure and its case depth is governed by the hardenability (chemical
composition) of the steel and process parameters adopted. Although prior art
utilize this technique commercially for medium and high carbon steels [1,2],
however, utilizing this method for low carbon steel is not known in the art.
The benefits attributed to the use of lasers are that they provide precisely
determined localized heat input, negligible distortion, ability to treat specific
areas, access to confined areas and short cycle times.
Although, Nd:YAG and C02 systems are known for a number of years, these
systems have several limitations such as high capital cost, perceived reliability of
equipment, low wall-plug efficiency, high size of equipment, low area coverage
rates and complexity of operation, which interalia restricted their adaptability in
industry. Further, such system when used with a laser source for 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 2 kW, diode laser to harden medium carbon steel to
achieve the case depths of up to 1mm at a speed of 400 mm/min, although no
hardness values were reported. An even energy distribution within the spot, and
a shorter wavelength produced by the diode laser, attribute many beneficial
points in using the diode laser beam for surface hardening, for instance,
increased process efficiency, lower reflectance compared to the other available
laser types [2].
Most of the prior art experimentations were however carried out for medium and
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 a continuous wave C02 laser
beam. The study 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.
Besides the above work on laser surface hardening, few patents have also been
published. For instance, [7] states that Long cylinder of medium carbon steel,
medium carbon alloy steel etc, were surface hardened by involving carbon-
nitrogen co-cementing treatment. Similarly, [8-9] used laser carburization
method for surface hardening of pure Iron and low carbon steel, whereas [10-
12] developed laser surface hardening method and apparatus for surface
hardening of gear and to improve fatigue properties of turbine blades alloy steel
respectively. A laser quenching method was introduced in patent [13] and it was
effectively used in [14]. Laser phase transformation and ion implantation process
were used for ferrous and non-ferrous metals to improve the hardness and
corrosion resistance as patented in [16].
The US patent US6218642, assigned to J.F.Helmold & Bro., Inc. discloses a
method of surface hardening of steel workpieces using laser beams so as to
obtain equivalent or superior ductility properties with superior wear resistance.
The selected surface areas of steel workpieces are heat treated using the laser
beam to increase the hardness of the area. Laser beam of less intensity is
subsequently applied, for relieving stress. Application of laser beam reduces
processing time without weakening metal section and its durability. The method
can be used for the cutting rules, knife blades etc.
The European patent EP2161095, assigned to Alstom Technology Ltd., discloses
method of surface treatment of turbine component using laser or electron
radiation. In this method, the surface of the steam turbine is remelted by laser
radiation or electron radiation and then surface-alloying is done to increase the
mechanical stability and the corrosion resistance of the surface of the steam
turbine. The method produces steam turbine part with good smoothness, high
strength and high corrosion resistance thus improves the efficiency of the turbine
blade. This method can be used for treating surface of a steam turbine made of
austenitic or ferritic-martensitic steel.
The European patent EP0893192, assigned to Timken Co, discloses the method
of imparting residual compressive stresses to steel machine components by
inducing martensite formation in a microstructure. In this invention, the steel
component, such as a bearing race, is locally melted using laser beam along its
surface so that the thickness of the melted region is substantially less than the
thickness of the component. The molten steel is rapidly solidified to transform
some of the austenite into martensite. After tempering, most of the surface is
transformed to martensite and the solidified steel acquires a residua!
compressive stress due to the increased volume occupied by the martensite. This
process improves fatigue performance and crack resistance of the component
and can be used to improve physical characteristics of the machine.
The Chinese patent CN101225464, assigned to Xi An Thermal Power Res. Inst.,
discloses a method to improve the anti-oxidation performance in high
temperature steam atmosphere of ferrite/martensite refractory steel. The
properties of quick heating and quick cooling of laser phase transition heat
treatment is utilized to form the steel surface into a fine-grain region. This
improves chromium element diffusion from basal body to oxygenation level,
thereby improving high temperature and steam oxidation resisting properties of
ferrite/ferrite refractory steel. Thus, this method can be used for improving the
properties of ferrite/ferrite refractory steel.
The main drawback of the prior art is that the laser beam hardening process
have only been used for medium and high carbon steels, which have delimited
the use of the process in automotive industry. In addition, the laser beam
hardening process emphasizes application of surface hardening only to improve
the surface related properties (for example, wear resistance, surface alloying,
cladding etc). Therefore, a need exists to propose a process for improvement of
overall property of steel sheet, adaptable in automotive industry.
OBJECTS OF THE INVENTION
It is therefore an object of the invention to propose an improved process to
enhance overall strength and fatigue life of low-carbon steel by laser surface
hardening for producing automotive components.
Another object of the invention is to propose an improved process to enhance
overall strength and fatigue life of low-carbon steel by laser surface hardening
for producing automotive components , which is enabled to develop a
sandwiched structure in the steel blank by using multiple process variables
during laser treatment for example laser power and scanning speed.
A still another object of the invention is to propose an improved process to
enhance overall strength and fatigue life of low-carbon steel by laser surface
hardening for producing automotive components, which generates dual phase
structure upto a certain depth of the steel blank from the surface using multiple
process variables.
SUMMARY OF THE INVENTION
Prior art by the present applicant teaches optimum parameters for surface
hardening by laser beam of hot-rolled commercial grade low carbon (LC) steel
for application on automotive component.
Accordingly, the present invention develops a combination of microstructure and
mechanical strength of the surface upto a specific depth in a low-carbon steel
blank so as to produce a graded and composite microstructured blank with
enhanced overall strength and fatigue life of the auto components manufactured
from the developed steel. The invention adapts different combination of process
variables during laser treatment of the low carbon steel blanks. The best
mechanical properties are identified in terms of phase transformation along the
thickness with a consideration of laser power, scan speed, chemistry of steel and
blank thickness.
The present invention emphasizes the laser surface hardening of a full size blank
(using same processing parameters as of prior art ), to improve the overall
strength and fatigue life of the blank which is used in manufacturing automotive
component. According to the invention, both surface of a hot rolled (HR) LC,
(low-carbon) steel blank is heat treated by a laser beam with the optimized
process variables, for example, laser power 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 dual phase structure with some grain refinement up to a
certain depth on the both sides. This sandwiched composite structure of the
blank provides an enhanced overall tensile strength (upto 22% increase in YS)
and fatigue life (103% increases in fatigue limit).
Variables
According to the invention, the best hardenable, steels (with better formability)
for the surface treatment namely HR(0.08%C), is identified. The selected grade
comprises Mn content (1.4%) with the presence of other elements. The table 1
shows a chemical composition of the grade of steel, considered for the invention.
The initial microstructure of the steel contains mainly ferritic structure. A laser
beam applied using several combinations of laser process variables to achieve a
definite surface temperature for phase transformation. The process variables for
laser surface hardening have been identified as 1200W of laser power and a scan
speed of lOmm/s. These variables can be selected with respect to the desired
depth of hardened layer and hardness level. Laser optics are arranged in such
way that the impinged 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 a 6-axis Robot with the movement of beam occurring
along the short axis of the rectangular beam.
The hardened depths for HR steel blanks was measured upto 0.3mm, and
achieved at optimum process condition of a laser power: 1200W and scan speed:
10mm/s.
Microstructure contains a combination of ferrite and martensite. This fraction of
martensite was enough to achieve 240Hv hardness level. These results are
unique for surface hardening of such low carbon (0.08%) steel.
Two different type of laser surface hardening profile was selected to improve the
overall mechanical property of the blank as shown in Fig. 1. The types are as
follows:
1) No gap between the tracks
2) 1mm overlap between the tracks
Surface hardening of each type of blank was done on one side as well as on
both sides (Fig. 1). The main application of these type of steel is to make auto-
component with improved strength and fatigue life. It will also give better wear
resistance for skin panel components.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS.
Figure 1- Schematic representation of a plan view of a laser track profile for
different samples of a blank surface a)3D view of the steel plates b) No gap
between tracks c) 1mm overlap between tracks.
Figure 2- Optical micrograph of the laser treated HR steel sheet (cross section),
surface hardening being done on both side of the steel blank.
Figure 3- SEM micrograph of a sandwich structure of both side laser treated
samples a) one edge; laser hardened b) middle; based material c) another edge;
laser treated.
Figure 4- Hardness profile along the cross section of the laser treated samples.
Figure 5- Stress-strain diagram a) before laser surface treatment, b) after laser
surface treatment
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 1: Chemical composition of the steel sheet used for experiments.
A cross-sectional macrostructure of a both-sided hardened sandwich structured
created in HR steel is illustrated in Fig. 2. At the sametime, hardness profile was
taken along the thickness of the sample (Fig. 4). The hardness increased to 240
Hv as compared to its base hardness 150Hv in the both side. However, hardness
value of the sandwiched structure in the both is not identical. The reason is that
the heat treatment on a first side affect the microstructure of the second side.
That is to say, when a first side is under treatment, because of heat transfer
effect the structure of the second side gets tempered. Therefore, the hardness
value of the second side gets reduced. The SEM micrograph shown in Fig. 3,
indicates formation of hard phases (for instance martensite) which is responsible
for the increased hardness value.
A tensile sample was fabricated from the laser surface hardened blanks. The
gauge length of the tensile samples were fabricated across a laser track. All the
tensile properties are shown in Table 2 compared to the base material property.
The overall tensile property of several fabricated samples show a substantial
improvement in terms of yield Stress, tensile strength and elongation. The 'No
gap' and '1mm overlap' samples showing almost 22% increase in strength.
Another important point was observed that the improvement of YS was much
higher than UTS in all the cases.
Table 2: Tensile property evaluation of the all laser treated samples.
Fatigue test has been done for Base steel and the sandwiched structure steel
with "No Gap' type of sample and the result is tabulated in Table 3. The fatigue
test parameters are as follows:
R- -1 Sinosoidal waveform, Frequency : 10 HZ
The as-received samples were tested at different amplitude of stress (from 340
MPa to 210MPa). The as-received samples failed at 132919 no of cycles (table
20), whereas the laser treated samples failed at 270262 with an amplitude of
stress of 210 MPa. Therefore, the results show that the fatigue life increased
approximately 103% with respect to the base materials.
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.ll.(1984),1935-1948.
7. Patent No: CN121115, Surface hardening treatment method for inner wall of
long cylinder, 1996-04-24.
8. Patent NO: JP9179776, Surface hardening method by carburization hardening
of pure Iron and low carbon steel by laser, 1984-10-12.
9. Patent No: JP59185723, Low strain surface hardening method of cold worked
parts, 1984-10-22.
10. Patent No: US4533400, Method and apparatus for laser hardening of steel,
1985-08-06.
11. Patent No: US4539461, Method and apparatus for laser gear hardening,
1985-09-03.
12. Patent No: US5073212, Method of surface hardening of turbine blades and
the like with high energy thermal pulses, and resulting product, 1991-12-17.
13. Patent No: US5182433, Method of laser quenching, 1993-01-26.
14. Patent No: US5313042, Laser hardening device, 1994-05-17.
15. Patent No: US6379479, Steel member surface treatment method, 2002-04-
30.
16. Patent No: US6454877, Laser phase transformation and ion implantation in
metals, 2002-09-24.
We Claim:
1. A multi-track laser beam process of surface hardening of a full size steel
blank of low-carbon steel for producing automotive components with
improved strength and fatigue life, the process comprising the steps of:
- providing a hot rolled low carbon steel blank in the form of flat sheet
having a chemical composition by weight percentage,
C,Mn,S,P,Si,AI,V,Nb,and Ti respectively of 0.08, 1.4, 0.005,0.014, 0.005,
0.04, 0.001, 0.001, and 0.002;
- optimizing laser processing variables to produce a temperature capable for
phase transformation of the initial microstructure of the steel sheet;
- selecting a laser track profile for surface hardening of the steel sheet;
- applying the selected laser processing variables in the form of laser power
(1200W) and scanning speed (10mm/s) combinations on the surface of
the steel sheet;
- selecting and adapting associated laser optics to operate the laser beam
such that an impingement laser spot size on the sheet is of rectangular
shape, wherein a 6-axis robot employed to carry the laser through a fiber
fixed on 6th axis enabling an movement of the laser beam under the
specimen along a short axis of the rectangular beam;
- controlling the surface temperature of the specimen to eliminate any
possibility of melting the sheet based on on-line surface temperature
effect and comparing with pre-stored data representing surface
temperature effect; and
- periodically reviewing the development of desired microstructure of the
sample, including measuring hardness level and fraction of different
phases.
2. The process as claimed in claim 1, wherein both surface of the steel sheet
is hardened and a composite sandwich structure is formed with improved
strength and fatigue life.
3. The process as claimed in claim 1, wherein an increase in YS of 22% and
Fatigue life of 103% of the overall steel sheets is obtained.
4. The process as claimed in claim 1, wherein the hardened surface of the
steel sample comprises complex phase structure.

ABSTRACT

The invention relates to a multi-track laser beam process of surface hardening of
a full size steel blank of low-carbon steel for producing automotive components
with improved strength and fatigue life, the process comprising the steps of:
providing a hot rolled low carbon steel blank in the form of flat sheet having a
chemical composition by weight percentage, C,Mn,S,P,Si,Al,V,Nb,and Ti
respectively of 0.08, 1.4, 0.005,0.014, 0.005, 0.04, 0.001, 0.001, and 0.002;
optimizing laser processing variables to produce a temperature capable for phase
transformation of the initial microstructure of the steel sheet; selecting a laser
track profile for surface hardening of the steel sheet; applying the selected laser
processing variables in the form of laser power (1200W) and scanning speed
(10mm/s) combinations on the surface of the steel sheet; selecting and adapting
associated laser optics to operate the laser beam such that an impingement laser
spot size on the sheet is of rectangular shape, wherein a 6-axis robot employed
to carry the laser through a fiber fixed on 6th axis enabling an movement of the
laser beam under the specimen along a short axis of the rectangular beam
controlling the surface temperature of the specimen to eliminate any possibility
of melting the sheet based on on-line surface temperature effect and comparing
with pre-stored data representing surface temperature effect; and periodically
reviewing the development of desired microstructure of the sample, including
measuring hardness level and fraction of different phases.