FIELD OF THE INVENTION
The invention relates to a technique of improving the hardness of nodular cast iron. More particularly the invention relates to a line beam based Laser Surface Melting (LSM) process to improve the hardness of nodular cast iron.
BACKGROUND OF THE INVENTION
Generally, Laser Surface Melting (LSM) process exhibits low thermal penetration resulting in negligible distortion of nodular cast iron materials, and is used for improving the tribological properties of materials and enhance their life against wear, erosion and corrosion. While at the same time, the process need to address the reflectivity, Beam shaping and melt pool shrouding. The prior art Laser Surface Melting (LSM) process uses circular beam with limited area of coverage with surface finish of around 25 pm. Considering less area of coverage, multiple passes of circular beam need to be used to cover wider surface areas to be treated. Hence, the productivity of prior art process is low. The present inventors note that line beam instead of circular beam if applied in a LSM process, is likely to increase the productivity. However, Laser Surface Melting (LSM) with line beams needs to be established with optimum parameters for nodular cast iron to improve the base material hardness value of 220 HV0.3.

OBJECTS OF THE INVENTION
It is therefore an object of the present invention to propose a line beam based
Laser Surface Melting (LSM) process to improve the hardness of nodular cast
iron.
Another object of the present invention is to propose a line beam based Laser
Surface Melting (LSM) process to improve the hardness of nodular cast iron,
which enhance the process-productivity.
SUMMARY OF THE INVENTION
Improving the surface conditions of materials subjected to abrasion, erosion and corrosion is a long-felt need of the industries to reduce the erosion rate of the components. One of the solutions in respect of nodular cast iron seems to be a technique to be improve the hardness of nodular cast iron. Accordingly, a line beam based Laser Surface Melting process is provided in which the process parameters for achieving 4.2 times higher hardness than that of the base nodular cast iron material have been optimized. Scanning Electron Microscope (SEM) image of the Laser Surface Melting sample according to the present

Invention displays very fine structure containing austenite dendrites with inter-dendritic network of carbides due to high self-cooling rate during solidification solidificotiQon. It was also confirmed through X-Ray Diffractometer (XRD) that the structure was changed from Body Centered Cube (BCC) to y Face Centered Cube (FCC - Fe3C) when the nodular cast iron sample was treated with Laser Surface Melting (LSM) by line beam. The micro hardness of Laser Surface Melting (LSM) zone of the sample increased considerably due to the presence of inter-dentritic structure. It was evident that the production rate of the process is increased around 70% than that of the prior art Circular beam. The Laser parameter optimized for line beam based Laser Surface Melting (LSM) process is applicable to improve the hardness of nodular cast iron of 920.8 HV0i3.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The invention can now be described in detail with the help of the Figures of the accompanying drawings in which:
Fig. 1 shows the Process flow for Laser parameter optimization of Laser Surface Melting (LSM).

Fig. 2 shows the line diagram of a prior art Laser Surface Melting (LSM) process with circular beam.
Fig. 3 shows the line diagram of Laser Surface Melting (LSM) process with line beam according to the present invention.
Rg. 4 shows a Scanning Electron Microscope (SEM) image of the nodular cast iron sample treated by Laser Surface Melting (LSM) with line beam exhibiting very fine structure containing austenite dendrites with an inter-dendritic network of carbides.
Rg. 5 shows an X-Ray Diffractometer (XRD) pattern of the sample of base material displaying that X-Ray Diffractometer (XRD) peaks comprises as Body Centered Cube (BCC- a Fe) and graphite nodules, which when treated by the inventive process of Laser Surface Melting (LSM) with line beam, the structure was changed from Body Centered Cube (BCC) to y face Centered Cube (FCC-Fe3C).
Fig.6 shows a Micro hardness chart for a sample treated according to the inventive process of Laser Surface Melting (LSM) with line beam.

DETAILED DESCRIPTION OF A PRERRED EMBODIMENT OF THE PRESENT INVENTION
Fig.l shows the process flow to optimize the parameters of Laser Surface Melting
(LSM) process for nodular cast iron to improve the hardness. Laser Surface
Melting was carried out over the surface of the samples with 3 KW fiber laser of
wavelength 1050 nm and the process parameters were optimized as shown in
Table 1 by evaluating the hardness of samples treated under varying parameter
conditions. Fig. 2 shows the line diagram of circular laser beam (1) focused on
the specimen (2) leading to the formation of melt pool width (3) of around 3mm.
Fig. 3 shows the line diagram of line laser beam (4) directed on the specimen
(5) leading to the formation of melt pool width (6) of around 20mm. Flg.4, Fig.5
and Fig.6 indicate the material characteristics evaluated by Scanning Electron
Microscope (SEM) and X-Ray Diffractometer (XRD) confirming that the surface
material property obtained via circular laser beam (1) is also being achieved via
line laser beam (4).

Fig. 4 provides the Scanning Electron Microscope (SEM) image of Laser Surface Melting (LSM) sample by line beam showing very fine structure containing austenite dendrites with an inter-dendritic network of carbides (7). From the micrograph as shown in Fig. 4, it was evident that the graphite nodules had dissolved in the molten ferrous material during treatment and rapid self-quenching had suppressed the re-formation of nodular cast iron. The absence of cracking during Laser Surface Melting (LSM) of cast iron was due to inter-dendritic structure (7). The X-Ray Diffractometer (XRD) analysis for base and laser treated sample by line beam were performed and the resulting diffraction patterns is shown in Fig.5. In untreated samples, most of the X-Ray Diffractometer (XRD) peaks comprises as Body Centered Cube (BCC - a Fe) and graphite nodules. After laser melting, the structure was changed from Body Centered Cube (BCC) to y Face Centered Cube (FCC-FeaQ. The presence of inter-dendritic structure (7) has possibly increased the hardness and the measured value of hardness is shown in Fig. 6.

Table 1. Laser surface melting parameters for Line Beam

Power Scanning Defocus(mm) Overlapping f Interaction [ Energy
(W) speed (mm/s) ratio % time(s) density(J/mm2)
1000 300 15 30 0.46 110.72
From Fig. 6 showing the graphical representation of micro hardness, the micro hardness of the Laser Surface Melting (LSM) zone is 920.8 HV0.3 for a depth of 1 mm while that of the untreated material is 220 HV0.3.
As the melt pool width obtained through line laser beam (4) input is higher than that of circular laser beam (1) input, it leads to decrease in the processing time for line laser beam (4) than that for circular laser beam (1). Hence, production rate could be improved by 70% for nodular cast iron in the case of line laser beam (4) when compared to circular beam.

WE CLAIM :
1. A Line beam Laser surface melting process applicable to nodular cast iron for increasing surface hardness of the base material including productivity enhancement, the process comprising the steps of:
- supplying an input power of 1000 watts for Laser Surface Melting (LSM) process by line beam;
- maintaining a scanning speed of the line laser beam at 300 mm/s wim the melt pool width of 20 mm leading to an interaction time of the laser beam on the material surface up to 0=46 seconds;
- restricting a Defocus depth to 15 mm in the Laser Surface Melting (LSM) process by the Line beam;
- maintaining an overlapping ratio at a maximum of 30%; wherein the energy density of line laser beam for Laser Surface Melting (LSM) process is 110.72 J/mm2.

2. The process as claimed in claim 1, wherein the hardness value of the nodular cast iron sample is increased as a result of very fine microstructure of austenite dendrites with an inter-dendritic network of carbides due to high self-cooling rate during solidification.
3. The process as claimed in claim 1, wherein the carbide formation of Y FCC (Fe3C) from BCC (a Fe) takes place due to process parameters utilized for nodular cast iron.
4. The process as claimed in claim 1 wherein the micro hardness of the laser treated surface is improved up to 920.8 HVa3. for depths of samples up to 1 mm.