In this article we will discuss about:- 1. Introduction to Laser Hardening 2. Metallurgical Variables of Laser Hardening 3. Advantages and Disadvantages.
Introduction to Laser Hardening:
Laser beams are invisible electromagnetic radiations in the infra-red portion of the spectrum, and are increasingly being used for surface-hardening of ferrous materials to improve mechanical properties like wear resistance and even fatigue resistance.
There are two main type of Lasers used- YAG Solid-state type and the carbon-dioxide gas type. The output of YAG laser has much shorter wavelength, 1.064 µm, whereas the carbon dioxide laser emits radiations with 10.8 µm wavelength. Carbondioxide laser is more commonly used and is suitable for surface hardening, particularly when the process requires more than 500 W of power.
The power density of laser beam is usually expressed as watts per square centimeter. The power densities used in laser surface hardening are in the range of 500 to 5000 W/cm2 with dwell times in the range of 0.1 s to 10 s. For carbon steels, power densities used are from 1000 to 1500W/cm2 with dwell time of 1 to 2 s.
During Laser surface hardening, a laser can generate very intense energy fluxes at the surface of the component, when the Laser radiations impinge on it, and are absorbed to generate heat energy. This heat is then conducted inside the component. When the power density of the laser beam is high, the rate of heat generation is much higher than the rate of heat conduction. The temperature of the surface layer increases rapidly to soon attain the austenitising temperature.
A moderate power density of 500 W/m2, results in temperature gradient of 500°C/mm. The laser beam may be moved over the surface of the component as illustrated in Fig. 8.78. The surface which meets the laser beam gets heated up. Once the beam passes over, the heated volume gets subsequently ‘self-quenched’. Thus, by selecting power density and the speed of the laser spot (i.e., the dwell time), a desired case depth can be hardened.
The case depth also depends on the hardening response of the ferrous material (i.e., its original microstructure, and its hardenability). The case depth is rarely more than 2.5 mm. The shallow hardening low and medium carbon steels have 0.25 mm and 1.3 mm as case depth respectively.
As the heating and the cooling rates are very high, even the non-hardenable mild steels can be hardened. The hardness obtained by laser-hardening is slightly higher than that obtained by conventional-hardening. As it leads to higher wear resistance, machine components like camshafts, crankshafts, gears could be surface hardened. Commonly the materials which are laser-hardened are plain carbon steels (AISI 1040, 1050, 1070), alloy steels (4340, 52100), tool steels, cast irons (grey, nodular, malleable).
Ferrous materials are not good absorbers of laser beams, and around 80 to 90% of the incident radiations of a CO2 laser are reflected. Chemical coatings such as manganese phosphate, and/or paints of graphite, silicon and carbon which are applied as spray-paint can be used to absorb up to 80-90% of laser energy, or use a Browster-angle treatment without coatings.
Metallurgical Variables of Laser Hardening:
Laser-surface-hardening is similar to any other surface-hardening method such as induction, or flame, except that the laser beam is used to generate heat here. The heating time to the austenitising temperature, particularly in laser heating, is very short-fractions of seconds to few seconds. The dwell time cannot be made very large as surface melting may occur which is undesirable.
As the heating rate is very high, higher temperatures are attained soon. The rate of austenitisation is fast, but the time of austenitisation is too short (Fig. 8.78 a). Thus, the original microstructure of ferrous material should be very fine-most preferably the hardened and tempered state.
Alloy steels intended to have higher hardenabilities should have very fine carbides particles even then their dissolution is difficult. Diffusion of carbon though faster than alloying elements requires longer dwell time (low speed of motion of laser spot) to obtain homogeneous austenite.
In one technique, the time above the transformation temperature (A3) can be skillfully increased. In Laser heating, the leading edge of the laser-spot has higher than normal power density. If the ratio of power density between the leading edge and the rest of the spot is about two to one, then the variation of temperature with time till the trailing edge has reached is illustrated in Fig. 8.78 (b).
Compare with Fig. 8.78 (a). The longer time of austenitisation (almost double) helps to have better diffusion of carbon even over longer distances. This results in obtaining homogeneous austenite even in nodular cast irons, low and medium carbon steels.
Higher peak temperatures obtained by high-power-density-laser beams result in more retained austenite in steels. The presence of larger amount of retained austenite is not desirable.
To increase the area of surface-hardening, the overlapping passes over the surface of the component result in soft tempered bands between the passes. To avoid this, the width of the laser beam should be made equal to the width of the component but it is limited by the available power of the laser.
The important variables are- Power density, travel speed (dwell time), uniformity of power density, size of laser spot, thermal properties of steel, hardenability of steel (which includes response of material to rapid heating and quenching i.e. original microstructure and type of phases present in it).
With the use of lasers of power density 500 W/cm or more, the temperature gradient at the surface of component becomes steep. As melting of the surface is to be avoided or rather the peak temperature should be much less than this, the speed of travel of laser spot has to be high. This limits the case depth obtainable. (Fig. 8.79). A ferrous material having poor hardenability such as plain carbon steels are laser surface hardened by high power density and high travel speed.
Lasers of lower power density have lesser temperature gradient at the surface and the temperature attained at the surface of component is less (than high power density), the dwell time of laser can be increased (taking care that melting does not occur) by decreasing the speed of travel of Laser spot.
As the dwell time is more, steels of high hardenability can be used here as homogeneous austenite can be obtained now. This increases the case-depth. Now as the case depth is more, the gradient of temperature is not that steep, the rate of heat removal from this region by the rest sections of the component is less. ‘Self-quenching’ is not efficient.
This may not be enough in many cases to produce martensite. Quenching from outside may become essential. Even in materials of low hardenability, low power density and low travel speeds may be used to increase the case depth, but it may require invariably quenching from outside. Quenching from outside leads to greater distortional effects.
For depth of hardening, Steen and Courtney’s relationship is:
Y = depth of hardening (mm),
P = Laser power (W),
Db = incident beam diameter (mm)
V = travel speed (mm/s)
but with a considerable scatter of experimental data. At a constant value of P/√DhV, the depth of hardening can vary by a factor of 2.
Advantages and Disadvantages of Laser Hardening:
1. Non-hardenable steels like mild steels can be surface hardened.
2. Hardness obtained is slightly higher than conventional hardening.
3 Closer control over power inputs helps in eliminating dimensional distortion.
4. Beam (with the help of optical parts) can easily reach the inaccessible areas of components, and re-entrant surfaces.
5. No vacuum or protective atmosphere is required.
6. The last optical element of the Laser and the component to be surface hardened may be far-placed.
7. Very long and irregular-shaped components can be hardened easily.
8. Laser-delivery systems are quite flexible.
9. Several jobs can be performed simultaneously with one laser by using several working places each with its own optical system.
10. Entire process can be controlled by microprocessors.
11. High productivity can be achieved as time is too small.
12. Distortion is minimal.
13. It is possible to give localised treatment by this method.
14. Normally no external quenching is needed, thus saves the problems related to coolants, storage, spilling, and cost.
15. No final machining is required after hardening.
16. There are no flames, no contamination etc., in this process.
17. There could be precise control over the areas to be hardened.
1. High initial cost particularly of large lasers.
2. Lasers use 10% of the input energy, i.e., there are inefficient.
3. The depth of case is very limited.
4. Working cost is high.
5. Difficult to surface harden high alloy steels.
6. Extra care is needed to avoid fusion.