Principle of Laser Hardening of Steel or Cast Iron
Steel is essentially an alloy of iron and carbon, and often also contains various other substances such as chromium, vanadium or titanium. Depending on the chemical composition and the temperature, an amazing variety of different variants of steel exist in thermal equilibrium, and non-equilibrium states of steel are also technologically very important. With various processes, often involving rapid heating and cooling, steel can be converted into other forms with other microscopic structures and substantially different properties in terms of hardness, strength, ductility, density, chemical robustness etc.
In particular, a substantial hardening of carbon-rich steel (with at least 0.8% of carbon) is possible by heating it to roughly 1000 °C (below the melting point) and thereafter cooling it with an appropriate speed. What happens microscopically is basically that the integration of carbon changes. At 1000 °C, one has the austenite form with a face-centered cubic (FCC) lattice, which can integrate a substantial amount of carbon, basically as iron carbide. If the steel is then cooled slowly, the iron is transformed into the body-centered cubic (BCC) lattice (ferrite). As that can accommodate less carbon, the carbon precipitates in the form of isolated grains of Fe3C, called cementite. The mixture of ferrite and cementite is called pearlite. In case of more rapid cooling, however, the carbon atoms do not have sufficient time to migrate to the cementite grains, and thus remain more dispersed in the ferrite. As a result of that, crystal defects within the (by far dominating) ferrite grains can no longer slip so easily, and the material (in that form called martensite) is correspondingly harder. At the same time, the steel surface becomes chemically more resistant.
Similar processes can be achieved with cast iron, containing more than 2% carbon.
A side effect results from the reduced density of martensite: if only the surface is transformed into that form, substantial internal mechanical stress results.
The Laser Hardening Process
The laser hardening process simply involves heating the surface with a moderately intense laser beam for a short while; the heat is then conducted downwards. When the laser beam is turned off or moved away, the surface rapidly cools, mainly by heat conduction into the bulk material (self-quenching).
While in some cases hardening is applied to a small limited area, in other cases it is applied to long stripes, or by scanning to larger areas. In the latter cases, the hardening is done sequentially by moving the laser processing head.
The laser hardening process is much faster than with traditional hardening methods. Depending on the process details, the hardening may occur up to a depth of about 3 mm in the steel, or somewhat less in cast iron. Further inside the material, the temperature excursion is not strong enough to cause hardening.
It is very advantageous that the heat can be applied in a very targeted and controlled manner. Therefore, laser hardening can be applied in cases where flame hardening, for example, would not properly work. Often, one needs less or no reworking after the process, since one directly obtains parts with a good quality. The shape of treated machine parts is hardly changed. The rapid processing, sometimes even “on the fly” (during movement), is another important advantage.
Laser Sources; Absorption of Laser Light
Depending on the circumstances, quite different laser powers between roughly 0.1 kW and 10 kW are applied. As the beam quality requirements are quite moderate, it is nowadays common to use direct diode lasers – a particularly low-cost and power-efficient solution: the wall-plug efficiency of such a laser source is often about 50%, or even around 70%. Unfortunately, the absorption of the laser light – typically at wavelengths between 0.8 μm and 1 μm – is not perfectly efficient due to the high reflectivity of metal surfaces in that spectral region. Therefore, in some cases one first applies an absorbing coating (e.g. of graphite) to the surface, e.g. increasing the absorptivity to around 85%. It can also help to produce a suitable surface microstructure, for example by aluminum oxide blasting.
In principle, one could use other types of laser diodes for other spectral regions with better absorption, but normally one achieves high enough output power and beam quality only with devices based on gallium arsenide technology, which is limited to the mentioned wavelength range.
Before direct diode lasers were sufficiently developed, diode-pumped solid-state lasers were widely used, which very easily reach the required beam quality. A somewhat better beam quality can actually help to obtain the ideal intensity profile by rapid scanning (see below).
It is common to apply flat-top beam profiles, i.e., with a quite uniform intensity over some area and very low intensity outside that. Such beam profiles are often naturally provided by the used laser diode sources (containing a large number of small emitters), and are most appropriate, because that way one achieves consistent temperatures over the full processed area.
Note that the temperature profile accurately reflects the applied intensity profile due to a more or less one-dimensional heat flow into the bulk material, provided that the width of the treated area is large compared with the depth of the processed material. Where this is not the case, it may be useful to shape the intensity profile accordingly, with somewhat lower intensity in the center region.
Instead of directly producing an appropriate beam profile, one may also use a more tightly focused laser beam in conjunction with a laser scanner. By controlling the scan pattern at a sufficiently high speed, one can flexibly generate a wide range of average intensity profiles.
It is beneficial to carefully control the temperature of the process by monitoring it continuously (via the generated heat radiation) and automatically adjusting the laser power or the movement speed accordingly. That leads to more reproducible high quality results. Such techniques are used in industrial laser hardening machines.
A process gas may not be needed for protecting the treated surface, but nevertheless one often uses a cross-jet in the laser processing head for protecting the optics against the deposition of material from fumes. Note that although the steel does not significantly evaporate in the process, there may be contaminants like oils or other dirt, causing fumes when being burned by the laser beam.
Applications of Industrial Laser Hardening
Typical industrial applications of laser hardening are the fabrication of machine parts which must withstand substantial forces during their operation. For example, that is the case for turbine blades, where the front part is subject to particularly high stress, or for bending tools. Another example is the improvement of camshafts of combustion engines and gear wheels, which can become much more long-lived with such hardening treatment of the surface. Numerous other applications exist in the automotive and aerospace industry and in other areas of industrial manufacturing.
Note that it is often desirable to harden only the surface (rather than the whole volume) in order to avoid detrimental effect of the hardening, and particular an increased brittleness.
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