Heat Flow in Laser Hardening,Heat Flow in Laser Hardening,Heat Flow in Laser Hardening,Heat Flow in Laser Hardening
In laser surface hardening, thermal energy is generated by absorption of the laser radiation at the surface. The increase of
temperature in the interior of the workpiece is by way of conduction only; no sources of thermal energy exist below the
surface. Thus, if the rate of absorbed power and the thermal properties of the material are known, it is possible, at least in
principle, to calculate the temperature distribution in the workpiece. This is of considerable value, because it is then
possible to predict the results of laser processing in advance and to calculate the optimum processing parameters, such as
power density, processing speed, and spot size.
The long wavelength electromagnetic radiation (infrared) from a typical carbon dioxide laser is not efficiently absorbed
by ferrous metals at room temperatures. It is, therefore, necessary to coat the workpiece with a substance that will aid in
absorbing the laser energy or to use a Brewster-angle treatment without coatings. Commonly used coating materials are manganese phosphate, graphite, or carbon-black paint. The paint, in the form of flat, black spray paint, is by far the most
convenient coating to use. It is easy to apply and is fairly insensitive to variation in coating thickness.
When laser energy is absorbed at the surface at a rate of 500 W/cm2 (3200 W/in.2) or more, surface temperature rises very
rapidly because the conduction of heat to the interior cannot keep up with the influx of energy to the surface. The higher
the input flux, the more rapidly the temperature rises in the surface layer, and as a consequence, the temperature gradient
in the workpiece will be steeper. The maximum surface temperature allowable is the melting point of the material,
although in practical applications temperatures should be held well below this value. This clearly acts as a constraint on
the depth of austenitization that can be achieved. If lower laser power density is used and the processing speed is
correspondingly decreased, the surface temperature will rise more slowly and the temperature gradient will be less steep.
This allows austenitization to a greater depth, as shown in Fig. 4. However, the rate of cooling by self-quenching will be
slower, and it may be insufficient to trap the dissolved carbon in a martensitic matrix without some carbon precipitation.
This would not allow the material to harden fully. Thus, the two physical limitations of melting temperature (Tm) and
necessary cooling rates act to impose a limit on the obtainable depth of case regardless of the power available for the
processing.

To obtain good self-quenching, it is generally necessary to use high power density and high processing speed for
steels of low hardenability. Steels of high hardenability can be processed to greater depth by relatively low power
densities and low processing speed. For many such steels, the slow processing rate will be necessary to give the material
time to form homogeneous austenite. Thus, deep case depth in plain carbon steels may be difficult to achieve. It is
possible to use external quench procedures, thereby obtaining deeper case depths in steels of low hardenability. This is
achieved at the cost of greater dimensional distortion because the total power input increases with decreasing speed for
constant maximum surface temperature.
Temperature Distribution. If the absorbed power density, laser spot dimensions, processing speed, and thermal
properties of the material are known, the temperature distribution in the workpiece can be calculated by means of several
expressions found in the literature. The simplest expression is obtained if it is assumed that heat only flows normal to the
workpiece surface, that is, one-dimensional heat flow.

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