Heat Treating with Lasers

reprinted from Advanced Materials & Processes, August 1998
by Leonard Migliore

One of the first production applications of lasers in material processing was for heat treating cast iron . While the use of lasers for welding and cutting has expanded drastically in the last 20 years, heat treating applications are rare. With the increasing availability of high-powered lasers, however, there has been renewed interest in using them as energy sources for thermal processing. Job shops and industrial manufacturers are doing laser heat treating today, and they are making it pay.

Laser heat treating's niche

You should be careful when considering lasers for heat treating: lasers are very expensive heat sources in comparison to flame, electrical resistance or electrical induction. This makes it inadvisable to use them when other sources will serve. There are, however, a number of situations where the special characteristics of lasers make them the right choice for selective heat treatment. The areas where lasers are effective are where very high concentrations of energy are needed or where the region to be processed is small or unusually shaped.

These conditions make it difficult or impossible to use more conventional sources. Radio Frequency (RF) induction heating is the only source that approaches the laser in energy concentration or spatial control. While it is a popular heat treating method, RF induction is difficult to use on internal diameters, tends to heat unwanted areas and can generate large amounts of electrical interference. Laser heat treating can, in many cases, be used to solve these problems.

Lasers as Energy Sources

Lasers are able to deliver substantial amounts of power in the form of light. Commercial neodymium YAG lasers are available with continuous outputs up to 4 kilowatts, while carbon dioxide lasers are available with powers up to 45 kilowatts. Both kinds of lasers are able to concentrate their output into very small areas. A 5 kilowatt laser beam can easily be focused to a spot 0.5 mm in diameter, producing an irradiance ( power per unit area) of 2.5 million watts per square centimeter (The correct SI unit is watts/square meter but most literature uses watts/cm2. Irradiance itself is often called "power density", which is more vivid but technically incorrect.). This concentration of light can be a powerful heat source. While light itself has no temperature, one may calculate a temperature equivalent to the irradiance using the Stefan-Boltzmann equation Eb = s T4 where Eb is the power radiated by a black body, T is the absolute temperature and s is the Stefan-Boltzmann constant, equal to 5.5 X 10-12 W/cm2 K. An irradiance of 2.5 million watts/cm2 is then equivalent to 26,000 K. If you doubt the applicability of this equivalence, put a piece of steel at the focus of a 5 kW laser and see how many milliseconds it takes to vaporize.

The good part for heat treating is that one may produce lower irradiances. If the 5 kilowatt laser beam in the above example is focused into a 1/2" square, the irradiance is 3000 W/cm2 and the equivalent temperature is 4,800 K, which is useful for processing steel. What is even more useful is that the beam can be precisely controlled with respect to its intensity, geometry and dwell time. This gives us a unique tool for localized heat treatment.

The Absorption of Laser Light

For a laser to do material processing, the workpiece must absorb the energy in the beam. Unfortunately, metals have some characteristic properties that interfere with this goal: they are completely opaque and they are efficient reflectors of light. Most metals reflect especially well in the infrared range where most high-power lasers operate. Figure 1 shows the reflectivities of some metals at the 1.06 micron wavelength emitted by industrial Nd:YAG lasers, and at the 10.6 micron wavelength emitted by industrial CO2 lasers. It is obvious that if one directs a laser beam onto a metal surface, most of it is reflected away. Since laser power is expensive (CO2 lasers have a wall plug efficiency around 10 percent. Nd:YAG lasers are closer to 1 percent) these losses are extremely undesirable.

The usual remedy for this problem is to coat the surface with an absorbing substance. The following materials have been used to improve absorption of laser light on metal are listed in table 1.

Table 1 Coatings that improve absorption of laser light
Graphite Applied as aerosol spray
Heat resistant
Absorption strongly dependent on thickness
May carburize surface
Messy and hard to remove
Black paint Applied by spraying
Burns away during heat treatment
High initial absorption
Oxides Applied in aqueous bath
Heat resistant
Inconsistent absorption
Not generally removed after processing
Phosphates Applied in aqueous bath
Breaks down at heat treating temperature
Inconsistent absorption
May cause surface cracking from phosphorus diffusion
Not generally removed after processing
None of these coatings is perfect. Even if they were, coatings add several steps to the heat treating process: They must be prepared, applied and removed. Since they greatly affect the coupling of energy to the work, the thickness and condition of the coating must be tightly controlled. When enough power is available, laser heat treating may be performed on bare metal to eliminate these problems.

What Kinds of Heat Treating are Appropriate for Lasers?

In general, lasers are not good heat sources for bulk heat treatment since laser light is very expensive in terms of dollars per joule. They are, however, appropriate for selective heat treatments such as localized hardening, tempering or annealing. Of these processes, transformation hardening of steel is the most common and the most interesting.

There are, of course, many ways in which localized hardening of steel is accomplished. These can be divided into diffusion methods such as carburizing, where the steel’s composition is locally changed to harden it, and selective hardening methods such as flame hardening, where heat is applied locally to transform an area.

Selective hardening is done with sources such as flame, electrical induction or lasers which can be set to heat only a portion of an article. In contrast to diffusion hardening, the entire part must be composed of a steel which can produce the desired hardness. The heat source raises the selected area to the austenitizing temperature, and rapid cooling produces martensite in the heated zone. Lasers, because of their ability to produce very high irradiance, are generally used for shallow hardening (1 mm or less case depth). For most such heat treatment, no external quenching is used: the steel is quenched by heat conduction to the body of the workpiece.

As noted above, lasers heat only the surface of the work. All sub-surface heating is by conduction. This limits the case depth, which is an inverse function of the irradiance (fig.1) . If surface melting is not desired, the maximum irradiation time (calculated for a surface temperature of 1450 °C) is shown in Table 2., generated by a calculation of heat conduction using the time limit imposed by the surface melting criterion, shows the relation of case depth and irradiance.

Microstructures: Since the entire intent of selective hardening is to transform a limited region, the heating time must be short: Eventually, conduction will heat areas away from the desired transformation zone. Austenitization times of a few seconds are typical. This makes the prior microstructure of the article important. Austenite picks up carbon by diffusion out of Fe3C. Coarse structures, where carbides are several microns apart, will take longer to reach equilibrium than structures such as pearlite with sub-micron diffusion paths. It is important, then, to insure that steels to be selectively hardened have suitable microstructures. The best structure for localized transformation is tempered martensite because the carbides are in the form of a precipitate with a spacing on the order of 10 nm. It takes carbon less than a microsecond to diffuse this distance . The optimum sequence for selective hardening is then:

1 Heat the entire part into the austenitic range for enough time to produce a uniform solution of carbon in iron
2 Cool rapidly enough to form martensite
3 Temper to decompose the martensite and produce the desired bulk properties
4 Heat the area that is to be selectively hardened
5 Quench the localized area to produce martensite
6 Temper if required

Setting up for Laser Heat Treating

Most laser heat treating is done on general-purpose laser processing stations. These stations are used for welding, drilling and cutting. They consist of a laser source, a beam delivery system, focusing optics and a numerically controlled table. A 5-axis (3 translation axes, 2 rotation axes) system is most appropriate for heat treating.

It is possible to harden steel with a 200 watt laser, but most practical heat treating jobs need at least 3 kilowatts. For a 0.8 mm case, the process rate is about P/4000 cm2/sec where P is the input power in watts. There is often a lot of power lost to beam shaping, so several kilowatts are needed to cover any reasonably large surface.

Heat treat patterns: Beam shaping itself is a big part of successful laser hardening. The great advantage of a laser is that its light can be made to form any desired pattern. The beam exiting the laser, however, is usually poorly formed for heat treating: it has a modal structure with hot spots and weak areas, and is never going to be the same shape as the part you’re trying to heat treat. A wonderful variety of devices, some spinning, some vibrating, some static, have been made to generate heat treating patterns. The most robust beam shaping optic is the segmented mirror (US Patent No. 4195913, Spawr Optical Research Inc.). As shown in Figure 2A, a segmented mirror is composed of many polished molybdenum facets arranged so that they superimpose their images on a plane in space. The advantage of this is that it is very stable and will always produce the pattern that was designed into it. This is also its disadvantage: it may not be adjusted to produce any other pattern.

Adjustable patterns can be obtained with scanners. Since the typical time constant for laser hardening is on the order of 0.1 second or more, a beam scanned at 30 Hz or so will have the same effect as one that doesn't change with time. Scanners give complete flexibility in making heat treat patterns, but are fragile given the severe conditions in which they are used. They are useful for developing heat treating processes, but are not good choices for production use. Figure 2B shows a 2-axis scanning system.

Beams may also be shaped with a kaleidoscope, which works like the toy: A diverging laser beam is sent down a reflecting tube (see Figure 2C). The tube is shaped to produce the desired heat treat pattern. If the beam goes through enough reflections, it emerges well-homogenized. Kaleidoscopes are relatively inexpensive to build and modify, but they lose a great deal of laser power because of the reflections.

Inside diameters are reached with periscopes, usually after the beam has been processed with one of the above devices. Periscopes generally have water-cooled copper mirrors since they are very near the heat treat area and tend to get hot.
Before parts get to the laser, they have to be coated (unless you use a Nd:YAG or diode laser, which are absorbed better than CO2 laser light). Applying and controlling the coating plays an important role in the success of the heat treating job. If paint or graphite is used, its thickness should be measured. Drying time is also significant. Conversion coatings need close control of the bath composition to maintain their absorptivity.

Examples of Laser Hardening

A) Rock Bit Bearing Race
The rotary cutters for rock bits used in oil drilling put heavy loads on their internal ball bearings. The cutter itself is 4140 steel. Prior to laser processing, the part is hardened and tempered to RC 40, then phosphate coated. A 3 kW laser beam shaped to a 10 mm X 15 mm rectangular pattern is delivered to the internal race with a periscope. Hardening is performed by rotating the cutter at 3.8 RPM for a surface speed of 10 mm per second. A case depth of 1 mm is achieved, increasing cutter performance. The beam is kept on for slightly more than one rotation of the part. There is a small tempered zone where the beam overlaps , but this has not been a source of trouble in this application. The matching race on the body of the bit is hardened by induction because access to the heat treat area is unrestricted.

B) Locomotive Engine Cylinder
Pearlitic cast iron cylinders for Diesel locomotive engines are internally hardened to improve their service life. After the parts are painted black, heat treating is performed by a 5 kW laser whose beam is shaped into a 20 mm square and delivered to the cylinder bore by a periscope. Since cast iron has some ledeburite eutectic, the surface temperature cannot exceed 1100 °C without causing melting. This limits the interaction time at 1250 W/cm2 to 1.5 seconds rather than 2.5, the limit for steel.

This job is performed by several dedicated systems. The lasers are on a mezzanine elevated above the work area. A cylinder is mounted on a rotary spindle and the periscope lowered into the bore. Rotation is timed with periscope motion to generate a spiral hardened zone 0.8 mm deep covering most of the bore.

The Future of Laser Heat Treating

Big lasers have had their chance in heat treating, and have seen only niche applications. It's not because they can't do the job; it's just that, most of the time, other methods are less expensive. This may change with the acceptance of high power laser diodes. These devices are much more efficient than CO2 or Nd:YAG lasers, with 30% to 40% conversion of electrical input to optical output. At the present time, the diodes are relatively expensive, but their costs are dropping rapidly. Laser diodes are available with several kilowatts of output power, and they emit in the near infrared (usually around 800 nm), which means their light is absorbed reasonably well by uncoated steel. Diodes have poor focusability compared to standard lasers, but this is not a problem for heat treating: an irradiance of 104 W/cm2 is easily achieved.

Laser diodes’ low operating cost and their ability to harden bare metal should expand the areas where laser heat treating is the most appropriate choice.

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