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Laser heat treating – a short course

Blue River Digital

Maybe you’ve heard a little about laser heat treating and you think it might be all smoke and mirrors. Or, maybe you’re surprised a laser could be used for something other than cutting metal, and you’d like to know how it might benefit your operation. Either way, the technology is tried and tested in multiple applications in nearly every industry in America.

Here’s how it works.

Laser heat treating is much the same as any other thermal hardening process, except that it uses the highly controllable energy of a CO2 laser. Regardless of the energy source, the object is to raise the temperature of the piece to the point where its internal structure begins to crystallize (the transformation temperature), then allow the piece to cool rapidly.

In conventional case-hardening, the cooling (quenching) is hastened by chilling the piece with oil, water, or forced air. When treated by laser, the workpiece can usually be self-quenched, or cooled in ambient air.

Used at full intensity, the laser is able to cut through a metal piece. Reduce the energy somewhat, and the beam melts the surface, making it possible to weld–or to create a tough clad coating. Reduce the beam energy further, or shorten its duration, and the surface is only heated; if brought to the transformation temperature, then cooled, the surface becomes harder than before.

A laser beam causes very rapid heat input into a small area of the work-piece. There is rapid transformation, then an extremely high cooling rate, as the heat is conducted into the workpiece by its own mass. The real advantage of this type of heat treating lies in the uniformity of the method and the precise control of the depth of the hardened area.

The depth of hardening is determined by a number of factors. The slower the treatment speed, the deeper the hardening effects. Power density and treatment speed determine the actual heat input into the workpiece surface.

BMR Group has over two decades of experience in laser heat treating. We know what works and what doesn’t.  Because we offer multiple hardening technologies to fit applications across most industries, you can be sure we’ll help you select the best technology for your components and your operating environment.

BMR Group has more expertise and experience with laser materials processing than any other company in the business. Need proof? Look at these articles.
The Art & Science of Laser Heat Treating
Job Shop Laser Heat Treating
Toughening Up Capstan Surfaces

If you have components that require case hardening and you’re tired of problems of distortion and failure, call BMR Group at 260-635-2195 and discover how easy it can be to gain the advantages of laser heat treating.

C02 Laser Design

Blue River Digital

The CO2 laser, once an industry work-horse, has been eclipsed by newer laser designs that are more narrowly focused toward specific uses. However, because of its unique design and function, the CO2 laser is still an irreplaceable tool for many industrial applications that cannot be matched by other technologies.  Optical systems can focus the light energy into small spot sizes useful in metal cutting, welding, cladding, and heat-treating applications. Here’s why it works so well in so many heat treating, welding, and other hard-surfacing applications.

Essential components

The CO2 laser is comprised of two basic parts. The first part includes the components that generate the laser energy, and the second part is the beam delivery system that translates the laser energy into a light beam that can be used. The energy-generating part of a CO2 laser consists of a lasing medium, an optical resonator to contain the lasing medium, and an excitation source for the lasing medium.

A lasing medium is the original source of the energy for the laser. The lasing medium in a CO2 laser is a mixture of carbon dioxide, helium, and nitrogen gas.

In an axial-flow type of CO2 laser, the gas is contained in a long double walled glass tube. This glass tube is the optical resonator and has reflecting mirrors mounted at each of its ends. High electrical voltage is the excitation source applied to the gas in the resonator to excite the CO2 molecules to a higher energy level.

Making light

As the gas mixture is electrically charged, the excited molecules try to find a more stable energy level. In the process, they emit energy in the form of photons. This is similar to a controlled chain reaction, as each emitted photon stimulates the emission of more photons from other excited molecules. Every photon emitted by the excited CO2 molecules has exactly the same wavelength.

As the energy beam develops from the excited photons, the mirrors at the ends of the optical resonator catch the photons traveling parallel to the axis of the resonator and reflect them back down the resonator. This process further amplifies the energy content of the beam by stimulating the emission of even more photons. The optical resonator makes all the energy flow in one direction, since all reflected photons are traveling in a direction parallel to the resonator’s axis.

The energy within the laser is of no value until it can be directed out of the unit and onto a work surface. An output coupler is used to get the energy out of the optical resonator so it can be used for some work application. To form the output coupler for a CO2 laser, one of the mirrors on the end of the resonator tube is made only partially reflective.

This partially reflective mirror allows about 50 percent of the laser beam to exit the optical resonator, and that portion is used for the laser’s intended application. The remaining 50 percent of the beam continues to stimulate photon emission in the excited CO2 molecules inside the resonator. The output coupler focuses the exiting beam to produce high energy densities in a small area.

Making heat

The laser beam is simply an energy source until it contacts the surface of a work piece. To make the light beam useful, lasers have optical systems that use reflectors and lenses to direct the beam from the optical coupler to the work piece.

Because a CO2 laser beam retains most of its power over long distances, the optical systems can direct the beam to a workstation located quite a large distance from the laser. The beam can even be split and shared among more than one workstation, as it is in BMR Group’s facility in Wolf Lake, Indiana.

BMR Group has used CO2 laser technology for two decades, helping customers conquer or alleviate wear problems caused by everyday operation in difficult production environments. If you’d like to learn how this same technology might help you improve your productivity and your bottom line, call BMR Group at 260-635-2195 or drop us an email explaining what you want to accomplish.

The basics of case-hardening

Blue River Digital

In surface heat-treating, or case hardening, the surface of a piece of steel is heated to a suitable temperature at which it begins to change its internal structure, and then it is cooled very rapidly. The heating and rapid cooling process causes a permanent transformation in the steel, and it becomes harder. This process is known as transformation hardening, because the molecular structure of the metal surface is transformed to another state.

The transformation involved when steel is hardened is actually a change in the arrangement of the atoms or a change in the crystallographic lattice network of the metal. This change occurs at critical transformation temperatures both when the metal is being heated and as it is cooling.

The transformation temperature is the temperature above or below which solid-state transformation takes place. The exact temperature depends on the alloy composition of the steel being treated. The changes and the structures they produce in the metal all take place while the metal is in the solid state. In other words, the transformation occurs before the surface material reaches its melting temperature.

If a piece of steel is heated to its transformation temperature and allowed to cool slowly in ambient air, it passes through several states or arrangements in its microstructure. One of these arrangements is called martensite, a microstructure required to form high hardness in ferrous metals. The greater the degree of martensite formation, the higher the hardness.

To obtain the desired martensite formation, the heated steel must cool at the critical cooling rate, which is the rate at which the heated material is cooled from the transformation temperature to the desired lower temperature. With most heat-treating methods, critical cooling rates are achieved after the heating process by quenching.

In quenching, the surface is sprayed or immersed in water or oil to make it cool very rapidly. Quenching causes the heat-treated area of the steel to bypass other possible states and go directly to martensite.

The most commonly used scale for determining the hardness of steel is the Rockwell C (RHC) scale. The requirements for typical industrial hardening applications are 56 to 62 points Rockwell C.

Although many types and compositions of steel are used, machinable steels usually need a minimum of 0.4% carbon content before they can be successfully hardened to this range by any heat treating method. The carbon content of the metal being treated is the most important factor in determining the maximum attainable surface hardness of a steel, and the higher the carbon content, the greater the potential hardness.

The depth of the hardened case depends on how far below the surface the martensite can form in sufficient quantities. The chemical composition or alloy content of the steel determines the depth to which the maximum hardness can be achieved on a given piece of steel.

The most widely used case hardening technologies are flame hardening, induction hardening, laser hardening.

BMR Group has used CO2 laser technology for two decades, helping customers conquer or alleviate wear problems caused by everyday operation in difficult production environments. If you’d like to learn how this same technology might help you improve your productivity and your bottom line, call BMR Group at 260-635-2195 or drop us an email explaining what you want to accomplish.