Laser heat treating simplified

Editor's Note: ILS Editor-in-Chief David Belforte asked Dr. Vivian Merchant to consider writing a back-to-basics feature on the subject of laser heat treatment because he is uniquely positioned as one of a select group of industrial materials processing professionals who have lengthy background and experience in this field as a process developer, researcher, and practitioner. His response, "Laser Heat Treatment Simplified," describes three processes: laser transformation hardening, laser annealing, and laser surface melting. Each is described in easy-to-understand terms for those who are not metallurgists, but who are charged with improving the wear and fatigue characteristics of metals.

Let’s start off by saying "Heat Treating" is far too general a term, although it is widely used in conjunction with laser processing. There is a myriad of ways that materials can be heat-treated by conventional means, and each of these ways results in a particular useful crystal or chemical structure of the treated material. Lasers have been applied to only three means of heat treating: laser transformation hardening, annealing, and laser surface melting. The first two are solid state processes, in which the material is heated by the laser beam, but not melted. The atoms in the metal crystals are rearranged by the laser treatment, improving the material properties. The third of the laser heat treatment processes does involve melting, but only at the surface of the material being treated. Details about these three processes are described below.

Laser transformation hardening
Laser transformation hardening is the use of laser radiation absorbed in a metal to change the orderly arrangement of atoms - the crystal structure - and produce a hard surface. This process can be applied only to particular types of steel, so it might seem limited, but these types of steel are widely used in industry. The hard surface results in a high resistance to wear and improved fatigue resistance, giving the material a long life time when used in high-wear situations. Several studies have estimated that excessive wear costs industry, and ultimately all of us, tens of billions of dollars per year.

In laser transformation hardening, a laser beam is scanned over the surface of the material to be treated; the surface is heated to a controlled depth by the absorption of beam energy. The processing conditions are chosen so that surface melting does not take place. The freezing of the material following surface melting would result in a rippling of the molten pool as it solidifies and ultimately a rough surface. Hence, surface melting is avoided. Instead, high temperature conditions are achieved so that the carbon in the steel diffuses around and becomes uniform through the material.

Initially the carbon is not uniform in the material but is concentrated in certain areas. One structure called pearlite contains bands of different kinds of steel; one band will be rich in carbon and the next band low in carbon, with these bands alternating through the material. One cannot see these bands except under a microscope after careful surface preparation, called "metallography". The banded structure typical of pearlite occurs whenever steel over a wide range of carbon contents is allowed to cool slowly, as when it is being poured from a vat in a steel plant.

The carbon atoms move around
When the absorbed beam energy heats a shallow surface layer of a metal to a high temperature, but without melting the surface, the carbon moves out of the high carbon bands and the distribution of carbon becomes uniform. This occurs because the carbon is a very light atom, and at high temperatures the carbon moves around in the crystal structure formed by the heavier iron atoms. This carbon atom movement is called "diffusion". When the material cools, the carbon becomes locked in place because the diffusion only takes place at high temperature. If the material cools very fast, then the carbon becomes locked in place with the uniform distribution created by the high temperature diffusion. This rapid cooling forms a crystal structure that would not normally exist at room temperature; it is called a "metastable" crystal structure. The crystal structure that would normally exist is the pearlite that was described above, and the metastable crystal is formed only because of the rapid cooling.

The rapid cooling comes about because the laser has heated only the surface of the material so that diffusion of carbon in the surface layer can take place. The laser beam is being scanned over the surface of the material, and as the beam moves away from a particular spot, that spot stops being heated. The bulk of the material below the surface layer was never heated because the beam wasn’t applied to the material for long enough. The heat content of the top surface layer is then conducted into the cooler bulk of the material. The top layer cools rapidly since most metals are good conductors of heat.

What’s needed for transformation hardening?
From the above description of the process, we can see there are several requirements to make transformation hardening occur. There must be a means of moving the beam over the surface of the material. This can be done by using a mirror to move the beam over a stationary part, or more often by mounting the part on a moving stage that passes the area of the part to be treated through the beam.

Another requirement is a uniform treatment. If one uses a beam focused to produce an energy density sufficient to heat the material at the center of the beam to the point where hardening occurs, the parts of the material that get the sides of the beam don’t get enough heat. If the beam power is increased so that these sides receive enough heat, then the part of the material that gets the center of the beam will likely melt, resulting in a rough surface when it resolidifies. To overcome this problem, laser transformation hardening is most often carried out using a special optic to produce a uniform energy density on the material surface. This special optic, called a "beam integrator", produces either a rectangular or a square distribution of the laser energy on the surface, often called a “top hat”. As this uniform distribution is moved over the surface, every point goes through the same temperature cycle. Because of the rectangular beam, each point that is heated stays at an elevated temperature long enough to ensure that the carbon diffuses around, producing a uniform distribution.

Since the laser is an expensive source of energy, it is worthwhile to ensure that the beam is efficiently used. Ferrous metals at room temperature are very efficient reflectors of certain wavelengths of laser radiation, those usually associated with high output power. To overcome this, the parts to be laser treated are usually coated with a thin layer of black absorbing material. Many different types of absorbing material have been tried, but aerosol paint available from the nearest hardware store is among the most effective.

Setting up the process
The above description shows that moving the beam over the surface at the right speed produces a uniform area of hardness. Moving the beam slower allows the heat to conduct further into the material, and extends the region where the transformation hardening occurs, or "case depth". But how does one find the right conditions of laser power and surface scanning speed to produce a hardened surface of the required depth? An approximate answer to this question can be found by calculations of the rate of flow of the heat into the material. Ultimately however, one has to go through a stage of process development, in which multiple trials are conducted under different conditions of speed and laser power. The samples produced in these trials are cut up and metallographically analyzed under the microscope. The hardness of the material is measured using a microhardness indenter where the point of a diamond is pushed into the material at high pressure. If this indenter is applied to a soft material, a large indentation results, but if it is applied to a hard material, the diamond can’t penetrate very far into the material and a small indentation results. Thus the size of the indentation in the material is an indication of the material hardness. The hardness can be measured at different distances into the material surface to determine the depth to which transformation hardening occurred.

The maximum hardness achieved doesn’t depend on the laser conditions but on the amount of carbon in the steel, determined in the original manufacture of the steel. However, if the laser conditions (power and speed) aren’t chosen carefully, the hardness may be less than what the steel is capable of. The power and speed also determine the depth to which the material is hardened. The process development stage described above is required to find the correct power and speed.

As an example, in one type of steel, a 3 kilowatt carbon dioxide laser beam was used with special optics to irradiate a square area of dimensions 12.7 by 12.7 mm on the surface of one type of steel. This beam was moved over the surface at a speed of 1.9 cm/sec and hardened the material to a depth of 1 mm.

In addition to carbon dioxide lasers, other types of lasers such as neodymium YAG, direct diode, and fiber lasers can be used for transformation hardening. The output of these lasers are at wavelengths that are shorter than that of the carbon dioxide laser and are more readily absorbed by the steel being hardened. With these laser types, the transformation hardening process can be carried out without painting the work piece beforehand, although some improvements do result from the painting operation.

Laser transformation hardening in production
Once the process is in production, one needs a quality assurance method to ensure the process remains good. If the final application can tolerate a rough surface, a surface hardening test can be performed using a machine called a Rockwell Indenter. This tells nothing about the depth of hardness, however. If the parts being treated are not too expensive, parts can be sacrificed and cut up and measured using the same procedures described above during the process development stage. If the parts are very expensive, dummy parts with the same geometry in the region being hardened can be used. It is important that the parts have the same geometry because the geometry determines the rate at which the heat is conducted away from the surface and hence the hardness that is achieved.

Laser transformation was first applied in the 1970s. Today, it is used in the manufacture of a wide variety of components. Examples include cylinder liners for diesel engines, grooves in pistons to hold piston rings, power steering mechanisms, and rollers for photocopying machines.

Annealing
Laser annealing of metals is much less common than transformation hardening, and only a couple of examples exist where it has been used. One example is the annealing of wires used for steel-belted radial tires. Wires are made by a "drawing" process, where the metal is forced through a small hole to produce material for the desired size. Actually, the metal is forced through a succession of holes each slightly smaller than the last, so the material size is reduced bit by bit until the desired size is achieved. This drawing process takes a lot of force, and this force does a lot of damage to the crystal structure of the material. Normally the atoms in the material are a set distance apart, but the enormous forces of the drawing process may leave the atoms displaced from their proper position. As a result, the strength of the material is significantly lower than it should be.

The manufacturer of the wire for the steel belted radial tires put the wire through an "annealing" process to restore the positions of the atoms in the crystal structure and to get rid of the distorted structure left by the drawing process. A 500 W carbon dioxide laser was used with a reflecting cone to concentrate the beam around the wire, which was pulled along the axis of the cone at speeds from 30 to 100 m/min. The fatigue strength of the laser-processed wire was three to four times larger than that of the non-processed wire.

Another example of laser annealing was the repair of a cast aluminum housing that had a misplaced hole. The incorrectly placed hole was filled by melted aluminum alloy which solidified to a hard structure. This hard structure was laser annealed to soften the metal so that the hole could be re-drilled in the right location.

Laser annealing is frequently applied after an ion-implantation operation in the manufacture of semiconductors. But processing of semiconductors, as distinct from processing of metals, is beyond the range of this article

Laser surface melting
In the description of laser transformation described above, it was mentioned that surface melting should be avoided. However, this isn’t always true. Surface melting results in a somewhat rough surface so another smoothing operation is required to produce a smooth surface again. Sometimes the improvement in material properties is sufficient so that the extra manufacturing operation to smooth the surface is worthwhile.

Laser surface melting has been used with cast iron components, in some of which the carbon is not distributed uniformly, but collected together in a carbon or carbide block, called an inclusion, inside the material. The material adjacent to the inclusions may be deprived of carbon and have poor properties. When the material is melted, all the constituents are dissolved and are uniformly distributed in the liquid. Rapid cooling of the liquid may not give sufficient time for the inclusions to reform, and the carbon content of the material becomes very high because the carbon formerly isolated in the inclusions is now uniformly distributed. The high carbon material becomes very resistant to wear.

Another material to which laser surface melting has been applied is nickel aluminum bronze. The material is widely used in marine applications, for valves and propellers for example. Examination of nickel aluminum bronze under the microscope shows that there are regions that are locally enriched in one or another of the constituent materials ? nickel, aluminum, copper, or tin. When immersed in sea-water, the electrically conducting liquid in conjunction with areas that are rich in different constituents becomes a battery. The electrical current from this battery leads to corrosion of the material. In addition the material is subject to a strange process known as cavitation erosion. The rapid motion of the propeller through the water can lead to the formation of bubbles. If bubbles right next to the material collapse, some of the water moves rapidly across the void that used to be the bubble and can strike the surface at a high enough speed to knock some of the material away. Near sea ports one often sees old ship propellers planted in concrete as curiosity items. Examination of these propellers shows there are areas on the edge of the propeller blades where material is missing, just as if a mouse or a rodent had been nibbling at the material. But the missing material is not due to mice; it is damage caused by the collapse of bubbles. It seems strange that bubbles can do so much damage. The same process occurs on the inside of valves when water passes quickly through them.

Laser surface melting of the nickel aluminum bronze produces a layer of liquid on the surface of the material in which all the constituents are uniform distributed. The rapid cooling as the heat is conducted away from the surface into the bulk of the material leads to a solid in which the constituents are evenly distributed. This laser modified material has been shown to have a significantly improved resistance to corrosion and to the erosion caused by the collapsing bubbles.

Another laser melting application is the remelting of cast iron cam shaft lobes to produce a harder surface. A slightly focused beam is scanned across the surface of the cam lobe as it is rotated under the beam. At the end of each scan, the dwell time is adjusted to prevent overheating on the next pass. Uniform 1 mm deep hard zones are produced with no overlap at the end of the lobe rotation.

Dr. V.E. (Vivian) Merchant ([email protected]) provides technical and marketing information, process development, and start-up assistance for high technology enterprises. His many years of industrial laser experience includes: investigating laser induced chemical reactions, building high-power laser systems for pipeline welding, hermetic sealing of electronic components for heart pacemakers, and developing laser transformation hardening and laser cladding applications to protect surfaces against wear, erosion and corrosion in the resource and defense industries. He is currently preparing a book on “The Art of Building Lasers” and is active on the American Welding Society’s subcommittee on Laser Beam Welding, Cutting, and Drilling. Dr. Merchant resides in Vernon, B.C., Canada.