laser hardening (original) (raw)
Definition: hardening materials (usually steel or cast iron) with a laser treatment
Category: 
laser material processing
Related: laser surface modificationlaser material processing
Page views in 12 months: 163
DOI: 10.61835/296 Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
Content quality and neutrality are maintained according to our editorial policy.
📦 For purchasing laser hardening, use the RP Photonics Buyer's Guide — an expert-curated directory for finding all relevant suppliers, which also offers advanced purchasing assistance.
Contents
What is Laser Hardening?
Laser hardening is an important example for laser surface modification, and more generally a kind of laser material processing.
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 outstanding variety of different variants of steel exist in thermal equilibrium, and non-equilibrium states of steel are also technologically critical. With various processes, often involving rapid heating and cooling, steel can be converted into forms with different microscopic structures and substantially different properties in terms of hardness, strength, ductility, density, chemical robustness etc.
Figure 1: Hardening of a gripper cam with a high-power diode laser. Source: Fraunhofer IWS, Dresden, Germany.
In particular, a substantial hardening of carbon-rich steels (or medium-carbon steels) is possible by heating it to roughly 1000 °C (below the melting point) and thereafter cooling it at 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.
With more rapid cooling, carbon does not have time to diffuse and form cementite. Instead, the austenite formed during laser heating transforms into martensite — a supersaturated, body-centered tetragonal phase. The associated lattice distortion and high dislocation density impede slip, so the surface becomes much harder and more wear-resistant. Note that laser hardening generally does not improve corrosion resistance; it mainly increases hardness and wear resistance and may introduce residual stresses.
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. In some cases, one subsequently applies tempering to reduce such stress.
Hardening processes are in some cases unwanted, e.g. in the context of laser welding and laser soldering. Such processes are then optimized accordingly.
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%. (The diodes themselves can even reach 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).
Beam Profiles
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 laser diode sources (containing numerous 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 an approximately 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 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.
Temperature Control
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. This leads to more reproducible high-quality results. Such techniques are used in industrial laser hardening machines.
Process Gas
In laser hardening, the use of a process gas is generally not required to protect the workpiece surface, since the treatment involves heating below the melting point and does not cause significant metal evaporation or oxidation. Nevertheless, a cross-jet — a stream of clean gas directed across the laser beam path — is commonly employed in the laser processing head. Its primary function is to protect the optical components (especially the protective window and focusing lens) from contamination by fumes or splatter.
Although the steel itself does not vaporize under typical hardening conditions, surface contaminants such as residual oils, oxides, or dust can burn or decompose under the laser beam, producing smoke or particulate matter. These by-products could otherwise deposit on the optics and degrade beam quality. In some cases, an inert gas (e.g., nitrogen or argon) is used in the cross-jet to further reduce oxidation or discoloration of the treated surface, particularly when aesthetic appearance or corrosion behavior are critical.
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 in 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) to avoid detrimental effects of the hardening, and particularly an increased brittleness.
Frequently Asked Questions
What is laser hardening?
Laser hardening is a laser surface modification technique that uses a laser beam to heat the surface of a material like steel or cast iron. The subsequent rapid cooling (self-quenching) by heat conduction into the bulk material increases the surface hardness.
What happens microscopically during the laser hardening of steel?
The laser heats the steel to form an austenite crystal structure. The subsequent rapid cooling prevents the carbon from precipitating as it normally would, instead forming a hard, strained crystal structure called martensite.
What are the main advantages of laser hardening over traditional methods?
Laser hardening offers precise, localized heat application, which minimizes part distortion and often eliminates the need for subsequent reworking. The process is also very fast and highly controllable.
What types of lasers are used for hardening?
High-power direct diode lasers are commonly used due to their high efficiency and cost-effectiveness. Diode-pumped solid-state lasers are also an option. The required beam quality is typically moderate.
Why is only the surface of a component hardened?
Surface-only hardening creates a component with a hard, wear-resistant exterior while maintaining a tougher, less brittle core. Hardening the entire volume could make the part too brittle for its intended application.
What are typical applications of laser hardening?
It is used to improve the durability of industrial machine parts subject to high stress, such as camshafts, gear wheels, turbine blades, and bending tools, particularly in the automotive and aerospace sectors.
Suppliers
Questions and Comments from Users
Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.
Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.
By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.