Definition of laser quenching
Laser quenching is the process of heating the surface of a material above its phase transition point using a laser.
As the material cools, austenite transforms into martensite, resulting in a hardened surface.
Laser quenching of tooth surfaces has the advantage of a fast heating and cooling speed and a short process cycle. Additionally, no external quenching medium is required.
This method also boasts unique benefits such as minimal workpiece deformation, a clean working environment, and the absence of the need for post-grinding finishing. Furthermore, the size of the treated gear is not restricted by the size of the heat treatment equipment.
1) Quality advantage
Laser quenching has a high power density, a rapid cooling rate, and does not require a cooling medium like water or oil. It is a clean and efficient quenching process.
Compared to induction quenching, flame quenching, and carburizing quenching, laser quenching results in a uniform hardened layer with higher hardness (usually 1-3HRC higher than induction quenching), minimal workpiece deformation, and precise control over the heating layer depth and trajectory. It is also easier to automate than other methods.
Furthermore, the design of induction coils is not necessary, as is the case with induction quenching, and the processing of large parts is not limited by the size of the furnace during chemical heat treatment methods such as carburizing and quenching.
Due to these advantages, traditional processes such as induction hardening and chemical heat treatment are being gradually replaced in many industries.
It is worth noting that laser quenching results in almost negligible workpiece deformation before and after the process, making it particularly suitable for surface treatment of parts with high precision requirements.
- Technical features
The depth of the laser-hardened layer depends on various factors including the composition, size, shape of the part and the laser process parameters, typically ranging from 0.3 to 2.0mm.
For instance, quenching of the tooth surface of large gears and the journal of large shaft parts results in a surface roughness that remains unchanged and meets the requirements of actual working conditions without the need for further machining.
Laser fused quenching technology involves heating the surface of the substrate to its melting temperature using a laser beam and then rapidly solidifying it through heat conduction cooling inside the substrate. The resulting fused and quenched structure is dense and comprised of a melt-solidified layer, a phase change hardened layer, a heat affected zone, and a substrate.
Compared to laser quenching, laser fused quenching results in a deeper depth of hardening, higher hardness, and improved wear resistance. However, this process can cause surface roughness damage and may require subsequent machining to restore it.
To reduce surface roughness and the amount of subsequent processing, the use of special laser fused quenching coatings can greatly reduce the surface roughness of the fused layer.
Laser melting treatment is now commonly used in the metallurgical industry for a variety of materials including rolls and guides, with a surface roughness that is similar to that achieved through laser quenching.
- Applicable material
Laser quenching has been successfully implemented for surface strengthening of consumable parts in industries such as metallurgy, machinery, and petrochemicals.
In particular, it has a remarkable effect in increasing the service life of wearing parts such as rolls, guides, gears, and cutting edges, resulting in significant economic and social benefits.
The use of laser quenching for surface strengthening of parts such as molds and gears has become increasingly widespread in recent years.
Laser quenching history and development
As early as 1980, the US Military Technology Research Laboratory’s Applied Technology Laboratory reported on the results of their laser hardening investigation of gears, with the laser hardening research project of gears also being undertaken by the Illinois Institute of Technology in Chicago.
Altegott and Patel of the Bell Aircraft Manufacturing Company collaborated and published a paper titled “Spindle Gear Laser Surface Hardening MM&T”, which reported the results of their experiments. The comparison of the anti-gluing life of the AMS (American Aerospace Materials Specification) 6265 spur gear and the flexural strength of the gear after laser surface hardening treatment showed that laser hardening provides significant economic benefits, replacing the energy of the carburizing treatment in aerospace devices. The effective hardening depth was found to be 0.66 to 0.86mm and the cost per piece was reduced by 37% to 78%.
In the late 1980s, James F Lewis of the California Institute of Mechanical and Electrical Engineering used a 5 kW laser to laser quench a large spline shaft. With a scanning speed of 4.32 to 7.62 mm/s and a spot diameter of 6.35 to 7.62 mm, a quenching hardness of HRC59 and a hardened layer with a depth of 0.762 to 0.864mm were achieved.
The US Military Research Institute utilized laser quenching to solve the problem of excessive gear deformation and noise caused by conventional heat treatment on heavy-duty large gears used in submarines and airplanes. These laser-hardened gears included the planetary gears of the AH-64 helicopter auxiliary power unit and the transmission gears of the main drive of the aircraft.
Laser quenching eliminates the need for grinding, thereby reducing production costs and increasing productivity.
Laser quenching characteristics
The parts that undergo laser quenching do not experience deformation, and the thermal cycle of laser quenching is fast. The process results in almost no damage to surface roughness and the application of a thin, oxidation-resistant coating.
Laser quenching is a clean and efficient process, with no cracking and precise, quantifiable CNC quenching, even for local grooves. It eliminates the need for cooling media such as water or oil and results in a higher quenching hardness compared to conventional methods, with a fine-structured and tough quenched layer.
Laser quenching is a rapid heating, self-cooling process that does not require furnace insulation or coolant quenching. It is a green, pollution-free heat treatment process that can easily perform uniform quenching on large mold surfaces.
The fast heating speed of the laser results in a small heat affected zone, and the surface scanning heating and quenching method minimizes deformation in the mold. Additionally, the small divergence angle of the laser beam and good directivity allow for accurate quenching of the mold surface through the light guiding system.
The depth of the hardened layer in laser surface hardening is typically 0.3 to 1.5 mm.
Laser quenching equipment
1) The laser
Currently, the main equipment used for laser quenching is a cross-flow CO2 laser. This laser works by having the working gas rapidly flow through the discharge region in a direction perpendicular to the optical axis, keeping the gas temperature in the cavity low and ensuring high power output. The beam mode is multimode.
When selecting lasers for laser quenching, it is important to consider the following aspects:
- Good beam quality, including mode and mode stability
- Stability of the laser output power
- High reliability, able to meet the demands of continuous operation in industrial processing environments
- Good maintenance and fault diagnosis capabilities
- Simplicity and convenience in operation
- Economic and technical capabilities and credibility of the equipment vendor
- Availability of protected sources for wearing parts of the equipment and unblocked supply channels.
2) The machine tool
The basic dimensions of laser machining machines are 5.5 meters in length and 2.6 meters in diameter. They can accommodate a wide range of special workpieces of various sizes.
The laser processing machine is designed as a double cantilever system, capable of performing multi-station laser processing.