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Laser surface engineering methods apply precise laser energy to metal surfaces to increase hardness and wear resistance. A recent study published in the "Metal" magazine found that the combination of laser surface engineering technology and nitriding creates excellent wear resistance for AISI P21 steel used in plastic molds.

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AISI P21 steel has good mechanical properties and excellent workability, so it is a common material for injection molds. In the mass production of plastic products, plastic injection molds need to be used millions of times.

This extremely high wear requirement means that the mold is often damaged due to wear, corrosion and fatigue during the injection molding process. Damaged molds must be replaced to ensure satisfactory quality in the production process, which increases the manufacturer's production costs.

Surface treatment can prevent mold damage, extend its service life and provide a long-term return on treatment investment.

Nevertheless, there are few studies on surface treatment and its influence on the microstructure, mechanical properties or functional properties of P21. This research led by researchers from Inha University and Korea Institute of Machinery and Materials provides new evidence for the surface treatment of P21.

Lasers are widely used in surface treatment applications due to their high precision, high speed and selectivity. The laser is a versatile tool with novel characteristics, so it is increasingly used in industrial metal processing operations.

During laser processing, the material can be cooled very quickly compared to other surface engineering techniques. This means that laser processing can be applied to a range of materials, even glass.

Laser hardening methods are commonly used for steel and cast iron materials. Controlled local heating maintains the metallurgical properties of the substrate while forming a hardened surface to improve wear resistance.

Laser surface engineering usually forms a hardened layer of up to 0.2 to 2.0 mm in the material, and the shape of the hardened area can be controlled by beam shaping optics.

Products used for laser heat treatment can also be used for heat-assisted roll forming, heat-assisted stamping of ultra-high-strength steel, heat-assisted bending of high-strength steel, and other surface structuring applications.

Nitriding is a heat treatment technique that obtains a hardened surface by diffusing nitrogen to the subsurface of the metal. Nitriding is usually performed using plasma arc and is widely used in manufacturing. Gears, die casting tools, camshafts, crankshafts, molds and other metal parts that require extremely high wear resistance all benefit from plasma nitriding.

Laser nitriding was first proposed in the early 1980s and has been the focus of research ever since. Laser nitriding has the potential to become a more precise process than plasma nitriding, so it may be suitable for more selective surface engineering applications.

Further reading: The meaning of laser surface functionalization

Usually, the parts to be processed are placed in a nitrogen chamber for laser nitriding. Use high-intensity laser, for example, excimer, CO 2 or Nd-doped yttrium aluminum garnet. The laser beam is directed to the target area, where it generates nitrogen plasma and melts a small part of the surface. Then, the precursor nitrogen diffuses to the molten area on the surface.

This technology limits the treatment to a few square millimeters with just a few injections; moving and withdrawing the sample to create a uniformly treated surface. This means that the material is only exposed to the laser for a short time, and the heating can be highly localized. Therefore, laser nitriding is suitable for heat-sensitive materials.

Traditional laser heat treatment and laser nitriding are applied to AISI P21 steel parts of plastic injection molding machines. For these two technologies, the team used high-power diode lasers to make high-durability plastic injection molds using P21 steel.

After applying the treatment, the team used transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) analysis to determine how the microstructure of the sample evolved during the treatment.

They compared the measurement results of the microhardness of the samples before and after the treatment, and also performed a needle-disk friction test on the base material, laser heat-treated P21 and laser-nitrided P21. The latter experiment was used to study the correlation between laser treatment and wear behavior.

TEM and EELS analysis showed that nitrogen (N) diffused into the molten P21 steel, and aluminum (Al) in the alloy also diffused into N. This is due to the fact that the chemical affinity between N and Al is stronger than iron (Fe), another main component of steel. This resulted in the formation of many aluminum-nitrogen (AlN) precipitates in the subsurface of the nitrided sample.

The microhardness measurement showed that the surface hardness of P21 steel increased from 409 HV to 536 HV after laser nitriding, while the hardness of other laser heat treatment methods used for P21 in the study did not increase. The wear resistance of laser nitriding P21 is also improved, while the wear resistance of the substrate and other heat-treated samples does not increase.

The effect of laser nitriding on P21 steel is attributed to the formation of hard AlN deposits in the subsurface of laser nitriding P21.

Lee, KH. etc. (2016). The structure and hardness of the surface melting and hardening zone of SM45C die steel use Yb:YAG disc laser. Welding and Connection Magazine. Website: https://doi.org/10.5781/JWJ.2016.34.1.75.

Meka, SR, etc. (2016). A dual-phase microstructure is produced by nitriding; nitriding of iron-based Fe-Mn alloys. Materials Science and Technology. Website: https://doi.org/10.1179/1743284715Y.0000000098.

Shin, WS., etc. (2020). The effect of laser heat treatment and laser nitriding on the microstructure evolution and wear behavior of AISI P21 die steel. Metal. Available at: https://doi.org/10.3390/met10111487.

Sim, A. etc. (2019). The effect of high-power diode laser laser-assisted nitriding on the surface hardening of aluminum-containing martensitic steel. Optics and laser technology. Website: https://doi.org/10.1016/j.optlastec.2019.03.040.

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Ben Pilkington is a freelance writer interested in society and technology. He likes to understand how the latest scientific developments affect us and imagine what might happen in the future. Since completing his postgraduate studies at Oxford University in 2016, Ben has been reporting on the development of computer software, the UK technology industry, digital rights and privacy, industrial automation, Internet of Things, artificial intelligence, additive manufacturing, sustainability and clean technology.

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