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Microsctructure of carbon steel subjected to laser surface processing.

© 2019 A*STAR Advanced Remanufacturing and Technology Centre

Laser heat keeps steel strong

30 Sep 2019

The strength of steel comes from carbon, which is lost when steel is heated, resulting in weakening of the alloy. A*STAR researchers have found a way to overcome carbon loss with lasers.

The mass production of steel during the Second Industrial Revolution transformed the world and ushered in the modern era. The addition of some carbon into steel makes it harder and stronger than pure iron, which is crucial across a wide range of industries such as construction and manufacturing.

However, when steel is heated to high temperatures, carbon atoms can diffuse away into the air in a process known as decarburization. “Decarburization is bad for steel as it weakens the steel surface. It can occur during any high-temperature steel treatment,” said Niroj Maharjan, a Development Scientist who works on the surface enhancement of materials at A*STAR’s Advanced Remanufacturing and Technology Centre (ARTC). As such, his team decided to use laser beams as a heat source to see if they could reduce the extent of decarburization.

The researchers, in collaboration with a Singapore-based industry partner, Precision Laser Solutions, tested laser treatment on carbon steel and low-alloy steel. They optimized the power density of the laser beam to 1.27 × 105 W/cm2 and delivered it to the work surface with a spot size of 0.5 mm, at a scanning speed of 10 mm/s. Because the laser heats the steel surface at a very small point then moves on, the heated point cools off very rapidly, thereby limiting decarburization.

The team reported no detectable carbon loss for the low-alloy steel, while the layer of carbon loss in the carbon steel was only a third of what they observed using an ordinary furnace.

“The rapid cooling also produces a very hard phase of steel called martensite which is very resistant to wear. We believe martensite formation suppressed the adverse effects of decarburization during the laser treatment process, and we saw no significant change in the wear performance of the carbon steel,” Maharjan said.

Nonetheless, Maharjan believes that carbon loss can be further reduced by applying a coating of graphite, or some other carburizing medium, to the surface. Not only would this strategy minimize carbon loss, it could also improve the surface properties of the material.

“In addition, we have seen encouraging initial results when we perform laser processing in a carburizing atmosphere, which also allows the steel surface to absorb further carbon. This has given us steel with even higher surface hardness, opening up various possibilities in the processing of high-quality steel,” he said.

The A*STAR-affiliated researchers contributing to this research are from the Advanced Remanufacturing and Technology Centre (ARTC).

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References

Maharjan, N., Zhou, W., Zhou, Y., Wu, N. Decarburization during laser surface processing of steel. Applied Physics A 124, 682 (2018) | article

About the Researcher

Niroj Maharjan

Development Scientist

Advanced Remanufacturing and Technology Centre
Niroj Maharjan was awarded the Singapore International Graduate Award (SINGA) by A*STAR to pursue his doctorate at the School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore, in 2014. During this period, he investigated the interaction of laser beams with metals, specializing in the surface hardening of steels. After obtaining his PhD degree in 2018, he joined the Surface Enhancement team at the Advanced Remanufacturing and Technology Centre (ARTC) as a Development Scientist, where he has been developing surface treatment capabilities such as laser hardening and laser shock peening.

This article was made for A*STAR Research by Wildtype Media Group