Transformation Hardening using a Laser

H. K. D. H. Bhadeshia

A laser is a controlled heat source which on traversing a steel surface, induces it to transform into austenite during heating and then into martensite during the cooling part of the thermal cycle. The method can be used to produce a layer of martensite on the surface of an appropriate steel. The component then has a hard layer on its surface, supported by a softer and more ductile core. The hard layer confers wear resistance and may also be used to control the state of stress of the component.

The micrographs below have been provided by Henrikki Pantsar (Lappeeranta University, Finland). The steel, which is quenched and tempered before laser treatment, has the chemical composition (wt%):

The Mn, Cr, Ni and Mo confer hardenability and the medium carbon concentration ensures a sufficiently high hardness in the martensite that forms due to laser treatment.

A diode laser (wavelength 808 and 940 nm) with a power of 1650 W, traverse speed of 0.63 m/min and a beam size on the work piece of 12 by 5 mm, was used in the experiments.

	The finer microstructure on the right is a consequence of the untempered martensite formed during the cooling part of thermal cycle experienced as the laser traverses. The surface hardness therefore increases to more than 800 HV.
	Higher magnification image. The finer microstructure on the right is a consequence of the untempered martensite formed during the cooling part of thermal cycle experienced as the laser traverses. The surface hardness therefore increases to more than 800 HV.

A steel bottle-opener hardened by rapid heating and cooling of the surface, using a diode laser. The sample has been donated by Rofin-Baasel U.K. The process can be seen in detail in a movie also provided by Rofin-Baasel U.K.

PT Group Home

Materials Algorithms