Laser transformation hardening was carried out on two types of steel that differed in hardenability, using a 1kW CO2 laser under various conditions, and the effects of processing parameters-such as scan rate and the defocussing of the laser beam-on the hardened case depth and width, microstructure, hardness distribution and wear properties were investigated.
The results obtained were as follows:
1) The maximum depth and width of hardened case decreased with increasing scan rate.
2) A two-step change in hardness was observed, in which the second step corresponded to an incompletely hardened layer between the maximum hardness layer at the surface and the matrix.
3) In the hardened surface layer, X-ray diffraction analysis revealed the existence of retained austenite.
4) Hardening occurs independently of hardenability in the steels tested, due to the fact that the cooling rate in laser hardening is much faster than the critical cooling rate of the steels.
5) The wear resistance of the laser-hardened steels was much better than that of the normalized steels. The improvement produced by laser hardening was particularly marked in sliding rates of 0.8m/s or less. This was attributed to the fact that the hardened area on the sliding surface increased with decreasing scan rate.
