A Method for Laser Heat Treatment Efficiency Evaluation in Multi-Track Surface Hardening

Laser surface hardening is nowadays an industrial emerging technique, which is gradually substituting induction and flame surface hardening thanks to its advantages related to power saving and process versatility. This manufacturing technology is a challenging process especially when it has to be applied on surfaces larger than the beam spot. In this case several adjacent passes must be performed in order to scan the whole surface to be treated. This strategy involves inevitably an intrinsic tempering effect due to the re-heating of the previously hardened material. The extent of the softening occurring depends on several parameters. First of all, it depends on the material and its initial state, then on process parameters related to the laser source, such as type, optical path and spot dimension and further on the adopted surface scan strategy of the beam. This last set of process parameters is represented by: laser beam speed, number of tracks, pass overlapping degree and tracks sequence. The hardness uniformity of the heat treated layer and the consequent effectiveness of the process depend strictly on the tempering degree occurring in the material. According to this it is important to find a practical method devoted to quickly characterize the result of a laser surface treatment in terms of tempered zones extension and distribution. This article proposes, then, the definition of a “Covering Uniformity” index (CU) which represents an engineering approach to this problem and it allows to easily determine the effectiveness of a particular laser hardening treatment. The CU index is based upon hardness measurements and it is related to the ratio between the extension of the tempered zone and the total extension of the treated area. In order to underline and demonstrate the intrinsic value of this parameter a set of experimental trials was carried out on AISI 1070 carbon steel, AISI 1040 carbon steel and AISI 420B martensitic stainless steel.