레이저 열원을 이용한 탄소강의 표면변태경화 공정에 관한 연구

Abstract

Laser, due to the unique beam property, has replaced most of the industrial process including a welding, cutting, drilling and surface modification. LTH(laser transformation hardening) is one branch of the laser surface modification processes. Compared to the conventional hardening methods such as induction and flame heating, LTH process has several superior characteristics : first, it is a non-contact process. second, it is easy to control the hardening depth through adjusting laser power. third, it dose not require the additional cooling system due to self quenching. forth, it is possible to harden the material with the localized and complicated shape. fifth, it hardly leads to thermal deformation and lastly, a prominent hardening result(hardness) can be obtained despite the materials with the low carbon content.

This study is related to the surface hardening of rod-shaped carbon steel by the laser-lathe composite processing system. The existing studies on the LTH process have been generally conducted by CO2 laser and optic head with an uniform power-intensity distribution. However, in this study, two types of lasers - Nd:YAG and high power diode laser(HPDL), which have a higher absorptivity and working flexibility comparatively were used as a heat source. Also, two optic heads with the gaussian circular beam and rectangular beam were applied to this experiment in order to compare the hardening characteristics with beam profiles.

The laser power, revolution counts(rpm), beam traveling speed and overlap rate were considered as the dominant process parameters, and the concepts of the circumferential speed and theoretical overlap rate were introduced to determine the working conditions, which were defined as a function of the revolution counts and beam traveling speed. The hardening depth and width depended on the heat input as a combination of these parameters.

Three issues were considered in this study with respect to the experiment results - hardening characteristics with the dominant process parameters, longitudinal and depth directional hardness distribution and behavior of the phase transformation in the hardened zone. Through these analysis of the hardened zone, an optimization of the process parameters by both optic heads could be achieved and a feasibility of the surface hardening by the gaussian circular beam was verified.

Finally, it is expected that this study provides more advanced information about the LTH process and contrives an improvement of the processing quality.