고출력 다이오드 레이저를 이용한 프레스 금형재료용 주철의 표면경화처리에 관한 연구

Abstract

Mold industry is essential to the basic industries like as automobile, ship and electronic parts and grows constantly with allied industries. But surface hardening treatment is necessary to improve the quality and prolong the life time of die because the die has enormous influence on the quality and cost of products. So far, Flame and induction heating have applied for surface hardening of die, but they involve many problems such as the distortion and limited shape. Therefore, laser surface treatment method is used to solve these problems and increases the surface hardness of die.

Laser surface hardening uses the laser heat source of high power-intensity to rapidly heat the surface of steel into austenitic region. And then due to the self-quenching which is occurred by rapid cooling with only high rates of heat transfer, the surface of steel transforms from austenite to martensite. The surface treatment has the very low thermal distortion because of fast thermal-cycle and it does not require the external cooling system and it is possible for localized heat treatment. High power diode laser especially is regarded as the most proper laser for surface hardening treatment by the specific characteristics of diode laser. Therefore, diode laser is applied to increase the surface hardness of die as the press die for car body.

This study is related to the surface hardening treatment to four types of cast irons for press die by using high power diode laser. Hardened width and depth was measured and hardened zone was analyzed by hardness test after surface hardening in order to research the optimum condition of heat treatment. The surface treatment is carried out with the change of beam travel speed and laser power. The slower beam travel speed was and the more laser power increased, the more hardened zone increased. But when the heat input was not controlled, the coloring or melting of surface was occurred.

Then it was found that flake graphite cast iron needs the higher heat input than spheroidal graphite cast iron by the hardening characteristics with material and form of die. And the proper heat treatment temperature for each material was drawn by heat-treating with temperature control. As a result of hardness measurement and observations on microstructure of hardened zone, hardness increased more than three times as compared with base metal because the lath martensite was formed on the matrix structure.