Laser Transformation Hardening of Carbon Steel Sheets

Abstract

Laser transformation hardening (or known as laser heat treatment) is an important technique to increase surface hardness by utilizing a high intensity laser beam and a material’s self-quenching capability. Compared to other traditional heat treatment techniques, laser heat treatment is especially useful when the target material needs to be heat treated selectively without affecting unnecessary regions because the laser beam is relatively small and has a high power density. Until now, there have been a lot of researches regarding laser heat treatment but to the best of our knowledge, the process map from bulk type to sheet type has not been reported so far. In this thesis, we have studied how to optimize the laser transformation hardening of carbon steel sheet. In the first chapter, we investigated a process map for diode laser heat treatment of carbon steels that gives an overall perspective of the laser heat treatment of carbon steels. Using a heat treatable region map, we conducted laser heat treatments on AISI 1020 and 1035 steel specimens using a 3kW diode laser and measured their surface hardness and hardening depths. The experimental results are in agreement with the carbon contents and carbon diffusion time in austenite and cooling time. In the second chapter, we investigated the effect of specimen thickness on hardening performance in the laser heat treatment of carbon steel using the process map considering thickness of plate. We conducted laser heat treatment on AISI 1020 steel specimens using the same 3kW diode laser system from the previous chapter and constructed surface hardness map. The hardness decreases as thickness decreases and we conjectured which one would be the most dominant factor in terms of enhancement of hardness. In the third chapter, based on the results from previous chapter, we investigated how to enhance surface hardness of carbon steel with four different types of heat sink: stainless steel, steel, copper and no heat sink. The primary factors of the process are the thermal conductivity and the thermal contact resistance of the heat sink. For experiment, we used 2mm thick DP590 and boron steel sheets. In this chapter, we found effective ways to enhance the hardenability of steel sheets and how large the effect of this enhancement is proportional to thermal conductivity of the heat sink. In the fourth chapter, we simulated 3D model using AbaqusTM commercial software and Fortran user subroutine to know the influence of thermal contact resistance and thermal conductivity using heat sink. From the simulation, we realized the phase mole fraction using TTT (Time Temperature Transformation) diagram and the deformation using the parameter of the thermal expansion coefficient and phase change expansion coefficient, transformation plasticity coefficient. We found the reason why thermal contact resistant and the thermal conductivity are efficient in terms of laser heat treatment.