Laser Surface Modification of Tool Steel
Date Issued
2007
Date
2007
Author(s)
Chiang, Kwo-An
DOI
zh-TW
Abstract
In this paper, the tool steels treated by laser surface modification was proposed. The influences of the modified coatings with different laser working parameters, including temperature distribution, micro-structure and mechanical properties by laser surface modification were investigated. There are a total of three topics discussed as following:
1.Laser transformation hardening of SK3 and SKD61 steel
When the laser was focused on the steel, the temperature of the steel was raised. Due to the effect of superheating, the critical phase transformation temperature in laser transformation hardening became higher than the austenized temperature in traditional quenching due to the higher heating rate and the shorter heating time by laser heating.
However, the critical phase transformation temperature of steel increased with increasing laser travel speed. The original structure of steel caused the different critical phase transformation temperature, the effect of superheating increased when the laser was treated on the steel which has bigger or more inhomogeneous grain structure. In this paper, the temperature of superheating of SK3 steel was increased compared with in the quenched, quenched-tempered and as-received conditions. When the laser was focused on the SKD61 steel, the hardened layer of the steel was decreased with increasing the laser travel speed and increased with increasing the laser input power.
However, it has a limitation in increasing the hardened layer with increasing the laser input power. Laser heating of SKD61 steel usually causes the formation of a melting layer on the steel surface which is of poor thermal conductivity and diffusivity.
Although the austenized temperature of SKD61 makes no different compared with the carbon steel, in order to raising the hardness of laser hardened layer, the heating temperature of SKD61 should be higher than the austenized temperature which causing the metallic carbides fully dissolved into the matrix when austening .
When the laser was focused on the SKD61 steel, the hardness of modified layer was higher in the quenched condition which has smaller metallic carbides and more homogeneous grain structure than quenched-tempered and as-received conditions for the same laser working parameters.
2.Laser Surface Alloying of Chromium-Electroplated Films on Steel
Laser surface alloying is a process for altering surface elemental compositions by using the energy of a high-power laser to melt a thin layer at the surface of a bulk material, while simultaneously adding alloying elements, to effect, upon freezing, an alloy or a compound. In this paper, we characterize the surface alloys of chromium on an SK3 steel substrate produced using different laser powers, travel speeds, and energy densities. We investigated the behavior of the rapidly solidifying laser-alloyed Cr layer, including its dissolution, distribution, and microstructure, using scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD). Several significantly different characteristics were present in the microstructures of the laser-alloyed chromium layers when the material was subjected to different laser energy densities. The alloy layer, which is defined by measurements of the diffusion depth and content of the alloy element—in this case, Cr—can be controlled by the laser energy density. The dissolution of Cr decreases but the diffusion range increases upon increasing the laser energy density. Laser surface alloying of Chromium-Electroplated films on steel causes the formation of a melting layer on the steel surface when the laser energy density is over 31.83 J/mm2 and the structure of the melt layer is different with changing the laser energy density. Due to the growth of grain structure with sufficient time by higher laser energy density, the structure of the melt layer tends to form a cell like or dendritic structure and the orientations of growth is apparently influenced with the gradient of temperature.
3. Laser brazing of super hard materials (tungsten carbide and poly crystalline diamond grits) with stellite12 alloy
Stellite12 cobalt base alloys with different WC or diamond grits contents were deposited on SK3-carbon tool steel by laser cladding. The behavior of WC particulates or diamond grits, including dissolution and distribution, and the microstructure of composite coatings with rapid solidification were investigated.
Several significantly different solidified microstructures were characterized by dendrites, eutectics, faceted dendrites and the retained WC particles in the laser cladding WC + Stellite12 coatings, under different laser energy densities. When WC was melted and dissolved into the Stellite12 melt pool, the basic structure of solidification, characterized by the matrix and faceted dendrites in various shapes, and the contents of the faceted dendrites remained nearly identical. The faceted dendrites contained the majority of W as well as some Cr, Co while more Cr and Co were located in the matrix. The X-ray diffraction analyses indicated the existence of σ-Co, M23C6, M6C and M7C3 (M=W, Cr, Co) in the Stellite12 with different WC contents when deposited on substrates by laser cladding. The faceted dendrites provided the coatings with excellent resistance during dry sliding wear test. A higher content of WC gave higher volume fractions of faceted dendrites that imparted excellent wear resistance to the coating.
The X-ray diffraction analyses indicates the existence of σ-Co(FCC matrix enriched with cobalt), the metallic compound carbides such as M23C6, M6C and M7C3（M＝W, Cr, Co, Fe）, and poly crystalline diamond in the laser brazed diamond grits with Stellite12 alloy . Metallurgical bonds are formed between poly crystalline diamond grits and the matrix of stellite12 alloys that provide better bonding strength than electro-plated diamond wheel and the diamond grits tend to form more cutting angles due to self-sharpening during dry sliding wear with the electro-plated diamond wheel. The diamond grits provide the coating with excellent resistance during dry sliding wear test. A higher content and bigger size of diamond grit impart better wear resistance to the coating.
Subjects
雷射相變硬化
雷射表面合金化
雷射熔覆
碳化鎢
聚晶鑽石
Stellite12鈷基合金
金屬碳化物
laser transformation hardening
laser surface alloying
laser brazing
tungsten carbide
poly crystalline diamond
Stellite12 cobalt base alloys
metallic carbide
Type
thesis
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