연속파형 Nd:YAG 레이저를 이용한 고온구조용 합금강의 표면경화열처리 및 용접 특성에 관한 연구

Abstract

In the modern industrial society, we can find infinite possibilities for applying high-density energy processing technology to the highly sophisticated industries. Especially, laser welding and surface hardening technology has been applied to the industry in general because of its ability to enhance wear resistance, corrosion resistance, and strength of various parts and structures. This study examines the laser processing property of KP-4, STD61 and Hastelloy C-276, which are mold steel and nickel alloy widely used in the molding, electric power generation and chemical plant industries, using the experimental methods with laser beam. In general, molding materials used for manufacturing molds should have high tensile strength, high wear resistance, and high corrosion resistance as well as high machinability for machining. Especially, surface hardness of the mold is an essential property in efficiently producing a large quantity of parts with the same quality. However, with conventional molding materials, it is not possible to make the mold that has completely satisfactory properties. For this, a method of concentrating an external heat source on the surface of the materials and then cooling it off is used to increase the hardness of surface layer. This method is called surface hardening technology. For this study, the shortcomings produced when a materials is surface-hardened by defocusing Gaussian beam was supplemented. In order to have the hardened width of a large area with even uniform hardness distribution, an optical system with elliptical profile was fabricated. On the basis of these, this study examined the hardening property of the selected materials with varying the laser processing variables. For the experiment, Plano-convex series lenses with varying ratios of lens diameter to lens focal length(F#) as varied to 2.8, 4.8, 5.8 and 8.5. As a result, when a single pulse beam was applied to the surface of die steel for plastic molding, the heat input absorbed by specimens were shown to be even for the lenses with F# of 5.8 and 8.5 even though their defocused distances were different. However, for the lens with F# of 2.8, the heat input of the specimen was remarkably reduced when the defocused distances were ±2mm. As a result of the surface hardening experiment with the same processing variable, it was shown that the lens with F# of 2.8 formed the largest surface hardening width and even surface roughness. Even though F# of the surface hardening lens was varied, the hardening depth was about 700㎛ and the mean hardness value was about Hv=600 when the laser beam travel speed was 0.3m/min and the defocused distances was 0mm. At the laser beam travel speed of 0.2m/min, the surface roughness became irregular as the thermal stress was concentrated on the hardening center during melting and re-solidification process regardless of F# of the surface hardening lens. Moreover, over-melted layers were formed in the microstructure, resulting in a small hardness value. For STD61(Hot work tool steel), laser surface hardening experiment was carried out using the lens with F# of 4.8. A processing technology that could contribute to extending the lifespan of molds by repairing damages or defects caused during industrial mold manufacturing or by repair-welding the parts damaged while the molds were in use was also studied. Laser repair welding is a method to grow a certain size of mixed layer by adding filler metal to the melting pool created during the interaction between parent material and laser beam. In this study, wire was used as the filler material for the laser repair welding, and the phenomenon in which the supplied filler material was melted and beaded down into the specimen was examined with varying laser powers and welding speeds. The ratio of bead depth to bead height, H/H and the ratios of melting width to melting depth, W/H and W/H, were used as processing variables. The optimal processing condition was found to be the laser power of 1,300W, the welding speed and feed wire supply speed of 0.5m/min and the defocused distances of +2mm. At this time, the heat input(E) was 426×103 J/cm2, and no internal defect occurred. The hardness value of the welded part was double that of the parent material at the center of the welding line, which was rapidly reduced after being increased up to Hv=600 in the direction of the base material. When repair welding was carried out as the optimal processing for the part that had an external defect with the radius of 2mm, the filler metal was melted, resulting in the volume smaller than the defect part and thus causing the part unfilled. Therefore, it was found to be necessary to carry out repair welding two to three times by multiple passes rather than do it only once by single pass. Hastelloy C-276 is a corrosion-resistant alloy at high temperature and thus very widely used in the chemical plant and the electric power generation industries. However, as this alloy is very expensive in comparison with steel, it is mostly used for the structures that have to undergo high temperatures such as heat exchanger. For other structures, it is welded partially by various methods so as to cut down on material costs. For example, such methods as thin-sheet metallic lining, clad plate construction, surface weld overlay and thermal spray process are used. In this study, the thin-sheet metallic lining method was used to examine the characteristics of laser welding of similar and dissimilar metals. When dissimilar metals, namely Hastelloy C-276 and SM45C, were butt-welded, flowing shapes were made in the direction from the center welding line toward Hastelloy C-276 from the welding line. When the laser power was 1,000W and the welding speed was 1.0m/min, the shapes of banding and rippling were formed on the welded part as the melting zone was solidified by crossing of dendrite structure micro segregation. This phenomenon was caused because solute concentration and solidification structure became unstable. When the laser power was increased to 1,200W or more, a type of defect "humping" occurred because of different physical properties of main ingredients of SM45C steel and Hastelloy C-276. The optimal processing condition for the butt welding of Hastelloy C-276 and SM45C steel was found to be the laser power of 1,000 W and the welding speed of 1.0 m/min. At this time, the power density, maximum stress and strain were 55.6×103 W/cm2, 478 MPa and 0.32 respectively. The optimal processing for the butt welding of Hastelloy C-276 and STS304 was the same as that of Hastelloy C-276 and SM45C steel. At this time, however, the power density, maximum stress and strain were 44.7×103 W/cm2, 614 MPa and 0.46 respectively. As a result of lap-welding STS304 and SM45C steel individually to Hastelloy C-276 with the sheet metal fixed on Hastelloy C-276, the welding of Hastelloy C-276 and SM45C steel happened to have solidification crack on the parts where the surfaces of welded materials were lapped during the laser processing. The microstructure of the welded part were uneven in equiaxed dendrite, cellular dendrite and columnar dendrite. Also, there was internal crack in the boundary where the columnar dendrite and the cellular dendrite met. Mean hardness value of the welded part was Hv=210 and on the part near the parent material of SM45C steel, a hardening layer of Hv=690~760 was formed. It seemed that this phenomenon occurred because Hastelloy C-276 whose main component of nickel was mostly included in the melting zone. On the other hand, the lap-welding of Hastelloy C-276 and STS304 did not have any internal defect unlike that of SM45C steel. However, wine-cup shaped beads appeared on the welded part in general. It was because the melting zone on the specimen welded to the parent material existed in highly ductile austenite with no phase transformation. As for microstructures of the welded part, equiaxed dendrites of regular size were formed at the center line of welding, and columnar dendrites were formed on its left and right sides in the direction to the parent material. The chemical composition of the welding part was different at the top and at the bottom. At the top of the welded part, iron was detected while at its bottom, iron was detected in a small amount but Ni and Mo were detected in large quantities in comparison with the parent material.