工具鋼之雷射表面改質處理

Laser Surface Modification of Tool Steel

江國安(Kwo-An Chiang)

指導教授 ： 陳永傳

國立臺灣大學/工學院/機械工程學研究所/博士(2007年)

https://doi.org/10.6342/NTU.2007.02854

摘要

本論文將工具鋼施以雷射表面改質處理，研究雷射加工參數對改質層之溫度分佈、顯微組織及機械性質的影響。論文分為三大部分：
（一）SK3碳工具鋼及SKD61模具鋼的雷射相變硬化處理
將SK3、SKD61鋼材施以雷射相變硬化處理後，從材料表面所經歷的熱歷程來探討雷射相變硬化處理時，過熱度對於相變硬化效果的影響。實施雷射相變硬化處理時，由於加熱速度極快，保溫的時間極短，故需要有適當的過熱度，使組織變為均勻的沃斯田鐵，冷卻後才能有效硬化。當其他條件不變時，相變硬化所需的過熱度會隨著雷射掃描速度的增加而增大。雷射相變硬化所需的過熱度與材料最初的組織有關，組織越粗大或越不均勻，需要的過熱度越大。在本研究中，SK3鋼材雷射相變硬化所需的過熱度，由大至小依序為：退火材、淬火回火材、淬火材。SKD61模具鋼經雷射相變硬化處理後，在相同的功率之下，硬化層的厚度會隨著雷射掃描速度的提升而降低，當功率增加時，硬化深度也會增加，但是硬化深度的增加有一極限，因為功率密度太高時，材料表面溫度便會超過熔點而熔融。影響模具鋼SKD61相變硬化的因素，主要取決於碳化物是否溶入基地中，雖然SKD61沃斯田鐵化溫度與碳鋼無異，但要使硬度提高，勢必要加熱至高於沃斯田鐵化溫度以上，使碳化物充分溶入沃斯田鐵之中，冷卻後才有硬化的效果。由於淬火材的碳化物最小，且分佈最均勻，故經過相同條件的雷射相變硬化處理後，所得到硬度最高。

（二）電鍍硬鉻之SK3工具鋼的雷射表面合金化處理
由於電鍍硬鉻具有良好的耐腐蝕性及高溫抗磨耗的性質，所以一般工業界經常對產品實施硬鉻電鍍處理，然而電鍍硬鉻層較易剝落，且較脆弱，是其美中不足之處。所以針對此問題，本研究將電鍍硬鉻後的工件再施以雷射表面合金化處理，利用雷射加熱，來使表面硬鉻層的鉻元素向內擴散，以期能改善電鍍硬鉻層的易剝落性，當雷射能量密度較低時，合金化處理後表層鉻元素的含量較高，但鉻元素擴散融入的區域範圍較小；能量密度升高時，表層鉻元素含量會降低，但鉻元素擴散融入的區域範圍會變大，而且鉻元素除了在熔融區有溶入底材外，也擴散至熱影響區。以31.83 J/mm2以上的能量密度進行雷射表面合金化處理時，表層會有熔融現象產生，且依能量密度的不同，熔融區的組織有極大的差別，能量密度較大時，晶粒有較充分時間成長，易趨向胞狀與樹枝狀組織，且受溫度梯度影響所呈現的方向性也較明顯。

（三）利用雷射加熱在碳工具鋼表面被覆超硬材料（碳化鎢、鑽石）與鈷基合金
以超硬材料（碳化鎢、鑽石）與鈷基合金為熔覆材料，經由雷射表面被覆處理的方式，在工件表面上進行超硬材料與鈷基合金之複合被覆處理。藉此結合碳化鎢與鑽石的高硬度、耐磨性以及鈷基合金的結合強度、耐蝕性等優點，以進一步提升工件的使用性能，如切削能力、耐磨性、使用壽命等。本研究以雷射表面熔覆的方式將Stellite12鈷基合金與碳化鎢或聚晶鑽石結合，實驗結果顯示，SK3鋼料表面以雷射熔覆碳化鎢與Stellite12鈷基合金時，熔覆層組織主要為σ-Co（FCC之富鈷相）與金屬碳化物，如M23C6, M6C，M7C3等（M＝W, Cr, Co）。隨著能量密度的提升，Stellite12合金基地會逐漸形成富鈷的基地，而晶間析出之硬化相會逐漸轉變為富鎢之第3相組織。由於富鎢之第3相組織具有優異的耐磨耗性，故碳化鎢與Stellite鈷基合金複合熔覆層之耐磨性比Stellite12鈷基合金熔覆層更好。在雷射熔覆聚晶鑽石與Stellite12鈷基合金方面，以雷射熔覆聚晶鑽石與Stellite12鈷基合金後，其熔覆層組織主要為鑽石與Co-Cr-W-C之四元合金，鑽石顆粒會與基地中的金屬元素相結合形成M7C3金屬碳化物，而降低鑽石與熔融金屬的介面能，且能以化學鍵結方式將鑽石包裹在鈷基合金內，故其結合強度高。將經過雷射熔覆聚晶鑽石與鈷基合金後的試片與市售電鑄鑽石砂輪相互對磨，試片表面的鑽石並沒有脫落，可見其結合強度比以電鑄方式結合者來得大，而且鑽石表面因其自銳性的影響，形成更多的切削角度。由耐磨耗試驗結果得知，熔覆層的耐磨性隨著聚晶鑽石的粒徑及添加量的增加而增加。此熔覆層是利用聚晶鑽石與鈷基合金相互產生的碳化物鍵結使聚晶鑽石緊緊鑲於鈷基合金內，以避免在磨耗時脫落或使裂縫延伸擴大，因而能有優異的耐磨耗性

關鍵字

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

並列摘要

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.

並列關鍵字

laser transformation hardening ； laser surface alloying ； laser brazing ； tungsten carbide ； poly crystalline diamond ； Stellite12 cobalt base alloys ； metallic carbide

參考文獻

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