SUS444不銹鋼薄板應用CO2雷射之銲接性質研究

Abstract

肥粒鐵系不銹鋼具有優異抗高溫氯氣氧化腐蝕性能，近年來常被運在太陽能熱水器面板、熱水槽、水管管路等。本研究針對SUS 444肥粒鐵系不銹鋼薄板施以CO2雷射銲接，利用改變不同銲接走速，探討不同入熱量對SUS 444不銹鋼銲件微觀組織及機械性質之影響。經雷射銲接之試片銲道以XRD分析及顯微鏡觀察組織變化，再分別進行拉伸測試、硬度測試與拉伸破斷面型態觀察，最後再探討銲接試片銲道附近析出物變化情況，與電化學測試做比較，以瞭解材料之抗腐蝕性變化。研究結果發現SUS 444不銹鋼在母材與銲接後之銲道皆為單相肥粒鐵相組織，利用CO2雷射銲接後，若銲接走速越慢，入熱量較高，則在銲道區域會形成粗大柱狀晶結構，導致機械性質較差。隨銲接走速提昇，銲道柱狀晶有細化現象，銲道硬度值越高，延伸率與拉伸強度亦有增加趨勢。此外，銲接走速降低也會影響銲件熱影響區析出物的型態，導致含鈮析出物團聚在晶界附近，降低材料的抗腐蝕性。

Ferritic stainless steels have recently been received increasing interest for application in the solar water heater panels and hot-water tank, due to the low thermal expansion coefficient, excellent resistance to high-temperature oxidation, and stress corrosion cracking. The objective of this study is to demonstrate the feasibility of CO2 laser welding for joining of SUS 444 ferritic stainless steel by different welding speeds. Microstructures and precipitations of the welds will be examined using optical microscopy (OM) and scanning electron microscopy (SEM). This experiment performs micro-hardness measurement in accordance with welding areas after CO2 laser welding. Moreover, the specimens have been cut vertical to weld pass after welding and then perform tensile test using universal material testing machine in order to explore the joining quality of welding joints. At the same time, SEM has been used to observe the fractured surface of the tensile testing specimens. Finally, electrochemistry experiments were performed in an aqueous solution of 3% NaCl to explore the Pitting corrosion resistance of the welds. The experimental results have found that microstructure of 444 stainless steel welds is completely ferrite single-phase structure, including base metal and welding path. However, the welding fusion zones present coarse columnar structure when the specimens have lower welding speed, which will result in the mechanical properties degradation of weldments. With the increase of welding speed, the solidification zones have fine columnar structure and the weldments show higher microhardness in the welding fusion zone. At the same time, the tensile strength and elongation have been promoted, and the fracture site appears in the base metal. Furthermore, the lower welding speed will result in the aggregation of Nb-contained precipitations in the grain boundaries of heat-affected zone, which will bring the pitting corrosion resistance degradation of weldments.