銲接製程參數對SS400與SM490A低合金鋼同質或異質銲接之影響

Abstract

中文摘要 為符合各種作業場所機具的要求，銲接技術已經是一門不可或缺的重要技術，在各種機械主體結構的銲接製程裡，須能有效掌握熱應力所產生的變形量與殘留應力，並進行有效設計。由於銲件的熱影響區及銲道具有與母材不同的顯微組織，使得熱影響區及銲道的機械性質、電化學行為特性異於母材。隨著銲接程序不同所導致熱影響區及銲道在顯微組織上的變化，常常是造成銲件在使用過程中劣化或損壞的重要原因之一。本研究主要探討在不同入熱量下的多道次銲接對合金性質所造成的影響，包括顯微組織、機械性質及腐蝕性質，並嘗試找出最佳化參數，將製程標準化後，訂定所有的銲接參數，確保不同的銲接機具與人員皆能達成相同的銲接品質，以符合工業上之需求。實驗分為SM490A同質銲接與SM490A對SS400異質銲接，其中同質銲接採用包藥銲線金屬電弧銲(Flux Cored Arc Welding)，異質銲接採用惰性氣體遮蔽電弧銲(Gas Shield Metal Arc Welding)。 熱影響區及中下層的銲道經過多道次銲接後，會受到一類似回火或正常化的熱處理過程，可消除區域上的不良組織(例如：費德曼肥粒鐵)，有效的提升韌性，並降低粗晶區的硬度。由衝擊試驗的結果顯示：由於晶粒細化程度的不同，較高入熱量的製程會有較高的衝擊值，而在同質銲接其拉伸強度會隨入熱量升高而下降，異質銲接則隨入熱量升高而上升。 腐蝕性質方面，在極化曲線中顯示，無論同質或異質銲接的銲道其腐蝕電流均比母材來得小，代表其腐蝕速率較慢，有較母材為佳的抗腐蝕性。隨著入熱量的上升，其銲道的腐蝕電位差異不大，而腐蝕電流下降，有較佳的抗腐蝕性質；而在循環極化曲線的測試中可以了解材料的抗孔蝕性，隨著入熱量的減小，銲道會從均勻腐蝕的性質轉變為有輕微的孔蝕現象。

Abstract To meet the needs of every machine in the working space, the welding technology is one of the most important industrial technologies. Thus, in the welding process of mechanics, we should know well the distortion and residual stresses induced from heat stress and design them properly. Because the weld metal and the heat affected zone have different microstructures from the base metal, their characteristics such as mechanical properties and electrochemical properties are also different from the base metal. The different welding procedures often cause the transformation in the microstructures of the weld metal and heat affected zone. Because of the transformation, it is probable for these welded machines to have cracks and damages when used. This thesis aims to investigate how, under different parameters, the multi-pass welding influences the properties of the alloy, including the microstructure, the mechanical property and the corrosion property. This thesis also tries to find a better set of welding parameters, work out a standard process and formulate all parameters. In doing so, the transformation, which causes the cracks and damages of machines, would be reduced, even though the machines are welded by different workers in different environments. These experiments described in this thesis include the similar welding of SM490A with FCAW and the dissimilar welding of SM490A and SS400 with GMAW. After a multi-pass welding, a thermal treatment, such as tempering or normalization, would happen to the heat affected zone and the under passes. The bad microstructures, such as Widmanstätten ferrite in the heat affected zone and reheated weld metal, would be eliminated, so the toughness and hardness of the heat affected zone would be improved. The result of the Charpy-notch impact test shows that, because of the difference in grain refinement, the higher heat input would produce higher impact energy. With the rise of the heat input, the tensile strength would decrease in the similar welding, and increase in the dissimilar welding. As to the corrosion behavior, the result of the potential-dynamic polarization curve tests reveals that, whether in the similar or the dissimilar welding, the corrosion current of weld metal is weaker than that of base metal. It means that the weld metal has a slower corrosion velocity and thus, has a better anti-corrosion property. The rise of heat input causes no obvious difference in the corrosion potential of weld metal, but the weld metal would have a better anti-corrosion property if the corrosion current decreases. It is to say that, under higher heat input, it has higher resistance to corrosion. As to the anti-pitting corrosion property, the result of the cyclic-polarization curve test indicates that, the corrosion behavior of weld metal would be transformed from uniform corrosion into lightly pitting corrosion.