316L不銹鋼與Inconel 600合金異質銲接之機械性質與抗腐蝕特性研究

Abstract

壓水式反應器的管路設備必須承受高溫高壓外，亦受水的化學作用與腐蝕的影響，因此經常使用具有高溫耐蝕性及高溫機械性質的不銹鋼或鎳基超合金做為結構件或管件材料，在較嚴苛的環境使用鎳基合金，而壓力槽體出水口則使用碳鋼或低合金鋼等材料，以降低成本。不同部位的異種金屬通常使用惰氣鎢極電弧銲接法接合，由於兩種合金之間存在組織、物理及機械性質等差異，在異質銲接後容易在界面處產生殘留應力而導致破壞。 本研究使用雙面式摩擦攪拌銲接技術與惰氣鎢極電弧銲接技術用於Inconel 600合金與316L 不銹鋼的異質接合，摩擦攪拌銲接法利用含鈷之碳化鎢製作攪拌頭，攪拌頭傾斜角1°、轉速設定800 rpm、進給速率50 mm/min，可以成功接合並無缺陷產生，惰氣鎢極電弧銲接則比較不同電流大小，並探討添加AMS 5786填料與否對銲道造成的影響。將各成功接合的試片進行顯微組織及機械性質之影響，並進行元素分析，最後進行殘留應力量測及抵抗沿晶腐蝕能力分析。 實驗結果顯示，惰氣鎢極電弧銲接之接合件銲道硬度、拉伸強度及伸長率都明顯下降，拉伸斷裂點位於銲道熔融區；而利用摩擦攪拌銲接之接合件銲道硬度、拉伸強度及伸長率都顯著的提升，拉伸斷裂點位於316L不銹鋼母材。顯示摩擦攪拌銲接具有優良的接合效果，並且可以降低銲件之殘留應力，但是在攪拌區進給邊的抵抗沿晶腐蝕能力則會下降。另外可以觀察到在異質接合後，Inconel 600合金在硝酸中的腐蝕速率高於316L 不銹鋼。Pressurized water reactor have to withstand extremely high temperature, high pressure, and the chemical reactions and decay effects cause by water. That is why the construction parts and pipe fitting are usually constructed by stainless steel and nickel base alloy, taking the advantages of their mechanical properties of resistance from heat and corrosion. Nickel-based alloy are used to constructed of nuclear reactors, and carbon steel or low alloy steel are used to constructed of reactor pressure vessel outlet to reduce costs. Gas tungsten arc welding is normally applied to the dissimilar alloy assembly. However, the dissimilar materials welding parts may easily trigger unfavorable destruction caused by the residual stresses at the joint because two alloys contain different organizational, physical and mechanical properties. In this study, we used GTAW process and double-sided friction stir welding to join the dissimilar materials of Inconel 600 alloy and 316L stainless steel. Cobalt-containing tungsten carbide stir rod was used for FSW of Inconel 600 alloy and 316L stainless steel, and a defect-free weld was successfully produced. A 1° tilt was applied to the stir rod during FSW, and the welding parameters used were a rotational speed of 800 rpm and a travel speed of about 50 mm/min. Using AMS 5786 filler and without filler in GTAW to compare the effects of different currents on the weldment. The specimens were analyzed for their microstructure, mechanical property and elemental analysis followed by the dissimilar welding of GTAW and double-sided type FSW. Determine whether the weldment is prone to stress corrosion cracking by measurement residual stress and analyzed resistance of intergranular corrosion.

Experimental results showed that the microhardness of fusion zone, tensile strength and elongation of GTAW dissimilar joint decreased apparently and the tensile fracture site was at the fusion zone of welding joint. However, an excellent joining effect was achieved by FSW, the microhardness of stir zone, tensile strength and elongation of FSW dissimilar joint have been improved. The tensile fracture site was located at the 316L stainless steel base metal, also indicating the superior joining effects by this welding approach. Using FSW process can reduce the residual stress of the weldment but the resistance of intergranular corrosion are decreased in advancing side of stir zone. It can also be observed that the corrosion rate of Inconel 600 alloy is higher than 316L stainless steel in nitric acid after dissimilar welded.