HSLA-80/Inconel-625異質焊接件與HSLA-80模擬粗晶熱影響區的腐蝕行為

Abstract

高強度低合金鋼HSLA-80為改善HY-80的可焊接性所設計之極低碳鋼種，具有優良的抗蝕性質，多做為海事船體構件使用；鎳基超合金Inconel-625在高溫、含氯環境下仍具備優秀的抗腐蝕能力，可作為傳輸流體管件材料。在不同設計需求下，可能須將不同金屬進行焊接接合，因此本研究中主要針對2.5 cm厚的HSLA-80/Inconel-625潛弧焊異質焊接件的各區域進行基礎性質與腐蝕性質分析，並進一步將HSLA-80基板進行熱處理以模擬粗晶熱影響區結晶結構後，進行電化學行為研究。 由於HSLA-80/Inconel-625異質焊接件經過數道次焊接，焊接熱影響會使不同位置HSLA-80的微結構經歷多次不同的熱循環，故本研究中以1/2厚度位置處進行探討。HSLA-80 熱影響區（Heat Affected zone，HAZ）部份在最靠近縫合線由初析肥粒鐵（Primary ferrite）相、變韌鐵、麻田散鐵/沃斯田鐵（Martensite/austenite constituent）相組成物所構成；由縫合線（Fusion line）往底材區方向移動，變韌鐵的數量逐漸減少，在1.5 mm處即全部由肥粒鐵所構成。由硬度掃描可獲得HAZ的範圍約為2.3 mm，與金相分析結果相同。HSLA-80底材區由準多邊形肥粒鐵（Quasipolygonal ferrites）相所構成。熔融區由晶粒成樹枝狀晶（Dendrite）鎳合金構成，同時Nb與Mo偏析樹枝狀晶之間。Inconel-625熱影響區與底材區皆為多邊形晶粒，晶粒大小無明顯差異，在底材區可觀察到約數μm大小顆粒的NbC與MoC偏析。 將HSLA-80/Inconel-625異質焊接件在0.6 M NaCl水溶液中進行動電位極化（Potentiodynamic polarizarion，PDP）、電化學阻抗圖譜（Electrochemical Impedance Spectroscopy，EIS）性質分析，可知電化學依序為熔融區≒Inconel-625部分>HSLA-80部分，而熱影響區的活性會微高於底材。藉由EIS輔助量測HSLA-80的伽凡尼腐蝕差異。在相同時間浸泡下，HSLA-80底材在與熔融區相接時會有比較劇烈的阻抗變化；在量測伽凡尼電流密度時，亦發現HSLA-80底材相接熔融區時會有較高的電流密度產生，代表熔融區作為伽凡尼陰極會產生較嚴重的伽凡尼腐蝕。此外，由於結晶結構差異，HSLA-80 HAZ有微區伽凡尼效應，使抗蝕能力較差。將HSLA-80/Inconel-625異質焊接件完全於浸泡0.6 M NaCl水溶液中進行腐蝕深度測試，發現HSLA-80區域皆有腐蝕發生，在HAZ具有最深的腐蝕深度，此即為HSLA-80相對焊接件其他區域皆為伽凡尼陽極的結果。 為了模擬 HSLA-80熱影響區的顯微結構，HSLA-80 底材以1200 °C進行快速升溫3分鐘和30分鐘，之後進行水淬冷卻至室溫。由X光繞射（X-ray diffraction analysis，XRD）結果顯示。金相結果顯示1200 °C 的熱處理可將由準多邊形肥粒鐵組成的HSLA-80，轉變成麻田散鐵、變韌鐵、初析肥粒鐵。在短時間的電化學分析中，極化阻抗、腐蝕電流與電荷轉移電阻皆顯示以1200 °C 3分鐘抗蝕性最差；但長時間的腐蝕特性測試中，長時間浸泡的電荷轉移電阻與重量損失法測試則顯示1200 °C 30分鐘具最低的抗蝕性。

HSLA-80, a high-strength low-alloy steel with ultra-low carbon content, was developed to improve the weldability of HY-80 and exhibits excellent corrosion resistance, making it widely used in marine hull components. Inconel-625, a nickel-based superalloy, demonstrates superior corrosion resistance in high-temperature, chloride-containing environments and serves as a material for fluid transmission piping. Under different design requirements, dissimilar metals may need to be joined through welding. Therefore, this study primarily focuses on the analysis of fundamental properties and corrosion characteristics across various regions of 2.5 cm thick HSLA-80/Inconel-625 dissimilar submerged arc weld (SMAW) joints. Furthermore, HSLA-80 base material was subjected to heat treatment to simulate the microstructure of the heat-affected zone (HAZ), followed by electrochemical behavior investigation. Since the HSLA-80/Inconel-625 weldment underwent multi-pass welding, the thermal effects of welding caused the microstructure of HSLA-80 at different locations to experience multiple thermal cycles. Therefore, this study examined the mid-thickness position (1/2 thickness). The HAZ of HSLA-80 closed to the fusion line (FL) consisted of primary ferrite phase, bainite, and martensite/austenite (M/A) constituent. Moving from the fusion line toward the base material region, the amount of bainite gradually decreased, and at 1.5 mm, the microstructure was composed entirely of ferrite. Hardness scanning revealed that the HAZ extended approximately 2.3 mm, consistent with metallographic analysis results. The HSLA-80 BM region consisted of quasi-polygonal ferrite phase. The fusion zone comprised dendritic nickel alloy grains, with Nb and Mo segregating between the dendrites. Both the Inconel-625 HAZ and BM region exhibited polygonal grains with no significant difference in grain size. In the base material region, NbC and MoC precipitates of approximately several micrometers in size were observed. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) analyses were performed on the HSLA-80/Inconel-625 dissimilar weldment in 0.6 M NaCl aqueous solution. The electrochemical nobility sequence was determined as: fusion zone ≈ Inconel-625 region > HSLA-80 region, with the HAZ exhibiting slightly higher activity than the base material. EIS was employed to assist in measuring galvanic corrosion differences in HSLA-80. Under identical immersion times, the HSLA-80 BM exhibited more pronounced impedance changes when coupled with the fusion zone. Galvanic current density measurements also revealed higher current densities when the HSLA-80 base material was coupled with the fusion zone, indicating that the fusion zone acting as the galvanic cathode produced more severe galvanic corrosion. Additionally, due to microstructural differences, the HSLA-80 HAZ suffered microgalvanic effects, resulting in inferior corrosion resistance. When the entire HSLA-80/Inconel-625 dissimilar weld joint was immersed in 0.6 M NaCl aqueous solution for corrosion depth testing, corrosion occurred throughout the HSLA-80 region, with the HAZ exhibiting the greatest corrosion depth. This result confirmed that HSLA-80 served as the galvanic anode relative to other regions of the weld joint. To simulate the microstructure of the HSLA-80 heat-affected zone, the HSLA-80 base material was subjected to rapid heating at 1200 °C for 3 minutes and 30 minutes, followed by water quenching to room temperature. X-ray diffraction (XRD) analysis was performed, and metallographic results demonstrated that heat treatment at 1200 °C transformed the HSLA-80 base material, originally composed of quasi-polygonal ferrite, into martensite, bainite, and primary ferrite. In short-term electrochemical analyses, polarization resistance, corrosion current, and charge transfer resistance all indicated that the specimen heated at 1200 °C for 3 minutes exhibited the poorest corrosion resistance. However, in long-term corrosion characterization tests, charge transfer resistance after extended immersion and weight loss measurements showed that the specimen heated at 1200 °C for 30 minutes possessed the lowest corrosion resistance.