INCONEL 718超合金銲接性之研究

Abstract

本研究Inconel 718超合金以TIG與PAW對接銲，在不同熱處理條件下施以拉伸試驗及微硬度試驗，比較銲接機械性質的差異；另外使用可調應變試驗以評估Inconel 718超合金銲接熱裂縫敏感性。金相與SEM觀察則用以研究上述試驗產生的微觀組織及裂縫。 拉伸試驗結果顯示，有填料銲件之機械性質明顯高於無填料的銲件，而三種不同熱處理條件皆有助於提高機械性質。TIG銲件機械性質比PAW銲件之機械性質為佳，但以相同熱處理條件後，兩者機械性質趨於一致。 可調應變試驗的結果顯示，相同外加應變量下，裂縫總長度會隨熱循環次數增加而增加，而在相同熱循環次數下，裂縫總長度均隨著外加應變量之增加而增加，故熱循環次數與外加應變量增加會提高熱裂敏感性。 金相顯微觀察，熔融區皆為樹枝狀組織。經三種熱處理後，CHT及1950H熱處理之熔融區為柱狀樹枝狀組織，而MHT熱處理熔融區為等軸晶組織。熱裂縫主要出現在銲道與熱影響區交界之部分熔融區，屬於沿晶裂縫，熔融區裂縫較粗 大，熱影響區微小且少。從破斷面分析，熱裂縫之銲道熔融區屬於凝固熱裂，在熱影響區屬於液化熱裂；銲接拉伸破斷面皆屬延性破壞。There are two purposes of this research: one is to compare the mechanical properties of the Inconel 718 Weldments using two different welding processes, Gas Tungsten Arc Welding(GTAW) and Plasma Arc Welding(PAW). The weldments were then undergone tensile test and microhardness test after different heat treatments. The other purpose is to evaluate the hot cracking susceptibility of the welded Inconel 718 using the Varestraint test. Metallographic and SEM observation were used to study the microstructure and cracking mechanism resulted from the above experiments. From the results of tensile tests, the mechanical properties of weld metal with filler metal is higher than that without filler metal. The mechanical properties of all the weldment with different heat treatments are better than those of the weldment without heat treatments. Among all of the GTAW and PAW weldments, the mechanical properties of GTAW weldment are better, but after heat treatments, the mechanical properties of both become the same. From the results of the Varestraint test, with the same augmented strain, the total crack length of the specimens increases with the increase of the number of thermal cycling. With the same number of thermal cycling, the total crack length increases with the increase of augmented strain. Therefore, the increase of multiple thermal cycling and augmented strain can raise the weld hot cracking susceptibility. From the metallographic microstructure observation, the fusion zone are all columnar dendritic. However, after three different hot treatments, the fusion zone becomes equiaxed after MHT treatment, while the fusion zone remains in columnar dendritic type after CHT or 1950H hot treatment. Hot cracks are intergranular type, mainly appear in partially melted zone which is between the fusion zone and the heat affected zone. The crack in fusion zone is much thicker and bigger while that in the heat affected zone is few and tiny. Besides, the fracture surface of cracking have displayed solidification cracking in the fusion zone and liquation cracking in the heat affected zone. The ductile fracture surface was observed in all tensile tested specimens.