HAYNES 230超合金銲接特性之研究

Abstract

在軍用飛行器及工業用渦輪機等高溫性能要求日漸嚴苛之下，一般合金材料實難以滿足其需求。但具有高溫強度及高溫穩定性、耐氧化腐蝕性、抗高溫潛變的超合金，即具有重要地位。而Haynes 230超合金為鎳鉻鎢鉬超合金，具有極佳的高溫特性，在高溫嚴苛環境仍具有優異的性質，是一極佳的研究對象。 本研究針對Haynes 230超合金施以惰性氣體鎢極電弧銲（GTAW）及電漿銲(PAW)後，另外本研究使用可調應變評估Haynes 230超合金銲接熱裂縫敏感性。並在銲後熱處理設計，透過微硬度試驗、拉伸試驗、光學顯微組織觀察及掃瞄式電子顯微鏡觀察，探討銲件之機械性質及分析銲件微觀金相組織結構，藉此找出較佳的銲接特性及較佳的銲接熱處理方式，以期得到最佳的銲接設計。 研究結果顯示，在點銲可調應變試驗中， Haynes 230超合金的裂縫隨著外加應變的增加而增加；但不會隨著熱循環次數的增加而增加。經微硬度試驗，填料試片的硬度高於無填料銲件。經完全固溶熱處理銲件伸長率大幅提昇，但抗拉強度、降伏強度及微硬度卻降低。然而實施應力釋放熱處理則使銲件抗拉強度增加，尤其是填料銲件的抗拉強度、降伏強度、伸長率等機械性質皆比未熱處理之銲件佳。經由SEM觀察點銲可調應變裂縫破斷面組織，銲道熔融區為凝固熱裂縫而熱影響區為液化裂縫。 然而銲件拉伸試驗破斷面經由SEM觀察，不論歷經何種熱處理方式，銲件之破斷面均呈現延性破壞之現象。

Superalloys Haynes 230 is one of several high strength precipitated hardened Ni-base superalloys suitable for service in the high temperature range. It has excellent corrosion and oxidation resistance as well as good tensile, fatigue, and creep properties at elevated temperature. TIG welding is one of the major welding processes which give a full penetrated, high-quality weldment. There are welding problems such as hot cracking and reduction of strength in the HAZ in the Ni-base superalloys. The objective of this research project is to determine the optimum welding parameters which can obtain full and uniform penetration of the weld fusion zone in TIG weldment of superalloys Haynes 230. Mechanical properties including tensile test and microhardness test were conducted. Metallurgical examination, X-ray analysis, and SEM observation were also used for microstructure analysis. Several heat treatments were conducted to determine the optimum condition to recovery the strength in the HAZ. In order to compare the hot cracking susceptibility of Haynes 230 and Inconel 718, a newly developed Varestraint testing machine was utilized. From the results of Varestraint test, the total crack length increases with the increase of augmented strain. But the total crack lengths have not been affected by multiple thermal cycling. The microhardness of fusion zone with filler metal is higher than the fusion zone that without filler metal. The ductility of weldment could be enhanced by solution heat treatment but ultimate tensile strength and yield strength were decreased. After stress relief heat treatment, the ultimate tensile strength, yield strength and ductility are better than the weldment without heat treatment. From the observation of SEM fractography results, 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.