高溫固溶處理對旋形麻時效鋼及EBW銲件之影響

Abstract

高旋形加工量C-250 麻時效鋼電子束銲件經時效熱處理後，因銲道強化元素徧析及晶界間生成多量的逆變態沃斯田鐵池，導致材料強度下降及延伸率嚴重不足。先前學者研究藉由製程技術改善銲件延伸率不足，但仍須施以高溫的均質化熱處理以消除逆變態沃斯田鐵池，卻導致材料偏脆化的負面影響，限制了麻時效鋼在航太科技工業上的發展。為改善旋形麻時效鋼銲後直接施以時效熱處理，材料延伸率偏低及銲道晶界間逆變態沃斯田鐵生成之問題。本研究以58% 旋形加工C-250麻時效鋼及電子束銲接後，除了施以標準時效熱處理外，特別規劃以不同前置「固溶」熱處理「溫度」及「時間」製程進行研究與實驗，以獲得提升延伸率及消除逆變態沃斯田鐵池之最佳化熱處理製程。 研究試驗顯示，為獲得較佳銲件延伸性及改善銲道晶界間逆變態沃斯田鐵池，經過時效（510℃、540℃）及三分鐘固溶至均質化溫度（810℃∼1150℃）不同熱處理後，以940℃/3min/AC(Air cooling)再施以時效熱處理效益為最佳，除銲件延伸率可滿足 AMS 6520D規範規格外，並大幅提升200%，且銲道晶界間逆變態沃斯田鐵開始減少及開始再結晶成板條狀麻田散鐵組織。當施以1150℃/3min/AC熱處理後，其結果與AMS 6512 E規範標準具有相同的效益，已可完全消除銲道晶界間逆變態沃斯田鐵池，但銲件有偏脆化之現象。 經研究結果，58% 旋形麻時效鋼電子束銲件經940℃/3min/AC後，再施以標準時效熱處理，機械性質均可滿足AMS 6520D規範值，大幅改善銲後直接時效之材料延伸率不足及銲道逆變態沃斯田鐵生成等問題，此縮短前置熱處理時間之有效製程將可提供後續工程應用上更多選擇。

After electron-beam welding (EBW) and highly deformed tubing, the C-250 maraging steel suffer a severe lack of percentage elongation. Its weldment shows reversion austenite formed at the intergranular boundaries of the steel after 480℃ aging treatment. This reversion austenite has created an adverse effect on the mechanical properties. Homogenization treatment can dissolve the reversion austenite pools but enhance the brittleness of the weldment. The objective of this research is to study the effect on 58% flow-formed of maraging steel and EBW weldment by various heat treatments and to solve the problem of loss of ductility. From the experimental results of material strength, the 480℃ aging treatment showed the best performance and can met AMS 6520D Standard Code for the maraging steel and EBW weldment that had received the flow forming amount of 58%. But the weldment suffers a severe lack of percentage elongation. In order to increase the ductility of the EBW weldment, several heat treatments were conducted, including over-aging treatment (510℃,540℃/6h/AC), solution and homogenization treatment (810℃∼1150℃/3min/AC). Among these, the 940℃/3min/AC + 480℃/6h/AC treatment showed the best performance. The amount of reversion austenite at the intergranular boundaries of the weldment can be decreased and the reversion austenite can be transformed into lath martensites. Besides, after three minutes solution/homogenization treatment before aging, the EBW weldment of 58% flow-formed maraging steel can meet the requirement of AMS 6512E Standard. This new technique of high temperature solution treatment can effectively solve the problem of lack of ductility of EBW weldment of C-250 flow formed maraging steel and can provide more choices of engineering applications.