旋形量與電子束能量對C-250麻時效鋼銲件機械性質之影響

Abstract

高強度麻時效鋼廣泛應用於航太科技工業之高壓及高溫載具推進器系統，為能提升製程效益及發揮優良的機械性質，應用流旋形冷作加工技術製造精密無縫之薄形管件。然而，昔日研究結果，經高旋形加工量及高能量電子束銲接製程後，均產生延伸率嚴重不足的問題，造成整體結構的脆弱點，限制了麻時效鋼的應用彈性，形成航太科技工業發展的瓶頸。 本研究應用順流旋形冷作加工及電子束銲接製程技術，規劃設計以不同旋形加工量、銲前消除加工應力、降低電子束熱輸入量、銲接前、後以低熱量電子束熱處理、不同時效熱處理及高溫環境試驗等有系統及關聯性之研究程序，以解決當前旋形麻時效鋼銲後延伸率嚴重偏低的問題，及高溫對機械性質的影響。 研究結果顯示，固溶C-250麻時效鋼適合應用經濟與易製性的順流旋形加工製程，其強度隨著旋形加工量而增加，延伸率則相對降低。經79%旋形加工及480℃時效熱處理後，拉伸強度提升12%，延伸率則大幅下降31%。經電子束銲接後，強度同樣隨著旋形加工量而增加，延伸率則相對下降，顯示麻時效鋼經高旋形加工量後，不適合直接施以電子束銲接製程。經一般熱輸入量電子束銲接後，銲道內由於合金成份的偏析，在正常的480℃時效熱處理後，銲道晶界間產生11%的逆變態沃斯田鐵池，導致延伸率(1.2%)嚴重降低僅有規範值的48%，對銲件機械性質造成負面的影響，限制了應用旋形加工與電子束銲接製程的發展。 經本研究有了重大的成果，藉由降低26%電子束熱輸入量，可有效的縮小銲道及熱影響區之截面積，同時可大幅減少銲道內55%的逆變態沃斯田鐵池，可提升拉伸強度約12％，但仍無法有效的消除逆變態沃斯田鐵池的生成。經銲前增加消除加工應力製程，已可有效解決延伸率不足的窘境，大幅提升108％，符合規範規格2.5%。同時研究第二種銲接製程，應用低熱量的電子束在銲道處施以續熱處理，以改變熱影響區的顯微結構，並促使晶粒細化之再結晶作用，使破斷的位置發生在α´+γ´雙相暗浸蝕區。使拉伸強度提升8％，延伸率更大幅提升100％，已達規範值規格2.4%。 79%高旋形加工量的麻時效鋼，經不同時效溫度熱處理後，450℃∼540℃範圍之機械性質均符合規範規格，其中以480℃時效熱處理條件為最佳。因大量的旋形冷作加工效應，使再結晶溫度降低至540℃時即已發生。旋形麻時效鋼及電子束銲件經高溫拉伸實驗，拉伸強度僅有300℃∼500℃符合規範規格。其延伸率隨著溫度升高從遞減至遞增，在400℃條件時延伸率為最低，旋形麻時效鋼及旋形銲件分別僅有規範值的88%及46%。在600℃以上時破斷均發生在暗浸蝕區外側的母材，且有Ti元素偏析集中形成破裂的起始點。 固溶C-250麻時效鋼適合應用順流旋形加工，製造高縮減率之精密無縫薄形管件，經簡易時效熱處理後，具有超高強度及優異的常溫與高溫機械性質，且適合應用在480℃以下的高溫環境。昔日旋形麻時效鋼因電子束銲接後，延伸率嚴重不足的瓶頸已獲得突破，使延伸率由1.2%提升至2.5%。就銲件使用之機械性質與穩定性，可優先採用「銲前消除加工應力＋降低熱輸入量電子束銲接」，或以「銲後低熱量電子束續熱」製程技術。並可依實際工程應用需求，與不同時效熱處理做最適化的組合，以獲得最佳的效益，以解決昔日航太與國防科技發展中棘手之低延伸率的窘境。

The high strength martensite steel is widely used in aerospace and defense industries, particularly the motor propulsion system. To elevate the efficiency of manufacturing process and to exploit the superior mechanical properties of the steel, cold flow forming technique is commonly employed to manufacture seamless tubing. However, subsequent to its electron-beam welding (EBW) and age hardening treatment, the highly deformed tubing is known to suffer a severe lack of percentage elongation. This fact has limited the applications of the maraging steel and utility of the flow forming process, creating a bottle neck of manufacturing in the aerospace and defense industries. In the present study, the cold forward flow forming technique and EBW process were employed to fabricate the tube and to join the tubing pieces, respectively. Various process parameters were incorporated to combat the problem of inadequate ductility associated with the weldment fabricated from the maraging steel. These included varying the input amount of cold forming, applying pre-EBW stress relief, varying the amount of the pre-EBW and post-EBW energies, and trying different age-hardening treatments and high temperature environmental tests. The present study showed that the C-250 maraging steel tube produced by the forward flow forming technique had an increased mechanical strength but a decreased ductility, as the amount of flow forming was increased. In the case of 79% flow forming input, the mechanical strength of the steel was elevated by 12% but the percentage elongation markedly deteriorated by 31% after the steel was aging treated at the temperature of 480□C. Subsequent to EB welding, likewise the maraging steel showed an enhanced mechanical strength but a deteriorated ductility. This suggested that the heavily flow formed C-250 maraging steel can not be used for a direct EB welding. Further investigation revealed that the maraging steel that had received a flow forming input of 79% and a regular EB thermal energy showed 11% reversion austenite formed at the intergranular boundaries of the steel after a conventional 480□C aging treatment. The formation of reversion austenite, which was related to segregation of alloying elements, had resulted in deterioration of mechanical properties, namely, reduction of percentage elongation (1.2%) accounting for only 48% of the value stipulated in the specification. This has created an adverse effect on the mechanical property of the weldment of the maraging steel, and thus has greatly limited the manufacturing process development for the highly efficient flow forming fabrication and EBW. Through reducing the EBW thermal energy input by 26%, not only the size of the weld metal but the total area covered by the reversion austenite pools formed was also reduced by 55%. Although the reversion austenite was not completely eliminated, the tensile strength of the steel was effectively raised by 12%. To solve the problem of inadequate ductility, an additional EBW stress relief was conducted before the welding fabrication. As a result, the percentage elongation was raised markedly by 108%, reaching a value of 2.5% and has thus met the AMS 6520D specification requirement. In order to increase the efficiency of manufacturing process and to develop a second EB welding route, a low thermal EB energy was applied for post EBW annealing. This was meant to alter the microstructure of the heat affect zone, namely to refine the gains through re-crystallization, such that the fracture line (the weakest link line) in the welded tensile specimens can be shifted outward to the dark etch area where (α□□’) dual phase was located. This resulted in an 8% increase of tensile strength and a remarkable 100% increase of percentage elongation, reaching 2.4% in value. For the maraging steel that had received the flow forming amount of 79%, the steel met the mechanical properties stipulated in the specification for the temperature range of 450℃∼540℃after its various aging treatments. Among these, the 480℃ aging temperature showed the best performance because heavy cold forming had enabled the steel to re-crystallize at a low temperature of 540℃ After high temperature tensile test, the flow formed maraging steel met the specification only in the 300℃∼500℃ testing range. However, the percentage elongation decreased to start with and then increased as the temperature was increased. The lowest percentage elongation occurred at testing temperature of 400℃,where the maraging steel and the weldment of the steel showed meager percentage elongation of 2.2% and 1.16% respectively, which accounted for 88% and 46% of their corresponding specification values. For tensile testing temperature beyond 600℃, fracture took place in the parent metal and the fracture was initiated at the spot where Ti segregated. The study has demonstrated that C-250 maraging steel is a suitable material for manufacturing seamless thin-wall tubing through the cold forward flow forming technique. After a simple 480℃ aging treatment, the steel can deliver superior room temperature and elevated temperature mechanical properties, which are fit for below 480℃ applications. However, the maraging steel has been found not suitable for EB welding directly after cold flow forming, because the weldment of the cold formed steel may suffer a serious inadequacy of percentage elongation. The study has demonstrated that percentage elongation of the maraging steel can be greatly improved if a pre-welding stress relief coupled with low EB thermal energy input or singly by post welding annealing with low EB energy is employed in the fabrication process. The purposes of the preceding additional processing steps were to alter the microstructure in the weld metal and in the heat affect zone. The improvement of the steel’s ductility resulted from the preceding process modification was also demonstrated in the fracture mode study of the present investigation. As a result of the present research, the sticky problem associated with lack of ductility of the maraging steel that used to trouble the aerospace and defense industries has been solved.