旋形量與熱處理對D6AC鋼電子束銲件機械性質之影響

Abstract

本研究目的為探討D6AC超高強度鋼在旋形加工、熱處理與電子束銲接後對機械性質的影響，進而獲得完整且最適化的關鍵製程技術，以提供國防與航太科技工業發展的依循。

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、電子束銲接、不同回火熱處理試驗等有系統及關聯性之研究程序，以解決當前旋形D6AC鋼銲後延伸率嚴重偏低的問題，及對機械性質的影響。

研究結果顯示，D6AC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78%旋形加工後，拉伸強度提升，延伸率則大幅下降。經電子束銲接後，強度同樣隨著旋形加工量而遞減，延伸率更相對下降，顯示D6AC鋼經高旋形加工量後，不適合直接施以電子束銲接製程。 經本研究有了重大的成果，78%高旋形加工量的D6AC鋼，經250℃∼650℃回火溫度熱處理後，其中以350℃回火熱處理條件為最佳。D6AC鋼及電子束銲件經250℃∼650℃回火熱處理後，其延伸率隨著溫度升高從遞減至遞增，在650℃條件時延伸率13.2%為最高。

D6AC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350℃回火熱處理後，具有超高強度及優異的機械性質。且昔日旋形D6AC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3.2%提升至8.1%，已解決昔日航太與國防科技發展中棘手之低延伸率的窘境。最後在電子束銲接後其必須透過回火熱處理，改善銲道與熱影響區之殘留應力，以強化機械性質。

The objective of this research is to investigate the effect of flow-forming process, heat treatment, and EB welding on the mechanical properties and microstructure of D6AC ultrahigh-strength steels. Therefore, a group of optimum manufacturing parameters can be determined and will be applied to the development of 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 D6AC steel. These included varying the input amount of cold forming, EBW and trying different post-weld heat treatments.

The present study showed that the D6AC 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 78% flow forming input, the mechanical strength of the steel was elevated by 0.4% but the percentage elongation markedly deteriorated by 8.1% after the steel was post-weld heat treatment at the temperature of 350℃. Subsequent to EB welding, likewise the D6AC steel showed an enhanced mechanical strength but a deteriorated ductility. This suggested that the heavily flow formed D6AC steel can not be used for a direct EB welding.

For the D6AC steel that had received the flow forming amount of 78%, the steel for the temperature range of after 250℃∼650℃ post-weld heat treatments. Among these, the 350℃post-weld heat treatment temperature had a great mechanical properties. However, the percentage elongation decreased to start with and then increased as the temperature was increased. The highest percentage elongation 13.2% occurred at testing temperature of 650℃.

The study has demonstrated that D6AC steel is a suitable material for manufacturing seamless thin-wall tubing through the cold forward flow forming technique. After a 350℃ post-welding heating, the steel can deliver superior mechanical properties. However, the D6AC 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. As a result of the present research, the sticky problem associated with lack of ductility of the D6AC steel that used to trouble the aerospace and defense industries has been solved. Post-weld heat treatment is necessary to reduce the residual stress and improve the mechanical properties of the EB weldment.