A572 Gr.65高強度低合金鋼銲接熱模擬顯微組織與機械性質之研究

Abstract

A572Gr.65合金結構鋼為一種重要的核能結構材料，當其作為工程的結構時，必須經過銲接接合的過程，但在銲接後時常發生龜裂現像。銲接接合時產生所謂的熱影響區，便是造成破壞的根源，為了提升材料在使用上的安全性，須對銲接所造成的熱影響區多加注意。本實驗將針對銲接熱影響區的顯微組織相變態及其與衝擊韌性之關係做一研究。實驗步驟為將取得的A572Gr.65利用熱膨脹儀施以10kJ/cm、20kJ/cm及30kJ/cm的銲接入熱量模擬，並使用光學顯微鏡及穿透式顯微鏡觀察各個顯微組織，藉此瞭解銲接入熱量對顯微組織之影響。接著再將不同銲接入熱量(10kJ/cm、20kJ/cm及30kJ/cm)後所得之試片，分別施以300℃後熱10min、30min和60min，瞭解不同時間的後熱對其顯微結構之影響。最後將不同銲接入熱量(10KJ/cm、20KJ/cm及30KJ/cm)所得之試片，施以300℃後熱30min後，分別再施以590℃銲後熱處理30min、60min和180min，藉此步驟探討在不同時間的銲後熱處理對其顯微結構之影響。接著將經過不同銲後熱處理時間的試片，施以衝擊試驗，藉此觀察顯微結構與衝擊韌性之間的關係，並找出較佳的銲後熱處理條件。

For the purpose of investigating the phase transformation of ASTM A572 grade 65 steel plates during the welding process, simulated heat affected zone (HAZ) experiments with heat inputs of 10, 20 and 30kJ/cm have been carried out by dilatometer. Furthermore, the evolutions of microstructure after post heat (at 300℃) and post weld heat (at 590℃) treatment within different soak time were also be concerned as well. The results indicate that the phase transformation temperature (Bainite starting temperature, Bs) of the steel plate would change with different heat inputs, and that the simulated HAZ microstructure gives large amount of lower bainite. Under the TEM (Transmission electron microscope) observation, it was found that with heat input of 10kJ/cm the significant amounts of auto-tempered martensite were obtained in the bainite matrix. Upper bainite became dominant while the simulated welding condition was changed to be 30kJ/cm. In order to investigate the tend of toughness versus variation of microstructual constituent for A572Gr.65, the thermal simulation for charpy impact test samples was carried out on a Gleeble machine. The result shows that specimens, which were experienced the heat input of 20kJ/cm, then held at 300℃ for 30min postheat treatment and at 590℃ for 180 min post-weld heat treatment, have higher toughness than those obtained from other heat treatment processes.