Study on Delayed Cracking of a Dissimilar-Metal Weld

本文以銲接模擬之方法，針對重型鋼結構之主要結構銲道，進行銲後發生延遲裂紋之可能性進行評估。研究過程中使用二維軸對稱模型作有限元素分析，除了對相異類別母材之多道次銲接進行結構與熱彈塑性之耦合分析外，亦在耦合計算過程中考慮了材料之相變態、殘餘相比例、結構拘束度與殘餘氫含量等影響因子。為分析可擴散氫之分佈模式，模擬「遮蔽金屬電弧銲接」之殘餘氫濃度被刻意的施加在第三道次之銲珠，並沿著該銲珠週圍的三個方向，共選定14個節點進行龜裂可能性分析。由分析數據中萃取出「銲後殘餘應力」、「殘餘可擴散氫」與「溫度梯度」等影響因素，並結合Alan Glover-Brian Graville模型以比對銲後發生龜裂之可能性。研究結果顯示，本論文所分析銲接結構選定的節點中，有6個節點被評估為會立即發生龜裂，龜裂率高達42.85％，其中4點歸咎於殘餘應力過高，另外2個節點可能肇因於位於熱影響區之粗晶區，在殘餘應力與殘氫量之共同作用下，以致在殘餘應力不高之情況下，在高溫冷卻過程中即發生龜裂。此一結果與台灣某工地發生鋼結構銲後龜裂之情形非常吻合。In the present dissertation, simulation is carried out to study the likelihood of occurrence of delayed cracking in a load-carrying weld. In particular, a big weld joining together two separate heavy decks to construct a huge annular steel structure in a construction site is studied in detail. A two-dimensional axial symmetric model is built for finite element simulation. The multi-pass welding joining two base metals made of different materials, the iterated and coupled calculation between structural deformation and thermal-elasto-plastic process, the material phase transformations, the residual phase percentage after phase transformation, the structural constraint, and the residual hydrogen concentration are all taken into consideration in the simulation. A hydrogen concentration model resulted from a real shielded metal arc welding process is also considered in the third weld bead for analyzing the diffusible hydrogen distribution pattern. Fourteen nodes in three specific directions around the third weld bead are chosen for evaluating the likelihood of delayed cracking. From the simulation, variation histories of the residual stress, the residual hydrogen concentration, and the temperature gradient during cooling are obtained. They are used for evaluating the likelihood of occurrence of delayed cracks based on a so-called “Alan Glover-Brian Graville” model. The result indicates that, among the selected fourteen nodes, six will crack after welding processes are completed. Four of them are owing to that their residual stresses are higher than the ultimate strength; and the other two is attributed to the combined effect of residual stress and diffusible hydrogen concentration. For the latter case, cracks occurs during the cooling process even the residual stresses are not substantially high. The evaluation result is very similar to cracks found in a construction site in northern Taiwan.口試委員會審定書 謝 i文摘要 iii文摘要 v錄 vii目錄 x目錄 xi號說明 xv一章 緒論 1.1 背景說明與研究目的 1.2 研究範圍與方法 4.3 論文架構與研究流程 5二章 文獻回顧 8.1 銲接過程與材料之可銲性 8.2 銲接延遲裂紋與致裂因子 9.3 材料之敏感性 9.4 銲接殘餘應力 12.5 可擴散氫 15.6 溫度場之影響 18.7 銲接模擬 19三章 銲接結構、銲接程序與銲接參數 29.1 銲接結構與銲道 29.2 銲接製程參數與銲接工序 29.3 銲接參數之影響 30.4 銲接參數與致裂因子間之相互關係 33四章 分析模型建立與模擬參數設定 42.1 分析模型及假設條件 42.2 模擬參數設定 444.2.1 材料性質設定 444.2.2 材料之熱傳性質 454.2.3 相變態過程之參數設定 454.2.4 結構計算之參數設定 494.2.5 熱源公式與參數設定 494.2.6 熱傳邊界條件 514.2.7 結構拘束設定 524.2.8 氫擴散參數、氫擴散負載與邊界條件之設定 524.2.9 銲接工序設定 554.2.10 網格元素之初始設定 554.2.11 求解參數設定 56五章 分析結果與討論 79.1 溫度場之分析結果 795.1.1 溫度分佈圖 79.2 相比例之分析結果 805.2.1 波來鐵+肥粒鐵（Phase 1）之比例分佈 815.2.2 變韌鐵（Phase 2）之比例分佈 815.2.3 麻田散鐵（Phase 3）之比例分佈 81.3 應力場分析結果 82.4 氫擴散之分析結果 83.5 位移與變形之分析結果 85.6 結果討論 865.6.1 銲後析出相比例 865.6.2殘餘應力與殘餘可擴散氫含量 86六章 結論與未來發展方向 114考文獻 11