【摘要】：S355J2W耐候鋼主要用于高速列車轉(zhuǎn)向架的制造,轉(zhuǎn)向架是保證列車運行穩(wěn)定性及安全性的關(guān)鍵部件。然而,S355J2W鋼在低溫下會發(fā)生韌脆轉(zhuǎn)變,使材料突然發(fā)生脆性斷裂。轉(zhuǎn)向架各部位的連接主要通過焊接,焊接缺陷及組織的不均勻性等因素均會增大焊接接頭的低溫敏感性,從而使接頭發(fā)生低溫脆斷的可能性增大。因此,有必要對S355J2W鋼焊接接頭的低溫性能進行研究。本文以S355J2W鋼MAG焊接接頭為研究對象,通過在不同的溫度下進行拉伸試驗、沖擊試驗及斷裂韌性試驗,研究強度、塑性及韌性等指標隨溫度的變化規(guī)律,并深入探究了接頭各區(qū)的韌脆轉(zhuǎn)變行為;通過斷口金相觀察討論了接頭各區(qū)解理斷裂的臨界事件,通過EBSD分析了母材和焊縫晶粒取向及裂紋擴展路徑;通過TEM對比分析了常溫及低溫下斷口附近的微觀組織及位錯形貌;采用有限元分析方法結(jié)合斷口細觀參數(shù)測量,計算了接頭各區(qū)的解理斷裂應力,并深入分析了微觀組織對低溫韌性的影響。低溫力學性能及韌性試驗結(jié)果表明,隨著溫度降低,接頭各區(qū)的強度增大而塑性、韌性降低,母材具有最好的低溫韌性和最低的韌脆轉(zhuǎn)變溫度,而焊縫低溫韌性最差,韌脆轉(zhuǎn)變溫度最高,為焊接接頭最薄弱環(huán)節(jié)。低溫下,接頭各區(qū)的微觀斷裂機制逐漸由韌性斷裂向脆性斷裂轉(zhuǎn)變。母材在-80～-115℃解理斷裂的臨界事件為晶粒尺寸的微裂紋穿過晶界擴展,在-196℃臨界事件轉(zhuǎn)變?yōu)槲⒘鸭y的形核,焊縫和熱影響區(qū)微觀組織的不均勻性造成解理斷裂臨界事件的差異性。母材中大角度晶界的比例較高,可有效阻礙裂紋擴展而使低溫韌性提升,焊縫中先共析鐵素體及側(cè)板條鐵素體內(nèi)部的小角度晶界對裂紋擴展阻礙作用較小,而針狀鐵素體區(qū)域高比例的大角度晶界有利于提升低溫韌性。母材在常溫下較大的塑性變形引起位錯增值,在-196℃下具有較低的位錯密度,Nb2C析出相對位錯的釘扎作用顯著。母材解理起裂源位于晶界,焊縫解理起裂源多為夾雜相誘發(fā)形成,熱影響區(qū)存在晶界和夾雜相兩種形核機制。母材細小的微觀組織使特征微裂紋長度較小,從而具有較大的解理斷裂應力,宏觀上表現(xiàn)為較好的低溫韌性;焊縫和熱影響區(qū)的中粗大的組織顯著降低了解理斷裂應力,從而具有較差的低溫韌性,組織的不均勻性造成了解理斷裂應力較大的離散性。
[Abstract]:S355J2W weather-resistant steel is mainly used in the manufacture of high-speed train bogie, which is the key component to ensure the stability and safety of train operation. However, the ductile-brittle transformation of S355J2W steel will occur at low temperature, which will cause brittle fracture of the material. The connection of each part of the bogie mainly through welding, welding defects and microstructure inhomogeneity and other factors will increase the low temperature sensitivity of the welded joint, thus increasing the possibility of low temperature brittle fracture of the joint. Therefore, it is necessary to study the low temperature properties of S355J2W steel welded joints. In this paper, the tensile test, impact test and fracture toughness test of MAG welded joint of S355J2W steel are carried out at different temperatures, and the variation of strength, plasticity and toughness with temperature is studied. The ductile-brittle transition behavior of the joint was deeply investigated. The critical events of cleavage fracture in each area of the joint were discussed by means of fracture metallographic observation, the grain orientation and crack propagation path of base metal and weld were analyzed by EBSD, and the microstructure and dislocation morphology near fracture surface at room temperature and low temperature were compared and analyzed by TEM. The cleavage fracture stress in each area of the joint is calculated by finite element analysis method combined with the measurement of fracture meso-parameters, and the effect of microstructure on low temperature toughness is deeply analyzed. The results of low temperature mechanical properties and toughness tests show that with the decrease of temperature, the strength and toughness of the joints increase and the toughness decreases. The base metal has the best low temperature toughness and the lowest ductile brittle transition temperature, while the low temperature toughness of the weld is the worst. The ductile-brittle transition temperature is the highest, which is the weakest link of welded joint. At low temperature, the microscopic fracture mechanism of the joint gradually changes from ductile fracture to brittle fracture. The critical event of cleavage fracture of base metal at-80 鈮，

本文編號：2487350