C-Mn钢和焊缝金属韧-脆转变微观断裂行为

对C-Mn钢和焊缝金属的COD,4PB,Charpy V和3NB试样在韧-脆转变温度区产生不同纤维裂纹长度后的断裂进行了卸载试验。通过对卸载试样纤维裂纹尖端及其两侧的微孔形态变化的观察和测量,断裂试样断口韧窝形貌以及起裂源粒子及其位置的观察和测量,对上述试样韧-脆转变区的微观断裂机理进行了分析研究。发现无论是裂纹试样还是缺口试样产生纤维裂纹后发生解理断裂的临界事件都是铁素体裂纹的扩展。韧-脆转变的主要影响因素为纤维裂纹扩展其前端三向应力度和最大正应力上升并超过临界值。韧性值的波动是由于在纤维裂纹扩展过程中,其尖端宽度的随机变化和材料中薄弱环节的随机分布引起的。

Micro-fracture Behavior of C-Mn Base and Weld Steel in the Brittle-ductile Transition Temperature Region

This paper presents COD, 4PB, Charpy V and 3NB tests of specimens of C-Mn base and weld steel, the tests are carried out in the brittle-ductile transition temperature region. Some specimens fracture after fiber cracks occurre and extend to various lengths and some specimens are unloaded prior to fracture. Through detailed observation of the variations in the shapes of micro-cavities at the tip or on both sides of fiber cracks occurring in unloaded specimens and the variations of shapes of dimples in various lengthes of fiber cracks on fracture surfaces, the micromechanism of the change from the fiber cracking to cleavage is analysed. The result reveals that the unstable extension of a ferrite grain-sized crack is the critical event for the successive cleavage fracture after a fiber crack has been initiated and propagated either in a notched or in a precracked specimen. The main factor promoting the change from the fiber crack to cleavage is the increas of the local normal stress ahead of the fiber crack which is caused by the increase of the triaxiality of stress and the apparent normal stress in remaining. The considerable fluctuation of toughness values in the transition temperature region is due to the random variation of the widths of the tips of fiber cracks during their extension and the random distribution of the weakest constituents in the microstruc-ture.