The effect of stress concentration geometry on the fracture behavior and fracture mechanisms of a two-step quenching and partitioning steel consisting of tempered martensite and retained austenite (RA) is investigated using double edge notched tension (DENT) specimens with different notch root radii. The initial notch opening is varied from a fatigue pre-crack to 15 mm radius. The fracture surface consists in flat triangular regions and slant zones. The fracture mode evolves from intergranular brittle fracture to ductile fracture with dimples. The ductile fracture process further changes from internal necking coalescence to a void sheet shear type coalescence when increasing the notch root radius. A similar transition of the fracture behavior is observed along the ligament of the DENT specimen. This change in the fracture mode results from a decrease of the stress triaxiality. The essential work of fracture is equal to $$40\hbox { kJ/m}^{{2}}$$ 40 kJ/m 2 . This is small when looking at the large value of the product of ultimate tensile strength and total elongation ( $$\sim 25, 000\, \hbox {MPa}{\cdot }$$ ∼ 25 , 000 MPa · %). The small fracture toughness is ascribed to an intergranular fracture mode, which results from the distribution of blocky RA islands along the boundaries of martensitic packets and from the large stress concentration near the notch tip. A proper control of both the amount and morphology of RA during microstructure design is thus essential to generate the best compromise between tensile properties and fracture toughness, and to avoid the relatively low toughness and more brittle failure observed when stress triaxiality increases.

中文翻译：

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两步淬火分区钢断裂机理对应力集中配置的敏感性

使用具有不同缺口根部半径的双刃缺口拉伸 (DENT) 试样研究应力集中几何形状对由回火马氏体和残余奥氏体 (RA) 组成的两步​​淬火和分隔钢的断裂行为和断裂机制的影响。初始缺口开口从疲劳预裂纹变化到 15 毫米半径。断裂面由平坦的三角形区域和倾斜区域组成。断裂方式由沿晶脆性断裂演变为带凹坑的韧性断裂。当缺口根部半径增加时，韧性断裂过程进一步从内部缩颈聚结转变为空隙板剪切型聚结。沿 DENT 试样的韧带观察到类似的断裂行为转变。断裂模式的这种变化是由应力三轴性的降低引起的。断裂的基本功等于$$40\hbox { kJ/m}^{{2}}$$40 kJ/m 2 。当查看极限拉伸强度和总伸长率的乘积的大值 ($$\sim 25, 000\, \hbox {MPa}{\cdot }$$ ∼ 25 , 000 MPa · %) 时，这是很小的。小的断裂韧性归因于晶间断裂模式，这是由于块状 RA 岛沿着马氏体包边界的分布以及缺口尖端附近的大应力集中造成的。因此，在微观结构设计过程中适当控制 RA 的数量和形态对于在拉伸性能和断裂韧性之间产生最佳折衷至关重要，