4Cr5MoSiV1热作模具钢700℃的低周疲劳行为

Low-cycle fatigue behavior of 4Cr5MoSiV1 hot-work die steel at 700 ℃

采用轴向应变幅控制的低周疲劳试验研究了总应变幅对4Cr5MoSiV1热作模具钢700℃低周疲劳行为的影响,包括循环应力响应行为、循环应力应变行为、循环迟滞回线和应变疲劳寿命行为等.结果表明:随着总应变幅从0.2%增大到0.6%,4Cr5MoSiV1钢在700℃时循环应力响应均表现为先循环硬化再循环软化的特性,并且应力幅最大值从220 MPa增大到308 MPa.同时,随着总应变幅的增大,4Cr5MoSiV1钢在700℃下的低周疲劳寿命由6750循环周次降低到210循环周次,且其过渡寿命约为1313循环周次.疲劳断口形貌分析结果显示,高温低周疲劳过程中裂纹主要萌生于试样表面处,且随着应变幅增大,裂纹源逐渐增多,疲劳条纹间距变宽,其断裂方式由韧性断裂转变为脆性断裂.透射电镜分析结果显示,循环软化可能与板条结构转变为胞状结构、基体发生位错湮灭、碳化物的析出和粗化有关.

4 Cr5 MoSiV1 hot-die steel exhibits excellent thermal fatigue and comprehensive mechanical properties, and it is widely used in hot forging die and hot extrusion die. Under actual service conditions, mold cavity temperature reaches 700 ℃ during mold operation. Mold cavity surface produces tension and compression strain owing to acute heat and cooling-constraints of subsurface layer,thereby resulting in local plastic deformation of mold and low-cycle fatigue. Therefore, low-cycle fatigue behavior of 4 Cr5 MoSiV1 steel at 700 ℃ is studied to provide reference data for component design and life prediction of 4 Cr5 MoSiV1 steel. The effect of total strain amplitude on low-cycle fatigue behavior of 4 Cr5 MoSiV1 steel at 700 °C has not been studied. The influence of total strain amplitude on the low-cycle fatigue behavior of 4 Cr5 MoSiV1 steel at 700 ℃ was studied using the low-cycle fatigue test with an axial strain amplitude control, including cyclic stress-response behavior, cyclic stress-strain behavior, cyclic hysteresis loop, and strain-fatigue life behavior.Results show that, with the increase of the total strain amplitude from 0.2% to 0.6%, the cyclic stress responses of 4 Cr5 MoSiV1 steel at 700 ℃ show the characteristics of cyclic hardening and recycling softening, and the maximum stress amplitude increases from 220 MPa to 308 MPa. As the total strain amplitude increases, the low-cycle fatigue life of 4 Cr5 MoSiV1 steel at 700 ℃ decreases from 6750 cycles to 210 cycles, and its transition life is approximately 1313 cycles. The results of fatigue fracture morphology analysis show that the crack mainly occurs on the surface of the sample during the high-temperature low-cycle fatigue. With the increase in the strain amplitude, the crack source gradually increases, the distance between fatigue stripes widens, and the fracture mode changes from ductile to brittle fracture. The results of TEM analysis show that the cyclic softening may be related to the change of lath structure to cellular structure,dislocation annihilation of matrix, carbide precipitation, and coarsening.