Effect Of Solution Treatment On Low-temperature Fracture Characteristics Of Ductile Ni-resist Iron

Ductile Ni-resist(DNR)iron has been widely used in industrial production due to its excellent mechanical properties and physical properties.At present,most of the production methods are based on the addition of a large amount of nickel and a small amount of manganese to obtain the austenitic matrix.In order to reveal the performance of the DNR iron at low temperature with high manganese content,the influence on microstructure and mechanical properties by manganese had been investigated.Solution treatment was also used to analyze the fracture characteristics so that a kind of DNR,with a proper content of manganese,has a similiar performance with DNR with high nickel content.It can be found that,although manganese can promote the formation of austenite,it also deformed the graphite spheroidization and led to the generation of a large amount of carbides in the grain boundary by speeding up the cooling rate of liquid iron.The carbides will affect the mechanical properties of the DNR iron.Solution treatment can dissolve carbides in the grain boundary,improving the impact toughness of DNR.However,the solution treatment has no obvious influence on the spheroidization grade of graphite.In addition,the graphite still restricts the improvement of properties.7Mn-10 Ni has the highest grade of spheroidization among the four components.It can be indicated that the initiation and expansion of the crack at room temperature occurs in the grain boundary in four components of DNR based on the observation of crack propagation path near the fracture initiated region of V-notched.However,the initiation and expansion of the crack became extending along the graphite after solid solution treatment,which was related to the change of mechanical properties.In order to analyze the impact toughness of 7Mn-10Ni-2.3C-3Si DNR at low temperature,four tests were carried out at-30 ℃,-60 ℃,-100 ℃ and-196 ℃,respectively.The results showed that the impact values decreased with the reducing of the temperature and were not related to the solution treatment.However,the properties of specimens,under solid solution,were higher than those without solution treatment.The variation of yield load Fy,maximum load Fm and ductile fracture zone Dmp1 during low temperature impact were analyzed by using the force deflection curve.It was found that the yield load Fy did not change obviously before and after solution treatment or the decrease of temperature.The ductile fracture zone Dmp1 was proportional to the toughness and maximum load Fm showed an increasing trend at first and then decreased.Based on the analysis of the variety of the fracture morphology,the fracture mechanism of 7Mn-10Ni-2.3C-3Si DNR iron was explained.The fracture of 7Mn-10Ni-2.3C-3Si DNR iron at room temperature showed a mixed fracture mode of dimple fracture and quasi-cleavage fracture.The micro-cracks in the matrix caused by carbides in grain boundaries led to this fracture at low temperature.However,after solution treatment,7Mn-10Ni-2.3C-3Si DNR showed obvious dimple fracture mode at room temperature,and the quasi-cleavage fracture morphology had disappeared.The intergranular fracture occured suddenly with the decreasing of temperature,and then the area of intergranular fracture expanded to develop a brittle fracture eventually.