Research On The Microstructure And Low-temperature Impact Fracture Behavior Of Ultra-low Temperature Ductile Ni-resist Iron

In recent years,there are more and more industry equipments working under ultra-low temperature such as the working temperature of huge ultra-low temperature boil-off gas(BOG)compressor is normally below-160℃,therefore there is a great demand for ultra-low temperature materials.Normally,the bearable temperature of ductile cast iron as ferrite matrix can reach to-60℃ at lowest,which may not meet the applied requirement for lower temperature.But ductile Ni-resist(DNR)alloyed iron has no ductile-brittle transition phenomenon with the decreasing of temperature and has good low-temperature mechanical properties,therefore,it has wide application for ultra-low temperature(below-100℃)industry manufacturing.At present,the research of DNR alloyed iron mainly focused on high temperature properties,and little research has been reported on ultra-low temperature,microstructure,impact fracture characteristic,instrumented impact fracture process and initial and metastable propagation of impact fracture.Thus,this study is concentrates on the microstructure and low-temperature impact fracture behavior of ultra-low temperature ductile Ni-resist iron.The research on microstructure and abrasive wear behavior of ultra-low temperature DNR alloyed iron indicates that it is mainly composed of austenite,graphite spheroid and carbide located at grain boundary,and the segregation distribution of Mn and Cr can lead to formation of M23C6-type(M=Fe,Mn,Cr)carbide of which micro-hardness can reach up to 1200 HV far higher than that of austenite matrix,consequently it can improve the macro-hardness.Meanwhile,element Cr has stronger carbide forming capacity than Mn,therefore,it has more effect on abrasive wear property.Through the research on worn surface,it is revealed that with different Cr additions this kind of material shows abrasive wear mechanism,among which grain-boundary carbide as hard phase facilitated by Cr element can improve the abrasive wear property.The investigation on low-temperature impact property of DNR alloyed iron with different alloy(Ni,Mn and Cr)additions using instrumented impact method indicates that impact property of DNR alloyed iron with different alloy additions has similar characteristics with the decreasing of temperature,and presents increase at first and then decline in the trend,meanwhile,the changes of Ni content has a positive effect on low-temperature impact property,but excessive addition of Mn and Cr will cause the deterioration of low-temperature impact property.The impact fracture morphology analysis under SEM shows that it presents the ductile fracture morphology characteristic as the centre of graphite spheroid between room temperature and-193℃,and the number of graphite spheroids on the fracture has a direct causal relationship with impact property,which means it has better property if there is more graphite spheroid.The change in the amount of carbides has no obvious effect on impact property at room temperature,but the effect presents increasing trend with the decreasing of temperature,and it lead to the vertical micro-crack on impact fracture at ultralow temperature(-193℃),which can seriously destroy impact property.Based on the research about low-temperature impact property regular of ultralow temperature DNR alloyed iron,it deeply analyze on instrumented impact curves,which further explores impact fracture process,and the result shows that a method of instrumented impact curves by combination of slope method and compliance changing rate method can effectively quantify the low-temperature impact fracture process.Among them,the impact fracture metastable propagation energy under high load can reach more than 60% of the total energy,and the both trend is consistent of each other(the increase at first and then decline in the trend with the decreasing of temperature),so the metastable propagation energy under high load is the main factor on low-temperature impact property.However,the reason why increase at first and then decline in the trend is due to the metastable propagation,average load under high load play the key role in low-temperature impact property from room temperature to-80℃,and when the temperature continues to fall down,the metastable propagation deflection under high load becomes the main factor.The research also found that even if Ni content changes the law still exists.Meanwhile,the geometric topography of the impact fracture is analyzed quantitatively using LSCM,and through the statistics on the surface roughness index of the metastable propagation deflection under high load at different temperatures verify the correctness of the above conclusion.Due to the metastable propagation energy under high load in instrumented impact curve has good relevance with the impact fracture initiation and metastable propagation process,further research shows that ultra-low temperature DNR alloyed iron with different Ni additions has better resistance ability of impact fracture initiation,and it affect by temperature in the early phase,but it affect by temperature and Ni content changes in the late phase.The graphite spheroid in the matrix(especially the adjacent graphite spheroid)and the grain-boundary carbides are the main effective factors on impact fracture path,but the decreasing of temperature and the Ni content increasing intensifys the brittle fracture tendency.Meanwhile,through the calculation on fracture toughness JBl0.2 of ultra-low temperature DNR alloyed iron with different Ni additions using Schindler method,it is revealed that dynamic fracture toughness presents the increasing trend with the decreasing of temperature,but this trend slows down significantly below-80℃.