Effect Of The Gas Content On Solidification Structure And Macrosegregation In The Liquid Metal

In the process of metal casting,there will be a lot of defects,such as segregation,porosity,crack,shrinkage cavity,etc.The segregation is one of the main defects in the casting process,especially the macrosegregation,which would not be dissolved in the subsequent process.The irreversible solidification defects have many pernicious effects to the service life of materials.This paper focused on the 35# steel,high manganese steel,magnesium and aluminum,and studied the impact of gas content on metal solidification structure evolution law and macrosegregation through the gas tester,OM,SEM and direct-reading spectrometer.The results are shown that:(1)The change of gas content has significant impact on solidification structure in liquid metal,especially the change of columnar grain.When the oxygen content increased from 1.0×10-3wt% to 2.0×10-3wt% in 35# steel,the columnar grain area was obviously increased,and columnar grain transformed from fine sliced to thick long strips.The average length of columnar grain increased from 728μm to 3213μm.When the oxygen content increased from 0.8×10-3wt% to 1.4×10-3wt% in the high manganese steel,the average length of the columnar grain increased from 799μm to942μm.The added value of the length of columnar grain before and after degassing for 35# steel(2485μm)is higher than high manganese steel(485μm),which is because the little change of oxygen content before and after degassing owning to Mn element.For non-ferrous metal sample,when the hydrogen content of pure magnesium increased from 7.0ml/100 g to 8.5ml/100 g,the average length and width of the columnar grain transformed from 1199μm and 824μm to 2815μm and 1077μm.Hydrogen content of pure aluminum increased from 0.13ml/100 g to 0.15ml/100 g,theaverage length and width of the columnar grain transformed from 1cm and 0.2cm to1.5cm and 0.5cm.The difference of average length of the columnar grain for 35# steel(2485μm)is greater than the pure magnesium(1616μm)before and after degassing,which is mainly because coefficient of thermal conductivity of 35#steel is smaller than pure magnesium leading to gas for ingot large effect on the solidification structure of the columnar grain.(2)The change of gas content not only affects the solidification structure in liquid metal,also has influence to the macrosegregation.Before and after degassing,the content of C,S,P element of 35# steel increased from 0.36%,0.037%,0.038% to0.03%,0.05%,0.06%,where the content of C,S,P element of high manganese steel increased from 1.0%,0.05%,1.0% to 0.05%,0.14%,0.10%.The content of Fe element of pure magnesium increased from 0.005% to 0.018%,where the content of Fe and Si element of pure aluminum increased from 0.02% and 0.75% to 0.08% and0.8%.(3)The change of gas content not only affects the solidification structure and macrosegregation,also has certain effect on performance.The results of salt fog corrosion experiment to pure magnesium specimens are shown that there is a very serious non-uniform corrosion phenomenon for pure magnesium with more gas,which is mainly occurred in the area of Fe element segregation.The non-uniform corrosion phenomenon was not discovered in samples with less gas content.