| 440C stainless bearing steel is a martensitic steel with a carbon content of 1.0%and a chromium content of 16%to 18%.440C steel has a wide range of applications and can be used to manufacture bearings that are resistant to impact and corrosion,such as rolling mill bearings,railway bearings,offshore oil rig bearings,and marine engine bearings.However,there are still many problems with the 440C steel produced by traditional smelting process.For example,there is a large amount of reticulated carbide in steel.After forging,the phenomenon of carbide segregation is serious;440C steel has a certain corrosion resistance as a stainless bearing steel,but bearing steel is used as a raw material for precision basic components.A stronger corrosion resistance is required to ensure that the produced bearings have excellent fatigue life.A large number of research results show that magnesium treatment of molten steel can effectively purify molten steel,refine microstructure and improve carbide distribution in steel.In this experiment,under the condition of pressure,the nickel-magnesium alloy with a mass fraction of 5%is added as the carrier of magnesium to the molten steel,which reduces the reaction area between magnesium and molten steel,and weakens the reaction degree of magnesium and molten steel,so as to improve the yield of magnesium.A total of 4 furnaces of 440C steel were smelted under laboratory conditions,and the magnesium contents in each furnace were:?0.0005%,0.0029%,0.0081%,and 0.0160%,respectively.And with the increase of magnesium content in steel,the content of oxygen,sulfur and nitrogen in steel decreased continuously,and the oxygen content decreased from 0.0013%to 0.0003%,the sulfur content decreased from 0.0032%to 0.0018%,and the nitrogen content decreased from 0.0088%to 0.0049%.At the same time,with the increase in the amount of magnesium added,the yield of magnesium is also rising.Inclusions and microstructure analysis show that magnesium can modify inclusions in steel and refine grains.When no magnesium is added,the inclusions in the steel are mainly MnS and Al2O3;When the magnesium content is 0.0029%,magnesium appears in the MnS inclusions in the steel,and becomes Mn-S-Mg inclusions.Mg-O-S composite inclusions also appear in the MnS;when the magnesium content is increased to 0.0081%,a single-phase small MgO inclusion and Mg-O-S composite inclusions begin to appear in the steel;when the magnesium content reaches 0.0160%,the inclusion type in the steel does not change significantly.With the increase of magnesium content,the number of inclusions per unit area decreases,and the average equivalent diameter of inclusions decreases.The as-cast ratio of the as-cast steel increased,the secondary dendrite spacing decreased,and the grain size of the steel increased from 13.03 to 14.89 in the tempered state.The results of mechanical properties experiments show that the strength and toughness of 440C steel increase with different degrees with the increase of magnesium content.With the addition of magnesium,the grains of 440C steel have been refined,and the conditions of carbides and inclusions in the steel have been improved,which has greatly improved the mechanical properties of 440C.The uniform corrosion experiment showed that the uniform corrosion resistance of 440C steel was improved with the increase of magnesium content.The uniform corrosion rate decreased from 0.000757 g·cm-2h-1 without magnesium to 0.000664 g·cm-2h-1.Pitting experiment showed that with the increase of magnesium content,the size of inclusions and carbides in steel decreases,reducing the chance of pitting.The pitting potential of 440C steel is increased enough to resist the corrosion of seawater,which can improve the life of 440C steel in seawater. |
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