Study On Mg Treatment Of 5CrNiMo Hot Work Die Steel Melted Under Pressurization

5CrNiMo commonly used in the manufacture of various large and medium-sized forging die a variety of engineering instruments and many mechanical parts.Therefor,its performance requires high strength and toughness.5CrNiMo die steel is likely to form the Wenstein body,when manufacture during processing is not suitable.Ferrite accumulation tends to occur after heat treatment,resulting in an uneven distribution of alloying elements in the austenite during high-temperature austenisation,with large grains and irregular randomness,followed by strong grain boundary inheritance sex and grain genetic,which adversely affect the mechanical properties.In this experiment,the low magnesium content of the nickel-magnesium alloy under the conditions of pressure as a metal magnesium added carrier to reduce the magnesium added to the molten steel after the reaction severity,increase the yield of magnesium.Experimental results show that Mg treatment has played a deoxy,desulfurization and metamorphism inclusions,the specific performance:when the Mg content of 0.069%,the total oxygen mass fraction was only 0.0005%,while the sulfur mass fraction was 0.0025%at the same time.On the other hand,the inclusions in the steel after being treated with a large amount of Mg are mainly MgO with small size and dispersed distribution.It was found that the diameter of inclusions before Mg treatment is mostly above 2 ?m and the proportion of inclusions with diameters below 2?m.However Mg treatment increased sharply from 38.60%to 70.60%.Further more,with the increase of Mg content in steel,the conversion behavior of oxides:Al2O3?MgO Al2O3?MgO.This is consistent with the thermodynamic calculation of FactSage in this experiment.The influence of the content of alloying elements on the precipitation phase was also analyzed by Thermo-Calc thermodynamic calculation.Mg treatment also had a significant effect on the microstructure and properties of 5CrNiMo steels.In details,Magnesium treatment can significantly refine the secondary dendrite arm spacing of as-cast steel,refine the pearlite sheet spacing of annealed steel and inhibit the growth of Westen's body tissue.On the other hand,the magnesium treatment can refine the grains of quenched steels,and the higher the magnesium content,the better the refining effect.Furher more,in terms of properties,the impact energy of steel increased gently and then decreased with the increase of magnesium mass fraction.In addition,the changes of tensile properties of steel was also gradually increased and then decreased with the increase of magnesium mass fraction.