The Effect Of Mg On The Inclusion, Microstructure And Mechanical Properties Of Cr14 Steel

Cr14 steel is a kind of high alloy ultra high strength steel with tempering hardening. The main problem encountered in the development process at present is how to improve the toughness of the steel. The main factors affecting the toughness of steel include cleanliness, size and type of precipitated phase, and the final structure of steel. Based on the Cr14 steel smelting in the laboratory, this paper did a research on the smelting process adding Mg in this kind of steel.In this paper, the effect of Mg on the inclusions, microstructure and mechanical properties of Crl4 steel was researched, and the effect of different tempering temperature on microstructure and mechanical properties of Cr14 was discussed as well.The clean degree and inclusions in the Cr14 steel with three different magnesium mass fraction were researched by analysis methods, including TC500, OM, SEM and EDS. Then, several quenching experiments under different temperature were done, and the best quenching temperature for Cr14 steel is founded 1050?. After cold treatment on -73?, different temperature, such as,450?,480?,500?,520?,540?, and 560?, were selected for tempering. The effects on microstructure with different Mg mass fraction and different tempering temperature, were researched by the methods of OM and X-ray diffraction Finally, the tensile testing machine and impact testing machine were used to test the room temperature tensile and impact, then the effect of different Mg content and tempering temperature on the mechanical properties were studied. The results showed that:(1) With the addition of Mg, the clean degree promoted, and the content of oxygen in steel reduced. The inclusions with Mg treatment were magnesia alumina spinel mostly. With the mass fraction of Mg increasing, the average diameter of the inclusions decreased from 1.570 ?m to 1.285 ?m, and the average mass fraction of Mg in the inclusions increased from 3.305% to 11.845%. The type of inclusions changed with the addition of Mg:when the mass fraction of Mg was 0.0006%, the inclusions were mainly MgO-Al2O3-SiO2 and MgO-Al2O3-MnS, and when the mass fraction of Mg was 0.0025%, the inclusions in steel changed to MgO-Al2O3.(2) The microstructure of Crl4 steel after tempering was tempered martensite and retained austenite. Under 450?,480? tempering, the steel with high mass fraction of Mg had a higher volume fraction of austenite. When the tempering temperature was higher than 500?, Mg had little effect on the volume fraction of austenite. When the time of tempering treatment was 4 hours, the unstable austenite discomposed completely between 480? and 500?, and the decomposition rate between this temperature was fast.(3) The Crl4 steel in this process had a high strength, the maximum tensile strength of which was up to 1890 MPa and a minimum of 1620 MPa. And the testing showed that the highest yield strength of the steel was 1240 MPa and a minimum of 905 MPa. If the tempering temperature was less than 480?, the tensile strength of the steel was lower than 500? tempered steel, but the yield strength was higher than that. The influence of Mg on the intensity of Cr14 steel was more complex, which showed different trends with different tempering temperature. But with the increase of Mg content, the impact toughness was improved significantly, and under 450? and 480? tempering was more obvious, the highest value reached 95 J. On the other hand, the tempering temperature had a great effect on the impact toughness. With 4 hours tempering, the impact energy changed quickly when the temperature was between 480? and 500?. Above this temperature, the impact toughness deteriorated rapidly, and reduced to 30 J?40 J.