The Study Of Inclusions And Tempering Property Of Secondary Hardening Ultra-high Strength Steel

The materials used in this experiment is a kind of secondary hardening ultra-high strength steel named SHS-1 provided by a certain steel factory in China,but its strength and toughness are not as good as foreign countries’.Aimed at improving its strength and toughness,this experiment will study the effect of inclusions and tempering temperature on them.At last,we come up with the following findings:Though study,the addition of Aluminum can easily produce Al2O3 type inclusions with bad anisotropic.Because of its morphology with lots of cutting-edges,the crack will be easily born and grow very fast on the inclusions.But after adding Lanthanum,a kind of ball-shape inclusions can be produced with better morphology and good anisotropic,so the mechanical properties will improve a lot.Besides,though CT 3D non-destructive test technology,inclusions with high and low density and pores could be found,the sizes are very small and contents of them are very low which could say that the metallurgical technology is within expected.What’s more,pores could get together in a certain area;the distribution of high-density inclusions has no rules;to the opposite,the low-density inclusions’distribution looks homogeneous.After the first tempering,we can find that the martensite boundary will become blurry,and the direction of martensite will get changed with the growth of tempering temperature.What’s more,the content of the retained austenite may reduce,and lots of alloying carbides can be found during the process.When the temperature arrives at 500℃,the mechanical properties will come to the best;with higher tempering temperature,the martensite and alloying carbides will coarsen a lot.At the same time,the reversed austenite can be found,so the mechanical properties will get worse.Based on the first tempering at 500℃,we put it under a very low temperature to reduce its content of retained austenite,and then started the secondary tempering process.When the secondary temperature is 500℃,the mechanical properties are better than other secondary tempering temperatures and the first tempering at 500℃.When the secondary temperature is 500℃,the microstructure is more uniform with smaller sized alloying carbides,and this is why the mechanical properties become better.Same with the first tempering process,when the secondary tempering temperature is too high,the martensite and alloying carbides will coarsen a lot,at the same time,the reversed austenite with long film shape will be born,so the mechanical properties will get worse.The tempering martensite,carbides,dislocation,retained austenite and reversed austenite in the secondary hardening ultra-high strength steel can have a big effect on the strengthening and toughening level.During the tempering process,the quenching martensite will change into the interweaving tempering martensite to improve the strengthening level,but when the temperature is too high,the martensite will get coarsening;the carbides will keep consistent with the matrix to get texture strengthened,but the consistence will disappear when the carbides get coarsened;the dislocation with high density may get stuck by the secondary phase particles;the retained austenite may increase the toughness as well as decrease the strength,otherwise,the reversed austenite can increase the toughness,but won’t change the strength.