Research On Refining Mechanism Of Ultrafine Grain Inoculants On High Speed Steels

W18Cr4V high speed steel(HSS)which contains W,Cr and V as the main alloying element is a kind of high alloy tool steel.W18Cr4 V HSS is used widely in the manufacture of materials such as cutting tools and molds because of their excellent combination of red hardness,high hardenability and good wear resistance.But the carbides of consecutive reticulation are distributed in the grain boundaries and the matrix is separated,which makes the toughness of HSS reduced.These disadvantages make W18Cr4 V HSS in the using process easy to be peeled off,and it is difficult to be fully used.Therefore many efforts have been made to improve the W18Cr4 V HSS' as-cast microstructure through controlling the morphology,distribution,and type of carbides.In all the structure optimization processing of HSS,the modification method by adding inoculants into the molten HSS is an effective approach for this purpose.The project aimed to improve the microstructure and mechanical properties of W18Cr4 V HSS by means of modification.Fe-Al-Cr-Nb-B-C and Fe-Al-Cr-Nb-C-N amorphous nanocrystalline inoculants were fabricated using the method of in-situ reaction and rapid solidification.The phase composition of Fe-Al-Cr-Nb-B-C and Fe-Al-Cr-Nb-C-N inoculants were analyzed.Fe-Al-Cr-Nb-B-C and Fe-Al-Cr-Nb-C-N amorphous nanocrystalline inoculants were joined during the W18Cr4 V high speed tool steel smelting.The effect of Fe-Al-Cr-Nb-B-C and Fe-Al-Cr-Nb-C-N amorphous nanocrystalline inoculants on structures and properties of W18Cr4 V was studied.The results show: the inoculants are a mixture of amorphous and nanometer crystal,and Fe-Al(-Cr)intermetallic compound are the matrix phases of Fe-Al-Cr-Nb-B-C and Fe-Al-Cr-Nb-C-N inoculants,while particulate nitride,carbide and oxide and rod-shaped carbide and boride are the important precipitation respectively.With the addition of inoculants,the grain size of as-cast HSS refined significantly,the Net thickness and dendritic spacing decreased significantly and the fishbone carbide tended to be broken,especially for Fe-Al-Cr-Nb-B-C amorphous nanocrystalline inoculants.It was found that the optimum quenching temperature of W18Cr4 V HSS is 1260?,oil-quenching;and the appropriate tempering temperature is 560?,thrice tempering.After modification,the quenching and tempering microstructure of W18Cr4 V HSS has been fined significantly,especially for Fe-Al-Cr-Nb-B-C amorphous nanocrystalline inoculants.The inoculants have increased the carbides and martensite content,and decreased the sizes of the carbides of W18Cr4 V HSS.More carbons and alloying elements can diffuse out of the matrix to form the secondary hardening during tempering,since the secondary carbides have been distributed within the HSS grain more homogeneously.And after refining,the hardness,red hardness,wear resistance and impact toughness of T1 HSS have been improved obviously.