Investigation On The Effect Of Silicon On The Formation And Transformation Of Carbides In M2High Speed Steel

With the rapid development of equipment manufacturing industry, high speed steel,as a very important tool material, has aroused more and more attention in the worldagain，which has been included in the “twelfth five-year development plan of scienceand technology” as an advanced materials in China.Generally, the eutectic carbide with net-shape formed during the solidificationprocess in high speed steel must be broken by multi-step forging and rolling. However,it is very difficult or impossible that a large amount of hot deformation for big-sectionparts made of high speed steel is employed. Even for the small-section parts, the breakof the carbides is not gratifying sometime. It has been indicated that silicon hasimportant effects on the type of carbides in as-cast high speed steel and thedecomposition of coarse M₂C eutectic carbide during heat treatment at highertemperature in some researches. But researches about these effects were limited andscattered. So, it is necessary to understand the rule of the effects of silicon on thecarbides in as-cast universal M2high speed steel and the decomposition of M₂C eutecticcarbide during heat treatment at higher temperature. Thus, it is possible to make aneffective refinement of coarse carbides in the big-section parts, even under the conditionof small amount of deformation. This research has higher academic value, it can beprovided theoretical basis for the establishment of appropriate heat treatment parametersin practical production. It can help for the development of other or new steel grade, so ithas application value and important meaning.In the present work, the effects of silicon on the as-cast microstructure of M2highspeed steel, decomposition and coarsening of the eutectic carbides during heat treatment,microstructure and mechanical properties of steel after deformation were investigatedsystematically by using optical microscope, scanning electron microscope, energydispersive spectrometer, X-ray diffractometer, differential scanning calorimeter andThermo-Calc software. Mechanism of the effect of silicon on formation andtransformation of carbide was discussed combing thermodynamics and kinetics. Mainresearch conclusions are as follows：Silicon addition had the important effects on the eutectic ledeburite and theeutectoid structure in M2high speed steel. The main eutectic carbides werelamellar-shape M₂C rich in molybdenum and tungsten in normal as-cast M2steel. The type and morphology of eutectic carbides had unobvious change in M2steel with0.8%silicon addition. When1.6%and2.4%silicon were added, eutectic carbides changedfrom lamellar-shape M₂C to herringbone-like M₆C. With the increase of silicon addition,eutectoid in as-cast microstructure grew out of nothing, and more and more flourishing.The eutectoid carbides changed from plate-like into filar-like as cluster, with carbideshell in the periphery of eutectoid carbides, which is M₆C. Silicon is a ferrite formingelement, it changes the microstructure of steel by changing partition coefficient andactivity coefficient of alloying elements. Silicon addition enlarges the ferrite phaseregion, and increase the amount of residual δ-ferrite when peritectic reaction ended，which transformed to δ-eutectoid structure.Silicon addition could make the eutectic carbides M₂C in high speed steeldecomposed under a lower temperature or with a faster speed. During heat treatment,phase transformation of lamellar-shape eutectic carbide M₂C took place, decomposedinto fine MC and M₆C carbides. With the increasing temperature or prolonged duration,the decomposition of carbide occurred obviously. For M2steel, the M₂C decomposedcompletely under （1100℃×4h,1150℃×2h or1200℃×30min） heat treatment. ForM2-0.8Si steel heated at1100℃for2h,1150℃for1h or1200℃for15min, no M₂Ccarbides were observed in samples. Silicon addition could accelerate the decompositionof M₂C carbides under the same heat treatment, which due to the increasing nucleationrate of new phase by silicon addition. Under those heat treatment processes, thedimension of carbides in M2formed by decomposition is larger than that of ones inM2-0.8Si, the difference in size is more than0.2μm, indicating that silicon could refinedthe carbides formed by decomposition.When the temperature was higher than1200℃, fine carbides by decomposition ofM₂C were coarsening. Silicon can’t prevent the coarsening of carbides obviously. Whensteels heated at1225℃for15min, the size of the decomposition products in M2andM2-0.8Si high speed steels was about0.63μm and0.65μm, respectively. They were assimilar as ones heated at1200℃for1h and larger than ones for15min. When theduration exceeded30min, the decomposition products occurred to coarsen obviously,the block-like or ceratoid carbides appeared, which their size more than2μm. Whenheating temperature was1250℃, the coarsening of carbides were remarkable, and theblock-like or ceratoid carbides with size of7μm also appeared, new eutectic carbidesMC formed by remelting in some area. In a word, the size of decomposition products inM2-0.8Si steel was not large than that of ones in M2steel when heating temperature lower than1225℃. Appropriate heat treatment temperature shouldn’t higher than1200℃.After forging, the size of carbides in M2-0.8Si steel forge stock was smaller thanthat of ones in M2steel, and carbides were more uniform. The large half-bakedherringbone-like carbides exist in M2-1.6Si and M2-2.4Si steels still because ofincomplete break of herringbone-like carbides. M2-0.8Si high speed steel （heated at1165℃for2.5h before forging） forging stock achieved high integrative mechanicalproperties, with hardness of66.7HRC, the bend strength of3250MPa. The carbideswere broken more after rolling, but some block-shape carbides exist in M2-1.6Si andM2-2.4Si bars still, which is bad to properties. M2-0.8Si high speed steel bar （heated at1165℃for2.5h before forging） had higher performance than other bars. The optimumheat treatment process for it is quenched at1180℃and tempered thrice for1h at540℃,obtaining bend strength of3250MPa and hardness of66.7HRC.“Phase transformation refinement” of lamellar-shape M₂C carbides made thesmaller carbide granules in bar after forging and rolling, herringbone-like M₆C wasbroken by “deformation refinement” only, they may appeared in the steel afterdeformation, which is not good to performance. The distribution of carbides inpost-deformation high speed steel can be improved, and the size of carbides is muchsmaller by “phase transformation refinement” and “deformation refinement” because ofproper silicon addition. Thus, it can be realized the refinement of carbides forbig-section parts with a small amount of deformation, which can reduce the practicalproduction costs and have important application value.