| Due to existence of hard and coarse primary borides, the toughness of hypereutectic high boron cast steels as well as their abrasion resistance at high impact conditions is decreased definitely, which limits its application in industrial conditions. In this paper, inoculating treatment, suspension casting and alloying treatment were used to refine the primary Fe2B borides and the peritectic Fe3(C,B) borides in the hypereutectic high-B cast steel. The results were drawn as follows:The dimension of the primary borides was refined as well as the morphology of the peritectic was altered in the hypereutectic high-boron cast steel(the HFBC alloy) after modified by the rare earth ferrosilicon and potassium/sodium salts.The contents of modifier at 0.6%RE or 0.3%K/Na have a maximum refinement making the primary seperately from 81.4μm undisposed to 30.4μm and 23.4μm. It was found that RE can purify molten steel by eliminating sulfur and oxygen. CeB6 andCeO2S can rifine the particals of both primary and peritectic borides by becoming their heterogeneous nucleation,because the mismatch between them is small enough. K/Na can easily cause constitutional supercooling leading the primary phase smaller by increasing it's crystal nucleus.As surface-active elements, K/Na make primary borides becoming balllike by absurbing onto the surface of the primary and liquid.Adding Nb,Ti into the alloy has a refinement to the primary,peritectic and eutectic borides,also changes the amount of the peritectic and eutectic ones. The contents of modifier at 0.9%Nb or 0.5%Ti have a maximum refinement making the primary seperately from 81.4μm undisposed to 50.4μm and 23.9μm. TiC and NbC serve as the heterogeneous nucleation of the primary making it smaller as the mismatch between them is small enough.Carbides on the edge of primary phase also impede it's growth.The development of primary boride slows down along with the high cooling rate.Primary boride has a sustained refinement with the increasing of suspending agent,and has a minimum at 2.8% content making the primary from 81.4μm undispo -sed to 20.2μm.Propose a formula to calculate the rate of refinement.It is the arithmetic mean of the commen refinement rate and its to the eth power,in which e is the natural background. Rate of refinement derived from this formula can better show the refinement ability of modifiers.Several multimodifiers have great refinement to the primary boride in the HFBC alloy. 0.9%Nb +0.4%RE multimodifier makes the primary refiner as well as change the shape of lateral section from quadrangle with sharp point into a nearly round shape, avoiding to lead to stress concentration caused by freezing and external force.The vary of shape is due to the shape point dissolves into liquid while freezing. The multimodi–fier make the druse of primary phase glue into the whole crystal causing the energy of system decreasing.The peritectic and primary borides dissolve in turn with increasing of the heat temperature. The peritectic resolves completely and the primary resolves into the bonelike of animal from square-cylinder shape at 1050℃.RE raises greatly the stability of borides at high temperature ,and the higher content the more significant effect.As the primary has a very good high temperature stability ,heat treatment is difficult to alter the shape and distribution of it. |
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