Abstract
The effects of different proportions of vanadium carbide(VC) particles instead of V powder on the density, microstructure and properties of powder metallurgy high-speed steel(PM HSS) were studied in this paper. The difference of strengthening phase composition and the mechanism of material properties under different sintering time were analyzed. The testing results show that the bonding state between elemental V and matrix is improved and the sintering densification of HSS is effectively promoted with the increasing proportion of VC particles. When holding time is 90 minutes, a large number of lath-like M2C carbides appear in the sample structure with VC particles. When holding time is extended to 120 minutes, the M2C carbides in the high-speed steel structure decompose completely, and a large number of fine M6C and MC carbides are produced. The mechanical properties of HSS can be improved by increasing the proportion of VC particles. The maximum strength of HSS is up to 2597 MPa when the proportion of VC particles is 150% and the sintering time is 120 minutes. The hardness of HSS is mainly related to density and properties of strengthening phase. The hardness of HSS prepared in this experiment is about 51~52 HRC.
Highlights
The testing results show that the bonding state between elemental V and matrix is improved and the sintering densification of HSS is effectively promoted with the increasing proportion of VC particles
When hold⁃ ing time is 90 minutes, a large number of lath⁃like M2C carbides appear in the sample structure with VC particles
When holding time is extended to 120 minutes, the M2C carbides in the high⁃speed steel structure decompose com⁃ pletely, and a large number of fine M6C and MC carbides are produced
Summary
图 3 为烧结保温时间为 90 min 时,添加不同比 例 VC 颗粒时高速钢的微观组织形貌。 各种类型碳 化物具体元素组成的能谱分析结果见表 3。 由背散 射电子成像原理及标准 M2 高速钢合金物相组成可 知[14] ,图 3 中呈块状或角状的亮白色组织为 W、Mo 等原子序数较大元素富集的 M6C 型碳化物;组织中 分布比较杂乱的棒状或板条状的白色碳化物类型为 时,试样抗弯强度与添加 VC 颗粒比例的变化曲线。 当试样中 V 质量分数为 2%时,随着 VC 添加比例的 增加,试样的强度呈现出明显的上升趋势,且保温时 间从 90 min 延长到 120 min 时,试样的强度均有一 定的提高,约 200 MPa 左右。 材料的强度主要与晶 粒大小、孔隙数量及形态、强化相尺寸及分布等因素 有关,结合相应试样的密度变化曲线可知,此时影响 不同 VC 试样抗弯强度的主要因素为内部孔隙的数 量。 当全部以单质 V 粉加入时,材料的致密化程度 较低,组织内部孔隙数量多,载荷逐渐增大的过程当 中,孔隙处不仅滋生了大量的微裂纹,更限制了硬质 相对于基体的强化作用,大大降低了材料的力学性 能。 延长保温时间至 120 min 时,此时虽然对于材 料的密度提升不明显,但是此时组织内部的孔隙更 加趋于球形,在一定程度上加大了裂纹产生的难度, 因而表现出抗弯性能的上升。 此外,对于添加 VC 颗粒的试样而言,延长保温时间使得组织内稳定性 较差的 M2C 碳化物的分解过程得以充分进行,分解 得到的细小 M6C 及 MC 组织与基体有着更好的界 面相容性,对材料的强化作用效果明显。 强度增加了 320 MPa,提高了 15%。 这表明延长保 温时间对于全部以 VC 颗粒作为钒源添加条件下试 样的抗弯强度提升有利,这与 VC 颗粒的溶解特性 有关。 V 为强碳化物形成元素,直接添加的 VC 颗 粒在 高温下仍能保持良好的热力学稳定性, 在 1 100°C 以上才开始逐渐溶解到基体当中,且即便加 热到晶界融化温度,仍会有一定数量的 VC 颗粒残 留。 当直接添加的 VC 比例增大时,在较短的烧结 保温时间内 VC 颗粒难以实现有效的扩散,对基体 的强化作用大大 受 限。 而当保温时间延长到 120 min 时,此时,组织内部的晶粒长大不明显,溶解到 基体当中[ V] 、[ C] 的量逐步增多,材料的强度迅速 提高[2] 。 此外,固溶到基体当中的[ V] 、[ C] 再次析 出成为 MC 碳化物时,大大改善了其与基体之间的 结合状态,组织当中的应力减小,材料的强度进一步 提高。
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