【作者】 陈勇；

【导师】 刘兰俊；

【作者基本信息】 合肥工业大学 ， 材料工程（专业学位）， 2016， 硕士

【摘要】 高铬铸铁磨球以其优良的耐磨性,在电力、建材、水泥和冶金矿山等行业有着大量的应用。本文针对淬火+回火态高铬铸铁磨球冲击韧性较低,在大型球磨机中使用时出现大量剥落和破碎的失效问题,从热处理工艺对磨球基体组织和碳化物的影响着手分析,制定了合理的等温淬火和分级奥氏体化等温淬火热处理工艺,提高了磨球的冲击韧性和抗冲击疲劳性。本文主要工作成果如下：1.淬火+回火态高铬铸铁磨球的基体组织为细针状马氏体,基体中弥散分布着大量的块状和杆状的二次碳化物,表层硬度约为60HRC,心部硬度约为56HRC,冲击韧性约为4.1J/cm2,硬度较高但冲击韧性偏低。2.高铬铸铁磨球在230℃、260℃和290℃的温度下等温淬火15～50min的试验结果表明,随着等温淬火温度的提高,磨球的硬度和冲击韧性均呈先升后降的趋势；随等温淬火时间的延长,磨球的硬度降低而冲击韧性先升后降。260℃等温淬火40min后,磨球获得细针状下贝氏体为主的奥贝氏体基体,基体中弥散分布着一定数量的圆粒状二次碳化物,磨球的强韧性配合较好,表层硬度约为58HRC,心部硬度约为55HRC,冲击韧性可达7.2J/cm2,冲击韧性较淬火+回火态提高了约75%。3.在950～830℃的二次奥氏体化温度范围内,高铬铸铁磨球的硬度随着二次奥氏体化温度的降低而降低。磨球经980℃×150min+920℃×30min的分级奥氏体化后在260℃等温淬火30～50min,尖锐的共晶碳化物棱角钝化,基体中弥散分布着大量的圆粒状二次碳化物,强韧性有一定的提高,表层硬度保持在59HRC左右,心部硬度保持在54.5HRC左右,冲击韧性保持在7.1～7.4J/cm2。4.落球疲劳试验结果表明,淬火+回火态高铬铸铁磨球出现大量的剥落甚至破碎的现象,而经过等温淬火和分级奥氏体化等温淬火后的高铬铸铁磨球极少出现剥落的情况,其抗冲击疲劳磨损能力大大优于淬火+回火态高铬铸铁磨球。对于服役于大直径球磨机的高铬铸铁磨球,等温淬火热处理工艺较好。5.通过数值模拟结合实验分析可知,磨球表层与内浇口的交界部位最后凝固,此处组织粗大,脆性较大且应力集中,在冲击下,脆性相易开裂形成裂纹,导致磨球在该部位首先发生剥落失效现象。更多还原

【Abstract】 High chromium cast iron grinding balls have a large number of applications in metallurgical mining, building materials, cement and power industries because of their excellent wear resistance performance. After quenching and tempering heat treatment, the low toughness high chromium cast iron grinding balls appeared plenty of spalling and broken failure problems when used in the large ball mill. Thus, to improve the impact toughness and impact fatigue resistance of grinding balls, the influence of austempering heat treatment process and multi-stage austenitizing austempering heat treatment process on the matrix microstructures and carbides of the grinding balls were experimentally investigated. In this paper, the main results are as follows:1. The matrix microstructure of high chromium cast iron grinding balls is fine acicular martensite and with a lot of dispersively distributed block and rod secondary carbides after quenching-tempering process. The surface hardness of the grinding ball is about 60 HRC, the core hardness is about 56 HRC, and the impact toughness is about 4.1 J/cm2.Hardness is high but impact toughness is low.2. The high chromium cast iron grinding balls were austempered at 230℃、260℃ and 290℃ for 15 min to 50min. The experimental results show that with the increase of austempering temperature, the hardness and impact toughness of grinding balls all raised first and lowered later; with the extension of austempering time, the hardness of the grinding balls reduced and the impact toughness raised first and lowered later. The matrix of grinding balls is austenite-bainite microstructure with main fine acicular bainite when austempered at 260℃ for 40min, in addition, a certain number of round granular secondary carbides dispersively distributed in the matrix. The hardness and impact toughness of the grinding balls cooperated better, the surface hardness is about 58 HRC, the core hardness is about 55 HRC, and the impact toughness can be up to 7.2 J/cm2 that increased about 75% relative to the quenching-tempering process.3. With the reduce of secondary austenitizing temperature, the hardness of the high chromium cast iron grinding balls was on the decline when the second austenitizing temperature was between℃ 950 and 830℃. By multi-stage austenitizing of 980℃× 150min+920℃×30min, then austempered at 260℃ for 30min to 50min, the shape of angled eutectic carbides were improved, with a large number of round granular secondary carbides dispersively distributed in the matrix. The hardness and impact toughness have a certain extent increase, the surface hardness keep around 59 HRC, the core hardness remain at around 54.5 HRC, and the impact toughness in the range of 7.1 J/cm2 to 7.4 J/cm2.4. The falling ball fatigue test results show that high chromium cast iron grinding balls appeared a large number of spalling and even broken failure problems after quenching-tempering process, but spalling rarely appeared after austempering process or multi-stage austenitizing austempering process. Thus, the impact fatigue wear resistance is greatly superior to the grinding balls after quenching-tempering process. Accordingly, austempering heat treatment process is better for high chromium cast iron grinding balls when served in the large ball mill.5. The numerical simulation and experimental analysis results show that the boundary between grinding ball and flow gate was the last solidification area, the surface layer microstructure of this area is coarse, brittle and stress concentrated, results in the brittle phase cracked easily and formed crack under impact, eventually led to spalling failure occur in this region firstly.更多还原