摘要
含碳化物奥铁体球墨铸铁(Carbidic Austempered Ductile Iron,简称CADI)是近些年由等温淬火球墨铸铁发展而来的新型耐磨材料,在冶金矿山等行业中有很好的应用前景。本文对CADI磨球的新型热处理工艺及磨球的失效机理进行了系统研究,主要内容包括:1)硝盐淬火介质冷却能力的研究;2) CADI材质与硝盐淬火介质间表面综合换热系数的测定;3) CADI磨球新型热处理工艺的试验研究;4) CADI磨球落球疲劳试验中的应力数值模拟;5) CADI磨球的失效机理研究等。
研究表明:1)在试验范围内(0~2%)随熔盐中含水量的增加,相同温度下盐浴冷却能力不断增强;在试验范围内(250℃~350℃)随盐浴淬火温度的提高,相同温度下盐浴冷却能力有降低的趋势;2)淬火介质的冷却特性曲线不代表热探头表面的淬火信息,淬火介质的冷却特性曲线有其特定的用途;3)在一维传热模型条件下,采用ANSYS软件求得了CADI热探头与淬火盐之间的表面综合换热系数,并验证了结果的可靠性;4)采用ANSYS软件模拟了CADI磨球淬火过程温度场,得出了磨球内外温度均匀所需时间;5)在试验范围内(4min~150min)随着盐浴淬火时间的延长,CADI磨球淬透性增强;磨球的硬度和冲击韧性均随盐浴淬火时间的延长而提高,6)在新型热处理工艺中,盐浴淬火480s时,φ100mmCADI磨球的体积硬度为HRC54.1,冲击韧性8.5J/cm2,排除试验误差的影响,可以认定新型热处理工艺已基本达到等温淬火工艺的水平;7)在试验范围内(1%~3%)随着熔盐中加水量的增加,磨球不同深度层硬度有上升的趋势,而磨球的冲击韧性有所降低;8) CADI磨球在单次落球试验冲击过程中,碰撞接触界面剪应力峰值出现在皮下略偏离接触界面中心位置,等效应力峰值出现在接触界面表层中心位置;残余等效应力峰值和残余剪应力峰值均出现在撞击部位周围挤压区。9) CADI磨球亚表层二次夹渣及一次夹渣的存在是导致其在球磨机中失圆失效的主要内因。
根据本文研究结果,采用“盐浴淬火+电阻炉保温”的新型热处理工艺来代替传统等温淬火工艺是切实可行的,该工艺在保证磨球热处理质量的同时极大提高了劳动生产率。CADI磨球在落球冲击过程中残余应力的累积、应力诱发马氏体相变和加工硬化等因素综合引起的材料疲劳损伤导致了磨球的表层剥落失效;磨球材质及其冶金质量也对磨球的剥落失效有重要影响;磨球亚表层夹渣物的存在是引发其剥落失效和失圆失效的主要内因。
Carbidic austempered ductile iron,so-called CADI,is a new kind of wear resistant material that derived from ADI in recent years.It has a good application and prospect in Metallugical and Mine Industries.This work focuses on the research of new-heat treatment technology and failure mechanism of CADI grinding ball. It mainly includes: 1) Research on cooling capacity of salt-bath quenching medium; 2) Determination of the heat-transfer coefficient between the CADI thermal probe and the salt; 3) Experimental investigation of the new heat-treatment technology on CADI grinding ball; 4) Stress numerical simulation of CADI grinding ball during the falling sphere test;5) Research on failure mechanism of CADI grinding ball.
The result showed: 1) In the test range(0~2%),as water in the salt-bath increased,the cooling capacity of the salt was intensified at the same temperature ; In the test range(250℃~350℃),the cooling capacity of the salt weakened at the same temperature as the austempering temperature enhanced; 2) The cooling curves of the quenching medium can not display the surface information of the thermal probe,but it has special uses; 3) The heat-transfer coefficient between the CADI thermal probe and the salt was determined under the one-dimensional heat transfer model by ANSYS program and the result was reliable; 4) The temperature field of the CADI grinding ball during the austempering was simulated by ANSYS program and the necessary time for the balance of the temperature field was derived; 5) In the test range(4min~150min),as the salt-bath austempering time increased,the hardenability of the CADI grinding ball enhanced while the hardness and impact toughness of the ball showed the same trend;6)In the new heat-treatment test,the mechanical properties of theφ100mm CADI grinding ball with the salt-bath austempering time of 480s were: AVH 54.1HRC, impact toughness 8.5J/cm2. It can be inferred that the new heat-treatment technic reached the level of the austempering technic excluding the impact of experimental error. 7) In the test range(1%~3%), as water in the salt-bath increased, the hardness at different depth of the grinding ball enhanced while the impact toughness of the ball decreased; 8) During the single drop in the falling sphere, the peak value of shear stress appeared in the area slightly deviated from the center of the contact interface while the maximum of equivalent stress just at the center of the contact interface.The peak of residual equivalent stress and residual shear stress both appeared in the neighboring extrusion zone of collision.9) The existence of slag in the subsurface of the ball was the real reason that caused the failure of the grinding ball in the ball mill.
According to the research, it is practical to replace the austempering technology with the new technique of“salt-bath and electrical resistance furnace quenching”, and the labour productivity was enhanced significantly while guaranteeing the heat-treatment quality.The auther believed that surface spalling failure of the CADI grinding ball resulted from the fatigue damage of the material that caused by the accumulation of the residual stress,stress-induced martensitic transformation,etc.At the same time, material and metallurgical quality really had effect on the spalling failure of the grinding ball.The existence of slag in the subsurface of the ball was the real reason that caused the spalling and the roundless failure of the CADI grinding ball.
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