灰铸铁制动鼓的热循环开裂分析及表面改性

Thermal Cycle Cracking and Surface Modification of Gray Cast Iron Brake Drum

针对重型卡车在连续刹车时频繁地冷热循环容易造成制动鼓失效开裂的问题,提出一种提高灰铸铁制动鼓抗热开裂性能的表面改性方法——搅拌摩擦焊。实验以灰铸铁(HT250)制动鼓为研究对象,采用循环淬火处理模拟制动鼓的服役工况,淬火处理时的温度为900℃,冷却方式为水冷。利用维氏硬度测试仪测量淬火前后试样的硬度变化,结合扫描电镜、能谱仪和金相显微镜表征试样的微观组织和裂纹形貌。实验结果表明:频繁的冷热循环是导致制动鼓开裂失效的主要原因,经历20次循环淬火处理后,裂纹开始在石墨与珠光体界面处萌生,且主要沿着石墨长度方向扩展,裂纹周边生成了氧化铁新生相;此外,循环淬火处理后灰铸铁的维氏硬度有所提高,但硬度分散性增大;经历相同循环淬火次数后,表面改性试样比原始试样的裂纹数量更少,且裂纹长度更短,这主要是因为表面改性后灰铸铁表面的石墨组织长宽比减小,且珠光体片层间距减小,提高了裂纹萌生和扩展的阻力,从而导致灰铸铁制动鼓的抗热开裂性能提升。

In order to solve the problem of brake drum failure caused by frequent hot and cold cycles during continuous braking of heavy truck,a surface modification method,i.e.,friction stir welding,was proposed to improve the thermal cracking resistance of the gray cast iron brake drum.In this experiment,the gray cast iron(HT250) brake drum was selected and the cyclic quenching treatment was used to simulate the service condition of the brake drum.The quenching treatment was set as 900 ℃ and the cooling method was water cooling.The Vickers hardness tester was used to measure the hardness changes of the samples before and after quenching treatment.Thereafter,the microstructure and crack morphology of the samples were characterized by the scanning electron microscope,energy dispersive spectrometer and metallographic microscope.It was found that frequent cold and hot cycling was the main reason for the failure of the brake drum.The experimental results showed that,after 20 cycles of quenching treatment,the cracks originated from the interface between graphite and pearlite,and then propagated mainly along the length of graphite.Besides that,the new phase of iron oxide is formed around the cracks.Moreover,the Vickers hardness of gray cast iron increased after cyclic quenching,but the hardness dispersion also increased.After the same numbers of cycle quenching,the number of cracks in the surface modified sample is less than that of the original sample,and the crack length was shorter.This is mainly because the graphite length width ratio on the surface of the gray cast iron decrease after the surface treatment,and the pearlite lamellar spacing decreased,which improve the resistance of crack initiation and propagation and thus enhanced the thermal cracking resistance of the gray cast iron brake drum.