NOS525钢热锻模具早期开裂失效分析

Failure analysis on early cracking of hot forging mold for NOS525 steel

采用直读光谱仪、数显洛氏硬度计、光学显微镜、扫描电镜及能谱仪,对NOS525钢热锻模具早期开裂失效件的化学成分含量、断口特征形貌及金相组织进行检测和分析。结果显示,显微组织中的马氏体针尤为细长,该组织属于晶粒粗大的过热组织,主要原因为:模具在热处理过程中高温区加热时间过长,造成组织晶粒的急剧长大,且随着基体组织中碳含量的增加,马氏体形态由条状转变为针状,材料强度降低且脆性增大。由扫描电镜检测得知,模具开裂断面呈准解理断口及出现条状撕裂棱,表明组织中存在低硬度的软点区,断口表面的撕裂棱特征形貌属于淬火后未转变的残余奥氏体组织。残余奥氏体在应力作用下易发生马氏体相变,从而增加组织应力,进一步加大了材料开裂风险,降低模具的使用性能。同时,热锻模具在使用过程中,工作面存在冷却不及时或冷却不彻底的现象,使得模腔凹槽部位产生明显的热疲劳裂纹。

The chemical composition content,fracture feature morphology and metallographic structure of the early cracking failure piece for hot forging mold of NOS525 steel were detected and analyzed by direct reading spectrometer,digital Rockwell hardness tester,optical microscope,scanning electron microscope and energy spectrometer.The results show that the martensite needle in the microstructure is particularly slender,and the microstructure belongs to the ovrheated structure with coarse grains.The main reason is that the mold is heated for a long time in the high temperature region during the heat treatment process,and the structure grains grow up rapidly.As the carbon content in the matrix increases,the martensite morphology changes from strip to needle,the strength of the material decreases and the brittleness increases.Furthermore,by the detection of scanning electron microscopy,the cracked section of the mold is quasi-cleavage fracture and appears strip-shaped tearing edge,which indicates that the soft point zone with low hardness occurs in the microstructure,and the tearing edge feature of the fracture surface belongs to the unconverted retained austenitic organization after quenching.However,the retained austenite is prone to martensite transformation under stress to increase the structural stress and the risk of material cracking,and the usability of the mold is reduced.At the same time,during the use of the hot forging mold,there is a phenomenon that the working surface is not cooled in time or the cooling is not complete,so that the groove of the mold cavity has obvious thermal fatigue crack.