制管用马氏体钢加工开裂影响因素分析

ANALYSIS OF INFLUENCING FACTORS ON PROCESSING CRACKING OF MARTENSITIC STEEL FOR PIPE-MAKING

虽然马氏体钢具有较高的强度和安全性,在汽车车身的应用比例越来越高,但因内部缺陷引发裂纹而导致低应力断裂的现象却很难避免。本文以两种超高强度级别的马氏体钢为研究对象,从延迟断裂的机理出发,利用金相显微镜、扫描电镜、拉伸试验机分析了不同酸浸及通氢条件下显微组织、断口形貌、断裂时间、拉伸性能的变化。结果表明,酸浓度0.5 mol/L时,即使浸泡时间短,试样开裂时间也会大大提前;断口属于典型沿晶断裂或穿晶断裂形貌,无塑性变形后留下的蜂窝迹象,在断口的周围还存在明显的结晶状颗粒。相同条件下,随电流密度的增大,原始组织中存在成分偏析的试样在1.5 mA/cm^(2)充氢电流密度下,几乎不存在塑性,未到弹性极限就已断裂;同时氢脆敏感性显著增加,应力集中现象更明显,裂纹源数量明显增多,尺寸也变大。综合认为环境(表面)腐蚀直接影响马氏体钢的开裂速率,马氏体钢的原始偏析组织及加工成型后的腐蚀侵蚀是造成零件开裂的主要原因。

Although martensitic steel has high strength and safety,the proportion of application in automobile body is higher and higher,but it is difficult to avoid the phenomenon of low stress fracture caused by internal defects.In this paper,two kinds of martensitic steels with ultra-high strength are studied,the changes of microstructure,fracture morphology,fracture time and tensile properties under different acid leaching and hydrogen permeation conditions were analyzed by means of metallographic microscope,SEM and tensile testing machine.The results show that when the acid concentration is 0.5 mol/L,the cracking time of the sample will be greatly advanced even if the soaking time is short,and the fracture surface belongs to the typical intergranular fracture or transgranular fracture morphology,and there is no honeycomb sign left after plastic deformation but obvious crystalline grains around the fracture surface.Under the same condition,with the increase of the current density,the samples with component segregation in the original structure have almost no plasticity at the current density of 1.5 mA/cm^(2),and break before reaching the elastic limit.At the same time,the hydrogen embrittlement sensitivity is increased,the stress concentration is more obvious,the number of crack source is increased obviously,and the size is also increased.It is considered that environmental(surface)corrosion directly affects the cracking rate of martensitic steel,the original segregation structure of martensitic steel and the corrosion after processing are the main reasons for the part cracking.