65Mn钢渗硼层的微观组织、硬度及生长动力学

Microstructure,hardness and growth kinetics of 65Mn steel borided layer

采用固体渗硼工艺对65Mn钢进行渗硼处理,并借助光学显微镜、X射线衍射仪、电子探针及维氏硬度计等手段系统研究了渗硼温度(800~1000℃)和渗硼保温时间(2~8 h)对65Mn钢渗硼层厚度、微观组织和硬度的影响规律以及渗硼层的生长动力学。结果表明,随着渗硼温度的升高或渗硼时间的延长,渗硼层的厚度不断增大,但当渗硼温度超过900℃时,渗硼层中黑色孔洞的数量、大小以及距离渗硼层表面的深度都逐渐增大。65Mn钢渗硼层都由Fe_(2)B柱状晶,以及位于Fe_(2)B柱状晶生长前沿及晶粒间的Fe_(3)(B,C)相、二元铁硅化合物和三元铁碳硅化合物组成,其维氏硬度(800~1590 HV0.05)远大于65Mn钢基体的硬度(238 HV0.05)。由于硬度较低的Fe_(3)(B,C)相和富硅相分布于高硬度的Fe_(2)B柱状晶晶粒之间,导致渗硼层的硬度并不随离渗硼层表面距离的增加而单调减小。渗硼层厚度的平方与渗硼时间呈线性关系,B原子在65Mn钢渗硼层中的扩散激活能为220.96 kJ/mol。

Effects of boriding temperature(800-1000℃)and boriding time(2-8 h)on the thickness,microstructure and hardness of the borided layers on 65Mn steel by pack boriding and its growth kinetics were systematically investigated by optical microscope,XRD,EPMA and Vickers hardness tester,etc.The results show that with the increase of boriding temperature or time,the thickness of the borided layers increases continuously,but the amount,size and depth away from the surface of borided layer of black holes also increase gradually when the boriding temperature exceeds 900℃.The layers are composed of Fe_(2)B columnar crystals,Fe_(3)(B,C)phase,Fe-Si binary compounds and Fe-C-Si ternary compounds,and the later three are formed between and in the front of Fe_(2)B columnar crystals.The hardness of the layers(800-1590 HV0.05)is greatly higher than that of the 65Mn steel(238 HV0.05).However,it is found that the hardness values of the borided layers are not monotonically decreasing when the distance from the surface of the layers is increasing,which should be attributed to the Fe_(3)(B,C)and Si-rich phases with lower hardness distributed between the Fe_(2)B columnar crystals with higher hardness.There is a liner relationship between square of the thickness of borided layer and boriding time,and the calculated diffusion activation energy of boron atoms in borided layer on the 65Mn steel is 220.96 kJ/mol.