材料期刊网

用电镜薄膜技术研究了6Cr4Mo3Ni2WV基体钢的淬火和回火态的组织结构,着重观察了硬度-回火温度曲线峰值附近的结构,指出合金的二次硬化主要由V_4C_3和M_2C沉淀产生.回火时沿马氏体内孪晶界和{112}晶面析出的片状渗碳体,低于450℃是稳定的,高于500℃是亚稳相,它在回火过程中转变为V_4C_3,M_2C和新的细化渗碳体,这对二次硬化也有贡献;但更主要的是这种转变使合金的冲击韧性α_K值显著提高.在600—650℃亚稳渗碳体的转变区复合转变为M_6C和Cr_7C_3,同时沿原始奥氏体晶界和亚晶界析出连续的等轴M_6C和Cr_7C_3,这一过程又使α_K值下降.

The microstructure changes in the tempering process of a quenched 6Cr4Mo3Ni2WV Matrix steel have been studied by TEM. Special attention has been paidto the observation of the microstuctures near the peak of the hardness-temperingtemperature curve. It is shown that, the secondary hardening of the steel is main-ly brought about by the precipitation of V_4C_3 and M_2C. With the tempering tempera-ture below 450℃ or so, the cementite lathes precipitated at the twin boundariesand {112} planes are fairly stable. However, they are transformed into a meta-stable phase, and finally are replaced by V_4C_3 M_2C and a new fine cementitewhen the tempering temperature is raised to beyond 500℃ This process certainlyleads to an increase in strength (secondary hardening), and what is more, it leadsto a substantial improvement in impact toughness in the meantime. The best impacttoughness is obtained by the time the cementite lathes are completely trans-formed. When tempering in the range of 600-650℃, the V_4C_3, M_ and alsothe fine cementite coalesced to equiaxed particles-M_6C or Cr_7C_3 at the austeniticgrain and sub-grain boundaries, and the impact toughness of the steel drops ac-cordingly.