A TEM OBSERVATION OF THE SECONDARY HARDENING PROCESS IN A 6Cr4Mo3Ni2WV DIE STEEL
Yu Xuejie Shanghai Institute of Iron ond Steel Research
Cite this article:
Yu Xuejie Shanghai Institute of Iron ond Steel Research. A TEM OBSERVATION OF THE SECONDARY HARDENING PROCESS IN A 6Cr4Mo3Ni2WV DIE STEEL. Acta Metall Sin, 1980, 16(1): 53-132.
Abstract 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.
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